Solar-powered seawater desalination plants offer a sustainable solution to the global water crisis. However, their processes still suffer from inefficiencies, leading to wastage and potential environmental impacts.
«By 2025, it is estimated that 1.8 billion people will live in areas facing ‘absolute water scarcity’ – with less than 500 cubic metres of freshwater per person per year – while two-thirds of the global population may experience ‘water stress,’ defined as having access to between 500 and 1,000 cubic metres of freshwater annually», warns the FAO (Food and Agriculture Organization of the United Nations).
Factors such as population growth, economic development relying on intensive water resource exploitation, and climate change are driving these phenomena, which are expected to worsen in the coming years due to the anticipated acceleration of global warming [source: United Nations Water].
In March 2024, the UN Environment Programme (UNEP) outlined seven actions to combat the global water crisis during the annual World Water Day, held on 22nd March. Among these actions is the «utilisation of non-conventional water sources» including seawater, provided that desalination processes are carried out sustainably. UNEP emphasises the need to «address the issue of toxic brine discharge into oceans and eliminate the risk of increased greenhouse gas emissions from the energy required to power the entire process». Toxic brinerefers to «a highly saline solution, with salt concentrations higher than seawater, typically at 3.5% salinity» which can cause significant harm to marine ecosystems.
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Desalination via solar heat: the importance of photothermal capacity in materials used
Desalinating seawater involves a process to remove either all or part of its salt content, making it suitable for drinking, agriculture, or industrial use. Among the various methods for desalination, the most environmentally sustainable is solar heat evaporation.
Recent advancements in solar evaporators focus on materials with high photothermal capacity, such as metallic nanoparticles, semiconductors, and polymers. A study published in Nature (“Flatband λ-Ti3O5 towards extraordinary solar steam generation“, 2023) highlights the critical importance of optimising the step that converts solar energy into steam to make solar-driven desalination more efficient and prevent energy waste.
«Most previous efforts have concentrated on capturing solar energy, while less attention has been given to enhancing the absorption of solar energy by the photothermal materials used» note the authors from Northeastern University and the Chinese Academy of Sciences in Shenyang, China.
The Chinese research team has proposed using powdered metallic material (λ-Ti3O5), which boasts a solar absorption rate of 96.4%. «By incorporating these powders into three-dimensional hydrogel-based porous evaporators with a conical cavity, we achieved an evaporation rate of approximately 6.09 kilograms per square metre per hour, with 3.5% saltwater» the research group explains.
Advances and current limitations of solar evaporators
The solar evaporators referenced by the Chinese scientists are three-dimensional, but this was not always the case. Up until five years ago, experimental photothermal evaporators were two-dimensional, consisting of a single flat surface. As researchers from the Future Industries Institute at the University of South Australia pointed out in their 2021 study “Sunlight to Solve the World’s Clean Water Crisis“, these early models «risked losing 10–20% of solar energy due to heat dissipation into the surrounding water and environment».
The breakthrough came with the development of the first three-dimensional evaporator, which features a fin-shaped design. This innovative structure redistributes excess heat away from the main surfaces and towards its “fins,” where water evaporation occurs. As a result, the evaporating surface remains cooler, preventing energy loss during the process.
This is just one of many examples of the ongoing evolution in seawater desalination through solar heat, leading to both significant improvements in evaporation efficiency and greater sustainability. Specifically, research has focused on developing solar evaporators with diverse structures – ranging from porous architectures to finned and columnar designs – while enhancing evaporation rates by, for example, optimising water channels and implementing multistage evaporation processes.
However, a recent study by the Waterloo Institute for Nanotechnology at the University of Waterloo (“Thermo-adaptive interfacial solar evaporation enhanced by dynamic water gating” – Nature Communications, 2024) highlights the challenges that still need to be addressed, particularly the issue of salt scaling within evaporator water channels. These deposits can cause malfunction and require regular maintenance.
To counter this, solutions have focused on «non-contact water channels and localised crystallisation, which prevent salt deposits».
Despite these advances, another major issue persists: the rigid structural designs and passive operational cycles of solar evaporators limit their prolonged functionality. This has led researchers to explore the concept of an “autonomous” solar evaporation process for desalinating saltwater. Here’s how this could work.
Saltwater desalination: focus on autonomous solar evaporation
The research team from the Canadian university drew inspiration from the natural water cycle to develop a solar evaporation structure that mimics how trees transport water from roots to leaves.
«We designed a dual-layer solar evaporator that induces water to evaporate, transports it to the surface, and condenses it in a closed cycle, preventing salt build-up that would reduce the device’s efficiency. Equipped with a dynamic thermal control system, it autonomously switches from water evaporation to salt washing», thereby minimising the risk of releasing the so-called “toxic brine” into the sea, which was previously mentioned.
