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4.4 Thermoelectric materials Thermoelectric materials are usually used for small applications where engines cannot be used due to their relatively larger size. The use of waste heat in combustion engines promises to be a high-volume field of application (Niu et al, 2009). Thermoelectric materials generate power based on the heat flux through thermoelectric elements. The heat flux is driven by temperature difference across the elements where voltages are produced based on actual temperature differences. For the opposite effect, it is also known as the Peltier–Seebeck effect, when a voltage is applied a temperature difference is created (Peltier effect). At the atomic scale an applied temperature gradient results in charged carriers in the material, either electrons or holes, to diffuse from the hot side to the cold side, hence the thermally induced current. A low thermal conductivity is desirable attribute (Agbossou et al, 2010). Most work on typical efficiencies of around 5–10% (Penella and Gusulla, 2007; Cook-Chennault et al, 2008). Commercial thermoelectric generators range from μW to kW in electrical output. Material properties are the key parameter for improving both output power (increase of thermal heat flow) and efficiency (improvement of Seebeck coefficient). The main problem is maintaining a high temperature gradient, for electrical energy conversion especially when the loading is time-variable, like solar heating. The harvested energy is directly proportional to the temperature gradient, where the proportionality coefficient depends the thermoelectric materials used. To maintain a relatively low temperature on the cold face of the thermoelectric generator, it is always connected to a metallic heat sink exposed to the air. There are two problems with this widely used system especially in solar applications: • The temperature on the cold face of the thermoelectric generator rises rapidly when the air around the heat sink is heated by solar radiation and the convection between them decreases; and • The system can only work during the day, when solar radiation acts directly as the heat source. A solution is to use phase change materials as a source of constant temperature heat sink. If a PCM has sufficient latent heat, a suitable fusion temperature and sufficient volume, the temperature on the cold face will remain relatively stable over the whole day, as the PCM’s temperature does not change enormously when it becomes liquid. When external heat source is removed, the role is reversed; PCM becomes the heat source while the external environment becomes heat sink. For solar-driven thermoelectric systems, the efficiency of whole system (ηsystem) can be described as: ηsystem = COP x ηpv where COP is the coefficient of performance for the thermoelectric refrigeration at usually less than 0.6 and ηpv is the efficiency of the photovoltaic cell with an average of 0.1. Hence, the average efficiency of a solar-driven thermoelectric system is usually less than 0.06 (Li et al, 2007). NASA and other space organisations started a dual-use technology project to develop solar-driven refrigeration technology. But due to the low COP or low energy 49PDF Image | EPSRC Thermal Management of Industrial Processes
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