Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation

Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation
energies
Article
Feasibility Study on Variable-Speed Air Conditioner
under Hot Climate based on Real-Scale Experiment
and Energy Simulation
Jaehun Lim 1,2 , Myung Sup Yoon 2 , Turki Al-Qahtani 3 and Yujin Nam 1, *
 1   Department of Architectural Engineering, Pusan National University, 2 Busandaehak-ro 63, Geomjeong-gu,
     Busan 46241, Korea; jake88@ktl.re.kr
 2   Korea Testing Laboratory, 87, Digital-ro, Guro-gu, Seoul 08389, Korea; msyun95@ktl.re.kr
 3   Saudi Standards Metrology and Quality Organization, Al Imam Saud Ibn Abdul Aziz Road,
     Al Mohammadiyah, Riyadh 11471, Saudi Arabia; ts.alqahtani@saso.gov.sa
 *   Correspondence: namyujin@pusan.ac.kr; Tel.: +82-51-510-7652; Fax: +82-51-514-2230
                                                                                                   
 Received: 19 March 2019; Accepted: 18 April 2019; Published: 19 April 2019                        


 Abstract: It is well known that inverter-driven variable-speed compressor (or inverter) air conditioners
 are more efficient than constant-speed compressor air conditioners. Therefore, most countries have
 employed part-load assessment test standards such as ISO 16358, EN 14825 and ASHRAE 116 in
 addition to the conventional ISO 5151 full-load test standard to appropriately evaluate the part-load
 efficiencies of variable-speed air conditioners. However, many countries in the Middle East and
 South America still use the ISO 5151 standard owing to policy or high- temperature environmental
 considerations. In this study, we experimentally verify the energy saving effect of the inverter air
 conditioner with respect to the constant-speed air conditioner under the Korean climate with distinct
 temperature changes of four seasons and under the hot climate of Saudi Arabia throughout the
 year. ISO 5151 defines test conditions for a single temperature, whereas ISO 16358, EN 14825 and
 ASHRAE 116 simulate seasonal efficiencies using interpolation of several climate test results. Herein,
 we directly employ the environmental changes during a day or season in a qualified test room with
 specific dimension. Using extensive regional and seasonal climate data for Saudi Arabia and South
 Korea, the changes in temperature conditions are applied directly to the outdoor side and appropriate
 building cooling load conditions are applied to the indoor side of the air-enthalpy-type test room.
 The energy savings of the inverter air conditioner were analyzed experimentally according to the
 spatial and temporal temperature changes. The energy reduction effects of the inverter air conditioner
 largely depended on the temperature and cooling load changes for a day or season. Furthermore, a
 feasibility study based on an energy simulation showed that the variable-speed air conditioner could
 be economical even in hot climates.

 Keywords: energy saving; inverter air conditioner; energy efficiency; Saudi Arabia; Korea




1. Introduction
     Air conditioners are essential not only in “hot” countries such as Saudi Arabia but also in Korea.
In Korea, air conditioners were luxury items until the 1980s, but gradually became popular since the
1990s. Nowadays, they are used in most homes owing to the increased temperatures in summer by
climate changes. The energy labelling program of air conditioners started in 1992 in Korea, as energy
resources were scarce and the demand for cooling energy in summer was a large portion of the total
electricity demand. In particular, the peak power demand management in summer was a big concern
every year.



Energies 2019, 12, 1489; doi:10.3390/en12081489                               www.mdpi.com/journal/energies
Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation
Energies 2019, 12, 1489                                                                                2 of 14



      Saudi Arabia is the world’s largest oil producer having approximately 20% of the world’s oil
reserves. Despite the supply of electricity by the abundant oil resources, the Saudi government has
been interested in energy conservation owing to the rapid industrialization and population increase in
the 2000s. Saudi Arabia’s crude oil production is not expected to rapidly increase. In addition, the
crude oil exports are expected to decrease with the increases in domestic demand. In this regard, Saudi
Arabia has mandated the minimum energy efficiency performance standard for room air conditioners
since 2013, as the autonomous Saudi Standards, Metrology, and Quality Organization (SASO) 2663/2007
standard has been amended to an enhanced compulsory SASO 2663/2012 standard [1].
      The energy efficiency regulation of air-conditioners is applied in most countries, not only in
these two countries. The definition of energy efficiency of an air conditioner is largely managed by
two approaches based on the simple energy efficiency ratio (EER) by International Organization for
Standardization (ISO) 5151 and seasonal EER (SEER) by ISO 16358, European (EN) 14825 and ASHRAE
116 [2–5]. Manufacturers have carried out extensive technological studies to improve the performances
of air conditioners. One of the most notable approaches is the compressor technology. A compressor
control technology has been developed, which could vary the speed of the compressor in response
to a cooling load. It provides a high efficiency of the compressor such as a high efficiency of the
compressor motor section, reduction in the mechanical friction of the rotating section, and reduction
in the pressure loss of the refrigerant entering and exiting sections. The variable-speed compressor
(or inverter compressor) can reduce the operation cost, compared to the constant-speed compressor.
In this regard, we need to assess the efficiency of the air conditioner during a part-load operation.
      Some countries in the Middle East and South America still prefer to use full-load evaluation
methods as the ISO 5151 full-load test is simple and the hot climate in most the seasons requires full
operation of the air conditioner. However, the winter period in Saudi Arabia does not require air
conditioning; in addition, some months before and after the summer peak require a part-load operation.
      Several studies compared performance differences between constant-speed and variable-speed
air conditioners. Chen et al. [6] carried out a test in a specific-size real room with an operation start at a
pre-heated temperature and evaluated the effects of the product temperature setting and space heat
load. Yurtseven et al. [7] studied real office usage circumstances in a relatively long-term power input
measurement. Al-tamimi et al. [8] analyzed the thermal performance and energy consumptions of air
conditioner devices for two identical houses located in the Dhahran area of Saudi Arabia for one year.
Khatri et al. [9] carried out a long-term field test to compare the energy savings of two types of air
conditioners in two specific rooms in India. However, these studies did not analyze the performance
differences under controlled temperature variations and building cooling load variations in accurately
controlled test rooms.
      In addition, the performances and economic efficiencies of the air conditioners have been
investigated. Wu et al. [10] contributed to the variable air conditioner efficiency standard by monitoring
the outdoor temperature distribution and operation time in China. Oropeza-Perez [11] analyzed the
economic efficiencies based on several variables such as the set temperature of the air-conditioner in
Mexico. Shah et al. [12] carried out a simple payback period analysis according to the efficiency and
type of the air conditioner in India.
      In this study, we analyze the energy saving effects of inverter and constant-speed air conditioners
during the cooling periods in Saudi Arabia and South Korea. People usually think that no difference in
energy efficiency between constant-speed and inverter-type air conditioners exists in countries with
hot climates such as Saudi Arabia. However, few experimental studies based on dynamic energy
simulations of variable air conditioners were carried out. In this study, dynamic energy simulation
results are applied to a real-scale environmental test chamber. Using the test results, the energy
consumptions are compared and the suitability of the constant and variable-speed air conditioners are
analyzed. The energy usage of each air-conditioner under the laboratory environmental conditions is
evaluated by simulating the climate of South Korea where the four seasons are evenly distributed and
hot climate of Saudi Arabia throughout the year.
Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation
Energies
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               12 FOR   1489 REVIEW                                                                                                             3 of 14
     Energies 2019, 12 FOR PEER REVIEW                                                                                                                3

2. Materials and Methods
    2. Materials
       Materials and
                 and Methods
2.1. Summary of the Experimental Approach
          Summary of
     2.1. Summary    of the
                        the Experimental
                            Experimental Approach
      In general, the condenser of the air-conditioner is installed at the outdoor side, while the
           In is
              general,         condenser of the air-conditioner is installed at the outdoor side, while the
                         theat condenser
evaporator       installed     the indoor side of the building, as shown in Figure 1. As the outdoor and
     evaporator is installed at the indoor side of the building, as shown in Figure 1. As the outdoor and
     evaporator
indoor temperatures are not fixed, i.e., they vary with time, the performance of the air conditioner,
     indoor temperatures are   are not fixed, i.e., they
                                                    they vary
                                                          vary with
                                                                with time, thethe performance of of the air
                                                                                                         air conditioner,
sensitive totemperatures
               the temperature,not   changes accordingly.      In thistime,
                                                                         regard, performance
                                                                                    a performance thetest ofconditioner,
                                                                                                              the air
     sensitiveisto  the temperature, changes accordingly. In         In this
                                                                         this regard,
                                                                               regard, aa performance
                                                                                           performance test of of the air
conditioner       carried out in a calorimeter-type      or air-enthalpy-type      test room (Figure 2) test
                                                                                                          where the
     conditioner is carried out in a calorimeter-type or air-enthalpy-type
     conditioner                                                 air-enthalpy-type test test room (Figure 2) where the
temperature environment is exactly controlled according to the ISO 5151 standard.
     temperature environment is exactly controlled
                                               controlled according
                                                            according to to the
                                                                            the ISO
                                                                                 ISO 5151
                                                                                      5151 standard.
                                                                                            standard.




