A solar-assisted double absorption heat transformer: Off-design performance and optimum control strategy

Abstract Using solar-assisted absorption heat transformers to provide high-temperature process heat over 100 °C has raised increasing concern in recent years for its energy saving and environmentally friendly characteristics. In this study, a solar-assisted double absorption heat transformer is proposed and analyzed theoretically. The optimum off-design performance is investigated to develop high-efficient control strategies under various climatic conditions (solar radiation intensity I and ambient temperature t amb ). In order to maximize the heating capacity and overall system thermal efficiency ( Q h and η sys ), the conjugate directions method is applied for searching the optimum operation parameters including the strong solution mass flow rate and bypass ratio ( ṁ sol and BP ). The results show that the η sys can reach 20.3% with the heating temperature at 130 °C under typical climatic conditions ( I  = 600 W·m −2 and t amb  = 25 °C). Under off-design conditions, the ṁ sol and BP exist optimum values, at which the Q h reaches the maximum value. Specifically, under typical operation conditions, the maximum Q h can be obtained as the ṁ sol  = 90 kg·h −1 and BP  = 0.8. Under higher I and t amb conditions, the control strategies of increasing the collector temperature and ṁ sol are very practical to enhance the system performance.