Analysis of Photovoltaic Collectors and Built Works...through concentrating lenses and tracking...

csdCenter for Sustainable Development

Alison Ledwith

Werner Lang

Instructor

Analysis of Photovoltaic Collectors

and Built Works

The University of Texas at Austin - School of Architecture - UTSoA

Analysis of Photovoltaic Collectors and Built Works

AuthorAlison Ledwith

Photovoltaic collectors, among other technological solutions, are used to create a sustainable built environment. Sustainability, as a lifestyle choice, is the conscious decision to preserve resources for future generations while maintaining a high quality living environment. One of the most important elements of sustainability is energy conservation; one of the current goals is a focus on renewable energy. The European Solar Charter has stated the goal of building technology with regard to the incorporation of sustainable systems:

“Buildings and open spaces should be designed in such a way that a minimum of energy is needed to light and service them in terms of harnessing heat for hot water, heating, cooling, ventilation and the generation of electricity from light. To cover all remaining needs, solutions should be chosen that meet the criteria of an overall energy balance and that comply with the latest technical knowledge on the use of

environmentally compatible forms of energy1.”

In order to realize this goal, both active and passive sustainable systems must be incorporated into building design. Passive systems for energy conservation are systems that neither utilize nor produce electricity, but are instead integrated into the existing architectural framework. This can include insulation, thermal mass, window types, daylighting, and natural ventilation. In contrast, active systems are a means of reducing energy consumption of a building through technology, especially decentralized renewable energy generation. Active systems include solar hot water systems, photovoltaic panels, wind turbines, radiant heat, and geothermal technology. The combination of these two systems can allow low energy buildings to be constructed. One of the current low energy standards is the Passivhaus; in contrast with the current European residence consuming 1,400W per

Fig. 01 Photovoltaic Collector

main picture of presentation


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capita, the Passivehaus consumes 35-550W per capita2. This reduction of energy consumption reduces greenhouse gas emissions, and the transition from non-renewable to renewable energy sources also aids in the environmental impact.

Solar energy can be used to provide heat and electricity through various active systems, including solar thermal panels, vacuum tube collectors, and photovoltaic cells. Solar thermal and vacuum tube collectors are used to heat media, usually water or air, that can be used to heat a building, power a climate control system, or fulfill domestic hot water requirements. On the other hand, photovoltaic cells are used to alleviate the dependence of a building on grid electricity. A photovoltaic effect is produced when incident sunlight causes electrons to move from one panel surface to another, thereby closing a circuit and producing electricity (see Fig. 02)3.

When used correctly, photovoltaics can reduce total energy consumption, on-grid consumption, and fuel emissions while maintaining or increasing occupant comfort. Skepticism is more prevalent than in other sustainable systems due to an increase in initial building cost. Available data have been used as a basis for the analysis of several related systems used to harness solar energy, for which built works have been studied for their sustainability, economic reality, and architectural success. In the end, solar energy generation has proven to be viable for many applications, and as the technology becomes more prevalent, market economics will allow its integration into architecture to become more widespread.

The many types of photovoltaic cells use different materials and processes to achieve the same result. The efficiency of the collector is intrinsically linked with the material and manufacturing process. A solar collector’s efficiency is the fraction of sunlight incident on the surface that can be converted to electricity for the end user. Theoretically, the efficiency of a photovoltaic collector should be 85% based on the temperatures of the sun, absorber, and surrounding air. The market standard material is silicon, whose efficiency is limited to 29% based on its smaller spectrum of usable sunlight4. Fig. 03 shows that not all wavelengths are absorbed equally by photovoltaic cells; certain portions of the spectrum are readily absorbed, and larger wavelengths cannot be absorbed using current processes.

Silicon cells can be monocrystalline, polycrystalline, or amorphous (see Fig. 04). The monocrystalline have an energy intensive production process in order to grow one large crystal of silicon per wafer, but they reach efficiencies of 20%. Polycrystalline is a composite material of silicon crystals that reaches efficiencies of 16%. Amorphous, or thin film cells, are produced by the deposition of a vapor onto glass or stainless steel. These cells reach an efficiency of 10%5. Finally, tandem cells, also known as multijunction, are used to increase the efficiency of a silicon photovoltaic cell. They are manufactured using multiple layers of material, some of which include thin-film cells over monocrystalline panels. The combination of the two types of cells results in an effective hybrid photovoltaic system.

