Mixtures of hydrocarbons with CO2 are efficient refrigerants and are less flammable

The high degree of flammability prevents the spread of hydrocarbons with excellent thermodynamic characteristics in those sectors of the refrigeration industry where a large amount of refrigerant charge is required.

To address safety and regulatory concerns, a team of scientists from Northeastern University in Boston, Massachusetts (USA) and Texas A&M University in Qatar studied the behavior of hydrocarbon-CO2 mixtures in a standard vapor cycle.

It was found that the mixtures perform well in terms of coefficient of performance (COP) and volumetric refrigeration capacity (VRC). A related study was published in the January 2020 issue of the Journal of Energy Resources Technology.

While COP is well known to HVAC & R professionals as a measure of energy efficiency, VRC, defined as the cooling effect of a unit volume of steam entering a compressor, is not used as often. However, this is an important characteristic that determines, in particular, the size of the equipment, and, consequently, its cost.

Mixing hydrocarbons with non-combustible CO2 has two advantages. First, the reduced risk of ignition compared to pure hydrocarbons expands the range of refrigerant applications. Secondly, the mixtures are characterized by a lower condensing pressure, and, therefore, a higher critical temperature compared to pure CO2, which also expands the field of application of refrigerants (primarily geographically).

The critical temperature of pure CO2, equal to 31 ° C, does not allow the refrigerant to work efficiently at ambient temperatures above this value, although technological tweaks can minimize the negative effect.

Scientists analyzed the behavior of 15 refrigerants in a standard vapor compression cycle with a coolant providing constant evaporator and condenser / gas cooler temperatures.

Two modes were simulated: tropical air conditioning and food refrigeration.

The researchers found that propane (R290), isobutane (R600a), propylene and dimethyl ether behave well when mixed with CO2, and as the proportion of CO2 in the mixture increases, COP decreases and VRC increases.

The best COP performance was demonstrated when simulating food refrigeration. Of the four mixtures tested, the dimethyl ether mixture showed the highest COP at low CO2 levels, and propylene at high CO2 levels. The researchers also included blends of CO2 with HFOs in the experiment, but they performed worse than blends with hydrocarbons.