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Filtration+Separation September/October 2011

Sustainable desalination:

Renewable energy in desalination systems T

here is an increasing use of renewable energy technologies in

desalination systems around the world. Anthony Bennett reports on

some of these projects and how they are meeting local needs.

Renewable technologies

With the exception of geothermal energy from within the Earth, the sun is our source of energy, whether it has been captured by plants using photosynthesis and stored within fossil fuels over millions of years, or utilised renewably at the present time. Renewable energy technologies include those which capture the sun’s energy directly – these are ‘solar energy’ technologies. When the sun’s energy acts upon the atmosphere, the oceans and the earth’s surface, indirect utilisation of the sun’s energy can be harnessed using

wind, biomass, hydro, tidal, ocean and a range of other emerging renewable energy technologies. We will now outline the various types of renewable energy technologies and provide examples of where these are being used to power desalination technologies.

Solar energy technologies

Direct energy from the sun can be harnessed using solar photovoltaic (PV) technology, as well as directly using solar stills, indirectly using solar ponds, and by using concentrated solar power (CSP). CSP systems use mirrors or

lenses to concentrate a large area of sunlight onto a small area. Electricity is produced when the concentrated light is converted to heat which drives a steam turbine connected to a generator. CSP is being widely commercialised for large scale use. However, solar PV is the established method currently commercially packaged alongside water treatment systems so we will concentrate on that technology here.

Solar PV is a method of generating electrical power by converting solar radiation into direct current (DC) electricity. It is usual to join several solar panels together, each panel

Renewable energy technologies are increasingly being used to power desalination projects.

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consisting of numerous solar cells. A Solar PV cell is a solid state device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Solar PV panels typically comprise a panel of crystalline silicon cells, an aluminum frame and a sheet of glass on the side facing the sun, allowing light to pass through while protecting the semi-conductor wafers from abrasion and impact damage. Various types of silicon wafer are used including monocrystalline, polycrystalline and solar thin film, a new technology where crystalline silicon thin films are attached to a glass backing. This technology combines the advantages of crystalline silicon as the solar PV cell material (including availability, non-toxicity, high efficiency, long-term stability) with the cost savings of the thin film approach.

Various companies provide pre-engineered hybrid systems that combine solar PV with water treatment technologies, either thermal or membrane based. For utilisation of the electricity, the DC current produced is transformed to alternating current (AC) using inverters.

One thermal desalination example that utilises solar PV is a filterless multi-stage flash evaporation technology developed through a strategic alliance between Australia Global Trading and CLLEEN™ Water and Power, a water treatment company based in Texas, USA. The technology has been introduced to the Australian resources sector. The CLLEEN Water and Power System is self powered using solar PV and batteries that store the solar energy for operation at night. The system can be mobile or stationary and lends itself to use in remote localities for desalination but also for use in the treatment of mining wastewater, exploration, gas, frac water treatment, humanitarian aid and in marine/offshore applications. The CLLEEN system

is pre-assembled in a standard container and evaporates seawater or wastewater at a typical rate of 1,090 m3/day.

McGrath [1] has described various membrane-based desalination plant applications in the Pacific Islands powered by renewable energy such as Nukubati Island resort off the north coast of Vanua Levu in Fiji. Here, all energy and water needs on the island are provided by a solar PV power plant. A partnership between Citor, an Australian reverse osmosis (RO) membrane desalination unit manufacturer and Solar Power Indonesia has provided customised solar desalination units. McGrath advises that island packages are provided with units smaller than the CLLEEN system that range from treatment capacities of 5 m3/day (3.1 kW power including pump feed) to 240m3 per day (100 kW power including pump feed). Across the Pacific region, successful implementation of renewable desalination systems is seen by McGrath as requiring government backing in the form of policy, finance and technological support, with opportunities for small scale demonstration projects funded by the international community. Each island nation will require a separate approach and assessment in order to most efficiently identify project opportunities.

