SOLAR ENERGY - ACTIVE USE OF IN BUILDINGS WHY HOW WHAT

SOLAR ENERGY - ACTIVE USE OF IN BUILDINGS WHY HOW WHAT
ACTIVE USE OF
               SOLAR ENERGY
                  IN BUILDINGS
                      WHY HOW WHAT



                GUIDE FOR THE ARCHITECT




DAGNE VILKA
HORSENS 2010
SOLAR ENERGY - ACTIVE USE OF IN BUILDINGS WHY HOW WHAT
Active Use of Solar Energy in Buildings   2010




         Active Use of Solar Energy in
                  Buildings
                                 Why? How? What?
                                    Guide for the architect




Author Dagne Vilka

Consultant Laila Olesen



Horsens, November 2010




                 Architectural Technology and Construction Management

                     Elective subject (Dissertation) – final – 7th semester

                          Via University College – Campus Horsens




This is a College assignment for examination use ONLY – no legal or technical validity is
claimed or assumed.


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                                                           List of Contents


List of figures ........................................................................................................................ 4
List of tables .......................................................................................................................... 5
Introduction .......................................................................................................................... 6
Definition of problem ........................................................................................................... 7


1. Solar energy’s role in global scale .................................................................................. 8
      1.1. History of solar energy ........................................................................................... 8
      1.2. Solar energy versus other energy sources ........................................................... 10
      1.3. Positive and negative aspects of solar energy ..................................................... 12
2. Solar energy properties and dependence of environmental issues ............................. 14
      2.1. Sun’s energy output ............................................................................................. 14
      2.2. Atmosphere, time and location influence o solar energy .................................... 17
3. Active solar energy systems ......................................................................................... 20
      3.1. Photovoltaic systems ........................................................................................... 21
            3.1.1.Solar radiation conversion to electricity ...................................................... 21
            3.1.2.Types of photovoltaic (PV) solar cells ........................................................... 22
            3.1.3.Design solutions for increasing efficiency .................................................... 23
            3.1.4.Efficiency affecting issues ............................................................................. 25
            3.1.5.System components ..................................................................................... 26
            3.1.6.Design variations in buildings ....................................................................... 27
      3.2. Photo‐electrochemical systems ........................................................................... 28
      3.3. Solar thermal systems .......................................................................................... 29
            3.3.1.Perforated plate collectors ........................................................................... 29
            3.3.2.Flat plate collectors ...................................................................................... 30
            3.3.3.Evacuated tube collectors ............................................................................ 32
            3.3.4.Batch collectors ............................................................................................ 33
            3.3.5.System components ..................................................................................... 34
      3.4. Solar thermoelectric systems ............................................................................... 36
      3.5. Solar cooling and other applications .................................................................... 38


Conclusion .......................................................................................................................... 40
References .......................................................................................................................... 41




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                                                        List of figures

Figure 1. Development of Active and Passive solar energy .................................................. 8

Figure 2. Seville’s Power Tower [3] ...................................................................................... 9

Figure 3. World Total Energy Production [4] ....................................................................... 10

Figure 4. Energy Consumption by Industry [9] ................................................................... 12

Figure 5. Largest Solar Powered Building [10] .................................................................... 13

Figure 6. Equilibrium of the Sun [11] .................................................................................. 14

Figure 7. Nuclear fusion ...................................................................................................... 14

Figure 8. Electromagnetic wave [15] .................................................................................. 15

Figure 9. Electromagnetic Spectrum Properties [17] ......................................................... 16

Figure 10. Light reflection [18] ........................................................................................... 16
Figure 11. Interaction of incoming solar radiation with the Atmosphere [20] .................. 17
Figure 12. Global yearly irradiance [21] ............................................................................. 18
Figure 13. Solar Geometry [22] .......................................................................................... 18
Figure 14. Yearly sum of global irradiation on a horizontal surface – Denmark [23] ......... 19
Figure 15. Office energy consumption [24] ........................................................................ 20
Figure 16. Residential energy consumption [25] ................................................................ 20
Figure 17. Positively and negatively charged atoms [26] ................................................... 21
Figure 18. Photovoltaic cell construction [27] .................................................................... 21
Figure 19. Electron flow [28] .............................................................................................. 22
Figure 20. Mono‐crystalline silicone ingot and wafers [29] ............................................... 22
Figure 21. Production process of typical crystalline silicone solar cells [30] ...................... 23
Figure 22. Amorphous silicon thin film cell [31] ................................................................. 23
Figure 23. Mono‐crystalline silicone cell fragment [32] ..................................................... 23
Figure 24. Poly‐crystalline silicone cell surface texture [33] .............................................. 24
Figure 25. Concentrated solar cell with focusing lenses [34] ............................................. 24
Figure 26. Concentrated solar cell with focusing mirrors [35] ........................................... 24
Figure 27. Solar panels on a tracking system [36] .............................................................. 24
Figure 28. Efficiency versus solar radiation [37, page 134] ................................................ 25
Figure 29. Dependence on Sun incidence angle [37, page 135] ........................................ 25
Figure 30. Sweeping snow from solar panels [38] .............................................................. 26
Figure 31. Off‐grid system example [39] ............................................................................ 26
Figure 32. Photovoltaic pavement [40] .............................................................................. 27

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Figure 33. Integrated Concentrating Solar Façade System [41] ......................................... 27

Figure 34. Photovoltaic façade cladding [42] ..................................................................... 27

Figure 35. Dye‐sensitized solar cells [44] ........................................................................... 28

