Theoretical analysis of a single-stage and two-stage solar driven flash desalination system based on passive vacuum generation
Introduction
Global resources of freshwater are becoming scarce and unevenly distributed with increasing population. The world population is growing approximately at a rate of 1.2% annually resulting in a net addition of about 77 million people every year. The population is expected to increase to around 8.9 billion by 2050 [1]. The earth's water supply is about 1370 million km3 out of which nearly 3% constitutes freshwater. Nearly 29 million km3 of freshwater is frozen in the form of glaciers and ice. Ground water, lakes and rivers together constitute just a little over 8 million km3 of freshwater. The critical water level to satisfy basic human needs is estimated to be 1000 m3/capita annually. It is projected that by 2050 about 1.7 billion people in 39 countries will fall below this level [2]. Thus, a logical solution to this crisis is to produce freshwater from the available resource of earth's saline water via desalination.
According to the World Health Organization, the permissible limit of salinity in water is 500 ppm (parts per million) for potable water, and for special cases up to 1000 ppm of total dissolved salts. Most of the water available has salinity up to 10,000 ppm and seawater normally has salinity in the range of 35,000–45,000 ppm. Desalination has evolved over the past few decades as a promising solution to water scarcity. The conventional desalination technologies include multi-stage flash (MSF), multi-effect distillation (MED), reverse osmosis (RO), electrodialysis (ED), vapor compression (VC) process and freezing.
Desalination is an energy intensive process. With the total installed capacity expected to increase drastically in the coming decades, the energy consumption for desalination will continue to rise and hence the amount of conventional hydrocarbon fuels required will substantially go up. In terms of oil consumption, it is estimated that about 203 million tons of oil per year is required to produce 22 million m3 per day of desalinated water [3]. With conventional hydrocarbon fuel shortages being inevitable unless radical changes occur in the demand or in the supply of non-conventional hydrocarbons [4], the energy–water link cannot be overlooked. In addition, the usage of fossil fuels continues to pollute the environment and adds to the cause of global warming. Thus, a feasible and promising solution is the use of renewable energy resources for desalination.
Desalination using solar energy is increasingly becoming an attractive option. Indirect collection systems can be seen as a combination of two systems, a collector to convert solar energy and the actual desalination plant to which the collected energy is supplied. Zejli et al. [5] designed a combination of a MED system with an open cycle adsorptive heat pump using internal heat recovery. The heat transfer fluid flowing through tubes in the adsorbent beds is heated up by a parabolic trough collector. Theoretical modeling was done, and variation of energy consumption and performance ratio (PR) with the number of effects is shown. PR is defined as the ratio of water produced to the required heat input. Hawaj and Darwish [6] coupled MED with a solar pond. A similar system studied by Tabor [7] optimizes the size of the pond and the number of effects used, taking into account the large variation of pond heat output between summer and winter. Garcia-Rodriguez and Gomez-Camacho [8] studied a solar parabolic trough collector field coupled to a conventional MSF plant and concluded that use of solar energy could compete with a conventional energy supply in MSF distillation processes in some climatic conditions. Lu et al. [9] did an experimental study of a small multi-effect, multi-stage flash distillation (MEMS) unit and a brine concentration and recovery system (BCRS) coupled with a solar pond which aimed at reaching zero-discharge desalination. Hawlader et al. [10] accomplished an experimental study of a single-effect solar assisted heat pump desalination system incorporating both flash and distillation techniques. Joseph et al. [11] did an experimental study of a single-stage flash desalination system working on flat plate solar collectors and obtained a maximum distillate yield of 8.5 l/d with collector area of 2 m2. Farwati [12] concluded from an experimental study that the yield from a MSF desalination system coupled with a compound parabolic collector is better than when coupled with a flat plate collector. Thomson and Infield [13] studied a photovoltaic-powered seawater reverse osmosis desalination system experimentally and their system showed substantial cost reduction to other PV–RO systems. Another similar experimental study by Laborde et al. [14] assisted by mathematical modeling outlined different parameters needed to be optimized with regard to power needs and energy consumption. Amara et al. [15] optimized the principal operating parameters in an eight-stage air solar collector heating–humidifying desalination system. Dai et al. [16] studied a humidification–dehumidification desalination system using flat plate collectors and validated their mathematical model with experiments. A similar numerical analysis done by Fath and Ghazy [17] showed that the dehumidifier effectiveness has an insignificant influence on system productivity.
Most of the conventional desalination systems can be classified as a combination of complex units with large output yields (MED, MSF plants) or simple systems with low output yields (solar stills). In order to increase the freshwater output, electrical equipments like vacuum pumps are added to the systems. This increases the energy requirement as well as demands more maintenance. The proposed concept in this paper moves towards the idea of decentralized–small scale desalination systems which requires only one water pump for the whole unit and also produces attractive freshwater yield.
Section snippets
Proposed desalination system
In the proposed flash desalination system, an innovative passive gravity based method is used for the production of vacuum. The concept was proposed by Sharma and Goswami [18]. A standing column of water is allowed to drop generating very low pressures in the headspace created. Conventional desalination systems require the use of vacuum pumps or steam ejectors to attain the same purpose. Based on this concept, a desalination system was investigated by Al-Kharabsheh and Goswami [19] which
Theoretical analysis
Mathematical modeling of the proposed desalination systems includes the analysis of each component of the system. Mass and solute conservation equations along with energy balance equations are formulated.
Method of analysis
The procedure followed in this analysis was to first write the equations in their differential form. All the components of the system are assumed to move from one static state at time ‘t’ to another static state at time ‘t + Δt’. In this method all the differential terms are approximated in finite difference form and the equations are solved simultaneously and iteratively till a convergence is obtained. All the system parameters are solved for time t, and then input parameters for time t + Δt are
Results and discussion
Two cases are studied; the first one assumes a constant heat source temperature while the second one takes into consideration a solar collector as the source of heat. The assumptions made here are: the heat loss from the system, the thermal capacity of the system material and the effect of non-condensable gases on condensation are neglected. The system parameters which are taken as constants for all the cases are given in Table 2.
Conclusion
An innovative desalination system is proposed which makes use of natural forces of gravity and atmospheric pressure to create vacuum under which saline water is flashed. The system can be coupled with low grade heat source like solar collectors to produce potable water. Single-stage and two-stage concepts of the desalination system were outlined. Mathematical models of the concepts were formulated and developed, and the results were presented for the systems coupled with: a) constant
Acknowledgements
We wish to thank Dr. Sanjay Vijayaraghavan (post doctoral associate at University of Florida, 2003–2006) for his insight and invaluable ideas in this work. He is presently working at Intel Technology India Pvt. Ltd, Banglore, India.
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