                                                Figure 1. Basic
                                                    Figure      model
                                                               Basic
                                                            1. Basic  for the
                                                                     model foranalysis.
                                                                               the analysis.
                                                                                   analysis.
                                                    Figure  1.       model for the




                                   Figure
                               Figure      2. Air-enthalpy
                                      2. Air-enthalpy type type  test room
                                                           test room   usedused  forexperiment.
                                                                            for the  the experiment.
                                   Figure 2. Air-enthalpy type test room used for the experiment.
           Unlike
      Unlike     thethe(T(T   out /T ) environmental setting of the ISO test methodology, we used the temperature
                           out/Tin) inenvironmental setting of the ISO test methodology, we used the
     in theUnlike
temperature outdoor
                in thetheside (Tand
                         outdoor out/Tin ) environmental
                                        the
                                       side  building   coolingsetting
                                              and the building      load
                                                                      coolinginofthe the
                                                                                   load    inISO
                                                                                       indoor thesidetest(Tout
                                                                                                    indoor      /qbuild
                                                                                                            methodology,
                                                                                                              side    (T).outIn      we
                                                                                                                                 other
                                                                                                                             /qbuild        used
                                                                                                                                            other the
                                                                                                                                     ). Inwords,     we
    temperature
     set the          in
             temperature  the   outdoor
                                 of  the     side
                                          outdoor and   the
                                                      side    building
                                                            for  a          cooling
                                                                   simulation         of load
                                                                                          the    in
                                                                                                heat
words, we set the temperature of the outdoor side for a simulation of the heat reservoir outside andthe   indoor
                                                                                                       reservoir     side
                                                                                                                     outside (T     /q
                                                                                                                                  and
                                                                                                                                out        ). In
                                                                                                                                         applied
                                                                                                                                      build       other
                                                                                                                                                    the
    words,thewe
     cooling
applied        loadsetcondition
                cooling theloadtemperature      ofinthe
                                      in the indoor
                                     condition           outdoor
                                                         side
                                                       the     for aside
                                                            indoor      sidefor
                                                                       simulationfora simulation
                                                                                          of the heat
                                                                                      a simulation     of infiltration
                                                                                                           the
                                                                                                           of theheatheat
                                                                                                                        reservoir       outside
                                                                                                                             orinfiltration
                                                                                                                                 generation.       and
                                                                                                                                                 orT out

generation. Tout and qbuild were provided time-dependently. In the test setup, Tout(t) was realized by a or
    applied
     and  qbuildthe  cooling
                  were     providedload    condition    in
                                          time-dependently. the   indoor
                                                                     In  the   side
                                                                                test   for
                                                                                      setup, a  simulation
                                                                                                 T out (t) was   of   the
                                                                                                                  realized   heat
                                                                                                                                by   infiltration
                                                                                                                                      a cooling     coil
    generation.
     andcoil
cooling   heater
               andatTheater
                        the
                      out  and  atqbuild
                                    thewere
                               outdoor          provided
                                            side,
                                          outdoor while      time-dependently.
                                                     side, qwhile
                                                             build (t)qbuild
                                                                        was (t)implemented  In the test
                                                                                  was implemented     by asetup,
                                                                                                             heater
                                                                                                             by      Tout
                                                                                                                        at(t)the
                                                                                                                  a heater      was    realized
                                                                                                                                atindoor
                                                                                                                                    the            byas
                                                                                                                                               side,
                                                                                                                                          indoor      a
side,cooling
     shown
      as shown coil
              in   inand
                  Figure    heater
                            2. The
                       Figure          at the
                                       largest
                                   2. The      outdoor
                                                difference
                                            largest       side,was
                                                      difference  while
                                                                      thatqthat
                                                                     was      build(t)
                                                                              the   thewas
                                                                                    cooling    implemented
                                                                                                coil coil
                                                                                          cooling     at the   theby
                                                                                                            at indoor   a side
                                                                                                                     indoorheater didatnot
                                                                                                                                 side     theoperate
                                                                                                                                         did    indoor
                                                                                                                                               not
operate and the refrigeration function was operated only by the indoor test unit (air-conditioner not
    side,  as  shown      in  Figure     2. The  largest   difference       was    that   the   cooling     coil  at  the   indoor      side   did
    operate and the refrigeration function was operated only by the indoor test unit (air-conditioner
Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation
heater, which could be easily controlled and had a quick response.

2.2. Air-Enthalpy-Type Test Room Specifications
     The Korea/International Laboratory Accreditation Cooperation (ILAC) accredited facility
Energies
following      12, 1489
         2019,the  ISO5151 standard was used as the air-enthalpy test room (Figure 3). The maximum         4 of 14

cooling capacity that could be measured was 18,600 W. The inner dimensions (W × D × H) of the
outdoor and indoor rooms were 4.1 m × 4.8 m × 3.6 m and 5.7 m × 4.8 m × 3.6 m, respectively. H
and the refrigeration function was operated only by the indoor test unit (air-conditioner indoor unit).
included an upper airflow passage above the punched ceiling. The walls of the test room were 75
Therefore, the temperature of the indoor room was decreased by the operation of the air conditioner
mm poly-urethane panels. The main instruments were shown in Table 1.
while the target temperature was set. The wall of the test room minimized heat penetration and air
leakage from the outside with the use of a sufficient thermal insulation material; the relevant test room
                             Table 1. Measurement error of measuring equipment.
specification is described in Section 2.2.
      In order toItem
                    measure the energy Model
                                           consumptionManufacturer        Measurement
                                                          in each case (constant     andUncertainty
                                                                                            variable-speed air
            Temperature (RTD)           Pt100            CHINO                      ±0.1 °C
conditioners), the outdoor temperature changes (Tout (t)) and indoor cooling load changes (qbuild (t))
                                                                                     ±0.6 V
were applied    in the air-enthalpy test
            Power consumption
                                          room shown Yokogawa
                                       WT230            in Figure 2. The outdoor±0.04 temperature
                                                                                          A
                                                                                                      data were
applied for the capital cities of Saudi    Arabia and South
                                      KDY-AC                   Korea during the cooling
                                                        Light star                  ±1.1 W season, while the
cooling load data    were obtained by DA100
               Data logger              TRNSYS modeling.
                                                       Yokogawa            0.05% of reading, 2 digits
      As the latent heat in the cooling load cannot be easily experimentally controlled according to the
2.3.
time,Heat
       theBalance  Evaluation
           latent heat        of the Air-Enthalpy-Type
                        was included                    Test Room
                                       in the sensible heat.       under the
                                                              The cooling    Steady
                                                                          load  wasState
                                                                                       simulated by a heater,
which   could  be easily controlled  and had  a quick response.
     In order to experimentally implement qbuild(t), it was necessary to evaluate the heating
components affecting the both inside and outside of the test room. As shown in Figure 2, the heat
2.2. Air-Enthalpy-Type Test Room Specifications
generation element in the test room was not only the heater, but also the turbo, air handling unit
(AHU) Theand
          Korea/International
              air sampler fans.Laboratory    Accreditation
                                  In addition,              Cooperation
                                                 heat penetration    from (ILAC)    accredited
                                                                            the outside  of thefacility following
                                                                                                 test room   wall
the ISO The
existed.  5151heat
                standard  was
                    balance of used   as room
                                the test  the air-enthalpy
                                                 was comparedtest room
                                                                  between (Figure  3). Thecomponents
                                                                              the heating    maximum cooling
                                                                                                          in each
capacity that could
configuration        be measured
                and cooling         wasof18,600
                             capacity      the airW. The inner dimensions (W × D × H) of the outdoor and
                                                   conditioner.
indoor  rooms  were  4.1 m ×  4.8 m ×  3.6 m  and
      Based on the measurement for each component, 5.7 m × 4.8the × 3.6 heat
                                                                m total        generation H
                                                                         m, respectively.    included
                                                                                           was  compared an upper
                                                                                                             with
airflow
the      passage
    tested  coolingabove the punched
                    capacity  of the air ceiling.  The walls
                                          conditioner,         of the
                                                        as shown    in test
                                                                       Tableroom   were the
                                                                               2, under  75 mm
                                                                                            steadypoly-urethane
                                                                                                     state before
panels.  The main instruments
the time-dependent    simulationwere
                                   tests.shown in Table 1.




