
Renewable Desalination: A Sustainable Solution for Fresh Water Production
Introduction
Water makes up 71% of the earth’s composition, with only 3% being fresh water and the remaining 97% being pure saltwater that cannot be used for domestic purposes. One method of utilizing this saline water is to remove salt particles and impurities from it to obtain fresh water ready for use, a process technically known as desalination.
This article focuses on water desalination using renewable energy, sometimes known as “renewable desalination.”
Global water withdrawals are over 4,000 billion m3 per year, with desalination being the most significant supply of water for drinking and agriculture in several countries, particularly the Middle East and Northern Africa (MENA). and world desalinated water production is now around 65.2 million m3 per day (24 billion m3 per year), or 0.6% of the world’s water supply. The MENA region accounts for around 38% of worldwide desalination capacity, with Saudi Arabia being the largest desalinator.
Techniques used for desalination of sea water
- Reverse osmosis (RO) is the main technology, accounting for 60% of global capacity.
- Multistage Flash (MSF)accounting for 26.8%.
- Multi-Effect Distillation (MED).
Technologies for Thermal Desalination:
Thermal desalination procedures involve distillation operations in which saline feed-water is heated to vaporize, forcing fresh water to evaporate and leaving behind a highly saline solution known as brine.
After that, freshwater is obtained by vapor cooling and condensation. The method of multi-stage flash (MSF) is separated into sections or phases. The boiling point of saline water ranges between 90 and 110 °C, with decreasing pressure through the phases. At each step, some of the water quickly evaporates, while the rest continues to flow through the subsequent phases.
Technology used in desalination
MSF is the major desalination method in the MENA area, where fossil fuels are abundant and cheap. Multi-Effect Distillation (MED), like MSF, is a multi-stage process type in which vapor from each vessel (stage) is condensed in the next vessel and evaporated again at a lower ambient pressure. Unlike MSF, MED enables the processing of feed-water without the requirement for extra heat for vaporization at each step.
The Vapor Compression (VC) distillation method is another thermal desalination system in which the heat for water evaporation originates from compression rather than direct heating. To increase overall efficiency, this procedure is typically utilized in conjunction with other processes (MED).
Membrane Desalination Technologies:
In this technique, the membranes separate the fresh water from the salty feed water. The feed water is brought to the surface of the membrane, which selectively passes the water and excludes the salts.
There are two types of desalination membranes:
The first type is Reverse Osmosis (RO): in which the pressure of seawater is increased above osmotic pressure, allowing desalinated water to pass through semi-permeable membranes, leaving behind solid salt particles.

But reverse osmosis plants are very sensitive to the quality of the feed water (salinity, turbidity, and temperature), unlike other distillation techniques, which are not demanding in this regard .
High salinity and high temperature feed water can hinder the osmosis process because they affect the osmotic pressure, which requires more energy.
Highly turbid feed water can cause contamination as the pores of the membranes become clogged with suspended solids.
The total dissolved solids in the Red Sea may reach 4100 ppm, while in the Arabian Sea they reach 45000 ppm. This is a very high percentage, given that the typical percentage is 35,000 ppm. This large increase in the percentage of pollution is a result of biological pollution, that is, the large diversity of living organisms.
To maintain reverse osmosis plants, per-treatment of feed water is required.
The advantage about reverse osmosis technology is that it can be used in rural areas where there is no alternative.
The second type is electrolysis (ED):This method uses electrical voltage to move the salt through the membrane, leaving fresh water.
Recently, ED has been used to desalinate brackish water on a large scale because the more salt in the feed water, the higher the electricity consumption.
Desalination with renewable energy
Since the need for desalinated water is increasing day by day, the trend has been to use renewable energy sources, which can ensure the sustainability of the production of fresh water.
It is expected to become of great economic value as renewable energy costs continue to decline and fossil fuel prices continue to rise.
The RO process accounts for 62% of renewable desalination, followed by thermal procedures such as MSF and MED.
The most common energy source is solar photovoltaic (PV), which is used in about 43% of current applications, followed by solar thermal energy and wind energy .
Mixing water desalination technology with renewable energy is an effective way to meet demand, whether for energy or water, in an economical and environmentally friendly way.
This mix depends on a combination of factors such as location, quality (salinity) of the feed-water inputs and freshwater outputs, available renewable energy sources, plant capacity, and the size and availability of the electricity network.
Desalination using Photovoltaic
Photovoltaic (PV) technology can be used directly in reverse osmosis (RO) and ED plants that use electricity for operation.

The most important examples of the use of photovoltaic stations in desalination are:
Canary Islands (PV-RO) seawater, 1-5 m3/day)
Riyadh, Saudi Arabia (PV-RO) (brackish water, 5m3/day)
and Ohshima Island, Japan (PV-ED) seawater, 10 m3/day)
Desalination with Solar Thermal
Solar heat is used as an energy source for seawater desalination by MSF and MED.
There are two types of solar heat energy:
Direct is when all components are integrated into a desalination plant. It can be coupled with either membrane or thermal desalination machines (eg,reverse osmosis, RO).
Indirect, in this way, the desalination plant consists of two parts: a solar heat collector and a distillation device.
Desalination with Geothermal energy
As geothermal energy can provide both electricity and heat, it may be used in conjunction with both thermal and membrane desalination systems. For MED desalination, low-temperature geothermal energy, generally in the 70-90°C range, is optimal.
The plant is a dual system that uses hot water from geothermal wells to power either an ORC or a MED .
The system has the potential to benefit the local population by generating desalinated water at a reasonable cost, i.e. USD 2/m3.
However, geothermal energy extraction is highly dependent on specific local circumstances, with often requires large upfront investment expenditures.
Desalination with Wind Power
Wind turbines’ electrical and mechanical power may be utilized to power desalination facilities, particularly RO and ED desalination units, as well as vapor compression (VC) distillation processes (particularly Mechanical Vapor Compression, MVC).
The mechanical energy of the wind turbine is used directly for VC in the MVC, with no additional conversion into electricity. In general, wind power-based desalination might be one of the most promising methods for saltwater desalination, particularly in windy coastal locations.
Wind-powered desalination plants have been installed all over the world, including :
Gran Canaria in the Canary Islands (Wind-OR, seawater, 5-50 m3/d), Fuerteventura in Spain (Wind diesel hybrid system, seawater, 56 m3/d), the Centre for Renewable Energy Systems Technology in the United Kingdom (Wind RO, seawater, 12 m3/d).
Wind desalination, like PV and CSP, has the disadvantage of being intermittent.
Cost of renewable desalination
Desalination needs a significant quantity of energy. RO uses just 3.5 to 5.0 kWh of electricity per m3. but MSF generally requires 80.6 kWh of heat energy (290 MJ thermal energy per kilogram) plus 2.5 to 3.5 kWh of electricity per m3 of water, so RO is used on a large scale.
The global production of around 65.2 million m3/d of desalinated water requires at least 75.2 TWh per year, which represents approximately 0.4% of global power consumption.
Desalination costs have dropped to USD 0.5/m3 in recent years, although market pricing for desalinated water normally ranges between USD 1/m3 and USD 2/m3.
As a result, desalination is now affordable to middle-income regions but not to the world’s poorest countries.
The economics of renewable desalination are influenced by the cost of renewable energy, as desalination prices are mostly controlled by energy costs. In general, the cost of renewable desalination remains greater than the cost of conventional desalination using fossil fuels as the energy input. However, the costs of renewable technologies are rapidly declining, and renewable desalination can already compete with conventional systems in remote regions where the cost of energy transmission and distribution is higher than the cost of distributed generation.