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Experimental and computational investigation of passive heat exchangers to enhance the performance of a geothermal thermoelectric generator
(Elsevier, 2024) Pascual Lezaun, Nerea; Alegría Cía, Patricia; Araiz Vega, Miguel; Martínez Echeverri, Álvaro; Astrain Ulibarrena, David; Ingeniería; Ingeniaritza; Institute of Smart Cities - ISC
Thermoelectric devices hold significant promise for generating electricity from geothermal heat, enabling the powering of measuring equipment in remote locations without the need for moving parts. Nevertheless, most developed geothermal thermoelectric generators employ fans and pumps to enhance heat transfer, thereby compromising the robustness and reliability inherent to thermoelectricity. Furthermore, there is a lack of research on passive heat exchangers for geothermal thermoelectric generators, particularly in studying their operation under a wide range of meteorological conditions. Therefore, this paper conducts a comprehensive analysis of passive heat exchangers for the cold side of the generators. Phase-change-based heat exchangers differing in their length and fluid are studied experimentally, along with a fin dissipator. Additionally, the influence of wind velocity on heat transfer and mechanical requirements is further explored through a Computational Fluid Dynamics model. The most significant outcome is quantifying the impact of the design parameters and operational variables on the electrical production of the thermoelectric generator. Accordingly, this research aims to broaden the application of these generators to extreme environments, such as Deception Island in Antarctica. Under average operational conditions, generators incorporating 400 mm water heat pipes generate 0.95 W per thermoelectric module, while those incorporating heat pipes with methanol achieve an average of 0.70 W. Moreover, water and methanol-based systems produce 120% and 60% more power than generators using a fin dissipator. Nonetheless, for temperatures beyond -6.5 °C, water might freeze and the methanol-based heat exchangers become more suitable.
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Design and analysis of a two-stage cascade system for heating and hot water production in nearly zero-energy buildings using thermoelectric technology
(MDPI, 2024-12-16) Ordóñez, Javier ; Díaz de Garayo, Sergio; Martínez Echeverri, Álvaro; Algarra Pérez, Fernando; Astrain Ulibarrena, David; Ingeniería; Ingeniaritza; Institute of Smart Cities - ISC; Gobierno de Navarra / Nafarroako Gobernua
This paper proposes an innovative system that integrates two thermoelectric heat pumps (one air–water and the other water–water) with two thermal storage tanks at different temperatures to provide heating and domestic hot water to a 73.3 m2 passive-house-certified dwelling in Pamplona (Spain). The air–water thermoelectric heat pump extracts heat from the ambient air and provides heat to a tank at intermediate temperature, which supplies water to a radiant floor. The water–water heat pump takes heat from this tank and provides heat to the other tank, at higher temperature, which supplies domestic hot water. The system performance and comfort conditions are computationally analyzed during the month of January under the climate of Pamplona and under different European climates. The COP of the system lays between 1.3 and 1.7, depending on the climate, because of the low COP of the air–water thermoelectric heat pump. However, it is able to provide water for the radiant floor and to maintain the temperature of the dwelling above 20 °C 99.8% of the time. Moreover, it provides domestic hot water at a temperature above 43 °C 99.9% of the time. Noteworthy is the fact that the water–water heat pump presents a COP close to 4, which opens up the possibilities of working in combination with more efficient heat pumps for the first stage.
Open Access
Net thermoelectric power generation improvement through heat transfer optimization
(Elsevier, 2017) Aranguren Garacochea, Patricia; Astrain Ulibarrena, David; Rodríguez García, Antonio; Martínez Echeverri, Álvaro; Ingeniería Mecánica, Energética y de Materiales; Mekanika, Energetika eta Materialen Ingeniaritza
Thermoelectric generation contributes to obtain a more sustainable energetic system giving its potential to harvest waste heat and convert it into electric power. In the present study a computational optimal net generation of 108.05 MWh/year was produced out of the flue gases of a real tile furnace located in Spain (the equivalent to supply the energy to 31 Spanish dwellings). This maximum generation has been obtained through the optimization of the hot and cold heat exchangers, the number of thermoelectric modules (TEMs) installed and the mass flows of the refrigerants, including the temperature loss of the flue gases and the influence of the heat power to dissipate over the heat dissipators. The results are conclusive, the installation of more TEMs does not always imply higher thermoelectric generation, so the occupancy ratio (δ) has to be optimized. The optimal generation has been achieved covering the 42 % of the surface of the chimney of the tile furnace with TEMs and using heat pipes on the cold side, which present smaller thermal resistances than the finned dissipators for similar consumptions of their fans. Moreover, the high influence of the consumption of the auxiliary equipment shows the importance of considering it to obtain realistic usable electric energy from real applications.
