Person: Alegría Cía, Patricia
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Alegría Cía
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Patricia
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Ingeniería
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ISC. Institute of Smart Cities
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0000-0003-1174-2037
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811965
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Publication Open Access Enhanced behaviour of a passive thermoelectric generator with phase change heat exchangers and radiative cooling(Elsevier, 2023) Astrain Ulibarrena, David; Jaramillo-Fernández, Juliana; Araiz Vega, Miguel; Francone, Achille; Catalán Ros, Leyre; Jacobo-Martín, Alejandra; Alegría Cía, Patricia; Sotomayor-Torres, Clivia M.; Ingeniería; Ingeniaritza; Institute of Smart Cities - ISC; Universidad Pública de Navarra / Nafarroako Unibertsitate PublikoaHeat exchangers are essential to optimize the efficiency of Thermoelectric Generators (TEGs), and heat pipes without fans have proven to be an advantageous design as it maintains the characteristic robustness of thermoelectricity, low maintenance and lack of moving parts. However, the efficiency of these heat exchangers decreases under natural convection conditions, reducing their heat transfer capacity and thus thermoelectric power production. This work reports on a novel heat exchanger that combines for the first time, phase change and radiative cooling in a thermoelectric generator to improve its efficiency and increase the production of electrical energy, specially under natural convection. For this, two thermoelectric generators with heat-pipes on their cold sides have been tested: one with the radiative coating and the other without it. Their thermal resistances have been determined and the electric power output was compared under different working conditions, namely, natural convection and forced convection indoors and outdoors. The experimental tests show a clear reduction of the heat exchanger thermal resistance thanks to the radiative coating and consequently, an increase of electric production 8.3 % with outdoor wind velocities of 1 m/s, and up to 54.8 % under free convection conditions. The application of the radiative surface treatment is shown to result in a more stable electrical energy production, suppressing the drastic decrease in the generated electric power that occurs in thermoelectric generators when they work under free convection.Publication Open Access 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 - ISCThermoelectric 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.Publication Open Access Experimental measurement of thermal conductivity of stereolithography photopolymer resins(Springer, 2022) Oval Trujillo, Añaterve; Rodríguez García, Antonio; Pérez Artieda, Miren Gurutze; Dung Dang, Phuc Yau; Alegría Cía, Patricia; Ingeniaritza; Institute of Smart Cities - ISC; Ingeniería; Gobierno de Navarra / Nafarroako GobernuaThe rise in the use of additive manufacturing highlights the importance of knowing the properties of the materials employed in this technology. Therefore, for the commercialization of thermal applications with this technology, heat management is essential. Here, computational modelling is often utilised to simulate heat transfer in various components, and knowing precisely the values of thermal conductivity is one of the key parameters. In this line of research, this paper includes the experimental study of three different types of resin used in additive manufacturing by stereolithography. Based on a test bench designed by researchers from the Public University of Navarre, which measures thermal contact resistances and thermal conductivities, the thermal conductivity analysis of three kinds of resin is carried out. This measuring machine employs the temperature difference between the faces and the heat flux that crosses the studied sample to determine the mentioned parameters. The thermal conductivity results are successful considering the constitution of the material studied and are consistent with the conductivity values for thermal insulating materials. The ELEGOO standard resin stands out among the others due to its low thermal conductivity of 0.366 W/m K.Publication Open Access Impact of a thermoelectric subcooler heat exchanger on a carbon dioxide transcritical refrigeration facility(Elsevier, 2022) Casi Satrústegui, Álvaro; Aranguren Garacochea, Patricia; Araiz Vega, Miguel; Alegría Cía, Patricia; Astrain Ulibarrena, David; Ingeniaritza; Institute of Smart Cities - ISC; Ingeniería; Gobierno de Navarra / Nafarroako Gobernua; Universidad Pública de Navarra / Nafarroako UnibertsitaTo improve the performance of vapour compression refrigeration cycles, the inclusion of a thermoelectric subcooler for low-medium power units has been the focus of recent studies due to its robustness, compactness and simplicity of operation. In thermoelectric systems, it has been demonstrated that the heat exchangers used in the hot and cold side of the thermoelectric modules have a critical impact in the performance of the system. This influence has not yet been studied for thermoelectric