Desalination removes salt and minerals from saline water, such as seawater, to produce freshwater. This technology is an increasingly important solution for water scarcity globally. However, the energy required to separate water from salt is the most significant factor limiting the widespread adoption and affordability of desalinated water. Energy consumption dictates a large portion of a plant’s operating cost and environmental impact.
The Thermodynamic Minimum Energy Requirement
The energy required for desalination is governed by the Gibbs Free Energy of separation. This principle defines the absolute minimum energy theoretically needed to separate pure water from dissolved salt. This minimum requirement is directly related to osmotic pressure, the natural force that drives fresh water toward the concentrated salt solution across a semipermeable membrane.
To desalinate water, a system must apply a force that overcomes this natural osmotic pressure. For standard seawater (approximately 35,000 milligrams of salt per liter), the theoretical energy limit is calculated to be around \(0.86 \text{ to } 1.1 \text{ kilowatt-hours per cubic meter}\) (\(\text{kWh/m}^3\)). This figure serves as the benchmark for a perfect, thermodynamically reversible process. All practical desalination methods require substantially more energy than this minimum due to irreversible processes, friction, and fluid losses.
Energy Consumption of Reverse Osmosis Systems
Reverse Osmosis (RO) is the most widely adopted membrane-based method for seawater desalination. RO functions by applying intense mechanical pressure to force water through fine membranes. This applied pressure must significantly exceed the water’s natural osmotic pressure, pushing the solvent through the barrier while leaving the dissolved salt ions behind. The electrical energy required for the high-pressure pumps is measured in kilowatt-hours per cubic meter (\(\text{kWh/m}^3\)) of product water.
Modern seawater RO plants have substantially reduced energy intensity due to advancements in energy recovery devices (ERDs). These devices capture the significant hydraulic energy remaining in the high-pressure brine stream rejected from the system. By transferring this recovered energy to the incoming feed water, ERDs dramatically reduce the demand on the main high-pressure pumps. Early RO plants without effective ERDs consumed as much as \(6 \text{ to } 8 \text{ kWh/m}^3\).
The best-performing, full-scale seawater RO facilities currently operate with a total specific energy consumption (SEC) ranging from \(2.5 \text{ to } 4.5 \text{ kWh/m}^3\). This range includes all necessary auxiliary processes, such as seawater intake pumping, pre-treatment filtration, and post-treatment conditioning. While the RO unit is the largest consumer, pre-treatment also contributes to the total electrical demand.
Achieving the lowest end of this range, sometimes as low as \(2.2 \text{ kWh/m}^3\), is typically accomplished by very large plants utilizing the most efficient pressure exchanger ERDs. These modern systems are approaching the practical thermodynamic limit, operating only about three times above the theoretical minimum energy requirement. Further energy reductions are difficult to achieve, as they require overcoming the fundamental physics of the separation process.
Energy Demand in Thermal Desalination Processes
Thermal desalination processes, such as Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED), use heat to evaporate water and then condense the resulting pure steam. Unlike RO, these methods primarily require thermal energy input, often supplied by low-grade steam from a nearby power plant. This makes the process suitable for co-generation facilities.
The energy demand for thermal methods combines the thermal energy used for evaporation and the smaller amount of electrical energy required for pumps. In MSF and MED systems, the thermal energy consumption is substantial, ranging from \(70 \text{ to } 120 \text{ kilowatt-hours thermal per cubic meter}\) of water. When expressed as an electrical equivalent, MSF plants can have a total equivalent consumption between \(13.5 \text{ and } 25.5 \text{ kWh/m}^3\), depending on heat recovery efficiency.
The electrical energy component for pumping in thermal plants is relatively low, typically \(1.5 \text{ to } 6.5 \text{ kWh/m}^3\). However, when the high thermal energy is converted for direct comparison with RO, the total energy footprint of thermal methods is generally much higher. These processes are most economically viable when an inexpensive or waste heat source is readily available, such as from an adjacent power generation facility.
Variables That Increase Energy Use
Several external factors determine how much energy a desalination plant consumes, often causing the real-world figure to rise above the ideal benchmark.
Salinity
The salinity of the source water is a direct influence, as higher salt concentrations lead to a higher osmotic pressure that must be overcome. For every increase in salt concentration, the necessary mechanical pressure applied in an RO system must also increase, leading directly to greater energy consumption.
Temperature
The temperature of the feed water also plays a significant role in the overall energy requirement. Colder water is more viscous, which increases the pressure needed to force it through RO membranes at the required flow rate. In thermal systems, a lower feed temperature means more energy is needed to pre-heat the water to the necessary boiling point for distillation.
Recovery Rate
The recovery rate is the percentage of incoming feed water converted into freshwater product. As more freshwater is extracted, the remaining brine becomes progressively more concentrated with salt. This increasing concentration causes the osmotic pressure to continuously rise, requiring the system to apply higher pressure to maintain separation and substantially increasing energy demand.