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Determination of the optimal discharge pressure of the transcritical CO2 heat pump cycles for heating and cooling performances based on new correlation

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Abstract

Considering the expansion of heat pump systems as one of the environment-friendly cycles, increasing these cycles' efficiency is significant. This paper is a comprehensive study of transcritical CO2 heat pump cycles in heating and cooling modes covering a range of ambient temperatures from −15 to 30 °C along with the range of 25 to 45 °C in cases of gas-cooler exit temperatures. In this study, the ambient temperature of 0 °C and gas-cooler exit temperatures of 30 and 35 °C were examined, and the best Coefficient of Performance and exergy efficiency were obtained at a discharge pressure of 7496 and 8896 kPa, respectively. The ambient temperature of 30 °C and gas-cooler exit temperatures of 45 and 50 °C were investigated, and the discharge pressure of 10,938  and 12,363 kPa have the highest Coefficient of Performance and exergy efficiency, respectively. The results show that the gas-cooler temperature positively affects the optimum discharge pressure. In conclusion, an equation for Optimum Discharge Pressure is presented as a general correlation to obtain this parameter for the heating and cooling modes in these commonly used cycles. This equation also presented with a deviation less than 5.66% of the optimum discharge pressure obtained from energy and exergy analysis. Therefore, it has been identified as an acceptable criterion in these cycles. Furthermore, by examining the amount of exergy destruction of these cycles' most essential components, it becomes clear that they are the most exergy destruction related to the compressor.

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Abbreviations

\(\mathop {Ex}\limits^{ \cdot }\) :

Exergy rate (kW)

h:

Enthalpy (kJ kg−1)

\(\mathop m\limits^{ \cdot }\) :

Mass flow rate (kg s−1)

P:

Pressure (kPa)

\(\mathop Q\limits^{ \cdot }\) :

Heat transfer (kW)

S:

Specific entropy (kJ kg−1 K−1)

T:

Temperature (K)

\(W\) :

Power consumption (kW)

\(\eta\) :

Efficiency

0:

Ambient

1–4:

State points in Fig. 1

a:

Each point in Fig. 1

mp:

Compressor

dest:

Destruction

eva:

Evaporator

gc:

Gas-cooler

is:

Isentropic

COP:

Coefficient of performance

ODP:

Optimal Discharge Pressure

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Authors and Affiliations

  1. Center of Computational Energy, Department of Mechanical Engineering, Hakim Sabzevari University, PO Box 397, 9617976487, Sabzevar, Iran

    Saeed Maddah

  2. Department of Medical Research, China Medical University Hospital, China Medical University, Taichung, Taiwan

    Mohammad Reza Safaei

  3. Mechanical Engineering Department, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia

    Mohammad Reza Safaei

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  1. Saeed Maddah
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Maddah, S., Safaei, M.R. Determination of the optimal discharge pressure of the transcritical CO2 heat pump cycles for heating and cooling performances based on new correlation. J Therm Anal Calorim 145, 1537–1546 (2021). https://doi.org/10.1007/s10973-021-10723-4

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