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Solar Tower of Power A world first in our backyard? Solar Energy. Wind Power. These are some of the phrases which spring to mind when environmentally-sensitive generation methods are mentioned. For decades these have been small scale, “fringe” technologies, too expensive and impractical to replace fossil fuel power. Back in our March issue we briefly mentioned a proposal to combine wind and solar power in a massive “power station”. It’s progressing beyond the drawing board . . . By Sammy Isreb M elbourne-based EnviroMission is an energy company with a difference. That much is obvious from their plans to build a 1000 metre tall ‘power station’ 70km east of the Victorian town of Mildura. By September 2005, all things going to plan, they aim to have built not only the world’s tallest manmade structure along with the world’s largest “greenhouse” – but a 200MW solar power station into the bargain. Scientific testing has already commenced at the proposed site. The Principle The Solar Mission project is based on the “Solar Tower” design by Professor Jorg Schlaich from the University of Stuttgart, Germany. The basic principle of operation is the use of the Sun’s radiation to heat a very large body of enclosed air. Being warmer than the surrounding atmosphere, this air will begin to rise. By causing it to flow through www.siliconchip.com.au windmill-style turbines on its journey up a tall chimney, electricity can be generated. Obviously, generating 200MW of power in this way is no mean feat. The ‘greenhouse’ collector will be a roughly circular canopy of transparent plastic material measuring approximately 5km in diameter. This canopy, or roof, will slope upwards towards the centre drawing in air from the edges. In the centre will reside the tower, a 1000-metre- tall structure with a base around 170 metres wide. On a sunny day, the air at the bottom of the tower will be around 35°C greater than the ambient air temperature, causing it to flow at roughly 15 metres per second. In the lower atmosphere, as a general rule, temperatures fall by around 1°C per 100 metres of altitude. Thus at the top of the tower, the ambient air temperature will be around 10°C cooler than that at the bottom, without even taking into consideration the heating effect of the greenhouse. July 2002  7 About 40 metres up from the ground, 32 Kaplan-style turbines placed in the chimney will be driven by the rising air, in turn driving generators. An increase in generated power could be achieved by either increasing the size of the solar collector or the height of the chimney, or both. Night Generation Here’s a somewhat simplified diagram showing how the massive Solar Tower works. And the beauty of the system is that it is so simple! SUNLIGHT ENTERS “GREENHOUSE” AND WARMS AIR INSIDE One of the most attractive features of the Solar Tower over that of traditional 1000m solar generation methods is its capacity to generate electricity under cloud AIR cover, or even during the night. In order to achieve this, sealed water tubes are placed under the canopy, filled only once during manufacture. During daylight hours incident solar radiation will heat this very large mass of water. At night, that heat will be released. Varying the amount of water under the canopy will alter the output versus time of day profile of the power station. Through this design, the Solar Tower technology avoids becoming a rapid peak generator, instead having the capacity to produce a much smoother load curve, with very low output variance. This aids in interconnection to the supply grid, avoiding the need to coordinate generation and demand peaks which normally plague green power production methods. Pilot Program For seven years a pilot 50kW prototype Solar Tower plant was successfully operated in Manzanares, Spain. Built by the Spanish government in collaboration with designer Professor Jorg Schlaich, the plant proved the technology to be technically feasible. Operated from 1982 until 1989, this pilot plant featured a 195-metre-tall chimney, with a collector diameter of 240 metres. Operational data acquired over this 7-year period has been used in the scaling and design of the proposed 200MW plant. It is this successful pilot plant operation which sets apart the Solar Mission project from other large scale speculative GENERATOR TURBINE POWER TO GRID AIR WARM AIR RISES UP CHIMNEY 5000m alternative energy generation projects. Site Determination Currently the Mildura site is the favored location for the Solar Mission plant, with geotechnical testing being undertaken to confirm its suitability. With lessons learnt from the Spanish plant, final site determination will be made using the following criteria: • Solar Radiation Levels • Weather Patterns • Geological Stability • Access to the Electricity Grid • Geographical features • Government and community support Economic Feasibility Calculating the production cost of Solar Tower electricity is much simpler than that of traditional coal-sourced electricity. While coal plants have many cost inputs, including fuel, mining and transport, plant maintenance and even mining site remediation, the major cost for the Solar Mission is the capital cost of production (land acquisition and building) and associated finance costs. And some Governments have started to place taxes on major polluters – coal-fired It’s not some crazy idea which will never work: these photos show the pilot plant built some 10 years ago at Manzanares in Spain. Yes, it does look like a greenhouse! 8  Silicon Chip www.siliconchip.com.au This “viewed from above” drawing gives an even better idea of the massive size of the project, both solar collector and chimney. Compare the road and building scale to that of the collector and tower! power stations are firmly in their sights. Since the operation and maintenance cost is comparatively low, a direct correlation between the prevalent interest rate and the electricity production cost can be made. With an interest rate of 11% and a 4-year cycle to production, the cost of ‘solar’ electricity is a mere 20% higher than coal generated power. With an interest rate of 8% (roughly that currently available), the cost of solar electricity will match that of coal-powered plants. Environmental Benefits Compared to Victoria’s coal-generation facilities, the 200MW Solar Mission project is relatively small. Howev- www.siliconchip.com.au er, it will provide enough electricity for around 200,000 typical Australian homes. Electricity demand, and thus selling price, is highest during the hottest days of the summer months, at which time the Solar Mission plant will be at its production peak. Each year it is estimated that the plant will reduce carbon dioxide output by an astounding 900,000 tonnes, satisfying both Australia’s Kyoto treaty obligations and the 1997 federal legislation stipulating that 9500GWh of the nation’s electricity must come from clean, green renewable sources by 2010. With such a focus on green electricity, a successful implementation by EnviroMission could very well make them leaders in this new market. Conclusion If the preliminary site testing yields positive results and all regulatory hurdles are met, a ‘world first’ in commercial green power generation technology could be up and running in Victoria by late 2005. If this large-scale project proves successful it could revolutionise environmentally-friendly power generation in temperate climates. Admittedly, Solar Tower technology will probably never fully replace the ease of tried and proved fossil fuel technologies but it will go a long way to redressing the incredibly heavily reliance on non-renewable technologies. And that’s a step in the right direction. Acknowldegement: Thanks to Solar Misson for the use of their illustrations. SC July 2002  9