                                 (a)                                              (b)
                Air-enthalpy-type test room: (a)
      Figure 3. Air-enthalpy-type            (a) Korea/International
                                                 Korea/International Laboratory Accreditation Cooperation
      (ILAC) accredited chamber; (b) test sample.
                                          sample.

                             Table 1. Measurement error of measuring equipment.

                Item                Model              Manufacturer             Measurement Uncertainty
        Temperature (RTD)              Pt100              CHINO                           ±0.1 ◦ C
                                                                                         ±0.6 V
                                    WT230                Yokogawa
       Power consumption                                                                 ±0.04 A
                                   KDY-AC                Light star                      ±1.1 W
            Data logger             DA100                Yokogawa                0.05% of reading, 2 digits


2.3. Heat Balance Evaluation of the Air-Enthalpy-Type Test Room under the Steady State
     In order to experimentally implement qbuild (t), it was necessary to evaluate the heating components
affecting the both inside and outside of the test room. As shown in Figure 2, the heat generation
element in the test room was not only the heater, but also the turbo, air handling unit (AHU) and air
sampler fans. In addition, heat penetration from the outside of the test room wall existed. The heat
balance of the test room was compared between the heating components in each configuration and
cooling capacity of the air conditioner.
Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation
Energies 2019, 12, 1489                                                                                              5 of 14



     Based on the measurement for each component, the total heat generation was compared with the
tested cooling capacity of the air conditioner, as shown in Table 2, under the steady state before the
time-dependent simulation tests.

                              Table 2. Measurement results of the test room components.

                 Categories                        Description               Measured (W)            Total (W)
                                               Cooling capacity
                                                                                   5680 ± 5          5680 ± 5
                  Test unit                       (Sensible)
                                               Cooling capacity
                                                                                   231 ± 10          231 ± 10
                                                   (latent)
                                                 Heater (EH )                  4175 ± 100
                                               AHU’s fan (EAF )                 1150 ± 20
           Indoor room heating
                                               Turbo fan (ETF )                 250 ± 10            5760 ± 145
               components
                                               Heat penetration                  15 ± 5
                                               Air sampler fan                  170 ± 10
                Condensate                              -                          213 ± 10          213 ± 10


     During the simulation test described below, the variable components such as the heater, AHU fan
and turbo fan were measured in real time with a power meter. By adjusting the power consumption of
each component affecting the cooling load in real time, the time-variant building cooling load could be
provided in the indoor side of the test room.

2.4. Climates in Korea and Saudi Arabia
      Korea is located in the mid-latitude temperate climatic zone; the four seasons, spring, summer,
fall, and winter are conspicuous. In winter (December–February), it is cold and dry under the
influence of the continental air from northern China and Siberia. In summer (June–August), hot and
humid weather is observed, attributed to the Pacific atmospheric pressure. In spring (March–May)
and autumn (September–November), many sunny days are observed owing to the influence of the
migratory anticyclone.
      The climate of Saudi Arabia is characterized by high temperatures during the day and low
temperatures during the night. Most of the country follows the pattern of the desert climate, except
the southwest region having a semi-arid climate [13].
      Table 3 shows the distributions of the monthly temperatures in Seoul and Riyadh, the capital cities
of South Korea and Saudi Arabia, respectively [14,15]. In Seoul and Riyadh, the cooling seasons are
June–September and March–November, respectively. In both cities, continuous air conditioning is not
required for all the cooling periods. In Seoul, the air conditioners operate intermittently throughout
the cooling period. In Riyadh, the air conditioners operate intermittently in March, April, September,
October and November.

              Table 3. Temperatures in Seoul and Riyadh (average high, daily mean, average low). *

