Back to Energy and the Environment: Sources, Technologies, and Impacts Home
Table of Contents
Solar Energy Give me the splendid silent sun with all his beams full-dazzling. ~ Walt Whitman (1819 - 1892) CHAPTER 10 Since ancient times, humans have demonstrated a basic understanding of solar energy by designing shelters that protected them from the cold of winter and the heat of summer. Socrates suggested that houses should be built facing south in order to maximize the heat of the winter sun. Anasazi Indians designed their dwellings under cliff overhangs, maximizing the solar gain in winter and providing shade in summer. The sheltering overhangs also reduced radiant heat loss at night and protected against cold winds. Furthermore, some houses were built with thick stone and clay walls and had small windows to retain heat during the day and release it gradually at night. Today, solar power has been proven to be highly effective not only in providing heat to buildings and swimming pools, but also in cooling applications and for the generation of electricity. As with many other sources of energy, there are limitations on where this energy source can be used. The amount and intensity of solar energy available varies depending on geographical location, the position of the sun in the sky, weather conditions, and the orientation of the collector and surrounding objects. Solar energy can be used worldwide, especially in regions with high insolation. This includes large areas of Africa, Australia, the Middle East, Central and South America, and the southwestern United States. In this chapter, we start by looking at the sun itself and the amount of radiation that can potentially be intercepted by various collectors on earth. We will discuss passive and active systems, as well as thermal and photovoltaic systems for utilizing solar energy for heating, cooling, and producing electricity. The Sun The sun is a 1.4 million kilometer wide sphere of extremely dense, hot gas that resides approximately 150 million kilometers from the earth (Figure 10-1). Composed predominately of hydrogen and helium, its central temperature and pressure are estimated to reach 15-25 million kelvins and 300 billion atmospheres – conditions suitable for nuclear fusion to take place. As we will discuss later in Chapter 11, fusion involves combining atoms of hydrogen into a helium nucleus and heat. The sun loses mass at a rate of about 4.3 billion kilograms every second while producing an enormous amount of energy.1 At its outer layer (called the photosphere), 1 Figure 10-1 The sun Source: NASA http://www.nasa.gov. Th is loss of mass is nothing to worry about! By some estimates, the sun will have enough energy to support life on earth for another five billion years. 219 ionized gases both absorb and emit a continuous spectrum of radiation, causing this layer to be essentially opaque. Temperatures drop to about 5,800 K at the surface of the sun. Solar Position The geometric position of the sun as seen from a particular place on the surface of the earth varies from day to day and hour to hour. At any given instance, the sun’s position can be fixed by two angles, altitude and azimuth (Figure 10-2). Altitude is the angle between the sun and the horizon. When the sun is on the horizon (at sunrise and sunset), this angle is zero. Solar altitude is at the maximum at solar noon. The complement of solar altitude angle, or the angle of the sun from a vertical line directly overhead, is called the zenith angle. A zimuth is the angle between a north-south line on the earth’s surface and the horizontal projection of the sun’s rays; it is measured from true south. By convention, solar azimuth is negative before noon and positive after noon. To calculate the sun’s angle we need to know not only the relative position of the sun, but also the geographical location of the observer on earth. Two angles define the position of any point on the surface of the earth. Figure 10-2 The position of the sun in the sky can be determined by knowing the solar altitude (q) and azimuth (f) angles. Latitude is the angular distance measured along a meridian north or south of the equator. All points on the equator have a latitude of zero; the north and south poles have latitudes of +90o (90o N) and –90º (90o S). Los Angeles is located at a latitude of 33.9º N. Longitude is the angular distance measured east or west from the Prime (International) Meridian -- an imaginary circle passing through Greenwich, England. Since the period of rotation of earth about its own axis is 24 hours, each hour covers 360/24 = 15º longitude. Los Angeles has a longitude of 118° W. Calculating the relative position of the sun in the sky from earth at different locations and times is outside the scope of this book, but readers are encouraged to refer to more advanced texts.2 Solar Insolation Solar insolation (not to be confused with insulation) is the amount of radiation arriving from the sun that collides with a flat surface. This includes the direct (beamed) radiation from the sun, scattered (diffused) radiation from the sky, and reradiated (or reflected) radiation from the surroundings. Both direct and diffused radiation depend on a number of 2 See for example, Duffie, J, and Beckman, William A., “Solar Engineering of Thermal Processes ,” 2nd. Ed., John Wiley and Sons, 1991. 220 Chapter 10 - Solar Energy factors such as the day of the month, the time of day, and the weather. The reflected radiation depends on the topography of the surrounding area, the amount of shade, and the reflectivity (also called albedo) of the earth. Albedo varies significantly with the type of material and whether the surface is exposed or covered by water, snow, or vegetation. Atmospheric conditions affect the intensity of the light as it reaches the collector. Factors affecting the light intensity are cloud cover, humidity, and atmospheric composition. It is a common experience that the sun rises higher in the sky in summer than winter. It may also be noted that, in the northern hemisphere, the sun rises south of due east in winter and north of due east in summer. As winter proceeds into spring and summer, the sun rises earlier in the morning, moves higher in the sky, reaches its highest point at noon, and sets later in the afternoon (See Figure 10-3). The opposite is true in the southern hemisphere.3 Unlike what some might suspect, the total solar energy incident on a south-facing window is greater during the winter than in the summer in spite of the fact that days are longer in the summer. Two factors contribute to this: 1) Although the sun rises earlier in the summer, it remains north of the window before it sets in the afternoon. In winter, the opposite is true -- sun rises and sets south of the window; therefore, south-facing windows are subject to incident sunlight for the entire length of the solar day. For example, a house in Long Beach, California located at 33° 48’ latitude in the northern hemisphere experiences about fourteen hours of daylight (5:30 am to 7:30 pm) on Summer Solstice ( June 21) and ten hours of daylight (7:30 am to 5:30 pm) on Winter Solstice (December 21). The south-facing windows in this house receive sunshine for only seven hours (8:30 am to 3:30 pm) during summer and receive the full ten hours of sunshine during the winter. 2) The summer sun is higher in the sky than the winter sun. In the winter, the lower sun strikes the windows more directly than in the summer when the sun is higher. For example, every square meter of the window of a house in Long Beach receives, on average, 220 watts of solar energy per hour of daylight in the winter, whereas the same window receives 40% less energy in the summer. Furthermore, because of the higher angle of incidence in summer, more of the sunlight is reflected off the glass than would be during the winter. Of all the radiation emitted by the sun, only a very small fraction is intercepted by the earth’s atmosphere. This amount is called the solar constant and represents the energy from the sun, per second, that falls on a surface perpendicular to the sun’s rays and just outside the earth’s 3 SUMMER W S θ N E WINTER W S θ N E Figure 10-3 Summer and winter sun paths in the northern hemisphere. From now on, we will limit our discussions to the northern hemisphere. 