Solar Desalination

Solar Desalination

Solar desalination is the desalination of water using solar energy. Renewable energy overcomes the usually high energy operating costs as well as greenhouse emissions of conventional reverse osmosis. Reverse Osmosis is currently the favoured technology for desalination, being the most cost-effective. Recently, there is evidence of growing research interest in the field. This is prompted by growing energy costs, demand growth in the face of depleted water stores, and the growing human pollution of many communities' water supplies.

The severity of fresh water shortage can easily be gauged by the fights that have erupted over water in the recent past in India and outside. Statistics emphasize the same—at least 40% of the world’s population lives without drinking water and roughly 80 000 habitations across the planet have no source of safe water. Of the 575 000 Indian villages, about 162 000 face problems of brackish or contaminated water and scarcity of fresh water.

Solar water distillation is a solar technology with a very long history and installations were built over 2000 years ago, although to produce salt rather than drinking water. Documented use of solar stills began in the sixteenth century. An early large-scale solar still was built in 1872 to supply a mining community in Chile with drinking water. Mass production occurred for the first time during the Second World War when 200,000 inflatable plastic stills were made to be kept in life-crafts for the US Navy.

Most people see strong potential for solar thermal (or wind) energy to be used in large scale desalination. This is particularly so for solar in arid regions due to the usual coincidence of water shortage, good solar radiation and seawater (or brackish) availability. While there are many theoretical models the process is not yet developed at large commercial level. A number of small solar desalination devices exist, and whilst larger plants are technically feasible, they fall down on energy cost comparisons.

In general, there are two different designs for solar stills: electrically and mechanically driven systems which utilize reverse osmosis and thermally driven systems.

Reverse Osmosis

Reverse osmosis is a pressure-driven process that forces the separation of fresh water from other constituents through a semipermeable membrane. This is the preferred method in large-scale desalination implementations where electricity is cheaply available. Here, solar energy is collected and converted into electrical or mechanical energy to initiate the process.

Solar Humidification-Dehumidification

Solar humidification

The solar humidification-dehumidification (HDH) process (also called the multiple-effect humidification-dehumidification process, solar multistage condensation evaporation cycle (SMCEC) or multiple-effect humidification (MEH), is a technique that mimics the natural water cycle on a shorter time frame by evaporating and condensing water to separate it from other substances. The driving force in this process is thermal solar energy to produce water vapor which is later condensed in a separate chamber. In sophisticated systems, waste heat is minimized by collecting the heat from the condensing water vapor and pre-heating the incoming water source. This system is effective for small- to mid- scale desalination systems in remote locations because of the relative inexpensiveness of solar collectors.

Problems:

There are two inherent design problems facing any solar desalination project. Firstly, the system's efficiency is governed by preferably high heat and mass transfer during evaporation and condensation. The surfaces have to be properly designed within the contradictory objectives of heat transfer efficiency, economy and reliability.

Secondly, the heat of condensation is valuable because it takes large amounts of solar energy to evaporate water and generate saturated, vapor-laden hot air. This energy is, by definition, transferred to the condenser's surface during condensation. With most forms of solar stills, this heat of condensation is ejected from the system as waste heat. The challenge still existing in the field today, is to achieve the optimum temperature difference between the solar-generated vapor and the seawater-cooled condenser, maximal reuse of the energy of condensation, and minimizing the asset investment.

Solutions

One solution to the barrier presented by the high level of solar energy required in solar desalination efforts is to reduce the pressure within the reservoir. This can be accomplished using a vacuum pump, and significantly decreases the amount of energy required for desalination. For example, water at a pressure of 0.1 atmospheres boils at 50°C rather than 100°C(Source: Wikipedia).