    City        Temp.         Jan.   Feb.    Mar.    Apr.   May. Jun.       Jul.     Aug. Sep.      Oct.    Nov.   Dec.
              High   (◦ C)      2      5      10      18     23      27      29       30      26     20      12      4
   Seoul      Mean (◦ C)       −2      0       6      13     18      22      25       26      21     15      7       0
              Low (◦ C)        −6     −3       2       8     13      18      22       22      17     10      3      −3
          High (◦ C)           19     23      27      32     38      41      43       42      40     34      27     22
   Riyadh Mean (◦ C)           14     17      21      27     32      34      36       36      33     28      22     17
          Low (◦ C)            11     15      21      26     28      29      27       26      20     16      11      9
      * Climate data from Wikipedia (“Climate of Seoul”, “Climate of Saudi Arabia”); originally from the Weather base.
Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation
multi-zone module by applying a Meteonorm climate (for 20 years average temperature). Based on
the transfer function method, a dynamic thermal energy analysis was performed in consideration of
various types of complex heat gain occurring in space and associated radiation, convection, heat
storage and heat dissipation of equipment [16,17]. The TRNSYS energy rate control method was
used to analyze the seasonal building cooling load. Modules of type 15-2, type 56, TRNSYS
Energies 2019, 12, 1489                                                                                                      6 of 14
three-dimensional (3D) plugin and Google SketchUp were utilized. We set a single zone of the same
size in Seoul and Riyadh; however, the size of the window was set according to each country (Figure
4).
2.5.The   architectural
      Summary              style
                   of Energy     changes according
                              Simulations                 to theCoolong
                                           for the Building        life andLoad
                                                                             environment aspect, and various studies
are being conducted on this [18,19]. However, the comparison in the form of the same building is
       The TRNSYS program (version 17) was used to obtain the building cooling load for the cooling
likely to be suitable for comparing the energy consumption by the type of air conditioner of this
periods. We applied the meteorological data of Seoul and Riyadh for several years embedded in the
study. The Saudi households typically use small windows to block direct solar radiation. The
TRNSYS program. TRNSYS (TRaNsient SYSTem Simulation), which is widely used as a building load
properties of glass and wall materials were applied by referring to the Korea’s architectural
calculation program, was developed by SEL (Solar Energy Lab) of University of Wisconsin, USA. The
regulation notice and Saudi’s paper [20,21]. The room temperature in the TRNSYS simulation was
calculation algorithm of building load in TRNSYS is based on ISO 13790 and ISO 7730. Simulation for
set to 26 °C, which was the same as the air-conditioner temperature setting during the experiment.
estimating the heating and cooling load on the building was performed through type 56. A multi-zone
The recommended room temperature is different for each country. In the case of Korea, 26 °C is
building analysis module of the TRNSYS program. Multi-zone module includes building properties
recommended according to the "Ministry of Land, Infrastructure and Transport", and 23–25 °C are
such as geometric dimensions and wall configuration data and windows. In this module, the heat
recommended according to the government guidelines in Saudi Arabia. For an equal comparison,
transfer by conduction, convection and radiation, heat gains were calculated considering the presence
we set the temperature as 26 °C in this study [20,22].
of occupants and equipment, and the storage of heat in the room air and building mass. Furthermore,
       According to ISO 7730, heat from the human body under light work circumstance is 65 W for
it is possible for the temperature, solar radiation, bearing etc. to multi-zone module by applying a
sensible heat and 55 W for a latent per person [23].
Meteonorm climate (for 20 years average temperature). Based on the transfer function method, a
       The building components for the TRNSYS simulation in Seoul and Riyadh are summarized in
dynamic thermal energy analysis was performed in consideration of various types of complex heat
Table 4.
gain occurring in space and associated radiation, convection, heat storage and heat dissipation of
equipment [16,17]. TheTable    TRNSYS   energysimulation
                                    4. TRNSYS    rate control      method in
                                                                conditions   was  used
                                                                               Seoul and toRiyadh.
                                                                                            analyze the seasonal building
cooling load. Modules of type 15-2, type 56, TRNSYS three-dimensional (3D) plugin and Google
           Input variables                               Seoul                                         Riyadh
SketchUp      were utilized. We set a single zone            of the same size in Seoul and Riyadh;                however, the
         Space (single zone)                       10 m × 8 m × 2.5 m                            10 m × 8 m × 2.5 m
size of the Window
               window was set according           to  each    country     (Figure
                                        60% of the southern wall area—15 m2        4).  The   architectural       style
                                                                                       30% of the southern wall area—7.5  changes
                                                                                                                             m2
according to the life and environment Located   aspect,between
                                                          and various
                                                                    floors studies are being     conducted
                                                                                              Located   betweenon     this [18,19].
                                                                                                                   floors
            Floor of space
However, the comparison in the form          (Upofand
                                                    thedown
                                                         same  insulations)
                                                                  building is likely to be  (Up  and down
                                                                                               suitable    forinsulations)
                                                                                                                comparing the
                 Glass                                1.20 W/m2K                                    1.720 W/m2K
energy consumption           by the type of air conditioner           of this study.    The  Saudi    households         typically
                 Wall                                 0.21 W/m2K                                    0.345 W/m2K
use   small
   Room      windows
          temperature      to block direct solar26radiation.
                        & humidity                    °C & 60 %R.H. The properties of glass 26    and°C wall    materials were
                                                                                                         & 60%R.H.
applied    by referring
       Heating             to the Korea’s architectural
                & Light element                              regulation
                                                        25 W/m   2          notice and Saudi’s paper  25 W/m[20,21].
                                                                                                                2       The room
                                                       4 persons       ◦
temperature in the TRNSYS simulation was set to 26 C, which was the same as the air-conditioner       4 persons
temperaturePersons
                 setting during the experiment.(Sensible: 65 W / person
                                                        The recommended room temperature      (Sensible: 65 W / person
                                                                                                          is different for each
                                        ◦        Latent: 55 W/person)                           Latent: 55 W/person)
country. InInfiltration
               the case of Korea, 26 C is recommended   0.4 (1/h)   according   to the  "Ministry    of0.4 (1/h) Infrastructure
                                                                                                         Land,
and Transport",                    ◦
                      and 23–25 C are recommended
             Ventilation                                0.6 (1/h)according to the government0.6          guidelines
                                                                                                           (1/h)         in Saudi
Arabia. For an equal comparison, we set the temperature as 26 ◦ C in this study [20,22].




                               (a)                                                              (b)
                              Figure 4. Models of households in (a) Seoul and (b) Riyadh.

      According toof ISO
2.6. Specifications       7730, heatAir
                      the Compared   from  the human body under light work circumstance is 65 W for
                                        Conditioners
sensible heat and 55 W for a latent per person [23].
      The building components for the TRNSYS simulation in Seoul and Riyadh are summarized in
Table 4.
Energies 2019, 12, 1489                                                                                     7 of 14



                            Table 4. TRNSYS simulation conditions in Seoul and Riyadh.

          Input variables                           Seoul                                   Riyadh
        Space (single zone)                  10 m × 8 m × 2.5 m                       10 m × 8 m × 2.5 m
             Window                 60% of the southern wall area—15 m2      30% of the southern wall area—7.5 m2
                                           Located between floors                   Located between floors
           Floor of space
                                         (Up and down insulations)                (Up and down insulations)
            Glass                                1.20 W/m2 K                             1.720 W/m2 K
            Wall                                 0.21 W/m2 K                             0.345 W/m2 K
 Room temperature & humidity                  26 ◦ C & 60%R.H.                         26 ◦ C & 60%R.H.
   Heating & Light element                         25 W/m2                                  25 W/m2
                                                  4 persons                                4 persons
              Persons                  (Sensible: 65 W/person Latent:           (Sensible: 65 W/person Latent:
                                                55 W/person)                             55 W/person)
            Infiltration                           0.4 (1/h)                                0.4 (1/h)
            Ventilation                            0.6 (1/h)                                0.6 (1/h)


2.6. Specifications of the Compared Air Conditioners
     For the comparison test of constant and variable-speed air conditioners in the test room, products
with similar specifications were selected (Table 5). The two products had similar capacities, power
inputs and EERs with relative differences within 5%. The purpose of this study was to evaluate the
energy saving effects of the variable-speed air conditioner in real climate conditions even though the
specifications were similar to those of the constant-speed air conditioner. To operate under the Korean
and hot Saudi Arabian climates, hot-climate (T3) air conditioners were employed.

                     Table 5. Specifications of the constant and variable-speed air conditioners.

                Contents                          Constant Speed                         Variable Speed
             Electricity                     Single phase, 230 V, 60 Hz            Single phase, 230 V, 60 Hz
 Rated cooling capacity (T1 climate)                   5130 W                                5275 W
  Rated power input (T1 climate)                       1520 W                                1,490 W
     Rated current (T1 climate)                          6.8 A                                6.7 A
         EER (T1 climate)                            3.375 W/W                             3.540 W/W
   Cooling capacity (T3 climate)                       4420 W                                4484 W
     Power input (T3 climate)                          1820 W                                1754 W
        Current (T3 climate)                            8.14 A                                 7.8 A
         EER (T3 climate)                            2.429 W/W                             2.556 W/W
      Maximum power input                              2610 W                                2900 W
         Maximum current                               11.68 A                                14.0 A
             Refrigerant                           R410a, 0.85 kg                        R410a, 0.95 kg
            Product type                       Wall mount split type                 Wall mount split type
  Annual energy consumption by
                                                   4104 kWh/year                         4023 kWh/year
        SASO energy label


3. Results

3.1. Simulation Results
      Before the experiments with the constant and variable-speed (inverter) air conditioners, TRNSYS
simulations were carried out for building cooling loads under Seoul and Riyadh temperature conditions
(Figures 5b and 6b). As the TRNSYS program contained long-term data (more than 10 years), it seemed
that the recent temperature increase owing to global warming is buried in Figures 5a and 6a. Therefore,
it seemed that 1–3 ◦ C were underestimated compared with the climate felt nowadays. The building
cooling loads of Seoul and Riyadh have maximum values of approximately 4000 W in August. The
two cities have similar maximum building loads as the humidity in summer is very low in Riyadh and
very high in Seoul.
temperature conditions (Figures 5b and 6b). As the TRNSYS program contained long-term data
(more than 10 years), it seemed that the recent temperature increase owing to global warming is
buried in Figures 5a and 6a. Therefore, it seemed that 1–3 °C were underestimated compared with
the climate felt nowadays. The building cooling loads of Seoul and Riyadh have maximum values of
approximately
Energies             4000 W in August. The two cities have similar maximum building loads as
         2019, 12, 1489                                                                    8 ofthe
                                                                                                14
humidity in summer is very low in Riyadh and very high in Seoul.
              o
          ( C)                                                                                       (W)

