Characterization of thermal contacts between heat exchangers and a thermoelectric module by impedance spectroscopy

Heat to electricity energy conversion efficiency of a thermoelectric (TE) device is not only influenced by the TE materials properties, but it also depends on the temperature difference between both sides of the TE legs. Keeping this temperature difference as close as possible to the temperature difference between the heat sink and the heat source is crucial to maximize the TE device performance. However, achieving this is quite difficult, mainly due to the thermal contact resistance at the interfaces between the TE module and the heat sink/source. In this study, it is analyzed the effect of this thermal contact resistance on the impedance spectroscopy response of a TE module that is thermally contacted by two aluminum blocks, which act as heat exchangers. A new theoretical model (equivalent circuit) that takes into account the thermal contact resistance is developed, which includes two new elements that depend on this parameter. The equivalent circuit is tested with experimental impedance measurements where the thermal contact is varied. It is demonstrated that using this equivalent circuit the thermal contact resistivity can be easily determined, which opens up the possibility of using impedance spectroscopy as a tool to quantify and monitor this crucial property for the TE device performance

Experimental evaluation of the CO2-based mixtures CO2/R32, CO2/R1234yf and CO2/R1270 in a transcritical refrigerating plant considering the effect of the internal heat exchanger (IHX)

Using CO2 mixtures results in a simple but effective solution to increase the critical temperature of CO2 and reduce its working pressures. However, the overall effect of these mixtures on the global performance of the cycle has yet to be explored experimentally in detail. Accordingly, this work tests three CO2-binary blends with R32, R1270 and R1234yf in a small-capacity vapour compression test rig considering the same working conditions of heat rejection temperature and cooling demand. These mixtures were compared with pure CO2 in two-cycle arrangements without and with the internal heat exchanger (IHX). The results without IHX provided COP enhancements of 6.4%, 9.7% and 15.3% with CO2/R1270, CO2/R1234yf and CO2/R32, respectively, compared to CO2 without IHX. Using the IHX and taking CO2 without IHX as a reference, the COP increments ranged from 6.7% with R744, 12.1% with CO2/R1270, 16.1% with CO2/R1234yf and 22.2% with CO2/R32

Alternative blends of CO2 for transcritical refrigeration systems. Experimental approach and energy analysis

As a natural refrigerant, Carbon Dioxide (CO2) has been extended in almost all refrigeration fields due to its environmental friendliness, high availability, and high technological development in the main components of the cycle. However, despite the benefits, CO2 is limited to high-capacity applications due to the cost of components and the complex cycle arrangements to overcome the low coefficient of performance (COP) at high ambient temperatures. A simple but effective method to solve this issue involves mixing CO2 with other refrigerants obtaining a new refrigerant mixture with higher critical points, lower working pressures and low global warming potential (GWP). Depending on the mixture percentage, the mixture flammability can be suppressed, and the performance of the refrigeration plant can be increased. Building on this, this work determines theoretically different CO2­based zeotropic blends as an alternative to pure CO2 with the restrictions of non-flammability, GWP below 150 and higher COP than pure CO2. The results suggest two blends of CO2/R1270 and CO2/R32 that have been prepared and energetically tested in a vertical display cooler using pure CO2 as a reference. The results revealed that CO2-blends reduce energy consumption by up to 17.2 % at the ambient temperature of 25 °C and up to 12.2 % at 30 °C. Moreover, the results with CO2-blends were closest to those obtained with pure R1270 and better than R134a under the same operating conditions

Análisis y evaluación experimental de un sistema en cascada R744/R290 empleando tres configuraciones de intercambiador intermedio

En este trabajo se analiza experimentalmente los efectos que tiene el uso del intercambiador interno (IHX) en un sistema de refrigeración en cascada, empleando R290 y R744 como fluidos de trabajo. Para ello, se han analizado tres posibles configuraciones empleando IHX en el circuito de baja temperatura, IHX en el circuito de alta temperatura, e IHX entre los dos circuitos. Las condiciones de ensayos analizadas cubren tres temperaturas de foco caliente (20, 30 y 40 ºC) y una temperatura de foco frío (-20 ºC) manteniendo la instalación en el interior de una cámara climática a las mismas temperaturas de foco caliente. Los resultados obtenidos ponen de manifiesto la conveniencia de utilizar IHX en los ciclos en cascada, puesto que ofrecen mejoras en términos de COP y potencia frigorífica generada con respecto al ciclo sin IHX.Los autores agradecen al Ministerio de Ciencia y Tecnología del gobierno español la financiación de este trabajo a través del proyecto RTI2018-093501-B-C21, así como la Universidad Jaume I por la financiación a cargo del proyecto UJI-B2019-56

Análisis experimental de un sistema de subenfriamento en un ciclo simple de compresión de vapor usando R744 transcrítico

En este trabajo se analiza experimentalmente un sistema de subenfriamiento en un ciclo simple de compresión de vapor que utiliza CO2 como refrigerante. Este sistema consiste en la extracción de líquido saturado del depósito intermedio de refrigerante. El líquido extraído se expande hasta la presión de evaporación y se hace pasar por un intercambiador (subcooler), donde subenfría el caudal de refrigerante que sale del gas-cooler, posteriormente el caudal extraído se mezcla con la corriente proveniente del evaporador. La mezcla resultante es recomprimida hasta la presión de gas-cooler. Se ha buscado el óptimo de la configuración base y de la configuración con subenfriamiento a tres niveles de temperatura de foco caliente de 28, 31 y 34 ºC. Posteriormente se han comprobado los resultados de los principales parámetros energético.Los autores agradecen al Ministerio de Ciencia y Tecnología del gobierno español por la financiación de este trabajo a través del proyecto RTI2018-093501-B-C21 a la beca PRE2019- 091617 asociada a dicho proyecto, y al proyecto de la Universidad Jaume I por su proyecto UJI-B2019-56