221 Did You Know That ...? Solar Energy: The Facts • Every second, about 4.3 million tons of solar mass is converted into massive amount of energy, totaling to equal to roughly one billion hydrogen bombs. • Photovoltaic modules covering 0.3% of the land in the United States could supply all its electrical demand. • The sun emits radiation at all possible wavelengths, from very short- wavelength, high-energy cosmic and gamma rays to very long-wavelength, low-energy radar and radio waves. Interestingly, a large fraction of solar radiation (48%) falls in the narrow band between 400-700 nm (4x10-7 to 7x10-7 m) which is visible to our eyes. Another 46% of the radiation is in the form of heat and falls within the infrared region. atmosphere. The solar constant is not truly “constant”, however. Because the actual orbit of the earth around the sun is elliptic, the average distance varies by about 3.4% during the year and the solar constant varies between 1,300-1,400 W/m2. Like the sun, the earth is also emitting radiation, but at a much lower rate. The earth’s (terrestrial) emission is in the infrared region of the spectrum and averages at around 172 W/m2. Vernal Equinox +23.5 -23.5 Summer Solstice 95.9 million miles 89.8 million miles Winter Solstice Autumn Equinox Figure 10-4 Earth travels around the sun along an elliptical orbit about once every 365 days. Question: The data collected on the motion of the earth around the sun shows that the distance between the earth and the sun is slightly shorter in the winter than in summer (Figure 10-4). Why then are summers warmer than winters? Answer: The earth’s rotational axis is not perpendicular to its orbit around the sun, but tilts by 23.5 degrees. In the northern hemisphere, the angle of the earth’s rotation tilts toward the sun in the summer and away from it in the winter. In southern hemispheres, the earth’s tilt is in the other direction and the seasons are reversed.4 To calculate the amount of energy reaching a surface, we not only need to know the position of the collector plate, but also its tilt – the angle that the collector makes with a horizontal plane. It is obvious that the highest amount of solar energy will be received by a collector that is perpendicular to the sun’s rays. The sun is, however, rarely directly overhead, and its position changes throughout the day, limiting the solar power averaged over a 24-hour period to only one quarter of the solar constant value, or 340 W/m2. Due to the absorption and scattering by particles and clouds, only 18% of this amount actually reaches the earth. Therefore, for maximum collection efficiency, many designs employ tracking systems that move the collectors to directly face the sun at all times. Tracking systems, however, can be very costly and are used only in special applications. Solar Heating and Cooling Heating and cooling of a space is needed when the temperature falls below The seasonal variation of the lengths of days and nights can be explained by noting the relative position of the sun with respect to the earth. Twice a year on the vernal (first day of the spring) and autumnal (first day of the fall) equinoxes, day and night become equal in length. The vernal equinox has been celebrated throughout history as the time of rejuvenation and rebirth, thus marking the start of a New Year in the Zodiac calendar. The summer solstice occurs around June 22, and represents the longest day of the year. The shortest day of the year f alls on winter solstice, at or around December 22 in the Northern Hemisphere. 4 222 Chapter 10 - Solar Energy Window Glass FYI ... W indow glass is a simple and economical way of getting both light and heat conveniently into a living room or an office building. However, in many instances, such as during the hot summer days, we would like to block the sun’s heat while still allowing the light into the room. This is traditionally done by laminating window glass with ultra-thin layers of silver, or by adding infrared-absorbing dyes that are broken down by strong sunlight and scatter light, thus giving the glass a smoky haze. A new technique has recently been developed that blocks the infrared by an order of magnitude and, at the same time, allows visible light to pass through.i The key is to sandwich small particles of lanthanum hexaboride in the glass. These particles strongly absorb the near-infrared radiation, but are so small in size (smaller than the wavelength of the visible light) they cannot scatter sunlight, leaving the glass highly transparent. Schelm, S. & Smith, G. B. Dilute LaB6 nanoparticles in polymer as optimized clear solar control glazing. Applied Physics Letters, 82, 4346 4348, (2003). i or rises above a desired value. To determine how much heating or cooling is necessary, we must determine how much heat enters or leaks out of a building. A simple procedure for the rough calculation of thermal load is presented in the box below. The more detailed calculation is outside the scope of this book, but the interested reader is referred to texts and journals dedicated to the topic.5 Passive Heating Passive heating refers to the collection of solar energy without use of electrical or mechanical power. The simplest form of passive heating is the direct gain of solar energy as it passes through window glass (Figure 10-5a). In a more complex system, a medium with a large thermal mass (such as a concrete wall, or a drum of oil or water) stores the solar energy before releasing it at a later time when it is needed. Because they have fewer (if any) moving parts, passive designs are simpler, more reliable, more durable, and cost less than active systems. Passive solar systems can be designed in a variety of forms.6 Solar rooms are south-facing rooms with large windows, thick walls, and well-insulated roofs. Large windows allow maximum gain of direct solar radiation. Double and triple glazing of windows can significantly reduce heat losses from the room, while only moderately reducing the solar gains. Thick walls have large masses and high thermal storage capacity. Solar chimneys allow cool ambient air into the bottom of a glass collector where it is subsequently heated by incident sunlight and then rises by natural convection to the top before being released into a room. Solar chimneys are advantageous in that they can eliminate direct sunlight and glare and reduce heat losses during the night. More elaborate designs allow the amount of circulating air (and thus heat input) to be adjusted 5 6 Figure 10-5 Passive and active solar systems See for example ASHRAE Journal of Heating, Ventilating, and Air Conditioning. A nderson, B., and Wells, M., Passive Solar Energy, Brick House Publishing, New Hampshire, 1981. 223 as necessary. Active Heating The main component of most active systems is rooftop solar collectors, where solar energy heats a working fluid like water and either stores it in a hot water reservoir or distributes it directly to interior spaces through pipes or ducts. For most applications, temperatures of about 100°C are sufficient and flat plate collectors are the most convenient. For applications that require higher temperatures, energy must be concentrated and solar concentrators such as lenses and mirrors are necessary. Flat plate collectors consist of a number of tubes through which a working fluid (such as water or air) is heated by solar energy. Tubes are arranged in parallel inside an airtight collector box that is covered by a sheet of glass or plastic and insulated on the back. To absorb the maximum amount of energy possible, the back-plate is painted black.7 The collectors are placed on rooftops or in open areas and tilted at an angle that maximizes solar insolation. Ideally, collector plates should be perpendicular to the sun’s rays at all times. This requires costly and complex tracking devices, however, so fixed flat plate collectors are often used instead. For optimal efficiency, collectors are faced south (north if in the southern hemisphere) and tilted at an angle equal to the latitude of their location. Evacuated tube collectors are similar to flat plate collectors except that the glass tubes are replaced with two concentrating tubes. Fluid flows inside the inner one while the outer tube is evacuated. This arrangement eliminates conduction and convection losses, which increases the collection efficiency. The most common uses of flat plate collectors are for domestic hot water systems, pools, spas, and space heating.8 Usually a gas or an electricpowered heater is added as a backup for periods when sufficient solar energy is not available. Evacuated-tube solar collectors can be used under cloudy conditions, so their year-round efficiency is higher. They are considerably more expensive and maintenance costs are higher. A typical domestic hot water system is shown in Figure 10-5b. Solar Cooling In Chapter 5, we reviewed the principle of operation of a refrigerator (or an air conditioning system). Cooling occurs as a result of a refrigerant removing heat from the refrigerated (air conditioned) space and becoming vaporized (step 1). The vapor is then heated to a high temperature and pressure by a compressor (step 2); it then enters a condenser where it is condensed and becomes liquid (step 3). Finally the liquid is expanded Hoagland, W., “Solar Energy,” Scientific American, V. 273, pp 170-173, September 1995. Solar water heaters, sometimes called domestic hot water systems, can be either passive or active and open or closed loop. Passive systems rely on the principle that water in the collector becomes lighter and rises as it heats, while cooler water in the tank sinks, causing circulation by natural convection (thermosiphon). No pump is needed for water circulation t hrough the collectors, making the system simpler and less expensive. For passive systems to work, the tank must be above the collector. Active systems use pumps to assist circulation. Open-loop s ystems are popular in mild climates where there is no danger of freezing. These systems require a circulation pump and are therefore inherently active. C losed-loop s ystems u se a mixture of water-glycol antifreeze mixture and thus can be used in area where freezing is a possibility. 