Free cooling guide
Free cooling guide
Table of contents 1. Introduction to the concept of free cooling ...3 The need for cooling in low-energy houses . . 4 Comfort and energy efﬁciency – the best ﬁt for low-energy houses . . 4 Investing for the future – the design of a low-energy house . . 5 2. Cooling loads in residential buildings . . 6 Factors inﬂuencing the sensible cooling load . . 6 Factors inﬂuencing the latent cooling load . . 7 The effect of shading . . 7 Room variation . . 8 Duration of the cooling load .
. 8 Required cooling capacity . . 9 3. The ISO 7730 guidelines . . 10 Optimal temperature conditions . . 10 Draught rate . . 11 Radiant asymmetry . . 11 Surface temperatures . . 12 Vertical air temperature difference . . 12 4. Capacity and limitations of radiant emitter systems . . 13 Heat ﬂux density . . 13 Thermal transfer coefﬁcient . . 13 Dew point limitations . . 13 Theoretical capacities of embedded radiant cooling . . 14 5. Ground heat exchangers . . 15 Ground conditions . . 15 Ground heat exchangers . . 16 Ground temperature proﬁle . . 17 Primary supply temperatures . . 17 Dimensioning of ground heat exchangers for free cooling .
. 17 6. Free cooling in combination with different heat sources . . 19 7. Choosing and dimensioning the radiant emitter system . . 20 Capacity of different radiant emitter systems . . 20 Radiant ﬂoor constructions and capacity . . 22 Radiant ceiling constructions and capacity . . 24 Capacity diagrams . . 24 Regulation and control . . 26 The self-regulating effect in underﬂoor heating ..27 Functional description of Uponor Control System . . 27 Component overview . . 29 8. Uponor Pump and exchanger group (EPG6) for ground sourced free cooling . . 29 Dimensions . . 30 Pump diagram . . 30 Control principle .
. 31 Installation examples . . 33 Operation of Uponor Climate Controller C-46 . . 36 Operation mode of Uponor Climate Controller C-46 . . 36 Dew point management parameters and settings . . 37 Heating and cooling change-over: external signal . . 38 Heating and cooling change-over: Uponor Climate Controller C-46 . . 38 2 UPONOR · FREE COOLING G UIDE
1. Introduction to the concept of free cooling Free cooling is a term generally used when low external temperatures are used for cooling purposes in buildings. This guide presents a free cooling concept based on a ground coupled heat exchanger combined with a radiant heating and cooling system. A ground coupled heat exchanger can for example be horizontal collectors, vertical boreholes or energy cages. A radiant system means that the ﬂoors, ceilings or walls have embedded pipes in which water is circulated for heating and cooling of the building. Under ﬂoor heating and cooling is the most well know example of a radiant system.
A radiant system combined with a ground coupled heat exchanger is highly energy efﬁcient and has several advantages. In the summer period, the ground coupled heat exchanger provides cooling temperatures that are lower compared to the outside air. The radiant system operates with large surfaces, which means it can utilize the temperatures from the ground directly for cooling purposes. The result is that free cooling can be provided with only cost being the electricity required for running the circulation pumps in the brine and water systems. No heat pump is required.
In the heating season the system is operated using a heat pump. As the ground temperature during winter is higher compared to the outside air temperature, the result is improved heat pump efﬁciency (COP) compared to an air based heat pump. In addition, the radiant emitter a system (under ﬂoor heating) operates at moderate water temperatures in large surfaces which further improves the heat pump COP. 3 UPO NO R · FREE COOLING GUIDE
The need for cooling in low- energy houses Today, there is a high focus on saving energy and utilising renewable energy sources in buildings.
The energy demand for space heating is reduced by increased insulation and tightness of buildings. However, increased insulation and tightness also increase the cooling demand. The building becomes more sensitive to solar radiation through windows and becomes less able to remove heat in the summer. More extreme weather conditions further contributes to the cooling needs and together with an even more increased consumer awareness of having the right indoor climate, the need for cooling also in residential buildings will become a requirement. Optimal architectural design and shading will help to reduce the cooling need, but simulations and practical experience show that such measures alone will not eliminate the cooling need.
Space cooling is needed, not only in the summer, but also in prolonged periods during spring and autumn when the low angel of the sun gives high solar radiation through windows. In order to meet the energy frame requirements of the building regulations, space cooling can be provided by utilising renewable energy sources such as ground heat exchangers for cooling purposes in conjunction with a radiant system with embedded pipes in the ﬂoor, wall or ceiling.
Cooling needs will differ between rooms and are highly inﬂuenced by direct solar radiation. Rooms with larger window areas and facing the south will generally have higher cooling requirements. In periods with high cooling loads, active cooling is normally required during both day and night time. Comfort and energy efﬁciency – the best ﬁt for low-energy houses Using shading will help to reduce the cooling demand. However, this forces occupants to actively pull down the shades e.g. when leaving the house. Also, shading will block daylight which increases electricity consumption on artiﬁcial light, and shading will block the view which may not be in the interest of the home occupant.
In fact many architects state that energy efﬁciency and comfort may conﬂict when deﬁning comfort in a broader sense, such as the freedom to design window sizes, spaciousness with increased ceiling height, daylight requirements and the occupant’s tendency to utilise open doors and windows. All such requirements put increased demands on the HVAC applications. Ground heat exchangers combined with radiant systems is the only “all-in-one” solution, with the ability to provide both heating and cooling. Such systems are more cost efﬁcient and simpler to install than having to deal with a separate heating and cooling systems.
