Tides in the Kimberley region of Western Australia average around 10m in height, some of the highest in the world.
This difference between high and low water is the basis of a unique proposal to generate electricity to one of the remotest areas in the country.
Australian company Tidal Energy Australia (TEA) has spent in excess of $A4.5M ($2.4M) over the past four years on detailed engineering studies and gaining statutory approvals. Managing director Peter Wood points out that tidal flow is the only completely predictable form of renewable energy. He has developed computer models forecasting electricity production over a 25 year period.
At present the Kimberley towns of Derby, Broome and Fitzroy Crossing are supplied with electricity by the Stateowned Western Power Corporation, using diesel powered generating stations. But the cost of diesel, with excise duty, means that electricity prices are extremely high, even with the state government subsidy.
For example Broome High School pays A$0.32kW/h. TEA claims that it can supply power initially at A$0.15kW/h and, adds Wood: 'That price can reduce in real terms as the burden of interest payment is progressively eliminated.'
However there is a rival scheme from the liquidified natural gas lobby, which is apparently preferred by Western Australian State Energy Minister, Colin Barnett. The offshore North West Shelf project, which lies 800km south of Broome has both the reserves and capacity to supply fuel for new gas fired power stations.
Barnett said recently that on a strict cost basis, tidal power ranked as the fourth most expensive tender, while gas was the cheapest. However, his analysis failed to incorporate environmental or regional benefits and took no account of federal grants, available only to renewable schemes.
His figures could also be challenged on other counts. The TEA project, costed at A$340M, has a 120 year design life, or around four times that of a gas or diesel system. Also missing from the equation is the cost of getting gas to the power stations. Once the gas is piped ashore it must be processed before road tankers start the 1,000km haul to deliver it.
Roads and bridges along the route will need millions of dollars spent on upgrading to withstand the loads and, as one Derby resident points out; 'what happens to supplies during the wet season when monsoon rain can flood hundreds of square kilometres in 24 hours?'
TEA's concept of using the tides to generate energy continuously is centered on a fortunate land formation.
Doctors Creek, a blind tidal creek system, is located on a 130km 2mudflat some 15km north of Derby. The mouth is divided by an area of elevated ground which splits the creek into two almost equally sized basins each 15km long with an average width of 500m. The proposal is to build a 1.7km long barrage across both mouths to control the water levels in the two creeks which are designated High Basin and Low Basin.
The High Basin side of the barrage will contain 14 sluice gates, each 10m wide by 6.5m high and the Low Basin will contain 10 similar gates. A channel behind the barrage connects the two creeks and incorporates the power station with six 8MW turbines.
Wood points out that this is regarded as only the first phase. 'We can increase output relatively cheaply by adding one or more turbines as required. And with further modifications to the layout, the potential maximum output could be increased from 200GWh per annum to in excess of 350GWh per annum.'
Essentially the design provides a permanent river of seawater flowing through the turbines between the two basins. During the incoming tide the High Basin gates are opened while the Low Basin gates remain closed. At low tide the Low Basin is allowed to drain. A constant head of about 6m is maintained between the High and Low Basins, creating a flow of about 500t/s between the two basins to drive the turbines.
Time difference between high and low water is six hours. The basins have the capacity to cope with this water movement until the next tidal movement triggers gate opening. Neap tides which occur on two days each fortnight are reduced in amplitude so at these times generating capacity could fail to meet demand. To supplement output at that time, the company plans to refurbish and upgrade the existing Broome power station to provide supplementary power.
Wood and TEA development manager Brendan Corry spent weeks crisscrossing the region searching for suitably sized rock armour to face both sides of the 7km long barrages. 'We spent so long we reckoned we could smell rock, but a source of good quality armour rock eluded us, ' says Corry.
Undaunted, the bidding consortium redesigned the barrages with a 1:10 slope that looks more like a beach than a conventional dam. This gentle incline means that locally available rip rap rock material can be used for facing.
Finding suitable fill material presented no problems. The Low Basin has huge bars of high quality sand at its mouth and this would be dredged and pumped into the barrage. An estimated 1M. m 3of sand is required for in-fill and its removal would add significantly to the holding capacity of the Low Basin.
Sheet piled walls founded deep in the impermeable clay strata will provide protection from scour. Construction of the sluices and turbine hall will take place in the dry within temporary sheet piled cofferdams.
With one power station replacing three existing plants, TFA will need to string more than 500km of 132kV line, forking out from Derby to Broome in the south and eastwards to Fitzroy Crossing, benefiting the small communities in between.
Some sections of the Kimberley region are affected by cyclones. In recognition of this, the transmission network has been designed in accordance with Australian Standards and includes provision for extreme wind speeds in the cyclonic zone near Broome.
It is obvious that Wood and his team are totally committed to seeing the scheme through to fruition. Leighton Contractors, the lead contractor in the consortium is guaranteeing not only construction costs and on-time delivery within 30 months of start up, but also the amount of electrical output. TEA has passed environmental approvals and Aboriginal Heritage surveys and, almost uniquely, the site is clear of contentious 'Native Title' claims.
Tourism provides a significant part of the region's income and the tidal basins offer recreational, fishing and aquaculture opportunities.
Wood stresses the important green benefits of tidal power. 'Two years ago, at the Kyoto conference in Japan, Australia agreed to control growth in greenhouse emission levels at 8% of the 1990 level. It is understood that Australia's emissions are currently 17% above the 1990 level. The Derby scheme would make a 200,000t reduction in CO 2emissions each year, ' he adds. The federal government in Canberra is prepared to back the scheme with grants in excess of A$60M from the Remote Power Generation programme.
Wilson Tuckey, Ministry for Forestry and Conservation in Canberra contends that the decisions on whether to adopt tidal power are more about financial policy than technology.
'We are obliged to purchase the electricity generated for the entire life of the project (120 years design life). Clearly the financial markets must extend their financing structures beyond the typical 25 year period applicable to conventional projects, or the early years of generation by tidal power become too expensive compared to fossil fuel based facilities.
'The Kimberley tidal energy region not only has the capacity to generate tens of gigawatts of tidal electricity, it has a known deposit of 1.4bn tonnes of bauxite, is close to under-developed gas fields, has abundant freshwater rivers and is within economic distance of a hot briquette iron plant providing electric arc feedstock. The Derby Tidal Power scheme will showcase the potential that exists.'
Tidal Energy Australia has formed alliances with some of Austra l ia's biggest compan ies .
Leighton Contractors formed part of the bidding consortium and assumes the role as lead contractor and project manager for all civil works. Kilpatrick Green is responsible for transmission lines and associated work while Pacific Hydro will act as operator of the tidal power station.
The overall design will be by Gutteridge Haskin & Davey. Babcock & Brown and ANZ (Australia & New Zealand) investment bank has provided financial advice.
Three companies have been asked to tender for turbine design and build; Alstrom, Kvaerner and Sulzer.
All three have offered a modified propeller design for ease of installation, maintenance and efficiency.