Wind energy technology

Wind power is the conversion of wind energy, an indirect form of solar energy, into a useful form of energy, such as using wind turbines to generate electricity, windmills for the direct utilisation of mechanical power, wind pumps for water pumping or drainage, or sails to propel ships.

Small wind facilities are used to provide electricity to isolated locations and utility companies are increasingly buying back the surplus electricity produced by small domestic wind turbines.

A large wind farm may consist of several hundred individual wind turbines which will be connected to the electric power transmission network. At the end of 2010, worldwide energy production was 430 TWh, which is about 2.5% of the global electricity usage. Several countries have achieved relatively high levels of wind power penetration though, such as Denmark at 21%.

The construction of wind farms is not universally welcomed because of their visual impact, but any overall effects on the environment from wind power are typically less problematic than those of any other power source. The intermittency of wind seldom creates problems when using wind power to supply up to 20% of total electricity demand, but as the proportion rises, increased costs, a need to upgrade the electricity distribution network, and a lowered ability to supplement conventional production using fossil fuels may occur. Power management techniques such as exporting and importing power to neighbouring areas or reducing demand when wind production is low, can mitigate these problems.

The islands of Cape Verde, the isolated archipelago off the west coast of Africa, are an example of a remote location with reliable trade winds but no natural water resources. The Island’s government is now actively pursuing options to expand the country’s power and water infrastructure rapidly to enable an increase in the production of potable water by 20% to alleviate a major water shortage on the island of Sao Vicente, which includes the second largest city and an expanding population, currently at 75,000. The island has reliable trade winds and sites available for wind turbines where the annual wind speed averages more than 9 m/s. Options are being considered to provide an integrated

The sun’s energy is captured directly using various types of solar energy technologies.

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system directly linking wind energy to desalinated water production.

An established example of a large scale desalination system powered by wind energy is the Kwinana Desalination Plant, located south of Perth in Australia. Described by McGrath [1], the system was built by Degremont for the Perth Water Corporation and the West Australian Government and utilises RO technology. Feed water is transferred into the plant at a rate of 0.1m3/s through a 200m pipe with the inlet positioned offshore in Cockburn Sound. The plant takes half an hour to process and distribute the desalted water. The electricity for the RO system is provided by Power Emu Downs Wind Farm, which has 48 wind turbines and is rated at 80MW overall, located at Cervantes, a three hour drive north of Perth. Power from the wind farm is fed into Western Power’s distribution network and provides enough renewable energy to power the desalination plant. The plant requires 26 MW to run and uses an estimated 4.1 kWh/m3 of water produced. The plant opened in April 2007 and was the first of its kind in Australia. Perth had experienced a 21% decline in rainfall over the previous 10 years, so this plant has helped supply potable and industrial water to the area.

A number of small scale wind turbine technologies, including wind/diesel

hybrid systems, can be used to power desalination systems. Typically, the small wind systems are utilised alongside diesel back-up systems to power the water treatment processes during the night.

Biomass cogeneration

The production of electrical energy and heat from waste biomass is termed biomass cogeneration. This is a growing area of development especially in the UK with the planned introduction of the UK government’s Renewable Energy Incentive in April 2012. Wastewater can be reliably treated by various companies utilising anaerobic digestion (AD) to produce a valuable biogas resource – an energy gain from the system. Using a combined heat and power (CHP) plant, heat energy is generated from the biogas and recovered for use within the process. If wastewater treatment systems can be located near thermal or membrane based desalination systems then the energy from the CHP system can be used directly to power the seawater desalination system. Alternatively, desalination technology could be utilised to treat and reuse the wastewater from the AD system and associated processes. Downstream technologies such as membrane bioreactor systems can be used as pre-treatment to nanofiltration or RO technology to provide a high quality treated water for use within the industrial site.

Hydro power

Hydroelectric power generation has been around for over 100 years. Hydro power uses flowing water to create energy that can be captured and turned into electricity. Hydro power provides the largest source of renewable energy in the US.

