Background on Report 3
Photovoltaic (PV) Solar Technology 5
Concentrated Solar Power (CSP) 8
Solar Heating and Cooling (SCH) Technology 10
Analysis and Recommendation 11
Figures and Tables
Figure 1: Diagram of a P-N junction in a Photovoltaic Cell 6
Figure 2: A Typical Parabolic Through CSP System 9
Figure 3: Wakiki Shore Apartments in Honolulu, Hawaii 11
Used in Contemporary Society
The use of fossil fuels results in combustion gases which contribute to global warming. Furthermore, these combustion gases result in air pollution which is detrimental to the health of humans. People nowadays are leaning more towards renewable energy. There is wind power, geothermal, hydroelectric, and solar energy technologies. Primarily, the focus of this study is on solar power technologies. Solar energy is harnessing the sun’s radiation for electricity and heat generation. Currently there are three major technologies currently used to harness it: (1) photovoltaic technology, (2) concentrated solar power, and (3) solar heating and cooling. This paper attempts to assess the different solar power technologies currently utilized and to recommend which solar technology is best in terms of (a) design, (b) employability, (c) cost and (d) maintainability.
Background on Report
Solar energy provided by the sun to the earth is estimated to be at 120 x 1015 Joules per second. This is an incredible amount of energy. Solar energy is abundant and at the same time it is clean energy. It does not produce combustion gases that can harm the environment. Use of solar power has been documented as early as 5th century B.C. in Greece (Chu, 2011). Greeks utilized the sun’s heat to warm their houses during winter. At France in 1878, August Mouchet invented a solar-powered steam engine. His design consists of parabolic dish concentrators to collect the sun’s heat energy. In the United States, solar heaters came to the market with the innovations by Clarence Kemp in1891 and William Bailey in 1908 (Jones and Bouamane, 2012). The heater design by Kemp was enclosed in glass to enclose the heat to the water tank. Bailey improved this by converting the water tank to small pipes. The higher surface area allows more efficient heating. A few years later after Bailey, Frank Shuman built as sizable solar steam engine in Egypt. His pilot plant produced approximately 600 lbs/ hr steam. This is equivalent to about 25 Hp power. His solar collector covered 10,000 square feet of land area.
Previous attempts on solar energy were documented. This inspired the “solar homes” movement in the United States. Homes powered by solar energy became common place during the 1940’s (Denzer, 2008). The architect George Keck and the Massachussetts Institute of Technology (MIT) constructed well-designed solar homes. Keck created the “House of Tomorrow” and “Sloan House” which consist primarily of glass. The designs of these houses allow warm temperatures even during winter. MIT, on the other hand, integrated pumps and storage tanks to the design equations. However, the solar house movement failed due to the emergence of interest in electric heating and cooling.
A breakthrough invention, the photovoltaic (PV) cell was developed in the Bell Laboratories during the postwar era. The doping process in which impurities such as germanium atoms are added to the crystalline structure of silicon. A P-N junction results which forces electricity to flow in one direction. In 1946, Russel Ohl invented the first solar cell. Since then, modifications and development has paved the way for current solar power technologies to emerge.
Currently, solar energy is third in terms of renewable energy capacity. The first is hydroelectric power followed at second place by wind power. There are three major classifications of technologies that take advanatge of solar energy. First is the photovoltaic (PV) cells. The advantage of this technology is the direct conversion of solar energy to electricity. It is also the most widely used of the three. Second is the concentrated solar power (CSP). This technology converts the heat energy to mechanical energy. The mechanical energy drives a turbine to produce electricity. Last is the heating and cooling systems. Specific applications include heating water and conditioning the air in residences and buildings. Usually, these systems are installed in independent houses. However, there are examples of large scale solar power utility plants in the United States and in other parts of the world. Man is beginning to understand the advantages of using renewable energy.
