Renewable Energy - Coursework Example

By submitting this work, I certify that this is all my own work. ……………………………………… signature if this is a non-electronic submission………………………………………………… SPLD Students Please tick this box if you are officially recognised by the University as an SPLD student. Abstract Of all the sources of energy in the world, solar energy provides the largest amount of energy. It offers significant potential for both immediate and lasting climate change mitigation. Conversion of solar energy consists of various types of technologies capable of meeting different energy service needs. Some of these technologies can deliver electricity, heat, natural lighting, fuel and cooling. The maturities of these technologies are not the same and their applicability depends on prevailing conditions as well as government policies to support their establishment. Both challenges and opportunities are involved in the incorporation of solar energy into broader energy systems. Introduction Of all the resources of energy, solar energy, which is a renewable source of energy, is the most abundant yet the least exploited. However, even though it’s the least exploited source of energy, there is a rapid growth of markets for solar technology. The major advantage with solar energy technology is its relatively smaller environmental burden. The cost of solar technologies has dropped drastically in the past few decades and supportive government policies and technological advances continue to offer the prospective for additional reductions in cost. Full exploitation of solar energy in future will depend on supportive public policies, cost reductions and the extent of continued innovation. Resource potential The source of solar energy is practically unlimited, and it is accessible and able to be used in many parts of the world. However, to be able to plan and design proper energy conversion systems, solar energy experts must be equipped with appropriate knowledge on how to harvest the energy. Trieb (2005) among others did research on the aspect of solar irradiation, which is an electromagnetic radiation from the sun’s rays. They found that outside the earth’s atmosphere, the solar irradiance on a surface perpendicular to the rays of the sun at a mean distance between the earth and the sun practically remains unchanged throughout the year. According to Bailey et al. (1997), its value is now accepted to be 1.367W/m2. “This figure is estimated to be about 1,000W/m2 on the earth’s surface when the sky is clear”. When the sun’s surface temperature is almost 6000K the electromagnetic irradiance from the sun is spread over wavelengths ranging from 0.3 to 3µm. About 50% of solar irradiance is infrared, 40% is visible light and 10% is ultraviolet radiation. However, it is difficult to evaluate solar irradiance at the earth’s surface due to its interface with the atmosphere which contains aerosols, clouds, trace gases and water vapor that are temporal and vary geographically. Solar irradiance is typically reduced by atmospheric conditions by almost 90% on cloudy days and about 40% on clear, dry days. According to Solomon et al (2007), solar irradiance on the earth’s surface is 198W/m2 on average. The two major components of solar irradiance striking the earth’s surface are: beam solar irradiance, which comes unswervingly from the sun’s disk and diffuse irradiance, which emanates from the whole of the sky other than the sun’s disk. The sum of these two components makes the global solar irradiance. Some of the factors that determine the magnitude of solar energy that can be used are: metrological conditions, demands for energy services and availability of land. The assessment methodologies and the technical potential do vary geographically. As described by Krewitt et al (2009). The solar electricity technical potential for photovoltaic (PV) cell and concentrating solar power (CSP) plant depends on factors such as future development of technology improvements, land use exclusion and availability solar irradiance. Possible impact of climate change on solar energy The climate change imposed by too much greenhouse gases in the atmosphere may affect cloud cover, water vapor content, rainfall and turbidity and this can influence the resource potential of solar energy across the world. Trieb (2005) conducted a study in changes in major climate variables such as cloud cover and solar irradiance. He found that on average, the variation pattern of monthly global solar irradiance does not go beyond 2% over some parts of the world and it also depends on the model. Technology and applications In solar heating system, the solar collector uses a carrier fluid such as water or air to convert solar irradiance into heat. Two factors that determine the choice of solar collector are: the desired range of temperature of the heat-carrier fluid and the service to be rendered by the solar collector. An unglazed collector, also referred to as an uncovered absorber, is likely to be limited to low-temperature heat production (Krewitt et al, 2009). Solar heating systems for production of hot water can be put into two categories: passive and active solar water heaters (Solomon et al, 2007). It is of importance to note that there are also active solar cooling systems for cooling the hot water produced by solar energy. In passive solar water heaters, the cold water is preheated as it goes through the solar collector. The heated water then flows to a standard backup water heater and stored inside the collector itself. In a thermosyphon system which is also a passive system, there is a separate tank directly above the collector that is used to store the heated water. This design is mostly applied in regions where freezing temperatures are unlikely and it has user-friendly and cost-effective advantages compared to a system that has a separate heat-exchanger tank set aside. It also fits well in households with momentous daytime and evening hot water needs. However, it is not appropriate in households with morning hot water needs because most of the collected energy can be lost by the tank overnight. Active solar water heater systems make use of controllers and electric pumps to circulate the carrier fluid via the collectors. There are three classifications of active water heating systems. Direct circulation systems rely on pumps to directly circulate pressurized water though the collectors. These systems are suitable in regions that do not have acidic or hard water and do not experience freezing temperatures. Indirect, also known as antifreeze circulation system, pumps the heat carrier fluid (glycol water mixture) through