Evaluation of thermoelectric modules for power generation
Introduction
The relatively low conversion efficiency of thermoelectric modules (∼5%) has been a major factor in limiting their applications in electrical power generation and has restricted their use to specialised situations where reliability is a major consideration [1]. However, one exception is the thermoelectric recovery of waste heat when it is unnecessary to consider the cost of the thermal input [2]. Consequently, the low conversion efficiency is not a serious drawback. The primary consideration in this application is to optimise the thermoelectric module to provide maximum power output. A previous investigation has shown that the power output of thermoelectric modules can be improved by optimising the thermoelement length [3]. Accompanying recent progress in this field is an urgent need for reliable information on the generating performances of thermoelectric modules. Generally, the power output and conversion efficiency provide a rough estimation of module performance when operating in the generating mode. However, additional information such as the power-per-unit-area, cost-per-watt, quality of module fabrication and reliability is required to evaluate its commercial potential for its intended application and to identify areas of further improvement in module performance. Furthermore, this information would also facilitate a direct comparison of currently available modules of different design and assembled using different fabrication processes.
Section snippets
Maximum power output and conversion efficiency
Two types of commercially available multicouple thermoelectric modules are shown schematically in Fig. 1a–b. Type A was originally designed for cooling applications and possesses significant inter-thermoelement separation. In this type of module, n- and p-type semiconductor thermoelements are connected electrically in series by highly conducting metal strips and sandwiched between thermally conducting but electrically insulating plates. Type B has been developed recently for power generation
Manufacture quality factor (MQF)
Maximum power output and conversion efficiency of a thermoelectric module provide useful information on its performance as a generator. However, as indicated by , , both the maximum power output and conversion efficiency depend upon temperature difference, thermocouple materials, module geometry and contact parameters, the latter being closely associated with the module fabrication process. The performance of a module, having thermoelements with a fixed geometry, fabricated from a given
Power-per-area and cost-per-watt
The power-per-area of a module is defined as the ratio P/Am, where P is the power output and Am the area of the module, while the power per area of thermoelements is defined as the ratio P/2NA, where A is the cross-sectional area of a thermoelement and N the number of the thermocouples employed in a module. There are basically two types of commercially available thermoelectric modules. Modules denoted I and II in Table 1 possess a type A configuration, while the module denoted III is type B. In
Reliability and failure mechanisms
The cost-per-watt of a module provides a measure of its economic performance to some extent. However, the ultimate cost of the electricity generated using a thermoelectric module is a function of the operating period and consequently, related to its reliability. In general, the cost of electricity generated thermoelectrically using waste heat is given by,In Fig. 6 is shown the cost of electricity as a function of the operating period for several modules at
Conclusions
Thermoelectric modules which were originally developed for cooling applications also exhibit a promising performance for electrical power generation using waste heat in the temperature range 300–400 K. The results reported in this paper show that a cost-per-watt of about £4/W can readily be obtained using commercially available modules with an appropriate thermoelement length. A detailed analysis of module performance indicated that a further reduction in cost-per-watt can be obtained using
Acknowledgements
This work is supported by New Energy and Industrial Development Organisation, Japan. Power output of a number of modules were measured by Dr. S.G.K. Williams and some reliability measurements was taken by Professor L.W. Fu.
References (9)
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