Furthermore, radiant systems are able to heat at a low supply temperature and cool at a high supply temperature. This ﬁts perfectly to the typical operating temperatures of a ground coupled heat exchanger. Furthermore, the connected heat pump will be able to run more efﬁciently and thereby consume less electricity. In addition, a radiant system provides no draught problems and provides an optimal temperature distribution inside a room. Last but not least, radiant systems provide complete freedom in terms of interior design, as no physical space is occupied inside the room. Even more important when looking at the lifetime and property value of a house, such systems have very low maintenance need and a lifetime that almost follows the lifetime of the building itself.
In today’s uncertain environment of future energy prices, free cooling and ground coupled heat pumps provides a high stability on the future energy costs of the building in question. It will most certainly meet today’s and future building regulations even in a scenario where future property taxation would be linked to energy efﬁciency. Hence, it is an investment that helps to maintain and differentiate the future property value.
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Investing for the future – the design of a low-energy house A radiant system, e.g. underﬂoor heating and cooling, coupled to a ground source heat pump, provides optimal comfort with high energy efﬁciency both summer and winter. In addition, due to the increased tightness requirements in low-energy houses, a ventilation system is necessary to maintain an acceptable indoor air quality. In order to keep the ventilation system energy efﬁcient, it should be coupled to a heat recovery ventilation (HRV) unit to minimise heat losses through the air exchange.
Energy sources for cooling There are several alternative HVAC applications available for cooling purposes. A district heating connection is an energy efﬁcient option for space heating, but cannot be used for cooling purposes. Alternative means of cooling could be an air-to-water heat pump, but no “free cooling” can be extracted from such a system, hence cooling can only be provided with the heat pump running causing a higher electricity consumption. Purely air-based systems like split units can also act as a cooling system but as can be seen from the picture below, the efﬁciency is considerably lower than for water-based cooling systems.
European seasonal energy efﬁciency ratio (ESEER) for different cooling systems. ESEER is deﬁned by the Eurovent Certiﬁcation Company and calculated by combining full and part load operating conditions. Correlation between average property m2 prices and energy class The ﬁgure above shows the correlation between property prices and the energy efﬁciency level of the property in Denmark. Properties with energy class A or B are on average 6% more expensive than energy class C and 17% more expensive than energy class D. DKK/m2 Energy class 5 10 15 20 25 Air to air heat pump Air to water heat pump Brine to water heat pump Free cooling 5 UPO NO R · FREE COOLING GUIDE
2. Cooling loads in residential buildings The design cooling load (or heat gain) is the amount of energy to be removed from a house by the HVAC equipment, to maintain the house at indoor design temperature when worst case outdoor design temperature is being experienced. As can be seen from the ﬁgure above, heat gains can come from external sources, e.g. solar radiation and inﬁltration and from internal sources, e.g. occupants and electrical equipment. Two important factors when calculating the cooling load of a house are: • sensible cooling load • latent cooling load The sensible cooling load refers to the air temperature of the building, and the latent cooling load refers to the humidity in the building.
Factors inﬂuencing the sensible cooling load • Windows or doors • Direct and indirect sunshine through windows, skylights or glass doors heating up the room • Exterior walls • Partitions (that separate spaces of different temperatures) • Ceilings under an attic • Roofs • Floors over an open crawl space • Air inﬁltration through cracks in the building, doors, and windows • People in the building • Equipment and appliances operated in the summer • Lights 6 UPONOR · FREE COOLING G UIDE
The effect of shading To reduce the cooling load from solar gains, the most efﬁcient and sustainable way is to use passive measures.
From an architectural point of view, shading can be created by building components and by using blinds. Depending on the type of blinds used, the solar gain can typically be reduced with up to 85% with external shading. The ﬁgures below show a building simulation example conducted on a low-energy single family house, where using different shading factors have been applied. Without shading; cooling loads up to 60 W/m2 .
Shading factor 50%; cooling loads up to 40 W/m2 . Shading factor 85%; cooling loads up to 25 W/m2 . As can be seen from the ﬁgures above, even with the most efﬁcient shading factor, the cooling load still amounts to 25 W/m2 . External heat gain Internal heat gain Transmission (Sensible) Solar Radiation (Sensible) Air Ventilation (Sensible) (Latent) (Sensible) (Latent) (Sensible) (Sensible) (Latent) Lighting Equipment People C O N D I T I O N E D S PA C E Total sensible Total latent Cooling Load 2% 5% 3% 10% 13% 15% 52% Heat from air ﬂows Heat from occupants (incl. latent) Heat from equipment Heat from walls and ﬂoors (structure) Heat from lighting Heat from daylight (direct solar) Heat from windows (including absorbed solar) and openings Factors inﬂuencing the latent cooling load Moisture is introduced into a room through: • People • Equipment and appliances • Air inﬁltration through cracks in the building, doors, and windows Internal gains in residential buildings are limited to the people normally occupying the space and household equipment.
In national building regulations, the load for internal gains in ordinary residential buildings is often mentioned (3-5 W/m2 ). In residential buildings, the cooling load primarily comes from external heat gains, and mostly from solar gains through windows and doors, transmission through wall and roof, and inﬁltration through the building envelope/ventilation. The ﬁgure below shows that about 2/3 of the cooling load comes from the solar radiation. 7 UPO NO R · FREE COOLING GUIDE
37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 Temperature [°C] Time [h] No window opening, no HRV by-pass Open windows, no HRV by-pass Open windows, with HRV by-pass UFH, no opening window Room variation There is a big variation in the cooling load from room to room, caused by the architectural design of the building. Large window areas facing the south and west are needed for daylight requirements and winter heat gains, but they also incudes high summer cooling loads. As a result of large south facing window areas, the cooling demand in south facing rooms are higher than in the north facing rooms.
