Seawater desalination technology has long faced high energy consumption. The most energy-intensive step in traditional seawater reverse osmosis systems is the high-pressure separation process, during which high-pressure pumps account for over 40% of electricity consumption. Many projects consequently face a high electricity bill dilemma during long-term operation.
Energy recovery devices are key to overcoming this dilemma and are a core technology that determines a project’s economics and sustainability. An energy recovery device for desalination can reuse the residual pressure in high-pressure concentrated brine, thereby significantly reducing RO energy consumption in the seawater desalination process.
This article introduces the technical principles of energy recovery devices in SWRO plants and compares the mainstream types. It will provide you with design ideas for optimized selection and energy reduction.
Why Energy Consumption Is Critical in SWRO？
Seawater desalination is essentially the exchange of energy for fresh water, and the physical process of pressurizing seawater with a booster pump is extremely energy-intensive. LCA shows that the OPEX of a seawater desalination plant often far exceeds its CAPEX.
Although the energy consumption ratio of different seawater desalination plants varies depending on the project size, electricity price, and system efficiency, even the most efficient plant still consumes 40% of its energy. In short, energy consumption directly determines the profitability of a desalination project.
The energy consumption of the high pressure pump is the primary energy drain. For example, in a medium-sized freshwater project with a capacity of 10,000 t/d, the motor power of the high-pressure pump could reach 800-1000 kW.
During RO, approximately 50%-60% of the concentrated brine leaves the membrane module under extremely high pressure. If an energy recovery device is not used to recover pressure, more than half of the input energy will be wasted. This is a direct loss of energy and a direct loss of operating costs. Therefore, energy consumption is crucial in SWRO.
ERD Demystified: What It Is and How It Works？
The ERD system is located between the concentrate outlet side of the seawater reverse osmosis membrane module and the feed seawater pipeline. The energy recovery unit captures residual pressure energy from the high-pressure brine flow and then transfers this energy to the newly entering seawater.
Its essence is an energy exchanger. Pressure energy transfer occurs between the high-pressure concentrated brine and the low-pressure feed seawater within its interior. The concentrated brine is then discharged at atmospheric pressure, while the new seawater is pressurized to near the pressure required by the membrane system and then pressurized again by a booster pump before entering the membrane module.
Types of Energy Recovery Devices
Different types of energy recovery units vary significantly in efficiency, stability, maintenance costs, and system integration complexity. Understanding these differences will help you make more informed choices during the project design phase. Currently, the mainstream energy recovery units include pressure exchangers, turbochargers, and Pelton turbines.
Pressure Exchanger (PX)
Introduction
A pressure exchanger is an energy recovery device based on the isobaric principle. Internally, it contains a precision ceramic or composite rotor with multiple flow channels. Driven by a motor, the rotor rotates at high speed, alternately contacting concentrated brine and low-pressure feed seawater. When the rotor’s flow channel reaches the high-pressure concentrated brine port, the high-pressure concentrated brine pushes the low-pressure seawater up, and then the rotor rotates again to discharge the atmospheric pressure concentrated brine, continuing the cycle.
PX has a high efficiency, reaching up to 98%. The fluid isolation design minimizes the mixing of the two fluids during pressure transmission. Chloride ion leakage can be controlled within 1%-3%. Currently, over 80% of newly built seawater reverse osmosis systems use isobaric pressure recovery units.
Advantages
Besides its high efficiency, the most obvious advantage of PX is isobaric recovery, with only a 2%-5% loss in outlet pressure. Furthermore, they offer high processing capacity per unit volume, are easy to install, and save floor space. In addition, it has a wide processing flow range and can be connected in parallel to meet the needs of ultra-large seawater desalination plants.
Disadvantages
Pressure exchangers are sensitive to the quality of pretreated water because the micron-sized gap between their rotor and stator is easily worn or jammed by particles in the incoming water. Furthermore, PXs have a higher initial investment and a higher unit price than other types and require more advanced maintenance techniques.
Adapted scenarios
PX is best suited for large-scale municipal water supply projects, industrial parks, and island water supply projects. It is particularly suitable for areas that are sensitive to energy consumption, have high electricity prices, or have limited land area. Its high efficiency can quickly cover the initial investment in terms of energy saving benefits, with a payback period of 1-3 years.
Turbocharger
Introduction
Turbochargers primarily utilize centrifugal force. They consist of a turbine end and a pump end connected on the same shaft. High-pressure concentrated brine enters the turbine end, impacting the turbine blades and causing them to rotate at high speed, driving the turbine and the coaxial pump-end impeller. The pump-end impeller then pressurizes the incoming seawater.
The efficiency of modern turbochargers fluctuates with changes in flow rate and pressure, with energy recovery efficiencies typically between 70% and 85%. A significant characteristic of turbochargers is that the concentrated brine and seawater do not come into direct contact, meaning there is no chloride ion leakage and no impact on the osmotic pressure of the reverse osmosis system. The unit requires no motor or external power source.
Advantages
Its structure is simple, with no complex control system and few potential points of failure. It also has lower pretreatment requirements due to its wider flow channels, making it less sensitive to influent particles and requiring less investment than a pressure exchanger. Turbochargers are easy to maintain, suitable for retrofit projects, and their modular design facilitates integration into existing systems.
Disadvantages
The efficiency of turbochargers varies with operating conditions, and the efficiency drops significantly when deviating from the design point, which places certain requirements on the stability of system flow and pressure. Its actual single-machine processing capacity is limited.
