Thermoelectric Topping Cycle for Trough Solar Thermal Power Plant
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1218-Z07-05
Thermoelectric Topping Cycle for Trough Solar Thermal Power Plant Andrew J. Muto1 and Gang Chen1 MIT Mechanical Engineering Department, 77 Mass Ave, Cambridge, MA 02139, U.S.A.
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ABSTRACT Solar thermal power generation is fast becoming cost competitive for utility scale electricity. Parabolic trough concentrators have proven economical and reliable but their efficiency is limited by the maximum temperature of the heated fluid. This work will explore the possibility of adding a thermoelectric power generator (TEG) as a topping cycle at high temperature to increase the overall efficiency of the system. In this design the perimeter of the receiver tube is covered with thermoelectrics so that the absorber temperature is raised and the energy rejected from the TEG is used to heat the fluid at its originally specified temperature. A heat transfer analysis was carried out on the design to determine the overall system efficiency. Our findings show the state of the art solar thermal trough collector is not a good application for a ZT=1 thermoelectric material. To increase the overall power output of the system by approximately 10% would require a ZT=3 material. INTRODUCTION Solar thermal power generation is fast becoming cost competitive for utility scale electricity with 380 MW electric, currently installed[1]. Solar thermal plants consist of large arrays of collectors which concentrate solar radiation onto a receiver. Fluid flows through the receiver and is heated to a temperature high enough to generate steam that is then used in a Rankine cycle to generate electricity. Solar thermal is attractive for utilities because it uses conventional Rankine cycle turbomachinery, and offers the possibility of 6-15 hours thermal storage so that steam can be generated even during the night[1]. The majority of existing solar thermal installations employ parabolic collectors because they are usually economically favorable over other collector types. Parabolic collectors can have an aperature width >5 m which focuses light onto to a tubular receiver, figure 1. The receiver consists of an outer glass tube and an inner metal tube carrying the heat transfer fluid, with diameters of 12 cm and 7 cm respectively. The inner tube is coated with a selective absorber surface, meaning that the surface has a high absorptivity α in the visible spectrum and a low emissivity ε in the IR spectrum. The annulus of the receiver is evacuated to eliminate air conduction and convection losses from the selective surface.
Figure 1. Left: Cartoon of a solar parabolic trough collector array. Right: photograph of the receiver tube.
A major factor limiting trough power plant efficiency is the maximum temperature of the heat transfer fluid which in most cases is oil which is stable up to 400°C. This work will explore the possibility of adding a solar thermoelectric topping cycle (STET) within the receiver to increase the overall efficiency. In this design the perimeter of the receiver tube is covered by thermoelectric (TE) elements thus raising the
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