Thermoelectric Generator Concept If two semiconducting materials connect a high and a low temperature reservoir, then due to this temperature difference an electric current is generated. The phenomenon behind this power production is known as the Seebeck effect. A schematic look at this system is presented in Figure 7.13. Unfortunately, this single-stage generator delivers very low output voltage. A simple way of overcoming this limitation is to have a multistage generator by staging a series of couples. Then the output voltage is N times the voltage of one couple where N is the number of couples. This implies that the electrical power output and the thermal input needed in the hot reservoir increases linearly with N. Figure 7.13 Model of a Thermoelectric Generator [Angrist, 1982] To have a better feeling of his technology's potential, let's summarize the relevant quantities that a designer would be interested in knowing in order to accomplish a specific task. The actual calculations can be found in. [Angrist, 1982] Let's consider a thermoelectric generator that operates between 27 'Cand 327 °C. The n-type material is made of 75% of Bi2Te3 and 25% of Bi2Se3 while the p-type is 25% of Bi2Te3 and 75% of Sb2Te3. The design for maximum power will able an output of 1.48 Vwith a current of 23.2 A. In this case the power delivered is 13.04 W. The thermal efficiency for this generator is 11.03%. This technology has up to now be used for radioisotope thermoelectric converters. It has provided power independently of solar flux. This has been particularly advantageous for deep space probes. This concept could nevertheless be used for generating energy from the sun by using the solar radiation as the heat source for the hot reservoir. However, we should remember that the radioisotope thermoelectric generator have two main disadvantages which are the high cost (10 M$/kW) and the high weight (200 kg/kW). Thermophotovoltaic (TPV) Generator Thermophotovoltaic energy generator is a system in which heat is first converted to radiant energy. After, it is converted to electrical energy by a photovoltaic cell (PV). The Figure 7.14 shows such a concept. Selective emitters based on the rare Earth oxides-solid are the most promising. Contrary to most solids that emit in a continuous band, the rare Earth materials emit in narrow bands which can be matched with the absorption band of the solar cell. The following materials promise the largest efficiency at a moderate temperature (1500 K). These are the oxides from neodymium (Nd2O3), holmium (HO2O3), erbium (Er2O3), ytterbium (Yb2O3).
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