A deep dive into desalination’s energy problem (and solutions)

TL;DR

• Water scarcity is a global problem that disproportionately affects arid regions like the Middle East. Approximately 1.1 billion people worldwide don’t have access to fresh water.

• Desalination is one solution to this problem. There are currently 21,000 desalination plants capable of producing water for more than 300 million people. However, desalination is responsible for 25% of the water industry’s energy consumption.

• While fossil fuel-powered reverse osmosis is one of the most used desalination methods, alternatives are emerging that are more energy and cost-efficient.

• The process can be improved by introducing next-generation filtration membranes, recycling waste energy through energy recovery devices, and using renewable power instead of fossil fuels.

• Innovative alternative methods may also provide a route forward; wave-powered desalination, forward osmosis and solar distillation could all provide fresh water without harming the environment.

The detail

Water scarcity is one of the most widespread and serious challenges being faced by people around the world.

It affects more than 40% of the world’s population – approximately 1.1 billion people don’t have access to fresh water and 2.7 billion experience water scarcity at least one month a year.

This is why attentions have turned to seawater, but there’s a problem. One litre of seawater typically contains around 35 grams of dissolved salt so, to meet standards set out by the World Health Organization, 98% of this salt must be removed for the water to be drinkable.

That’s where desalination comes in; the process of removing salt from seawater.

It’s estimated that more than 300 million people currently rely on desalinated water and this demand is only set to increase. There are more than 21,000 desalination plants, twice as many as there were a decade ago, desalinating more than 200 million cubic metres of water each day. In the next five years, the market is expected to grow 8.8% annually and will be worth $28.83 billion by 2030.

It's perhaps unsurprising that desalination is concentrated in arid regions, most notably the Middle East. Nearly two-thirdsof the region’s inhabitants live in areas where water is scarce. As a result, the Middle East is responsible for around 40% of the world’s desalinated water capacity, with production of desalinated seawater in the region expected to increase almost fourteen-fold by 2040.

Saudi Arabia is the world’s biggest consumer of desalinated water, with four out of five litres of fresh water consumed coming from the sea, followed by the United Arab Emirates, the USA, Kuwait, Qatar, Japan and Spain. When it comes to how this desalinated water is used, 62.3% is used by humans, 30.2% goes into industry, 4.8% enters the energy sector, and 1.8% is used in agricultural irrigation.

High costs and high consumption

The biggest cost factor in desalination is energy. All the world’s desalination plants combined meet less than 1% of global water demand and consume 25% of the energy used by the water industry.

Estimates suggest desalination consumes more than 200 million kWh every day, and in 2020, only 1% of that power came from renewable sources. This is largely due to the Middle East’s reliance on fossil fuel-based desalination, which accounts for 90% of the thermal energy used for desalination globally. Israel’s Hadera desalination plant, for example, is responsible 0.5% of the entire country’s energy consumption.

It is not just an environmental issue, as the energy required for desalination also drives up prices. Energy accounts for between a third and just over half of the total cost of desalinated water and the power demand for a state-of-the-art desalination plant is close to 3 kWh per cubic metre.

This is not to say that desalination plants do not yield economic benefits. The Carlsbad Desalination Plant, based outside San Diego in California, produces approximately 50 million gallons of fresh water each day. Not only does it provide clean water to 300,000 people, but it also adds $50 million annually to the region’s economy.

Finding alternatives to fossil fuels

Reverse osmosis is the most frequently used method to desalinate seawater. The technology was first developed in 1965 and is based on passing seawater through membranes that filter it and trap the salt. Unfortunately, the pressure required to force the water through the membrane uses a vast amount of energy. This is the method employed by many desalination plants in the Middle East, with around 70% of its desalination processes driven entirely by fossil fuels.

Reverse osmosis is not inherently ‘bad’, but to be more sustainable, the process needs to be powered by renewable energy instead of fossil fuels.

The switch is already underway. A giant plant being built in Al-Kharji, Saudi Arabia plans to use a reverse osmosis process driven entirely by solar energy and should have a capacity of 60,000 cubic metres a day, meeting the needs of the entire city. Solar PV panels are also being applied to multiple reverse osmosis desalination projects. In Dubai, for example, is priced at less than $0.02 per kWh, meaning the country’s Hassyan plant could close its financing round at a record low tariff of $0.36 per cubic metre.

