Water as an adsorptive for adsorption cycles operating at a temperature below 0 °C

Abstract

From the thermodynamic point of view, water is the best working fluid for adsorptive heat conversion (AHC) cycles. Here we propose to use an aqueous salt solution instead of pure water for preventing its freezing in the evaporator/condenser at a temperature below 0 °C. The thermodynamic aspects of this approach are comprehensively investigated. The comparison with methanol and ammonia is made when possible. The effect of salt solution on the cycle boundary pressures and useful heat is studied for common (cooling and heat storage) and innovative (“Heat from Cold”, HeCol) AHC cycles. The experimental study on water sorption dynamics under the reduced vapour pressure over eutectic solutions of NaCl and CaCl2 shows a significant slowing of desorption (for the HeCol cycle) and adsorption (for the heat storage cycle) at long times. However, the specific power at a 70% conversion (0.3–0.6 kW/g) still remains promising for practical applications.The proposed substitution of pure water in the condenser/evaporator with an aqueous salt solution is proved to be very profitable from the thermodynamic point of view and quite acceptable from the dynamic one. This could allow the temperature range of water application to be expanded for AHC cycles working at a temperature below 0 °C.