Optimal design of a hybrid thermal- and membrane-based desalination unit based on renewable geothermal energy

Abstract

• A renewable-based hybrid desalination system consisting of HDH and RO is evaluated. • Two different operating modes of the introduced system are investigated. • Exergoeconomics is used to identify the optimal working condition of the system. • Maximum exergy efficiency of the plant is 28.86% for the second operating mode. • The first operating mode can provide freshwater at a maximum rate of 22070 m 3 /day. In this article, exergoeconomic analysis of a hybrid desalination system driven by renewable geothermal energy source is investigated. Freshwater and electrical power are the products of this system. Humidification – dehumidification (HDH) unit works in parallel with reverse osmosis (RO) unit to produce freshwater and Kalina cycle is the power generator unit. Thermoelectric generator (TEG) is responsible for providing the energy required by RO unit. As both HDH and TEG can operate with low-grade energy, two different modes are suggested for the proposed system. In the first mode, the geothermal low grade energy stream is passed to TEG unit, while in the second, it goes through the HDH unit. The effect of different parameters on four main objective functions is examined in parametric study. Moreover, with the aid of selection maps, the objectives of modes can be compared in different operating conditions. The maximum attainable exergy efficiency is calculated to be 28.86% for mode 2. Single objective optimization studies revealed that the first mode produces the largest amount of freshwater at a rate of 22,072 (m 3 /day), while the minimum achievable freshwater cost and total product cost rate can be obtained in the second mode, which is calculated to be 21.94 (¢/m3) and 37.19 ($/GJ). Besides, Sankey diagrams for exergy flow and mass flow of saline water are presented to unravel the impact of each sub-system on exergy and salinity.