• Energy Research

Abstract Liquid-to-liquid thermoelectric generators are now being considered for the purpose of converting low cost heat to electricity for local energy uses. The importance in investigating their system efficiency lies in the fact that the generator's purpose is to maintain a heat source and a heat sink for its embedded thermoelectric modules. Of particular importance is the generator's ability to maintain an asymmetric thermal field across its embedded modules since this mechanism partially dictates the devices' thermal to electric conversion efficiency. Indeed, since the modules' semiconductor materials' ability to generate an electromotive force is dependent on the quality of the thermal dipole across the material, gains in thermoelectric generator energy conversion efficiency are made possible with thermal system management. In an effort to improve the system conversion efficiency of a liquid-to-liquid thermoelectric generator (TEG), the present work builds upon recent advancements in TEG inner pipe flow optimisation by investigating the thermoelectric power enhancement brought upon by flow impeding panel inserts in a thermoelectric generator's flow channels for fixed thermal input conditions and with respect to varying insert panel densities. The pumping penalty associated with the flow impedance is measured in order to present and to discuss the net thermoelectric power enhancement.

Abstract The conversion of solar energy to electricity currently relies primarily on the photovoltaic effect in which photon bombardment of photovoltaic cells drives an electromotive force within the material. Alternatively, recent studies have investigated the potential of converting solar radiation to electricity by way of the Seebeck effect in which charge carrier mobility is generated by an asymmetric thermal differential. The present study builds upon these latest advancements in the state-of-the-art of thermoelectric system management by combining solar evacuated tube technology with commercially available Bismuth Telluride semiconductor modules. The target heat source is solar radiation and the target heat sink is thermal convection into the ambient air relying on wind aided forced convection. These sources of energy are reproduced in a laboratory controlled environment in order to maintain a thermal dipole across a thermoelectric module. The apparatus is then tested in a natural environment. The novelty of the present work lies in a net thermoelectric power gain for ambient environment applications and an experimental validation of theoretical electrical characteristics relative to a varying electrical load.

Abstract The current state-of-the art in photovoltaic technology observes that the photoelectric conversion efficiency decreases with increasing material temperature. A proposed solution is to use the thermoelectric effect as a heat pump to manage the excess thermal energy of a photovoltaic cell. Previous investigations into photoelectric–thermoelectric hybrid systems have been either limited to theoretical analyses or experimental set-ups in which external refrigeration pumps are employed or an external heat source – other than photoelectric waste-heat – is used to drive the thermoelectric effect. The present work experimental investigates the viability of converting photoelectric waste-heat into electricity by way of the thermoelectric effect in an effort to better manage a photovoltaic module’s conversion efficiency. To this end, the electrical performance of a photovoltaic module and a thermoelectric module which are thermally in series is reported and discussed.

Abstract Thermoelectric power production has many potential applications that range from microelectronics heat management to large scale industrial waste-heat recovery. A low thermoelectric conversion efficiency of the current state of the art prevents wide spread use of thermoelectric modules. The difficulties lie in material conversion efficiency, module design, and thermal system management. The present study investigates thermoelectric power improvement due to heat transfer enhancement at the channel walls of a liquid-to-liquid thermoelectric generator brought upon by flow turbulating inserts. Care is taken to measure the adverse pressure drop due to the presence of flow impeding obstacles in order to measure the net thermoelectric power enhancement relative to an absence of inserts. The results illustrate the power enhancement performance of three different geometric forms fitted into the channels of a thermoelectric generator. Spiral inserts are shown to offer a minimal improvement in thermoelectric power production whereas inserts with protruding panels are shown to be the most effective. Measurements of the thermal enhancement factor which represents the ratio of heat flux into heat flux out of a channel and numerical simulations of the internal flow velocity field attribute the thermal enhancement resulting in the thermoelectric power improvement to thermal and velocity field synergy.

Thermoelectric behavior in semiconductor materials is investigated for the purpose of converting thermal energy to electrical energy. This paper measures and quantifies the variations in the internal electrical and thermal resistances of a thermoelectric module with respect to the circuit’s electrical load in order to better characterize its behavior in a closed circuit. Since open-circuit measurements fail to accurately describe thermoelectric material properties in power generation mode, the load-bearing Seebeck coefficient and the load-bearing figure-of-merit are defined, measured, and discussed. It is shown that the load-bearing Seebeck and the load-bearing figure-of-merit are more accurate parameters in measuring a module’s ability to operate in thermoelectric power generation mode. In particular, there exist peak load-bearing characteristics that are not identified using the conventional open-circuit definitions of the Seebeck coefficient and the figure-of-merit. The results show that the peak load-bearing figure-of-merit is compared with the peak power output showing a misalignment between the two quantities, the significance of which is discussed.

Abstract The electrical characteristics of a solar thermoelectric generator (STEG) are measured. The STEG device is novel in that it requires no external pumps such as a water refrigeration unit to maintain a thermal differential. The heat source is simulated solar radiation heating a double walled evacuated tube. The heat sink is a CPU cooler subjected to simulated wind energy. The peak power performance of the STEG for a fixed heat input and a varying forced convection brought upon by the artificial wind is reported. The STEG’s thermoelectric power output under naturally occurring solar and wind energy is also reported. A discussion on the thermoelectric effect presents experimental results showing the effect of varying the circuit’s load resistance on a Bi 2 Te 3 module’s electromotive force, electrical resistance, and thermal resistivity.

Abstract Due to an abundance of low cost waste-heat in the industrial and residential sector, many studies in recent years have focused on applications of low grade heat for local energy needs. These include heat reutilization, thermal conversion to mechanical energy and thermal conversion to electricity. The thermoelectric effect presents a promising potential for effective conversion of low grade waste-heat yet is currently limited in application due to a conversion efficiency that is not cost effective. The present work focuses on mechanical methods to improve the thermal tension driving the electromotive force responsible for thermoelectric power production. More specifically, flow impeding geometries are inserted into the flow channels of a liquid-to-liquid thermoelectric generator thereby enhancing the heat transfer near its embedded thermoelectric modules. Consequentially, the thermal dipole across the modules is increased improving the overall power output. Care is taken to measure the adverse pressure drop caused by the use of the flow impeding geometries in order to evaluate the net power output. This net thermoelectric power output is measured, reported and discussed for a fixed inlet temperature difference, a fixed electrical load, varying flow rates and varying insert geometries.

This paper builds upon the recent progress made in the field of thermoelectric energy conversion when using Bismuth Telluride Bi2Te3 semiconductor modules. These commercially available modules have been the subject of many recent studies in which the common goal is to better understand their thermoelectric behavior when converting a low cost heat source to electricity. The present experimental work investigates the thermopower properties of a single module relative to the electrical load resistance with the use of two experimental apparatuses. The first test stand is built with a precision control of the injection and rejection of heat to and from the module; the second test stand is a novel demonstration of the module’s application to thermoelectric solar energy conversion. The thermopower characteristics of the module are measured over a wide range of thermal input conditions. The results highlight the importance in calibrating to an optimal electrical load for peak power output. A normalized thermopower theoretical evolution curve relative to load resistance is presented. Furthermore, a method of thermoelectric recovery of solar radiation is demonstrated using laboratory controlled working conditions.