Designing a Mechanically Robust Thermoelectric Module for High Temperature Application

We report a numerical study on the impacts of variations in the geometry, boundary conditions, and the coefficient of thermal expansion of the materials on the maximum shearing stress in thermoelectric power generator module (TEM) for high temperature applications. The maximum shearing stress in the TEM is evaluated for different designs focusing on their dependency on the fill factor. Although predictions by the previously developed analytical modeling are in partial agreement with numerical results, simplifying assumptions for the analytical model can limit the range of validity. Our numerical analysis shows that reduction of the fill factor alone under all the circumstances will not reduce the maximum shear stress. Imposing mechanical constraints at the boundaries, increasing the number of legs (6 × 6 in the analysis), and engineering the coefficient of thermal expansion are some of the key parameters controlling the maximum shearing stress and its changes with the fill factor

A Three-Dimensional Transient Model for Evaluating the Performance of Cement-Based Thermoelectric Module

The thermoelectric module (TEM) is a device that integrates multiple thermoelectric (TE) elements to realize the mutual conversion of heat and power. Due to the advantages of no moving parts and flexible expansion, the application of conventional Bi2Te3-based TEM in buildings has attracted the attention of researchers. On the other hand, the TE behavior of hardened cement composites was found by combining conductive additives with cement. Therefore, a new study on cement-based TEM for building energy harvesting and temperature control is proposed. To simulate the performance of cement-based TEM, a three-dimensional heat transfer model considering temperature-dependent TEM characteristics was established. The validity of the model is verified by comparing the results with commercial simulation software and experiments. Different from the existing analytical models and commercial software, the customized model has greater scalability, optimization, and control flexibility. Through parametric studies, the model can guide the design of TEM and the development of TE cement

Role of Thermal Conductivity for Thermoelectrics with Finite Contacts

The role of thermal conductivity in the performance of thermoelectric (TE) devices as compared to other material properties such as Seebeck coefficient and the electrical conductivity will be discussed. A TE energy conversion system that includes thermal contacts for the hot side and the cold side with finite heat transfer performances is considered. Some of the trends in electronics cooling applications have been described. In this article, the effect of material properties as a function of energy current flow direction will be focused. TEs have an advantage as solid-state heat energy conversion devices especially for applications with spatial constraints. A commonly accepted application is the Peltier cooling of high power or high heat flux electronic devices such as laser diodes (\u3e1 W/mm2). These applications lead to a limited heat transfer performance in both hot and cold contacts. Based on a generic one-dimensional model for TE systems, thermal conductivity appeared to be the most important property to be improved to get better energy conversion performance among three TE properties including Seebeck coefficient and electrical conductivity. For TE cooling, thermal conductivity of the TE material is the most cost sensitive in terms of mass use of the material, for minimizing the power consumption to pump the heat from the target device at the constrained temperature. Typically in electronics, the device temperature is constrained, such as 65oC–85oC. Hence the maximum cooling to reach minimum target temperature is not always required. Achieving this target temperature with minimum electrical power input for a given power dissipation from the target device is required. The difference in impact between thermal conductivity and the other material properties is on the thermal resistance of the TE element similarly to the energy harvesting system case. By changing any properties, a coefficient-of-performance (COP) of cooling yields the same as ZT remains the same. This article will summarize the role of thermal conductivity as the thermal resistance match with finite external contacts in the TE microcooler, while relating the effects of the thermal conductivity, Seebeck coefficient, and electrical conductivity to the finite thermal resistance of the TE microcooler

Energy Optimization for Transcritical CO2 Heat Pumps for Combined Heating and Cooling and Thermal Storage Applications

A transcritical heat pump (THP) cycle using carbon dioxide (CO2) as the refrigerant is known to feature an excellent coefficient of performance (COP) as a thermodynamic system. Using this feature, we are designing and building a system that combines a water-to-water CO2 heat pump with both hot and cold thermal storages know as Thermal Battery (TB) (Blarke, 2012). Smart and effective use of intermittent renewable energy resources (for example solar and wind power) is obtained supplying water heating ( \u3e 70 oC) and cooling services ( \u3c 10 oC) for residential and commercial buildings. Our fundamental hypothesis is that if electricity generated by intermittent sources is destined for thermal end-uses an efficient conversion of electricity to thermal energy and storage enables a flexible power supply. Thermal storage is more cost-effective than any electro-chemical or mechanical storage technology. The usability and the cost effectiveness are critical for smart grid policies on large-scale integration of intermittent renewables. In this paper, we present an analytic thermodynamic model that predicts the effect of temperature and flow rate of hot and cold water circulation on system COP. The analytical predictions are consistent with the experimental results (Sarkar, 2010)

