Desalination removes salt and minerals from seawater or brackish water to produce fresh, usable water. This technology is increasingly relied upon in arid regions and coastal communities worldwide to ensure a stable water supply. Desalination uses a lot of energy, although modern advancements have dramatically lowered the energy footprint. Understanding the energy cost requires looking at the physics of separation and the technology used in commercial plants.
Why Energy Is Necessary to Separate Salt and Water
The energy demand in desalination is a direct result of overcoming osmotic pressure. Water molecules naturally want to move from an area of low salt concentration to an area of high salt concentration, aiming to equalize salinity across a semipermeable barrier. This natural drive, a function of chemical potential difference, must be reversed to separate the components.
The thermodynamic minimum energy required to separate water from salt is equivalent to the Gibbs free energy of separation. For typical seawater, this theoretical minimum energy is approximately 1.1 kilowatt-hours (kWh) for every cubic meter of fresh water produced. This figure represents the absolute lowest amount of energy needed under perfect, reversible conditions, regardless of the technology employed.
In a practical desalination process, a much greater force must be applied to push the water molecules in the opposite direction, against the osmotic pressure gradient. This applied pressure must be high enough to overcome not only the natural osmotic pressure but also the resistance and inefficiencies within the system. The high concentration of salt in seawater, typically around 35 grams per liter, creates a significant natural pressure that must be physically overpowered by pumps or heat. Any real-world system will always consume more energy than the theoretical minimum due to factors like friction, imperfect components, and the need for a constant, high driving force.
Energy Consumption Across Major Desalination Technologies
The actual energy consumption of a desalination plant depends heavily on the technology chosen, which generally falls into two categories: membrane-based and thermal-based. Membrane technologies, primarily Reverse Osmosis (RO), are now the most common globally due to their superior energy efficiency. RO works by forcing water through a semipermeable membrane under immense pressure, relying solely on electrical energy to power high-pressure pumps.
Modern, large-scale seawater RO plants typically consume between 3.0 and 4.5 kWh of electrical energy per cubic meter of water produced. This represents a dramatic reduction from the 1970s, when RO systems required up to 20 kWh/m³. This efficiency improvement is why RO accounts for approximately 69% of the world’s installed desalination capacity.
In contrast, thermal desalination methods, such as Multi-Stage Flash (MSF) distillation and Multi-Effect Distillation (MED), rely on heating the water to create vapor, which is then condensed into fresh water. While these processes produce very high-quality water, they require a significant amount of thermal energy, which is often supplied by steam from a power plant. When factoring in both the electrical energy for pumps and the thermal energy for heating, the total equivalent energy consumption for thermal methods can be much higher, sometimes reaching up to 25.5 kWh/m³ for older MSF plants. The dominance of RO lies in its ability to convert relatively cheap electrical energy into the required mechanical work, avoiding the energy-intensive phase change of boiling water.
Strategies for Reducing Energy Requirements
Reducing the energy footprint of desalination plants has centered primarily on increasing the efficiency of the Reverse Osmosis process. A major breakthrough was the widespread adoption of Energy Recovery Devices (ERDs) in seawater RO plants. These specialized components capture the significant hydraulic energy remaining in the highly pressurized brine stream that is rejected from the system.
Pressure exchangers, the most efficient type of ERD, can recover energy with an efficiency exceeding 96% and transfer it directly to the incoming feedwater. This recovered energy substantially reduces the load on the main high-pressure pumps, which are the largest consumers of electricity in the plant. Furthermore, optimizing the feedwater quality through advanced pre-treatment processes is another effective strategy for energy savings.
When the source water is properly filtered and treated, it prevents fouling and scaling on the RO membranes, which allows the plant to operate at lower, more stable pressures. Lowering the required operating pressure directly translates to less energy needed by the high-pressure pumps. Beyond process optimization, integrating renewable energy sources, such as solar or wind power, has become increasingly common to offset the demand for grid electricity. This integration helps make the total operation more sustainable.