High efficiency membrane distillation modules for solar applications
Author: Niels Brand
Summary: Normally gas powered technology and/or fossil fuels are used to be able to provide electricity and potable water (RO) in remote off-grid locations. Membrane Distillation (MD) shows a lot of potential for off-grid desalination of brackish or seawater. MD can be applied in these locations and/or cooperate with RO and other pressure driven technologies to increase water recovery and/or provide clean water where scarcity is rapidly increasing. MD being a thermally driven technology will require a source of thermal energy. Waste heat from a variety of source such as diesel generators and solar collectors can be used as input thermal energy for the process of MD The drawback of this solution is that the amount of thermal energy available is limited. Producing the highest possible amount of water using available solar energy is desirable in these applications. For locations with lower solar irradiance high thermal efficiency becomes increasingly more important in order to be keep the amount of solar collectors i.e. investment costs, required ground area, thermal storage and consequently the Cost Of Water to a minimum. This high thermal efficiency is also associated with Gain Output Energy (GOR). The cost of water (COW) is highly dependent upon installation size (Flux) and the costs of heat (GOR) i.e. the cost of solar collectors. For the COW there is the trade-off between installation size and operational costs. COW of $2.72 - $2.79 were calculated under the same boundary conditions for a location in Abu Dhabi. The COW is optimal within a range of operational conditions. MD can be successfully applied within this range of operational parameters to match available thermal input. A feature ideal for solar applications. Experimental results from a 24m² Aquastill module for thermal efficiency are provided matching with parameters used to determine the COW. Production rates and thermal efficiency are primarily related to water circulation flow and secondarily related to applied vacuum pressure.