In addition, the desired temperature levels of each room may differ ranging from the highest temperature requirements in the bathroom, to the lowest temperature requirements in the bedroom.
Duration of the cooling load The ﬁgures below show the duration of over-tempera- ture with different shading and ventilation strategies. The data originates from a full year building simulation of a low-energy single family house in Northern European climatic conditions (Denmark). Without shading; over-temperature up to 2 300 hours per year. 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 Temperature [°C] Time [h] No window opening, no HRV by-pass Open windows, no HRV by-pass Open windows, with HRV by-pass UFH, no opening window 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 Temperature [°C] Time [h] No window opening, no HRV by-pass Open windows, no HRV by-pass Open windows, with HRV by-pass UFH, no opening window Shading factor 50%; over-temperature up to 1 100 hours per year.
Shading factor 85%; over-temperature up to 800 hours per year. The simulations show that without active cooling there will be a signiﬁcant amount of time with over- temperature (assuming that the maximum temperature allowed is 26 °C). All the cases also show that with radiant ﬂoor cooling, it is possible to keep the temperature below 26 °C all year round. National building regulations across Europe have already started to implement maximum duration periods of over- temperature. In Denmark, the requirement in the 2015 standard is that a temperature above 26 °C is only allowed for maximum 100 h during the year and above 27 °C for maximum 25 h during the year.
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5000 4500 4000 3500 3000 2500 2000 1500 1000 500 Capacity [W] January February March April May June July August September October November December Cooling Heating 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 Capacity [W] January February March April May June July August September October November December Cooling Heating 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 Capacity [W] January February March April May June July August September October November December Cooling Heating Required cooling capacity Based on the peak load calculations of the building, the heating and cooling system can be designed.
The HVAC system should be designed to cover the worst case (peak load). The ﬁgures below show an example of the variation of the needed capacity to cover the heating and cooling loads.
Required heating and cooling capacity Low energy building, shading in-between windows. Window opening and HRV by-pass are used during cooling season Low energy building, external shading. Window opening and HRV by-pass are used during cooling season As can be seen, the cooling capacity peaks are actually higher (up to 4 kW), than the heating capacity peaks (up to 3.5 kW) under any shading conditions (excluding domestic hot water). Although, the heating period still remain longer than the total cooling period, it is interesting to note that the cooling period extends into early spring and late autumn.
Low energy building, no shading. Window opening and HRV by-pass are used during cooling season 9 UPO NO R · FREE COOLING GUIDE
In order to provide thermal comfort, it is necessary to take into account local thermal discomfort caused by temperature deviations, draught, vertical air temperature difference, radiant temperature asymmetry, and ﬂoor surface temperatures. These factors can inﬂuence on the required capacity of the HVAC system. Optimal temperature conditions EN ISO 7730 is an international standard that can be used as a guideline to meet an acceptable indoor and thermal environment.
These are typically measured in terms of predicted percentage of dissatisﬁed (PPD) and predicted mean vote (PMV). PMV/PPD basically predicts the percentage of a large group of people that are likely to feel “too warm” or “too cold” (the EN ISO 7730 is not replacing national standards and requirements, which always must be followed). PMV and PPD The PMV is an index that predicts the mean value of the votes of a large group pf persons on a seven-point thermal sensation scale (see table below), based on the heat balance of the human body. Thermal balance is obtained when the internal heat production in the body is equal to the loss of heat to the environment.
PMV Predicted mean vote PPD Predicted percentage dissatisﬁed [%] +3 Hot +2 Warm +1 Slightly warm 0 Neutral -1 Slightly cold -2 Cool -3 Cold Seven-point thermal sensation scale The PPD predics the number of thermally dissatisﬁed persons among a large group of people. The rest of the group will feel thermally neutral, slightly warm or slightly cool.
The table below shows the desired operative tempera- ture range during summer and winter, taking into con- sideration normal clothing and activity level in order to achieve different comfort classes. Class Comfort requirements Temperature range PPD [%] PMV [/] Winter 1.0 clo 1.2 met [°C] Summer 0.5 clo 1.2 met [°C] A < 6 - 0.2 < PMV < + 0.2 21-23 23.5-25.5 B < 10 - 0.5 < PMV < + 0.5 20-24 23.0-26.0 C < 15 - 0.7 < PMV < + 0.7 19-25 22.0-27.0 ISO 7730 basically recommends a target temperature of 22 °C in the winter, and 24.5 °C in the summer. The higher the deviation around these target temperatures, the higher the percentage of dissatisﬁed.
The reason for the different target temperatures is because that the two seasons apply different clothing conditions as can be seen in below ﬁgure: Operative temperature for winter and summer clothing Dissatisﬁ ed [%] PPD PMV Operative temperature [°C] Basic clothing insulation: 0.5 Predicted Percentage of Dissatisﬁ ed [%] Basic clothing insulation: 1.0 Metabolic rate: 1.2 3. The ISO 7730 guidelines 10 UPONOR · FREE COOLING G UIDE
Radiant asymmetry When designing a radiant ceiling or wall system, make sure to stay within the limits of radiant asymmetry. As can be seen in the ﬁgure below, the radiant asymmetry differs depending on the location of the emitter system, and whether it’s used for heating or cooling. With the insulation levels typically used today, radiant asymmetry does normally not cause any problems due to the moderate heating and cooling load the emitter has to perform. However, especially when using ceiling heating, a calculation must be made for a given reference room.