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Study of a complete thermoelectric generator behavior including water-to-ambient heat dissipation on the cold side
(Springer US, 2014) Aranguren Garacochea, Patricia; Astrain Ulibarrena, David; Martínez Echeverri, Álvaro; Ingeniería Mecánica, Energética y de Materiales; Mekanika, Energetika eta Materialen Ingeniaritza
The reduction of the thermal resistances of the heat exchangers of a thermoelectric generation system (TEG), leads to a significant increase in the TEG efficiency. For the cold side of a thermoelectric module (TEM), a wide range of heat exchangers has been studied, form simple finned dissipators to more complex water (water-glycol) heat exchangers. As Nusselt numbers are much higher in water heat exchangers than in conventional air finned dissipators, convective thermal resistances are better. However, to conclude which heat exchanger leads to higher efficiencies, it is necessary to include the whole system involved in the heat dissipation, that is, TEM-to-water heat exchanger, water-to-ambient heat exchanger, as well as the required pumps and fans. This paper presents a dynamic computational model able to simulate the complete behavior of a TEG, including both heat exchangers. The model uses the heat transfer and hydraulic equations to compute TEM-to-water and water-to-ambient thermal resistances, along with the resistance of the hot side heat exchanger at different operating conditions. Likewise, the model includes all the thermoelectric effect with temperature-dependent properties. The model calculates the net power generation at different configurations, providing a methodology to design and optimize the heat exchange in order to maximize the net power generation for a whole variety of TEGs.
Open Access
Thermoelectric self-cooling for power electronics: increasing the cooling power
(Elsevier, 2016) Martínez Echeverri, Álvaro; Astrain Ulibarrena, David; Aranguren Garacochea, Patricia; Ingeniería Mecánica, Energética y de Materiales; Mekanika, Energetika eta Materialen Ingeniaritza
Thermoelectric self-cooling was firstly conceived to increase, without electricity consumption, the cooling power of passive cooling systems. This paper studies the combination of heat pipe exchangers and thermoelectric self-cooling, and demonstrates its applicability to the cooling of power electronics. Experimental tests indicate that source-to-ambient thermal resistance reduces by around 30% when thermoelectric self-cooling system is installed, compared to that of the heat pipe exchanger under natural convection. Neither additional electric power nor cooling fluids are required. This thermal resistance reaches 0.346 K/W for a heat flux of 24.1 kW/m2, being one order of magnitude lower than that obtained in previous designs. In addition, the system adapts to the cooling demand, reducing this thermal resistance for increasing heat. Simulation tests have indicated that simple system modifications allow relevant improvements in the cooling power. Replacement of a thermoelectric module with a thermal bridge leads to 33.54 kW/m2 of top cooling power. Likewise, thermoelectric modules with shorter legs and higher number of pairs lead to a top cooling power of 44.17 kW/m2. These results demonstrate the applicability of thermoelectric self-cooling to power electronics.
Open Access
Experimental and computational study on thermoelectric generators using thermosyphons with phase change as heat exchangers
(Elsevier, 2017) Araiz Vega, Miguel; Martínez Echeverri, Álvaro; Astrain Ulibarrena, David; Aranguren Garacochea, Patricia; Mekanika, Energetika eta Materialen Ingeniaritza; Institute for Advanced Materials and Mathematics - INAMAT2; Ingeniería Mecánica, Energética y de Materiales
An important issue in thermoelectric generators is the thermal design of the heat exchangers since it can improve their performance by increasing the heat absorbed or dissipated by the thermoelectric modules. Due to its several advantages, compared to conventional dissipation systems, a thermosyphon heat exchanger with phase change is proposed to be placed on the cold side of thermoelectric generators. Some of these advantages are: high heat-transfer rates; absence of moving parts and lack of auxiliary con- sumption (because fans or pumps are not required); and the fact that these systems are wickless. A com- putational model is developed to design and predict the behaviour of this heat exchangers. Furthermore, a prototype has been built and tested in order to demonstrate its performance and validate the compu- tational model. The model predicts the thermal resistance of the heat exchanger with a relative error in the interval [?8.09;7.83] in the 95% of the cases. Finally, the use of thermosyphons with phase change in thermoelectric generators has been studied in a waste-heat recovery application, stating that including them on the cold side of the generators improves the net thermoelectric production by 36% compared to that obtained with finned dissipators under forced convection.