Figure 36. Perforated plate collector’s principal scheme [47] ........................................... 30

Figure 37. Flat plate collector composition [48] ................................................................. 30

Figure 38. Flat plate collectors installed on a wall [50] ...................................................... 31

Figure 39. Evacuated tube collector mounted on a roof [51] ............................................ 32

Figure 40. Evacuated glass tube composition [52] ............................................................. 32

Figure 41. Evacuated‐tube’s geometry [53] ....................................................................... 32

Figure 42. Integrated tank solar evacuated‐tube collector [54] ........................................ 33

Figure 43. “Do It Yourself” batch solar collector [55] ......................................................... 33

Figure 44. Flat plate collector closed loop system [56] ...................................................... 34

Figure 45. Solar thermoelectric system collector types [57] .............................................. 36

Figure 46. The solar furnace at Odeillo [58] ....................................................................... 37

Figure 47. Evaporative cooling system principle [59] ......................................................... 38

Figure 48. Desiccant cooling system [60] ........................................................................... 38



                                                     List of Tables

Table 1. Comparison of renewable energy sources [6, page 31, 7] ................................... 11




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                                        Introduction


As conventional sources soon won’t be able to meet the demands of future society,
development and use of renewables is increasing. On one hand the rapid growth of
population, rising standards of life and on the other hand environmental problems caused
by pollutions and ineffective resource use are issues that influence each other. In between
them is energy production – the consumption is rising, but producing more of it by using
non‐renewable sources, the environment is suffering and no long term existence is possible.
To keep the balance between rising consumption and decreasing resources, renewables are
used that make the environment, society and economics sustainable.

In the building industry, that is one of the main energy consumers, new sustainable projects
are not thinkable without use of renewables, especially solar energy. As it is such an
abundant and infinite source, remarkable savings or even earnings can be achieved not only
financially but also materially. To do this, the Sun can be used in two ways – active and
passive. Passive use is more known as solar architecture and deals with shapes and materials
that during exploitation don’t depend on other energy inputs provided by men. Tough active
use is supported by different technological systems, therefore more efficient and the subject
described in this research.

Although the handling of active solar systems may seem to be the responsibility of technical
engineers, the concept of a project is still defined by architects. Solar technologies must be
implemented in a project from the very beginning and many things have to be subordinated
according to them to achieve maximum efficiency.




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                                    Definition of problem


Some say that a good architect must have knowledge in all sectors of life and in nowadays
situation one of the main issues is the use of renewables. As the field of active use of solar
energy is relatively new, more and more inventions appear every day and it is getting hard
to keep up with the nowadays tendencies. Not to get confused by the clever marketing
strategies of different novelties, basic knowledge is needed to be able to objectively
evaluate all the products and optimally use them.

This research is a summary of all issues related to active use of solar energy in buildings that
in many cases are missed. All chapters together offer the basic information needed to be
able to orientate in this field. The main questions that define the issues about active use of
solar energy are:
     Why should solar energy be used?
        Although everywhere is promoted that use of this energy type is good, not
        everybody knows what are the real reasons and therefore the real gains can be
        missed. Also the awareness of importance in global scale is essential, to keep up
        with the nowadays tendencies.
     How does solar energy work and what are the issues that affect it?
        The efficiency of solar systems is not only dependent on the physical characteristics
        of different technologies, but also issues like time, region, climate etc. By
        understanding the principles of solar energy properties, optimal evaluation by
        making the choice of a suitable system can be done.
     What are the solar system types and how can they be used?
        Solar energy cannot be converted only with solar panels placed on roofs. There are
        numerous other types of solar systems and ways how to use them. In chapter 3 all
        types are described to show the diversity and enable designers to find the right
        solution for their region and give the directions how to use them.

Architects, that define and modify the surrounding environment, are the third party that
brings the inventions from laboratories to the customers. Although the scientists and
engineers are well educated in this field and deliver good inventions, the current architect’s
education is incomplete concerning renewable source use. Therefore this research is also a
suggestion of the basic information that students and employees of building industry should
know when developing projects.




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                 1. Solar energy’s role in global scale
Solar energy is the scientific name for the energy we get from the Sun in form of radiant light
and heat. Nowadays everybody knows that this is an important source that should be used
in all possible ways beginning from simple actions like letting sunlight in your room to
establishing plants that collect solar energy and conduct to the customers. The keyword is
“saving energy” and Sun is one of the best sources to do that, but not everybody knows why.


1.1. History of Solar energy

The role of Sun was already recognized by the ancient civilizations, although there was no
awareness that Sun’s energy can run a steam engine or produce electricity to run a
household or vehicle. The big burning ball in the sky was a deity in different cultures world‐
wide. It came every morning and disappeared every evening, ruling people’s lives. During
time the importance grew and fell and it seems that nowadays it is close to the level that
ancient cultures had, only not in the field of religion but science.

Passive use of solar energy ‐ light and warmth is used ever since we, humans, are aware of
ourselves. The first steps to use the Sun for producing something was to concentrate the
Sun’s rays to gain fire and use this fire in different religious and household needs – 7th
Century BC. During time, when more complicated buildings with glass windows developed,
the function of the glazed surface was not only giving a possibility to look through the
building envelope but also gain warmth from the Sun, by situating these surfaces to the side
where the Sun shines at most. The passive use of sunlight was intentional and used in more
and more ways, and these methods are used also nowadays in solar architecture [1].




