(642c) Humidification/Dehumidification for Low Cost, Energy Efficient, Zero Liquid Discharge Desalination Using Solar Thermal Sources

Humidification/Dehumidification (HDH) has developed into a robust technology for small-scale desalination. HDH has multiple advantages such as low capital and maintenance costs and compatibility with low grade heat sources such as non-tracking solar thermal collectors. Despite the substantial advantages offered by HDH, significant shortcomings hinder its application. A primary limitation of HDH is recovery ratio for a single cycle. Since energy for evaporation is generally provided by sensible heat of the feedwater itself, the maximum potential freshwater recovery is limited by the ratio of heat capacity times viable operating temperature difference and heat of vaporization. The ratio of recovered to feed water volume must be < 14% for a single cycle operating under normal conditions. Here we introduce and analyze novel approaches to increase recovery ratio of an HDH cycle via introduction of additional sensible heat to the system allowing independent control of the sensible to latent heat ratio. The decoupling of latent and sensible heat allows for operation with zero liquid discharge. We address material constraints to deal with precipitates, and we consider the coupling of this system with low cost solar thermal sources. The efficiency of HDH is primarily limited by the exchange of heat from condensing vapor to the feedwater in the dehumidification stage. There is generally a tradeoff between heat exchanger size and complexity and heat transfer efficiency. Thus, the overall efficiency of HDH is constrained for current economically viable heat exchanger implementations. We also consider approaches to enhance sensible heat recovery from condensation using low capital cost, high-efficiency heat exchangers.