Sub-cooling benefits explained by Jeff Gingras, Systemes LMP

By Clémence Girard-Reydet, Dec 11, 2013, 11:08 5 minute reading

In an exclusive interview with, Jeff Gingras, Vice President of Sales at Systemes LMP, a leader in transcritical CO2 refrigeration solutions, talked about the main advantages of sub-cooling, as well as the types of sub-cooling, which applications it can be used for and its future in warm ambient climates. How does sub-cooling optimise the efficiency of a CO2 transcritical system?

Jeff Gingras: Mechanical sub cooling increases the quality of refrigerant prior to the expansion valve. This methodology helps us achieve a higher percentage of liquid than flash gas and increases the refrigeration capacity as well as the Energy Efficiency Ratio. When using mechanical sub-cooling, we will be able to obtain 21% more efficiency and consume less energy than a conventional synthetic system in warmer climates. This is strictly based on Power consumption. What different types of sub-cooling exist today for a transcritical system?

JG: There are many different types of sub-cooling in a transcritical phase on the market today. Here is a brief list of available sub-cooling technologies:

  • Natural sub-cooling (During the cooler periods of the year)
  • Water in an evaporative heat transfer
  • Mechanical sub cooling using CO2
  • External chillers using a synthetic coolant
  • Internal heat exchangers between gas cooler outlet and suction line. (Low amount of sub-cooling can be obtained with this technology)

We feel that it is very important to increase the quality of liquid in the flash tank, which can be achieved consistently during the entire year and is not climate dependant, through mechanical sub-cooling. With the different sub-cooling options available, which one would be the most efficient sub-cool design on the market today?

JG: We feel that all of the sub-cooling technologies will help you increase the efficiency in a transcritical system. With our technology, we use a CO2 compressor to mechanically improve the quality of the liquid to 85% liquid and only 15% flash gas. When there is an increase in the quality of liquid, it is possible to reduce the number of transcritical compressors and thus increase the Energy Efficiency Ratio. In our opinion, mechanical sub-cooling is the most efficient because of the quality of CO2 and the lower power consumption that will be obtain. In which applications would sub-cooling be the most advantageous?

JG: The ideal application for a mechanical sub-cooling would be for climates that have a high ambient temperature. With the use of a mechanical sub-cooling, our transcritical systems will run as efficiently as a conventional refrigeration system using a synthetic refrigerant. For example, an installation done in New Orleans in which the highest recorded temperature was 102°F, the transcritical compressors would have an Energy Efficiency ratio of 4.9 Btu/W. When mechanical sub-cooling is applied, the EER would be 9 Btu/W for the entire system. If the installation was done with a conventional synthetic refrigerant, like R507, the EER would be 7.8 Btu/W. It can be seen from the previous example, that when sub-cooling is properly applied to a Transcritical System, the energy savings are superior to those of the conventional synthetic refrigerant system. How does sub-cooling technology compare to ejectors or parallel compression in terms of increase in efficiency of a transcritical CO2 cycle?

JG: Ejectors can increase the efficiency in a transcritical cycle between 7% and 18%. Using ejectors is only beneficial for high temperature and air conditioning. Parallel compression is not something we have been using. In our opinion, the compressors in this technology could not obtain the same energy efficiency as a mechanical sub-cooling, especially in southern climates.

The use of Ejectors or Parallel compression in a transcritical phase will increase the quality of CO2, but it will not improve the energy consumption. When mechanical sub-cooling is applied, a significantly better quality of CO2 refrigerant is obtained. When mechanical sub-cooling is correctly used, the quality of liquid in the flash tank will be 85% and 15% flash gas. This is how we achieve energy savings and are more efficient compared to a conventional refrigeration system that uses a synthetic refrigerant in warmer climates. What increase in cost would a sub-cooling system add to a CO2 transcritical system?

JG: When mechanical sub-cooling is used, there is an incremental cost, as more components are being utilised and a different control strategy is needed; however, the total compressor number is reduced. It is very hard to evaluate the increase in cost, as you need to take into consideration the energy savings that will be obtained. The longer the system operates in a transcritical phase, the more savings will be achieved. Preliminary calculations show approximately a 2.5 to 3 year pay back when mechanical sub-cooling is installed. Do you think sub-cooling will become standard technology for CO2 transcritical refrigeration in warm ambient climates?

JG: We feel that mechanical sub-cooling will be a standard in warmer climates because of the savings that can be obtained. Without a good quality of liquid, it is very difficult to use a transcritical system in southern climates. It is our opinion that the mechanical sub-cooling approach has the best investment to energy saving ratio.

Depending on the climatic conditions, if a transcritical system is required, sub-cooling is the only solution, as the operating pressures would be such that they would be detrimental for the equipment used. With our technology, we are able to maintain lower operating pressures, thus allowing operation in warmer climates.

About Systemes LMP

“Made in Quebec” is a symbol of Systemes LMP’s commitment to quality. For well over a decade, the company has been among the market leaders in manufacturing energy-efficient and reliable cooling modules and heat recovery systems for commercial and industrial refrigeration. Systemes LMP provides one of the most adaptable transcritical refrigeration solutions on the market, tailor-made solutions to fit specific customer needs. One of the company’s priorities is environmental sustainability, which is reflect in its products optimised for natural refrigerant CO2.


By Clémence Girard-Reydet

Dec 11, 2013, 11:08

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