Desalination is a viable nexus technology: but local conditions are key

Juan A. de La Fuente and Baltasar Peñate

The world population is expected to increase from the current 8.5 billion to 11.2 billion by 2100 (World Population Prospects, United Nations 2017). By 2050, global demand for energy will nearly double, while water demand is set to increase by over 50%. To overcome the increasing constraints the world faces, we need to rethink how we produce and consume energy in relation to the water sector.

The authorities responsible for water and energy are generally separated. Each has its own priorities and there seems to be little incentive to collaborate in the planning and development of new policies. At the same time, the water and energy sectors have always operated independently and there may be some resistance to a better integration of both sectors. Often, studies on the interconnection between water and energy have been initiated and driven by specific local circumstances, such as water and energy crises.

Seawater desalination is an important option for addressing the world's water supply challenges, but current desalination plants use huge quantities of energy causing several environmental issues. The energy intensity of desalination processes has dramatically decreased over the past 30 years, from slightly more than 15 kWh/m3 in the 1970s to approximately 2.5 kWh/m3 today thanks in large part to reverse osmosis (RO) technology improvements. Still, several physical constraints limit the ability to reduce the energy intensity of RO much further. This means that energy efficiency in RO has almost reached its biophysical limits.  

Brine discharge into the sea can have a negative environmental effect on the marine ecosystems due to its high salt concentration and other chemicals. Devices like Venturi diffusers for brine discharge can be used to improve the dilution process and reduce their environmental impact. It has been shown that the capacity to improve the dilution of Venturi system is greater than 2.3 times the dilution obtained with conventional diffusers. Another option could be the valorisation of brine, by using it for the culture of the microalgaes for the production of molecules such as β-carotene and polyunsaturated acids. The biomass obtained can be used in animal nutrition and Nutraceutics.

It is very likely that the water issue will be considered, like fossil energy resources, to be one of the determining factors of world stability. Desalination processes involve a recurrent energy expense which few of the water-scarce areas of the world can afford. Even if oil were much more widely available, could we afford to burn it in such a manner so as to provide everyone with fresh water? Given the current understanding of the greenhouse effect and the importance of carbon dioxide levels in the atmosphere, environmental pollution caused by burning fossil fuels for desalination is a major concern.

Renewable energy (RES) technologies, mostly solar and wind energy systems, can provide access to a cost-effective, secure and environmentally sustainable supply of energy that can be used for water desalination. As RES technologies continue to improve, and as freshwater becomes scarce and fossil fuel energy prices rise, utilising RES for desalination becomes more viable economically. RES may provide water desalination cost reductions due to lower greenhouse gas emissions. For example, a seawater RO desalination system operating on traditional fossil fuel-based energy sources produces 1.78 kg and 4.05 g of CO2 and NOx per 1 m3 of desalted water, which can be reduced to 0.6 kg/m3 – 0.1 kg/m3 and 1.8 kg/m3 – 0.4 kg/m3, respectively, with electricity generated from wind or solar energy  (Raluy RG, Serra L, Uche J. 2005. Life cycle assessment of desalination technologies integrated with renewable energies. Desalination 183(1–3):81–93).

On the other hand, the role that desalination could play in the integration of electricity produced by renewable sources in the electricity grid is also an interesting topic.

The major constraint on increasing penetration of RES is their availability and intermittency, which can be addressed through using energy storage or smart control, when available, to balance renewable energy generation with energy demand.

The Canary Islands archipelago in Spain is a perfect example of how a region with water shortage and presence of RES resources has alleviated its water scarcity problem using desalination technologies, exploiting in turn the sun and wind resources available in the area.

The water – energy nexus has been one of the key R&D lines of the Canary Islands Institute of Technology (ITC). The ITC has developed and tested prototypes of different renewable energy driven desalination systems, operating in off-grid mode, since 1996. The ITC facilities in Pozo Izquierdo (Gran Canaria Island) are an ideal platform for testing RES desalination systems thanks to the excellent local conditions: direct access to seawater, annual average wind speed of 8 m/s, average daily solar radiation of 6 kWh/m2. Up to 18 different combined systems of renewable energy generation and desalination processes have been tested at the ITC.

Depending on the local environmental conditions, regulation and policy, desalination is a viable technology where RES resources are readily available. With planning and an adequate policy, desalination should be an alternative water resource. However, the energy dependence and the relatively high water cost must be analysed on a case by case basis before proposing specific arrangements.