Other Solar Thermal Technologies
Abstract
There are two other systems for generating electricity from solar energy. The solar chimney uses a large circular greenhouse to contain air that is heated by the Sun. At its center is a tall tower. Hot air funneled from the greenhouse into the chimney creates a massive updraft that can be used to drive steam turbines to generate electricity. The solar pond is a much more low-technology approach consisting of a large pond filled with brine and exposed to the Sun. When the brine is heated, a hot layer is created at the bottom of the pond and the temperature difference between the bottom and the top of the pond is used to drive a small heat engine. Neither has yet been developed commercially.
Keywords
Solar chimney; greenhouse heat collector; updraft; wind turbines; solar pond; brine; organic Rankine cycle turbine
In addition to the mainstream solar thermal technologies discussed in Chapters 4–6, there are two other approaches that exploit the heat in sunlight to generate electricity. These are the solar chimney and the solar pond. Neither has been deployed commercially. The solar chimney involves construction of a large greenhouse that acts as a solar collector, heating the air within the structure. This hot air is funneled toward the center of the structure to a large chimney through which the hot air ascends, creating a powerful updraft. This updraft of air is then used to drive wind turbines to produce electricity. The solar pond, meanwhile, is a low technology approach to generating electricity. It involves a large pond that absorbs heat from the Sun, leading to the creation of a temperature difference between its upper and lower layers that can be used to produce electricity from a small heat engine.
The Solar Chimney
The solar chimney concept involves the construction of a large, normally circular, open-sided greenhouse. The glass cover of the greenhouse retains the air that is heated by sunlight during daylight hours. The roof of the greenhouse slopes slightly upwards toward the center of the structure, encouraging the hot air to flow in that direction.
At the center, the greenhouse structure is punctured by a tall chimney. The hot air flows up this chimney, creating a powerful updraft. At the same time, more air is drawn into the greenhouse from the open sides. The current of air created by this structure can be used to drive wind turbines. These turbines can either be placed within the chimney itself, or more economically, they can be placed at ground level around the perimeter of the chimney. Energy storage can be added to the scheme by installing bags containing water on the floor of the greenhouse. These heat up during the day and release the heat at night, maintaining the air flow to drive the turbines.
This concept has been tested at a small scale. However, commercial plants based on the idea would need to be massive, and this has so far prevented a commercial project from being commissioned. A schematic of a solar chimney power plant is shown in Fig. 7.1.
The single solar chimney pilot scheme so far constructed was developed in Spain by a German company called Schlaich Bergermann Solar. The prototype was built between 1981 and 1982 at Manzanares and was funded by the German government. This pilot scheme had a tower 190 m high and 10 m in diameter. It was surrounded by a glass collector with a radius of 122 m. A turbine was installed at the base of the tower and was able to produce an output of 50 kW. The project operated until 1989. The company is still promoting the technology with proposals for plants of 200 MW in generating capacity and chimneys of up to 1 km in height.
Various other companies and organizations have taken up the solar chimney concept. These include an Australian company that proposed building a 200 MW scheme in the state of Victoria in 2004. Two African countries, Namibia and Botswana, have also taken an interest in the concept, and a small test facility was operated briefly in Botswana in 2005. Meanwhile, the Namibian government approved a scheme to build a 400 MW tower, 1.5 km high, with 37 km2 of greenhouse that would have been used to grow crops as well as generate electric power. In spite of these ambitious schemes, no large-scale solar chimney has been built.
The Solar Pond
The solar pond is a low-technology solution to solar heat collection for power generation. The concept upon which it is based has been tested at several locations, but it has never proved commercially viable.
The solar pond consists of a large, watertight excavation or container. The pond, usually around one to two meters deep and several thousand square meters in surface area, is filled with a brine made from salt (sodium chloride) and water. The surface of the brine is usually covered with an impermeable transparent material to prevent evaporation. The pond, once established, is exposed to sunlight, which heats up the brine.
Under normal circumstances, with fresh water, a pond heated in this way would establish a vertical thermal gradient so that the hottest water was at the top of the pond and the coolest at the bottom. However, the brine causes an inversion so that a hot layer of concentrated brine collects at the bottom of the pond while a layer of cooler, much less concentrated—and hence less dense—brine forms at the top. In between these two a gradient zone is established in which both temperature and salinity change. This helps keep the more dense, hot brine at the bottom.
The vertical temperature gradient within the pond can be used to drive a heat engine. To achieve this, hot brine is withdrawn from the bottom of the pond and used to heat and evaporate a thermodynamic working fluid. The gaseous fluid is then used to drive a turbine, creating a pressure gradient across the turbine by condensing the working fluid using cooler brine from the top of the pond.
The temperature gradient from top to bottom of the pond is likely to be no more than 50°C at best, with the hot brine reaching up to 80°C while the surface temperature is around 30°C. The best way to exploit this small temperature difference is by using an organic Rankine cycle turbine system. This is a closed cycle system based around a small turbine that is like a steam turbine. The working fluid in the cycle is a low boiling point organic fluid that both boils and condenses within the temperature range of 30°C to 80°C.
Apart from its simplicity, the solar pond has the added advantage of a built-in heat storage system. Depending on the size of the pond relative to the turbine used to extract energy, the complete system should be able to generate power around the clock.
Solar Pond Schemes
Large solar pond projects have been limited. One of the most significant was developed close to the Dead Sea in Israel during the early 1980s. The pond was constructed by excavating a cavity and lining it with rubber sheets to create a surface of 7000 m2. Once established, the temperature at the bottom of the pond stabilized at between 70°C and 80°C while the temperature at the surface was between 20°C and 30°C. This pilot scheme was able to generate 150 kW of power using an organic Rankine cycle turbine built by a company called Ormat, which developed the scheme. Subsequent to this a 5 MW project was built at Beit Ha’Arava and operated until 1988. The pond for this scheme had a surface area of 210,000 m2 and overall efficiency was around 1%. The eventual aim of these projects was to harness the Dead Sea for power generation, but the scheme was apparently dropped by the beginning of the 1990s.
Another, smaller solar pond was constructed at the University of Texas, El Paso. The pond was a converted fire protection pond and had an area of 3000 m2. The project operated intermittently between 1986 and the mid-1990s. A project was also completed in Bhuj in the state of Gujurat, India, in 1993, but this only supplied hot water.