Siân Green

A 200 MW solar tower project is taking shape in Australia. The project involves the construction of a 1 km-tall tower that will be the world’s tallest man-made structure, and will be one of the world’s largest non-hydro renewable energy generators.

Development of the world’s first commercial-scale solar tower plant took a step forward in August when the Australian government granted the project ‘Major Project Facilitation’ status. The 200 MW, A$800m ($431m) plant is scheduled for operation in late 2005 or early 2006, and will make a major contribution to Australia’s green energy drive.

Construction of the power plant could start as early as January 2003. The project is a major step forward in the development of solar tower technology, which consists of a ground-level ‘greenhouse’ area which heats up air and forces it through a tall, narrow chimney. The flowing air drives turbines to produce power.

The Major Project Facilitation award recognizes the significance of the project to Australia and to green power technology development in general. Although the solar tower concept was first designed in the early 1980s, its use has so far been restricted to small-scale demonstration projects. The plant will, say its developers, compete with other forms of generation in Australia’s power market.

The power plant is being developed near Buronga in Wentworth Shire, 25 km from the town of Mildura in Victoria state. It will consist of a 1000 m-high tower – the world’s tallest man-made structure – and a collector system with a radius of 3.5 km. It will produce enough power for 200 000 homes, and will offset the emission of almost 900 000 t of greenhouses gases per year.

Green commitment

Although Australia has not signed the Kyoto Protocol, it has made a commitment to reduce greenhouse gas emission through its Mandated Renewable Energy Target (MRET), under which two per cent of power generated must come from renewable sources by 2010. The plant will generate 560 GWh per year, accounting for around seven per cent of the MRET target.

Figure 1. The solar tower in New South Wales, Australia, will stand 1000 m tall and generate 200 MW
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The solar tower project is being developed by Australian company EnviroMission, which owns the exclusive Australian license to the technology through an agreement signed with German structural engineers Schlaich Bergermann, the designers of the technology.

Stuttgart-based Schlaich Bergermann engineers and constructs lightweight towers, roof, bridges and buildings, and senior partner Jörg Schlaich is the designer of the original solar tower technology. The company constructed a 50 kW solar tower plant in Manzanares in Spain, the prototype for the Australian project.

Between Schlaich Bergermann and EnviroMission, more than $37m has already been invested to prove the technology’s viability through research and development, pilot plant operations and feasibility studies for commercial plant construction. EnviroMission recently raised A$650 million through a listing on the Australian stock exchange.

Figure 2. The tower acts like a magnet in attracting sun rays from the air and from the large body of air underneath the collector system
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Further financing for the plant will be obtained on a project finance basis, according to EnviroMission. Due to its Major Project Facilitation status, the project can offer potential investors incentives, such as accelerated depreciation. In addition, suppliers and companies active in the project will be expected to offer equity to help spread the risk. The plant will also receive renewable credits – revenue generated by a levy on all non-renewable forms of generation.

EnviroMission completed an optimization study of the project in May 2002. The company states that the study showed “favourable results” which “confirm the economic viability” of the plant. A key part of this are the income streams from power sales as well as various other sources.

At the beginning of August 2001, EnviroMission signed a memorandum of understanding with Ergon Energy to negotiate a power purchase agreement for the sale of power from the plant. EnviroMission is also undertaking similar negotiations with other utilities. Such contracts will help EnviroMission obtain project financing.

Figure 3. Heated air is drawn towards the centre of the tower, passing through turbines to generate electricity
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The project is also expected to bring revenues from non-energy sources, however. These could include viewing platforms on the tower as well as leasing land for agriculture under the outer edges of the ‘greenhouse’ collector structure.

EnviroMission says that it wants to build five solar tower plants in Australia by 2010. Energen Global Inc., which owns 39 per cent of EnviroMission, is set to launch the technology in the USA, having recently established an office in California. Energen Global owns the license for the technology in the USA, Mexico, China, India, Pakistan, Egypt, Jordan, Vietnam and Sri Lanka.

Plant design

The solar tower plant will be constructed on Tapio Station, a large grazing holding in New South Wales on which EnviroMission has an option on a 10 000 ha area. The site was chosen by the company predominantly for its proximity to the national grid, being only 7.5 km from the Buronga terminal station. The site is close to Mildura, a major regional town, and the plant will be able to take advantage of premium prices for green electricity in the New South Wales market.

The plant will consist of a collector system standing slightly above the ground and rising towards the central tower. The collector will be composed of a transparent material with heat enhancing properties. The tower will be 1000 m in height; it will be constructed from reinforced concrete and will be strengthened by horizontal metal supports that can also act as platforms. The air temperature under the collector roof will be around 30°C and the wind speed will be about 32 km/h.

The sun’s rays will heat the large body of air underneath the collector system. As the hot air attempts to rise, it is forced towards the central tower, passing through 32 shrouded wind turbines set horizontally in the transition area. The tower creates an updraft, drawing air inwards. The turbines will be purpose-built from lightweight alloy materials, consisting of ten blades and coupled to synchronous generators. The turbines will accept air at around 60-70°C.

One innovative aspect of the design of the tower is that it will be able to generate electricity 24 hours-a-day. It will be most efficient during the day when the sun’s rays are most intense, which is also when electricity prices reach a peak.

The use of heat-storing material on the ground underneath the collector will enable the plant to generate power throughout the night.

Low maintenance

The design of the plant at Tapio Station draws on experience gained from the now-decommissioned prototype plant at Manzanares in Spain. This 50 kW project operated for 15 000 hours over a period of seven years from 1982 to 1989. It had a collector roof 240 m in diameter and a 197 m-high tower. This project, says EnviroMission, proved the concept of the design, which has now been adapted for the project in Australia.

Based on the Manzanares experience, EnviroMission believes that the solar tower plant will need minimal maintenance due to the absence of pressure parts and elevated temperatures. The turbines, it says, are more reliable than wind turbines and overall plant availability is expected to be close to 99 per cent. The collector will also need little maintenance.

Additional infrastructure

The solar tower project is likely to require a construction workforce of around 2000 or possibly more. This, says the local planning department, will mean that up to 800 new dwellings will be needed as well as additional public transport and education facilities, an improved road network and improvements in local, state and federal government services. The plant is also expected to attract a considerable amount of tourism to the area.

Remaining hurdles to the project’s progress include infrastructure issues such as the supply of water to the site and upgrading roads for heavy traffic, and native land rights, as the site is part of the Barkindji land claim.

EnviroMission expects the project financing to be closed by the end of 2002, and to have gained planning approval by February 2003. Site work will then commence in March 2003 with construction completed by the southern hemisphere summer of 2005/6. Currently, the company is in negotiations with construction companies and potential suppliers of materials such as glass and fluorinated polymer film for the collector, and steel, and is also finalizing the finer details of the plant’s design.

EnviroMission CEO Roger Davey is pleased with the solar tower project’s progress as the construction phase draws closer. “I believe it is probably almost there already. If you likened it to a horse race, I think we only have a couple of hurdles left to jump.”