Thermoelectric generation from solar water heater excess heat

Abstract

Solar water heater installations in hot climate zones such as Malta produce significant amount of excess heat such that some suppliers suggest to partial shading of the collectors to avoid operation at the upper limit of the system. Thermoelectric generation could be the ideal candidate to recover some of this heat and avoid having to shade the solar collector. A literature review on thermoelectric generation was carried out to better understand the principal of operation and how to improve the performance at the design stage. Thermoelectric conversion occurs when a thermoelectric module is subject to a temperature difference across its surfaces and heat is allowed to flow across. As a result an electromotive force is generated across its terminals. A detailed explanation of the process is presented in the literature review and theory section. A thermoelectric cooler module operates in reverse mode, i.e. when voltage is applied across its terminals, current flows through and as a result heat flows from one side of the module to the other. Thermoelectric generator modules are specifically built for power generation but alternatively thermoelectric cooling can be used in reverse operation. The advantage of using cooling modules for power generation is a reduction of 90% in modules cost. This figure is based on the purchase cost of the modules used in this project. Lower temperature applications are less common due to reduced thermoelectric conversion efficiency. However higher temperature studies were analyzed to assess the conclusions presented and acquire data to compare with the results of this project. Five different setups were built to increase knowledge on thermoelectric generation. Based on this data a final setup of 4 thermoelectric modules was built and connected to a flat-plate solar water heater. Performance comparison between cooler and generator modules is presented. Data from different setups was compared to each other and to results presented in the literature review. The results of all setups were superior or equivalent to those present in the literature review. However this does not necessarily mean that this project produced better results. The performance of a thermoelectric setup is assessed by the generated power against temperature v difference. It is explained that the difference in performance may be related to the ambiguity of temperature measurement. The ambiguity is in how temperature sensing is done, that is, at the cooling medium and hot source entry point or directly over the module surfaces.