The evaporator is constructed using porous nickel foam and features two strategic layers: the upper layer is assembled with polydopamine nanospheres – a synthetic polymer derived from dopamine – while the lower layer is engineered with sporopollenin, a biological polymer found in the outer walls of pollen grains.
Specifically, the upper layer acts as a photothermal interface, while the lower sporopollenin layer, which is thermo-responsive, «functions as a control layer». This dual mechanism ensures a continuous flow of saltwater without interruptions, while simultaneously self-regulating to address salt accumulation.
The new solar evaporator for seawater desalination also features a hollow structure, which provides thermal insulation. During testing, the design – according to the research team – «delivered high solar steam conversion performance, with a steam generation rate of up to 3.58 kg per square metre and a solar-to-steam efficiency of 93.9%».
The device is powered by solar energy (converting around 93% of sunlight into energy), contributing to the reduction of greenhouse gas emissions. It can produce approximately 20 litres of freshwater per square metre, «the amount recommended by the World Health Organization to ensure each person has enough water to drink and bathe without concern each day».
Glimpses of Futures
Transforming seawater into freshwater through solar evaporation has become an increasingly common practice in recent years. However, despite its noble aim of sustainably providing potable water from natural sources, this process risks becoming a source of environmental problems if its challenges are not addressed head-on.
hief among these issues are the discharge of toxic brine into the oceans and the greenhouse gas emissions produced by the energy required to power desalination processes.
With the aim of anticipating potential future scenarios, let’s use the STEPS framework (Social, Technological, Economic, Political, Sustainability) to analyse the impacts that the evolution of this new solar evaporation system for seawater desalination could have.
S – SOCIAL: in the future, the development of a more environmentally sustainable method for seawater desalination could broaden its range of applications. For instance, it might also prove useful for purifying contaminated freshwater and removing heavy metals. Additionally, another potential social benefit from the further development of this solar desalination technology could be the construction of a prototype that operates directly at sea, allowing for large-scale testing. If these trials prove successful, the device could eventually be deployed (in a sustainable manner) to supply freshwater to coastal communities in regions most affected by water scarcity.
T – TECHNOLOGICAL: the field of nanotechnology, particularly the search for new nanomaterials with enhanced solar energy absorption, will have the greatest influence on the evolution of the desalination device developed by the University of Waterloo. Future advancements are expected to bring new metallic nanoparticles, as well as innovative synthetic and biological polymers, which will further refine the solar evaporator. One of the key challenges that remains is finding the most effective solutions to manage salt build-up within the evaporator channels and its discharge into the sea, which could cause imbalances in marine ecosystems.
E – ECONOMIC: what is the economic impact of producing desalinated seawater? According to the Asociación Española de Desalación y Reutilización, whose aim is to «promote the appropriate use of seawater and brackish water desalination, and the reuse of treated wastewater for sustainable water management», the cost of producing desalinated seawater currently ranges between €0.5 and €1.0 per cubic metre. A litre of desalinated water «costs between €0.0003 and €0.0010» (noting that Spain, a pioneer in desalination in Europe, has been building desalination plants for over 60 years). These costs are relatively low, and solar evaporators like those developed by the Canadian research team, which aim to reduce heat loss and are powered by photovoltaic panels, could potentially lower them even further.
P – POLITICAL: in socio-economically fragile countries, water crises are often exacerbated by a lack of environmental protection policies, poor water management, and destructive interventions in freshwater ecosystems. For billions worldwide, this translates into a lack of clean water and safe sanitation facilities. The UN World Water Development Report 2024 reveals that 2.2 billion people lack access to bacteriologically safe water, while 3.5 billion are without safe sanitation. Clean water is one of the 17 Sustainable Development Goals (SDGs) of the 2030 Agenda (Goal 6), and with less than six years remaining to achieve this goal, effective water collection and purification technologies are urgently needed. The solar evaporation system discussed could, in the future, become portable, making it ideal for use in remote regions, such as drought-stricken areas of Africa, where access to freshwater is almost non-existent.
S – SUSTAINABILITY: seawater desalination does not always utilise environmentally friendly technologies, materials, and procedures. The challenges include the formation of excessively saline water condensate (discharged into the oceans), the risk of increased greenhouse gas emissions due to the high energy requirements of desalination plants, and heat loss resulting from poor absorption of solar energy by inefficient photothermal materials. The adoption of solar evaporators like the one proposed by the Canadian scientists could address these limitations, contributing not only to the production of clean freshwater but also to greater environmental sustainability.
