Thermodynamic study of multistage absorption cycles using low-temperature heat

Abstract

The use of low-temperature heat (between 50 and 90°C) is studied to drive absorption systems in two different applications: refrigeration and heat pump cycles. Double- and triple-stage absorption systems are modelled and simulated, allowing a comparison between the absorbent–refrigerant solutions H2O–NH3, LiNO3–NH3 and NaSCN–NH3. The results obtained for the double-stage cycle show that in the refrigeration cycle the LiNO3–NH3 solution operates with a COP of 0.32, the H2O–NH3 pair with a COP of 0.29 and the NaSCN–NH3 solution with a COP of 0.27, when it evaporates at −15°C, condenses and absorbs refrigerant at 40°C and generates vapour at 90°C. The results are presented for double- and triple-stage absorption systems with evaporation temperatures ranging between −40 and 0°C and condensation temperatures ranging from 15°C to 45°C. The results obtained for the double-stage heat pump cycle show that the LiNO3–NH3 solution reaches a COP of 1.32, the NaSCN–NH3 pair a COP of 1.30 and the H2O–NH3 mixture a COP of 1.24, when it condenses and absorbs refrigerant at 50°C, evaporates at 0°C and generates vapour at 90°C. For the double- and triple-stage cycles, the results are presented for evaporation temperatures ranging between 0 and 15°C. The minimum temperature required in the generators to operate the refrigeration and heat pump cycles are also presented. Copyright © 2002 John Wiley & Sons, Ltd.