Analysis of a solar-powered membrane distillation system

Nowadays, in dry and rural areas, solar-powered membrane distillation (SPMD) technology is considered a feasible means for the production of pure water from brackish water. Prior to the design and construction of a SPMD pilot plant, there is a need to predict its performance theoretically by means of a computational simulation program. Unlike previous approaches followed by other investigators to develop a mathematical model that can describe the components of a SPMD pilot plant, the developed mathematical model in this study is based on the fact that the SPMD process by nature is unsteady. The performance of a proposed SPMD pilot plant is then obtained by means of a numerical solution of the model with the aid of a simulation computer program. The results reveal that the proposed SPMD pilot plant has some unique features, which differ from a similar MD process operated at steady-state conditions in a laboratory. The analysis of the system has shown that heat recovery via an external heat exchanger is not only possible, but even effective, and an economical way to intensify the SPMD process. The plant productivity can be improved by increasing the heat-exchanger capacity (KA), decreasing the flow rates of both feed and permeate or otherwise by increasing the effective surface area of the membrane. The achieved enhancements in the SPMD pilot plant productivity are directly related to an improved heat recovery rate in the heat exchanger. However, further analysis reported in this paper shows that the increase in KA and membrane area should be optimized for any planned capacity in the design of a SPMD pilot plant.

Being an energy-sensitive process, the SPMD plant shows that its performance is remarkably influenced by the amount of energy transported to the feed. With a certain solar collector area, heat recovery from the permeate to the feed is the only way to improve energy efficiency of the system, and in this way plant capacity can be promoted. When a heat recovery exchanger is used in the plant, this promotion can be concretely attributed to the accordingly increased feed temperature and the prolonged period of MD running time within a day. However, improving the KA of the heat recovery exchanger is not always proportionally rewarded. The plant capacity tends to plateau with the increase of KA, and an economically optimal KA should exist for a certain system. This analysis is very important in determining the size of the heat recovery exchanger. Unlike previously reported MD processes, reducing the flow rates of feed and permeate will make the daily capacity of a SPMD plant linearly increase. This is due to the unchangeable amount of radiation energy within 1 day and the accordingly upgraded heat recovery rate. Also, the flow resistance is minimized when flow rates are reduced. The generally accepted idea that plant capacity is proportional to membrane area may not be always true. The effectiveness of improving plant capacity by increasing the membrane area largely depends on heat recovery: the larger the KA value of the heat recovery exchanger, the better the reward from increasing membrane area. When KA is small enough, adding membrane area has almost no significance for improving plant capacity. The MD starting temperature, tfstart, has almost no relation to plant capacity. Here, a better choice is to consider saving energy consumed by the feed and permeate pumps. A higher tfstart means less running time of the pumps, which is what we prefer.