The solar tower takes a slightly different approach to solar thermal power generation. Whereas the parabolic trough array uses a heat collection system spread throughout the array, the solar tower concentrates heat collection and utilisation at a single central facility.
The central facility includes a large solar energy receiver and heat collector which is fitted to the top of a tower. The tower is positioned in the centre of a field of special mirrors called heliostats, each of which is controlled to focus the sunlight that reaches it onto the tower-mounted solar receiver.
The mirrors used as heliostats must be parabolic in section, just like the trough mirrors, but because they have a very long focal length they appear almost flat. Each mirror has to be able to track the sun independently so that the incident light remains directed at the solar receiver. The heliostat field can be very large, large enough to supply energy to generate several hundred megawatts of electricity.
The most technically demanding component of a solar tower system is the heat capture and transfer system. At the top of the solar tower is a solar receiver containing tubes through which a heat transfer fluid flows. This has to be capable of absorbing the heat from the whole heliostat field. Once heated, the fluid is pumped to a heat exchanger where the heat is used to generate steam for a steam turbine.
This arrangement is much like the parabolic trough power plant, but modern designs include a crucial difference. The heat transfer fluid in a solar tower is not pumped directly from the solar receiver to the heat exchanger. Instead it is taken to a high-temperature storage tank, a heavily insulated tank where the hot fluid can be stored for up to 24 h. From here the fluid is taken as needed and pumped through the heat exchanger to generate steam, and then it is stored in a low-temperature reservoir. Fluid from this low-temperature reservoir is supplied to the solar receiver to be reheated.
By careful sizing of the storage system and power generation system, a solar tower can be constructed so that it can supply power continuously, not just during daylight hours. This means that the plant can be employed like a normal base-load fossil-fuel-fired power station, making it much more flexible and therefore much more valuable on a grid system.
A key component of the solar tower is the heat transfer fluid. The most successful has proved to be a molten salt comprising a mixture of sodium and potassium nitrates. This will melt at 220°C. It is normally kept at around 300°C in the low-temperature reservoir and is heated to 550°C in the solar receiver for storage in the high-temperature reservoir.
No commercial solar tower power plants have been built but a number of pilot-scale projects have been operated. Most important of these have been two projects at Barstow in California, called Solar One and Solar Two. Solar One operated from 1982 until 1988. It was later cannibalised to build Solar Two which started up in 1996 and operated until 1999. Both had power generating capacities of 10 MW.
Solar One used water as its heat transfer medium but it was converted to a molten salt system with two storage tanks for Solar Two. The latter comprised a 91-m high tower surrounded by 2000 heliostats with tracking systems which were computer controlled. At the top of the tower was the solar receiver, a system of vertical pipes which carried a molten salt. This molten salt reached a temperature of 565°C when heated by the sun. Storage capacity was 30,000 kWh.
The solar tower is a source of very high-grade heat. While pilot plants have operated with temperatures of around 550°C it is quite plausible to raise the temperature to 1000°C. Such a high temperature could be used to heat air to drive a gas turbine, instead of for raising steam. A gas-turbine-based system could prove more efficient than the current steam turbine solar tower design, but the scheme has yet to be proved. There may also be ways of combining a solar tower with a fossil fuel power plant in a hybrid arrangement similar to that being considered for the ISCC power plant described above.
The solar tower concept has never been proved commercially but it is considered to be perhaps the most cost effective of the solar thermal technologies. However it will be the second or third decades of the twenty-first century before it reaches commercial maturity.
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