Solar desalination is a process of removing salts from sea water with the help of solar energy to yield pure water. This process of desalination has become more critical in supplying drinking water across the globe. Reports state that the amount of fresh water available in the world per person was 4000 m3 a year in the early 1950s. Now, the amount of fresh water available globally has reduced to 1000 m3 per person per year, which has resulted in water scarcity.
Some communities - coastal areas and remote desert regions, in particular - were using solar stills during that period. However, solar stills became less viable owing to the development of several cheap water pumps and pipelines and low energy costs in the 20th century. This trend is reversed again now as the fossil resources are readily depleting day by day. Therefore, various desalination plants have been built with new sustainable technologies to overcome the problem of water scarcity in future.
Types of Solar Desalination
The following are the two major types of solar desalination methods:
Reverse Osmosis
Reverse osmosis is a pressure-driven process that separates fresh water from other substances via a semi-permeable membrane. This method is mostly employed in large-scale desalination systems that readily produce electricity.
Solar Humidification-Dehumidification (HDH) Process
The solar humidification-dehumidification (HDH) process is a method that operates similar to that of the natural water cycle in a short time interval by evaporating and condensing water for the separation of salt and other substances from it. Thermal solar energy acts as a driving force in this process which initially produces water vapor and later condenses in a separate chamber. In certain HDH systems, heat released from the condensation of water vapor is collected and the incoming water source is pre-heated in order to minimize the wastage of heat. Owing to the inexpensiveness of solar collectors, these systems are suitable for small- to mid- scale desalination systems.
Environmental Impact
Solar desalination is primarily a zero-carbon emission process and the advancements in solar technology enables overcoming previously existing problems like dust and high temperatures, which affected the efficiency of previously used solar panels. In 2011, the Environment Agency-Abu Dhabi (EAD) tested cutting-edge solar technologies for desalinating water in the desert. The trial conducted at 30 sites in the Emirate of Abu Dhabi was said to be the largest across the globe. Each unit set at the solar desalination facilities in Sweihan and Hameem could generate 35 kW/h of energy on average and thus produce 1050 kW/h of energy on the whole. This shows that the negative impact of desalination process on the environment as well as the cost of producing water can be reduced using the solar desalination technology.
In another research carried out by Jijakli et al from the Masdar Institute of Science and Technology in 2011, three desalination based alternatives such as a solar still, a photo-voltaic (PV) powered reverse osmosis (RO) unit and water delivered from a central RO plant using truck were compared and assesed for their environmental footprint. It was found that the PV-RO unit had the lowest impact on environment. This study helps in promoting the low-carbon desalination technologies.
Benefits of Solar Desalination
Some of the key benefits of solar desalination include the following:
- It uses free solar energy for operation.
- Solar desalination plants are inexpensive, light in weight and easy to transport.
- The plants can be set up easily either onshore or offshore.
- Low maintenance costs.
- Environment-friendly.
Problems Involved in Solar Desalination
The primary concerns of the solar desalination projects include the following:
- The system tends to be inherently inefficient due to the requirement of a large amount of heat for the evaporation of water.
- It is hard to dissipate this waste heat into the environment.
However, the heat dissipation problem can be minimized by protecting the chamber where condensation takes place from direct heat of the sun or by using an optical system to raise the temperature of the system. The system’s inefficiency in evaporation can be resolved by properly designing the surfaces of the system with respect to the heat transfer efficiency.
Conclusion
World's water resources include 97% of sea water and 3% of fresh water. According to predictions by the World Resources Institute, at least 3.5 billion people in the world will likely experience water shortage by 2025. As most of the world's water resources are covered by sea and oceans, desalination is the best method for producing drinking water. However, most of the desalination technologies are expensive and depend on fossil fuels. The future demand for water may in turn deplete the fossil fuel reserves thereby increasing greenhouse gas emissions. Hence, the utilization of renewable energies such as solar energy for powering desalination plants could be a sustainable solution.
Recent reports state that the Saline Water Conversion Corporation (SWCC) will likely build three new solar-powered desalination plants in Farasan, Dhuba, Haqel in order to reduce Saudi's oil dependency. In addition, the governor of the Corporation, Abdul Rahman Al-Ibrahim confirmed that another desalination plant being constructed in Ras Al-Khair would be capable of producing a million m3 of water for use in Riyadh.
Several prototypes were evaluated by the researchers including a low-cost system that directly uses solar energy and an efficient system that employs solar photovoltaic panels. The low-cost system was found to be capable of producing 4.95 lt of fresh water per day per m2 of evaporator area and upon installation of a reflector, nearly 7.5 lt of fresh water per day per m2 could be produced. The second system, on the other hand, was capable of producing 12 lt of water per day per m2. Therefore, these results ensure a strong potential for desalination processes that make use of renewable energy sources.
Sources and Further Reading