subcooling systems in vapour compression cycles. This work, for the first time, evaluates the impact that the heat exchangers of a thermoelectric subcooler, included in a transcritical carbon dioxide refrigeration cycle, have, in the performance of the refrigeration cycle. The influence is quantified in terms of: optimum working conditions, coefficient of performance and cooling capacity. The results show that, through an optimization of the heat exchangers of the thermoelectric subcooler, the performance improvements on the coefficient of performance using this technology are boosted from 11.96 to 14.75 % and the upgrade in the cooling capacity of the system rises from 21.4 to 26.3 %. Moreover, the optimum gas-cooler working pressure of the system is reduced and the optimum voltage supplied to the thermoelectric modules increases.Publication Open Access Design and optimization of thermoelectric generators for harnessing geothermal anomalies: a computational model and validation with experimental field results(Elsevier, 2024) Alegría Cía, Patricia; Catalán Ros, Leyre; Araiz Vega, Miguel; Erro Iturralde, Irantzu; Astrain Ulibarrena, David; Institute of Smart Cities - ISC; Universidad Pública de Navarra / Nafarroako Unibertsitate PublikoaThermoelectric generators have been recently proved to be a feasible alternative to harness hot dry rock fields with very promising results transforming the geothermal heat into electricity. This research deepens in the study of these generators, developing a versatile computational model that serves as a tool to design and optimize this type of thermoelectric generators. This tool is important to develop this thermoelectric technology on a large scale, to produce clean and renewable electrical energy especially in the Timanfaya National Park, in Lanzarote (Spain), where some of the most important shallow geothermal anomalies in the world are located, in order to promote self-consumption in this zone. However, it could be employed in other areas with different boundary conditions. The model, based in the finite difference method applied to the thermal-electrical analogy of a geothermal thermoelectric generator, has been validated with the experimental field results of two thermoelectric generators installed in two different zones of geothermal anomalies. It has achieved a relative error of less than 10% when predicting the power and between 0.5–1.6% in the annual energy generation, what makes it a very reliable and useful computational tool. The developed model has been employed for the first time to estimate the electrical energy that could be generated if harnessing the characterized area of anomalies in Lanzarote. Here, given the continuity of geothermal energy, 7.24 GWh per year could be generated, which means annually 1.03 MWh/m2.Publication Open Access Thermoelectric generator for high temperature geothermal anomalies: experimental development and field operation(Elsevier, 2023) Alegría Cía, Patricia; Catalán Ros, Leyre; Araiz Vega, Miguel; Casi Satrústegui, Álvaro; Astrain Ulibarrena, David; Institute of Smart Cities - ISC; Universidad Pública de Navarra / Nafarroako Unibertsitate PublikoaIn the current climate and energy context, it is important to develop technologies that permit increase the use of renewable sources such as geothermal energy. Enhancing the use of this renewable source is particularly important in some places, due to its availability and the enormous dependence on fossil fuels, as is the case of the Canary Islands. This work proposes the use of thermoelectric generators with heat exchangers working by phase change to transform the heat from the shallow high temperature geothermal anomalies on the island of Lanzarote directly into electricity, since the use of conventional geothermal power plants would not be possible because they would damage the protected environment. To bring this proposal to reality, this work has succeeded in developing and field-installing a geothermal thermoelectric generator that operates without moving parts thanks to its phase-change heat exchangers. This robust generator do not require maintenance nor auxiliary consumption, and produces a minimal environmental impact, it is noiseless, and the use of water as working fluid makes it completely harmless. The developed device consists of a thermosyphon as hot side heat exchanger, thermoelectric modules and cold side heat exchangers also based in phase change. Tests were carried out in the laboratory at various heat source temperatures and varying the number of thermoelectric modules. It was determined that installing more modules decreases the efficiency per module (from 4.83% with 4 modules to 4.59% with 8 modules at a temperature difference between sources of 235 °C), but for the number of modules tested the total power increases, so the field installation was carried out with 8 modules. After the good results in the laboratory, it was satisfactorily installed at Timanfaya National Park (Lanzarote, Spain) in a borehole with gases at 465 °C. This generator presents a maximum output power of 36 W (4.5 W per module), and is generating 286.94 kWh per year, demonstrating the great potential of the developed thermoelectric generators to build a