Desalination using renewable energy in Australia
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DESALINATION USING RENEWABLE ENERGY IN AUSTRALIA
D.G. Harrison, G.E. Ho, and K. Mathew
Environmental Science Division, Murdoch University, Murdoch, Western Australia, 6150
The history and performance of renewable energy driven desalination plants in Australia are reviewed. Included are the 3 500 m 2 of solar stills built in South Australia in 1966, the two small scale photovoltaic reverse osmosis (RO) plants currently operating in Central Australia, a larger containerised photovoltaic RO plant from Western Australia, a Wind Powered RO Plant in Shark Bay, Western Australia, and two experimental Windmill powered RO plants designed at Murdoch University.
The continuing research and development program at Murdoch University on the application of energy recovery systems, pump optimisers and array trackers for photovoltaic RO plants is also described.
Desalination; renewable energy; solar: wind.
In the arid interior and northern coastal areas of Australia, fresh surface water is rare and decreasing and ground-water is often brackish or contains high levels of nitrates and fluorides. Transportation costs are high and fuel for generators is expensive and delivery unreliable. Water supply plants are also often a long distance from the communities that they service. Accordin~y, the use of renewable energy sources for water treatment for these remote communities is a realistic option.
There are a number of ways that renewable energy can be harnessed for desalination. It can be used directly. such as the solar still which uses the heating capacity of sunlight the evaporate water. It can be converting directly to mechanical energy to drive pumps as seen in the Murdoch University, Wind powered RO plants. It can be converted to electrical energy via photovoltaics, wind turbines, solar ponds, water turbines, wave or tidal power, and the electrical energy used to drive pumps, as with reverse osmosis, or provide a field, as with electrodialysis.
Because renewable sources fluctuate diumaUy, as with solar power, or more randomly as with wind power, there are also a number of ways this conversion can be controlled. The plant can be run at a steady rate with the renewable power connected to the town grid so that power can be exported or imported to the plant as required. The electrical energy can be stored chemically in storage batteries and run at a steady rate, either 24 hours per day or run at set hours per day using a smaller bank. The plant can be controlled to only operate when required power levels are available and dump excess power to maintain steady production for interrupted periods, working best with solar power. It can, of course operate using the power as it becomes available but usually needs some internal control to maintain system efficiency.
Denham, Shark Bay
The Water Authority of Western Australia commissioned an RO plant at Denham in Shark Bay, Western Australia in 1989 producing 130m 3 product per day from brackish feed of 4,500ppm (TDS). When a second plant of 168m 3 per day was commissioned in 1991, the power requirements exceeded the diesel grid's capacity and a supplementary 30 kW Westwind turbine was installed by the Authority to power the plants. Being grid connected, the plant imports power when required and exports power back to the grid when excess power in generated (Brown, 1991).
The turbine exceeded the manufacture's specifications by 5% and is producing 32kW and annually produces 60-65 MWh compared to the manufacture's specification of 45-50 MWh. This is almost twice the world average for a 30kW turbine. It has a 47% utilisation rate reflecting the consistency of the wind at the site.
The 1991 buy-back rate from Western Power, the State power authority, was 18 cents/kWh while the cost of production was 12.5 cents/kWh and the cost of power from the grid was 29 cents/kWh. As a result, the Water Authority only pays for one third of the value of power used, but produces approximately 80% of its total power usage. The turbine system will pay for itself in six years and save the Authority $16,000 per annum, or $180,000 over the life of the turbine.
Annual savings in emissions are estimated at; Sulphur Dioxide 400 - 640 kg Nitrous Oxide 240 - 480 kg Carbon Dioxide 60,000 - 100,000 kg Particulates 32 - 72 kg
No major operational problems have occurred over the succeeding years.
