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 Unibertsita
To 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.
Open Access
Experimental assessment of a thermoelectric subcooler included in a transcritical CO2 refrigeration plant
(Elsevier, 2021-05-25) Aranguren Garacochea, Patricia; Sánchez, Daniel; Casi Satrústegui, Álvaro; Cabello, Ramón; Astrain Ulibarrena, David; Ingeniería; Ingeniaritza; Institute of Smart Cities - ISC
This study brings an experimental research that has tested a real transcritical CO2 vapor compression cycle that includes a thermoelectric subcooler at the exit of the gas-cooler of the refrigeration plant. The aforementioned technology hybridization increases the COP of refrigeration systems as long as the subcooling system is properly designed and operated. The experimental facility studied has been tested under constant ambient conditions (30 °C and relative humidity of 55%) and maintaining the evaporating temperature at -10 °C; while the voltage supplied to the thermoelectric modules and the thermal resistances of the heat exchangers located at the thermoelectric subcooler have been experimentally modified. The voltage supplied to the fans located at these heat exchangers was modified implying thermal performance deviation of the heat exchangers and a variation on the power consumption of the cooling facility. The results show an experimental increase on the COP of 11.3% while the cooling capacity increases a 15.3% when the thermoelectric modules are supplied with 2 V and the fans with 9 V. Moreover, the importance of optimizing the voltage supplied to the thermoelectric modules and to the auxiliary consumption of the thermoelectric subcooler is addressed along this research.
Open Access
Improvements in the cooling capacity and the COP of a transcritical CO 2 refrigeration plant operating with a thermoelectric subcooling system
(Elsevier, 2019) Astrain Ulibarrena, David; Merino Vicente, Amaya; Catalán Ros, Leyre; Aranguren Garacochea, Patricia; Araiz Vega, Miguel; Sánchez, Daniel; Cabello, Ramón; Llopis, R.; Ingeniaritza; Institute of Smart Cities - ISC; Ingeniería
Restrictive environmental regulations are driving the use of CO 2 as working fluid in commercial vapour compression plants due to its ultra-low global warming potential (GWP 100 = 1) and its natural condition. However, at high ambient temperatures transcritical operating conditions are commonly achieved causing low energy efficiencies in refrigeration facilities. To solve this issue, several improvements have been implemented, especially in large centralized plants where ejectors, parallel compressors or subcooler systems, among others, are frequently used. Despite their good results, these measures are not suitable for small-capacity systems due mainly to the cost and the complexity of the system. Accordingly, this work presents a new subcooling system equipped with thermoelectric modules (TESC), which thanks to its simplicity, low cost and easy control, results very suitable for medium and small capacity plants. The developed methodology finds the gas-cooler pressure and the electric voltage supplied to the TESC system that maximizes the overall COP of the plant taking into account the ambient temperature, the number of thermoelectric modules used and the thermal resistance of the heat exchangers included in the TESC. The obtained results reveal that, with 20 thermoelectric modules, an improvement of 20% in terms of COP and of 25.6% regarding the cooling capacity can be obtained compared to the base cycle of CO 2 of a small cooling plant refrigerated by air. Compared to a cycle that uses an internal heat exchanger IHX, the improvements reach 12.2% and 19.5% respectively.
Open Access
Enhancement of the power-to-heat energy conversion process of a thermal energy storage cycle through the use of a thermoelectric heat pump
(Elsevier, 2024) Erro Iturralde, Irantzu; Aranguren Garacochea, Patricia; Sorbet Presentación, Francisco Javier; Bonilla-Campos, Íñigo; Astrain Ulibarrena, David; Institute of Smart Cities - ISC; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
The principal strategy for achieving a neutral climate entails enhancing the share of renewable energies in the energy mix, in conjunction with promoting innovation in efficient technologies. Thermal energy storage systems have the potential to efficiently handle the intermittent nature of renewable energy sources. Furthermore, these systems can effectively handle shifts in both heat and electrical demand. Thus, efficient power-to-heat technologies are needed to boost thermal energy storage. This manuscript explores the potential of utilising a thermoelectric heat pump system in conjunction with electric resistances for charging a thermal energy storage. In order to achieve elevated temperatures, the thermoelectric system integrates thermoelectric heat pump blocks in a two-stage configuration. Air has been employed as a heat transfer medium for sensible heat storage. Higher airflow rates improve the performance of thermoelectric heat pump system. Moreover, its impact on the optimal voltage supply of the thermoelectric system is observed when it is combined with an electric resistance to achieve elevated temperatures. In comparison to the basic charging process that solely relies on the electric resistance of a thermal energy storage at 120 °C, a significant 30 % increase in power-to-heat energy conversion has been achieved by including the thermoelectric heat pump system. In fact, it efficiently elevates the temperature from the initial ambient temperature of 25 °C to a remarkable 113.1 °C, achieving a coefficient of performance of 1.35 with an airflow rate of 23 m3/h. Therefore, the use of this technology to enhance a complete process of storing excess renewable energy in the form of heat for subsequent use in both heat and electricity through a combined heat and power cycle is demonstrated.
