Numerical Study of Dynamic Adaptability for Flat-Plate Solar Evaporators Subjected to Transient Radiation Conditions

Sustainable and more efficient thermal technologies can contribute to meeting carbon reduction goals and reduce the electric energy demand around the world. A large part of thermal and refrigeration systems uses heat exchangers to carry out evaporation processes of different working fluids. One of the main advantages of these subsystems is that they can use renewable thermal energy sources such as solar radiation. However, the design of the thermal and refrigeration devices is normally given for specific operating conditions, while the source of solar radiation is variable over time. System high sensitivity to operating conditions leads to the need of real time adaptability to variable energy sources, to maintain their efficiency. In this study, the dynamic performance of a flat-plate solar evaporator with R1234yf as working fluid is assessed by means of a validated two-phase mixture numerical model at typical solar radiation conditions in Ciudad de México (CDMX), México. The behavior of the heat exchanger is assessed in terms of temperature and pressure, and an adaptability mechanism is proposed by varying the inlet mass flow according to the solar energy availability. In consequence, complete evaporation process and admissible outlet pressure range are guaranteed. Results show that the system is stable for a wider operation time range in comparison to the base system. This analysis can contribute to expanding the operating ranges and improving the efficiency of novel thermal systems such as solar ejector refrigerators or direct-expansion solar heat pumps.