7 8 224 Chapter 10 - Solar Energy in an expansion valve, allowing the refrigerant to cool to the evaporator temperature (step 4). The refrigerant is now ready to remove additional heat and repeat the cycle. The point to remember is that some energy in the form of electricity or heat is needed to carry out step 2. The source of this energy can be natural gas, oil, or in this case, solar energy. Solar Concentrator When temperatures higher than 100°C are demanded, solar energy must be concentrated.9,10 The degree to which solar energy is concentrated by a given collector is called the concentration ratio and is defined as: C.R. = Collector Aperture Area Receiver Surface Area (10-1) Concentrators come in two basic configurations: parabolic troughs and parabolic dishes (Figure 10-6). Parabolic troughs are basically long sheets of metals bent along an axis to concentrate sunlight on a tube containing a liquid and placed along the focal line. Parabolic dishes are similar except that they are bent to form dishes. Since parabolic dishes distribute the energy over a smaller collector surface (around a focal point), they have a higher concentrating power than parabolic troughs. Furthermore, it is a matter of common sense that the bigger and more curved the collector area is, the higher the degree of concentration and collection efficiency will be. As Table 10-1 shows, parabolic dishes with concentration ratios as high as 1000:1 can be constructed; these can heat water to superheated steam at 1,200°C. Temperatures as high as 4,000 oC have been reached by a combination of various concentrator methods (Figure 10-7). Figure 10-6 Parabolic trough and parabolic dish. Table 10-1. Solar Concentrators CR Flat Plate Collectors Parabolic Trough Parabolic Dishes Power Tower 1 10-50 30-1000 --Tmax 100°C 400°C hmax 20% 56% 1200°C 75% 2000°C 85% Electrical Power Generation In Chapter 5, we saw that whenever there is a temperature gradient, a heat engine can be designed that converts thermal energy to work. According to the second law of thermodynamics, the efficiency of any heat engine can be improved by increasing the temperature differences between the two heat reservoirs. In the case of fossil or nuclear power plants, the chemical energy in the fossil fuel or the nuclear energy in the uranium is used as a heat source to boil water into steam. Solar power plants work in a similar fashion except that the heat energy comes from the sun. Solar Thermal Power Systems There are several ways that thermal energy from the sun can be used to produce electricity; these include solar chimneys, solar collectors, and solar power towers. It was shown earlier that solar chimneys can be used to heat a room by M anchini, T., et al., Solar Thermal Power, in Advances in Solar Energy, Vol. 11, American Solar Energy Society, Boulder, CO, 1997. R eaders interested in detail designs of solar concentrators should refer to Solar Energy Systems Design by W.B. Stine and R.W. Harrigan (John Wiley and Sons, Inc. 1985. An updated copy of text is available electronically at http://www.powerfromthesun.net). 9 10 Figure 10-7 The largest solar furnace in the world is in Odeillo, France. A heliostat consisting of 63 mirrors (not shown here) automatically track the sun and reflect its rays into a fixed parabolic mirror reaching temperatures of 3800°C for a maximum of 1 MW of power. 225 drawing hot air through a duct. Alternatively, the rising current of air can drive a gas turbine to produce electricity; a conversion efficiency of 2-3% can be achieved.11,12 The capital cost is relatively high, but operating costs are very low, fuel is free, and the power station has a long lifetime. To increase efficiency, a heat exchanger can be placed at the outlet of the turbine to capture the remaining thermal energy in the air column and preheat the incoming air. The solar trough system consists of a trough with a parabolic cross section and a reflective surface on the inside. The sunlight impinges on the surface and is reflected to a receiver tube placed along the focal line of the parabola, where a working fluid is heated to about 400°C. The receiver is blackened to increase its absorption efficiency. The heat is transferred through a series of heat exchangers to convert water to superheated steam, which can be used directly or to power a turbine/generator system to generate electricity. The SEGS (Solar Electric Generating Station) plants are the world’s largest parabolic trough facilities; they operate at three sites in California’s Mojave Desert and range in size from 14 MW to 80 MW for a combined electric generating capacity of 354 MW. Each plant consists of a collector field of many troughs aligned on a northsouth axis, which reflect sunlight onto pipes filled with oil. Each trough is mounted on a tracking device that can follow the sun from east to west during the day to ensure that the sunlight is continuously focused on the receiver pipes (Figure 10-8). Because oil loses its heat quickly, gas-fired generators are used to assist solar collectors in meeting demand during nights and cloudy days. Solar power towers consist of the same main components as other power plants, except that the steam generator is replaced with a large array of mirrors, called a heliostat, arranged around a tower. A central processor controls each mirror individually in order to track and at all times focus the sun’s rays to a receiver located at the top of the tower. A fluid that circulates through the receiver carries the heat and passes it through heat exchangers to heat water to steam. Steam subsequently expands in a steam turbine, which is coupled with a generator to produce electricity. Using this concept, two demonstration plants, called Solar One and Solar Two, were designed by the Sandia National Laboratory and operated by Southern California Edison in Daggett, California. Solar One was a 10 MW plant where water was heated directly in the solar receiver to generate steam. The plant operated from 1982 through 1988 and was eventually decommissioned and replaced by Solar Two, in which water was replaced with a mixture of molten nitrate salt as the working fluid (Figure 10-9). The advantage of molten salt is that it remains liquid to a higher temperature, and thus its thermal efficiency is increased (Figure Figure 10-8 SEGS Solar field in Mojave Desert, California. Image courtesy of SunLabs, Dept. of Energy Figure 10-9 Solar Two. Source: National Renewable Energy Laboratoty. Figure 10-10 Molten salt was substituted for water as the working fluid used in Solar Two demonstration project. 11 12 Schlaich, J., The Solar Chimney: Electricity from the Sun, C . Maurer, Geislingen, Germany, 1995. Von Backström, T.W. and Gannon, A.J., “Compressible Flow through Solar Power Plant Chimneys”. A SME Journal of Solar Energy Engineering , Vol. 122, No. 3, pp. 138-145, 2000. 226 Chapter 10 - Solar Energy 10-10). Solar Two had an additional advantage in that it could generate 7 MW of electricity for three hours after sunset. The Solar Two plant operated from 1996 to 1999, after which point it was decommissioned due to a lack of funding. Based on lessons learned, BrightSource Energy is building the largest series of solar installations with capacity of 1,300 megawatts in Mojave Desert. When completed in 2013, the facility will provide electricity to 845,000 Southern California homes, doubling the total US solar output.13 Similar plants are being built in Spain, Australia, and elsewhere. Dish/engine systems are similar to trough systems, except they use parabolic mirrors. Due to the higher concentrating power of dishes and their ability to track the sun in all directions, higher temperatures (750°C) are achieved. Since dishes have a smaller aperture than trough reflectors, these systems are best suited for small-scale power production (10-50 kW) as a stand-alone unit in remote areas, away from power grids. The system can, however, be expanded by adding more modules as the load increases (Figure 10-11). Furthermore, these systems can be used in solar-only applications or hybridized with fossil fuels during periods of high-energy demand or at night. Unlike parabolic troughs and central tower systems that run steam turbines, dish systems use Stirling engines14 or gas turbines. Hybrid systems integrate solar thermal technology with existing coal-fired plants or high efficiency combined-cycle plants. This allows distribution, higher thermal efficiency, and continuous operation of the plant even after the sun has set. W hich of the solar thermal power plant schemes is the best is not obvious. Each system has its advantages. Towers and troughs are best suited for a large grid-connected power system in the 30-200 MW range, whereas Table 10-2. Characteristics of Concentrating Solar Power Systems Parabolic Trough Status Plant Size1 Operating Temp (°C) Concentration Ratio Peak Efficiency Annual Efficiency Capacity Factor 1 3 2 Figure 10-11 25 kW dish Stirling system. Source: University of Nevada, The Center of Energy Research (http://www.me.unlv.edu/ research/cer/research.htm) Power Tower Demonstration Stage 10-200 MW 565 ---23% 14-19% 20-77% Dish / Engine Prototype Stage 5-25 kW 750 30-1000 29% 18-23% 25% Commercially Available 30-320 MW 390 10-0 21% 14-18% 25-50% The upper range indicates 2030 projections 2 Projected based on pilot-scale testing 3 The % of the year the technology can deliver solar energy at the rated power BrightSource Energy Inc., http://www.brightsourceenergy.com. Stirling or “Hot Air Engine” refers to a class of engines in which heat is provided from a non-combusting external source such as nuclear or solar energy. Because the temperature of heat source and sink can be controlled and kept uniform throughout heat supply and heat rejection part of the cycle, thermal energy of Stirling cycle can reach that of the Carnot cycle. 