Solar chimneys, are one of the least publicized forms of solar power plants. The power plant consists of a very large glazed solar collector with a chimney in the center through which the hot air generated in the collector rises. The hot air wind drives turbo-generators located at the base of the chimney. The floor of the collector, is lined with heat absorbing media, which absorb heat during the daytime and release the heat during the night, so that the power is produced on a continuous basis. Cold air enters the collector, with an upward sloping ceiling, from its outer perimeter, is heated and rises through the tower at 50 feet per second (15 meters per second). The solar tower works on the principle that the higher temperature of the air in the collector compared to the outside temperature causes a density difference, which causes the air, heated to about 100 oF (38oC) hotter than the outside air, to rise, creating a solar wind that drives the turbines.
Recent improvements in the conceptual design, specifically, potential improvements in the performance of the roof area of the collector and a new method of storing heat have enabled modularization to units as small as 25 megawatts (MW). The smaller units are said to greatly reduce development costs, while still providing much lower capital costs for larger installations. Having a shorter chimny (still very tall) will reduce the profile of the plant which should reduce permiting and regulatory problems. Solar tower power plant modules, unlike most forms of renewable energy, produce a very significant quantity of electricity, 25 to 200 MW or more. The two disadvantages of the system are that they requires a large land area and are quite expensive.
A 50 kw demonstration prototype was built in Spain in the 1980's and operated for seven years. A 200 megawatt plant, described in this 2001 article from Wired, has been under development in Australia for the last five years by EnviroMission Limited. A recent announcement was made by the California parent company, SolarMission Technologies, that it is going to "build, own, maintain and operate 2,600 megawatts of solar towers". A group of Chinese investors has invested US$2 million in the project's development. There are also plans to invest a further US$8 million for development of a solar tower in China.
A more detailed description of the process can be found here at the SolarMisssion website. On Feb. 10, 2005 SolarMission was to purchase the site in Australia for the first plant. Details for a 200 MW Solar Tower power station are as follows (details have not been released for the 25MW module):
- Tower: 3000 feet high, 400 feet Diameter
- Concrete: 750,000 cubic yards
- Collector: 3.5 miles in diameter, glass/polycarbonate/plastic film
- Turbines: 32 units, 6.25 megawatts each.
- Land: 20.25 square miles
- Construction time: 34 months
- Jobs: Construction 2,700+, 15 ongoing
- Output: 200 megawatts.
Several options are available for the operation of the plant. By placing a shutter in the tower the hot air can be released at the most desirous time: during peak power demand or it can be throttled during the day so as to maintain an even flow throughout the 24 hour day providing perhaps a better base load plant. The size of the collector can be varied to allow more or less heat to be collected during the day for use at night or for periods when the sun is covered with clouds. The amount and type of heat storage media can be varied to increase the collectors ability to store heat.
Two of the real advantages that I see for the system are that it runs continuously and it should require minimum maintenance. The only significant machinery are the wind turbines and generators which are very well developed. The modular nature of the turbines and generators could mean that they could be maintained one module at a time with little, if any, interruption to the plants operation.
I would like to acknowledge the Alternative Energy Blog whose June 1 post brought this technology to my attention.