Refrigerant blends to challenge hydrocarbon efficiencies

SPAIN: Researchers have identified a number of refrigerant mixtures which could provide a more efficient alternative to pure hydrocarbons in small refrigeration systems.

Isobutane (R600a) has replaced R134a as the dominant refrigerant in domestic fridge/freezers, while propane (R290) has been widely adopted as the preferred environmental option for stand-alone commercial refrigeration appliances. This is particularly so in Europe where the F-gas regulations (517/2014) ban the use of refrigerants with a GWP higher than 150 in small capacity refrigeration systems.

In addition to being very low GWP, the hydrocarbons refrigerants are known for their energy efficiency. As these gases fulfil the requirements of all the environmental legislation, until now it seems that there has been little effort to improve the performance of the refrigerant. This is despite figures that estimate that more than 1.5 billion hydrocarbon fridges globally account for approximately 2.6% of the world’s electricity consumption.

Scientists from the Thermal Engineering Group at the Mechanical Engineering and Construction Department of Jaume I University in Valencia have now identified a small number of refrigerant mixtures that could, theoretically, provide more energy efficient options to R600a and R290.

The group studied 110,880 refrigerant blends were evaluated thermodynamically against R600a and R290 for refrigeration purposes.

The refrigerants considered for the blends were R290 (propane), R600a (isobutane), R600 (butane), R1270 (propylene), R152a, R32, R1234yf, R1234ze(E), R1233zd and R744 (CO2). Only blends with a maximum of three components were considered. The maximum GWP of any potential mixture was set at 150 and maximum effective glide allowed in the evaporator was 10K.

From those, only the blends exhibiting theoretical COP increments from 0 to 15% and variations in volumetric cooling capacity from -30 to 30% compared to R600a and R290 were selected. Finally, remaining mixtures were optimised again with a mass fraction variation of each component of 0.5%.

Blends of R1234yf/R600a and R1270/R600a were found to offer a small increase in COP, between 0.3% and 0.6% and between 0.1% and 0.8% respectively, compared to R600a. A small increase in VCC of between 5.9% and 6.4% for the R1234yf/R600a blend and between 6.3% and 11.2% for the R1270/R600a mixture was noted.

Blends of R1270/R600, R152a/R600, R1234zeE/R600 and R290/R600 achieved increased COPs of between 1.7%-5.3%, 3.3%-7.6%, 2.5%-4.4%, 2%-4.5%, 1.6%-8.6%, respectively, but the VCC decreased significantly by as much as 28%.

Of the possible alternatives to R290, blends formed by a small proportion of R744 with R290, R1234yf, R152a or R1234ze(E) achieved higher COPs of between 3.4% and 11.6%. The VCC, however, differed considerably between the blends. It also identified the blend of R32 and R290, which achieved a rise in COP and VCC of between 0.8% and 2.3% and 8.8% and 13%, respectively.

The researchers insist that it’s clear that there are some refrigerant mixtures which could offer a small increase in COP compared to pure hydrocarbons in small systems. They accept, however, that experimental validation will be needed to confirm the real possibilities of the identified blends.

The full paper can be found here.