13 14 227 dish/engine systems are modular and can be used singly in remote applications or be grouped together to power a small community and expanded as more power is needed. Table 10-2 summarizes characteristics of concentrating thermal power systems.15 Ocean Thermal Energy Conversion (OTEC) Oceans cover over 70% of the earth’s surface area and are a huge source of thermal energy. Ocean Thermal Energy Conversion (OTEC) takes advantage of the temperature difference between the warm surface waters and cold deep waters of oceans to drive a steam turbine and produce electricity. Tropical waters are especially suitable because of the higher surface temperatures (Figure 10-12). Ocean waters between the tropics of Cancer and Capricorn (latitudes between 23°N and 23°S) typically have highest surface temperatures around 25°C-30oC (77°F-86oF), while the water below 900 meters is only 5°C (41°F). The temperature differences are very steady over time between days and nights and from season to season. French engineer Jacques D’Arsonval was the first to note the huge potential for using thermal gradients in the ocean to produce power. His proposed system was not implemented until 1930 when his student, George Claude, built the first OTEC plant at Mantanzas Bay, off the coast of Cuba. Although the system functioned, the work needed to run the pumps and other devices (work input) was more than the work output. Furthermore, the difficulties in lubricating underwater pipes, salt corrosion, and growth of algae (biofouling) made the system inoperable and the project was abandoned. Many of the technical difficulties facing Claude have since been resolved. Recently, there has been a renewed interest in OTEC plants and demonstration plants have been constructed near Kailua-Kona on the island of Hawaii. In the United States, Florida and Puerto Rico could also utilize OTEC Technology. The Indian government has also actively sought to invest in this technology and has built a 1-MW floating OTEC plant. OTEC plants can be constructed to operate in either closed or open cycles. In open-cycle systems, water itself is the working fluid. Open-cycle systems work by boiling the surface water at very low pressures (flash evaporation). After expansion through the turbine, steam is condensed back to liquid water via a condenser, which is then cooled by deep seawater before being discharged. The output of the condenser is desalinated water, one of the desirable byproducts of the OTEC system. Closed-cycle systems use the ocean’s warm surface water to vaporize a working fluid, such as ammonia or another refrigerant. The vapor expands through a turbine, turning a generator to produce electricity. The vapor is eventually condensed by transferring its heat to the cold water of the deep ocean, before being pumped back to the evaporator to complete the cycle. A schematic of a Figure 10-12 The map shows the contours of oceanic temperature differences between the surface and 1,000-m depth. Orange and red colors are regions with a DT of at least 20°C. Figure 10-13 Closed-Cycle OTEC 15 Taylor, Craig, et al., “Concentrating Solar Power in 2001,” SolarPaces Corp, 2001. 228 Chapter 10 - Solar Energy closed-cycle OTEC plant is shown in Figure 10-13. In addition to open and closed cycles, OTEC can also be designed as a hybrid. In a hybrid cycle, steam is generated by flash evaporation of the warm surface water and then used as a heat source for a closed cycle. Example 10-1: W hat is the maximum theoretical efficiency of an Ocean Thermal Energy Conversion (OTEC) plant? Solution: The maximum efficiency is limited to its Carnot efficiency, which can be calculated as: η Ideal, OTEC = 1- (273+5) = 6.71% (273+25) Because of the power required for pumping cold temperature water from the bottom of the ocean and other frictional losses, the efficiency of a practical OTEC device is lower than this value at around 2-3%; this is far below the 30-35% thermal efficiencies commonly found in conventional oil- or coal-fired steam plants. Unlike conventional plants, however, there are no fuel costs. Advantages and Disadvantages OTEC could potentially fulfill a major part of our energy demand with few adverse environmental effects. Because of the huge water resources at our disposal, the technology potential could be significant, especially for those countries that have few fossil resources. Among features that distinguish OTEC from other renewable sources of energy is the wide range of side benefits it offers. Besides desalination, OTEC can provide industrial cooling, space air conditioning, and can allow chilled soil agriculture and aquaculture.16 Cooling – After steam is expanded in the turbine, it is condensed in the condenser. The condensate is relatively free of impurities and can be used in a number of applications such as air conditioning, desalination, and chilled soil agriculture. Aquaculture – Deep water is not only clean, but also has a significantly higher amount of nutrients that allow enhanced growth of marine algae and many marine organisms, such as salmon and lobster, that could not otherwise grow in a tropical environment. Sink - Deep seawater is a huge heat sink for many industrial applications that require dumping of waste heat. The environmental impact associated with OTEC operation is relatively minimal and limited to some changes in local marine life and the release of CO2 as the cold water from the deep oceans are brought to the warm Daniel, H. T., “Ocean Thermal Energy Conversion: An Extensive, Environmentally Benign Source of Energy for the Future,” Natural Energy Laboratory of Hawaii Authority, KailuaKona, HI 96740. 16 229 water surface. This release is however negligible compared to the amount of CO2 release that would result from the use of fossil fuel to generate a comparable amount of energy. The major obstacle to OTEC development is the initial cost and limit of suitable sites near shorelines. The OTEC cold water pipes make up almost all of the costs, so OTEC sites cannot be more than a few kilometers away from the shoreline. Most suitable sites are in areas away from population centers which make the power transmission costs even higher. It is likely that the best solution is to use the electricity generated by offshore OTEC platforms to produce alternative fuels such as methanol and hydrogen which can be loaded onto tankers or flow through pipes.17 It has also been suggested that the solar thermal energy stored in the reservoirs behind hydroelectric dams (thermocline) be used to increase the power output of conventional hydroelectric plants. The data suggests that, for typical thermoclines of 15°C, heat engines can effectively increase the gravitational potential (head) by tens to hundreds of meters and thus can double the hydropower efficiency for many hydroelectric power plants.18 Solar Ponds Unlike ocean waters where warmer water is at the surface, solar ponds are shallow lakes of salty water where higher temperatures are in the bottom layers and colder temperatures are near the surface. As solar radiation penetrates shallow waters, it is absorbed at the bottom, raising its temperature. If the water is fresh, the buoyancy causes mixing of the water and temperature will soon become uniform throughout. If water contains some salt, it becomes heavier than fresh water and sinks to the bottom, retaining the heat and temperature gradient (Figure 10-14). Temperatures as high as 95°C can be reached at the bottom layers. The temperature difference between the bottom and the surface layers can be used to design and construct large-scale power production and desalination plants. Heat is extracted from hot salty fluid at the bottom of the pond and passed through heat exchangers to heat a working fluid, causing it to evaporate. The vapor is used to drive a turbine, similarly to conventional steam power plants. Fresh water is produced as the byproduct of these processes. To operate the plant continuously, the salt concentration gradient must be maintained. As a result of convection, there is always some diffusion of salt from the bottom to top layers. To maintain stability, salt must be added at the bottom and removed at the top. Larger ponds and calmer wind conditions are preferable. Larger ponds have a larger surface to perimeter ratio, and convective effects are less important. To prevent wind from disturbing surfaces and to reduce salt mixing, smaller ponds install Avery, W. H., and Dugger, R. D., “Hydrogen Generation by OTEC Electrolysis and Economical Energy Transfer to World Markets via Ammonia and Methanol,” NREL Business/ Technology Books: ISBN 0899343317, 1997. 