Fig. 02 Process of Electricty Generation through a Photovoltaic Cell

Fig. 03 Percentage of Solar Radiation Usable by Photovoltaics

Fig. 04 Individual Silicon-Based Photovoltaic Cells


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In addition to silicon-based panels, alternate semiconductor cells are used to improve the efficiency of solar collectors. Cadmium telluride and copper-indium-gallium-selenide panels are commonly used in building construction and have efficiencies of 12-16% (see Fig. 05). These developing systems have efficiencies equivalent to many of the silicon systems, but they also use incredibly rare materials. It has been estimated that the Earth’s indium will be exhausted within a decade, while selenium and tellurium are also in short supply6. This is one instance where the sustainability of a system should reflect global availability of resources. In the same way that many feel the need

to reduce the dependence on fossil fuels due to their short supply, one should conserve the supply of these elements for future generations.

Organic photovoltaic cells are systems where unconventional compounds are used to generate electricity by mimicking photosynthesis. The most promising cells are known as bulk-heterojunction cells, and they are built from polymers and carbon nanostructures that simulate the processes in traditional photovoltaic cells7. Similarly, dye-sensitized solar cells use anodes and electrolytes to generate electricity through a photo-electrochemical process (see Fig. 06)8. The efficiencies of

these collectors range from 6-10%9. Overall, photovoltaic cells have been cited with efficiencies as high as 41.1%; this cell is a III-V multi-junction solar cell on a germanium substrate10. A III-V semiconductor is a system used primarily in space applications due to its beneficial characteristics in extreme temperatures and resistance to ultraviolet radiation11. Finally, additional efficiency can be achieved through concentrating lenses and tracking solar panels; while the increase in energy production depends on climate and number of tracking axes, moving panels usually have a 25% increase in energy production12.

The highest efficiency panels are not the widely available; still, even for the published efficiencies of marketable systems, there are several reductions that must be taken into account. Most importantly, industry studies have suggested that photovoltaic panels lose efficiency when mass-produced. The information gathered from Sun Technics suggests that the average mass-produced module has an efficiency that is sixty percent of the laboratory result, when taking eleven commonly used materials into account (see Fig. 07)13. Many manufacturers publish the laboratory efficiencies rather than the mass production efficiencies, even though the installed efficiency is required to perform a life cycle energy analysis of the building. Secondly, the climactic conditions can decrease the perceived efficiency. Since the panels are tested under ideal laboratory conditions, the efficiencies published do not include allowances for changes in weather conditions.

Fig. 05 Copper-Indium-Gallium-Selenide Thin-Film Photovoltaic Cell

Fig. 06 Dye-Sensitized Organic Photovoltaic Cell

Solar Cell Material

Cell conversion efficiency ɳz

Laboratory Result

Cell conversion efficiency production

ɳzModule performance Mass production ɳm

Monocrystalline silicon 24.70% 18.00% 14.00%Polycrystalline silicon 19.80% 15.00% 13.00%Hand-drawn silicon 19.70% 14.00% 13.00%Crystalline thin-film silicon 19.20% 9.50% 7.90%Amorphous silicon 13.00% 10.50% 7.50%Hybrid HIT (Heterojunction with Intrinsic Thin Layer) solar cell 12.00% 10.70% 9.10%Copper indium diselenide (CIS) 20.10% 17.30% 15.20%Copper indium gallium selenide (CIGS) 18.80% 14.00% 10.00%Cadmium telluride CdTe 16.40% 10.00% 9.00%III-V Semiconductors 35.80% 24.70% 27.00%Dye-sensitized solar cells 12.00% 7.00% 5.00%

Fig. 07 Table of Solar Cell Materials and Recorded Efficiencies under Several Critera


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When choosing a photovoltaic system for a building, the most important consideration is the total energy consumption. The intent of sustainable systems is to produce clean energy and to realize lower energy usage. An analysis of the energy embedded versus the energy produced is therefore required to understand the systems. In order to determine this information, one must understand the functionality of a solar system. The nominal value of electricity generation for a solar panel is the kilowatt peak (kWp), which is the power produced at standard test conditions, known as peak irradiance. These conditions are 25 degrees Celsius external temperature and 1000 W/m2 solar irradiance. The performance will decrease in extreme temperatures and with lower irradiance. Regardless of the manufacturing and disposal processes, the lifespan of most solar technologies is equivalent. A solar panel, for instance, is only guaranteed to be effective for twenty to twenty-five years before it must be replaced14. The solar panels are known not to function at peak efficiency throughout their entire usable life; indeed, most solar panels drop to eighty percent of their original efficiency by the end of the twenty-five years15.

The lifespan and efficiency of the panel can be used to determine the life cycle energy consumption. One of the most reliable means of describing the amount of embedded energy is the energy payback ratio. This is a quantity that combines the initial energy involved to manufacturer the device, the energy required to run the device, the energy required to dispose of the device, and

the expected lifespan of the system. The result is a single value: the ratio of energy produced to total energy consumed over the expected lifespan of a system16. The understanding of time has been removed from this quantity in order to better describe the energy balance of the system.