                 Figure 1. Development of Active and Passive solar energy


The turning point in use of Sun energy was when scientists started to develop devices that
collected energy from the Sun and directed it for further use, starting from the 18th century.
The reason for the first attempts was the search for new discoveries after the Scientific
Revolution in the 16th Century. The first significant invention, in 1860’s made by August
Mouchet, was a solar‐powered motor of 0.5 horsepower and a steam engine that was
powered by the Sun. In the 1870’s William Adams used the same principle of the steam

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engine, but concentrating the Sun’s rays with
mirrors to achieve higher efficiency – 2.5
horsepower. This principle is also used nowadays
and known as the Power Tower Concept (Figure
2). In the next decade the first solar cell was born
with the conversion rate of 1‐2% (Charles Fritz)
and the first solar system for heating water was
installed (Charles Tellier) [2]. Despite the

promising achievements none of the inventions
                                                              Figure 2. Seville’s Power Tower [3]
where developed further, because the use of
fossil energy, at that time, was more efficient.

The use of solar energy, as we know it now, started to develop rapidly in the 20th century
after World War II. Till then attempts to produce energy from Sun for commercial use where
minor or unsuccessful, but the discoveries continued in the laboratories. The first kick was
when scientists in 1954 discovered a semiconductor material that increased the efficiency of
solar cells to 6%. It was silicone ‐ the basis material of solar cells and solar panels nowadays.
Two years later, the first commercial solar cell was available but the price was high – 300$
per watt and the popularity was low. Further development was promoted by the space
programs – there was a need of an energy source that is renewable in space, the efficiency
of the solar cells rose. The importance and development of solar energy increased rapidly
after the OPEC oil embargo in the 1970’s. This incident showed the world that the gain of
fossil sources is unstable and there is a need energy sources that can replace it. The Solar
technologies became more and more efficient, the price decreased and new inventions
showed up more often.

Nowadays solar energy is a common used source. The driving force for development and
establishment of renewable sources is not only the political reasons, but also economical,
social and environmental aspects. The continuous developments and technological progress
promotes wide range of products and it starts to get confusing for the customer.




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1.2. Solar energy versus other energy sources

The diversity of sources from whom to gain energy is wide. These sources can be classified in
two groups – renewable and non‐renewable. The non‐renewable sources are mainly fossil
fuels ‐ coal, petroleum and natural gas (remains of the decomposition of plants and animals,
ironically, by the help of Sun’s energy) and nuclear power – energy gained from uranium via
nuclear fission. These are the conventional sources and as history shows – although cheap
upon others because of the high energy density, unreliable, with the tendency of producing
pollutants, when used and, as the classification says, non‐renewable – the source will expire.
On the other hand, renewable sources agree with the main requirements for nowadays
situation. The main advantages are endlessness of delivered energy (can be recycled) and
pollutant free use. For now the efficiency is comparatively low and thereof also the price is
higher, but the potential is big. The commercial use of these sources is recent and
development for higher efficiency is happening in full swing. Witch of the sources is going to
be the most used in future depends on the regions, their politics and environmental aspects.

 Today’s situation shows that we are using non‐renewable sources more than the renewable
(Figure 3). The awareness of need of changes is relatively new, so the shift is only in the
beginning. The rise of population and social development is an additional burden to the
energy production for both ‐ conventional and renewable sources. The difference here –
conventional sources have no long term perspective.




                          Figure 3. World Total Energy Production [4]


The renewable energy sources have way more types to offer than the conventional ones,
that also makes them more accessible in different regions and independent from the fuel
market. The mainstream forms of renewable energy are:
     Wind power – airflow used to run wind turbines ;
     Hydropower:
           o Hydroelectric energy ‐ production of electrical power through the use of the
               gravitational force of falling or flowing water;
           o Dam less hydro systems that use the kinetic energy of water flow;



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Active Use of Solar Energy in Buildings   2010


            o   Ocean energy in ways of tidal power, thermal energy conversion, marine
                current power;
       Solar energy:
            o Passive – use of Sun without active mechanical systems;
            o Active:
                     Photovoltaic systems – energy converted to electricity
                     Solar Thermal systems ‐ energy converted to thermal energy;
       Biomass – releasing energy by burning plants;
       Geothermal energy ‐ collecting heat from Earth’s core and also more shallow
        seams.[5]

It is interesting to mention that wind, hydro and biomass sources are directly or indirectly
driven by the Sun. The cause for wind is pressure differences of gasses that are dependent
on the level heated by the Sun. The wind and Sun affect the movement of water in different
scales and the energy, that biomass releases by burning, comes from the Sun. Of course
many other forces influence these energy sources, but the role of Sun is unquestionable.




                 Table 1. Comparison of renewable energy sources [6, page 31, 7]


In Table 1 basic criterion are collected to get a better overview of the main characteristics of
each source. The overall status of solar energy is positive because of the high amount of
energy available, variations of gained energy types and low impact on environment. Very
important, in this case, is the establishment in small scale, in means of good possibilities to
use this source for personal use. The price factor shows the average situation now, but it is
variable with the tendency to decrease for most of the sources with the lapse of time. The
main disadvantage on the background of other energy sources is the supply frequency –
solar energy is weather dependent.




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1.3. Positive and negative aspects of solar energy

Continuing the research, we are taking a closer look in the different aspects that make this
energy source so promising in the future development of energy production in global scale
and the building sciences from the viewpoint of an architect.