18 McNichols, J. L., et al., “Thermoclines: A Solar Thermal Energy Resource for Enhanced Hydroelectric Power Production”, Science , 203, pp 167-168, January 1979. 17 Figure 10-14 Solar pond 230 Chapter 10 - Solar Energy Solar Crock Pot* FYI ... I ndian workers have devised a cooking system that makes no demands on the critically short supply of wood, their traditional energy source. They dug a pond about 1 m deep and 8 m in diameter and lined it with impermeable plastic. The pond was filled with water and enough salt poured in to give a saline concentration of 20 percent at the bottom. The concentration of salt gradually decreased from the bottom to the top of the pond. The salt gradient prevents convection, as the salt water is too heavy to rise. Thus the water at the bottom absorbs solar heat energy throughout the day and reaches a temperature over 80oC at the bottom. Food is placed in sealed pots and left in the pond for a whole day. This kind of cooking is equivalent to an electric “crock pot” which takes 75 W for 8 hours. * Excepts from a physics text currently under preparation jointly by this author and Professor Igor Glozman of Department of Physics, Highline Community College, Des Moines, Washington 98198. suppression rings that cover pond surface. Additionally, many ponds contain a large amount of salt making them naturally suitable as solar ponds. The largest solar ponds are in Israel, where the hot, dry climate is ideal for their operations (Figure 10-15). Solar ponds have found numerous applications in drying, desalination, process heat, refrigeration, and power generation. Their cost is considerably lower than that of flat plate and photovoltaic solar collectors. Besides the initial cost of construction, the only cost associated with solar ponds is that of maintenance. This includes preventing growth of algae in the upper convection layer and maintaining the salt gradient. Photovoltaics Photovoltaic (PV) systems convert solar energy directly into electricity using specially designed collectors known as solar cells (Figure 10-16). The photovoltaic effect was first observed by Alexander Becquerel in 1839, although it took another century before any serious attempt was made to design a practical device. In 1968, Glaser19 proposed to place a satellite solar power system in a geosynchronous orbit that collected sunlight and beamed it to collectors that then generated electricity (See box “ Solar Power Satellites”). By the mid-1970s, solar cells were primarily used in space for powering the instruments aboard space stations and in satellites. As the technology matured and the cost of production decreased, solar cells found new applications in everyday appliances, from pocket calculators and wristwatches to small remote applications such as highway call boxes, message boards, and traffic signals. Worldwide, one gigawatt of electricity is generated by photovoltaics. Japan currently leads in solar cell manufacturing and controls nearly 40% of the global solar cell market; Germany is second and the United States is third. Compared to fossil fuels, the cost of electricity generation by solar cells is expensive, but can quickly become competitive as manufacturing costs drop. Because they can be installed at any capacity, photovoltaic power is an ideal method for powering remote homes or villages too remote to connect to a grid. 19 Figure 10-15 The largest solar pond operated to-date is a 210,000 square-meter pond at the Dead Sea, Israel, which generates 2.5 MW of peak electric power. Figure 10-16 Photovoltaic Cells Source: National Renewable Energy Laboratory, Photographic Information Exchange (http://www.eere.energy.gov/pv). Glaser, P. E., “Power from the Sun: Its Future,” Science, Vol. 162, pp. 957-961 (1968). 231 Principle of Operation To understand the photovoltaic (or photoelectric) effect, it is best to visualize sunlight as small packets (quanta) of light called photons. The other equally valid view considers light as a wave. Photons are emitted at different frequencies and, therefore, have different wavelengths and energies. When photons strike a cell, some are absorbed, some are reflected, and some pass through. Only photons beyond a certain frequency have sufficient energy to dislodge electrons off a substrate and flow into an external circuit producing electricity. To facilitate the movement of electrons, an electric potential must be constructed. This can be done by sandwiching two layers of silicon wafers together; one layer is doped with a material that has one less electron - is positively charged (p-layer) - and the other contains a material with an extra electron (n-layer) in their valence shell. To make a positively charged semiconductor, silicon is doped with impurities such as boron, aluminum, or indium. To make a negatively charged semiconductor, silicon is doped with impurities such as arsenic, antimony, or phosphorus. Commercial silicon cells are made by pressing two thin silicon wafers together, a very thin top layer doped with a small amount of phosphorous (n-type) and a thicker bottom layer doped with a small amount of boron (p-type). W hen an n-type and a p-type semiconductor are pressed together, the imbalance of electrons across the p-n junction causes electrons to migrate from the n-type to the p-type material. If a wire connects two layers across a load (for example a light bulb or an electric motor), an electric field is created. As long as there is solar energy available and the circuit is closed, FYI ... Solar Power Satellites (SPS) A solar panel facing the sun in near-earth space receives about 1,350 watts of sunlight per square meter. Because of the sun’s angle and the attenuation by the atmosphere, the radiation received on the earth’s surface is considerably reduced. Furthermore, solar cells can only work during the day, so they can only receive sunlight half of the time. If solar collectors are placed in a geostationary orbit* (36,000 kilometers high), all of the extraterrestrial radiation can be collected and converted to electricity. This electricity can then be beamed as microwaves, which can pass unimpeded through clouds and rain and be received on the earth before it is converted back into electricity. This electricity is in the form of direct current, which can then be converted to the 50- or 60-cycle alternating current electricity used in homes. Because the panel always faces the sun, electricity would be generated 24 hours a day and there will be no need for storage. Such a system has indeed been proposed by the US Department of Energy and NASA. A preliminary design consisted of a 5 km x 10 km rectangular solar collector and a 1 km diameter circular transmitting antenna array. The SPS would weigh 30,000 to 50,000 tons and provide 5 billion watts of electricity, equivalent to ten 500 MW conventional coal or nuclear power plants. We are not likely to see such a huge orbiting solar collector beaming energy anytime soon. What probably makes more sense is to build a small proof-of-concept demonstration before extensive resources are dedicated toward fully-operational solar power satellites. * Alternatively, some have proposed to place the collectors on the surface of the moon. Source: Glaser, P.E., “Power from the Sun: Its Future,” Science, Vol. 162, pp. 856-861, 1968. 232 Chapter 10 - Solar Energy Metals, Nonmetals, and Semiconductors E Digging Deeper ... very atom is comprised of a nucleus and a number of electrons orbiting the nucleus. Electrons can only move in certain orbits. To move an electron from a lower orbital to a higher orbital, it must absorb a photon of energy proportional to the difference between the two energy levels. Similarly, an electron jumping from a higher to lower orbit emits a photon of light with a frequency proportional to the energy difference. Electrons naturally occupy the lowest available energy levels (valence bands), so upper energy levels (conduction bands) remain vacant. The energy needed to move the electrons from the highest valence band to the lowest conduction band is called gap energy. The greater the gap energy, the more difficult it is to move electrons and the less conductive (insulator) a material is considered. In non-metals, energy levels are separated by large gaps; therefore the transition from one energy level to the next is associated with frequencies that construct a discrete band. Metals, on the other hand, produce a continuous band because the electrons are free to move through the body of metal and pass through several energy levels. Metals have a continuous conduction band and are therefore good conductors of electricity. Semiconductors have intermediate gap energies. Because of its abundance and low price, silicon is the most popular semiconductor material. Atoms of silicon have four electrons in the outer valence shell and are therefore electrically neutral. Silicon is an intrinsic