Wind-Solar Hybrid Desalination System Design for Off-Grid Water Supply

No power grid? Is diesel too expensive? Worried about the operation at night and on cloudy days? This article will help you determine the most suitable solution, as well as the system’s design logic, cost, and payback period.

Why Use a Wind-Solar Hybrid System for Desalination?

For off-grid desalination projects, the biggest challenge isn’t the desalination process itself, but rather where to get the electricity.

The traditional solution is diesel generators, but diesel transportation costs are high, and prices fluctuate greatly in remote areas. Therefore, many people have considered using solar energy. Solar photovoltaic (PV) technology is mature, and prices are decreasing year by year, making it a very good option. But what if there’s no sun at night or on cloudy days?

This is why wind-solar hybrid desalination systems are needed. A typical hybrid desalination system integrates both solar and wind power generation. Combined with energy storage, they can provide a 24/7 off-grid water supply.

For islands, coastal areas, and remote communities—regions without a power grid or with an unreliable grid. The wind-solar hybrid power supply is currently the most practical off-grid power solution for desalination.

When to Combine Wind and Solar?

Not all projects require wind-solar hybrid power. Sometimes solar power alone is sufficient, while other times wind power is more cost-effective. The key choice depends on your project’s specific circumstances.

When conditions such as good sunshine, low wind speeds, primarily daytime water use, sufficient size, and a limited initial investment budget are fully met, solar power alone may be sufficient. 

When conditions such as poor sunshine, high wind speeds, limited land, and mature wind turbine maintenance capabilities are fully met, wind power alone may be sufficient.

A wind-solar hybrid seawater desalination system is strongly recommended if any of the following conditions are met:

• Requires nighttime operation
• High diesel prices
• Unstable reliance on a single resource.

Wind-Solar Hybrid System Design Logic

The core of the hybrid renewable energy desalination system design is to convert water demand into electricity demand and then distribute the electricity demand to solar and wind power. The overall approach involves five steps.

Step 1: Define Daily Energy Demand

The first step in the design is to determine how much fresh water the project needs to produce each day, which forms the basis for subsequent calculations.

Step 2: Calculate Total Energy Requirement

Reverse osmosis is currently the most energy-efficient seawater desalination technology, with a unit power consumption of approximately 2.9 kWh/m³. Taking into account factors such as water intake, pretreatment, and high-pressure pumps, the estimated range in engineering design is usually 3-5 kWh/m³.

Step 3: Solar Contribution Calculation

Solar photovoltaic power generation depends on installed PV capacity and peak sunshine hours. The estimation formula is: Daily power generation = Installed capacity x Peak sunshine hours x System efficiency. The system efficiency is typically taken as 0.75-0.85 to account for losses.

Step 4: Wind Energy Contribution Calculation

Estimating wind power output is relatively complex because wind speed and power generation have a cubic relationship; doubling the wind speed results in an eightfold increase in power generation. A rough estimate is that under an average annual wind speed of 6 m/s, a 20 kW wind turbine generates approximately 35,000-45,000 kWh annually. 

In actual engineering projects, wind turbine selection must be based on calculations using the wind speed distribution curve (Weibull distribution) of the project site.

Step 5: Hybrid Energy Ratio Calculation

The wind-solar ratio is the core of the entire system design logic, optimized based on the following variables:

• Local solar resources: Better sunshine conditions allow for a higher proportion of solar power.
• Local wind resources: Higher and more stable wind speeds allow for a higher proportion of wind power.
• Energy storage costs: Expensive batteries necessitate integrating multiple power generation sources to reduce the need for storage.
• Land use restrictions: With limited land, wind power has a greater advantage.
• Nighttime water usage: Higher nighttime water usage necessitates wind power or more energy storage.

A common design starting point is 60% solar power and 40% wind power, then adjustments are made based on the aforementioned variables. In areas with high wind speeds, the proportion of wind power can be increased to 50-70%. In practical engineering, through algorithm optimization, the optimal wind-solar ratio with the lowest LCOE can be found while meeting load requirements. And get the number of solar panels and wind turbines needed.

