Maximization of power generation from thermoelectric generators operating under constant heat flux

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

Transient response of a thermoelectric generator to load steps under constant heat flux

Most waste heat recovery applications involve a heat source that provides a limited heat flux that can be converted into electricity by a thermoelectric generator (TEG). When a TEG is used under limited or constant heat flux conditions the temperature difference across the device cannot be considered constant and will change depending on the electrical current generated by the TEG. This phenomenon is induced by the Peltier effect, which works against power generation and deviates the optimum operating point from the commonly known maximum power point (MPP). This point, dictated by the maximum power transfer theorem, is achieved when the source equivalent series resistance and the load resistance are equal, in conditions of constant temperature difference. Hence maximum power point tracking (MPPT) algorithms that regulate the TEG at half of the instantaneous open-circuit voltage are optimized only for applications where the TEG operates under constant temperature difference but are not ideal for constant heat flux conditions. Hill climbing MPPT methods, e.g., perturb-and-observe (P&O) or incremental conductance (IC), can reach the MPP more accurately if the sampling time is extended to the thermal time constant of the system. This article presents an analysis of the transient electrical and thermal response of a TEG to a load change. This investigation results fundamental to the design of MPPT algorithms such P&O or IC for TEGs operating under constant heat flux. A step-up (boost) dc-dc converter controlled by P&O is used to demonstrate the effects of the sampling time over of the transient response and hence the tracking performance of the MPPT algorithm

Multi-Output Power Converter, Operated from a Regulated Input Bus, for the Sireus Rate Sensor

This paper describes a DC to DC converter designed to meet the power supply requirements of the SiREUS Coarse Rate Sensor (CRS) which is a 3-axis MEMS Rate Sensor (MRS) that uses a resonating ring gyro and will be used in different ESA missions. The converter supplies +5V, −5V, 3.3V, 1.8V and 40V and it has been designed and prototyped by Clyde Space Ltd with the EQM and FM units being manufactured by Selex ES. The first model was designed for a 28V un-regulated bus and the second model presented here has been designed for a 50V regulated bus. PWM voltage regulation was not used because of the noise requirements and the regulated input bus allowed an unregulated power stage approach. There are also stringent volume and interface constraints, which also affected the design. For such reasons, a fixed dutycycle, quasi-resonant single-ended topology with output linear regulators has been implemented; having the advantages of providing low switching losses, low radiated and conducted noise and no over-voltage failure mode. This paper highlights the techniques used to satisfy stringent noise and protection requirements of the load

Multi-Output Power Converter, Operated from a Regulated Input Bus, for the Sireus Rate Sensor

This paper describes a DC to DC converter designed to meet the power supply requirements of the SiREUS Coarse Rate Sensor (CRS) which is a 3-axis MEMS Rate Sensor (MRS) that uses a resonating ring gyro and will be used in different ESA missions. The converter supplies +5V, −5V, 3.3V, 1.8V and 40V and it has been designed and prototyped by Clyde Space Ltd with the EQM and FM units being manufactured by Selex ES. The first model was designed for a 28V un-regulated bus and the second model presented here has been designed for a 50V regulated bus. PWM voltage regulation was not used because of the noise requirements and the regulated input bus allowed an unregulated power stage approach. There are also stringent volume and interface constraints, which also affected the design. For such reasons, a fixed dutycycle, quasi-resonant single-ended topology with output linear regulators has been implemented; having the advantages of providing low switching losses, low radiated and conducted noise and no over-voltage failure mode. This paper highlights the techniques used to satisfy stringent noise and protection requirements of the load

Multi-Output Power Converter, Operated from a Regulated Input Bus, for the Sireus Rate Sensor

This paper describes a DC to DC converter designed to meet the power supply requirements of the SiREUS Coarse Rate Sensor (CRS) which is a 3-axis MEMS Rate Sensor (MRS) that uses a resonating ring gyro and will be used in different ESA missions. The converter supplies +5V, −5V, 3.3V, 1.8V and 40V and it has been designed and prototyped by Clyde Space Ltd with the EQM and FM units being manufactured by Selex ES. The first model was designed for a 28V un-regulated bus and the second model presented here has been designed for a 50V regulated bus. PWM voltage regulation was not used because of the noise requirements and the regulated input bus allowed an unregulated power stage approach. There are also stringent volume and interface constraints, which also affected the design. For such reasons, a fixed dutycycle, quasi-resonant single-ended topology with output linear regulators has been implemented; having the advantages of providing low switching losses, low radiated and conducted noise and no over-voltage failure mode. This paper highlights the techniques used to satisfy stringent noise and protection requirements of the load