semiconductor, i.e. at room temperatures, electrons do not have sufficient energy to jump the gap. By raising the temperatures, some electrons gain enough energy to overcome the gap and move into the conduction band. If the source of energy is removed, the electrons fall back into their previously occupied valence band. Depending on the processing method, silicon can have different structures: Monocrystalline silicon is grown from a single atom of silicon and has a perfectly uniform molecular structure, making it ideal for the efficient transfer of electrons through the material. Single crystalline silicon is difficult to manufacture and very expensive. Polycrystalline silicon consists of, not one, but a number of crystals. It is much cheaper than single crystalline silicon to produce, but the different crystalline structures introduce boundaries that increase internal resistance and impede the flow of electrons. The greatest advantage of polycrystalline silicon is that it can be deposited monolithically as a thin film on a glass substrate. Unlike single crystalline silicon, which must be grown from a single atom (ingot), layer upon layer of polycrystalline thin-films of the required materials can be sequentially deposited to a desired thickness. Polycrystalline silicon cells are less expensive than monocrystalline cells, but their efficiencies are also lower. Amorphous silicon does not have a crystalline structure and atoms are not arranged in any particular order. Amorphous silicon contains large numbers of structural and bonding defects, where electrons and holes recombine and limit the maximum current that can flow. It is the cheapest, however, making it suitable for use in low-power consumer devices such as wristwatches and calculators. Amorphous silicon cells can be sprayed onto glass plates and onto a variety of flexible substrates like metal foils and plastic foils, eliminating much of the production cost. the current will continue to flow. Current is directly proportional to how much light strikes the module. To increase the current output, the top surface is coated with an anti-reflection material. Without the coating, an additional 30% of solar energy would be lost due to reflection. A typical commercially available cell, 10 cm x 10 cm, produces about 1.5 watts (0.5 volts and a current of about 3 amperes) of electric power when exposed to strong sunlight. A typical setup is shown in Figure 10-17. To achieve the proper current and voltage, cells are connected in particular configurations. Normally, modules are designed to supply electricity at a certain voltage by connecting a sufficient number of cells in series. The current can be adjusted by connecting a number of such modules in parallel. PV panels can then be grouped together to form large solar farms 233 Sunlight Contact Anti-reflective Coating Semi Conductor Back Contact Figure 10-17 Schematic representation of a solar cell 233 which generate electricity that can be used locally, stored in batteries, or fed into the commercial transmission grid. The output of photovoltaic cells is in the form of DC (direct current) that can be stored in batteries or used as a source for powering various devices. If it is to be used to generate electricity, the output must be converted to AC (alternating current) and amplified by transformers to high voltages before being transmitted through commercial electrical grids. Collection Efficiency Solar cells are often characterized by their conversion efficiency – defined as the percentage of incident power that is converted into electric power. The efficiency of solar cells is limited by the number of photons of a precise frequency that can knock out the electrons from the junction. Photons of lower frequency do not have sufficient energy, whereas higher frequency photons may cause overheating of the substrate and subsequent loss of efficiency. Because of its abundance, silicon remains the most common cell material used today. However, higher efficiency cells have been constructed of gallium arsenide, cadmium telluride, cadmium sulfide, and other materials (commonly called III-V materials because they are impregnated with material from groups III and V of the periodic table). Efficiencies in the range of 5% for amorphous silicon and 17% for monocrystalline cells can be expected for commercially available silicon solar cells. Efficiencies as high as 33% may be obtained under concentrated light conditions. To increase the flux of photons, scientists have developed a spherical cavity treated with highly reflective coating and lined with multiple solar collectors. Different cells are made from different materials with different energy gaps. Each cell is covered by a filter that allows only the light with the appropriate frequency to pass through. The remaining photons are reflected back and forth within the cavity until the proper cells absorb them. Efficiencies as high as 48% have been reported.20 Another innovation, the cascade or tandem cell, is constructed by stacking several single-junction cells with different band gaps on top of each other. The top layer has the highest band gap, capturing the high-energy photons and allowing the photons with lower frequencies to reach the lower cells with smaller band-gaps. The major problem with flat panel cells is that, once they are installed with a particular orientation, they only collect maximum sunlight if the sun is directly overhead. As the sun’s incident angle changes, the light intensity drops and less power is produced. The basic idea behind the spherical cells is that a spherical receiver can collect light from all directions. This allows the capture of direct beams of light as well as light diffused from clouds and reflected from buildings, thus spherical cells would have efficiencies up to 50% greater than those of flat cells. An attempt to improve efficiency 20 L eine, J. D. et al., Proc. 15th IEEE Photovoltaic Specialist Conference, Las Vegas, IEEE, 141, 1991. 234 Chapter 10 - Solar Energy and reduce costs, a group of Japanese scientists have recently succeeded in developing spherical cells using single crystal silicon droplets (0.5-2 mm in diameter) to make the p-layer (Figure 10-18). A thin layer of n-type film is diffused and, except for a small opening for electrodes, covers the surface of the microsphere. The microspheres can be lined up along a string and connected in a series or parallel configuration with fine copper wire and are mounted on a white resin reflection plate covered with a transparent layer. The spherical solar cell module is highly flexible and can be made to match the contours of buildings or cars, thus providing integrated power sources in windows, roofing materials, canopies and other surfaces.21 Another promising technology is the multi-junction solar cell. Unlike conventional photovoltaic cells that absorb light only in the red part of the light spectrum, these cells consist of several microthin layers of light absorbing materials sandwiched together. Each layer absorbs a different color of light, increasing the overall efficiency. The technology has been implemented on the Mars Rover, but remains too expensive to be used in commercial products.22 Thermophotovoltaics Photovoltaic cells principally respond to light in the visible wavelengths. Thermophotovoltaic (TPV) cells work on the same principal, except that they are sensitive to infrared energy and use heat instead of light as their energy source.23 The source of heat could be a burning gas, a radiant furnace, waste heat, or another source in the 700-1,700°C range. TPV can also work by focusing solar radiation on to an intermediate absorber, which then re-emits it as thermal radiation. These devices are capable of converting as much as 1-10 watts of power per square centimeter, a far better power density than solar cells, which can only deliver about 100 mW of power per square centimeter. Because thermophotovoltaics are solid state devices (no moving parts), they have found applications in heating, cooling, and as sensors for temperature stabilization. Thermophotovoltaics (TPV) have been used for many years by NASA to power space crafts. The substantial drop in price of semiconductor ceramic material has prompted investigators to develop TPV systems that use waste heat from furnaces and burners to generate power, or make heat pumps that can provide cooling or heating. Bismuth-telluride materials are among the most favored because they can operate at temperatures as low as 150-200 degrees centigrade. Electrically Conductive Plastics Instead of conducting electricity and producing light, as is done in flat-panel Kyoto Semiconductor Corp., Japan (http://www.kyosemi.co.jp). Olson, J, and Kurtz, S., “Multi-Junction, Monolithic Solar Cell Using Low-Band-Gap Materials Lattice-Matched to GaAs or Ge,” Patent no. 6,281,426, National Renewable Energy Press Release (http://www.nrel.gov/news/press/2007/502.html). 23 I n principle, these devices are made by linking together a large number of modern semi-conductor thermocouples in a series. A thermocouple consists of two electrical conductors of d ifferent materials joined together at both ends. If the junctions are kept at different temperatures, an electric current is generated and flows through the conductors. 