title: Maximization of power generation from thermoelectric generators operating under constant heat flux
Creators: name: Compadre Torrecilla, Marcos
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: MPPT, TEG, constant heat flux, boost converter, power electronics.

description: Thermoelectric generators (TEGs) are used to convert thermal energy into electricity. TEGs present an emissions-free source of power and, despite the low efficiency they offer, with typical values of 5%, they can be used to harvest waste-heat energy in different type of applications. The high robustness presented by TEGs allows their use in low-maintenance applications. TEGs can operate under two different conditions: constant temperature gradient or constant input heat flux. When a TEG operates under constant temperature gradient, the input heat flux varies with the electrical operating conditions of the TEG devices. Under these conditions the TEG is modeled by a constant voltage source with a constant resistance in series with the voltage source. When operated under constant heat flux, the temperature gradient of the TEG changes with the electrical operating conditions of the device. In this situation of constant heat flux, both the equivalent voltage source and the resistance in series with it change their values with the electrical operating point. The location of the Maximum Power Point, or MPP, of the TEG is different in both operating conditions. In constant temperature gradient the MPP is located at half of the instantaneous open-circuit voltage of the TEG, whereas under constant heat flux the MPP is located at an electrical point higher than half of the instantaneous open-circuit voltage. DC/DC converters are mainly used to operate TEGs at the MPP and Maximum Power Point Tracking (MPPT) techniques are used to operate the TEG at the MPP. Due to the difference in the location of the MPP between constant temperature gradient and constant input heat flux, the MPPT techniques will be different between these two operating conditions. This thesis focuses in the study of the location and MPPT techniques for TEGs operated under constant heat flux. A computational model of the TEG for its operation under constant heat flux is first developed. The model of the TEG is then interfaced with the model of a boost, or step-up, converter, which implements a new MPPT algorithm to operate the TEG at the true MPP. The output energy of the power converter is used to charge a lithium-ion (Li-Ion) battery. The complete model of the TEG system is then used to compare the new algorithm proposed in this thesis against two state-of-the-art algorithms: the Fractional Open-Circuit method and the Perturb and Observe method. The comparison is made under three different input heat flux profiles: constant heat flux, ramp-varying heat flux and step-changing heat flux. The last chapter of this thesis presents a hardware implementation of the TEG system and the MPPT power converter. Experimental results are presented for the new and the two state-of-the-art algorithms and a comparison between the three algorithms are presented for the three different input heat flux profiles described previously. The TEG model and the MPPT algorithm presented in this work can be applied to any TEG applications where the TEG operates under constant heat flux.