In a comparison of solar photovoltaic cells to other means of generating electricity, the solar cells have an energy payback ratio of 6-9, which is lower than hydropower and wind by higher than conventional fuel-based systems17. Hydropower and wind systems are more favorable due to their longer life expectancy, whereas conventional systems are less efficient because they must rely on fuel to produce the energy. Since oil and coal have embedded energy, there is an additional energy drain on the system. A ratio as low as 0.7 was determined for a standard oil-fueled power plant, which means that the system never breaks recovers the energy infused into the system (see Fig. 8).

For a study of various photovoltaic

materials, the embedded energy varies greatly depending on many factors. First, the manufacturing process depends on the type of cell and the electricity used to power the process. One common theory for reducing the initial energy placed into the system is solar panel breeding. That is, solar panels are manufactured and that energy is used to produce more solar panels. One study determined that the ratio of energy embedded to energy produced for solar panels increased by a factor of 2.8 when the manufacturing process is converted from fossil fuel energy to sustainable energy18. The use of energy-consuming materials, transportation, and other energy sources prevent the ratio from increasing infinitely, but solar panel breeding is a promising way of increasing the lifecycle efficiency of the systems.

Second, the type of cell determines the total energy consumption of the system. For instance, a comparative study of multicrystalline silicon modules, thin-film indium-gallium-phosphide modules, and a tandem

Energy TechnologyEnergy Payback

Ratio

Hydropower with reservoir 48-260Hydropower with run of river 30-267Photovoltaic 6-9Onshore wind power 34Offshore wind power 18Offshore wind power 18Biomass- direct wood fired 27Biomass - integrated biomass gasification combined cycle 15Oil-fired plants 0.7-2.9Coal-fired plants 2.5-5.1Coal gasification combined cycle 3.5-7.0Conventional boiler with carbon capture and geo-sequestration 1.6-3.3Natural gas-fired combined cycle 2.5g y

Fig. 08 Tabulated Energy Payback Ratios for Various Systems


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system of the two showed that the time to recover the embedded energy was 3.5 years, 6.3 years, and 5.3 years respectively19. The tandem cell is more efficient than the silicon cell, but it requires more energy to produce and a greater payback time. This case study is indicative of the market situation; materials that have been used longer, such as multicrystalline silicon, have been optimized for better energy performance. Materials such as indium-gallium-phosphide are still being refined, and as such

the manufacture process requires extra energy (see Fig. 09). Finally, the location plays a role in the time necessary to recover the embedded energy; locations that receive more incident sunlight will recover the embedded energy in less time.

Embedded energy is always discussed in parallel with greenhouse gas emissions due to an increased awareness in the potential for global warming. One analysis of a project in Singapore suggested that the energy payback

time was only a few months greater for an oil-fired plant than for a photovoltaic plant. However, the lifecycle greenhouse gas emissions are four times higher for the oil-fired plant20. This only verifies that the amount of time it takes to recover embedded energy does not always equate to an environmentally friendly system. Solar panels in Germany, for instance, can be used to reduce 10.1 tons CO2 per kWp installed even though the higher latitudes reduce the energy payback ratio of the system21.

One of the biggest concerns with photovoltaic technology relates to its emissions; while the installed system produces clean energy, the carbon dioxide equivalent emissions during the manufacture of the systems are high. It has been theorized that the emissions could be reduced from 217g-CO2/kWh to 68g-CO2/kWh. This would be accomplished through the reduction of manufacturing energy consumption by fifty percent, the minimization of aluminum support structures, and the improved efficiency of the solar cell22. Even at its current levels, the greenhouse gas emissions from the systems are favorable in comparison to other systems; the emissions from oil-fired steam turbines and natural gas-fired combined cycle electricity production are 937g-CO2/kWh and 493g-CO2/kWh, respectively (see Fig. 10)23.

In short, the proper selection of a solar panel based on the embedded energy and reduction of greenhouse gases is not straightforward, and this leads to confusion in the proper selection of a system. Photovoltaic technology is generally an improvement in energy usage over most conventional systems. Despite

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Multicrystalline Silicon

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Fig. 09 Abstraction of Embedded Energy versus Time for Three Photovoltaic Cells. System Payback Time Depends on Initial Energy and Efficiency of Collector.