Open Access
Experimental analysis of one and two-stage thermoelectric heat pumps to enhance the performance of a thermal energy storage
(Elsevier, 2023) Astrain Ulibarrena, David; Aranguren Garacochea, Patricia; Erro Iturralde, Irantzu; Chavarren Oroz, David; Alzuguren Larraza, Iñaki; Ingeniería; Ingeniaritza; Institute of Smart Cities - ISC
This experimental study demonstrates the possibility to enhance the performance of a low-temperature thermal energy storage system (~160 ¿C) based on airflow heating using electrical heaters by including thermoelectric technology. An improvement of the 17 % on COP is reached by using an optimized thermoelectric heat pump system to preheat the airflow, consisting of three one-stage and three pyramidal two-stage thermoelectric heat pumps sequentially installed along the airflow that is heating. This research experimentally analyses and compares the COP of three different configurations of thermoelectric heat pumps: one-stage, square two-stage, and pyramidal two-stage thermoelectric heat pumps. The experimental study aims to characterize the operation of each configuration for heating an airflow of 16.5 m3/h at 25 ¿C as ambient temperature. To that purpose, the airflow inlet temperature, voltage supply, and voltage ratio between stages have been modified. The experimental results show that for 25 ¿C as inlet temperature the one-stage thermoelectric heat pump has the best performance with a maximum generated heat of 78 W. Whereas, a two-stage thermoelectric heat pump is required when the inlet temperature increases. At 40 ¿C as inlet temperature, the square two-stage configuration provides the best performance with a voltage ratio of 2, which reaches a COP of 3.29 generating only 20 W of heat. However, the pyramidal two-stage configuration is able to achieve the maximum heat outputs with a voltage ratio of 1, generating 172; 161; 149 and 138 W, with corresponding COP values of 1.17; 1.16; 1.14 and 1.11 for inlet temperatures of 25; 40; 55 and 70 ¿C. This configuration is the one that achieves the greatest COP values with high inlet temperatures.
Open Access
Performance assessment of an experimental CO2 transcritical refrigeration plant working with a thermoelectric subcooler in combination with an internal heat exchanger
(Elsevier, 2022) Casi Satrústegui, Álvaro; Aranguren Garacochea, Patricia; Araiz Vega, Miguel; Sánchez, Daniel; Cabello, Ramón; Astrain Ulibarrena, David; Ingeniaritza; Institute of Smart Cities - ISC; Ingeniería; Gobierno de Navarra / Nafarroako Gobernua; Universidad Pública de Navarra / Nafarroako Unibertsitate Publikoa
Regulations in the refrigeration sector are forcing the transition to low global warming potential fluids such as carbon dioxide in order to decrease direct greenhouse gases emissions. Several technologies have arisen over the past years to compensate the low performance of the transcritical carbon dioxide vapour compression cycle at high ambient temperatures. For low-medium power units, the inclusion of a thermoelectric subcooler or an internal heat exchanger have been proven as effective solutions for enhancing the coefficient of performance. However, the combination of a thermoelectric subcooler and an internal heat exchanger working simultaneously is yet to be explored theoretically or experimentally. This work presents, for the first time, an experimental transcritical carbon dioxide refrigeration facility that works simultaneously with a thermoelectric subcooler and with an internal heat exchanger in order to boost the cooling capacity and coefficient of performance of the refrigeration system. The experimental tests report improvements at optimum working conditions of 22.4 % in the coefficient of performance and an enhancement in the cooling capacity of 22.5 %. The 22.4 % increase in coefficient of performance would result in a decrease of energy consumption along a reduction of the greenhouse gases emissions. The proposed combination of a thermoelectric subcooler and an internal heat exchanger outperforms each of the technologies on their own and presents itself as a great controllable solution to boost the performance and reduce the greenhouse gasses emissions of transcritical carbon dioxide refrigeration cycles.