collectors. The heat is transferred by heat exchangers from fluid to water for use. In the drain back system, water is circulated through the collectors by pumps. Water in the piping system and in the collector will drain back to the reservoir when the pumping stops. This eliminates the risk of freezing in regions with low temperatures (Bailey et al, 1997). Solar cooling can be classified into: solar thermal refrigeration, solar electric refrigeration as well as solar thermal air-conditioning. In the first classification, the refrigeration effect is achieved through thermal gain process. The three common options in this category are; solar adsorption refrigeration, solar absorption refrigeration and solar mechanical compression refrigeration. In the second classification, the PV panels are used by the solar electric compression refrigeration to power a conventional refrigeration machine. In the third classification, the conditioned air is directly supplied through the solar thermal gain by desiccant cooling According to Krewitt et al (2009), the use of PV panels to power solar electrical air-conditioning is of little importance. This is because in the developed world, where there is a very efficient electricity grid, photovoltaics are used to produce a lot of electricity, some of which is fed into the national grid. Photovoltaic electricity generation Photovoltaic solar technologies exploit the photovoltaic effect to produce electricity. When light shines on a semiconductor material such as silicon, it produces electron-hole pairs that are spatially separated by an intrinsic electric field produced by introducing impurities into the p-n junction (Nelson, 2003). This creates positive charges on one side of the unction and negative charges on the other side. A voltage is created by this resulting charge separation. The efficiency of conversion of a solar cell is the ratio of output power from the solar cell with unit area (W/m2) to the incident solar irradiance. The two factors that determine the maximum efficiency are: device design and properties of the absorber material. The efficiency of a solar cell can also be increased by stacking together specially selected absorber materials that can absorb much of the solar frequency band since each material collects solar photons of dissimilar wavelengths. Existing photovoltaic technologies crystalline cells, thin-film copper based cells such as cadmium telluride (Cd-Te) and thin-film silicon. Wafer-based silicon technology consists of solar cells made of multi-crystalline or mono-crystalline wafers that are about 200µm thick. Single-junction wafer based silicon cells have been proofed to have energy conversion efficiencies of 20.5% for multi-crystalline silicon cells and 25.3% for mono-crystalline cells (Trieb, 2005) under standard test conditions Several wafer-based crystalline silicon photovoltaics with higher efficiencies have been developed. Interdigitated back-contact (IBC) solar cells and heterojunction solar cells are examples of these higher efficiency cells. These cells have an advantage of lower temperature coefficient. An IBC solar cell, where both the emitter and base are connected at the back of the cell, has the advantage of no shading of the front of the cell by a top electrode (Taguchi et al 2009). Such a cell has an efficiency of up to 26%. Emerging photovoltaic technologies are still being fabricated in the laboratories and could be hitting the market in the next few years. They are based on cheap supplies and processes and include equipment such as organic solar cells, low-cost editions of inert thin-film technologies and dye-sensitized solar cells. Production of solar fuel The capacity to store solar energy in form of fuel may be advantageous for both the transportation industry and the high-efficiency electricity generation. The ongoing development of high-temperature solar collectors will benefit solar fuel processes in the future. Although hydrogen is considered to offer most attraction for the future by many researchers, transitional and intermediate strategies have also been put in place. The photo electrochemical (PEC) cell is the future technology innovation for electrolysis. The cell shall transform solar irradiance into chemical energy. It is made using an electrode that absorbs light from the sun, a membrane separating oxygen and hydrogen and two catalytic films. These cells can also use a semiconductor material as a solar light-absorbing anode (Bailey et al, 1997). Advantages of solar energy Advantages of solar power include: unlimited and renewable energy source, its free once the solar panel has been installed, it causes no pollution, can be used in remote area where electricity from the power grid can hardly reach and has high returns on investments due to low maintenance costs and free power it generates. Disadvantages of solar energy The disadvantages are: Installation of solar panels can be expensive; some countries are not well exposed to sunlight; the output of solar power stations is low compared to the output of conventional power stations of similar size and can only be harnessed only on sunny days. Conclusion The future deployment of solar energy can only be realized through effective cost reduction. Reduction of these costs can be achieved if both the existing and emerging solar technologies reduce their costs along their learning curves. The real potential and costs of putting into practice solar energy still remain unknown because the main settings that exist are all based on a single solar technology: photovoltaic. More research needs to be conducted so as to come up with more solar technologies to be able to fully exploit solar energy. References Bailey, D. Curtis, H. and Jenkins, P. (1997): NASA Technical Memorandum 113155 – Solar Cell Calibration and Measurement Techniques: IECEC-97534, NASA, Cleveland, OH, USA Krewitt, W., Nienhaus, K. Capone, C. and Graus, W. (2009): Role and Potential of Renewable Energy and Energy Efficiency for Global Energy Supply. Climate Change: Federal Environment Agency, Dessau-Roßlau, Germany Nelson, J. (2003). Over the limit: Strategies for high efficiency: In the Physics of Solar Cells: Imperial College Press, London, England Solomon, S., Qin, D. Alley, B. (2007): The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change: Cambridge University Press Taguchi, M., Tsunomura, Y. and Nakashima, T. (2009): High-efficiency HIT solar cell on thin (< 100 µm) silicon wafer: European Photovoltaic Solar Energy Conference, Hamburg, Germany Trieb, F. (2005): Concentrating Solar Power for the Mediterranean Region: Oxford University Press Read More