For seawater desalination, purchasing equipment is only a one-time investment. The real key to profitability lies in operating costs, with electricity typically accounting for more than half of those costs.
However, in practice, you’ll find that different suppliers can quote energy consumption figures that differ by as much as double, some at 3kWh/m³, others at 6kWh/m³. Which method should we use for our project? This question directly relates to daily electricity bills and whether the project is worthwhile. Understanding energy consumption is crucial to understanding your operating costs.
What Is the Energy Consumption of a Desalination Plant?
This is the most frequently asked question. Let’s use industry-recognized standards: the energy consumption range for an SWRO system is approximately 3-6 kWh/m³, while that for a BWRO system is approximately 0.5-2 kWh/m³.
It’s important to note that you can’t simply compare the two based on the kWh figures. Because BWRO and SWRO systems have different water qualities and pretreatment configurations, a direct comparison will lead to incorrect conclusions.
Why is there such a large range in energy consumption? Mainly because energy consumption is highly dependent on project conditions and technical configurations.
What Affects Energy Consumption?
To answer the question above, the energy consumption of a seawater desalination system depends on the following key factors:
TDS
This is easy to understand: the higher the salinity of seawater, the greater the osmotic pressure, and the higher the pressure required to overcome this pressure, thus increasing energy consumption. Standard seawater TDS is approximately 35,000-45,000 mg/L.
If your project site has high salinity, such as in the Red Sea region, energy consumption will definitely be higher than the theoretical value. Conversely, for low-salinity seawater like the Caspian Sea (<16,000 mg/L), optimized energy consumption can be as low as 1.809 kWh/m³. The impact of TDS is significant.
Pressure
RO systems require pressure higher than the osmotic pressure to force water through the membrane. The standard operating pressure for seawater desalination is typically 50-75 bar.
Higher pressure results in a larger water production rate per unit time, but also higher energy consumption. This creates a balance problem: too low a pressure results in no water production, while too high a pressure leads to high electricity costs.
Recovery rate
Recovery rate = Product water volume / Feed water volume.
When improving the recovery rate, the more concentrated the concentrate, the higher the osmotic pressure, and naturally, the greater the energy consumption. Standard SWRO systems typically have a recovery rate of 35-45%.
Therefore, if you see a supplier with exceptionally low energy consumption, always ask about their recovery rate. A low recovery rate will lower the kWh/m³, but it will increase the total water intake, leading to higher pretreatment costs.
Temperature
As water temperature increases, water viscosity decreases, leading to increased permeate flow through the membrane. A suitable temperature can reduce operating pressure for the same permeate flow rate. One study showed that increasing the feed water temperature from 5°C to 30°C increased the permeate flow rate by 0.929 m³/h.
Membrane efficiency
Different RO membranes have different permeability under unit pressure. Using a high-flux membrane can reduce RO system energy consumption to approximately 2.22 kWh/m³.
Furthermore, membranes age over time, and energy consumption increases annually. This should be considered during operation and maintenance.
Energy Recovery Device (ERD)
ERD works by recovering pressure from the high-pressure concentrate in an RO system and reusing it to boost the feed water, thus significantly reducing the energy consumption of the high-pressure pump. Equipping an SWRO system with an ERD can reduce energy consumption by up to 60%, and modern isobaric ERDs can achieve efficiencies of over 98%.
How Is Energy Actually Calculated?
kWh/m³ is just a number; what you need is to convert it into how much electricity you’ll have to pay each day. The logic behind energy consumption calculations isn’t complicated:
Daily electricity consumption (kWh) = Daily water production (m³) × Specific energy consumption (kWh/m³)
Electricity bill = Daily electricity consumption × Local electricity price
Seawater desalination has a theoretical minimum energy consumption. According to the second law of thermodynamics, under conditions of 25℃ and 50% recovery rate, the theoretical minimum energy consumption for producing 1 m³ of fresh water from standard seawater is 1.1 kWh/m³. The energy consumption of a real SWRO system is typically 2.5-4.0 kWh/m³ (with an ERD).
A common misconception is that electricity bills are calculated by directly multiplying the supplier’s kWh/m³ by the electricity price. The values provided by suppliers are usually measured under standard operating conditions. It is recommended to leave a 10-20% margin on the supplier’s quotation to cover differences in project site conditions.
Can Solar Desalination Change Energy Consumption?
Many people have a misconception about this, believing that using solar energy will reduce the energy consumption of the equipment. But frankly, solar energy only changes the cost structure, not the energy consumption itself.
The amount of electricity a set of RO systems uses to produce 1m³ of water is determined by technical parameters such as pressure and recovery rate. Whether you connect to mains power, use diesel fuel, or use solar energy, the equipment will still consume the same amount of electricity.
The advantage of solar energy is that it saves you trouble in the long run. The prices of grid electricity and diesel fuel fluctuate due to policy and market conditions, while solar desalination equipment is the cheapest energy source for the desalination system. Although the initial investment in solar panels is significant, the long-term operating costs can be kept very low.
But solar power generation is intermittent; at night or on cloudy days without sunlight, the system must rely on battery storage or switch back to the grid and diesel generators to provide backup.
Real-world Examples
Seawater desalination plants of different scales and in different regions exhibit significant differences in energy consumption and cost performance. Below are some project case studies for reference:
The Aqaba desalination plant in Jordan achieves low energy consumption and cost by integrating a wind power-diesel-battery hybrid energy system. This plant has a production capacity of 109,500 m³/d, a specific energy consumption of 2.67 kWh/m³, and a water production cost of approximately US$0.85/m³.
Freshwater resources are scarce in the Canary Islands of Spain, making seawater desalination the primary source of water supply. In 2024, the DESALO 2.0 desalination plant officially commenced operation on Gran Canaria, with a production capacity of 2,500 m³/d, a specific energy consumption of 1.83 kWh/m³, and an estimated water production cost of approximately US$0.55-0.80/m³.
Conclusion
In conclusion, the energy consumption of seawater desalination directly determines your long-term operating costs. Understanding the key factors affecting energy consumption, mastering the calculation formulas, and differentiating between different project environments are crucial for making accurate decisions during procurement.
NEWater offers customized energy consumption assessments and solution design to help you calculate your electricity bills. Contact us, send us your project parameters, and receive a personalized energy consumption report.
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