Efficiently maximising power generation from thermoelectric generators

Thermoelectric generators (TEGs) convert the thermal energy flowing through them into DC electrical energy in a quantity dependant on the temperature difference across them and the electrical load applied, with a conversion efficiency of typically 5%. Nonetheless, they can be successfully employed to recover energy from waste heat and their use has increased rapidly in recent years, with applications ranging from microwatts to kilowatts, due to energy policy legislations and increasing energy cost determined by climate change, environmental issues and availability of energy sources. The performance of TEGs, subject to thermal and electrical effects, can vary considerably depending on the operating conditions, therefore it is necessary to measure and characterise their performance, and to understand their dynamic behaviour and interaction with the other parts of the system. Based on this knowledge it is then desired to develop an effective electronic system able to control these devices so as to maximise the power generated and increase the overall efficiency of the system. Several TEGs can be electrically connected in series and/or parallel (forming an array) to provide the required voltage and/or current. However, TEGs are usually employed in environments with time-varying temperatures, thermal powers and electrical loads. As a consequence in most TEG systems the individual thermoelectric devices can be subject to temperature mismatch due to operating conditions. Therefore it is of relevant importance to accurately simulate the evolution of thermoelectric systems during thermal and electrical transients. At the same time accurate experimental performance data are necessary to permit precise simulations. Unfortunately, there is still no standardised method to test the electrical and thermal performance of TEGs. This thesis tackles these key challenges and contributes to the pool of existing knowledge about TEGs dealing with four main topics: testing of thermoelectric generators, simulation of thermoelectric generating systems, design and production of power electronic converters for thermoelectric generators, and physical applications of thermoelectric generators. After an introduction to the physical phenomena underlying the operation of TEGs, this thesis describes the innovative test system built at the University of Glasgow to assess the performance of TEG devices in the ”real-world”. The fixture allows a single TEG device to be tested with thermal input power up to 1 kW and hot temperature up to 800◦C with minimal thermal losses and thermal shock; the mechanical clamping force can be adjusted up to 5 kN, and the temperatures are sensed by thermocouples placed directly on the TEGs surfaces. A computer program controls all the instruments in order to minimise errors and to aid accurate measurement and test repeatability. The test rig can measure four TEGs simultaneously, each one individually controlled and heated. This allows testing the performance of TEG arrays under mismatched conditions, e.g., dimensions, clamping force, temperature, etc. Under these circumstances experimental results and a mathematical analysis show that when in operation each TEG in the array will have a different electrical operating point at which maximum energy can be extracted and problems of decreased power output arise. This thesis provides the transient solution to the one-dimensional heat conduction equation with internal heat generation that describes the transfer and generation of heat throughout a thermoelectric device with dynamic exchange of heat through the hot and cold sides. This solution is then included in a model in which the Peltier effect, the thermal masses and the electrical behaviour of the system are also considered. The resulting model is created in Simulink and the comparison with experimental results from a TEG system confirms the accuracy of the simulation tool to predict the evolution of the thermoelectric system both in steady-state and during thermal or electrical transients. This thesis presents an investigation of the optimum electrical operating load to maximise the power produced by a TEG. Both fixed temperature difference and fixed thermal input power conditions are considered. Power electronic converters controlled by a Maximum Power Point Tracking (MPPT) algorithm are used to maximise the power transfer from the TEG to the load. The MPPT method based on the open-circuit voltage is arguably the most suitable for the almost linear electrical characteristic of TEGs. An innovative way to perform the open-circuit voltage measurement during the pseudo-normal operation of the power converter is presented. This MPPT technique is supported by theoretical analysis and used to control an efficient synchronous Buck-Boost converter capable of interfacing TEGs over a wide range of temperatures. The prototype MPPT converter is controlled by an inexpensive microcontroller, and a lead-acid battery is used to accumulate the harvested energy. Experimental results using commercial TEG devices demonstrate the ability of the MPPT converter to accurately track the maximum power point during steady-state and thermal transients. This thesis also presents two practical applications of TEGs. The first application exploits the thermal energy generated by a stove to concurrently produce electrical energy and heat water, while the second application recovers the heat energy rejected to ambient by a car’s exhaust gas system to generate electrical energy for battery charging.