Adapted scenarios
Turbochargers are best suited for small to medium-sized projects, especially retrofit projects with limited space and scenarios where system stability requirements exceed extreme SWRO energy efficiency.
While less efficient than pressure exchangers, turbochargers offer higher reliability and greater tolerance for pretreatment conditions, remaining competitive in specific applications. These include situations with significant fluctuations in pretreated water quality, limited pretreatment conditions, or special scenarios with zero tolerance for chloride ion leakage. It is a simple, reliable, and easy-to-maintain solution that project owners can prioritize.
For a small project of 1000 m³/d, the annual electricity cost difference between pressure exchangers and turbochargers is approximately $10,000-$20,000 (calculated at $0.1/kWh), allowing for a comprehensive evaluation based on equipment investment, maintenance costs, and project lifespan.
Pelton Turbine
Introduction
The Pelton turbine originates from the mature Pelton turbine technology in the hydropower industry and was a mainstream solution for energy recovery in seawater desalination in the 1980s.
Unlike conventional turbochargers, the Pelton turbine disc has double-lobed water buckets evenly distributed along its edge. The nozzle sprays high-pressure concentrated brine into a high-speed water column. The spoon-shaped design of the water bucket causes the water flow to rotate almost 180 degrees. The change in water flow generates an impact force, which drives the turbine disc to rotate at high speed. The following process is similar to that of a turbocharger.
The impact design of the Pelton turbine means that its efficiency is affected by the nozzle design, the shape of the jet, and the precision of the jet matching with the turbine. Energy recovery efficiency is typically between 80-90%.
Advantages
It is mature, reliable, and has extensive operational experience. It can maintain high efficiency within a design flow range of 50%-150% and is highly adaptable to flow fluctuations.
Disadvantages
The Pelton turbine has relatively low efficiency and high system energy consumption. The high-pressure pump also has a large load, requiring the pressure needed to provide 100% feed flow, and therefore requires a large pump size.
Adapted scenarios
Pelton turbines are suitable for the continued operation of existing projects or for retrofit evaluation under specific circumstances.
Energy Consumption Comparison of Different Types of ERD
From an energy balance perspective, the total system energy consumption is the sum of the high-pressure pump energy consumption and the booster pump energy consumption. With energy consumption, the high-pressure pump only handles the net feed flow.
The energy consumption performance of different types of ERD is compared as follows:
| Parameter | Typical efficiency | Energy consumption （kWh/m³） | Energy-saving range |
Without ERD | \ | 6-8 | \ |
Pressure exchanger | 95%-98% | 2.5-3.5 | 50%-60% |
Turbocharger | 70%-85% | 3.5-4.0 | 40%-48% |
| Peloton Turbine | 80%-90% | 3.5-4.5 | 35%-45% |
ERD Selection for SWRO Plants
Improper selection of energy recovery devices may lead to problems such as pressure fluctuations, decreased efficiency, and increased maintenance costs. So, how do we choose the most suitable energy recovery unit for a specific seawater reverse osmosis project?
Plant capacity
For large-scale projects, pressure exchangers are the preferred choice.
For medium-sized projects, if electricity prices are high and long-term operating costs are a concern, pressure exchangers are the first option. If initial investment is more important and system simplicity is required, turbochargers are the better choice. For small projects, turbochargers are the best choice, as they better match the technical and economic considerations.
Pelton turbochargers can also be considered depending on civil engineering requirements and installation complexity.
Recovery rate
System recovery rate affects water production efficiency. High recovery rate designs utilize pressure exchangers.
For standard recovery rate designs, all three types of ERD can be used. Pressure exchangers offer the best energy savings, turbochargers provide a balanced solution, and Pelton turbines are suitable for specific retrofit projects.
Pressure level
Seawater reverse osmosis systems typically operate at pressures of 55-70 bar. High-pressure systems are commonly used in high- or low-salinity sea areas and employ PX and Pelton turbines from established high-pressure product lines.
Standard pressure systems can be used in all three scenarios, where PX offers advantages.
Low-pressure systems are commonly used for brine desalination or low-salinity seawater treatment, where turbochargers offer a cost advantage.
Future Trends in Desalination Energy Efficiency
Ultra-efficient ERD
Energy Recovery, a leading global manufacturer of ERD, has launched its next-generation pressure exchanger, the PX Q650. This new model achieves a peak efficiency of 99%, the theoretical limit, while reducing the volumetric mixing ratio to 2% and significantly increasing single-unit capacity. This translates to less chloride ion leakage and more favorable feed water quality for reverse osmosis systems.
Smart plant optimization
Future SWRO energy efficiency will rely more heavily on real-time optimization of ERD.
For example, dynamic efficiency optimization, where next-generation intelligent ERD are equipped with multi-sensor monitoring systems. It senses changes in inlet water pressure and salinity in real time and automatically adjusts motion parameters to maintain optimal efficiency. This type of dynamic optimization can improve system energy efficiency by an additional 1%-2% throughout the year.
Hybrid systems
ERD is now operating in conjunction with a wider range of energy systems, such as wind power and solar power. When renewable energy sources fluctuate, the intelligent control system can adjust the number and operating status of ERD in real time and respond in coordination with high-pressure pumps and booster pumps.
For off-grid or weak-grid areas in Southeast Asia and Africa, this means that ERD is not only energy-saving equipment but also a key technological component for achieving energy independence.
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