Energy TechnologyGreenhouse Gases (Tons CO 2 /GWh)

Hydropower with reservoir 4-18Hydropower with run of river 9-18Photovoltaic 44-217Onshore wind power 9.7Offshore wind power 16 5Offshore wind power 16.5Biomass- direct wood fired 400Biomass - integrated biomass gasification combined cycle 50Oil-fired plants 937Coal-fired plants 1001-1154Coal gasification combined cycle -Conventional boiler with carbon capture and geo-sequestration 340Natural gas-fired combined cycle 440g y

Fig. 10 Tabulated Energy Payback Ratios for Various Systems


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the justification of the embedded energy and environmental benefits, the reality of the construction industry dictates that the financial returns must also be equally favorable if photovoltaic systems are used extensively. It is true that such systems result in an increase in initial building cost, but the cost is recovered rapidly. It is necessary to conduct a life cycle assessment of the system cost in order to determine that the system is a worthwhile investment. Such an analysis involves system cost, operating costs and revenues, government subsidies, and environmental factors.

When discussing photovoltaic cells for electricity generation, the discussion of revenue generated versus cost becomes more complicated. There is an initial cost and revenue associated with the systems, but the issue of government involvement becomes more prominent. The estimated initial installed cost of a photovoltaic system is 4.7€/Wp for a centralized power plant and 5.3€/Wp for a decentralized installation on a rooftop24. For Austin, Texas, subsidies are available for select projects. A rebate of $2.50 per installed watt peak is provided to residential owners, with a maximum annual payout of $15,000 and a maximum life payout of $50,000. Loans and rebates are provided to commercial customers on a case by case basis25. In addition to these credits, the consumer benefits from savings through a lower energy bill, but there is no added incentive for saving grid energy. In Austin, small energy quantities are billed at 3.55 cents/kWh26. This equates to little revenue from energy savings compared to the investment cost in

photovoltaic panels. These policies provide some assistance in the purchase of panels, but they are not widespread across the United States.

In contrast, the government policies regarding solar energy systems in Germany provide much more financial assistance. The Renewable Energy Act guarantees property owners that feed-in tariffs will be provided to owners of photovoltaic arrays. That is, a system linked to the state energy grid can produce electricity, send the electricity to the grid, and generate revenue per kWh produced. As of July 1, 2010, the tariffs lowered but maintained the same structure; payouts for own consumption from building arrays, ground-mounted arrays on non-agricultural land, and roof-mounted arrays all receive revenue. The tariffs range from €24.16 to €32.88 for energy sold back to the state and €13.19 to €20.88 for energy produced and consumed on-site27. All tariffs decrease when the system produces more kW. For energy sold

to the state, whether the system is mounted on the roof or ground is a determining factor to account for the increased cost of roof mounting. In addition, the energy consumed on-site receives an 8 cent per kWh increase if more than thirty percent of the power produced is consumed on-site (see Fig. 11). By providing monetary assistance over the entire lifespan of a system, this means of subsidy has encouraged owners throughout Germany to install photovoltaic panels.

Using the above information, a comparative life cycle cost analysis can be performed for a sample system to determine how quickly the system could recover cost in both regions and compare to energy costs with no subsidies. This analysis will be run assuming a German climate and the system installed on a family house in Bad Sobernheim. The system had an installed power of 4.59kWp and an energy yield of 4,360kWh/year28. Using the above conversions, the

Photovoltaic Location Feed-In Tariff


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installation cost of the system is €24,327 and the energy cost saved in Austin is €123.39 using energy rates and currency conversions on July 6, 2010. Additionally, a rebate of approximately €9451.45 would be provided as a tax credit. In contrast, the system would produce €1,433.57 of feed-in tariff revenue per year. Therefore, the system in the United States receives its subsidies more quickly, but the system in Germany earns more revenue over the life cycle of the panel. This case study is indicative of the difference between the systems in the two countries (see Fig. 12).

Through an understanding of the types of photovoltaic cells, total embedded energy, greenhouse gas emissions, and lifecycle costs, one can better understand their implementation into architecture. Since all of these values are favorable for photovoltaic cells, it seems clear that architects would choose to implement them into new and existing structures. However, the integration into building design and cultural lifestyle is more complex than simply understanding the efficiency of the system. In many cases, photovoltaic cells are only placed on rooftops or in less prominent locations; in these instances the discussion relates exclusively to the revenue-generating ability of the system. For many buildings, however, the goal is much greater than to build a power plant within a structure. The design of a building must be taken into account; a good design should allow the system to be optimized while complementing and improving the aesthetic of the building. Through a study of utilitarian implementations of the systems and high architecture

case studies, one can clearly see that the systems can be effectively implemented to meet society’s requirements.

