Research On The Key Techniques Of High Power Thermoelectric Conversion And Its Testing Methods

With the implementation of the national energy conservation and emissions reduction as well as the development of environmental protection economy, improving power usage effectiveness has become the key subject in the energy field. Thermoelectric conversion technology provides recycling of low grade energy, such as industrial waste heat, waste heat and automobile exhaust gas emission heat. It covers thermoelectric materials, thermoelectric module, hot and cold side heat transfer, output control and characteristic parameter test. Based on current technology, the researches were carried out on high power thermoelectric conversion conditions, which were high heat flux and large temperature difference between hot and cold side, high output power, greater influence of the structural parameters on the TEM output. We have built TEM accurate calculation model for high power output and optimized TEM structural parameters and determined the optimum condition. Moreover, the conventional TEM power output testing technologies were analyzed systematically and from the perspective of thermoelectric conversion and the heat transfer mechanism were showed that there were large measurement errors when these test methods were applied to high power TEM. Aiming at high-power TEM, a rapid estimation method suitable for engineering and an accurate measurement method for reducing the test time consuming have been proposed. And the results verified by practical measuring are accurate, which provide reliable and effective testing methods and evaluation basis for high-power TEM characteristic parameters. Finally, based on the research foundation above, the 300 W thermoelectric conversion system consisting of mult TEMs was designed and then on test and analysis to gain some useful conclusions which have a guiding significance for the high-power thermoelectric conversion system design. The main contents of this research are as follows:The accurate TEM calculation model at large temperature difference, considering nonlinear and air heat transfer around thermoelectric couples of TEMs, has been presented which could be used to modify the traditional computing methods of output power and conversion efficiency and to reduce the theoretical calculation error of high-power TEM. Combined with actual temperature characteristic of thermoelectric materials and based on large temperature difference of high-power TEM and heat transfer between cold and hot side currently only through legs, we analyzed the impact of air medium on cold and hot side heat transfer at large temperature difference and combined the thermoelectric technology with heat transfer theory to propose a new and practical calculation model for large power TEM considering the heat transfer of air. This new model decreased the computing errors of traditional output power and conversion efficiency. By introducing the integrated thermal resistance(ITR) parameters closely associated with TEM structure, the TEM calculation model with ITR, legs geometry size, thermoelectric materials(Seebeck coefficient, thermal and electrical conductivity) as characteristic parameters has been deduced to implement output performance unity of various types of TEM. By comparing typical TEM characteristic parameter measurement, the calculation error of the newly-built model conversion efficiency is much smaller than that of the existing model, especially under large temperature difference. The calculation results are more accurate and reliable. The new model taking TEM cold and hot side(not junction) temperatures and ITR which represents comprehensive structure, as variable is more practical, and realizes the combination of the structure parameters and characteristic to lay a theoretical basis for TEM structure design optimization.On the basis of the new model combined with classical theory, the research on the design and optimization of high-power TEM was carried out, including material optimization and hot stress analysis of P-N couples at large temperature difference and analysis of ITR impact on properties. The optimization for ITR was presented and the conventional current corresponding to maximum output power was modified to obtain the best working current of high-power TEM, considering the parasitic and contact thermal and the additional resistance. This established a theoretical basis for TEM maximum power outputting and had a guiding for high-power thermoelectric conversion system and the matching design of the load and is consistent with theoretical analysis after actual testing and validation.Another main contents of this research is high-power TEM key structure parameters and output performance test. Through combination of thermoelectric theory and heat transfer with testing technology, an accurate method to test actually TEM integrated thermal resistance(ITR) in the new model, which is closely related to TEM optimization, has been implemented for the first time. Especially, by using symmetric pressure regulation system, the test process simulates TEM pressure operation state, which TEM contact thermal resistances are consistent with the actual ones. So, the data obtained are more real and reliable, which has provided a design basis for high-power TEM structure optimization and has significant for TEM structural improvement.The existing power output testing methods were also analyzed systematically, and from the theoretical and testing mechanism it was pointed out that conventional volt-ampere characteristic method and short circuit current method for low resistance and high-power TEM had defects of great error in the test. The fundamental cause that measuring results of short circuit current method was greater than the actual value was analyzed from the two aspects of constant and variable material properties and the more practical maximum output power correction method has been first deduced, which is very suitable for engineering application. It reduced the measuring error of a high-power TEM using traditional short-current and Volt-ampere characteristic method. Measurement accuracy has been improved significantly and test time-consuming is less than classical method’s.In view of the uncertainty of test results of present commonly used conventional volt-ampere characteristic method and by combining theoretical analysis with classic load matching method, a new accurate testing method for TEM maximum power output has been proposed, which not only keeps the accurate advantage of the classic method, but also simplifies the testing process to save much testing time by selecting the appropriate load values. This method is especially not affected by such additional resistances as the lead resistance, switch contact resistance and so on, thus effectively solving testing accuracy of the low internal resistance TEM influenced by the these resistances, so it is a practical test method for high-power TEM.Based on the above theory and test plan, we have established the comprehensive test platform for module maximum output power and conversion efficiency of thermoelectric module, including actual test system and test method, to provide an effective experiment means and powerful data support for high-power TEM application.Taking flat wall-like conversion system as our object, the high-power thermoelectric conversion system has been designed. By integrating TEM, hot and cold side heat exchanger, the maximum power point tracking(MPPT) controller and circulating cooling system, the 300 W power thermoelectric conversion system with multi-module has been designed and completed, and the hierarchical design and the optimization scheme have been put forward for high-power thermoelectric conversion system with a complex temperature field which is of greater heat source temperature fluctuation and with multiple and variety TEMs, which explores a feasible way for the design and the implementation of the k W higher-power thermoelectric conversion system.