Open Access
Influence of temperature and aging on the thermal contact resistance in thermoelectric devices
(Springer, 2020) Rodríguez García, Antonio; Pérez Artieda, Miren Gurutze; Beisti Antoñanzas, Irene; Astrain Ulibarrena, David; Martínez Echeverri, Álvaro; Ingeniaritza; Institute of Smart Cities - ISC; Ingeniería
During thermal design in the first phases development, thermoelectric systems, such as thermoelectric generators, the most important parameter affecting the performance is thermal resistance of the components. This paper focusses on the thermal contact resistance (TCR), analyzing the influence of aging and temperature on different thermal interface materials (TIMs), i.e., thermal paste, graphite and indium. In previous papers, TCR has been studied depending on parameters such as surface roughness, bonding pressure, thermal conductivity and surface hardness. However, in thermoelectric applications, a relevant aspect to consider when choosing a TIM is aging due to thermal stress. The exposure of this type of material to high temperatures for long periods of time leads to deterioration, which causes an increase in the TCR impairing the conduction of the heat flow. Therefore, there is a need to study the behavior of TIMs exposed to temperatures typical in thermoelectric generators to make a correct selection of the TIM. It has been observed that exposure to temperatures of around 180°C induces a significant increase in the thermal impedance of the three TIMs under study, although this effect is much more relevant for thermal paste. The contact, comprising steel, thermal paste and ceramic, presents a 300% increase in the thermal impedance after 70 days of aging, whereas that exceeds 185% for the contact of aluminum, thermal paste and ceramic. In the tests with exposure temperature of 60°C, there is no observed decrease in the thermal impedance.
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Heat pipes thermal performance for a reversible thermoelectric cooler-heat pump for a nZEB
(Elsevier, 2019) Aranguren Garacochea, Patricia; Díaz de Garayo, Sergio; Martínez Echeverri, Álvaro; Araiz Vega, Miguel; Astrain Ulibarrena, David; Ingeniaritza; Institute of Smart Cities - ISC; Ingeniería
The nZEB standards reduce the energy demand of these buildings to a minimum, obtaining this little energy from renewable resources. Taking these aspect into consideration, a thermoelectric cooler-heat pump is proposed to achieve the comfort temperature along the whole year. The same device can provide heat in winter and it can cool down the buildings in summer just by switching the voltage supply polarity. Heat pipes are studied to work on both sides of the thermoelectric modules in order to optimize the heat transfer as these devices present really good thermal resistances and they can work in any position. However, they present pretty different thermal resistances if they work on the cold or on the hot side of the modules. A methodology to thermally characterize these heat exchangers working in both orientations is proposed and a validated computational model is developed to optimize the thermoelectric cooler-heat pump for a nZEB application. The number of thermoelectric modules, the position of the device, the ambient temperature and the air mass flow determine the operation and consequently they need to be studied in order to optimize the application.
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Study of thermoelectric systems applied to electric power generation
(Elsevier, 2009) Rodríguez García, Antonio; González Vian, José; Astrain Ulibarrena, David; Martínez Echeverri, Álvaro; Ingeniería Mecánica, Energética y de Materiales; Mekanika, Energetika eta Materialen Ingeniaritza
A computational model has been developed in order to simulate the thermal and electric behaviour of the thermoelectric generators. This model solves the non linear system of equations of the thermoelectric and heat transfer equations. The inputs of the program are the thermoelectric parameters as a function of the temperature and the boundary conditions, (room temperature and residual heat flux). The outputs are the temperature values of all the elements forming the thermoelectric generator, (performance, electric power, voltage and electric current generated). The model solves the equation system using the finite difference method and semi-empiric expressions for the convection coefficients. It has been built a thermoelectric electric power generation test bench in order to validate and determine the accuracy of the computational model, which maximum error is lower than 5%. The objective of this study is to create a design tool that allows us to solve the system of equations involved in the electric generation process without needing to impose boundary conditions that are not known in the design phase, as the temperature of the Peltier modules. With the computational model we study the influence of the heat flux supplied as well as the room temperature in the electric power generated.
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Zero-power-consumption thermoelectric system to prevent overheating in solar collectors
(Elsevier, 2014) Martínez Echeverri, Álvaro; Astrain Ulibarrena, David; Rodríguez García, Antonio; Ingeniería Mecánica, Energética y de Materiales; Mekanika, Energetika eta Materialen Ingeniaritza
Highly promoted by the European Union Climate and Energy Package for 2020, solar collectors stand out as the most promising alternative to meet water heating demands. One of the most limiting problems in these systems involves the overheating of the working fluid, resulting in rapid fluid degradation, scaling and premature component failure. This paper presents the computational design of a zero-power-consumption system that combines thermoelectric-self-cooling technology and thermosyphon effect to dissipate the excess heat from a real solar-collector installation. Thermoelectric self-cooling is a novel thermoelectric application proven to enhance the heat dissipation of any hot spot without electricity consumption. The simplest design outperforms currently-used static and dynamic dissipaters for overheating protection in solar collectors, since it increases the global heat transfer coefficient of a static dissipater by 75 % and requires no electricity. Likewise, the final design presents a global heat transfer coefficient of 15.23 W/(m2K), 155 % higher than that provided by static dissipaters, forming a reliable, robust and autonomous system that stands out as a promising alternative to prevent the overheating of solar collectors.