When designing radiant cooling systems, the dew point is normally reached before radiant asymmetry problems occur.
Can be calculated according to ISO 7726. Dissatisﬁ ed Floor temperature Local discomfort caused by warm and cool ﬂoors 0.4 0.05 0.2 0.15 0.25 0.35 0.2 0.3 0.5 4 1 1.5 2 2.5 3 3.5 4.5 3.0 K 4.0 K 5.0 K 6.0 K 7.0 K 8.0 K 9.0 K 10.0 K Maximum air velocity, 0.5 m from wall [m/s] Recommended comfort limit for sedentary persons Height of cool wall [m] Δt (wall-room) Draught rate Radiant systems are low convective systems and will not create any problems with draught. However, down draught from a cold wall can put a limitation to the system. A cold wall can create draught as we know from windows.
When designing wall cooling, the velocity on the air need to be within the recommendation (Class A is 0.18 m/s).
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Surface temperatures For many years, people have chosen underﬂoor heating systems as the preferred emitter system, because of the perceived comfort of walking on a warm ﬂoor. Similarly, the question is if the occupants complaint about discom- fort when utilising the ﬂoor to remove heat (cooling). According to ISO 7730, the lowest PPD (6%) is found at a ﬂoor temperature of 24 °C. A typical ﬂoor cooling system will have to operate with a minimum ﬂoor temperature of 20 °C, where the expected PPD would still be under 10%. As will be seen later, such ﬂoor temperatures still provide a signiﬁcant cooling effect, due to the large surface area being emitted.
Vertical air temperature difference The comfort categories are divided into A, B and C depending upon the difference between the air temperature at ﬂoor level and at a height equivalent to a seated person. As can be seen below, the temperature difference must be under 2°C in order to reach category A.
Category Vertical air temperature difference a °C A < 2 B < 3 C < 4 a) 1,1 and 0,1 m above ﬂoor A study done by Deli in 1995 shows the correlation between the ΔT ﬂoor surface/room (difference between the ﬂoor surface temperature and the dimensioned room temperature) and the vertical air temperature difference. Vertical temperature proﬁle with different emitter systems [°C] 18 20 22 26 24 Ideal heating Underﬂoor heating Radiant ceiling heating External wall radiator heating Temperature proﬁle radiant cooling [°C] 18 20 22 26 24 Radiant ﬂoor cooling Radiant ceiling cooling Radiant wall cooling 1 80 2 4 6 20 8 0 5 10 20 30 35 25 15 0 9 18 36 54 63 45 27 [°C] [°F] 60 40 10 Dissatisﬁ ed [%] Radiant temperature asymmetry [°C] Warm ceiling Cool wall Cool ceiling Warm wall Correlation between the temperature difference ﬂoor surface to room and the vertical air temperature difference (Deli, 1995).
The study concludes that up to a ΔT 8K, the comfort category is still A. This would equal a ﬂoor temperature of 20 °C and a dimensioned room temperature of 28 °C. The dimensioned room temperature must be below 26 °C and similarly above a ﬂoor temperature of 20 °C in order to reach comfort class B. Hence, the vertical air temperature difference will in practice not cause a indoor climate below category A. As the pictures below show, different emitter systems provide different temperature gradients in a room. Clearly, a radiant heating system in the ﬂoor provides a temperature gradient closest to the ideal.
Similarly, a radiant cooling system in the ceiling provides a temperature gradient closest to the ideal. 0,5 1 1,5 2 2,5 3 2 4 6 8 10 A B ΔT ﬂoor surface room Vertical air temperature difference [K] 0,1 - 1,1 m 12 UPONOR · FREE COOLING G UIDE
Thermal transfer coefﬁcient The thermal transfer coefﬁcient is an expression of how large an effect per m2 the surface is able to transfer to the room, per degree of the temperature difference between the surface and the room. The ﬁgure below shows the thermal transfer coefﬁcient for different surfaces for heating and cooling respectively. Due to natural convection, the ﬂoor provides the best thermal transfer coefﬁcient for heating while the ceiling provides the best thermal transfer coefﬁcient for cooling.
Dew point limitations In order to secure that there is no condensation on the surface of the emitter in the room the supply water temperature should be controlled so that the surface temperatures of the emitter always is above dew point.
In the diagram below, the dew point temperatures can be found under different levels of relative humidity (RH): 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 40 45 50 55 60 65 70 75 80 Dew point temperature [°C] Relative humidity RH [%] Room temp. 26 °C Room temp. 25 °C Room temp. 24 °C Room temp. 23 °C All emitter systems, whether it is pure air-based, radiators or pure radiant systems, are bounded by their ability to transfer energy. The capacity of any radiant emitter systems is limited by the heat ﬂux density, which differs depending on the location of the emitter, i.e. ﬂoor, wall or ceiling.
The heat ﬂux density can be used to calculate the capacity of the emitter, also known as the thermal transfer coefﬁcient. Speciﬁcally regarding cooling, any radiant emitter will need to work within the dew point limitations in order to avoid moisture on the surface and within the construction.
