New research from Spain has sought to understand if dimethyl ether and ternary mixtures of carbon dioxide, dimethyl ether and butane could represent an alternative to propane in heat pump systems. Their analysis has shown dimethyl ether could be the best alternative to R290, as it offers the highest coefficient of performance values for both cooling and heating, and avoids issues related to temperature glide.
Researchers from Spain’s Jaume I University have tested different refrigerants that might be alternatives to propane (R290) in existing refrigeration and heat pump systems.
The scientists investigated, in particular, the performances of dimethyl ether (RE170) and ternary mixtures of carbon dioxide, dimethyl ether, and butane (R744/RE170/R600).
“The use of dimethyl ether (DME) has regained again attention during the last years, since it is a fluid with excellent thermophysical and environmental properties (low global warming potential and zero ozone depletion potential),” the scientists said. “However, although this search has been considered from a theoretical perspective as a replacement for R290, it has not yet been experimentally validated.”
The researchers started with a thermodynamic analysis of the different refrigerants using the Refprop v.10 software. Different ratios of the ternary mixtures were simulated in a simple vapor compression cycle operating at an evaporation temperature of 0 C and condensing temperature of 50 C. Superheating and subcooling values are set to 4 K and 1 K, respectively.
“Concretely, the model indicates that the alternative fluids could offer theoretical coefficient of performance for refrigeration (COPR) increments between 8.3% to 13.5% and coefficient of performance for heat pump (COPHP) increments of between 6.5% to 10.3%,” the team noted. “As the thermodynamic properties are estimated with Refprop due to the absence of adjusted binary mixing coefficients, this work has addressed the analysis using an experimental approach.”
The experiment
The experimental approach considered R290 and RE170 as pure fluids and 11 blends composed of different proportions of RE170/R600/R744. They were tested in an experimental plant that was adapted for the use of R290. The plant is described as a water-to-water single-stage vapor compression cycle with an electronic expansion valve (EXV).
“Condenser and evaporator, built in our lab, are concentric tube-in-tube heat exchangers with the refrigerant flowing through the inner tube and the secondary fluid through the annular space,” the academics said. “These heat exchangers are divided into steps to measure the temperature evolution of the refrigerant along the heat exchanger and to guarantee an accurate thermal measurement in the secondary fluid.”
All pure fluids and blends were tested using three approaches: in the drop-in test, R290 was swapped for the new refrigerant without changing the system settings. That is, keeping the test plant with the compressor speed of 2,100 rpm; in the fixed cooling capacity test, the compressor speed was adjusted for the new fluids to have the same cooling power as R290; and in the fixed heating capacity test, it was adjusted for the same heating power as R290.
The results
According to the results, nearly all the mixtures and RE170 offer higher coefficient of performance (COP) values for heating and refrigeration than R290, while also suffering large reductions in capacity. “In the drop in tests, RE170 offered COP increments in relation to R290 of 29.8% (COPR) and 17.99% (COPHP) with capacity reductions of 17.3% and 24.7%, respectively. In this test, mixture RE170/R600 (92.5/7.5) obtained a 22.8% increase in COPR and a 21.1% increase in COPHP, also with a loss of capacity,” they said.
“When the fluids were tested under the fixed cooling capacity scenario, the increments in COPs were reduced. RE170 offered 12.7% COPR and 8.4% COPHP increments with similar values for the mixture RE170/R600 (92.5/7.5). In this case, the compressor speed needed to increase >30%,” they added. “At the fixed heating capacity scenario, again the best fluid was RE170 with COP increments in relation to R290 of 12.8 % (COPR) and 5.1 % (COPHP) with compressor speeds higher up to 41%. Some mixtures offered similar results.”
Concluding their results, the academics added that the use of pure RE170 is recommended as the best overall alternative to R290, since it offers the highest COP values for both cooling and heating, and avoids issues related to temperature glide. However, they also affirmed that in systems where compressor size or speed is a limiting factor, mixtures such as RE170/R600/R744 (85/10/5) or (83/9/8) could be a suitable option, as they provide similar performance with slightly lower compressor speed.
Their findings were presented in “RE170 (Dimethyl Ether) and ternary mixtures (R744 / RE170 / R600) as alternatives to R290 for refrigeration and heat pump applications,” published in the International Journal of Refrigeration.
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