               40                                                                                   4,000




                                                                                    Cooling Load
Temperature




               30                                                                                   3,000



               20                                                                                   2,000

                                                        Jun        Jul
                                                        Aug        Sep                                                                            Jun        Jul
               10                                                                                   1,000                                         Aug        Sep



                  0                                                                                       0
                      0      4         8           12     16       20        24                               0       4         8          12      16       20       24
                                           Daily hour(h)                                                                            Daily hour(h)
                                             (a)                                                                                     (b)
         2019, 5.
     Figure
Energies       5. (a)
               12 FOR Seoul
                    (a)PEER outdoor
                        Seoul       temperatures;
                               outdoor
                             REVIEW               (b) Seoul
                                       temperatures;        building
                                                       (b) Seoul     coolingcooling
                                                                  building   loads obtained by the TRNSYS
                                                                                    loads obtained  by the                                                               8
                  simulation.
                  TRNSYS   simulation.
              o
              ( C)                                                                                    (W)

               40                                                                                   4,000
Temperature




                                                                                     Cooling Load




               30                                                                                   3,000



               20                                                                                   2,000



                                            Mar         Jun          Sep                                                             Mar        Jun           Sep
               10                                                                                   1,000                                                     Oct
                                            Apr         Jul          Oct                                                             Apr        Jul
                                            May         Aug          Nov                                                             May        Aug           Nov

                  0                                                                                       0
                      0      4         8           12     16        20       24                               0        4        8          12       16       20      24
                                           Daily hour(h)                                                                            Daily hour(h)


                                           (a)                                                                                   (b)
                  Figure 6.6. (a)
                  Figure       (a) Riyadh outdoor temperatures;
                                   Riyadh outdoor temperatures; (b) Riyadh building
                                                                (b) Riyadh building cooling
                                                                                    cooling loads
                                                                                            loads obtained
                                                                                                  obtained by
                                                                                                           by the
                                                                                                               the
                  TRNSYS simulation.
                  TRNSYS     simulation.

3.2.
3.2. Experimental
     Experimental Results
                  Results
     Figures
      Figures 7–9
                7–9 show
                    show experimental
                           experimental results
                                             results for
                                                       for comparison
                                                            comparison of  of the
                                                                               the constant-speed
                                                                                    constant-speed and and inverter
                                                                                                            inverter airair
conditioners.
conditioners. TheTheindoor
                       indoorbuilding
                                buildingloads   and and
                                             loads    outdoor    temperatures
                                                             outdoor              were consistent
                                                                         temperatures                with the with
                                                                                          were consistent       TRNSYS the
simulation   results (circle
TRNSYS simulation             points
                        results (circleand    rectangular
                                         points               in the figures).
                                                  and rectangular                 The indoor
                                                                       in the figures).          building
                                                                                           The indoor       loads loads
                                                                                                         building  were
experimentally    implemented     through     the  heater   at the upper   end   of the  AHU,
were experimentally implemented through the heater at the upper end of the AHU, as shown in      as shown   in  Figure   2,
where
Figure the   powers
         2, where  theofpowers
                         other heating
                                 of other elements       (e.g., AHU
                                            heating elements       (e.g.,and
                                                                          AHUturbo
                                                                                 andfans)
                                                                                       turboinfans)
                                                                                                the indoor    side were
                                                                                                     in the indoor   side
simultaneously    measured    and  the  test  was  controlled    by  the  AHU   heater.   The  dotted
were simultaneously measured and the test was controlled by the AHU heater. The dotted lines with      lines with  small
oscillations  represented   the indoor    roomroomtemperature      which    varied   in the                  ± 1.5 ◦ C as
small oscillations   represented   the indoor             temperature     which   varied  inrange   of 26.0
                                                                                             the range   of 26.0 ± 1.5  °C
the test product   temperature    was   set  at 26 ◦ C. Both   the  constant-speed     and  inverter   air conditioners
as the test product temperature was set at 26 °C. Both the constant-speed and inverter air
were  automatically
conditioners           operated during
               were automatically          the experiments
                                      operated     during theand      only the indoor
                                                                  experiments     and onlybuilding   loadbuilding
                                                                                             the indoor    and outdoor
                                                                                                                     load
temperature
and outdoorwere      applied were
               temperature    according
                                     appliedto the  TRNSYS
                                                according     tosimulation
                                                                 the TRNSYS  results.
                                                                                 simulation results.
     The largest difference in the graph is the variation in the power input. The constant-speed air
conditioner exhibited simple on/off control whereas the inverter (variable speed) air conditioner
exhibited changes corresponding to the building load or outdoor temperature. In the case of the
inverter air conditioner in Riyadh, the power input in May was highly activated during the daytime
(12–19 h), whereas it was activated at any time in August. In the hottest season (August) in Riyadh
and Seoul, the building loads were similar under the considered simulation conditions (Section 3.1).
                                                                              o                                                                                      o
         (W)                                                                 ( C)                   (W)                                                              ( C)
                          Outdoor temperature(TRNSYS result from Fig.6(a))                                        Outdoor temperature(TRNSYS result from Fig.6(a))
5,000                     Indoor cooling load(TRNSYS result from Fig.6(b))    60        5,000                     Indoor cooling load(TRNSYS result from Fig.6(b))   60
                          Outdoor temperature(Tout)                                                               Outdoor temperature(Tout)
                          Indoor cooling load(qbuild)                         50                                  Indoor cooling load(qbuild)                        50
4,000                     Indoor temperature(T )                                        4,000                     Indoor temperature(T )
The largest difference in the graph is the variation in the power input. The constant-speed air
  conditioner exhibited simple on/off control whereas the inverter (variable speed) air conditioner
  exhibited changes corresponding to the building load or outdoor temperature. In the case of the
  inverter air conditioner in Riyadh, the power input in May was highly activated during the daytime
  (12–19
Energies    h),12,
         2019,  whereas
                   1489 it was activated at any time in August. In the hottest season (August) in Riyadh
                                                                                                      9 of 14
  and Seoul, the building loads were similar under the considered simulation conditions (Section 3.1).
                                                                                   o                                                                                 o
     (W)                                                                        ( C)      (W)                                                                     ( C)
                  Outdoor temperature(TRNSYS result from Fig.6(a))                                  Outdoor temperature(TRNSYS result from Fig.6(a))
   5,000          Indoor cooling load(TRNSYS result from Fig.6(b))                 60   5,000       Indoor cooling load(TRNSYS result from Fig.6(b))                 60
                  Outdoor temperature(Tout)                                                         Outdoor temperature(Tout)
                  Indoor cooling load(qbuild)                                      50               Indoor cooling load(qbuild)                                      50
   4,000          Indoor temperature(Tin)                                               4,000       Indoor temperature(Tin)
                  Power Input                                                                       Power Input
                                                                                   40                                                                                40
   3,000                                                                                3,000
                                                                                   30                                                                                30
   2,000                                                                                2,000
                                                                                   20                                                                                20

   1,000                                                                                1,000
                                                                                   10                                                                                10

      0                                                                            0       0                                                                         0
           0        4           8          12         16          20          24                0     4           8          12         16          20          24
                                    Daily hour(h)                                                                     Daily hour(h)


