A dual-mode system for water purification and cooling using a thermal desalination technique coupled to a heat pump unit

Abstract

Humidification-dehumidification is one of the most suitable small-scale desalination methods based on air humidification and condensation. The major energy-consuming components for a humidification-dehumidification process are a heat source for the humidifier and a cooling source for the dehumidifier. To improve the performance of these processes, the fluids were preheated using different energy sources or waste heat. In recent years, preheating air or saline water using heat from a heat pump's condenser has been explored to improve the system's performance and to produce cooling and desalination simultaneously. However, separate heat exchangers and additional solar collectors/heaters were used to preheat the second fluid. In this work, the air-cooled condenser of a heat pump has been replaced by an evaporative condenser and used as a humidifier. This configuration has been scarcely explored in the literature and requires a single heat exchanger to preheat the fluids. On the other hand, the heat pump's evaporator was used to produce low-temperature coolant, circulated through a cooling coil where the humid air from the evaporative condenser is cooled and dehumidified to produce fresh water and cooling simultaneously. This modification ensures reduced space requirement and better system performance than other heat pumps coupled to humidification-dehumidification systems for simultaneous cooling and desalination. Also, the present system operates flexibly to switch between desalination and cooling mode (mode-1) to cooling mode (mode-2). Towards this, a mathematical model of the system has been developed and analyzed under various operating conditions. The system's performance, which includes desalination yield, cooling output, gained output ratio, and coefficient of performance under each mode of operation, is studied for different ambient conditions and evaporator temperatures. The maximum desalination yield, cooling output, gained output ratio, and coefficient of performance of mode-1 are 4.55 LPH, 0.82 kWth, 3.79, and 0.75, respectively. The maximum cooling output and COP of mode-2 are 3.79 kWth and 3.82, respectively. The exergy efficiencies for mode-1 and mode-2 are 32.7 % and 29.7 %, respectively.