Researchers experimentally evaluated a two-stage cascade refrigeration cycle using R290 in the high-temperature stage and R170 in the low-temperature stage for ultra-low-temperature operation. The natural refrigerant pair was tested at evaporation temperatures of -80 °C and -70 °C and compared with the conventional R404A/R23 combination.
Tests were conducted at condensation temperatures of 25 °C (77 °F), 28 °C (82.4 °F), and 31 °C (87.8 °F). The R290/R170 system achieved up to 40% higher COP than the R404A/R23 baseline and provided higher cooling capacity despite operating with a lower refrigerant mass flow rate.
The study also identified the inlet of the low-temperature expansion valve as a critical safety point because of extremely low material temperatures associated with subcooling down to -40 °C (-40 °F). This was considered particularly relevant when using flammable natural refrigerants.
To address the risk, the researchers proposed an interstage heat exchanger connecting the high-temperature-stage liquid line with the low-temperature-stage vapor line. The configuration maintained high-risk points within safe operating temperatures and provided an adequate compressor suction temperature for lubrication, while the maximum COP reduction was 6.5%.
A Total Equivalent Warming Impact assessment found that the R290/R170 system reduced equivalent CO2 emissions by about 39% compared with the conventional refrigerant pair. The experimental work was conducted using a laboratory-scale prototype and evaluated mass flow rate, cooling capacity, compressor power consumption and COP.