System Configuration for Desalination Applications

A complete wind-solar hybrid seawater desalination system consists of a power generation unit, an energy storage unit, a control unit, and a desalination unit.

Power generation unit

Electrons in the semiconductors of the solar panel are activated by sunlight. Free electrons migrate, generating an electromotive force and forming an electric current. This is how a solar PV array converts solar energy into direct current. Its actual configuration depends on local sunshine conditions and water demand.

A wind turbine system uses electromagnetic induction to convert wind energy into mechanical energy, and then into electrical energy. The actual selection of wind turbines must be based on the project site’s wind speed distribution curve.

Energy storage unit

Battery storage ensures a stable power supply to the system even in the absence of wind and sunlight. Whether energy storage is needed depends on the proportion of nighttime water consumption in the project and the complementarity of wind and solar resources. For projects that require continuous 24-hour operation, it is highly recommended to configure an energy storage system.

Control unit

The Hybrid controller manages the input of solar and wind power, preventing overcharging and over-discharging of the batteries. It is the central control unit for the entire power generation system.

The Energy Management System (EMS) intelligently switches the system’s operating mode based on weather conditions, load demand, and energy storage status to ensure the stable operation of the desalination system.

Desalination unit

The core components of the RO desalination unit include a pretreatment system, a high-pressure pump, a reverse osmosis membrane module, and a post-treatment system. It is crucial for achieving the separation of salt and freshwater.

The Energy Recovery Device (ERD) is a key piece of equipment for reducing desalination operating costs, lowering system energy consumption by 30-50%.

Cost Structure and Investment Breakdown

CAPEX

OPEX

Off-grid desalination cost comparison

In most remote areas, wind-solar hybrid power generation has high CAPEX but extremely low OPEX. In areas with high diesel transportation costs, the payback period is faster. Over a 20-year lifespan, the total cost of a wind-solar hybrid system is typically lower than that of a diesel system. 

The table below compares the costs of wind-solar hybrid solutions and diesel solutions:

ROI and Payback Period

Having clarified the cost structure, the crucial question now arises: how long does it take for this system to recoup its investment?

Based on multiple academic studies and engineering case studies, the typical payback period for a wind-solar hybrid seawater desalination system is between 3 and 7 years. The factors influencing the ROI analysis are as follows:

• The more diesel fuel costs in the region, the greater the benefits of replacing diesel and the faster the return on investment.
• The larger the water demand scale, the lower the unit cost and the faster the return on investment.
• The higher the system utilization rate, the faster the return on investment can be achieved through continuous operation compared to intermittent operation.
• The higher the availability of wind/solar resources, the fewer devices are needed, the lower the CAPEX, and the faster the return on investment.

Best Applications for Wind-Solar Desalination Systems

On Trinéfé, the largest island in the Canary Islands of Spain, the supply of freshwater for tourists and residents has become a problem. The main issues are high energy consumption, reliance on imported fuel, and an isolated power grid. 

However, the region boasts the richest wind energy resources in the archipelago, and a PV-Wind hybrid system has been implemented, enabling a freshwater plant with a maximum capacity of 20,000 m³/d.

Residents of an Indonesian island have long faced shortages of both fresh water and electricity. Diesel transportation is costly, and supply is unreliable. 

The island ultimately opted to build a 720kW wind-solar-storage integrated off-grid power generation system, which produces 175,000 tons of pure water annually and generates over 1 million kilowatt-hours of electricity annually.

To address its increasingly severe water shortage problem, Morocco is vigorously promoting seawater desalination and mandating that newly built seawater desalination plants use clean energy. 

The Agadir project is a benchmark project in this field and is Africa’s first seawater desalination project directly powered by wind-solar power. After expansion, the total capacity reaches 400,000 m³/d, and clean power supply reduces desalination costs by 15-25%.

Conclusion

Having satisfied the aforementioned logic for technology selection, we can understand that a wind-solar hybrid desalination system is one of the most practical solutions for providing freshwater to off-grid areas. Solar and wind power are naturally complementary, and when combined with appropriate energy storage and ERD, a stable 24/7 water supply can be achieved. 

Furthermore, the total cost is significantly lower than that of diesel fuel, making it a long-term, controllable, and low-carbon path.