Sun is the biggest energy supplier on Earth. Every hour the Earth receives 10,4 EJ (equal to
2,9×1012 kW) of Sun energy. The amount of Sun energy we receive each day is just a little
less than the estimated energy contained in the world’s coal reserves (expected to last for
next 130 years)[8]. Although a big part of the solar energy is lost in different reflecting,
conducting and absorbing processes in the Earth’s atmosphere, the remaining amount is
enough to produce energy savings in different levels. This quality ensures a good chance to
be a premier source when replacing the use of conventional energy sources by the rising
requisition of mankind. Besides this, solar energy has also other significant characteristics:
     Renewable source – the solar energy comes from the Sun that is going to stay as it is
         for the next 5 billion years;
     Variety – solar energy can be used for electricity generation, heating and day‐
         lighting;
     No tie to fuels – by the generation of energy no fuels are involved – no fuel costs, no
         pollution by combustion processes, no dependence of the fuel market;
     The source is accessible world‐wide;
Also as history shows, the role of solar energy is rising and implication in different fields
related to energy issues is inescapable.

In the world‐wide energy consumption scene
about 40% is used in the building industry (Figure
4). Production of materials, construction and
maintenance of all the buildings in the world
consume big amounts of energy, therefore the
representatives of this industry have significant
influence on the global energy situation. This why
it is important for engineers, contractors,
planners and especially architects ‐ the creators
of the building concepts, to understand the role
of solar energy. To convince to give preference
to solar energy over other sources, here are the Figure 4. Energy Consumption by Industry [9]
main advantages:
      Saves money ‐ Solar energy is an endless and free source that can partly or totally
         overtake the supply of electricity, heating spaces and lighting of a building in ways of
         solar architecture (passive use) and photovoltaic and thermal systems (active use).
         Also the pay‐back period can be short (depends on the consumption) and if the
         systems produce more energy than needed, the overstock can be sold;
      Environmentally friendly – during the production of energy no water or air is
         polluted and the process is noise‐free, thereof it contributes the protection of
         environment;
      Independence ‐ solar systems don’t necessary have to be connected to power lines
         or other networks. Installed efficient, they can provide households with all the
         necessary energy independent from others;




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       Maintenance – solar systems are
        generally simple in use and upkeep
        and can also be implemented in the
        total building maintenance in cases
        where solar systems replace or are a
        part of building components.
       Design diversity – active and passive
        use of solar energy can be carried out
        in vide variety of ways. Implemented
        solar systems can positively affect the
        design and even raise its quality.
                                                       Figure 5. Largest Solar Powered Building
Unfortunately solar energy has also its                  [10], Dezhou, Shangdong Province in
                                                                   northwest China
problematic sides. But since the positive aspects
clearly place this source in the leading role in the future, there intensive researches are done
to solve or learn to deal with these disadvantages. The main weak points and their potential
solutions are:
     Environmental unfriendly materials and byproducts used in the production of solar
         devices. Mostly solar cells and panels that produce electricity are composed of these
         kinds of materials. Although the amount of risky materials is small, the solution is to
         raise the efficiency of these devices, in that way reducing the cell material needed.
     Low efficiency and high cost – these are permanent parameters that will change
         during time. The efficiency problem is the most investigated issue and good
         solutions are available already now. The cost is dependent on the commercial use of
         solar technologies and has a tendency to decrease.
     Solar energy dependence on climatic conditions, regions, daytime and pollution –
         this is the biggest disadvantage, also in global scale. The original energy output of
         Sun is decreased by these factors and they hinder a constant energy gain on Earth.
         This is also a reason for the need of storing energy; therefore additional devices to
         solar systems are needed. Evaluation of the regional characteristics and ability to
         choose devices appropriate to these conditions is the solution and also the subject
         of the next chapters.

All energy sources, no matters if conventional or renewable, have their pros and cons. Some
are more perspective than others, but as long as we learn to use them wisely and for the
right reasons, global problems won’t affect us and we will have a safe future.




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        2. Solar energy properties and dependence of
                     environmental issues
In this chapter we are analyzing the energy types the Sun is emanating and the supplied
energy’s activity under different environmental impacts. This knowledge is important,
because it helps to understand the energy conversion processes and eases to make the right
choice of solar devices.


2.1. Sun’s energy output

In this case the featured facts of the Sun are minor. The understanding of how the energy is
produced is more relevant since this information should be used in further research of
effective use of solar systems. The information’s language is kept simple to ease the
understanding for readers without special knowledge about this subject.

From the very beginning the reason
why Sun is able to produce energy is
related to the birth of this star. Sun
took shape from particles of space
(nebula ‐ big dust and gas cloud) slowly
joining together and forming a sphere
by the forces of gravity 4.5 billion years
ago. The sphere rotated and this
rotation caused the mass to compress
more and more. The compression
pushed together the gases in the core,
temperature and pressure rose till
nuclear fusion occur and energy was
produced (similar reactions are used in
nuclear power plants on Earth). In its            Figure 6. Equilibrium of the Sun [11]
turn the energy raised the pressure
even more and stopped the compression. The pushing and pulling forces in the mass
stabilized and the Sun took constant size as we know it now (Figure 6).


So, nuclear fusion, possible only by
extremely high pressure and temperature,
is the main energy creator in the Sun. In this
process two or more atomic nuclei join
together and form a new nucleus and
energy. The Sun consists of 74% hydrogen,
25% helium and seven other elements that
form the rest. In the nuclear fusion two
hydrogen nuclei join and form helium, but
as the new nuclei – helium is lighter than               Figure 7. Nuclear fusion
the two hydrogen nuclei the remaining
mass is converted into energy (E=mc2). The fresh helium nucleus joins with another helium
nucleus forming full value helium that contains also two hydrogen nuclei. These hydrogen
particles split away and the process starts from the beginning (Figure 7). This continuous


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chain will last till there is enough hydrogen in the Sun, because not always the hydrogen
joins with another nucleus of its type. If it comes to reactions where other particles join, no
new hydrogen is formed [12]. This why slowly the hydrogen amount decreases and in 5
billion years other reactions will start to occur and the Sun will change its characteristics ‐ for
us disadvantageous. But till then the reactions provide us with energy and this can clearly be
seen as a renewable energy source.

