Direct Thermoelectric Generators

In the last decade, much effort has been expended on developing novel thermoelectric (TE) materials of increased intrinsic conversion efficiency [9] at same time, the design of the system architecture plays an important role in optimizing the thermal exchange and in maximizing the conversion performance [10]. For that reason, in this section we report an example of a detailed thermal management analysis with the heat re-flowed in the system. 

The efficiency in TE power generation depends on the intrinsic characteristics of the material and on the temperature ratio between the hot and the cold sides [11]  

Here we will concentrate on system design and a typical value of ZT = 1 will be chosen for all working temperatures. The electric power generation as a fraction of the thermal heat flux is given by the generic relationship for thermal cycles [14]  

The basic hypothesis is that the temperature profiles of the two flows are parallel. This means that the heat exchanged at the hot side equals that exchanged at the cold side. This is an approximation since extracting electric power from the heat flux implies a difference between the two heat fluxes. However, the hypothesis, with only a minor influence on the final result, makes the equations compact and easy to manage. 

The temperature gap (AT) between the two flows is chosen as the controlling parameter it determines both the enthalpy feed and the Carnot efficiency of the thermoelectric element. The value of AT is related to the heat exchanger efficiency Tiexc or e-NTU (normal thermal unit), the ratio of the heat exchanged to the total exchangeable heat. This relationship comes from the definition of e-NTU for the exchanger efficiency and, in this specific case, it has the following form [16]  

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Figure 4.10 Direct thermoelectric generators will compete in efficiency with internal combustion engines when thermoelectric materials with figures of merit ZT of the order of 2 are available at temperatures above 900 K.

Antimonides of formulas CdSb and Cd2Sb2 have been reported. Both are usually prepared by direct union of the elements, the former is a hole-type semiconductor (9), with properties shown in Table 1, and finds use as a thermoelectric generator. Reagent-grade material costs 2.00/g in small lots. The band gap energy is 0.46 eV (2.70 J.m) (31) is 138 kj/mol (33.0 kcal/mol). Dicadmium triantimonideCd2Sb2, is a metastable, white... 

The most important isotope of plutonium is Pu = 24,200 years). It has a short half-life so only ultra traces of plutonium occur naturally in uranium ores, and most plutonium is artificial, being an abundant byproduct of uranium fission in nuclear power reactors. The nuclear reactions involved include the radiative capture of a thermal neutron by uranium, U( , y) U the uranium-239 produced is a beta-emitter that yields the radionuclide Np, also a beta-emitter that yields Pu. To date, 15 isotopes of plutonium are known, taking into account nuclear isomers. The plutonium isotope Pu is an alpha-emitter with a half-life of 87 years. Therefore, it is well suited for electrical power generation for devices that must function without direct maintenance for time scales approximating a human lifetime. It is therefore used in radioisotope thermoelectric generators such as those powering the Galileo and Cassini space probes. 

These anodes are coupled to the structure via the external source of electrical power. This source can be in the form of batteries, thermoelectric generators, generators or photovoltaic cells. Most commonly, however, alternating current line voltage is converted to direct current by a rectifier. 

The other primary thermoelectric phenomenon is the Peltier effect, which is the generation or absorption of heat at the junction of two different conductors when a current flows in the circuit. Whether the heat is evolved or absorbed is determined by the direction of the current flow. The amount of heat involved is determined by the magnitude of the current, I, and the Peltier coefficients, 7T, of the materials ... 

The thermoelectric semiconductor converts directly thermal energy into electric one. It can generate electric power even when the temperature difference between the heat source and atmosphere is much smaller compared with the one needed in conventional thermal power generation. 

An alternative approach is to employ the thermal energy from the solar concentrator to generate electricity directly by means of a PV cell based on a semiconductor with a low band gap e.g., 0.6 eV) that is sensitive to infrared radiation. Such thermo-photovoltaic cells are already in small-scale specialized use by the military, although with the heat provided by a propane burner, rather than from the Sun. This is an alternative to the thermoelectric effect for converting heat to electricity. 

Fig. 6.1 a The Seebeck effect an electrical potential difference is induced across a thermoelectric material when a temperature difference is imposed generating an electrical current, i.e. solid state conversion of heat into electrical energy, b The Peltier effect a p-n junction absorbs or releases heat when current passes through the junction, in the respective directions, c An example of a thermoelectric module consisting of an array of p- couples connected electrically in series and thermally in parallel... 

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