Experimental study and semi-empirical model of a thermostatic expansion valve of a R290 direct-expansion solar heat pump

Abstract

The expansion device plays a crucial role in the operation of heat pumps and refrigeration systems, making its study integral to advancements in the fields of refrigeration and heating. Among various expansion devices, the Thermostatic Expansion Valve (TEV) stands out due to its rapid response time and mechanical nature, which grants it high control capacity over refrigerant superheating. In this study, was developed a semiempirical mathematical model for a TEV incorporated in a DX-SAHP utilizing R290. The research was carried out in an experimental way, conducted using a fully instrumented DX-SAHP. A series of experiments tested different operational modes for water heating, subject to varying solar radiation intensities. A uniform methodology collected operational parameters, resulting in 150 experiments. The semiempirical model creation involved combining basic equations from the literature describing TEV operation and formulating an equation using multiple linear regression based on experimentally evaluated parameters. These parameters included solar radiation incidence, inlet and outlet pressures, and superheating degree. The regression model achieved an adjustment higher than 96 %. Combining this regression with equations suggested by previous researchers, resulted in a model with an adjustment exceeding 94 %. Furthermore, this study culminated in the derivation of a generalized model primed for broad application across various heat pump configurations for water heating purposes. Notably, the model underwent rigorous validation across a separate set of experiments, reaffirming its reliability and versatility. While excelling in steady-state operations, the model's transient data analysis falls short, as intended.