The most common type of hydro power plant uses a dam on a river to store water in a reservoir; this is called Impoundment hydropower. Water released from the reservoir flows through a turbine, spinning it, which activates a generator to produce electricity. The most famous hydroelectric system is probably the Hoover Dam in the US.

Ocean thermal energy

Low temperature thermal desalination (LTTD) is a type of desalination that utilises ocean thermal energy conversion (OTEC), a base-load renewable energy that provides for continuous operation of the desalination system. This process utilises the natural temperature gradient in the ocean but could also be applicable to any system where there is a temperature difference greater than 8°C, such as where waste heat is produced or generated in biomass cogeneration. An example of a commercial system has been developed by GEC. The system can also be adopted to concentrate fluids, thereby reducing shipment costs, and producing pure water at the same time.

Wind energy provides 2.5% of global energy requirements.

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The Ocean is the Earth’s largest solar collector and due to its mass, there is little temperature difference between day and night. The surface layer, the euphotic zone, receives all the solar energy and extends from the ocean surface to a depth of 200 metres. It is the warmest layer, and depending on geographical location, can reach temperatures of over 30°C (36°C has been recorded in the Persian Gulf). The deep layer, the disphotic zone, occurs at depths from 200m to 1,000m and is sometimes referred to as the twilight zone since the sunlight is very faint. Due to the lack of solar energy, the water temperature decreases rapidly with increasing depth. This phenomenon is referred to as the thermocline. The temperature differences below depths of 1000 m are small and therefore not considered for OTEC.

The LTTD process from GEC comprises a flash evaporator and a condenser. Warm seawater from the euphotic zone is pumped to the evaporator and cold seawater from the disphotic zone is pumped to the condenser, which generates pure water typically with a conductivity of 5 μS/cm. LTTD has the advantage of low maintenance compared to other desalination methods as scaling is greatly reduced at the low temperature in which the process operates, and filtration is unnecessary.

OTEC has been used to generate electricity for a number of years. The ‘working fluid’ within the process is pumped from a condenser to an evaporator and the warm seawater is used to evaporate it. The vapour rotates a turbine thereby generating electricity. The vapour then enters the condenser where the cold seawater condenses it back to a fluid. The fluid returns to the pump and the cycle is repeated. With its high latent heat, ammonia is the working fluid of choice in OTEC. At specific pressures, ammonia evaporates at a temperature lower than the warm seawater, and condenses back to a liquid at a temperature higher than the cold seawater.

Additional technologies

Other renewable energy technologies include hydrogen fuel cells, geothermal processes, various types of wave energy utilisation systems and osmotic power.

Conclusions

Commercial systems that link together renewable energy and desalination technology into one packaged process tend to be small-scale and for use in remote locations where a mains electricity supply is unavailable. These examples combining solar PV, small scale wind and ocean thermal energy conversion with thermal and membrane-based desalination technologies demonstrate that applications tend to currently be niche and specialised. There is scope, however, for the direct connection of

larger scale renewable energy producing systems to larger water treatment plants. This has been done at many sites using the power distribution network, such as the example in Perth, but projects similar to the one described in Cape Verde are likely to be seen in the short term. There is also the potential for the larger scale development of systems utilising ocean thermal energy conversion for the desalination of seawater. Additionally, the various government incentives available for the utilisation of renewable heat and power from waste and wastewater are likely to result in a number of exciting developments in the medium term. •

References

[1] McGrath, C (2010), Renewable Desalination Market Analysis: Oceania, South Africa, Middle East & North Africa, ProDes Project, Aquamarine Power Ltd

Further information

ProDes Project (Promotion of Renewable Energy for Water production through Desalination)www.prodes-project.org

Solar Power Indonesiawww.solarpowerindonesia.com

Citorwww.citor.com.au

CLLEEN Water and Powerwww.waterdesalinationplants.com

GECwww.otec.ws

Contact:Anthony Bennett

www.clarityauthoring.com

Anthony Bennett is a filtration and separation specialist at Clarity Authoring.

The ocean is the Earth’s largest thermal collector.