This summer, the Spanish firm Abengoa announced it had signed an agreement with a Moroccan government agency to forge ahead with the first phase of a project to build the world's largest renewable energy-driven seawater desalination plant.

Abengoa will undertake the engineering, construction, operation and maintenance of the plant for 27 years. The project will produce 275,000 cubic meters (m3) of desalinated seawater daily, to supply 150,000 m3 water for drinking as well as 125,000 m3 for irrigation of 13,600 hectares of farmland near Agadir, a coastal town in western Morocco. The contract provides for a possible future capacity expansion up to 450,000 cubic meters a day.

According to the Moroccan government, the electricity to power the plant will come in by planned new high-tension wires from the Noor Ouarzazate solar power plant (pictured at top) nearly 400 kilometers (249 miles) east of Agadir.

Agadir, Morocco. Abengoa's new desalination plant will serve the people of this city - and irrigate 13,600 hectares of nearby farmland, too

But is this new plant is a harbinger of a boom in desalination plants powered by sun or wind? Is there an emerging business case for unsubsidized renewable-energy-driven desalination plants?

Desalination by the numbers

At present, less than 1 percent of the world's population depends on desalinated seawater for its daily fresh-water supply. There are around 21,000 large desalination plants in operation; most are in the Middle East.

While the Agadir plant will draw seawater from the ocean and turn it into fresh water, "only about half the desalination plants in the world do that. The rest process water from other impure sources, such as brackish groundwater or polluted river water," according to Klemens Schwarzer, a scientist at the Jülich Solar Institute in western Germany, 60 kilometers west of Cologne.

The Via Maris seawater desalination plant near Tel Aviv, Israel, uses the reverse osmosis (RO) process. It provides about 100,000 m3 of freshwater daily. The Agadir plant will use similar technology.

Can scarce fresh water be made abundant?

In theory, the potential for increasing global freshwater supplies by using desalination technologies is enormous. About 97.5 percent of the 1,385 million cubic kilometers of water on Earth is salty seawater. The remaining 2.5 percent is freshwater, but around 90 percent of that freshwater is locked into the ice caps of Antarctica, Greenland, or other glaciers. Humanity's total annual water use is, in turn, a small fraction of the remainder.

Given these numbers, could desalination plants drawing on seawater turn the world's deserts and semi-arid drylands into thriving green plantations?

The short answer is: In theory yes, but in practice, not easily - because "it takes a lot of energy and equipment to make fresh water from seawater," Schwarzer said. "That means it's expensive."

The cost of desalination always must be compared with the cost of piping or trucking fresh water from somewhere it can be obtained without needing desalination - i.e. from lakes, rivers, or freshwater aquifers.

The Noor Ourzazate solar power plant uses concentrated sunlight to heat a carrier oil. The heat is then transferred to water via heat exchangers, and the hot water then drives a steam turbine. It's a solar thermal power plant, not a photovoltaic power plant

Technologies for desalination

A bewildering variety of desalination equipment exists, but are only two main kinds of process for turning saltwater into fresh, drinkable water: Thermal desalination and "reverse osmosis" (RO) desalination. Both are energy-intensive.

Thermal desalination works by causing water to evaporate, leaving behind salt and other impurities. RO works by using a multi-stage filtration process culminating in the use of high-pressure pumps to force salty water through a membrane whose mesh is so fine that water molecules can pass through, but salt and other impurities cannot. Abengoa's Agadir plant will use RO.

According to Schwarzer, thermal desalination tends to generate purer water than RO desalination.

On average, each human being directly or indirectly uses 3.8 cubic meters of water each day, when everything from washing and drinking through agriculture and industrial water use is counted. That means Abengoa's Agadir plant, once complete, will produce enough to cover the needs of about 72,500 average global citizens.

Pressure filters comprising part of a desalination plant at Beckton, England. This reverse osmosis plant turns mixed river and tidal water from the River Thames into drinking-water, at a rate of 150,000 m3 per day

Since there are about 7.5 billion people in the world, a back-of-the-envelope calculation shows that it would take nearly 104,000 plants the size of the one being built in Agadir to provide freshwater for everyone on Earth.

The business case for solar desalination depends on location

Most of the large desalination plants in oil-rich countries like Saudi Arabia are thermal rather than RO plants. They use waste heat generated as a byproduct in oil-fired electricity generating plants, Schwarzer explained. That means the energy input needed for desalination is nearly free-of-charge.

The heat driving the desalination process could also be provided by large arrays of sunlight-concentrating mirrors - but it's expensive to make and install those mirrors.

That's one reason why the answer to the question posed at the top – "is the business case for solar desalination technology finally strong enough to unleash a boom?" – is no, not in countries like Saudi Arabia where natural freshwater is scarce, but oil and gas are cheap, seawater is readily accessible, and waste heat from fossil-fueled power plants is abundant.

Gas-rich Qatar has built a number of gas-fired thermal desalination plants - with money it got from selling gas to Europe. In early 2017 it completed its first major reverse osmosis seawater desalination plant, Ras Abu Fontas A3, big enough to supply water to a million people in Doha

But what about regions where both freshwater and fossil fuels are scarce and expensive?

"In those contexts, solar desalination may make sense," Schwarzer said. "Morocco imports most of its fossil fuels. It has a great deal of solar and wind energy that can be tapped, and it's beginning to do that."

Africa's financial challenge

There is an enormous unmet need for clean freshwater in sub-Saharan Africa too, Schwarzer added. With abundant sunshine, solar desalination makes technical sense for the continent, at least near the coasts. It could also be used to purify polluted river water. But most Africans can't afford desalination equipment.

Ethiopian children at a water well. Clean freshwater is the most crucial limited resource in Africa

People need at least 5 liters a day of water for personal use. Schwarzer's group at Jülich Solar Institute has developed small, decentralized multi-stage thermal distillation units that produce 10 cubic meters of freshwater per day - or 10,000 liters – at a cost of about two euro cents per liter. It's enough to provide drinking and cooking water for a village of about 2,000 people.

Two cents per liter sounds, to European ears, like almost nothing, "but it's actually quite a lot," Schwarzer said. "Consider a family of eight people - two adults, six kids. That's 40 liters, or 80 cents a day. For an African family earning just a couple of euros a day, it's too much."

The solution, he said, will have to entail some form of partial subsidy to cover the cost of building and providing desalination equipment.