Final Report: Thermoelectric Power Harvesting SystemsEPA Grant Number: SU834328
Title: Thermoelectric Power Harvesting Systems
Investigators: Jones, Matthew , Allred, Jacob , Chamberlin, Skyler , Christiansen, John , Edwards, David , Lefevre, Jeremy , Naegle, Stephen
Institution: Brigham Young University
EPA Project Officer: Nolt-Helms, Cynthia
Project Period: August 15, 2009 through August 14, 2010
Project Amount: $10,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2009) RFA Text | Recipients Lists
Research Category: Nanotechnology , Pollution Prevention/Sustainable Development , P3 Challenge Area - Energy , P3 Awards , Sustainability
It is generally acknowledged that the resources and processes currently used to produce electrical power are not sustainable. The importance of inexpensive and reliable energy supplies to the global economy and to social stability cannot be overstated. Social and political instability in oil exporting countries and the impact of current energy use patterns on the environment require immediate efforts to use energy resources wisely and to extract useful power from all potential sources.
The opportunity to extract useful electrical power from waste heat has led to the rise of a field known as energy harvesting. Thermoelectric power harvesting systems convert thermal energy available in a fluid stream directly into electrical power. Likely sources are the effluent from coal fired or nuclear power plants. Other sources include the waste heat from alternative power sources such as solar thermal or geothermal power plants or waste heat exhausted from a wide variety of industrial processes. It is even conceivable that exhaust streams from common household appliances (gas-fired furnaces, water heaters, clothes dryers, etc.) could be economically converted into electrical power. The development of robust, economically viable thermoelectric power harvesting systems will reduce the consumption of fossil fuels by increasing the overall efficiency of power producing and power consuming systems. This project focused on the development of tools needed to perform system-level optimization of power harvesting systems based on thermoelectric generators (TEGs).
TEGs are robust, direct energy conversion devices, and the fact that these devices produce electrical power without moving parts makes them particularly attractive in applications where reliability and maintenance are of the utmost importance. Although the fundamental operating principles of TEGs are well understood, their low efficiencies have limited their use to date to highly specialized applications. Recent advancements in the areas of nanotechnology and thin film manufacturing processes, however, have improved the performance of thermoelectric generators.
Although improved device performance is critical to the development of more efficient power harvesters, the coupling of the device to a heat source and a heat sink also is critical. The objective of this research was to test the hypothesis that optimization of the thermal pathways between a TEG and its source and sink can significantly improve the efficiency of TEG-based power harvesting systems.
TEGs are a type of heat engine. This means that when they are thermally coupled to a thermal source and a thermal sink, a fraction of the heat flowing through the device is converted into electrical power. The amount of power produced by a power harvester may be expressed as
where ε is the second law or exergetic efficiency of the device, TL is the temperature on the cold side of the device, TH is the temperature on the hot side of the device, and QH is the rate heat is extracted from the source and channeled through the device. It should be noted that the middle term on the right hand side of Equation (1) is the Carnot efficiency.
Equation (1) shows that the power output of the harvester will be maximized by maximizing the second law efficiency of the device, by minimizing the ratio of temperature on the cold side of the device to the temperature on the hot side of the device and by maximizing the rate heat flows from the thermal source through the device. Increasing the second law efficiency of TEGs has been the focus of a significant research effort over the last couple of decades. Improvements have been achieved by engineering materials such that their thermal conductivity is decreased while maintaining relatively high electrical conductivity. These advances are highly significant, but Equation (1) clearly shows that optimization of the thermal pathways that connect the device to its thermal source and sink also is critical to maximizing the power produced by an energy harvesting system. At first, it may seem that it would be ideal to design the thermal pathways such that TH is as high as possible and TL is as low as possible, thereby maximizing the Carnot efficiency of the system. Note that the maximum possible value for TH is the temperature of the source, Ts, and that the minimum possible value for TL is the ambient temperature, Ta. It is important to realize, however, that the rate at which heat flows from the source through the device depends on the magnitude of the difference between Ta and TH. These considerations indicate that the thermal pathways of an energy harvesting system must be carefully engineered to ensure that the product of the Carnot efficiency and the heat rate from the source are maximized.
Because optimization of the thermal pathways in an energy harvesting system is critical, it is necessary to be able to accurately model the rate at which heat can be extracted from the source and channeled through the TEG. A model capable of predicting the rate at which heat can be extracted from an air stream with a finned heat sink has been developed using the concept of thermal resistances. The accuracy of this model has been verified by comparison with a detailed, three-dimensional model, which was obtained using commercial computational fluid dynamics (CFD) software. The thermal resistance model also was verified by comparing its predictions with measurements obtained from a test station, which has been designed and built to test the performance of waste heat recovery systems.
The power produced using various heat sink combinations under various operating conditions was measured using the test station. The performance of a power harvesting system equipped with finned heat sinks was as great as 26 times that of the unfinned system. Power harvesting systems equipped with carefully designed fin arrays produce two to three times as much power as systems equipped with course fin arrays. These results clearly demonstrate the need to carefully engineer the thermal pathways connecting the device to the thermal source and the thermal sink.
Three-dimensional, CFD simulations of the heat flowing through a power harvesting system with a course 3 by 3 fin array demonstrated that the thermal resistance model predicted the heat flowing through the TEG to within 17 percent on average. The output of a power harvesting system with 3 by 3 fin arrays on both the hot and cold sides was measured in the test station under six different sets of operating conditions, and these results were compared to values predicted by the thermal resistance model. The measured and predicted results agree to within the uncertainty of the thermal resistance model. Based on the agreement between the predictions of the thermal resistance model and both the CFD simulations and the experimental results, it was concluded that the thermal resistance model is valid.
Thermoelectric generator technology is being extensively researched, and the efficiency of these devices is improving. The overall efficiency of a TEG-based power harvesting system, however, depends not only on the efficiency of the TEG itself, but also on the extent to which heat can be extracted from an exhaust stream and channeled through the device. It is, therefore, very important to carefully engineer the thermal pathways that connect the device to its thermal source and to its thermal sink in order to maximize the power harvested by the system as a whole.
Proposed Phase II Objectives and Strategies:
The objective of Phase 1 is to create the tools and methodology needed to design heat sinks for any given operating conditions and thereby maximize the output from TEG-based power harvesting systems. This objective was achieved by the development of the thermal resistance model. This model was validated using CFD simulations and verified by comparing its predicted power output with experimental results.
The proposed research objectives for Phase II are to:
- Increase the accuracy of the thermal resistance model.
- Investigate the use of different types of fin arrays.
- Investigate the use of pyroelectric generators (PEG) and compare their performance to the performance of TEGs.