Reduce energy at higher pressures using CO₂ as a secondary coolant

Tuesday, November 16, 2010

CO2 as a refrigerant for low temperature applications has developed rapidly over the last few years. Indirect chiller applications with secondary coolants such as water based brines have also gained global popularity in high and low temperature industrial applications as well as in consumer applications (supermarkets).

An obvious drawback of chillers is their low energy efficiency - chillers generally run at a lower suction pressure due to additional temperature difference in the refrigerant/ secondary coolant heat exchangers, and pumps for secondary coolants consume significant amounts of energy. 

This is where the ideal use for CO2 as a secondary cooling fluid has been developed. 

In a typical layout of a low/medium temperature system with CO2 as secondary cooling fluid, three main areas can be highlighted for possible energy savings: 

1. Reduction of the pumping energy

The required mechanical pumping energy is related to the mass flow of the fluid that is brought into circulation. With water-based brines the volume flow is much higher because only the specific heat of the fluid can be used to remove heat from the installation. With CO2 the latent heat is used, which, is much higher than the specific heat, resulting in the considerably lower flow. Comparisons of the required power between CO2 and some most used brine types are shown in the graph below.

Another important issue is that the pumping energy from centrifugal pumps returns to the system in the form of additional heat. This needs to be rejected by the refrigeration unit as well, which means additional power consumption of the refrigeration system.

Pumps for CO2 circulation need on average only 10% of the required power than with normal brines

2. Increase of the suction pressure

With the assumption that similar chillers are used for both brine and CO2 applications, the influencers on the suction pressure are temperature differences in evaporators and in the cascade heat exchanger. It could be assumed that CO2 temperature in the system is basically constant, as CO2 acts as volatile brine and pressure drop have an only minor influence on the temperature increase. Water-based brines need to have a temperature difference between inlet and outlet. For standard brines the usual glide is 4 K. If the CO2 temperature is kept at the brine average the direct theoretical result is a 2K higher evaporating temperature. In practice, this difference is bigger, as the internal heat transfer coefficient for CO2 is much higher than for brines.

3. Reduction of radiation losses

Line losses contribute quite significantly to the energy consumption of the installation and could equal to 5-15% of the refrigeration load. A major contributor to those losses is heat gain in pipes. It is especially true for pumped systems, as both feed and return pipes are cold and require insulation. Obviously with an increased diameter the heat gain increases. No doubt that both on heat gain and on investment of piping including insulation CO2 outperforms all other brines and HFC refrigerants. Pumping systems with CO2 used as a volatile brine offer great potential for energy savings in a number of applications. Besides being more energy-efficient, CO2 systems are relatively simple and offer possibilities for further optimization.

All it needs to bring this into practice is a new perspective.

* For calculation purposes, a cold store of 500 kW high temperature and 500 kW medium temperature was considered. Hycool was used for the medium temperature and propylene glycol was used for the high-temperature installation. Savings of 20 % to 24 % are the result when using CO2 as circulating fluid. Full comparison data are available from the local Danfoss office.