Desalination of seawater/brackish by multi-effect solar still coupled with a point focus Fresnel lens

Abstract

The world is fronting a drinking water and energy crisis. Continuous population growth, uneven distribution of water resources and periodic droughts has motivated researchers to look for effective and inexpensive water desalination technologies. Conventional desalination processes are energy intensive and not affordable in many countries. Therefore, there is a great need to exploit renewable energy resources to improve the economic viability of seawater desalination. Solar energy is one of the possible renewable energy resources that could be used in thermal desalination processes. Small passive solar stills appear to be a good option for remote areas which suffer from shortage of infrastructure. But high production cost, low efficiency, low productivity and a large footprint are the main hurdles for wide adoption of these small plants. The available literature shows that one of the possible solutions is the use of multi-effect active solar stills coupled with solar collection systems. The current study involves the exploration of a new active solar thermal desalination system. A small multi-effect solar still (3 stages) was designed, fabricated and coupled with a point focus Fresnel lens. The system was tested in field conditions of The Petroleum Institute Abu Dhabi, UAE. Moreover, a transient mathematical model was developed to study the system behavior during different climate conditions. Maximum specific production capacity of 7.3 kg/m2 day (based on evaporation area) was achieved during the tests conducted on developed MES system. This represents almost two folds of improvement when compared to a single basin solar still (3-4 kg/m2 day). The overall efficiency for the system was 57% which compared to the efficiency of simple solar still (35%), shows an improvement of 62 %. The quality of distillate produced from the MES system was well within the range of WHO drinking water standards. The 1st stage of MES showed 14% improvement in distillate production (7% overall) when the temperature of 2nd stage was decreased by adding cold water during operation. This proves that the production of MES will greatly improve when operated as continuous system. The developed mathematical model showed 8.8%, 9.3% and 13.6% average deviation in distillate prediction from 1st, 2nd and 3rd stage respectively, when compared to experimental results. The average deviation of 4.8% for total yield of system is far less than the previously reported values of 11-45%. The seasonal variations predicted by the developed model showed that the system can achieve a maximum specific production capacity of 10.4 kg/m2 day in the month of June which shows almost a 3 fold improvement of output when compared to a single basin solar still. The optimization of systems' physical dimensions for the enhancement of output suggests that the angle of inclination and water quantity in 3 rd stage should be lower than the 2nd stage. The optimum stage spacing that will result in maximum output was found to be 10 cm through simulations conducted with different feasible stage spacing. The optimum number of stages and evaporation area was determined to be 5 and 1.25 m2 respectively. However the cost-optimized parameters were determined to be 5 stages, an evaporation area of 1 m2 and 2.2 m2 of Fresnel lens. These optimized parameters will result a MES output of 24 kg/day, unit production cost of $ 0.0224/kg and a payback period of 11 months.

Date of Award	Jan 2013
Original language	American English
Supervisor	Fawzi Banat (Supervisor)