New research from Spain has tried to understand whether dimethyl ether and ternar mixtures of carbon dioxide, dimethyl ether and butane can represent an alternative to propane in heat pump systems. Their analysis has shown that Dimethyl ether could be the best alternative to R290, because it offers the highest performance coefficient for both cooling and heating, and avoiding problems with regard to temperature glide.
Researchers from Spanish Jaume I University have tested various coolants that may be alternatives to propane (R290) in existing cooling and heat pump systems.
The scientists in particular investigated the performance of Dimethyl ether (RE170) and Ternar mixtures of carbon dioxide, dimethyl ether and butane (R744/RE170/R600).
“The use of Dimethyl ether (DME) has again shown attention in recent years, because it is a liquid with excellent thermophysical and environmental properties (low potential for global warming and zero ozone exhausting potential),” the scientists said. “Although this search from a theoretical perspective is considered a replacement for R290, it has not yet been validally validated.”
The researchers started a thermodynamic analysis of the various coolants using the REFPROP V.10 software. Different proportions of the Ternar mixtures were simulated in a simple vapor compression cycle that worked at an evaporation temperature of 0 C and condensation temperature of 50 C. Overheating and subcooling values are set to 4 K and 1 K. respectively.
“In concrete terms, the model indicates that the alternative liquids can offer the theoretical performance for cooling (COPR) increases between 8.3% to 13.5% and the performance determinations for heat pump (COPHP) increases between 6.5% to 10.3%,” the team noted. “Because the thermodynamic properties are estimated with Refprop because of the absence of adapted binary mixing coefficients, this work has treated the analysis with the help of an experimental approach.”
The experiment
The experimental approach considered R290 and RE170 as pure liquids and 11 mixtures composed of different proportions of RE170/R600/R744. They were tested in an experimental plant that was adapted to use R290. The factory is described as a water-to-water single vapor compression cycle with an electronic expansion valve (EXV).
“Condenser and evaporator, built in our laboratory, are concentric tube-in-tube heat exchangers where the coolant flows through the inner tube and the secondary liquid through the ring-shaped space,” the academics said. “These heat exchangers are divided into steps to measure the temperature evolution of the coolant along the heat exchanger and to guarantee an accurate thermal measurement in the secondary liquid.”
All pure liquids and melanges were tested with the help of three approaches: in the drop-in test, R290 was exchanged for the new coolant without changing the system settings. That is, maintaining 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 liquids to have the same cooling capacity as R290; And in the fixed heating capacity test it was adjusted for the same heating capacity as R290.
The results
According to the results, almost all mixtures and RE170 offer a higher performance coefficient (COP) values for heating and cooling than R290, while also a major reduction in capacity. “In the decrease in the tests, RE170 COP steps with regard to R290 of 29.8% (COPR) and 17.99% (COPHP) with capacity reductions of 17.3% and 24.7% respectively offered a loss of capacity,” also said in this test, “.
“When the liquids were tested in the scenario with fixed cooling capacity, the increases in the police were reduced. RE170 offered 12.7% Copr and 8.4% COPHP increases with comparable values for the mixture of RE170/R600 (92.5/7.5). In this case, the speed of compressor must be speeded up >30 %, “they added.” In the scenario with fixed heating capacity, the best liquid RE170 with COP steps in relation to R290 of 12.8 % (COPR) and 5.1 % (COPHP) with compressor speeds higher up to 41 %. Some mixtures offered similar results. “
By closing their results, the academics added that the use of Pure RE170 is recommended and the best alternative to R290, because it offers the highest COP values for both cooling and heating, and avoiding problems with regard to temperature glide. However, they also confirmed that in systems where the compressor size or speed is a limiting factor, mixtures such as RE170/R744 (85/10/5) or (83/9/8) can be a suitable option because they offer comparable performance with slightly lower compressor speed.
Their findings were presented in “RE170 (Dimethyl ether) and Ternaire Mixtures (R744 / RE170 / R600) as alternatives to R290 for cooling and heat pump applicationsPublished in the International Journal of Refreration.
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