The energy released by the nuclear fusion in the core of the Sun is in form of gamma‐ray
photons and neutrinos. The neutrinos are particles with minimal energy and leave the star
as nothing happened. But the photons, that have the most energy, start a long journey
through the photosphere to the Sun’s surface. This travel can take to 1 million years because
the particles are hindered by the star’s gravitational attraction. During this process the
photons loose energy for the burdensome movement and in the end they reach the surface
as visible photons [13].

The photon is the key word in understanding the tie between the energy produced in the
core and energy we receive on the Earth. The definition says: “The photon is the quantum of
the electromagnetic interaction and the basic "unit" of light and all other forms of
electromagnetic radiation and is also the force carrier for the electromagnetic force.”[14].
That means that the energy from the core is electromagnetic radiation and its main
characteristics are:
     It consists of electric and magnetic
        energy that are bounded and form a
        wave depending on their power
        (Figure 8);

       Ability to diffuse in matter or vacuum
        (in space with the speed of light);
       Different wavelength;
       Than shorter the wave than more
        energetic the radiation and opposite;
       The waves can be reflected, scattered             Figure 8. Electromagnetic wave [15]
        and absorbed (depends on the
        irradiated mass properties). If absorbed, the energy is converted to heat or
        electricity; if scattered or reflected – the energy is redirected or redistributed [16].

So the electromagnetic radiation, produced in the center of the Sun, travels to the star’s
surface then further on in all directions in space where it is absorbed by objects that are in
the way of this radiation. And Earth is that kind of an object.




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                         Figure 9. Electromagnetic Spectrum Properties [17]


As said before, the wavelength of the radiation varies (electromagnetic spectrum, Figure 9).
Sun emits all types of electromagnetic radiation only the quantities of each type are
different. Mostly it’s the radiation with wavelength from 100 to 106 nm that includes
ultraviolet radiation, visible light and infrared radiation. Since all waves absorbed heat up
the mass the main function of solar radiation is to maintain the life and motion of processes
on our planet.

We feel the solar radiation in form of heat and light. Light
is a small part of the spectrum that is visible by the human
eye. The wavelength is from 400 to 700 nm and the
intensity of sunlight can also be an indicator for the power
level of the received radiation. Visible light is the part of
solar radiation with the highest energetic potential
(ultraviolet radiation is more energetic than light but most
of it is absorbed by the ozone layer), so than more light we
get than more energy is available. The visible spectrum
contains all colors and our brain is able to convert these
waves, striking our eyes, in information – visual reflection
of the scene. The Sun emits all spectrums, so theoretically
it contains all the colors, however it is white and shades in
different tones only when there is a filter in front of it like
the atmosphere, air pollution etc.

So to produce energy, electromagnetic radiation in the
spectrum of solar radiation is used. Sun is reliable in
means of emitting constant energy flow, but sadly till the
energy reaches the Earth’s surface it is weaken
dramatically. The energetic intensity of the radiation,         Figure 10. Light reflection [18]
before breaking into the Earth’s atmosphere is 1,366
kW/m2, also called the solar constant. But the intensity when reaching the planet’s surface is
only up to 1000 W/m2 in clear sky conditions. The reasons for this decrease are discussed in
the next paragraphs.

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2.2. Atmosphere, time and location influence on solar radiation

The efficiency is the most important issue that defines the choice of use of solar energy.
Apart from the potential of technologies it is affected by environmental issues like the
climatic processes in the atmosphere, location and time.

The electromagnetic radiation, emitted by the Sun, reaching the Earth’s atmosphere is
affected already by the uppermost layers – exosphere and thermosphere. Here the gamma‐
ray, X‐ray and ultraviolet radiation (wavelength less than 200 nm) are interacting with
oxygen O2 and nitrogen N and turned into heat. Only the ultraviolet radiation with
wavelength more than 200 nm, together with all the bigger wavelength solar radiation types
continues the travel. The next stop is the ozone layer in the stratosphere where the rest of
ultraviolet radiation is reacting with oxygen and forming the ozone itself (only 1‐3 % of
ultraviolet radiation penetrates). Further down the infrared solar radiation is absorbed
mainly by water vapor and carbon dioxide (the main gases of greenhouse effect). The Water
vapor is available in big amounts in the lowest layer of atmosphere – the troposphere, in
form of clouds, air humidity etc. The only radiation type that is able to resist absorption is
visible light. But it too can be reduced ‐ by the reflection of airborne dust and clouds (able to
reflect better than to absorb) [19]. Also radio waves reach the Sun, but they are minor in
connection with solar energy because of their low energy level. Further on the radiation is
absorbed by the Earth’s surface (also reflected ‐ by snow, ice and light colored objects)
where it releases the energy by heating the surface. Then this warmth, in form of infrared
radiation, is emitted by the Earth and sent back to the atmosphere. An objects ability to
reflect or absorb solar radiation is called its albedo. It varies between 0 (total absorption)
and 1 (total reflection).