                             (a)
  Energies 2019, 12 FOR PEER REVIEW                                                                                       (b)                                             9
  Energies 2019, 12 FOR PEER REVIEW                                                                                                                                       9
                  Figure
                Figure 7.7.Experimental
                            Experimentalresults
                                         results for
                                                  for Riyadh
                                                      Riyadh in
                                                             in May:
                                                                May:(a)
                                                                     (a)Constant
                                                                         Constantspeed;
                                                                                  speed;(b)(b)
                                                                                             variable speed.
                                                                                               variable speed.
                                                                                   o                                                                                  o
      (W)                                                                       ( C)       (W)                                                                     ( C)
                   Outdoor temperature(TRNSYS result from Fig.6(a))             o
                                                                                                     Outdoor temperature(TRNSYS result from Fig.6(a))              o
     (W)                                                                         C)
                                                                               ( 60                  Indoor cooling  load(TRNSYS result                              C)
                                                                                                                                                                  ( 60
   5,000           Indoor cooling
                  Outdoor          load(TRNSYS result
                             temperature(TRNSYS      resultfrom
                                                             fromFig.6(b))
                                                                  Fig.6(a))
                                                                                          (W)
                                                                                        5,000       Outdoor   temperature(TRNSYS       resultfrom
                                                                                                                                               fromFig.6(b))
                                                                                                                                                    Fig.6(a))
                   Outdoor    temperature(T      ) result from Fig.6(b))        60                   Outdoor
                                                                                                    Indoor     temperature(T
                                                                                                           cooling  load(TRNSYS    ) result from Fig.6(b))            60
   5,000          Indoor  cooling load(TRNSYS out                                       5,000                                   out
                   Indoor cooling  load(q      )                                                     Indoor cooling
                                                                                                    Outdoor          load(qbuild))
                                                                                                              temperature(T
                  Outdoor temperature(T   build )                               50                                             out                                    50
                   Indoorcooling
                  Indoor   temperature(T    )
                                  load(qbuild
                                             out
                                                                                                     Indoorcooling
                                                                                                    Indoor  temperature(T     )
                                                                                                                    load(qbuild
                                                                                                                            in )
                                          in )
   4,000                                                                                4,000
                                                                                50                   Power temperature(T
                                                                                                            Input                                                     50
   4,000           Power  Input
                  Indoor temperature(Tin)                                               4,000       Indoor                 in
                                                                                                                             )
                  Power Input                                                   40                  Power Input                                                       40
   3,000                                                                        40      3,000                                                                         40
   3,000                                                                        30      3,000                                                                         30
   2,000                                                                        30                                                                                    30
                                                                                        2,000
   2,000                                                                        20      2,000                                                                         20
                                                                                20                                                                                    20
   1,000                                                                                1,000
                                                                                10                                                                                    10
   1,000                                                                                1,000
                                                                                10                                                                                    10
      0                                                                          0          0                                                                      0
      0 0           4           8      12      16                20           240           0 0       4           8      12      16                 20          240
        0           4           8 Daily12
                                        hour(h)16                20           24              0       4           8 Daily12
                                                                                                                          hour(h)16                 20          24
                                    Daily hour(h)                                                                     Daily hour(h)
                                        (a)                                                                               (b)
                                        (a)                                                                               (b)
                 Figure 8. Experimental results for Riyadh in August: (a) Constant speed; (b) variable speed.
                 Figure
               Figure 8. 8.Experimental
                            Experimentalresults
                                         resultsfor
                                                 for Riyadh
                                                     Riyadh in
                                                            in August:
                                                               August: (a)
                                                                        (a)Constant
                                                                            Constantspeed;
                                                                                     speed;(b)(b)
                                                                                                variable speed.
                                                                                                  variable speed.
                                                                                   o                                                                                 o
     (W)                                                                        ( C)       (W)                                                                    ( C)
                  Outdoor temperature(TRNSYS result from Fig.5(a))              o                    Outdoor temperature(TRNSYS result from Fig.5(a))             o
     (W)                                                                       ( 60
                                                                                 C)       (W)                                                                      C)
                                                                                                                                                                 ( 60
   5,000          Indoor cooling
                 Outdoor          load(TRNSYS result
                           temperature(TRNSYS       resultfrom
                                                            fromFig.5(b))
                                                                 Fig.5(a))              5,000        Indoor cooling
                                                                                                    Outdoor          load(TRNSYS result
                                                                                                              temperature(TRNSYS       resultfrom
                                                                                                                                               fromFig.5(b))
                                                                                                                                                    Fig.5(a))
   5,000          Outdoor
                 Indoor     temperature(T
                         cooling load(TRNSYS out
                                                ) result from Fig.5(b))         60      5,000        Outdoor
                                                                                                    Indoor     temperature(T
                                                                                                            cooling load(TRNSYSout
                                                                                                                                   ) result from Fig.5(b))        60
                  Indoor cooling
                 Outdoor          load(qbuild))
                           temperature(T                                                             Indoor cooling
                                                                                                    Outdoor          load(qbuild))
                                                                                                              temperature(T
                                            out                                 50                                            out                                 50
   4,000          Indoor cooling
                 Indoor  temperature(T     )
                                 load(qbuild
                                         in )                                           4,000        Indoor cooling
                                                                                                    Indoor  temperature(T     )
                                                                                                                    load(qbuild
                                                                                                                            in )
                  Power temperature(T
                         Input                                                  50                                                                                50
   4,000         Indoor                   )                                             4,000        Powertemperature(T
                                                                                                    Indoor  Input            )
                                        in                                                                                 in
                 Power Input                                                    40                  Power Input                                                   40
   3,000                                                                        40      3,000                                                                     40
   3,000                                                                        30      3,000                                                                     30
                                                                                30      2,000                                                                     30
   2,000
   2,000                                                                        20      2,000                                                                     20
                                                                                20                                                                                20
   1,000                                                                                1,000
                                                                                10                                                                                10
   1,000                                                                                1,000
                                                                                10                                                                                10
      0                                                                          0          0                                                                      0
      0 0           4           8      12      16                 20          240           0 0        4          8      12      16                20           240
        0           4           8 Daily12
                                        hour(h)16                 20          24              0        4          8 Daily12
                                                                                                                          hour(h)16                20           24
                                    Daily hour(h)                                                                     Daily hour(h)
                                     (a)                                                                               (b)
                                     (a)                                                                               (b)
                 Figure
               Figure 9. 9. Experimentalresults
                         Experimental    resultsfor
                                                 forSeoul
                                                     Seoul in
                                                            in August:
                                                               August: (a)
                                                                       (a)Constant
                                                                           Constantspeed;
                                                                                    speed;(b)
                                                                                           (b)variable
                                                                                               variablespeed.
                                                                                                         speed.
                 Figure 9. Experimental results for Seoul in August: (a) Constant speed; (b) variable speed.
        However
      The  largest the   power variations
                     difference   in the graphwereis different   owingintothe
                                                      the variation          the power
                                                                                  outdoorinput.
                                                                                            temperature      differences.
                                                                                                   The constant-speed
        However     the  power   variations   were  different    owing   to the   outdoor   temperature      differences.
airAlthough
    conditionertheexhibited
                   building cooling
                               simple loads
                                        on/offwere    similar
                                                control        in this
                                                         whereas     thestudy,   the (variable
                                                                          inverter   outdoor temperature         in Saudi
                                                                                                speed) air conditioner
   Although
   Arabia was thesignificantly
                   building cooling
                                higherloads
                                         than  were insimilar
                                               thatbuilding    inUsually,
                                                        Korea.loadthis study,   the outdoor
                                                                           the outdoor    unit temperature
                                                                                                of theInairthe  in Saudi
                                                                                                             conditioner
exhibited   changes    corresponding      to the                     or outdoor     temperature.                case of the
   Arabia  was significantly
   was affected                 higher
                  by the dry-bulb       than that inrather
                                      temperature      Korea. Usually,     the  outdoortemperature
                                                                                          unit of the air    conditioner
inverter  air conditioner    in Riyadh,    the power inputthan      by the
                                                                in May   was wet-bulb
                                                                               highly activated         in the
                                                                                                    during     theoutdoor
                                                                                                                   daytime
   was  affected  by  the dry-bulb    temperature    rather than   by  the   wet-bulb   temperature
   test room during the cooling test in which no condensation on the outdoor unit heat exchanger        in  the  outdoor
   test
   wasroom     during the cooling test in which no condensation on the outdoor unit heat exchanger
        observed.
   was observed.
        The power inputs in the above graph could be averaged to obtain the daily average power
        The power
   consumption.       inputs
                   Table      in thetheabove
                          6 shows             graph
                                        monthly        could
                                                   power      be averaged
                                                          consumption,         to obtain
                                                                            obtained       the daily average
                                                                                       by multiplying    30 (days) power
                                                                                                                     with
   consumption.    Table  6 shows   the monthly   power   consumption,      obtained   by multiplying
   the daily average power consumption. During the cooling seasons of Seoul (four months) and Riyadh     30  (days)  with
   the daily average power consumption. During the cooling seasons of Seoul (four months) and Riyadh
Energies 2019, 12, 1489                                                                                                                               10 of 14