One of the most prominent uses of photovoltaic technology in Germany is the large-scale generation of power on the roofs of commercial buildings. This is especially common in Freiburg, a city known for its widespread use of photovoltaic cells. The rooftop of a building is empty space and many owners desire to earn more revenue from a structure. The parking garage is a common typology for these systems due to its utilitarian nature. One example of such a structure is the ISES Solar Carport (see Fig. 14). This building uses solar modules on the roof of a parking garage in order to earn more revenue for the owner. The panels used are polycrystalline with an overall efficiency of 16%, which means that these panels are less energy intensive than their monocrystalline counterparts and average in efficiency compared to other polycrystalline panels. 108 of

Fig. 12 Comparative Cash Flow Diagrams for Austin, Texas and Germany

Austin, Texas

Germany

Initial Investment: �24,327

Initial Investment: �24,327

Value of Energy Savings: �123.39/year

Feed-In Tariff Revenue: �1,433.57/year

Tax Rebate: �9,451.45

Fig. 13 Building Integrated Photovoltaics, Freiburg, Germany


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these photovoltaic panels have been installed on the roof of the building to generate a peak power of 14.04 kWp.

The resulting system produces 12,635kWh/year and saves 8.2 tons of carbon dioxide emissions per year29. This system is located in the


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has already been determined to be beneficial; doubt arises when considering the energy payback time of the LED lights, which require a large amount of energy to manufacture. Since the whole system uses no grid energy, lasts for a long time, and does not consume nearly as much energy in its construction, the system would most likely be able to recover its embedded energy. The exact length of time or energy payback ratio could not be located; the primary selling points of this system are its reduction of carbon dioxide emissions and low cost. For the cost analysis, the lights themselves are more than twice as expensive as conventional lights, but the lack of wiring and extensive construction allows a reduction of initial investment cost of 40%, assuming a system of 20 lights. Over the life cycle of the system, there are no electricity costs and fewer lamp replacements, but there are more overall maintenance costs. In the end, there is a 33% reduction in operating costs and a 38% reduction in life cycle cost to use Ecolights (see Fig. 16).

While these utilitarian applications of photovoltaic cells are a prominent point in the global market, the more important case studies are buildings of architectural intrigue. The concept of buildings as a means of power generation has inspired many to work on a means of seamless integration with design. In addition, there is an ultimate goal of creating self-sufficient buildings; that is, all of the power will come from building integrated photovoltaics and other renewable sources. Built projects in Freiburg and other regions where the political climate motivates

their usage have photovoltaics for a percentage of their power, but more fascinating are the buildings designed to use only electricity from the sun. The Solar Decathlon is a biannual competition in Washington, D.C. where university groups design solar-powered projects, and the Team Germany buildings are excellent examples of a move toward

total electricity generation.

Team Germany first entered in 2007 and won the competition with a design that successfully used building integrated photovoltaics to produce its electricity. The design functions because passive and active systems are used together to reduce the electric load. The building was

Fig. 16 Ecolight: Photovoltaic-Powered LED Streetlight System


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designed for the climate conditions in Darmstadt, Germany; Washington, D.C.; and Phoenix, Arizona. Submitted proposals were evaluated for their architectural expression, engineering quality, marketability, public relations, lighting design, availability of hot water, appliance selections, maintenance of indoor comfort requirements, quantity of surplus energy available to power an electric automobile, and net surplus of energy at the conclusion of the competition (see Fig. 17).

The overall premise of the building design is versatility through the layering of space. Since the houses are limited to 800ft2 by the competition rules, providing a livable space for two people is a goal that must be met through multifunctional spaces31. The designers chose to have a core space with a kitchen and bathroom as the only permanent installation. All other rooms are multipurpose; by raising the floor on a platform, furniture was installed in the plenum and can be revealed as necessary (see Fig. 18). This, for instance, eliminates the need for a dedicated bedroom while simultaneously providing space for the building mechanical systems. The building incorporates an exterior porch that provides a porous threshold to the interior space. The inner skin of the building is composed entirely of glass, and the outer skin of wooden louvers. This allows for both daylighting and solar shading, depending on the time of year and user preference. By utilizing minimal spaces and providing a double skin façade, the building is able to cut its required energy consumption through passive means.

