Finding the most sustainable — and financially viable — means of re-using seawater has become an imperative. Dr Kapil Garg, Research Fellow in Heat Engineering, Cranfield University, investigates.
An infrastructure of desalination technologies will be essential for the wellbeing of populations, alongside economic prosperity, and in some cases, social and political stability.
The current output of desalination plants globally is more than 90 million cubic metres per day, and growing populations and their demands for higher standards of living mean this figure will need to grow exponentially to meet needs – and wants. In the Middle East and North Africa, the gap between demand for water and actual supply is said to be around 42 cubic kilometres per year, according to the World Bank figures, and expected to increase by five times that by 2050
The race is on to find the most cost-effective, low-carbon desalination plant: urgently needed technology for ensuring drinking water supplies globally. Solar-powered desalination plants, the theory and practice, have been under development since the start of the 21st century. The challenge continues to be around gaining hard evidence of viability in terms of costs and scale.
Research has demonstrated that MED technology is a viable option involving low energy demand. The water production costs, though, are still relatively high. The integration of the MED with low temperature (60 °C–95 °C) solar collectors means a cost of between 2$/m3 and 3.6$/m3; for medium temperature (165 °C–200 °C) it is 1.4$/m3–3.1$/m3; and high temperature (370 °C–530 °C): 1.8$/m3−2.2$/m3. In general, that would mean a payback period in terms of investment into infrastructure of four to 16 years.
A new large-scale MED pilot is planned for the island of Tenerife, one of the Canary Islands located off the coast of Africa. REWAISE (Resilient Water Innovation for Smart Economy) is an EU-funded project being run by Cranfield University alongside Solar Water plc. This involves a large-scale thermal desalination demonstrator.
The proposed prototype is a 20-metre solar dome. A concentrated solar power field of parabolic trough collectors provides the thermal energy needed for the plant’s operations, around 4.6 MWth from the hot Canary sun. The power is integrated with an MED system which works at a relatively low temperature to avoid the potential risks of scaling and fouling in the works, as well as reducing overall energy consumption. The projected daily production of fresh water from the dome is 216 m3 (only making a contribution to the island’s drinking water needs at this stage, but demonstrating the potential for scaling up and extending the number of domes). Compared with a more standard reverse osmosis process used for desalination, there is up to a 90% reduction in the production of carbon.
The new generation of solar desalination may be based on multi-effect distillation (MED) technology. In MED, feed water is evaporated in several stages under reduced temperature and pressure. In the first stage, seawater is heated and evaporated by the heat from the external source — in this case either solar, geothermal or waste heat from any industrial plants. For the remaining stages, feed water is evaporated by using the latent heat of freshwater vapours flowing from the previous stage. In this way, the latent energy of the vapours is used to evaporate the seawater, and freshwater condensation is collected from each stage. But in the last stage, vapours produced are condensed by the incoming seawater. In the case of solar-driven-MED units, energy to evaporate feedwater in the first stage is supplied by using a solar-thermal technology such as a parabolic trough collector or evacuated tube collector etc.
Reverse osmosis (RO) technology is more energy-efficient and widely implemented than MED technology due to the low cost of water production, although carbon emissions are higher. Also, solar-driven MED plants are important for those regions with severe water shortage but a higher incidence of solar energy. In the case of Gulf countries, where seawater can have harsh characteristics, such as high temperature, residuals of boron, high TDS and bromides, and severe fluctuations in seawater intake quality, the RO technology faces operational challenges. Also, with thermal storage technology becoming more mature, the challenge of operating solar-driven MED plants for 24 hours a day can be easily addressed.
The proposed prototype consists of a 20-metre solar dome (geodesic in design) integrated with 3 stages of the MED unit. Inside the dome, a large proportion of the feed water is aimed to be evaporated and the vapours act as the heat source for the feed water to be evaporated in the MED unit. A concentrated solar power field of parabolic trough collectors provides the thermal energy needed for the plant’s operations, around 4-5 MWth from the hot Canary sun. This thermal power is supplied to the feed water through a suitable heat exchanger placed in a trench under the dome and this desalination system works at a relatively low temperature (around 65-70oC) to avoid the potential risks of scaling and fouling in the works, as well as reducing overall energy consumption. The projected daily production of fresh water from this unit is approximately 200 m3 without storage. Compared with a more standard reverse osmosis process used for desalination, there is up to a 90% reduction in the production of carbon.
The challenge with this — and all other forms of desalination technology — is finding the working balance between an effective low-carbon process and one that is economically viable for industry and investors. One important way of improving viability is the deployment of renewable energy driven desalination systems at a faster pace. For example, Morocco via NOORo project with CSP technology and DEWA is using PV technology at Mohammed bin Rashid Al Maktoum Solar Park. As the costs reduce, the pace of deployment will inevitably accelerate. Also, capturing and re-using the waste heat available from industries and power plants could be an alternative to respond towards the decarbonisation of this sector.
With an infrastructure of sustainable desalination technology in place, the global water supply system will have much improved resilience to extreme weather events and shocks. But major investment is needed: a will in the industry to support innovation.
More details at cranfield.ac.uk/themes/energy-and-sustainability.