Study finds R717 and R600 more efficient in geothermal heat pumps

Study finds R717 and R600 more efficient in geothermal heat pumps

In a new study of the geothermal heat pumps, systems using ammonia (R717) or n-butane (R600) were found to be more efficient than those employing R22 or R134, as well as more economical.

The study, “Development of geothermal heat pumps by using environment friendly refrigerants,” was published August 7 in The Journal of Renewable and Sustainable Energy by Iranian researchers Vahid Saeidi, Mostafa Mafi and Ali Mohammadi. (See https://aip.scitation.org/doi/full/10.1063/1.5023657.)

The researchers studied heat pump performance in a 14-story residential building in Qazvin, Iran, with a cooling load of 915 kW (260 TR). They found that the ammonia system with a horizontal closed-loop ground heat exchanger had a COP in the cooling mode of 4.663, which was 5.8% higher than that of a comparable R134a system; n-butane’s COP was 4.606, which was 4.5% higher.

The paper also reported similar results for a vertical closed-loop and open-loop ground heat exchangers.

The COP improvements resulted in “reducing the operating cost of the system compared to conventional artificial refrigerants,” the paper said.

The study also found R717 and R600 to be better than R22 and R134a in terms of initial system cost.

In the study, the temperature of the refrigerant inlet to the evaporator is 2°C (35.6°F). The outlet temperature of the ground closed-loop heat exchanger is 42°C (107.6°F) in the horizontal mode and 39°C (102.2°F) in the vertical mode, while it’s 37°C (98.6°F) in the open-loop heat exchanger.

The paper noted that the results depend on climatic conditions, the soil type, and the condition of the design working system. As a result, “the output data are unique to the same area or case study, and their comparison with the results of other conditions are not approved,“ it said.

The authors advocated for using the earth “as a renewable heat source/sink which both increases efficiency and creates a diversity of energy sources.” Geothermal heat pumps can reduce energy consumption up to 44% compared to air source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment, according to the National Renewable Energy Laboratory.

They also advocated for natural refrigerants. “The big challenge in the coming years in the HVAC and refrigeration industry is to establish natural refrigerant technology to substitute CFCs, HCFCs and HFCs refrigerants,” said Mafi, one of the paper’s authors, in an article in ScienceDaily.com.

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