Solar-Combined Thermoelectric Power Generation Simulator

Photovoltaic (PV) devices are gaining popularity in harnessing solar energy as a form of sustainable energy source to generate electricity. However, these devices including tandem PV cells are limited to utilizing only high energy photons from the solar spectrum. This curtails their efficiency restricting them from being employed in mega Watts scale power generation. This study develops a software tool that allows engineers to tap into the wasted wavelengths of the spectrum by adding a thermoelectric (TE) module and a bottoming steam turbine cycle thus spreading the use of the spectrum. The tool allows investigating how power output and thus overall efficiency can be enhanced by combining these systems. In the TE device, solar heat develops a temperature gradient to generate electricity via the Seebeck effect. A steam-driven Rankine cycle through a heat exchanger connects to thermal storage at the bottom side of the TE. This storage allows dispatchability for off-sunlight power demand at a modest cost. The simulation tool built computes expected power output and efficiency at each individual stage of the combined system. The user is at liberty to manipulate material properties such as the band gap of PV materials which is a key parameter to optimize the PV efficiency. Test runs indicate that overall efficiency of power generation has increased up to 50% by the combined system for 1000 suns using optimized band gap and TE module design. This system can be used as a basis for future models in high efficiency distributed energy production

Analysis of a Data Center Using Liquid-Liquid CO2 Heat Pump for Simultaneous Cooling and Heating

Liquid–liquid CO2 heat pump systems are a promising technology for commercial building applications, which require simultaneous heating and cooling. This paper presents the investigation of a data center on the Purdue University, West Lafayette campus. The data center located in the Department of Mathematics is the most energy intensive data center on campus. The cooling load of the data center is approximately 750 kW/hour. The heating season in West Lafayette is 7 to 8 months and the heating load of the buildings is very high during the coldest months. The heating load of the Mathematics building can go to as high as 600 kW/hour during the coldest days of the year. To suffice this simultaneous cooling and heating demand, a liquid-liquid CO2 heat pump is proposed. Presently, the cooling load of the data center is met by eight electrically driven and four steam-driven chillers and the heating load is satisfied by two coal fired and two natural gas boilers. Simulations are performed to compare the proposed CO2 heat pump system with the present system. The assessment shows noteworthy fuel savings and reduction in the CO2 emissions with the system working with a coefficient of performance (COP) of 6.19. If the CO2 heat pump system is installed, 574.92m3/day of natural gas and 751.68 kg/day of coal could be saved on a cold day. The system has the potential to reduce CO2 emissions by 2980.76 kg/day

Study on Energy-Saving Performance of a Novel CO2 Heat Pump with Applications in Dairy Processes

In dairy processes, there are significant simultaneous heating and cooling demands. A novel type of transcritical CO2 heat pump system is proposed, and its features and benefits are introduced. Bassed on the technical characteristics, primary energy-savings, and operating cost aspects, the CO2 heat pump system is simulated and compared to current heating and cooling systems used in dairy plants. The results show that the highest primary energy-saving rate of the CO2 heat pump is 51.5%. For fluid milk and cheese manufacturing processes, the primary energy-saving is 36.2% and 45.1%, respectively. In addition, the operating cost savings of fluid milk and cheese production are evaluated based on the cost structures in the states of Wisconsin, California and New York

Cost Optimization of Thermoelectric Sub-Cooling in Air-cooled CO2 Air Conditioners

