Evaluation of a heat pump coupled two-stage humidification-dehumidification desalination system with waste heat recovery

Abstract

Humidification and dehumidification (HDH) desalination technology has attracted wide attention in the field of small-scale dispersed fresh water demand, because of its advantages such as simple configuration, low investment, and utilization of low-grade renewable energy and waste heat. In this paper, a heat pump coupled two-stage humidification-dehumidification desalination system with waste heat recovery is proposed, where the waste heat is used to heat the feed seawater of the first-stage humidifier, and the brine leaving the bottom is further heated by the condenser of heat pump and then taken as the feed of the second-stage humidifier, while the evaporator of heat pump is used to recover the heat of humid air from the first-stage dehumidifier. Performance evaluation of the proposed system with various working fluids are conducted regarding the fresh water production mpw, gained output ratio (GOR), recovery ratio (RR), specific entropy generation stot, and unit cost of fresh water production Zpw, and parametric studies are performed to identify the key operating parameters. Results show that the largest entropy generation occurs in evaporator, followed by the first-stage humidifier and condenser. Taking R22 as the working fluids is corresponding to the largest mpw, while the lowest Zpw and stot, and the highest GOR are obtained by R600. Higher spraying temperature of the first humidifier T4 is conducive to improving the mpw and Zpw, but leading to a larger stot. The GOR increases first and then decreases with the rise of T4, and the peak value is 4.57 at T4 = 340.65 K. The mass flow rate ratio of the first stage MR1 has significant effects on the system performance. When MR1 is about 3.5, the mpw, GOR and RR reach the peak. However, the bottom value of Zpw is 8.74 $/m3 corresponding to the MR1 of 2.2–2.5. With the increase of compression ratio (CR) of heat pump subsystem, the mpw, GOR and RR climb up first and then decrease, while Zpw and stot get raised.