Heat ﬂux density The ability of a surface to transfer heating or cooling between the surface and the room, is expressed by the heat ﬂux density. According to EN 1264/EN 15377, the values below can be used to express the heat ﬂux density. Floor heating, ceiling cooling: q = 8.92 (θs,m - θi )1.1 Wall heating, wall cooling: q = 8 (| θs,m - θi |) Ceiling heating: q = 6 (| θs,m - θi |) Floor cooling: q = 7 (| θs,m - θi |) Where q is the heat ﬂux density in W/m2 θs,m is the average surface temperature (always limited by dew point) θi is the room design temperature (operative) 4. Capacity and limitations of radiant emitter systems 10 5 15 Surface heating and cooling Floor Ceiling Wall Heating Cooling [W/m2 K] Thermal transfer coefﬁ cient 13 UPO NO R · FREE COOLING GUIDE
Emitter surface and humidity Design temperatures for cooling systems are speciﬁed according to the dew point. The dew point is deﬁned by the absolute humidity in the room and can be estimated from the relative humidity RH and the air temperature. The cooling capacity of the system is deﬁned by the difference between the room temperature and the mean water temperature. Often standard design parameters for cooling systems are an indoor temperature of 26 °C and a relative humidity of 50%. At the dew point, condensation will occur on the emitter surface. In order to avoid condensation, the emitter surface temperature has to be above the dew point temperature.
For radiant ﬂoor cooling a minimum surface temperature of 20 °C is required, which means that only when the relative humidity exceeds 70% in the room, the risk of condensation occurs, because that corresponds to a relative humidity of 100% at the emitter surface. Radiant cooling from the ceiling is limited by the radiant asymmetry between the surface of the emitter and the room temperature recommendation is that it should not exceed more than 14 K. For standard conditions (26 ºC, 50% RH) the surface of the emitter usually reaches the dew point before the radiant asymmetry limit. Distribution pipes and manifolds In any cooling system where you have distribution pipes or manifolds you have to be aware of that these parts of the system also have a risk of condensation because they sometime operates below the dew point.
Insulation of distribution system is often necessary in order to avoid condensation.
Design temperature The design supply water temperature of the system depends on the type of surface used, the design indoor conditions (temperature and relative humidity) and the cooling loads to be removed. It should be calculated to obtain the maximum cooling effect possible from the system. The capacity and mean water temperature for radiant ﬂoor cooling depends on the ﬂoor construction, pipe pitch and surface material. To have the highest possible capacity of the system you should design your ﬂoor construction so the surface temperature is equal to the minimum temperature of 20 °C.
The capacity and mean water temperature for radiant cooling from the ceiling is calculated, or can be read directly, in the capacity diagram of the cooling panels.
To have the highest possible capacity of the system you should design as close to the dew point as possible. Theoretical capacities of embedded radiant cooling Taking both ISO 7730 (surface temperatures, radiant asymmetry, and down draught) and the dew point limitations into account, the following surface temperature limitations exist. Surface temperature limitations With these surface temperature limitations in mind, the maximum capacities of different radiant emitter systems can be calculated. The results are shown in the ﬁgure below.
Maximum heating a cooling capacities In theory, the highest heating capacity can be achieved from the wall. Since space is limited due to windows and other things hanging on the wall, the real heating capacity from walls is signiﬁcantly reduced. Hence, the biggest capacity can be achieved by heating from the ﬂoor, and cooling from the ceiling. In practice, either a ﬂoor system or a ceiling system is installed and used for both heating and cooling. A ﬂoor system should be chosen if the heating demand is dominant and a ceiling system should be chosen if the cooling demand is dominant.
35 25 15 45 30 20 40 Floor Ceiling Wall Heating Cooling Parimeter Temperature [°C] 80 40 120 60 20 100 140 180 160 200 Floor Ceiling Wall Heating Cooling Parimeter Heating and Cooling Capacity [W/m 2 ] 14 UPONOR · FREE COOLING G UIDE
5. Ground heat exchangers Ground conditions When planning the use of ground heat exchangers, the ground conditions are of fundamental importance. Determining the ground properties, with respect to the water content, the soil characteristics (i.e. thermal conductivity), density, speciﬁc and latent thermal capacity as well as evaluating the different heat and substance transport processes, are basic pre-requisites to determine and deﬁne the capacity of a ground heat exchanger. The dimensioning has a signiﬁcant impact on the energy efﬁciency of the heat pump system. Heat pumps with a high capacity have unnecessary high power consumption when combined with a poorly dimensioned heat source.
With a higher water concentration in the ground, you get a better system capacity. Horisontal collectors are hence depending on the ground’s ability to prevent rain water from mitigating downwards due to gravitation. The smaller the corn size in the soil, the better the ground can prevent rain water from gravitation. Hence clay will provide a better performing ground heat exchanger than sand. Vertical collectors are depending on being in contact with ground water. Hence the depth of ground water levels has an important impact on the performance of a vertical ground heat exchanger. In addition to the water concentration, different ground types have different thermal conductivity.
For example rock has a higher thermal conductivity than soil, so ground conditions with granite or limestone will give a better performing ground heat exchanger than sand or clay.