(12–19 h), whereas it was activated at any time in August. In the hottest season (August) in Riyadh
and Seoul, the building loads were similar under the considered simulation conditions (Section 3.1).
      However the power variations were different owing to the outdoor temperature differences.
Although the building cooling loads were similar in this study, the outdoor temperature in Saudi
Arabia was significantly higher than that in Korea. Usually, the outdoor unit of the air conditioner was
affected by the dry-bulb temperature rather than by the wet-bulb temperature in the outdoor test room
during the cooling test in which no condensation on the outdoor unit heat exchanger was observed.
      The power inputs in the above graph could be averaged to obtain the daily average power
consumption. Table 6 shows the monthly power consumption, obtained by multiplying 30 (days) with
the daily average power consumption. During the cooling seasons of Seoul (four months) and Riyadh
(nine months), the monthly power consumptions of the constant- and variable-speed air conditioners
were compared (Figures 10 and 11). Considering the air conditioners’ specifications in Table 5, the
variable-speed air conditioner had a higher EER than that of the constant-speed air conditioner by
approximately 5%. Therefore, the relative difference was subtracted by –5% in Table 6. In Riyadh,
the variable-speed air conditioner has a smaller energy consumption by 18.3% in August and 47.1%
in November. This implies that the variable-speed air conditioner provides a higher energy saving
than that of the constant-speed air conditioner in the part-load condition. Seoul in August is less hot
than Riyadh (Figures 5a and 6a), which leads to a larger energy saving (36.3%) of the variable-speed
air conditioner. In Saudi Arabia and Korea, total inverter energy savings of 28.9% and 44.5% were
Energies 2019, 12 FOR PEER REVIEW                                                                          10
achieved during the cooling season, respectively. These results were very similar to those in the
study for
28.9%    andIstanbul   and Saudi
               44.5% were         Arabia,
                            achieved      where
                                       during  theenergy  savings
                                                    cooling        of 11–38%
                                                              season,         and 22–50%,
                                                                      respectively.        respectively,
                                                                                    These results        were
                                                                                                  were very
obtained    for  an inverter air conditioner in a real office  usage [6,8].
similar to those in the study for Istanbul and Saudi Arabia, where energy savings of 11–38% and 22–
50%, respectively, were obtained for an inverter air conditioner in a real office usage [6,8].
                                           Table 6. Monthly power consumption (kWh) during the cooling season.

    City              Table 6. Monthly
                Air Conditioner Type power
                                       Mar. consumption
                                             Apr. May.(kWh)
                                                        Jun. during
                                                                Jul. theAug.
                                                                         coolingSep.
                                                                                 season.
                                                                                      Oct.                                                     Nov.   Tot.
  City         AirConstant
                  Conditioner   Type
                            speed                                              Mar.
                                                                                  -   Apr.
                                                                                         -    May.
                                                                                                 -     Jun.
                                                                                                        341    Jul.
                                                                                                                490    Aug.
                                                                                                                        542    Sep.
                                                                                                                                319    Oct.
                                                                                                                                          -    Nov.
                                                                                                                                                  -    Tot.
                                                                                                                                                      1692
                  Constant
                  Variable speed
                            speed                                                --     --      --     341
                                                                                                        150    490
                                                                                                                249    542
                                                                                                                        318    319
                                                                                                                                138      --      --    1692
                                                                                                                                                      855
   Seoul          Variable speed                                                 --     --      --     150     249     318     138       --      --     855
  Seoul      (Con.-Var.)/Con.   × 100%                                                                  56.0   49.2    41.3    56.7                   49.5
              (Con.-Var.)/Con. × 100%
                −5% compensation                                                 --     --      --     56.0
                                                                                                        51.0   49.2
                                                                                                               44.2    41.3
                                                                                                                       36.3    56.7
                                                                                                                               51.7      --      --    49.5
                                                                                                                                                      44.5
                −5% compensation                                                 -      -       -      51.0    44.2    36.3    51.7      -       -    44.5
                  Constant speed                                                305    421     603      672     745     747     595     470     330   4888
                  Constant speed                                               305    421     603      672     745     747     595     470     330     4888
                  Variable speed                                                149    237     395      459     529     573     435     298     158   3233
   Riyadh         Variable speed                                               149    237     395      459     529     573     435     298     158     3233
 Riyadh      (Con.-Var.)/Con.   × 100%                                         51.1   43.7    34.5      31.7   28.9    23.3    26.9    36.6    52.1   33.9
              (Con.-Var.)/Con. × 100%                                          51.1   43.7    34.5     31.7    28.9    23.3    26.9    36.6    52.1    33.9
                −5%  compensation
                −5% compensation
                                                                               46.1
                                                                               46.1
                                                                                      38.7
                                                                                      38.7
                                                                                              29.5
                                                                                              29.5
                                                                                                        26.7
                                                                                                       26.7
                                                                                                               23.9
                                                                                                               23.9
                                                                                                                       18.3
                                                                                                                       18.3
                                                                                                                               21.9
                                                                                                                               21.9
                                                                                                                                       31.6
                                                                                                                                       31.6
                                                                                                                                               47.1
                                                                                                                                               47.1
                                                                                                                                                      28.9
                                                                                                                                                       28.9

                                                                        800                                           Constant speed
                                               Power consumption[kWh]




                                                                        700                                           Variable speed

                                                                        600
                                                                        500
                                                                        400
                                                                        300
                                                                        200
                                                                        100
                                                                          0
                                                                              Jun.           Jul.           Aug.           Sep.
                                                                                                    Month

                                      Figure 10.
                                      Figure     Monthlypower
                                             10. Monthly powerconsumption
                                                               consumptionduring
                                                                          during the
                                                                                  the cooling
                                                                                      cooling season
                                                                                              season in
                                                                                                      in Seoul.
                                                                                                         Seoul.


                                                                                                                              Constant speed
                                     800
                                                                                                                              Variable speed
              wer consumption[kWh]




                                     700
                                     600
                                     500
                                     400
                                     300
                                     200
200




                                                Powe
                                                       100
                                                         0
                                                                    Jun.          Jul.             Aug.            Sep.
                                                                                          Month
Energies 2019, 12, 1489                                                                                                                      11 of 14
                                         Figure 10. Monthly power consumption during the cooling season in Seoul.

                                                                                                                     Constant speed
                                        800
                                                                                                                     Variable speed

               Power consumption[kWh]
                                        700
                                        600
                                        500
                                        400
                                        300
                                        200
                                        100
                                          0
                                              Mar.           Apr.      May.   Jun.         Jul.     Aug.    Sep.          Oct.   Nov.
                                                                                         Month

                                        Figure 11. Monthly
                                                   Monthly power
                                                           power consumption during the cooling season in Riyadh.

