Boosting the COP of Small-Capacity Transcritical CO2 Systems

A subcooling system equipped with thermoelectric modules improves cooling capacity up to 33%, and COP upto 24%, a study found.

© Jakub Jirsak/ 123RF.com

It is well known that at high ambient temperatures, transcritical CO2 refrigeration systems typically operate in “transcritical mode,” causing low energy efficiencies in refrigeration facilities.

To address this issue, manufacturers have developed several energy-enhancing technologies, including ejectors, parallel compressors and subcooler systems, among others. These have been used especially in large centralized systems with positive results. 

However, according to a 2019 study, these measures are not suitable for small-capacity systems due mainly to the cost and the complexity of the enhancements. The study presents a new subcooling system equipped with thermoelectric modules, “which, thanks to [their] simplicity, low cost and easy control, are very suitable for medium and small capacity plants.”

The study – “Improvements in the cooling capacity and the COP of a transcritical CO2 refrigeration plant operating with a thermoelectric subcooling system” – was published in Applied Thermal Engineering, Volume 155, June 5, 2019, pages 110-122. The authors were D. Astrain, A Merino, L. Catalán, P. Aranguren, M.Araiz, D. Sánchez, R. Cabello and R. Llopis.

According to the study’s abstract, the developed methodology (computational tool) finds the gas-cooler pressure and the electric voltage supplied to the ther- moelectric modules that maximizes the overall COP (coefficient of performance) of the system; it takes into account the ambient temperature, the number of thermoelectric modules used and the thermal resistance of the heat exchangers included in the modules.

The maximum improvement in the cooling capacity with this technology is 33%, and the maximum COP gain is 24%, the study said.

The study found that, with 20 thermoelectric modules, there is an improvement of 25.6% in cooling capacity and 20% in COP, compared to the base cycle of CO2 in a small cooling system refrigerated by air. Compared to a cycle that uses an internal heat exchanger, the improvements drop to 19.5% and 12.2%, respectively.

This article originally appeared in the March issue of Accelerate Magazine.

By Michael Garry

Mar 24, 2020, 11:14




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