Switching to renewables isn’t the only way that desalination plants can curb their carbon footprint. A next-generation membrane is also helping to lower costs by improving the energy efficiency of desalination technology. These graphene oxide membranes are a vast improvement on traditional polymer-based membranes as they offer superior filtration capabilities. Thanks to their higher permeability, these membranes can allow more water molecules to pass through while still effectively blocking salts and other contaminants. These membranes could halve the amount of energy required for reverse osmosis desalination.

Energy recovery devices are also helping desalination plants become more efficient. This is one method being employed by the Carlsbad plant, which has taken steps to reduce its carbon footprint by implementing state-of-the-art energy recovery technology. These systems are designed to capture and reuse energy that would otherwise go to waste during the desalination process. It can be achieved in several ways, from introducing Francis or Pelton turbines to using isobaric devices that can achieve up to 97% efficiency. A study found that using an energy recovery system could reduce the energy consumption of a seawater reverse osmosis desalination plant from 4.5 kWh per cubic metre to 2.5 kWh per cubic metre. Financially, this could represent a saving of $7 million.

New processes with potential

There are also alternative processes that could hold promise. Forward osmosis is a water separation technique that, like reverse osmosis, uses a semipermeable membrane but harnesses the natural energy of osmotic pressure to separate water from dissolved solutes. The osmotic pressure is used to transport water through the membrane and keep the dissolved solutes on the other side. As it’s based on natural energy, forward osmosis is much less energy intensive.

While solar PV can be used to power reverse osmosis plants, the sun can also desalinate on its own. Solar distillation imitates the water cycle, evaporating seawater in large facilities, which is then condensed and collected as fresh water. The only energy required comes from the sun’s heat, but large amounts of land are required and it’s not as efficient as other methods. Indeed, the conversion rate when using photovoltaic cell water desalination results in a loss of approximately 40%. However, an approach like multi-effective solar desalination has the benefit of being able to run on a 24-7 basis and produce 85 cubic metres of fresh water a day.

Innovative start-ups are finding more new ways to power desalination. Oneka, for example, has pioneered wave-powered desalination technology. It uses floating buoys on the ocean floor to capture wave power and drive a pump that forces seawater through filters and reverse osmosis membranes. The fresh water is then piped back to shore using the natural motion of the waves. The technology needs waves just one-metre high to work and its floating unit require 90% less coastal land than a typical desalination plant.

Dutch start-up Desolenator, meanwhile, is building on solar distillation to produce low-cost water to communities and businesses. Its system collects heat and electrical energy from solar panels and uses it to power a thermal evaporation system. Any excess heat is stored in hot water tanks and electricity held in batteries so that the process can continue throughout the night. Each of its modular plants can produce up to 250,000 litres of fresh water each day, using 100% solar power.

Smaller initiatives are emerging out of universities. A solar power system developed by King’s College London uses specialised membranes to channel salt ions into a stream of brine to leave behind fresh water. The system is designed to adjust to variable sunlight levels without affecting the volumes produced and could be 20% cheaper than traditional desalination methods. MIT has also created a device that circulates water which, when combined with heat from the sun, evaporates the water and leaves the salt behind. It’s believed that if the device was scaled up to the size of a small suitcase, it could produce four to six litres of drinking water every hour at a price cheaper than tap water.

In Israel, Sorek 2 is an example of how desalination could change. It is expected to be the world’s largest desalination plant with a capacity of 200 million cubic metres of water per year and has been designed with sustainability in mind. Not only does it feature highly efficient pumps and motors, variable frequency drives, energy-efficient reverse osmosis membranes, and isobaric chambers energy recovery devices, but it will also use a patented steam-direct solution that offers significant energy savings. It can also operate more efficiently by generating clean water during periods of low energy consumption and shutting down during peak hours.

The shift towards building desalination plants that prioritise sustainability from the outset shows that the industry is aware of its energy and cost problem.

Thankfully, innovative solutions have emerged that are tackling the issues head on. In isolation, each may have limitations, but when used in combination, next-generation membranes, solar distillation, forward osmosis, wave power and energy recovery devices all have the potential to transform the desalination industry.

— Lew 👋

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