3.3. Economic
3.3. Economic Analysis
              Analysis
      In Korea,
     In           most of
         Korea, most     of the
                             the household
                                   household airair conditioners
                                                    conditioners in  in the
                                                                        the market    are inverters
                                                                             market are    inverters and
                                                                                                      and thethe actual
                                                                                                                  actual
operation   time  is supposed     to be remarkably     different from   the test condition.    As
operation time is supposed to be remarkably different from the test condition. As the electricity the electricity  rates
in Korea
rates     are high,
       in Korea   are the conditioners
                       high,              do not do
                              the conditioners     operate   during during
                                                       not operate    the whole
                                                                              theday,  butday,
                                                                                   whole    onlybut
                                                                                                  when   it iswhen
                                                                                                      only     very ithot.
                                                                                                                        is
very hot. However, we carried out an economic analysis in both countries. The resultsanalysis
However,    we  carried  out   an  economic   analysis  in  both countries.   The  results  of the economic       of the
were   basedanalysis
economic      on the experimental
                       were based onresults.
                                         the experimental results.
      Based  on a simple  payback
     Based on a simple payback period period  analysis, the initial
                                                   analysis,   the investment    and running
                                                                    initial investment    and costs   werecosts
                                                                                                 running      calculated
                                                                                                                   were
to compare    the economic     benefits  of the  different  types of  air conditioners.
calculated to compare the economic benefits of the different types of air conditioners.
      The initial
     The   initial investment
                   investment costcost was
                                        was the
                                             the average
                                                  average price
                                                            price of
                                                                  of anan 18,000
                                                                          18,000 British-thermal-unit      (BTU)/h air
                                                                                  British-thermal-unit (BTU)/h         air
conditioner   sold  in Korea    and  in Saudi  Arabia;  the  annual    operation  cost was   calculated
conditioner sold in Korea and in Saudi Arabia; the annual operation cost was calculated based on         based    on  the
monthly power consumption (Tables 7 and 8). To calculate the annual operation cost, electricity rates
proposed by the power authorities of both countries were employed [24,25].

                                              Table 7. Calculation results of the simple payback period (Seoul).

                                                                                                  Running Cost
          Categories                                         Initial Cost (USD)                                             Payback Period (Years)
                                                                                                   (USD/year)
       Constant speed                                                   727                           291                           Basis
       Variable speed                                                  1182                           104                            2.5


                                              Table 8. Calculation results of the simple payback period (Riyadh).

                                                                                                  Running Cost
          Categories                                         Initial Cost (USD)                                             Payback Period (Years)
                                                                                                   (USD/year)
       Constant speed                                                  432                            238                           Basis
       Variable speed                                                  635                            157                            2.5


    The results of the economic analysis for both types (constant, variable) of air conditioners are
shown in Figure 12.
period for the compressor), in Korea and Saudi Arabia, the inverter air conditioner is 1400 USD and
600 USD more economical than the constant-speed air conditioner, respectively.
      A total of 21 million of split and window air conditioners are used in Saudi Arabia, while the
inverter type is rarely used owing to its high initial cost. If all of the constant-type air conditioners
were replaced
Energies            by inverter-type air conditioners, the power consumption could be reduced by
         2019, 12, 1489                                                                               34.8
                                                                                                   12 of 14
TWh per year (the average electric consumption can be obtained by the employed models) [26].

                 4000                                                                        4000

                 3500                                                                        3500

                 3000                                                                        3000
     Cost(USD)




                                                                                 Cost(USD)
                 2500                                                                        2500
                 2000                                                                        2000
                 1500                                                                        1500
                                                               Costant speed                                                               Costant speed
                 1000               2.5 years                                                1000
                                                               Variable speed                                   2.5 years                  Variable speed
                 500                                                                         500
                   0                                                                           0
                        0   1   2    3     4      5    6   7    8     9     10                      0   1   2   3      4      5    6   7    8     9     10
                                                Year                                                                        Year
                                           (a)                                                                         (b)
                            Figure 12.
                            Figure 12. Periods of recovery
                                       Periods of recovery of
                                                           of the
                                                              the initial
                                                                  initial investment:
                                                                          investment: (a)
                                                                                      (a) Korea;
                                                                                          Korea; (b)
                                                                                                 (b) Saudi
                                                                                                     Saudi Arabia.
                                                                                                           Arabia.

4. Conclusions
      With respect to the constant-speed air conditioner, the payback period of the inverter type was
2.5 years in both countries. For the whole lifetime (10 years, assumed using the usual warranty period
      In this study, the energy saving effect of the inverter (variable-speed) air conditioner with
for the compressor), in Korea and Saudi Arabia, the inverter air conditioner is 1400 USD and 600 USD
respect to the constant-speed air conditioner was analyzed. The ambient temperature and building
more economical than the constant-speed air conditioner, respectively.
cooling load were obtained using TRNSYS climate data and simulation. The results were obtained in
      A total of 21 million of split and window air conditioners are used in Saudi Arabia, while the
the precisely controlled air-enthalpy-type test room (outdoor side: Temperature-controlled, indoor
inverter type is rarely used owing to its high initial cost. If all of the constant-type air conditioners
side: Cooling-load-controlled) with specific dimensions, which contained the air conditioner set at
were replaced by inverter-type air conditioners, the power consumption could be reduced by 34.8 TWh
the temperature of 26 °C throughout the day. For the Korean and Saudi Arabian cooling periods of
per year (the average electric consumption can be obtained by the employed models) [26].
four and nine months, respectively, TRNSYS simulations and experiments in the air-enthalpy test
room
4.      were carried out. In standards such as ISO, EN and ASHRAE, methods are introduced to
   Conclusions
determine the part-load efficiency of an air conditioner. However, in this study, we directly
      In thisthe
measured      study, the air
                 actual  energy   saving effect
                             conditioner        of the
                                           energy      inverter (variable-speed)
                                                    consumption                  air conditioner
                                                                   using a real climate          with respect
                                                                                         environment   rather
to thecalculation
than    constant-speed
                   of theair conditioner
                          SEER    based onwas
                                            someanalyzed.   The ambient temperature and building cooling
                                                   test points.
load The
      wereconclusions
             obtained using
                         of thisTRNSYS   climate
                                 study can         data and simulation.
                                            be summarized     as follows: The results were obtained in the
precisely controlled air-enthalpy-type test room (outdoor side: Temperature-controlled, indoor side:
Cooling-load-controlled) with specific dimensions, which contained the air conditioner set at the
temperature of 26 ◦ C throughout the day. For the Korean and Saudi Arabian cooling periods of four
and nine months, respectively, TRNSYS simulations and experiments in the air-enthalpy test room
were carried out. In standards such as ISO, EN and ASHRAE, methods are introduced to determine
the part-load efficiency of an air conditioner. However, in this study, we directly measured the actual
air conditioner energy consumption using a real climate environment rather than calculation of the
SEER based on some test points.
      The conclusions of this study can be summarized as follows:

1.           The variable-speed (inverter) air conditioner was more effective than the constant-speed air
             conditioner in the months and location of part-load operation; in all of the preset test cases, it led
             to a smaller energy usage.
2.           Inverter energy savings of 18.3–47.1% and 36.3–51.7% were observed during the Riyadh’s
             (March–November) and Seoul’s (June–September) cooling months, respectively.
3.           The energy savings depended on the month; larger energy savings were observed before and
             after the summer peak. Larger energy savings were observed in the months and location of
             part-load operation.
4.           With respect to the constant-speed air conditioner, the payback period of the inverter type was
             2.5 years in both countries. For whole lifetime (10 years, assumed using the usual warranty period
             for the compressor), in Korea and Saudi Arabia, the inverter-type air conditioner is 1400 USD
             and 600 USD more economical than the constant-speed air conditioner, respectively.

    It is widely known that no differences in energy efficiency between constant-speed and
inverter-type air conditioners exist in countries with hot climates such as Saudi Arabia. However,
Energies 2019, 12, 1489                                                                                         13 of 14



according to the results of this study, even in hot countries, the building load and outside temperature
change with time. Consequently, energy saving was obtained by part-load operation of the inverter
air conditioner in the morning and night periods. The actual energy consumption analysis through
the proposed approach can provide good empirical evidence to establish air conditioner policies in
different countries.

Author Contributions: J.L., M.S.Y. and Y.N. defined the performance analysis models, developed the methodology,
and wrote the full manuscript. The author T.A.Q. checked the contents regarding Saudi Arabia.
Funding: This research was funded by Basic Research Project (No.CBS18126) of the Korea Testing Laboratory (KTL).
Conflicts of Interest: The authors declare no conflict of interest.

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