              Figure 11. Interaction of incoming solar radiation with the Atmosphere [20]




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As a result of the atmosphere’s impact we get three kinds of solar radiation:
      Beam irradiance – directly from the Sun, minimally affected by the atmosphere;
      Diffuse irradiance – solar radiation scattered by the atmosphere (clouds);
      Reflected irradiance – coming from the Earth as reflected radiation and infrared
         radiation and depending on the surfaces albedo (comparatively small and
         dependent on the received radiation).
This distribution is useful in defining the choice of solar systems – concentrating devices can
efficiently use beam irradiance while diffuse and reflected radiation will be left unattended
and vice versa if non‐concentrating devices used [6, page 66]. All these three types form the
total energy quantity that is only about a half of the energy of the point of the solar
constant.

Unfortunately the solar radiation is not only
reduced by the atmosphere but also is irregular in
means of time ‐ daytime and season. Because of
the Earth rotating, the solar radiation is available
only at daytime – when the Sun is over the
horizon and reaching the maximum irradiance
level in the noon. And, because of the Earth’s
rotation around the Sun in elliptical track, the
distance to Sun varies (depending on the season)
and therefore also the solar constant (Figure 12).
To get more detailed information about these
differences it is useful to analyze a region by its
geographical latitude, height over see level and
the surfaces albedo.
                                                            Figure 12. Global yearly irradiance [21]

The Sun’s angle of altitude varies by
daytime and latitude ‐ as longer the
day and steeper the Sun shines to
the horizontal surface, than higher
the solar radiation supply and
absorption. These conditions are
common in the world areas that are
closer to the equator (latitude 0°).
But also good performance of
energy supply from Sun can be
observed in the far north regions
where at summer time the day lasts
for six months (midnight Sun).                        Figure 13. Solar Geometry [22]
Although the radiation level received
is dramatically low, solar systems, set up upright, can collect the energy from direct sunlight
and the reflected energy from the ice and snow. The height over the sea level is important
simply because as higher the surface than shorter the distance for radiation to go through
the atmosphere. All these geographical location issues are less of importance in regions
where cloudy weather is common – the impact of water level in air is bigger, but this
information can be useful if passive solar energy principles are used.




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However, the climate characteristics, height over see level, surface’s albedo, location
latitude and angular altitude vary from region to region. To simplify the search for the actual
received solar radiation maps and tables can be used, where all these aspects are gathered
and the average energy units shown (Figure 14). Usually they are arranged by concrete
locations and regions.




        Figure 14. Yearly sum of global irradiation on a horizontal surface – Denmark [23]




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Active Use of Solar Energy in Buildings    2010



                       3. Active solar energy systems
Active use of solar energy is one of two main possibilities how to improve a building by using
the Sun. It is, in comparison with the other way – passive use, more efficient and is not only
able to save but also to earn. Active solar systems convert the solar energy, with help of
different technologies, to useful energy that can be used to support the building service
systems.

The electromagnetic radiation emitted by the Sun can be converted in an active way in two
main energy types – electricity and thermal energy. Exactly these are the two types that
buildings (residential 37% and commercial 35%) are consuming the most (Figure 15 and 16).
Mainly energy is used for space heating, lighting and support of electrical equipment.




        Figure 15. Office energy consumption                    Figure 16. Residential energy
                                                                     consumption [25]

So logically follows that, if the owner of a building wants to save money by using renewable
energy, these are the fields that must be affected and solar energy is the best way to do it.

The conversion process can be direct, where solar technologies transform the Sun’s energy
to heat or electricity, or performed in several steps like, for example, the solar thermal‐
electric systems. The indirect conversion system consists of more components that require
more space but on the other hand, the direct conversion systems are smaller, simpler and
can be easily integrated in buildings.

In the next sections the main conversion system types are explored:
      Photovoltaic systems;
      Photo‐electrochemical systems;
      Solar thermal systems;
      Solar thermoelectric systems;
      Solar cooling and other applications.
Each of them has their own characteristics and ways to be used. Which one is better upon
the other depends on criterions like efficiency, price, availability and all in all the customer’s
needs.




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3.1. Photovoltaic systems

The direct process of solar energy conversion to electricity is generally maintained by
photovoltaic solar cells and is called the photovoltaic effect. It occurs when specific materials
are affected by sunlight and electric current is generated. To understand what exactly
happens in the cells, it is advantageous to start from the very basics.


3.1.1. Solar radiation conversion to electricity
As taught in the physics classes in school,
every atom consists of a nucleus and its
surrounding electrons connected together
with the power of valence band. Every
“normal” atom’s nucleus has around it certain
number of electrons. But the electrons have
unstable characteristics and under certain
circumstances they can leave the nucleus and
join other normal atoms in that way forming
                                                     Figure 17. Positively and negatively charged
positively and negatively charged atoms.
                                                                      atoms [26]
Positively charged atoms have a lack of
electrons and negatively charged atoms excessive electrons (Figure 17). Because in nature
everything tends to be in equilibrium the contrary charged atoms are attracted to each
other similar as magnets and tend to exchange the electrons to get back to the normal state.
This exchange of electrons in larger atom quantities is called the flow of electricity. A similar
process occurs also in the solar cell but only a little more complicated. A force that drives
this flow is called voltage (V), the flow itself – current measured in amperes (A) and the
relationship between the voltage and the current is expressed in watts (W) – power the
force consumes by moving the flow.