Experimental evaluation of alternative CO2-based blends for transcritical refrigeration systems

This work presents two refrigerant CO2 blends with R32 and R1270 designed with a theoretically nonflammability and a GWP below 150. Both mixtures were tested in a small-capacity vapour compression system with and without IHX, considering the same working conditions of heat rejection temperature and demanding conditions at the evaporator. Taking the pure CO2 results as a reference in a cycle without IHX, the blend of CO2/R32 enhanced the COP up to 18.1% with an optimal pressure reduction of 19.0% and a power consumption minimization of 17.1%. The mixture of CO2/R1270 presented a COP enhancement of 8.4% with an optimal pressure reduction of 8.0% and a power consumption minimization of 7.9%. In all cases, the use of the IHX resulted positively with increments compared to the pure CO2 base cycle of 6.4% for pure CO2, 13.3% for CO2/ R1270, and 24.7% for CO2/ R32

Optimización energética de los refrigerantes R152a, R1234yf, R290, R1270, R600a y R744, como alternativa al R134a en un armario de refrigeración vertical

Con la entrada en vigor de la normativa Europea F-Gas (2014) y la posterior enmienda de Kigali al protocolo de Montreal (2019), los equipos frigoríficos autónomos han sufrido cambios para poder sustituir los refrigerantes originales R134a y R404A, a otras sustancias con menor impacto medioambiental tales como R290, R600a o incluso R744 (CO2). Sin embargo, existen otras sustancias que también pueden ser consideradas como alternativas, como es el caso de los fluidos puros R1270, R152a y R1234yf. En esta ponencia se analiza energéticamente cómo funciona un armario de refrigeración vertical autónomo, cuando la carga de refrigerante es optimizada para los fluidos anteriormente mencionados. Dicha optimización busca minimizar el consumo energético del equipo en unas condiciones ambientes de 30ºC y 60% (Clase climática III), manteniendo una temperatura promedio de producto en torno a 3ºC. Tomando como referencia el refrigerante R134a, los resultados obtenidos muestran un ahorro energético del 27.5 %, 26.3 %, 13.7 %, 3.9 % y 1.2 %, para los refrigerantes R290, R1270, R152a, R744 y R600a, respectivamente, mientras que el uso del refrigerante R1234yf, supone un incremento del 4. 1%.Los autores agradecen al Ministerio de Ciencia y Tecnología del gobierno español por la financiación de este trabajo a través del proyecto RTI2018-093501-B-C21, y a la beca PRE2019- 091617 asociada a dicho proyecto. Además, también agradecen a la Universidad Jaume I su financiación a través de los proyectos UJI-B2019-56 y UJI-B2021-10

Estudi i disseny d'instal·lacions d'energia renovable per al desenvolupament rural d'un mas aïllat sense connexió a les xarxes elèctriques, d'aigua i de sanejament en el marc del projecte IN2RURAL

Treball Final de Grau en Enginyeria en Tecnologies Industrials. Codi: ET1040. Curs: 2016/201

Thermal contact resistance evaluation of a thermoelectric system by means of three I-V curves

Research data for this article: Data not available / Data will be made available on requestTo achieve a suitable performance in a thermoelectric (TE) device it is important to minimize the thermal contact resistances between the device external surfaces and the heat exchangers of the system (heat source and heat sink). Despite its relevance, there are not many methods available for the evaluation of the thermal contact resistance, and the existing ones typically employ complex setups. Here, we present a new method to determine the thermal contact resistance of a TE device thermally contacted to a heat sink and a heat source. The method is based on performing three current-voltage (I-V) curves at different system conditions under a small temperature difference. First, an I-V curve with a high voltage scan rate, which avoids the variation of the initial temperature difference, provides the ohmic resistance. A second I-V curve performed with a constant input heat power (or the device suspended) provides the TE resistance. Finally, a third I-V curve with a constant temperature difference between the heat exchangers allows obtaining the thermal contact resistance. Using this method, a thermal contact resistivity value of 3.57 × 10−4 m2KW−1 was obtained for a commercial Bi-Te TE module contacted with a heat source and a heat sink using thermal grease as thermal contact interface material, which is in good agreement with reported values. The new method is highly advantageous, since neither involves complex setups nor requires the measurement of heat fluxes. Moreover, it measures directly under operating conditions for small temperature differences

Mélanges alternatifs à base de CO2 pour les systèmes frigorifiques transcritiques

The extensive use of CO2 in commercial refrigeration system has grown in the last two decade thanks to the development of new components and configurations that allows working in transcritical conditions efficiently. However, using these arrangements increases the cost and complexity of the refrigerating plant, making it challenging to implement them in medium or low-capacity systems. As an alternative, CO2-based binary mixtures report attractive improvements that allow for enhancing the COP of the system by minimising its complexity and maintaining the safety and environmental conditions of CO2. This manuscript analyses five binary mixtures of CO2 with the refrigerants R32, R152a, R1234yf, R1234ze(E) and R1270, determining the optimal mixture composition for maximising the COP of a CO2 transcritical refrigeration plant in a wide range of environmental temperatures (0 to 40°C). Fixing the operating conditions for a medium-temperature application, the binary mixtures of CO2/R32 (81/19% in mass) and CO2/R1270 (92.5/7.5% in mass) reported the best COP enhancements results with increments up to +21.4% and +8.7%, respectively, at high environmental temperatures