Soil type Thermal conductivity (W/m K) Clay/silt, dry 0.5 Clay/silt, waterlogged 1.8 Sand, dry 0.4 Sand, moist 1.4 Sand, waterlogged 2.4 Limestone 2.7 Granite 3.2 Source: VDI 4640 15 UPO NO R · FREE COOLING GUIDE
Ground heat exchangers With ground heat exchangers, a distinction is made between horisontal and vertical collectors. These can be further classiﬁed as follows: Horisontal: • Horisontal or surface collectors • Energy cages Vertical: • Boreholes • Energy piles and walls The suitability of the different collectors depends on the environment (soil properties and climatic conditions), the performance data, the operating mode, building type (commercial or private), the space available and the legal regulations.
Horisontal collectors Collectors installed horisontally or diagonally in the upper ﬁve meters of the ground (surface collector). These are individual pipe circuits or parallel pipe registers which are usually installed next to the building and in more rare cases under the building foundation. Energy cages Collectors installed vertically in the ground. Here, the collector is arranged in a spiral or a screw shape. Energy cages are a special form of horisontal collectors. Boreholes Collectors installed vertically or diagonally in the ground. Here one (single U-probe) or two (double U-probe) pipe runs are inserted in a borehole in U-shape or concentrically as inner and outer tubes.
Energy piles Collectors build into the pile foundations that are used in construction projects with insufﬁcient load capacity in the ground. Individual or several pipe runs are installed in foundation piles in a U-shape, spiral or meander shape. This can be done with pre-fabricated foundation piles or directly on the construction site, where the pipe runs are placed in prepared boreholes that are then ﬁlled with concrete. Most often energy piles are used for larger commercial buildings. 16 UPONOR · FREE COOLING G UIDE
Ground temperature proﬁle The ﬁgure below shows a generic temperature proﬁle in the ground for each season during the year. The closer to the ground surface, the higher the inﬂuence from the outside temperature and solar radiation. Hence not surprisingly, the highest temperatures are found in late summer and the lowest temperatures in late winter. The reason for the temperatures being higher in late autumn than late spring, has to do with the ground’s ability to store energy. After a warm summer period, the ground remains relatively warm during the autumn. Ground temperatures stabilize below 10-15 m.
It is clear from these ground temperature proﬁles that the cooling capacity is higher below 15 m. Hence vertical collector systems provides a better cooling capacity than horisontal collector systems.
Primary supply temperatures The temperatures mentioned in the previous section are often referred to as the undisturbed ground temperature. Depending on the thermal resistance between the collector and the surrounding ground, the temperature of the ﬂuid in the collector will be higher than the surrounding ground. 0 20 20 15 10 5 0 20 10 15 5 10 15 5 1. February 1. May 1. November 1. August Temperature (earth’s surface) [°C] Depth in soil [m] Temperature (depth) [°C] Dimensioning of ground heat exchangers for free cooling The ﬁrst thing to decide is whether the ground heat exchanger shall be used for heating only or for both heating and cooling.
As demonstrated in this guide, new built low energy houses will often have substantial cooling loads. It is therefore highly recommendable to use the ground heat exchanger for free cooling in the summer period. A combined use for heating and cooling also balances of the ground temperature during the year and leaves the ground environment undisturbed. Existing guidelines for dimensioning ground heat exchangers are typically based on the peak load for the heating demand. But in order to ensure that adequate cooling capacity is available in the summer season, it is recommend doing a design check for the maximum cooling load as well.
Dimensioning for the heat load should be done based on the peak load for space heating plus the domestic hot water need. As a heat pump is used for covering the heat load, the COP of the heat pump on the coldest day (design day) should be applied in the design calculation. In addition to this, the speciﬁc characteristic of the chosen heat exchanger and the thermal conditions in the ground must be taken into account. Dimensioning for the cooling load should be done based on valid information of the maximum cooling load in the building. Free cooling operates without a heat pump. It is therefore vital that the thermal capacity of the ground heat exchanger is able to fully cover the max cooling load (no COP is included).
In residential buildings in Northern Europe the cooling need will normally be covered with the capacity derived from the heating requirements. But a design check is always recommended.
In special cases in residential buildings and typically in ofﬁce buildings, the cooling need will be dominant and thus the design driver. In such case vertical collectors are normally recommended as the deeper ground temperatures are sufﬁciently stable and independent of surface temperature and solar radiation. If a horizontal system is chosen, the space requirements can be a capacity limitation. Designing for inadequate cooling capacity on the warmest summer days may then be necessary compromise, but should be evaluated carefully.
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Dimensioning examples In order to dimension ground heat exchangers cer- tain information has to be considered.
First of all an estimation of the physical properties of the ground is needed. Normally its possible to obtain local ground data (thermal conductivity etc.) from local databases or authorities. The ﬁgures below show the capacity for different collectors. Horisontal collectors Energy cage Vertical collectors Pipe size 25, 32 and 40 mm Normal 32 mm XL 32 mm 40 mm Capacity cooling 7-28 W/m2 800-1120 W 1000-1500 W 30-70 W/m Dimensioning temperature, supply/return 17-20 °C 14-17 °C 10-13 °C 10-13 °C *) Energy cage; normal height is 2.0 m, and XL height 2.6. Required depth is 4 m. Flow and pressure drop in the collector When the cooling need is deﬁned, the ﬂow can be calculated.
When using ground collectors, the water used has to be mixed with anti-frost liquid. Hence, the speciﬁc heat capacity and density in the brine is Cooling need [kW] Ethanol Monoethylenglyciol Propylenglycol Flow [kg/s] Flow [l/s] Flow [kg/s] Flow [l/s] Flow [kg/s] Flow [l/s] 2 0.16 0.15 0.18 0.19 0.17 0.18 3 0.24 0.23 0.27 0.28 0.26 0.27 4 0.32 0.31 0.36 0.38 0.34 0.36 5 0.40 0.38 0.45 0.47 0.43 0.45 6 0.48 0.46 0.54 0.56 0.51 0.54 different from the physical properties of pure water. The table below shows the required ﬂow of often used brines for providing different cooling capacity. When calculation the pressure loss in the collector the ﬂow is divided equally up in the number of loops.