Photovoltaic (PV) Solar Technology. Photovoltaic cells, more popularly known as solar cells, take advantage of special properties of semiconductor materials. There is direct conversion of solar energy to electrical energy. This physical phenomenon was first discovered by a French physicist, Edmund Becquerel in 1839 (Hersch and Zweibel, 1982). Becquerel observed a specific amount of voltage when one of the electrodes he was working on was illuminated. In 1880, selenium was used but the efficiency is only in the range of one to two percent. In the 1930’s, the process was improved with the introduction of Chrozalski method. With this method, silicon of high purity can be manufactured at lower cost. At first, the PV cells produced only had within 4 percent efficiency. This was improved to 6-11 percent by research done in the Bell Laboratories (Chu, 2011).
In terms of design, PV technology takes advantage of semiconductor properties and P-N junction (Hersch and Zweibel, 1982). The semiconductor, silicon is widely used in modern set-ups. Silicon atoms have four valence electrons each. These atoms bond together to form a tetrahedral crystal structure. When light passes through a material, it is either reflected, absorbed or go straight through. The absorbed light energy excites the valence electrons of silicon. At low energy, the electrons vibrate and thereby cause heat. As light energy is increased, the electrons break free from their bonds creating an empty hole behind. This creates electron flow. As some electrons vacate their holes, other excited electrons transfer to these new holes in the crystalline structure.
Figure 1: Diagram of a P-N junction in a Photovoltaic Cell (National Technical Information Service, 1982)
Still, another mechanism which is the potential barrier allows for current to flow. This barrier is formed by doping which is adding impurities to the silicon structure. Two types of dopants are used: (a) the negative-carrier dopant and (b) the positive-carrier dopant. In theory, negative carrier dopants are electron donors while positive carrier dopants are electron acceptors. Negative-carrier dopants usually are atoms containing one extra valence electron as compared to silicon. One example is phosphorus which contains five valence electrons. The resulting doped crystal is said to be n-type. On the other hand, positive-carrier dopants are atoms which contain one less valence electron than silicon. For example, boron contains three valence electrons. This results in a p-type doped crystal. When the two types of doped crystals are aligned, a junction or potential barrier is formed. In the presence of light energy, electrons from the n-type crystal travel to the holes in the p-type crystal. This electron jump results in sufficient amount of voltage (See Figure 1 for schematic diagram). The PV cell formed is then integrated in a circuit to allow flow of electrons and thereby utilize the electrical energy.
In terms of employability, the materials for PV cells are cost-effective to install. The mineral quartzite contains about 99% silicon. This is the material used as raw material for most manufacturing process including the Chrozalski method. Aside from the doped crystals mentioned earlier, antireflective coating is added to allow more energy absorption by the solar cells. Electrical contacts are also installed to allow use in electric circuits. Another design constraint is the energy storage system utilized. PV systems are installed with inverter with the distribution grid, batteries, above-ground hydro-pumping, and even fuel cells. Furthermore, new developments particularly in the manufacturing side of the PV cells led to the availability of this technology to independent power producers.
With regards to cost, PV cells require high capital compared to solar heating of water and of enclosed space. In 2000, the world has only 800 MW of photovoltaic capacity. In 2008, the PV solar energy capacity of the whole world dramatically increased to around 14,500 MW (Chu, 2011). Among the countries, Germany leads the capacity installations at 36% (of the total 14,500-MW capacity). Spain and Japan lag behind at 23 and 15% respectively. This proves that the photovoltaic industry is growing. This is due to the economic incentives given by government to promote renewable energy sources. This is part of society’s determined effort to sustainable development in the energy sector. On the average, the cost of PV solar energy range from $3.00 to $4.69 per Watt. This cost pertains to the installation cost of systems. The cost is multiplied by the number of watts capacity for a specific power plant.
In terms of maintainability, PV systems require minimal maintenance since there are no moving parts involved (Solar Energy Industries Association, 2012). It can last for 20+ years according to industry experience. However, regular cleaning needs to be done to allow efficient collection of solar radiation. Modern trends suggest that maintenance cost is the major cost after installation of the PV systems. Additional components for energy storage would require funds. However, this can be addresses through producing power for the public grid. In Germany for instance, independent power producers (residential and industrial) generate electricity from solar energy. They distribute it into the public power grid to compensate maintenance cost and derive sufficient profit (Chu, 2011).
