Maximum power point tracking converter based on the open-circuit voltage method for thermoelectric generators

Thermoelectric generators (TEGs) convert heat energy into electricity in a quantity dependant on the temperature difference across them and the electrical load applied. It is critical to track the optimum electrical operating point through the use of power electronic converters controlled by a Maximum Power Point Tracking (MPPT) algorithm. The MPPT method based on the opencircuit voltage is arguably the most suitable for the linear electrical characteristic of TEGs. This paper presents an innovative way to perform the open-circuit voltage measure during the pseudo-normal operation of the interfacing power electronic converter. The proposed MPPT technique is supported by theoretical analysis and used to control a synchronous buck-boost converter. 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 prove that the converter accurately tracks the maximum power point during thermal transients. Precise measurements in steady state show that the converter finds the maximum power point with a tracking efficiency of 99.85%

The effect of temperature mismatch on thermoelectric generators electrically connected in series and parallel

The use of thermoelectric generators (TEGs) to recover useful energy from waste heat has increased rapidly in recent years with applications ranging from microwatts to kilowatts. Several thermoelectric modules can be connected in series and/or parallel (forming an array) to provide the required voltage and/or current. In most TEG systems the individual thermoelectric modules are subject to temperature mismatch due to operating conditions. Variability of the electro-thermal performance and mechanical clamping pressure of individual TEG modules are also sufficient to cause a significant mismatch. Consequently, 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.<p></p> This work analyses the impact of thermal imbalance on the power produced at module and system level in a TEG array. Experimental results clearly illustrate the issue and a theoretical model is presented to quantify the impact. The authors believe the experimental results presented in this paper are the first to validate a rigorous examination of the impact of mismatched operating temperatures on the power output of an array of thermoelectric generators

Constant heat characterisation and geometrical optimisation of thermoelectric generators

It is well known that for a thermoelectric generator (TEG) in thermal steady-state with constant temperature difference across it the maximum power point is found at half of the open-circuit voltage (or half of the short-circuit current). However, the effective thermal resistance of the TEG changes depending on the current drawn by the load in accordance with the parasitic Peltier effect. This article analyses the different case in which the input thermal power is constant and the temperature difference across the TEG varies depending on its effective thermal resistance. This situation occurs in most waste heat recovery applications because the available thermal power is at any time limited. The first part of this article presents the electrical characterisation of TEGs for constant-heat and it investigates the relationship between maximum power point and open-circuit voltage. The second part studies the maximum power that can be produced by TEGs with pellets (or legs) of different size and number, i.e. with different packing factors, and of different height. This work provides advice on the optimisation of the pellets geometrical parameters in order to increase the power generated, and consequently the thermodynamic efficiency, and to minimise the quantity of thermoelectric material used, for systems with limited input thermal power.</p

Multi-disciplinary robust design of variable speed wind turbines

This paper addresses the preliminary robust multi-disciplinary design of small wind turbines. The turbine to be designed is assumed to be connected to the grid by means of power electronic converters. The main input parameter is the yearly wind distribution at the selected site, and it is represented by means of a Weibull distribution. The objective function is the electrical energy delivered yearly to the grid. Aerodynamic and electrical characteristics are fully coupled and modelled by means of low- and medium-fidelity models. Uncertainty affecting the blade geometry is considered, and a multi-objective hybrid evolutionary algorithm code is used to maximise the mean value of the yearly energy production and minimise its variance

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

A combined heat and power system for solid-fuel stoves using thermoelectric generators

Solid-fuel stoves are used in developing countries, remote locations, and in general more commonly due to convenient fuel cost. The possibility of using the stove heat to heat water and produce electricity represents an added benefit. This work presents an application of thermoelectric generators to a solid-fuel stove to concurrently charge a lead-acid battery and transfer heat to water for heating or household use. The feasibility of the proposed CHP system is demonstrated for a common solid-fuel stove. This system produces an average of 600 Wth and 27 Wel during a 2-h long experiment, in which the TEG efficiency is around 5% and the MPPT efficiency of the power converters used is demonstrated

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&amp;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&amp;O or IC for TEGs operating under constant heat flux. A step-up (boost) dc-dc converter controlled by P&amp;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

Outdoor performance of a reflective type 3D LCPV system under different climatic conditions

Concentrating sunlight and focusing on smaller solar cells increases the power output per unit solar cell area. In the present study, we highlight the design of a low concentrating photovoltaic (LCPV) system and its performance in different test conditions. The system essentially consists of a reflective type 3.6× cross compound parabolic concentrator (CCPC) designed for an acceptance angle of ± 30°, coupled with square shaped laser grooved buried contact (LGBC) silicon solar cells. A heat exchanger is also integrated with the PV system which extracts the thermal energy rejected by the solar cells whilst maintaining its temperature. Indoor characterization is carried out to evaluate the system performance under standard conditions. Results showed a power ratio of 3.12 and an optical efficiency of 73%. The system is placed under outdoor environment on a south facing roof at Penryn, UK with a fixed angular tilt of 50°. The high angular acceptance of the system allows collection of sunlight over a wider range. Results under different climatic conditions are presented and compared with a non-concentrating system under similar conditions. On an average, the LCPV system was found to collect an average of 2.54 times more solar energy than a system without the concentrator