For vertical collectors the total pressure loss is normally very low hence the pressure is equalized and it is only the pressure loss in the feeding pipe has an inﬂuence. For horisontal collectors and partly energy cages the pressure loss has to be calculated in order to be sure that the pump will be able to circulate the water through the collector and the cooling exchanger including manifolds and valves.
Example: 4 kW installations Horisontal collector extraction power 15 W/m2 Liquid Monoethylenglycol Total ﬂow 0.38 l/s, 1.37 m3 /h Diameter of collector Ø 32 mm In the diagram below, the pressure loss in the ground collector should be maximum 34 kPa at the dimensioning conditions, and the ground collector should be dimensioned so that the pressure loss in each loop is less than 34 kPa. Pump diagram Available pressure for the primary circuit. CP1 CP2 0 0.5 1 1.5 2 2.5 3 50 40 30 20 10 Pressure loss [kPa] Rate of ﬂow [m3 /h] 18 UPONOR · FREE COOLING G UIDE
Free cooling in combination with diﬀerent heat sources Heating mode, the free cooling is deactivated Cooling mode, the free cooling is activated The illustrations below shows a ground heat exchanger combined with a radiant system in heating mode and cooling mode. In this example a ground sourced heat pump is providing heating to domestic hot water (DHW), space heating, and for heating up the incoming ventilation air. This could of course be utilized with other heat sources such as boilers or district heating. Free cooling is provided through a special pump and exchanger group (see chapter 8) that supplies cold water/brine from the ground heat exchanger directly to the radiant emitter system and possibly the incoming ventilation air.
In cooling mode, the heat pump will only be active for domestic hot water generation. As one can see from the grey connection lines the pump and exchanger group is not active in heating mode. Similarly, the connection lines from the heat pump (or any other heat source) to the emitter systems are in- active in cooling mode.
If a boiler or district heating system is used as heating source, the ground heat exchanger will only work during cooling (also known as a bivalent system). If a ground source heat pump is used as heat source, the ground ground heat exchanger will work both during heating and during cooling (also known as a monovalent system). 19 UPO NO R · FREE COOLING GUIDE
Embedded emitters are the key to any radiant system. In order to have an energy efﬁcient and comfortable solution, the emitter system has to be designed to the construction but also to the task it has to solve.
There are many types of constructions for ﬂoor, wall and ceilings. Uponor offers emitters that can meet the requirements of all types of installations. All emitters are able to provide heating and cooling. However, some emitters are more efﬁciently than others. The most efﬁcient cooling system is placed in the ceiling, but the heating efﬁciency is lower whereas an emitter system in Capacity of different radiant emitter systems In order to calculate the capacity of the radiant emitter, it is important to know the construction in which the embedded emitter is integrated, including the surface material on top of the construction.
In general, there are three factors that inﬂuence on the capacity of a radiant emitter system: • Thermal resistance in the surface construction RB • Pipe pitch, i.e. the distance between the pipes T • Thermal conductivity in the construction material In practice, this means that when designing the ﬂoor construction, the performance of the radiant system can be optimised by choosing the right construction, pipe layout and surface material.
Floor installation Wall installation Ceiling installation the ﬂoor has the highest heating efﬁciency, but with a lower cooling efﬁciency. Another important factor is the supply water temperature. Radiant emitter systems operate on a relatively low temperature for heating, and a relatively high temperature for cooling. A radiant system should be designed for the lowest possible temperature for heating and the highest possible temperature for cooling. This secures a heating/cooling system with high energy efﬁciency and optimal conditions for the heating and cooling supply.
Example: ﬂoor construction 7.
Choosing and dimensioning the radiant emitter system 20 UPONOR · FREE COOLING G UIDE
Pipe pitch, i.e. distance between the pipes The pipe pitch, i.e. the distance between the pipes in the embedded construction, not only has an inﬂuence on the capacity, but also on how equal the surface temperature is. This is especially important from a comfort perspective. The diagram shows the capacity of a concrete ﬂoor construction with =1.8 W/(mK), and with different kinds of surface material. The diagram illustrates the variation of the capacity depending on the pipe pitch. A short distance between the pipes, gives a higher capacity and vice versa. For a combined heating and cooling system, it is recommended to use a relatively small distance 300 mm between the pipes, in order to utilise free cooling and maintain an even surface temperature.
Thermal conductivity in the construction The thermal conductivity in the construction has an effect on the system’s ability to distribute heating and cooling in the thermal mass. A construction with a low thermal conductivity requires a smaller pipe pitch, in order to obtain an equal surface temperature variation. RλB = 0 RλB = 0.05 RλB = 0.10 RλB = 0.15 qCN (RλB = 0.15) qCN (RλB = 0) ΔθCN Y = Speciﬁc thermal output qc [W/m2 ] X = Temperature difference between room and cooling medium [θc K] 45 40 35 30 25 20 15 10 0.1 0.15 0.2 0.3 0.4 0.5 0.25 0.35 0.45 Thermal output q [W/m 2 ] Pipe spacing T [m] θm 15.5 °C, 14 mm parquet θm 15.5 °C, 7 mm parquet θm 15.5 °C, 10 mm tiles θm 18.5 °C, 14 mm parquet θm 18.5 °C, 7 mm parquet θm 18.5 °C, 10 mm tiles Floor surface temperature limit 20 °C Thermal resistance in the surface construction The thermal resistance in the surface construction has a big inﬂuence on the performance of the emitter.
