Quenching a thirst

Demand for desalination systems is growing rapidly around the world, but one of the biggest challenges facing the industry is the need to be more energy efficient and environmentally-friendly. Can innovative desalination technologies, coupled with renewable energy systems, satiate the demand for sustainable desalination solutions?

Siàƒ¢n Green

With the advent of desalination technology in the 1960s, the production of drinking water for populations living in areas with scarce water supplies became simple. Certain regions – in particular the Middle East – became core markets for suppliers of desalination equipment and services, and today, continue to be the mainstay of this industry.

“These traditional markets are areas that have very scarce water resources and so have relied on desalination systems for many years,” says Dr. Corrado Sommariva, divisional director of Mott MacDonald and president of the European Desalination Society. “The desalination market is growing very rapidly. In the UAE, the market is now worth $1000 million per annum.”

This rapid growth in the Middle East is being driven mainly by economic and population growth. Overall, the desalination market worldwide is estimated to be growing at some 10-12 per cent per year, and this is partly due to demand for desalination systems in a number of relatively new markets.

“These new markets are those which until recently relied mainly on traditional water resources, but which are now having to turn to desalination to supplement those supplies,” says Sommariva. “The USA and Australia, for example, are important markets now. They are turning to desalination because the cost of using natural water resources is rising, while the cost of desalination systems has fallen.”

Figure 1. Basic functionality of Enercon’s reverse osmosis (RO) desalination plant
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According to Sommariva, the cost of generating drinking water by desalination has fallen, largely due to new processes and an increase in energy efficiency. The cost of reverse osmosis (RO) desalination in particular has fallen because the amount of energy required to produce 1 m3 of drinking water has fallen from 7 kWh to 4 kWh. In addition, says Sommariva, life expectancy of plant has increased from 15 to 25 years, and there is a lot of competition between the main technologies.

Sommariva’s observations are echoed by Frank Hensel of the desalination sales management team of German wind turbine manufacturer Enercon. “Traditional sources of drinking water include ground water and rain, but in many parts of the world, population growth, industrial development and tourism are placing a strain on these resources.”

A key problem in the water industry is that groundwater is contaminated by seawater intrusion or industrial pollution, while in some countries – for example, Spain and Portugal – rainfall has diminished in recent years. Other areas where desalination technology is now in demand include Greece, Italy and North Africa. According to Sommariva, these countries are becoming increasingly aware of the environmental cost of exploiting natural water resources.

And while desalination can provide populations with a sustainable way of producing drinking water, there is a growing desire in the desalination industry to make the process more efficient and environmentally-friendly. The main focus of the industry is on reducing thermal wastage, says Sommariva, and there is also a growing interest in desalination powered by renewable energy systems such as wind turbines and photovoltaics.

Sustainable solutions

Enercon has been active in desalination since the 1990s. The company has recognised the need in the industry for sustainable desalination solutions, and this has been the main focus of its research and development efforts. These efforts culminated earlier this year in the launch of a RO desalination system. The technology is unique, not only because it uses a new energy recovery system, but also because unlike most RO systems, it does not use any chemicals.

“In developing new technology, Enercon’s philosophy is to avoid the creation of new environmental problems,” says Hensel. “The RO plant therefore uses no chemicals, just physical processes.”

In RO plants, the brackish water or seawater is pressurized and flows over a membrane. The structure of the membrane retains the dissolved salts, and the result is pure drinking water. In the Enercon system, feedwater flows through filters and a UV disinfection system to the energy recovery system. The pump pressure of 20 bar is transferred to sea water (56 bar) or brackish water (28 bar) and flows to the RO membranes. Here, feedwater separates into drinking water and brine. Drinking water leaves the system and the brine, still under pressure, flows back to the energy recovery system to support the process.

Figure 2. Enercon’s energy recovery unit links with the desalination process forming a continuous cycle
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In the absence of any energy recovery system, about 75 per cent of the energy input into a RO plant would go to waste, according to Enercon. The company’s energy recovery unit is therefore essential to increasing the efficiency of the process. It consists of a low pressure pump and a piston-type accumulator (PTA). The PTA transfers the pressure of the pump to the feedwater and recovers energy from the pressurized brine.

According to Hensel, the performance of the energy recovery system means that the company’s RO plant has a very low energy consumption – less than 2.5 kWh per 1 m3 of desalinated water (RO unit).

Wind power

In addition, Enercon’s RO plant has no fixed operating point. Water production can range between 12.5 and 100 per cent of nominal capacity by adjusting the piston speed in the energy recovery system. This means that output can be adjusted to match water demand, and that the RO plant can be powered by wind turbines, the output from which tends to fluctuate.

This, says Hensel, reflects Enercon’s holistic approach. “We want to be able to support a community’s needs for water and power, and a wind turbine-powered desalination plant can be run to supply and store energy and water precisely according to demand. For example, the desalination plant could run overnight and the water stored for the next day, while electricity produced by the wind turbines is fed to the grid during the day when power demand is highest.”

This illustrates that one of the main applications of Enercon’s wind turbine desalination system is for supplying the needs of remote communities, including tourist complexes or hotels. Other markets for the technology include islands, which tend to have good wind conditions. “Our focus at the moment is markets where demand for drinking water is around 500-5000 m3/day,” says Hensel. In the longer term, the company is hoping to aim for larger markets.

The key question, however, is whether such systems are suited to large-scale desalination applications. Some in the industry are sceptical, but Enercon believes that its desalination solution can go far. The company has been operating and testing a wind turbine powered desalination system on a Greek island since 2001. It will not give a great deal away about the plant, except to say that it used sea water to supply drinking water to the local population, and that it proved the reliability and performance of its RO plant, energy recovery system, and of the wind turbine-desalination system as a whole.

In addition, wind energy systems are becoming more and more efficient, and unit sizes getting larger, says Hensel. This, combined with the increasing efficiency and reduced cost of RO systems, means that wind turbine desalination systems have the potential to play a significant role in the market. “Oil and gas prices around the world are rising and the need for an alternative solution is quite clear,” says Hensel. “Enercon has an excellent reputation in the wind turbine market and we believe we can extend this to the desalination market.”

Containerized solution

Nevertheless, Enercon believes that its first commercial order will be for a grid-connected RO plant, rather than one powered by wind turbines. Its desalination plant is manufactured and tested at its site in Germany. The plant is modular, comprising a number of 20-foot (6 m) containers, each containing a separate part of the plant. This design enables easy worldwide transport and fast installation. “Providing the site is ready, we can have the plant operational within two weeks of delivery,” notes Hensel.