One site dedicated to photovoltaic technology is the DeSoto Next Generation Solar Energy Center in Arcadia, Florida. It is a 25 MW plant developed by Florida Power and Light Company for the electric needs of its consumers. Several other PV plants are operational in different US states and in other countries particularly Germany, Spain and Japan.
Concentrated Solar Power (CSP). CSP technology is based on optics. The technology uses mirrors to concentrate light energy to a particular point to heat water into steam. The steam, in turn, drives the turbine for power generation. Light energy is converted to heat energy. This heat energy is converted to mechanical energy which is then converted to electrical energy by a generator. To sum up, concentrated solar power technology converts solar energy to electricity. Examples of these technologies are: the parabolic dish, the parabolic through and the solar tower systems (Bosshard, 2006).
Figure 2: A Typical Parabolic Through CSP System (Chu, 2011)
In terms of design, CSP systems generally consist of: (a) a concentrating collector, (b) a heat engines, and (c) a power generator. Through a series of mirrors of different forms and configurations, light is concentrated at a particular point. For instance, in parabolic through systems, the parabolic mirrors are the concentrating collectors (See Figure 2). Mirrors focus sunlight on tubes that contain a working fluid. The tubes are already part of the heat engine. The working fluid is usually oil with special properties designed for the application. The oil transfers the heat to turn water into steam (after passing through a heat exchanger). The steam propels the turbine. Generally, the heat engine part is comparable to that which uses fossil fuels. The difference is that hot water is utilized instead of hot combustion gases. The last part of the CSP system is the power generator. The turbine is connected to an alternator which then produces electricity.
In terms of cost, the installation cost of CSP systems is comparatively low than PV technology (Chu, 2011). However, this is offset by maintenance. In comparison to PV systems, maintenance of CSP systems cost more since there are moving parts and thermal fluids involved in the fluid. Thus, the system requires preventive maintenance. And this implies additional cost.
One example of CSP is the Ivanpah Facility in Mojave Desert, California. It is a 392-MW plant which was completed in 2010. It uses solar tower technology which consists of 173,500 heliostats and three solar towers.
Solar Heating and Cooling (SCH) Technology. The Solar Energy Industries Association (SEIA) cited that almost 44% of the total energy consumption by residential, commercial, and industrial sectors is attributed to heating and cooling purposes. Solar heating and cooling technology provides a sustainable and long-term solution for society’s heating and cooling needs (Bosshard, 2006). Popular applications include domestic water heating, space heating, swimming pool heating, air conditioning, process heating, steam generation, and air heating.
In terms of design, SHC is a direct conversion of light energy to thermal energy. No electrical input is required. This is usually the case for heating applications. However, SHC technology is not only focused on heating. One classic example of SHC technology is the heat pump. A heat pump is generally a device that allows thermal energy transfer in two directions. This means that the system can act both as heater and as cooler, as the situation needs. Considered as a refrigeration cycle, heat pump generally requires energy. Its advantage is its being cost-effectiveness. The heat pump energy consumption is lower compared to a combined heater and air conditioning system all year round.
In Honolulu, Hawaii, the owners of Waikiki Shore Apartments installed a solar water heating system at the roofs of their building (See Figure 3). It has 60 solar collectors which contribute to $35,000 of annual savings.
Figure 3: Wakiki Shore Apartments in Honolulu, Hawaii (SEIA, 2013)
Analysis and Recommendation
Among the three major technologies (PV, CSP and SHC) in solar energy, it is the PV technology that revolutionized solar energy. In terms of design, employability and overall cost, businessmen are inclined more with PV technology since it is already tested and requires low maintenance efforts. The concepts behind how PV technology works were discussed and explained in the paper. Moreover, PV technologies are being studied extensively. At present, ongoing researches are geared towards organic photovoltaic cells (bio-inspired photosynthetic systems), photoelectrical cells, and dye-sensitized solar cells. The future is indeed bright for solar power as renewable source of energy.
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