Only two other operational wind powered RO plants appear in the literature. GKSS installed a wind powered sea-water RO system on an island in the North Sea using an Allgaier/Hiatter 6kW turbine to power 30m 2 of GKSS plate module membranes to produce 6m 3 of product per day (Petesen et aL, 1979). A stand-alone wind powered system similar to the Denham plant is proposed for Jordan. (Habali & Saleh, 1994)
Research at Murdoch University
Two experimental Windmill powered RO plants designed at Murdoch University used standard wind-mills to pump brackish ground-water into pressure vessels that could be pumped up to an operating pressure of 1200kPa and automatically discharged to a cut off pressure of 600kPa. Pumped water was diverted via a bladder type pressure relief valve to ensure over-pressure of the membranes did not occur. Recover)' remained at less than 10% to protect the'membranes as no pre-treatment was used.
In strong winds, the system operated in steady-state conditions when the pressure vessel acted only as surge suppressors to even out the flow, and a lot of water bypassed the RO modules as over-pressure. In lower wind speeds, the vessel could gradually pump up to pressure and cycle throughout the day. Production rates were 213L per day from brackish water, though target rates were 500-800L per day (Robinson, 1992).
The system suites adaptation to existing wind-miUs where a dual supply is desired. The reject stream is mixed with over-pressure by-pass water and is quite useable for stock, while the high quality water meets relevant drinking water quality guidelines and in quantity terms, is sufficient for a family if lower quality water is used for toilet flushing etc.
Output from the system can be expanded by adding more RO modules in parallel, as the accumulators present the same flow rates and pressure regardless of the module bank.
The Solar Energy Research Institute of Western Australia (SERIWA) constructed a photovoltaic powered containerised desalination plant in 1982 in Perth (James, 1983). It produced 400-700 L/day from a 1200 W peak array. Low pressure membranes were not available, cellulose acetate being used, and no energy recovery device was installed. The array had one-axis adjustability and fed a 32V DC motor driving a positive displacement pump through a battery bank and controller. The unit was sold after testing and is still in use providing drinking water for a road-house on the north-west coast.
Gillen Bore, Central Desert Region, South Australia
Highlighting the advances made in desalination technology, a Suntec Ltd. system installed at Gillen Bore in central Australia near Alice Springs produces 1200L/day from 1600ppm TDS bore water using 8 solar panels (520W peak) and has performed satisfactorily since September 1993 (Genders, 1995). Approximately half the TDS is attributed to NaC1 but nitrates are the main source of concern. Excessive nitrate consumption can cause a condition called methaemaglobinaemia in infants and nitrates are commonly higher than recommended in Central Australia. Strangely enough, the condition itself is almost unrecorded in Aboriginal communities even when consumption is known to be high.
The RO unit is located on a 6m tank stand between a feed tank and a permeate tank. The reject water is pumped back into the feed tank as it is only slightly more concentrated than the feed. The feed water is distributed to the community for normal usage but drinking water is supplied separately from the permeate tank.
The unit has shown only a 7% annual loss in performance and are still in use. The pre-filters have been changed only once and no other pre-treatment is used. The only damage the unit has suffered was a pump which required replacement after a visitor to the community turned the main supply valve off.
Solar distillation plants are very common throughout the world but large ones are rarely built now, having been overtaken by the technical advances of RO. Solar stills of the simple basin design seem on the surface to be the cheapest, lowest maintenance and most appropriate desalination technology available. In practice. they prove to be costly, high maintenance items with poor return for outlay.
Many of the old plants were built to designs of the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO). One plant of 3 500 m 2 was built in Coober Pedy in South Australia in 1966 to the CSIRO Mark II design and fed with 18,000ppm (TDS) feed they produced a nominal flow of l lkL per day product at 25% recovery. Plagued by storm damage, structural failure, sealing problems, leakages, dust accumulation and salt skins, they were replaced by an RO plant in 1969, not without its own operational problems (Makestas, 1985). Coober Pedy now has an up-dated RO plant run from the town's diesel grid.
The International Perspective
The Mobil seawater desalination plant at Jeddah, Saudi Arabia meets the drinking needs of 250 people by producing 3.2m 3 per day from an 8 kW peak photovoltaic array. No energy recovery devices were available at the time of commissioning (Bosch, 1982).
The GKSS system operating in Concepcion del Oro,