21 22 Figure 10-18 Spherical Solar Cells. Image Courtesy of: Kyosemi Semiconductor Corp., Japan (http://www.kyosemi.co.jp/). 235 FYI ... Solar Sail Its time to set sail for the stars ~ Carl Sagan D id you know that solar light can exert a gentle pressure just like water squirting out of a nozzle onto a plate? A team of Russian and American scientists have pioneered Cosmos-1, the first solar sail spacecraft which uses the impact of light particles or photons off of a reflecting surface to propel a spacecraft without heavy and expensive onboard fuel sources. Sunlight pressure is enough to accelerate the spacecraft at the rate of 100 mph a day. That doesn’t sound like much, but it can quickly add up over time. The effort opens a new chapter in space travel, offering a hope for future interplanetary missions. The first such spacecraft was launched on June 21st, 2005 from a Russian submarine in the Arctic Barents Sea. Unfortunately, the spacecraft was lost during the launch and experiment had to be postponed. Source: Matloff, G. L., Deep Space Probes: To the Outer Solar System and Beyond, Springer-Verlag New York, LLC, 2005. 2nd edition. displays, electrically conductive plastics run in reverse - they absorb light and produce electricity. Conventional solar cells are relatively expensive. The low cost and inherent flexibility of the electrically conductive plastics mean that they can be deposited as coatings hundreds of times thinner than the silicon crystals used in conventional solar cells and can be used on a range of materials such as glass and roofing tiles. Efficiencies as high as 3-4.5% have been reported.24 Environmental Impacts Photovoltaics are considered to be among some of the most environmental friendly energy alternatives. Their operation produces no pollution, does not contribute to global warming, and is noise free. Although PV operation is generally clean, its manufacturing (and disposal) is associated with the production of some of the most toxic materials such as cadmium and arsenic. Furthermore, some fossil or other non-renewable energy sources are normally used during their production. With proper safety precautions, the total emission is, however, small as compared to fossil burning. Summary Solar technology can be used for space heating and cooling, to generate electricity, or in various industrial and scientific applications such as drying, desalination, and manufacturing. It also can provide power to operate electrical appliances, and for telecommunication and lighting in remote areas. For solar energy to capture a major utility market share, large power plants with capacities comparable to a coal or nuclear plant in the range of 100-200 MW must be developed. The major drawback of solar energy is the diffuse nature of sunlight that requires a large surface area for capturing 24 Fairley, P, “Solar on the Cheap,” Technology Review, January-February 2002. 236 Chapter 10 - Solar Energy enough light. Another problem is that solar energy is intermittent, available only during daylight and in favorable atmospheric conditions. One way to address this difficulty is to store the power in large, bulky, and expensive batteries. Another is to design a pumping storage water facility that stores water in an elevated reservoir during the day and releases it downhill to turn a water turbine and a generator as needed. Many homes use a backup system to supplement their primary solar systems when necessary. Compared to current PV technologies with typical efficiencies of 5-17%, solar thermals are more efficient and can reach efficiencies of up to 30%. Unlike solar concentrating technologies that rely on direct radiation, photovoltaic cells rely mainly on indirect radiation and therefore can be used in areas with few hours of sunlight. Currently, none of the solar electric generating stations can compete economically with the low cost of electricity generation using conventional coal and nuclear power plants. Both the capital and the operating costs are much higher, and the solar stations take up a much larger land area. Further research is needed to design larger heliostats and mirrors, as well as lenses with better optical properties and lower costs. Additional Information Books 1. Markvart, T., and Castanar, L., Solar Cells: Materials, Manufacture and Operation, Elsevier Publishing Company, 2005. 2. Galloway, T., Solar House, Elsevier Publishing Company, 2004. 3. Stine, W. B., and Harrington, R. W., Solar Energy Systems Design, John Wiley and Sons, Inc., 1985. Periodicals 1. Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society, covers solar, wind and biomass energies. Government Agencies and Websites 1. National Renewable Energy Laboratory: Solar Research (http:// www.nrel.gov/solar). 2. Energy Efficiency and Renewable Energy: Solar Energy, US Department of Energy (http://www.eere.energy.gov). Non-Government Agencies and Websites 1. American Solar Energy Society (http://www.ases.org). 2. Solar Electric Power Association (http://www.solarelectricpower.org). 3. California Solar Center (http://www.californiasolarcenter.org). 237 Exercises I. Essay Questions 1. What does each of the angles of altitude, azimuth, latitude, and longitude represent? What angles are needed to locate the position of the sun from a point in space? From a point on the surface of the earth? What are factors that affect solar insolation? 2. How do solar chimneys work, and how can they be used for generating electricity? 3. What are advantages and disadvantages of passive and active solar energy systems? 4. What are the principles of operations of OTEC and solar ponds? 5. How do photovoltaics work? What are their advantages over solar thermal for power applications? 6. What are the advantages of electrically conductive plastics over solar cells? II. Multiple Choice Questions 1. The sun is a sphere consisting mainly of a. A solid core covered by a layer of molten liquid and hot gases b. A liquid core covered by a mixture of hydrogen and helium gases c. A mixture of hot helium and hydrogen gases d. A mixture of noxious ammonia and sulfur gases e. The same material that makes the planet earth 2. Solar thermal energy has its origin in the heat released by the a. Various chemical reactions at the center of the sun b. Various chemical reactions at the surface of the sun c. Fission reactions occurring at the center of the sun d. Fusion reactions occurring at the center of the sun e. Latent heat as liquid hydrogen converts into 238 gaseous form 3. Solar energy is ultimately responsible for which kind of energy? a. Fossil fuel b. Photovoltaic c. Wind d. Wave e. All of the above 4. The sun’s interior and exterior temperature are roughly a. 5 million and one million degrees celsius b. 10 million and 10,000 degrees celsius c. 20 million and 5,500 degrees celsius d. 50 million and 10 million degrees celsius e. 5,000 and 300 degrees celsius 5. Seasons are primarily a result of a. Rotation of the earth b. Rotation of the sun c. Varying distance of earth from the sun d. Tilt of the earth e. Orbiting of the earth around the sun 6. Which of the following is not a form of solar energy? a. Biomass b. Wind c. Wave d. Hydropower e. Geothermal 7. The fraction of solar energy that falls in the visible range is a. Less than 1% of the sun’s total radiant energy b. About 10% of the sun’s total radiant energy c. About 50% of the sun’s total radiant energy d. About 90% of the sun’s total radiant energy e. More than 99% of the sun’s total radiant energy 8. In what frequency range does the earth radiate the most? a. Cosmic ray b. Ultraviolet c. Visible Chapter 10 - Solar Energy d. Infrared e. Radio wave 9. The infrared portion of solar radiation consists of roughly a. 1% of total radiation b. 10% of total radiation c. 25% of total radiation d. 46% of total radiation e. 90% of total radiation 10. The angular location of a point on earth relative to the equator is called the a. Latitude b. Altitude c. Zenith d. Meridian e. Azimuth 11. The angle between a north-south line on the earth’s surface and the horizontal projection of the sun’s rays is a. Latitude b. Altitude c. Zenith d. Meridian e. Azimuth 12. The sun’s location relative to an observer on the ground can be determined by a. Azimuth and altitude b. Latitude and altitude c. Azimuth and latitude d. Latitude and longitude e. All four angles 13. Albedo is a. The fraction of incident light that is absorbed by earth b. The fraction of incident light that is reflected by earth c. The fraction of incident light that is transmitted through the atmosphere and reaches earth. d. An organism exhibiting deficient pigmentation e. Unnaturally strong sexual drive in humans 14. Solar insolation at a point on the earth depends strongly on a. The day of the month, the time of the day, and the weather b. The topography of the area and the amount of shade c. The albedo of the earth d. The type of clothing we wear e. All of the above 15. The best orientation for installing fixed flat plate collectors is a. Parallel to the ground surface, because maximum radiation occurs