Solar Thermal Power Systems

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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 Table 10-1. Solar Concentrators CR Tmax hmax Flat Plate Collectors 1 100°C 20% Parabolic Trough 10-50 400°C 56% Parabolic Dishes 30-1000 1200°C 75% Power Tower --- 2000°C 85% 9 Manchini, T., et al., Solar Thermal Power, in Advances in Solar Energy, Vol. 11, American Solar Energy Society, Boulder, CO, 1997. 10 Readers 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). Figure 10-6 Parabolic trough and parabolic dish. (10-1) 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. 226 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 north-south 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 10-10 Molten salt was substituted for water as the working fluid used in Solar Two demonstration project. Figure 10-9 Solar Two. Source: National Renewable Energy Laboratoty. 11 Schlaich, J., The Solar Chimney: Electricity from the Sun, C. Maurer, Geislingen, Germany, 1995. 12 Von Backström, T.W. and Gannon, A.J., “Compressible Flow through Solar Power Plant Chimneys”. ASME Journal of Solar Energy Engineering, Vol. 122, No. 3, pp. 138-145, 2000. 227 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. Which 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 13 BrightSource Energy Inc., http://www.brightsourceenergy.com. 14 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. 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) Table 10-2. Characteristics of Concentrating Solar Power Systems Parabolic Trough Power Tower Dish / Engine Status Commercially Available Demonstration Stage Prototype Stage Plant Size1 30-320 MW 10-200 MW 5-25 kW Operating Temp (°C) 390 565 750 Concentration Ratio 10-0


30-1000 Peak Efficiency 21% 23% 29% Annual Efficiency2 14-18% 14-19% 18-23% Capacity Factor3 25-50% 20-77% 25% 1 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 228 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

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