Fig. 17 Team Germany 2007 Solar Decathlon House

10.03

7.21

FOOTPRINT CALCULATION:

On Feburary, the 1st Mike Wassmer agreed on only counting the square footage of the louvers in the South, West and East but on the North facade.

fooprint of building:

10.03 m x 7.21 m = 72.31 sqm32.99 ft x 23.6 ft = 778.4 sqft

louvers:single louver:0.63 m x 0.17 m = 0.11 sqm 2.07 ft x 0.56 ft = 1.16 sqft

all louvers:9 x 0.11 sqm = 0.99 sqm9 x 1.16 sqft = 10.44 sqft

footprint alltogether:

72.31 sqm + 0.99 sqm = 73.3 sqm778.4 sqft + 10.44 sqft = 788.84 sqft

07/08/07 EZ

1 1:50Floorplan / opened louvers

G1.1FootprintG1.1

Floorplan with opened louvers

Fig. 18 Team Germany 2007 Solar Decathlon House - Floor Plan


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The purpose of the Solar Decathlon house is to provide for its needs through solar energy, and the building utilizes several systems in order to achieve this goal. The largest power supply comes from the roof, which uses building integrated photovoltaic cells. The cells are applied at a three degree angle to allow for easier transport, even though this is not the optimal angle for a photovoltaic panel. The panels are monocrystalline back-side contacted silicon, a high efficiency configuration that produces 8.2kWp over the forty modules. This system is applied with a traditional aluminum frame over an opaque, vacuum-panel insulated roof with a space to allow for air flow beneath the panels. This is necessary to prevent overheating of this type of solar panel (see Fig. 19)32.

The roof is covered by two other forms of solar collectors. One system is a translucent glazing system with thin-film photovoltaics of cadmium telluride (see Fig. 20). These panels provide a total power of 1.1kWp33. Thin-film collectors and alternative semiconductors tend to have lower efficiencies than monocrystalline silicon collectors. Combined with a lower surface area, this system provides much less electricity than the monocrystalline silicon. Over the bathroom area, two solar thermal collectors were provided to heat the water, as this process is much more energy efficient than generating electricity from photovoltaic panels to power a water heater. This system feeds into a storage tank, which is used for domestic hot water and the heat exchanger. The photovoltaic system is used to supplement via an electric coil when the heat from the system is

Fig. 20 Team Germany 2007 Solar Decathlon House - Outdoor Space

Fig. 19 Team Germany 2007 Solar Decathlon House - Roof Detail


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a different time of day, the entire system can be connected to the house via one inverter, which lowers the system cost34.

The photovoltaic cells power every system in the house. Evaporative cooling from the ceiling replaces traditional air conditioning to minimize the energy impact. Battery storage in direct current is provided to allow the building to function when no solar radiation is available. Computerized controls maintain the ventilation, shading, and battery systems to ensure that the house functions effectively. All of these systems contribute to this house meeting the Passivhaus standard of construction, meaning that the system uses less than 120 kWh/m2 for primary energy embedded in the building35. One major difficulty with Solar Decathlon houses and sustainability is the need for multiple stages of assembly and

transport; the German team chose their method of transportation with an emphasis on minimizing transportation energy36. The cost of the building, in contrast, is not practical for typical construction. The building itself cost $1,378,297, which translates to $1,723 per square foot. The German team projected that the cost of the building would fall to $337,768 by 2015, the target year of the competition, but this is still $422 per square foot and considered very high end construction37.

The same university sponsored another Solar Decathlon team in 2009. This team also won the competition through the construction of a surPLUShome that has a negative 421 kg CO2 equivalent per square meter carbon footprint over a twenty year lifespan38. One common suggestion for maximizing solar power gain is to cover every

not sufficient.

The final type of photovoltaic system is integrated into the louvers. The louvered panels slide and rotate, but they also contain thin-film photovoltaic cells on the surface (see Fig. 21). The louver system provides a total of 1.9kWp, thereby surpassing the translucent panels. However, in order to combine the need for both user and automated control of the shading system with the desire to produce electricity, the designers needed to invent a new means of attaching the system. Every two solar panels are connected in series, for a total of 15 module groups on each panel. Since each strip of photovoltaic produces 1.9Wp power, each shutter will contribute 58Wp power. The cells are incorporated on all louver systems except the north façade in order to maximize the electricity generation (see Fig. 22). Because each face of the building will be in peak sun at

Fig. 21 Team Germany 2007 Solar Decathlon House - Louvers Fig. 22 Team Germany 2007 Solar Decathlon House - Section Through Louvers


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surface in photovoltaic panels, and this is precisely what the Decathlon entry chose to do. The roof utilizes the highly efficient collectors, which are comprised of 40 monocrystalline silicon panels of 18% efficiency, for a total power output of 12kWp and a total energy generation of 9960kWh per year39. This is a similar configuration to the previous solar home, but it takes advantage of two additional years of panel efficiency research. This system is used in combination with vacuum insulation and meets the Passivhaus heating standard with a heating load of 14.8 kWh/m2 per year.