when the sun is directly overhead b. Tilted at an angle roughly equal to the latitude of the location where they are going to be installed c. Tilted at an angle roughly equal to the complement of the latitude of the location where they are going to be installed d. Perpendicular to the ground surface to occupy the least space e. It does not make any difference, because the sun is traveling across the sky 16. Flat plate collectors a. Are large arrays of photovoltaic cells b. Work by using sunlight to electrolyze water to its components hydrogen and oxygen c. Are faced toward the wind to catch the most w ind d. Are useful for heating domestic water heating systems e. None of the above 17. The majority of the solar radiation reaching the earth’s surface is a. Ultraviolet and infrared b. Infrared and radio wave c. Visible and ultraviolet d. X-ray and gamma ray e. Visible and infrared 18. An overhang on a southern-facing window is useful because a. It blocks the summer sun while enhancing 239 b. c. d. e. heating in the winter It prevents rain from hitting the side of the house It prevents squirrels from looking in It makes for more space in the ceiling for insulation It allows rain to drain efficiently e. Oceans with a minimum of 50oC difference between surface and deep water temperatures 24. OTEC technology can be used to a. Generate a large amount of electricity without the environmental impacts associated with fossil fuels b. Produce a large amount of drinking water c. Produce some fruits, vegetables, and marine organisms otherwise impossible to produce in a tropical environment d. Act as a large heat sink for many industrial processes e. All of the above 25. The main advantage of solar cells over other methods of electricity generation is that a. They are cheaper than the power the utilities can produce by burning coal b. Most utility plants are located in sunny areas c. They are clean sources of energy with no known adverse environmental impact d. Their power correlates with the utilities’ daily load patterns, because the power is available when it is needed most – during daylight hours e. Both c and d 26. Which one of the following ways of harnessing solar energy does not involve thermal energy? a. Active heating b. Passive heating c. Indirect generation of electricity d. Direct generation of electricity e. Passive cooling 27. PV cells are most frequently used in a. Consumer products, such as calculators, wristwatches, and solar-power radios b. Remote power applications of 100 W or less c. Areas close to or far from utility grids d. Satellites e. All of the above 28. The most common material used in PV modules today is a. Single crystalline silicon 19. For maximum efficiency, flat plate solar panels a. Must be horizontal b. Must face south and vertically c. Must be painted white d. Must face north e. Must be perpendicular to the sun’s rays at all times 20. The best solar system for producing process steam at temperatures of 300-400oC is a. Flat plate collector b. Parabolic trough c. Parabolic dishes d. Heliostat with flat mirrors e. Photovoltaics 21. The primary drawback of electricity produced by solar energy is that it is a. Intermittent b. In the wrong frequency c. Unpredictable d. Dispersed e. All of the above 22. Some of the byproducts of an OTEC plant include a. Desalination b. Space air conditioning c. Chilled soil agriculture d. Industrial cooling e. All of the above 23. OTEC systems a. Can be used anywhere in the northern hemisphere b. Can be used only with areas with hot summers c. Is best in latitudes of between 10o N and 10o S d. Only in Hawaii 240 Chapter 10 - Solar Energy b. c. d. e. Polycrystalline silicon Amorphous silicon Cadmium telluride Gallium arsenide remote areas away from power grids. 8. Both solar ponds and OTEC plants exploit the difference between higher surface temperatures and colder temperatures in the bottom layers. 9. The higher the frequency of photons, the more energy and the higher efficiency solar cells will have. 10. Since the output of solar cells is in the form of direct current, they cannot be directly fed into electrical grids. IV. Fill-in the Blanks 1. The angular distance measured east or west from the Prime (International) Meridian is called the ___________. 2. The amount of solar energy that reaches a surface on earth depends on the position of the surface and __________. 3. The first OTEC plant was constructed off the coast of ____________. 4. For a solar pond to work, there must be a higher concentration of _________ at the bottom than at the top. 5. A typical 10 cm by 10 cm commercial cell produces as much as _________ watts of electrical power. 6. ___________ cells are advantageous over flat panel cells, because flat panel cells only collect maximum sunlight if the sun is directly overhead. 7. Current PV technologies have typical commercial module conversion efficiencies between ____________ percents. 8. OTEC systems using water as the working fluid are of the ___________ type. 9. _______________ plastics are special kinds of plastics that collect light and convert it to electricity. 241 29. Which of the following statements is correct in regard to solar ponds? a. For a solar pond to be effective, surface water temperature must be about 50oC warmer than the water at the bottom of the pond. b. Temperature as high as 95oC can be reached at the bottom. c. Works best in areas of high winds. d. Works best in the same geographic areas that OTEC technologies are applicable. e. All of the above. 30. Thermophotovoltaics a. Is another name for solar cells b. Works best with visible lights c. Is most sensitive to ultraviolet lights d. Used mainly when waste heat is available e. Operates only when temperatures in excess of 2000oC are available III. True or False? 1. The Sun is the ultimate source of all our energy. 2. Albedo is another name for reflectivity of the surfaces on the earth. 3. Factors affecting light intensity are cloud cover, humidity, and atmospheric conditions. 4. The total energy incident on a southward-facing window is greater during summer than winter. 5. OTEC technology is an economical way of producing power from a few kW to hundreds of MW. 6. The fraction of solar energy in the visible range is called solar insolation. 7. Since dishes have a smaller aperture than trough reflectors, these systems are best suited for smallscale power production or as a stand-alone unit in 10. The higher the ____________, the higher will be the collection efficiency of the incident solar radiation. V. Project I - Solar Power Generation The objective of this project is to design a power generation plant capable of producing 10 MW of electricity. The power plant is located in a tropical region along a coast with plenty of sunshine. The ocean water is 27°C at the surface but drops to 12°C at a depth of about 500 meters. On the average, 1,200 W/m2 of solar insolation is available. Three options are being considered: a. Photovoltaic solar cells using poly-crystalline silicon cells with an efficiency of around 15%. b. A closed OTEC system using ammonia as the working fluid. Ammonia has a maximum temperature of 25 degrees and a minimum temperature of 15 degrees centigrade. 30% of the power generated is wasted through various frictional losses. c. A solar heliostat composed of plane and parabolic mirrors. The heliostat can heat water to steam at 500°C. Calculate: 1. For option a, the total area of photovoltaic cells required. 2. For option b, the theoretical maximum and actual efficiency of the OTEC plant. 3. For option c, calculate: a. The ideal Carnot efficiency (Eq. 5-1), b. The total amount of heat that must be disposed into the atmosphere. Hint: Recall that efficiency is the ratio of work delivered per heat input, and that conservation of energy requires that the part of energy not converted to work must be disposed into the atmospheric sink. Project II - Solar Photovoltaics for Homes You are asked to evaluate the economics and the environmental merits of building a solar electric system for your home instead of buying electricity from your local utility. Obtain a copy of your electric bill (desirably for a peak month) and note the following: a. The monthly electric bill ($) b. The total electric consumption for the month (kWh) Calculate: 1. Average cost of unit electricity ($/kWh) 2. Average power (kW) 3. Average solar insolation for your area. Insolation maps and tables are generated from historical data and are readily available for many cities. US data can be found from the National Renewable Energy Laboratory web site at (http://rredc.nrel.gov/solar/pubs/redbook). 4. Pick a commercial manufacturer and determine cost, peak and average power delivered, and efficiency. How much solar cell do you need? What is the cost? 5. Calculate peak power available from your solar system. 6. Estimate the total cost by adding the cost of the cell, the inverter, and other control devices you may need. Add 50% for the cost of installation. 7. What is the payback period? 8. Assuming your utility company uses natural gas to produce electricity, how much less carbon dioxide are you producing annually (tons/year) when you choose the solar option? 242