The photovoltaic cell is usually made of
several layers of materials as seen in
Figure 18. The uppermost that faces the
Sun is the protective layer that prevents
from mechanical damage. It is attached to
the cell with a transparent adhesive.
Further down is an anti‐reflection layer
that minimizes the reflection and
promotes the absorption of light. In the
middle of the cell is the most important
layer made of a semiconducting material
where the flow of electrons occurs. That,     Figure 18. Photovoltaic cell construction [27]
in its turn, is covered from both sides
with metallic contacts that collect the electrical current and conducts it to an external
circuit. The metallic contact in the upper part between the anti‐reflection layer and
semiconducting material is in form of a mesh so that it doesn’t stop the incoming light.
Under the semiconducting material the metallic contact is in form of foil.

So the solar electromagnetic radiation penetrates the upper layers of the cell till it hits the
semiconducting material. This solar radiation is the “certain circumstance” mentioned
before that activates the electrons. The irradiated electrons become so energetic that they


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are able to escape the valence band of an
atom and flow freely in the material. But
for the flow of electricity also a force is
needed that guides all the electrons in a
common direction. Therefore the
semiconducting material is made of two
parts – an n‐type semiconductor that is
doped with positive particles and p‐type
semiconductor that is doped with
negative particles. These both materials
form a gradient of electrical potential that
force the electrons. So to sum up, the                    Figure 19. Electron flow [28]
sunlight hits the lower part– the p‐type
material that is saturated with electrons (negatively charged atoms) and sets them free
leaving empty vacancies. The charged particles move to the n‐type semiconductor material
where a lack of them is (positively charged atoms) ‐ this makes the electric current. The
characteristics of the material prevent many of the electrons to join back to the nucleus in
the n‐type layer and they can continue the way through the metallic contacts to an external
circuit (Figure 19). In the external circuit the powered electrons leave their energy by driving
electrical devices and weakened come back through the circuit to the p‐type material in the
cell and by the power of valence band that is now stronger, are placed back in the empty
vacancies they left before. And then again and again they are charged by the solar radiation
and able to escape.


3.1.2. Types of photovoltaic (PV) solar cells

Nowadays for commercial use various types of PV technologies are available. The most
important criterions in comparing them are the devices cost and efficiency that is the
relation between solar energy received on the cell and the electrical power output. The most
popular electricity producing cell types are:
     Single‐crystalline or mono‐crystalline cells –
        the electricity production process described
        before is on the basis on these types of cells.
        These were the first ones invented and
        therefore the most common used. The
        whole development of PV industry was
        based on these cells. The semiconducting
        material is produced by growing a single
        ingot crystal from melted high purity silicone
        (Czochralsky process). Further the crystal is
        sliced in thin wafers (Figure 20). The
        production process is slow, energy intensive       Figure 20. Mono‐crystalline silicone
        and a lot of waste materials remain,                      ingots and wafers [29]
        therefore the cost is high, but the efficiency nowadays has increased up to 25%.

       Multi‐crystalline or polycrystalline cell production is more simple – melted high
        purity silicone is casted into ingots and afterwards sliced (Figure 21). In this process
        multiple crystals form and therefore the material conductivity is lover (up to 20%),
        but the production is faster and of low cost.



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Active Use of Solar Energy in Buildings   2010




               Figure 21. Production process of typical crystalline silicone solar cells [30]


       Amorphous silicon cells are produced by
        depositing the high purity silicone vapor on very
        thin films of steel. In that way only 1 % of
        material is used in comparison with the silicone
        crystalline     technologies.     Thereby     the
        production cost is again lower, but the
        efficiency also decreases. The present efficiency
        is 13 %, but the other properties like weight,
        low cost are good enough reasons for these
        cells to be able to challenge the before          Figure 22. Amorphous silicon thin film
        mentioned technologies.                                         cell [31]

       Other thin films are made of materials like Copper Indium Diselenide, Cadmium
        Telluride and Gallium Arsenide. This technology is still in development but this type
        has the biggest potential in increased efficiency in future. For now the main
        obstacles are increasing costs and complicated production technologies. The silicone
        cell industry is more stable and therefore more popular.


3.1.3. Design solutions for increasing efficiency

In addition to the potential of cell
materials to convert solar radiation to
electricity       supplements        and
modifications of the system can be
used to increase the efficiency. Starting
from the smallest scale, the PV cell
itself can be and is manipulated. As
before mentioned the solar cell has an
anti‐reflecting layer that prevents the
sunlight to escape back to the                  Figure 23. Mono‐crystalline silicone cell fragment [32]
environment – to reflect. This
reflection is one of the main factors


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Active Use of Solar Energy in Buildings   2010


why solar cells are not able to use all
received energy. Therefore the surface of
PV cells is made uneven, so that the cell
surface area is bigger and more planes
available for sunlight to hit. For example,
mono‐crystalline cell’s surface forms a
pattern of inverted pyramids in a grid
(Figure 23) and poly‐crystalline cells have a
                                                 Figure 24. Poly‐crystalline silicone cell surface
honeycomb texture (Figure 24) (attempts
                                                                  texture [33]
forming the surface in the same pattern as
mono‐crystalline cells have led to damages in the material). Yet these additional features are
already included in the materials efficiency percentage.

Continuing in bigger dimensions, the sunlight can be manipulated with concentrators.
Concentrated cells are a combination of classic silicone cells and focusing optics like glass
lenses or bent reflecting surfaces like mirrors. The solar radiation hits the subsystem and is
guided to a photovoltaic cell. The concentration ratio is the relation between the area of the
focusing optic and the area of the PV cell. In this technology smaller amount of cells is
needed on the same area as for non‐concentrating solar panels and the energy supply is
bigger. However the money saved on the decreased PV material amount has to be spent on
the focusing optics and tracking systems that are needed to achieve the optimal angles. Also
there is minimal energy gain if only diffused radiation is available (cloudy weather).