The four facades of the building have also been equipped with multicolored photovoltaic cells in an effort change the perception of the photovoltaic panel. Since the thin-film could be applied to any shape substrate, the team chose to use cylindrical shapes in some regions to break the traditional morphology40. In addition to this goal, the panels were used to create additional energy resources. The building succeeds in generating 7kWp from 250 cadmium-indium-gallium-selenide thin-film modules, which have an efficiency of only 10%. Since the building generates 13,690kWh per year and only requires 4,100kWh per year, the remaining electricity can be sold to the German energy grid for a profit of €5358 per year41. The solar energy systems are used to power a heat pump, to provide heating, cooling, ventilation, and to provide domestic hot water42. Energy is conserved through phase changing walls and ceilings, which are assembled near louvered vents to expel heat from the photovoltaic cladding system43.

Part of the competition for 2009 was to

Fig. 23 Team Germany 2009 Solar Decathlon House - Perspective



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well, but the 2007 house would be more pleasant to inhabit.

The 2009 Solar Decathlon house is an excellent example of the limits of technology to dictate the aesthetic of a building. This thoughtfully designed building uses photovoltaic technology of limited efficiency, which requires a designer to either settle for a lower amount of electricity generated or to find a way to make photovoltaic panels the entire aesthetic of the building. At some point, the architect must recognize all of the technical realities of the use of active sustainable systems and decide how to best employ them for the good of the project. As the technology continues to improve, the efficiencies of the panels will increase while the energy payback time, life cycle cost, and environmental impact will decrease. Research from June 2010 suggests that a photovoltaic cell will have an efficiency as high as 66% through the use of quantum dot technology. This cell uses semiconductor nanocrystals: electrons in quantum dot lead selenide are capable of cooling more slowly than conventional photovoltaic panels, which allows more of them to be transferred to the titanium oxide terminal of the circuit45.

While this particular technique may not solve the balance of efficiency versus embedded energy, it is verification that there is potential for solar collectors efficient enough to have a presence on a building without dominating the aesthetic. For an architect to successfully incorporate photovoltaic technology into a building, a numerical understanding of the impact of the technology is essential. However,

as the case studies have shown, photovoltaic cells can be beneficial, profitable decisions, but care must be taken to use them in an aesthetically pleasing way. It has been shown through the Solar Decathlon 2007 house from Germany that a Passivhaus standard design can easily incorporate solar energy systems and use them to enhance the aesthetic. It has been shown through the 2009 version that it the surPLUShome standard requires many more panels, and this tends to dominate the design. As efficiency increases, the principle of house as energy-generating device will become more feasible; greater architectural depth will be achieved once cost, environment, and energy are effectively balanced.

...

Notes

1. Thomas Herzog, et al, Solar Charter: European Charter for Solar Energy in Architecture and Urban Planning. Peter Green, trans. (Munich: Prestel Verlag, 2007), 44.

2. Klaus Daniels and Ralph E. Hammann. Energy Design for Tomorrow (Stuttgart: Edition Axel Menges, 2009), 141.

3. Clive Beggs. Energy: Management, Supply and Conservation (Boston: Elsevier, 2009), 87.


5. Ibid. 286-289.

6. Ibid. 290-291.

7. Peter Fairley, “Solar-Cell Squabble,” IEEE Spectrum, April 2008, http://spectrum.ieee.org/energy/renewables/solarcell-squabble/0.


9. Ibid. 290.

generate the most electricity surplus from the building, which is why the building suffers architecturally in comparison to the 2007 version. Whereas the 2007 focus was on building integrated photovoltaics, the focus here is on maximizing the surface area for photovoltaics. Despite this goal, the roof panels are placed flat, which results in a 16% loss of energy generation. Also, the designer chose thin-film photovoltaic materials for the façade when more efficient panels are available. A reasonable cost was not a priority, as the system cost over $850,000 to construct44. From the documentation of the building, the German team designed to maximize interior space and to create photovoltaic panels as a building aesthetic. The building is much more enjoyable from the interior, but the spaces are also completely removed from the solar systems. In the end, the 2009 house from a technical perspective fares



15


25. Marco Raugei and Paolo Frankl, “Life cycle impacts and costs of photovoltaic systems: Current state of the art and future outlooks,” Energy no. 34 (2009), 394.

26. Austin Energy. http://www.austinenergy.com/ (accessed 5 July 2010).

27. Ibid.

28. Angelika Nikionok-Ehrlich, “Hopes dashed,” New Energy, March 2010, 16.

29. TRITEC. Product Catalogue 2010. Basel: TRITEC International, 2010.

30. TRITEC. Ecolights. Basel: TRITEC International, 2010.

31. Manfred Heggar, ed, Sunny Times: Solar Decathlon House Team Deutschland 2007. Barbara Gehrung, trans. (Wuppertal: Verlag Muller + Busmann, 2008), 10.