In the diagram, an example of a cooling curve where different thermal resistance values from 0.00 to 0.15 m2 K/W are shown. The curve shows that higher resistance gives a lower capacity. All constructions with embedded radiant emitter systems will have a surface resistance that has to be considered. In order to get the highest efﬁciency, the resistance value has to be as low as possible. Field of characteristic curves of a cooling system For dry constructions, high performance material like heat distribution plates in aluminium or similar are used to ensure optimal heating and cooling distribution.
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Surface material Tiles 10 mm = 1.0 W/mK Surface material Wood 14 mm parquet = 0.014 W/mK Installation principle Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Wet ﬂoor installation 42 40 33 24 Installation integrated in construction 42 40 33 24 Installation on the joists 28 20 27 19 Dry ﬂoor installation 28 20 27 19 Installation between the joists 24 17 18 14 Floor installation Radiant ﬂoor constructions and capacity Radiant ﬂoor systems are far more common than ceiling or wall systems, and can be used for cooling and heating.
A radiant ﬂoor system can be installed in wet constructions using concrete and screed, and in dry constructions with heat emissions plates. A radiant ﬂoor has a cooling capacity of up to 42 W/m2 limited by a surface temperature of 20 °C. The most efﬁcient installation is in a wet construction with con- crete or screed, because of its high heat conductivity, using a relatively short distance between the pipes, and a surface material with a low thermal resistance. In the ﬁgure below, an overview of the capacity in the most common ﬂoor installations is shown with mean water temperatures of 15.5 °C and 18.5 °C corresponding to supply temperatures of 14 °C and 17 °C with a T of 3 K over the emitter loops.
Figures are based on a room temperature of 26 °C and a surface temperature of 20 °C.
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Wall installation Installation principle Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Dry wall installation 45 32 Wet wall installation 60 45 Stud wall installation 42 34 Radiant wall constructions and capacity Radiant wall systems are typically used as a supplement to ﬂoor and ceiling emitter systems for rooms with a higher need for cooling/heating. Instead of dimensioning the ﬂoor or ceiling system according to the room with the highest peak load, it can be designed according to the average and the peak room(s) can be supplemented with a wall emitter.
A radiant wall system will be limited by the architecture and by the furnishing. Radiant wall systems have a cooling capacity of up to 60 W/m2 (active area) limited Surface material Plaster 10 mm = 0.7 W/mK Surface material Plaster 11 mm = 0.24 W/mK Surface material Plaster 11 mm = 0.23 W/mK by a surface temperature of 17 °C, in order to be within the limits of radiant asymmetry and to prevent draught. In the ﬁgure below, an overview of the capacity of the most common wall systems is shown with mean water temperatures of 15.5 °C and 18.5 °C corresponding to supply temperatures of 14 °C and 17 °C with a T of 3 K over the emitter system.
Figures are based on a room temperature of 26 °C and a surface temperature of 20 °C .
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Installation principle Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Cooling effect q [W/m2 ] θm 15.5 °C Cooling effect q [W/m2 ] θm 18.5 °C Wet ceiling installation 75 55 Dry ceiling installation 59 42 Suspended ceiling installation 97 67 Ceiling installation Radiant ceiling constructions and capacity Radiant ceiling systems are the most efﬁcient systems for cooling, but can also be used for heating. Ceiling systems have originally been developed for ofﬁce environments, but are also available for residential constructions using wet plaster or dry gypsum panels.
Radiant ceiling systems have a cooling capacity of up to 97 W/m2 . It is important to note that especially for ceiling cooling, the surface temperature of the system is in peak often very close to the dew point. Special attention has to be taken for adequate dew point control.
In the ﬁgure below, an overview of the capacity in the most common ceiling systems is shown, with mean water temperatures of 15.5 °C and 18.5 °C corresponding to supply temperatures of 14 °C and 17 °C with a T of 3 K over the emitter system. Figures are based on a room temperature of 26 °C and a surface temperature of 16 °C. Capacity diagrams Uponor offers a wide range of embedded emitter systems adapted to different kinds of constructions in the ﬂoor, wall or ceiling. Whenever the choice of system has been selected, detailed diagrams can be used in order to make the planning of the capacity.
The diagram and example on next page shows a ﬂoor construction with the cooling and heating output of the emitter system.
Dimensioning diagram for cooling Analogue to dimensioning for heating, the following parameters must be considered: 1. Cooling effect of the radiant area qc [W/m2 ] 2. Thermal resistance in the surface construction RB [m2 K/W] 3. Pipe pitch, i.e. centre distance between the pipes T [cm] 4. Difference between room temperature and mean water temperature θc. = θi - θc [K] 5. Recommended minimum surface temperature (20 °C) 6. Difference between room temperature and surface temperature θv - θr, m [K] If three of the parameters above are known, the remaining parameters can be calculated using the diagram to the right.
Surface material Plaster 10 mm = 0.7 W/mK Surface material Plaster 11 mm = 0.23 W/mK Surface material Plaster 11 mm = 0.24 W/mK 24 UPONOR · FREE COOLING G UIDE