A high-efficiency multi-function system based on thermal desalination and absorption cycle for water, water-cooling or water-heating production

Abstract

Water-refrigeration cogeneration systems combining thermal desalination and absorption cycle have been identified as an effective and promising way of utilizing low-grade heat. To take full advantage of the synergy that can be achieved by the combination and overcome the disadvantages of the existing systems, this study proposes and analyzes a multi-function system composed of a single-effect water-lithium bromide absorption heat pump, a single-effect water-lithium bromide absorption refrigeration heat pump and a low-temperature multi-effect evaporation desalination unit. Consuming mainly low-grade heat, the system can operate in at least three modes: water-refrigeration mode for summer, water-only mode for spring/autumn and water-heating mode for winter, thus capable of meeting the requirements of different seasons. The mathematical model is presented and validated, and thermodynamic performance is studied. The results show that all the modes have good energy/exergy performance, as demonstrated in a case study where the energy- and exergy-based coefficients of performance of the absorption unit are 1.60 and 0.62, 1.75 and 0.87, and 1.75 and 1.01 for the three modes, respectively, and the energy saving rates are all as high as 42.7% compared with conventional systems. The proposed system also features great operational flexibility. In water-refrigeration mode, the refrigeration load rate can be regulated from 20% to 100% for a water load rate within 55.2%–110%. In water-heating mode, the heating load rate can be regulated between 0 and a top limit allowed which reaches 256% under a water load rate of 50%, peaks at 459% under 90% and decreases to 358% under 110%. Therefore, the system has good internal synergy, reflected in the whole-year multi-purpose operation, efficient energy utilization, and great operational flexibility.