32. Ibid 30.



35. Daniels, Klaus and Ralph E. Hammann. Energy Design for Tomorrow (Stuttgart: Edition Axel Menges, 2009), 136.


37. Team Deutschland. “Project Cost Summary.” Solar Decathlon 2007. http://www.solardecathlon.de/wp-content/uploads/sd07_tu-darmstadt_charges.pdf (accessed October 17, 2007 via www.archive.org).


39. Ibid. 35.

40. Jeffrey R.S. Brownson and Lisa D. Iulo. “Upsetting the Balance Beam: System Integrative Photovoltaics as Purposeful Manipulation of Energy Demand and Microclimate in the Built Environment.” Solar

2010 Conference Proceedings. http://www.ases.org/papers/243.pdf (accessed July 7, 2010).


42. Ibid. 17.

43. Jeffrey R.S. Brownson and Lisa D. Iulo. “Upsetting the Balance Beam: System Integrative Photovoltaics as Purposeful Manipulation of Energy Demand and Microclimate in the Built Environment.” Solar 2010 Conference Proceedings. http://www.ases.org/papers/243.pdf (accessed July 7, 2010).

44. “Solar Decathlon 2009: Team Germany and the benefits of Imagination.” Greenline: Design + Technology + Sustainability. http://greenlineblog.com/2009/10/solar-decathlon-2009-team-germany-and-the-benefits-of-imagination/ (accessed July 7, 2010)

45. William Tisdale, et al, “Highly Efficient Solar Cells Could Result from Quantum Dot Research,” The University of Texas at Austin, June 2010, http://www.utexas.edu/news/2010/06/17/quantum_dot_research/ (accessed July 7, 2010).

...

Figures

1. “Media Photo Gallery,” Oak Ridge National Laboratory, http://www.ornl.gov/ornlhome/photos.shtml (accessed July 7, 2010).

2. Nissin Electric Co, “Principle of Electricity Generation by Photovoltaic Cells,” Virtual Center for Environmental Technology Exchange, http://www.apec-vc.or.jp/e/modules/tinyd00/index.php?id=74 (accessed July 7, 2010).


4. Ibid. 288.

5. Eric Wesoff, “Why Have Investors Flocked to CIGS Solar?” Green Tech Media, http://www.greentechmedia.com/green-light/post/why-have-investors-flocked-to-cigs-solar-1101/ (accessed July 7, 2010).

10. Peter Fairley, “Solar-Cell Squabble,” IEEE Spectrum, April 2008, http://spectrum.ieee.org/energy/renewables/solarcell-squabble/0.

11. Marion Hopf, ed. Annual Report 2009 (Freiburg, Germany: Fraunhofer Institute for Solar Energy Systems ISE, 2010), 2.


13. Jürgen Heup, “Energy with a twist,” New Energy, March 2010, 86.


15. Q-Cells, Q.PRO: Multicrystalline Solar Modules (Bitterfeld-Wolfen, Germany: Q-Cells SE, 2010).

16. Ibid.

17. Chris Lund and Wahidul Biswas, “A Review of the Application of Lifecycle Analysis to Renewable Energy Systems,” Bulletin of Science, Technology &Society, June 2008, 202.

18. Ibid. 206.

19. Ibid. 203.

20. A. Meijer, M.A.J. Huibregts, J.J. Schermer, and L. Reijnders, “Life-cycle Assessment of Photovoltaic Modules: Comparison of mc-Si, InGaP and InGaP/mc-Si Solar Modules,” Progress in Photovoltaics: Research and Applications no. 11 (2003): 275.

21. R. Kannan, K.C. Leong, R. Osman, H.K. Ho, and C.P. Tso, “Life cycle assessment study of solar PV systems: An example of a 2.7kWp distributed solar PV system in Singapore,” June 2005, http://www.sciencedirect.com (accessed July 5, 2010), 560.


23. Ibid. 203.

24. R. Kannan, K.C. Leong, R. Osman, H.K. Ho, and C.P. Tso, “Life cycle assessment study of solar PV systems: An example of a 2.7kWp distributed solar PV system in Singapore,” June 2005, http://www.sciencedirect.com (accessed July 5, 2010), 559.


16


Decathlon 2009 (Wuppertal: Verlag Muller + Busmann, 2010), 27.

...

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9. Graph by Alison Ledwith


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21. Ibid.

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23. Ibid.

24. Ibid.

25. Manfred Heggar, ed. Sunny Prospects: The surPLUShome of Team Germany for the Solar


17


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