Performance investigation of a solar-assisted direct contact membrane distillation system

Abstract

This paper presents a solar-assisted direct contact membrane distillation (DCMD) system with novel energy recovery concepts for a continuous 24-hour-a-day operation. A temperature modulating scheme is introduced to the solar-thermal system that supplies feed seawater to the DCMD modules. This scheme attenuates extreme temperature fluctuations of the feed water by storing the collected energy during solarpeak hours and reutilizing it throughout the day. Thus, the energy savings is realized yet the feed seawater temperature is maintained within the preferred range. Additionally, the system employs heat recovery from the permeate and brine streams to the feed seawater. The simulations for such a system with a shelland-tube type DCMD modules are carried out to examine the spatial property variations and the sensitivity of system performance (i.e., transmembrane pressure, permeate flux and performance ratio) to the operating conditions (inlet temperature and flow rate) and the fiber dimensions (fiber length and packing density). It is found that there are trade-off between mean permeate flux and performance ratio with respect to permeate inlet temperature and flow rate and between total distillate production and performance ratio with respect to packing density. For the solar-assisted DCMD system having evacuated-tube collectors of 3360 m 2 with 160 m 3 seawater storage tanks and 50 DCMD modules, the annual solar fraction and collector efficiency are found to be 77% and 53%, respectively whilst the overall permeate production capacity is 31 m 3 /day. The overall specific thermal energy consumption of DCMD system with heat recovery is found to be 436 kWh/m 3 and it is about 43% lower as compared to the system without heat recovery. It is observed that the specific thermal energy consumption decreases significantly by 55% with increased collector area from 1983 m 2 to 3360 m 2 whereas the specific electrical energy consumption increases slightly by 16%.

Optimising thermal efficiency of direct contact membrane distillation by brine recycling for small-scale seawater desalination

“…A lower STEC value (i.e. 1.62.2 MJ/L) can be obtained from pilot-scale DCMD processes with energy recovery and better insulation [33,34]. However, the inclusion of an external energy recovery system is not suitable for a small-scale system and is beyond the scope of our study.…”

“…It is important to note that the recovery of latent heat from the distillate stream back to the feed stream by using a heat exchanger was not implemented in this study. The thermal efficiency of the DCMD process coupled with the heat exchanger can be improved significantly [25,33,36], but will also increase the system complexity, rendering it unsuitable for small-scale operation.…”

Tailoring surface charge and wetting property for robust oil-fouling mitigation in membrane distillation

“…For example, a distinct advantage of MD is the ability to harness low-grade thermal energy such as waste heat, geothermal energy, and solar thermal energy for desalination [4][5][6]. Such an advantage, together with the small system footprint and low capital cost, renders MD an appealing process for sustainable and off-grid desalination [7][8][9][10][11]. On the other hand, MD has also been identified as one of the only few promising technological candidates for desalinating shale gas produced water which, in certain cases, is too saline for reverse osmosis to treat [12][13][14].…”

Performance comparison between overlapped and woven spacers for membrane distillation

“…Its main attractive feature is that the process can be driven by low grade thermal energy [2], which makes it a potentially attractive candidate for pairing with renewable energy sources, such as solar thermal or geothermal energy, and/or taking advantage of waste heat, if any of these are available [3][4][5][6][7][8][9][10].…”