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
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
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.

Astrain Ulibarrena, David

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