larger-scale renewable installation.Publication Open Access Experimental development of a novel thermoelectric generator without moving parts to harness shallow hot dry rock fields(Elsevier, 2022) Alegría Cía, Patricia; Catalán Ros, Leyre; Araiz Vega, Miguel; Rodríguez García, Antonio; Astrain Ulibarrena, David; Ingeniaritza; Institute of Smart Cities - ISC; IngenieríaNowadays, geothermal energy in shallow hot dry rock fields is not exploited enough due to the high economic and environmental impact as well as the lack of scalability of the existing technologies. Here, thermoelectricity has a great future potential due to its robustness, absence of moving parts and modularity. However, the efficiency of a thermoelectric generator depends highly on the heat exchangers. In this work, a novel geothermal thermoelectric generator is experimentally developed, characterizing different configurations of biphasic heat exchangers to obtain low thermal resistances that allow the maximum efficiency in the thermoelectric modules. As a result, robust and passive heat exchangers were obtained with thermal resistances of 0.07 K/W and 0.4 K/W in the hot and cold sides, respectively. The geothermal thermoelectric generator was built with the most effective heat exchangers and was experimented under different temperature and convection conditions, generating 36 W (17 W by a prototype with 10 modules and 19 W by a prototype with 6 modules) for a temperature difference of 160 °C between the heat source and the environment. Furthermore, the experimental development showed that it is possible to increase electricity generation with a more compact generator, since a decrease in the number of modules from 10 to 6 increases the efficiency from 3.72% to 4.06%. With this research, the feasibility of a novel and robust geothermal thermoelectric generator whose working principle is phase change has been experimentally demonstrated, as well as the importance of compactness to maximize its efficiency and thus, power generation.Publication Embargo Diseño, análisis y optimización de un generador termoeléctrico mediante calor geotérmico de origen volcánico(2020) Alegría Cía, Patricia; Astrain Ulibarrena, David; Araiz Vega, Miguel; Escuela Técnica Superior de Ingeniería Industrial, Informática y de Telecomunicación; Industria, Informatika eta Telekomunikazio Ingeniaritzako Goi Mailako Eskola TeknikoaFrente al actual problema energético al que nos enfrentamos, la termoelectricidad aplicada a la energía geotérmica tiene un gran potencial de futuro. Esta tecnología conlleva grandes ventajas como, entre otras, su robustez, fiabilidad, no necesita mantenimiento ya que es una tecnología sin partes móviles, ausencia de ruidos, y es una energía totalmente renovable y limpia. Sin embargo, la eficiencia de un generador termoeléctrico depende en gran medida de los intercambiadores de calor. Por ello es de gran importancia que estos sean lo más eficientes posible. El objetivo de este proyecto es diseñar y estudiar un prototipo de generador termoeléctrico que sea capaz de transformar en electricidad un gradiente de temperaturas entre el ambiente y el calor del suelo del Parque Nacional de Timanfaya. Para ello, se han diseñado y construido intercambiadores de calor específicamente para esta aplicación. La función de estos intercambiadores será transportar el calor con la máxima eficiencia posible desde el interior del sondeo (foco caliente) hasta los módulos termoeléctricos, utilizando una parte de este calor para generar electricidad y otra parte para disiparlo al medio ambiente a través de intercambiadores en el lado frío. El principio de funcionamiento de todos estos intercambiadores será el intercambio de calor por termosifón y cambio de fase. Para lograr el diseño completo, se propusieron y valoraron diferentes soluciones para las diferentes partes que componen el prototipo. Finalmente, se realizó un estudio económico.Publication Open Access Generadores termoeléctricos sin partes móviles para yacimientos geotérmicos superficiales de roca caliente seca: diseño, desarrollo experimental, instalación en campo y evaluación del potencial en la isla de Lanzarote(2022) Alegría Cía, Patricia; Astrain Ulibarrena, David; Catalán Ros, Leyre; Ingeniería; IngeniaritzaEn el actual contexto climático y energético, es importante desarrollar tecnologías que permitan utilizar las fuentes de energía renovables que actualmente no se pueden aprovechar. La contribución de las renovables a la producción eléctrica mundial ha aumentado un 8% en la última década, pero la energía geotérmica, la única fuente renovable que no depende de la climatología y que además es totalmente estable y continua, contribuyo en el año 2021 con menos de un 3 %. Existen territorios en los que impulsar la utilización de esta fuente renovable cobra especial importancia, por su disponibilidad y por la enorme dependencia de los combustibles fósiles, como ocurre en las Islas Canarias, especialmente en Lanzarote, donde existe un gran campo de Roca Caliente Seca. En esta tesis se plantea la utilización de generadores termoeléctricos para transformar el calor de las anomalías geotérmicas superficiales de la isla de Lanzarote directamente en electricidad, ya que el