Strategies to power desalination with solar technology

04 July, 2020
As the most water-stressed regions on earth often enjoy a high percentage of sunny days, harnessing the power of the sun seems an obvious fit for desalination’s energy requirements.

Desalination of seawater is becoming essential as demand for freshwater expands well beyond the capacity of current reserves to sustain it. But on its own, desalination threatens to help feed the catastrophic climate change that is helping to accelerate water scarcity around the globe. 

Desalination is notoriously energy intensive, and energy consumption represents 50% to 60% of total operation costs for a plant. For early adopters of the technology, such as Saudi Arabia and other oil-rich states in the Middle East and North Africa, that wasn’t a problem. 

But with the growing understanding of a carbon-fueled climate catastrophe, using oil to power desalination is increasingly not a viable option, however low the price of oil sinks. 

Combining solar and desal

On the other hand, whether desalination is based on thermal technology or reverse osmosis the falling costs and climate-friendly nature of renewable energy make it a potentially attractive option for powering the process. 

And as the most water-stressed regions on earth often enjoy a high percentage of sunny days, harnessing the power of the sun seems an obvious fit for desalination’s energy requirements. 

There are a number of combinations of desalination and solar technology available to developers. The two main approaches are using the power of the sun to directly heat saltwater, or using electricity generated by photovoltaic (PV) solar cells to power reverse osmosis. 

Pros and cons of PV solutions

As the price of PV solar continues to fall, it has become an increasingly alluring proposition for powering desalination. Indeed, work has already started on a PV solar-powered desalination plant in the King Abdullah Economic City (KAEC) in Saudi Arabia, scheduled to go live this year. 

Moreover, there seems to be a recognition in the kingdom that the days of burning cheap oil to produce drinking water may be coming to an end. But there are some limitations for PV in the desalination arena that need to be addressed. The most obvious is lack of power at night. 

In addition, any reduction in sunlight during the day due to cloud cover, for example, could be problematic. If the pumps of a reverse osmosis plant stop, there is danger of damage to its essential membranes.

As a result, shortfalls in electricity would have to be mitigated by some form of backup energy storage, or through combination with other renewables such as wind.

In the case of the KAEC facility, the desalination plant will simply switch to grid power during the hours of darkness, expanding the plant’s carbon footprint. Elsewhere, lithium ion battery energy storage becoming increasingly affordable to developers. 

Currently a more expensive option, fuel cells could become an effective energy storage technology for solar as economies of scale bring down costs. 

As the price of PV solar continues to fall, it has become an increasingly alluring proposition for powering desalination.

Addressing thermal 

Despite enthusiasm for the reverse osmosis-driven Saudi KAEC project, Gulf states have mostly relied mainly on thermal-based desalination and predominantly multi-stage flash (MSF). This process currently produces around a quarter of the planet’s desalinated water. 

And although MSF is largely being replaced by more energy-efficient reverse osmosis in new projects, the latter is not ideal for warm, high-saline, polluted seawater prone to algal blooms, as found in in the Arabian Gulf. 

The more advanced thermal technique of multiple-effect distillation (MED) operates at lower temperatures than MSF. By reusing energy in successively lower temperature distillation stages, MED has lower overall energy requirements than MSF. 

And because of its lower operating temperatures, it could be well served by tapping waste heat from concentrated solar power (CSP). 

The advantages of concentrated solar

CSP covers a range of technologies, and perhaps the most exciting of all for desalination, or any operation that requires energy around the clock, are those that are integrated with energy storage. 

In such systems, the sun’s rays are used to heat molten salt that can retain its thermal energy for many hours after sunset, and thus continue to produce steam which drives electricity-producing turbines late into the night. 

The storage capacity also acts as a buffer, so that any short-term interruptions to sunlight, such as a cloud cover, won’t result in a system-jolting loss of electricity. 

Reverse osmosis plus CSP with storage

Although there are clear advantages to the co-location of desalination and CSP, there are also drawbacks in siting a CSP-plus-storage projects on the coast, where the majority of desalination plants are. 

Salt corrosion from sea breezes and suboptimal conditions for solar collection may mean that CSP could have a more effective role if based inland, dispatching electricity to reverse osmosis desalination plants rather than supplying a thermal plant with waste steam. 

And as we’ve seen, a plant with built-in energy storage could ensure that production of vital drinking water is not interrupted. 

Building a CSP plant with storage is expensive, and there is currently only around 6GW in operation, with around 1.5GW under construction and another 1.6GW planned. But interest in the technology is growing in several markets worldwide.

For example, one of the major opportunities for CSP development in conjunction with desalination is in Saudi Arabia. 

The kingdom’s National Renewable Energy Program has set a target of 9.5GW of renewable energy capacity by 2023, including potentially 1GW of CSP, as the country looks to diversify away from hydrocarbon resources. 

Meanwhile, the country’s daily demand for desalinated water is around 7.5 million cubic metres. That is predicted to rise by 30% up to 2030, and CSP-powered desalination could plug the gap in a climate-friendly way. 

Such a strategy would allow Saudi to save its oil reserves for export while increasing freshwater reserves. It would also potentially help cut the cost of CSP, which at present is relatively costly because a paucity of projects is preventing the emergence of economies of scale. 

A final benefit for the kingdom is that CSP plants offer significant opportunities for local supply chain development, creating jobs. 

And what works in Saudi would also work in many other countries that could benefit not just from water but also from carbon-free energy and extra employment.