Performance tuning of solar-assisted heat pumps via refrigerant pressure and solar radiation control

Document Type

Article

Publication Date

Spring 4-17-2026

Abstract

n recent years, solar energy has become a major contributor to domestic water heating in low- and medium-temperature ranges. Similarly, heat pumps have attracted researchers to investigate and optimize their performance. Considerable research has been conducted on direct expansion solar-assisted heat pump (DX-SAHP) systems to improve the system performance; however, the literature lacks the investigation of the effect of varying the refrigerant operating pressures on the system performance and lifespan. Consequently, this work experimentally investigates the optimal initial refrigerant evaporating pressure of R134a to serve as a reference for refrigerant charging during system startup, commissioning, and maintenance. Eighteen indoor experiments were conducted at various initial refrigerant evaporating pressure and solar irradiance levels, and their outputs were statistically analyzed using response surface methodology—central composite design (RSM-CCD) to determine the optimal range for initial refrigerant evaporating pressure. The outputs of the RSM-CCD optimization analysis indicate that the optimal range for the initial refrigerant evaporating pressure is between 20 and 21 psig. Consequently, six outdoor experiments were carried out to validate the optimal initial refrigerant evaporating pressure. Based on the results, it is recommended to initially charge the system to 21-psig refrigerant evaporating pressure (at nearly zero solar radiation condition) to achieve the maximum coefficient of performance (COP), minimum water heating time, extend system lifespan and avoid frost formation at startup.

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