empleo de centrales geotérmicas convencionales no sería posible debido a que requerirían de perforaciones en la roca y grandes instalaciones que perjudicarían al medio ambiente. Pero todos los generadores termoeléctricos existentes en la literatura emplean partes móviles para hacer circular fluidos calorportadores, lo que conlleva consumos auxiliares y necesidad de mantenimiento, perdiendo la ventaja intrínseca de la termoelectricidad. Además, apenas existen estudios experimentales y todavía menos en campo. Para llevar esta propuesta a la realidad, en esta tesis se ha conseguido desarrollar e instalar en campo dos generadores termoeléctricos para geotermia que funcionan sin partes móviles gracias al cambio de fase en sus intercambiadores de calor. Son generadores robustos, que no necesitan mantenimiento ni consumos auxiliares, además de ocasionar un mínimo impacto ambiental ya que se reduce el impacto visual, no producen ruidos, y el uso de agua como fluido de trabajo los hace totalmente inocuos. El primero de ellos, para zonas con anomalías de media temperatura, cuenta con dos prototipos compuestos cada uno por un termosifón de cobre en el lado caliente, y 10 y 6 módulos termoeléctricos respectivamente, con sus correspondientes intercambiadores en el lado frio formados por un disipador de aletas con cuatro heat pipes con aletas en su zona de condensación. En primer lugar se diseñó y se caracterizó en el laboratorio para posteriormente instalarlo en un sondeo del que salen gases a unos 170 °C del Parque Nacional de Timanfaya, constituyendo el primer generador termoeléctrico para geotermia sin partes móviles instalado y testado en campo, donde lleva en funcionamiento ininterrumpido desde agosto de 2020, y ha conseguido una potencia pico de salida de 39,99 W (2,49 W por modulo), y una energía generada anual por unidad de superficie ocupada de 180,16 kWh/m2. Tras demostrar la viabilidad de esta tecnología para transformar el calor geotérmico superficial de Lanzarote en electricidad, se planteó un reto mayor, el de desarrollar un generador adaptado para zonas de alta temperatura. Dicho dispositivo termoeléctrico cuenta con un único termosifón de acero y ocho módulos termoeléctricos con unos intercambiadores de calor similares a los del caso de media temperatura. Se realizaron ensayos en el laboratorio a varias temperaturas del foco de calor con entre 4 y 8 módulos termoeléctricos, determinando experimentalmente que al instalar más módulos disminuye la eficiencia por modulo, pero para el numero de módulos instalado la potencia total aumenta, por lo que la instalación en campo se realizó con 8 módulos. Tras los buenos resultados en el laboratorio, se pudo transportar hasta dicho parque e instalarlo de manera satisfactoria en un sondeo con gases saliendo a 465 °C. Dicho generador presenta una potencia pico de 4,5 W por modulo, un 80% mayor que el de media temperatura, y una potencia de salida media por módulo de 4,12 W, lo que supone una energía generada al ano de 382,59 kWh/m2, demostrando el gran potencial que tienen en Lanzarote los generadores termoeléctricos desarrollados. Con los dos generadores instalados en campo, se desarrolló un modelo computacional basado en el método de las diferencias finitas y en la analogía térmica-eléctrica, que se consiguió ajustar mediante los datos experimentales en campo y que presenta una elevada fiabilidad. Este modelo es capaz de simular las potencias de salida de los generadores termoeléctricos con errores relativos inferiores al +- 10%, y de reproducir las temperaturas en las caras de los módulos termoeléctricos, así como la energía generada a lo largo del ano con un error menor del 1,6% en el prototipo de media temperatura y del 0,5% en el de alta. Finalmente, empleando este modelo computacional, se realizó un calculo estimado del potencial de generación eléctrica con los datos disponibles de superficie de anomalías geotérmicas en el Parque Nacional de Timanfaya, donde se podrían llegar a generar entre 3,61 GWh y 11,35 GWh al ano, con un LCOE medio de 4,8 cente/kWh. Además, esto se puede extrapolar al resto de la isla, ya que se han detectado más anomalías fuera de dicha zona y que, de ser calculadas con exactitud, se podrían aprovechar mediante una instalación a mayor escala de generadores termoeléctricos como los desarrollados en esta tesis doctoral.Publication Open Access Experimental development of a novel thermoelectric generator without moving parts to harness shallow hot dry rock fields(2021) Alegría Cía, Patricia; Rodríguez García, Antonio; Catalán Ros, Leyre; Astrain Ulibarrena, David; Araiz Vega, Miguel; Ingeniería; Institute of Smart Cities - ISC; IngeniaritzaNowadays, geothermal energy in shallow hot dry rocks is not exploited enough due to the high economic and environmental impact as well as the lack of scalability of the existing technologies. Here, thermoelectricity has a great future potential due to its robustness, absence of moving parts and modularity. With this research, the feasibility of a novel and robust geothermal thermoelectric generator whose working principle is phase change has been experimentally demonstrated, as well as the importance of compactness to maximize its efficiency and thus, power generation.