This paper presents a cost-effective enhancement of a trans-critical carbon dioxide (CO2) cycle in air-conditioning mode by utilizing a thermoelectric sub-cooler. It is well-documented that the cooling COP of the transcritical CO2 cycle decreases as the ambient air temperature significantly increases above the critical temperature of the refrigerant. A high gas cooler outlet temperature limits the enthalpy of evaporation so that the air-conditioning cooling performance is reduced. Sub-cooling is known as a mitigation method to this problem. However, adding a small-scale heat pump to a residential or light commercial air conditioner can be quite costly. Therefore, a thermoelectric solid-state sub-cooler is proposed. The thermoelectric cooling (TEC) devices utilized in small temperature differences ranging from 5 to 15 oC can be quite efficient since the intrinsic heat loss of the TEC by heat conduction in reverse direction of pumping heat is minimal. Based on the prior work, the optimum design for cost-per-performance shows that the cost for sub-cooling is dominated by the heat exchangers and it is not by the thermoelectric material itself. The TECs are compact and have a low thickness, which is in the range of a few mm. Hence the TEC modules can be integrated into the form factor of a plate heat exchanger. In this study, the cooling COP of the CO2 air conditioner is enhanced by approximately 12% using an optimally designed thermoelectric sub-cooler at an ambient temperature of 35 oC. This potential improvement is based on a figure-of-merit (ZT) of currently available thermoelectric materials (ZT~1). The seasonal primary energy efficiency and the cost performance of the optimized TE sub-cooled CO2 heat pump system will be presented in comparison to other compact sub-cooling technologies

Experimental Study of a CO2 Thermal Battery for Simultaneous Cooling and Heating Applications

This paper presents experimental investigations of the dynamics of a transcritical CO2 heat pump system with two thermal storages for simultaneous cooling and heating application. The preliminary results of the thermal battery are provided using a small-scale test bed that shows the accelerated penetration of renewable energy sources for building heating and cooling applications. The experimental system consists of a CO2 heat pump system with a compressor of 3 kW (1.02x104 BTU/hr) cooling capacity and two water tanks. During operation, the compressor and expansion valve are considered quasi-static. Thermal sensors are located in each of the two tanks to monitor the temperature gradient of water along the vertical orientation of the tank which impacts the overall system performance. Experiments are carried out under different water circulation flow rates for both the gas cooler and the evaporator in the heat pump, as well as under various discharge pressure conditions controlled by different charging rates and expansion valve openings. The impacts of water circulation flow rate and valve opening are reported in an effort to find the optimum coefficient-of-performance (COP). The results show that increasing the water inlet temperature in the gas cooler raises the discharge pressure significantly and drops the COP, whereas increasing the water temperature of the evaporator raises the discharge pressure relatively moderately. Although a larger water flow rate enhances the heat exchanger capacity and system COP, a smaller water flow rate seems to be preferable to maintain the thermal profile of the water tanks and to provide a more stable COP. At higher gas cooler water inlet temperature, the COP tends to increase with closing expansion valve. In this particular setup, the best COP is found to be approximately 7.0 at a specific expansion valve opening and at a discharge pressure between 75 and 83 bars (1088 to 1204 psia). The heating COP negatively corresponds to the water temperature at the gas cooler inlet. Experiments suggest the need of a proper control strategy and a matched tank capacity design. Based on these results, a 20% power enhancement may be possible by controlling the hot and cold water flow rates

Performance Modeling and Analysis of a Thermoelectric Building Envelope for Space Heating

To provide energy-efficient space heating and cooling, a thermoelectric building envelope (TBE) embeds thermoelectric devices in building walls. The thermoelectric device in the building envelope can provide active heating and cooling without requiring refrigerant use and energy transport among subsystems. Thus, the TBE system is energy and environmentally friendly. A few studies experimentally investigated the TBE under limited operating conditions, and only simplified models for the commercial thermoelectric module (TEM) were developed to quantify its performance. A holistic approach to optimum system performance is needed for the optimal system design and operation. The study developed a holistic TBE-building system model in Modelica for system simulation and performance analysis. A theoretical model for a single TEM was first established based on energy conversion and thermoelectric principles. Subsequently, a TBE prototype model combining the TEM model was constructed. The prototype model employing a feedback controller was used in a whole building system simulation for a residential house. The system model computed the overall building energy efficiency and dynamic indoor conditions under varying operating conditions. Simulation results indicate the studied TBE system can meet a heating demand to maintain the desired room temperature at 20 °C when the lowest outdoor temperature is at -26.3 degrees C, with a seasonal heating COP near 1.1, demonstrating a better heating performance than electric heaters. It suggests a potential energy-efficient alternative to the traditional natural gas furnaces and electric heaters for space heating