    Figure 25. Concentrated solar cells     Figure 26. Concentrated solar cell with focusing
         with focusing lenses [34]                           mirrors [35]


Similar as in the concentrating systems, Sun tracking
installations may be added (Figure 27). Here it would be
useful to mention, although obvious, that solar cells are
only small (100 cm2) parts of the PV systems. One cell
usually is able to produce only about one watt power.
Because of this small amount and need to suit the systems
voltage to the standard electricity handling equipment’s
voltage, many cells are connected in series and parallel
circuits forming panels (modules). But also one panel is
usually insufficient to deliver the desired energy amount,
so the needed numbers of panels are connected together
in one or several arrays. Further on these arrays can be
mounted whether on a fixed system or on Sun tracking
systems. There are two types of them – one axis trackers               Figure 27. Solar panels on a
that follow the Sun from east to west and two‐ axis                        tracking system [36]

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Active Use of Solar Energy in Buildings   2010


trackers that additionally follow in vertical axis according to the sunlight angle of altitude in
different seasons. Tracking systems can increase the efficiency up to 40% in year. The
movement of the systems is done by motors that, logically, are driven by the energy gained
from the solar panels. The new trackers also include sensors that lead the panels towards to
the brightest points in the sky in case of cloudy weather. By placing panels on Sun tracking
systems that are fixed to building elements like walls or roofs, additional sound insulation
might be needed, because the moving installation can be noisy and vibrate.


3.1.4. Efficiency affecting issues

For potential buyers to ease to compare the PV systems efficiency, in data sheets producers
present the power output of their panels. Normally than bigger the panel is, then higher the
energy output. But in reality, the gained energy amount is smaller. It is because the panels
are tested in laboratories under a united system. The simulated sunlight flux is 1000 W/m2
that corresponds to the emitted Sun energy amount at clear sky conditions at noon at sea
level. But these kinds of situations rarely occur in real life, so basically the presented power
output is the maximum possible for a panel. Outside the laboratory sunlight’s flux is affected
by factors, described in chapter 2, like clouds, air humidity, pollution etc.

The biggest affect on the power output potential is the sunlight’s flux characteristics –
intensity and angle. Solar intensity meaning is that than brighter the Sun shines than more
energy is received to convert. But the maximum efficiency of PV systems is reached
relatively fast and it differs just a little by the emitted solar radiation highest amounts, that
means that the potential of the panels is the almost the same in clear as in cloudy weather
conditions (Figure 28). The sunlight’s flux angle meaning is that than steeper the flux, than
more radiation can be absorbed by the cells surface (Figure 29).




  Figure 28. Efficiency versus solar radiation        Figure 29. Dependence on Sun incidence
                 [37, page 134]                                 angle [37, page 135]


Also a related issue is the shadow effect. By objects stopping the flux, not only the energy
supply increases but also the whole system can be affected. Normally the cells in panels are
connected with strings where the voltage (force) drives the current (flow of electricity). If
one of the cells is shadowed it loses voltage, but it still has to carry the current of the cells in
the string so it acts as a load. The other cells have to produce more voltage and that
decreases the string current. The total energy loss in the string, because of the one cell, is
bigger than the loss of energy that could be produced by the shadowed cell. If more cells are
shadowed the whole string can stop to work.




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Active Use of Solar Energy in Buildings   2010


An issue not directly connected to the Sun’s flux is the temperature effect. In most solar cells
(except amorphous silicon) the rise of temperature for every degree decreases the power
output 0.4‐0.5%. Temperatures in the panel rise because of the movement of particles in the
material, triggered by the incoming radiation. This problem has caused a development of a
new product – photovoltaic thermal hybrid solar collector. It’s a combination of PV cells and
solar thermal collector, where the liquid of the collector absorbs the heat in that manner
cooling the PV cells.

Not less important, by planning and installing solar
systems things have to be taken into account like
precipitation in form of snow and hail or black frost.
Normally, solar panels are covered with the protection
layer that can stand hail in the size of golf balls. The
problem of snow is that it can pile on the panels and
must be solved according to the system construction;
however, in small amounts it melts away fast the same
as ice [37]. Also in areas with high pollution dust may
pile on panels, so regular control and surface cleaning
is advisable. Solar systems should also be provided with
lightning arrestors since the electricity flow in the         Figure 30. Sweeping snow from
system can attract the lightning.                                     solar panels [38]



3.1.5. System components

The solar panels alone are not able to
provide a household with electricity –
additional components are needed for
conversion, transmission and storage.
Solar power technologies can be
designed in two ways – grid‐connected or
off‐grid systems.

If a household is not connected to the
regional grid system photovoltaic solar
panels need also supplemental devices
like:
      Inverter – converts the direct
        current (DC) from solar panels to
        alternating current (AC), which is
        normally used in all household
        devices;
      Battery – it operates in DC
        current so it is placed before the
                                                  Figure 31. Off‐grid system example [39]
        inverter. In batteries excess
        power is stored that might remain if the whole produced power is not consumed by
        the household. Also a battery charger is needed to run the charging process. If no
        energy is produced by the solar panels, the stored power in the battery is used
        (through the inverter) to run the household;
      Dump heaters – if a battery is fully charged and still excess power remains it is led to
        dump heaters that discharge the power in form of heat.


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