ACTIVE USE OF SOLAR ENERGY IN BUILDINGS WHY HOW WHAT DAGNE VILKA GUIDE FOR THE ARCHITECT

Transcription

1 ACTIVE USE OF SOLAR ENERGY IN BUILDINGS WHY HOW WHAT GUIDE FOR THE ARCHITECT DAGNE VILKA HORSENS 2010

2 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. 2

3 List of Contents List of figures... 4 List of tables... 5 Introduction... 6 Definition of problem Solar energy s role in global scale History of solar energy Solar energy versus other energy sources Positive and negative aspects of solar energy Solar energy properties and dependence of environmental issues Sun s energy output Atmosphere, time and location influence o solar energy Active solar energy systems Photovoltaic systems Solar radiation conversion to electricity Types of photovoltaic (PV) solar cells Design solutions for increasing efficiency Efficiency affecting issues System components Design variations in buildings Photo electrochemical systems Solar thermal systems Perforated plate collectors Flat plate collectors Evacuated tube collectors Batch collectors System components Solar thermoelectric systems Solar cooling and other applications Conclusion References

4 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] Figure 4. Energy Consumption by Industry [9] Figure 5. Largest Solar Powered Building [10] Figure 6. Equilibrium of the Sun [11] Figure 7. Nuclear fusion Figure 8. Electromagnetic wave [15] Figure 9. Electromagnetic Spectrum Properties [17] Figure 10. Light reflection [18] Figure 11. Interaction of incoming solar radiation with the Atmosphere [20] Figure 12. Global yearly irradiance [21] Figure 13. Solar Geometry [22] Figure 14. Yearly sum of global irradiation on a horizontal surface Denmark [23] Figure 15. Office energy consumption [24] Figure 16. Residential energy consumption [25] Figure 17. Positively and negatively charged atoms [26] Figure 18. Photovoltaic cell construction [27] Figure 19. Electron flow [28] Figure 20. Mono crystalline silicone ingot and wafers [29] Figure 21. Production process of typical crystalline silicone solar cells [30] Figure 22. Amorphous silicon thin film cell [31] Figure 23. Mono crystalline silicone cell fragment [32] Figure 24. Poly crystalline silicone cell surface texture [33] Figure 25. Concentrated solar cell with focusing lenses [34] Figure 26. Concentrated solar cell with focusing mirrors [35] Figure 27. Solar panels on a tracking system [36] Figure 28. Efficiency versus solar radiation [37, page 134] Figure 29. Dependence on Sun incidence angle [37, page 135] Figure 30. Sweeping snow from solar panels [38] Figure 31. Off grid system example [39] Figure 32. Photovoltaic pavement [40]

5 Figure 33. Integrated Concentrating Solar Façade System [41] Figure 34. Photovoltaic façade cladding [42] Figure 35. Dye sensitized solar cells [44] Figure 36. Perforated plate collector s principal scheme [47] Figure 37. Flat plate collector composition [48] Figure 38. Flat plate collectors installed on a wall [50] Figure 39. Evacuated tube collector mounted on a roof [51] Figure 40. Evacuated glass tube composition [52] Figure 41. Evacuated tube s geometry [53] Figure 42. Integrated tank solar evacuated tube collector [54] Figure 43. Do It Yourself batch solar collector [55] Figure 44. Flat plate collector closed loop system [56] Figure 45. Solar thermoelectric system collector types [57] Figure 46. The solar furnace at Odeillo [58] Figure 47. Evaporative cooling system principle [59] Figure 48. Desiccant cooling system [60] List of Tables Table 1. Comparison of renewable energy sources [6, page 31, 7]

6 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. 6

7 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. 7

8 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 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 worldwide. 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 7 th 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 18 th century. The reason for the first attempts was the search for new discoveries after the Scientific Revolution in the 16 th 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 8

9 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 where developed further, because the use of fossil energy, at that time, was more efficient. Figure 2. Seville s Power Tower [3] The use of solar energy, as we know it now, started to develop rapidly in the 20 th 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. 9

10 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; 10

11 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. 11

12 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, 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 daylighting; 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; 12

13 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. Unfortunately solar energy has also its problematic sides. But since the positive aspects Figure 5. Largest Solar Powered Building [10], Dezhou, Shangdong Province in northwest China 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. 13

14 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 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 turn the energy raised the pressure Figure 6. Equilibrium of the Sun [11] 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=mc 2 ). 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 14

15 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. 15

16 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 10 6 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/m 2, also called the solar constant. But the intensity when reaching the planet s surface is only up to 1000 W/m 2 in clear sky conditions. The reasons for this decrease are discussed in the next paragraphs. 16

17 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 gammaray, X ray and ultraviolet radiation (wavelength less than 200 nm) are interacting with oxygen O 2 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] 17

18 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. 18

19 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] 19

20 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 thermalelectric 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. 20

21 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 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 positively and negatively charged atoms. Positively charged atoms have a lack of Figure 17. Positively and negatively charged atoms [26] 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 21

22 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 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. 22

23 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 mentioned technologies. Figure 22. Amorphous silicon thin film 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 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 environment to reflect. This reflection is one of the main factors Figure 23. Mono crystalline silicone cell fragment [32] 23

24 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 honeycomb texture (Figure 24) (attempts forming the surface in the same pattern as Figure 24. Poly crystalline silicone cell surface texture [33] 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 with focusing lenses [34] Figure 26. Concentrated solar cell with focusing 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 cm 2 ) 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 that follow the Sun from east to west and two axis Figure 27. Solar panels on a tracking system [36] 24

25 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 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/m 2 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 [37, page 134] Figure 29. Dependence on Sun incidence 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. 25

26 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 %. 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 system can attract the lightning. Figure 30. Sweeping snow from solar panels [38] 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. 26

27 Diodes used to close or open the chain to prevent battery overload or isolating the batteries from solar panels at night, that then act like a load; Mode controller controls and commands the processes in the whole system so no energy loss, overcharge or other errors occur. The systems may vary depending on the amount of power need, panel design, current direction etc. For example, also backup generators may be added to support the system, solar tracking motors and so on (Figure 31). In the grid connected systems the same devices are needed, except batteries, their chargers and dump heaters, because the excess power is led to the regional grid system the grid fulfills the function of storage. Before conducting the power, the energy must be converted from DC to AC with the inverter. However, smaller batteries might be needed to start up the system in the morning or for similar small loads [37, page 144]. By connecting the system to the grid in some countries net metering of feed in tariff programs are used. Both of them buy the excess energy when produced and support the household by selling energy back. The difference is that the net metering program is obsolete and the price of both energy types sold and bought is the same. The new feed in tariff program, established in some European countries, allows the small producer to earn by selling the excess for a higher price. The politics under this program are to encourage house owners to use renewable energy sources more Design variations in buildings. Photovoltaic solar systems are light in weight and dimensionally variable and therefore easy to integrate in different parts of buildings. Also the design, installation and start up of the system can be done in short time. Photovoltaic cells integrated in roofs and windows, panels replacing wall cladding, shades made of PV materials, pavements with built in cells are examples that show that the PV systems are able to adjust to different architectural elements and forms. Though, when developing a project and integrating PV technologies in the design, attention must be paid to prevent efficiency loss. For fixed solar systems the most suitable angle must be chosen, because the primary aim of the technology is not the look, but energy gain. Figure 32. Photovoltaic pavement [40] Figure 34. Photovoltaic façade cladding [42] Figure 33. Integrated Concentrating Solar Façade System [41] 27