The solar updraft tower (SUT) is a design concept for a renewable-energy power plant for generating electricity from low-temperature solar heat. Sunshine heats the air beneath a very wide greenhouse-like roofed collector structure surrounding the central base of a very tall chimney tower. The resulting convection causes a hot air updraft in the tower by the chimney effect. This airflow drives wind turbines, placed in the chimney updraft or around the chimney base, to produce electricity. As of mid 2018, although several prototype models have been built, no full-scale practical units are in operation. Scaled-up versions of demonstration models are planned to generate significant power. They may also allow development of other applications, such as to agriculture or horticulture, to water extraction or distillation, or to remediate urban air pollution. Commercial investment may have been discouraged by the high initial cost of building a very large novel structure, the large land area required, and the risk of investment. A few prototypes have been built in Spain in 1981, in Iran in 2011, and in China in 2010 (see below), and projects were proposed for parts of Africa, the US and Australia. In 2014, National Geographic published a popular update, including an interview with an informed engineering proponent. A solar updraft tower power plant can generate electricity from the low temperature atmospheric heat gradient between ground or surface level and structurally reachable altitude. Functional or mechanical feasibility is now less of an issue than capitalisation. A comprehensive review of theoretical and experimental aspects of solar updraft tower power plant (SUTPP) development is available, recommending commercial development. A review of progress in demonstration and modelled data was presented in 2020 by Dogan Eyrener, and included in publication of proceedings. A review of combined technologies to address intermittency of power output, of hybrid solar updraft tower with complementary technologies was published in 2022. Combined, multiple or hybrid technologies include combined updraft-downdraft towers, and solar updraft-gas turbine waste heat transfer.
Design Power output depends primarily on two factors: collector area and chimney height. A larger area collects and warms a greater volume of air to flow up the chimney; collector areas as large as 7 kilometres (4.3 mi) in diameter have been discussed. A larger chimney height increases the pressure difference via the stack effect; chimneys as tall as 1,000 metres (3,281 ft) have been discussed. Heat is stored inside the collector area allowing SUTs to operate 24 hours a day. The ground beneath the solar collector, water in bags or tubes, or a saltwater thermal sink in the collector could add thermal capacity and inertia to the collector. Humidity of the updraft and release of the latent heat of condensation in the chimney could increase the energy flux of the system. Turbines with a horizontal axis can be installed in a ring around the base of the tower, as once planned for an Australian project and seen in the diagram above; or—as in the prototype in Spain—a single vertical axis turbine can be installed inside the chimney. A nearly negligible amount of carbon dioxide is produced as part of operations, while construction material manufacturing can create emissions. Net energy payback is estimated to be 2–3 years. As of 2019, the efficiency of SUTs, which primarily depends on chimney height, was less than 2%. Since solar collectors occupy significant amounts of land, deserts and other low-value sites are most likely. Improvements in the solar heat collection efficiency by using unglazed transpired collector can significantly reduce the land required for the solar array. A small-scale solar updraft tower may be an attractive option for remote regions in developing countries. The relatively low-tech approach could allow local resources and labour to be used for construction and maintenance. Locating a tower at high latitudes could produce up to 85 percent of the output of a similar plant located closer to the equator, if the collection area is sloped significantly toward the equator. The sloped collector field, which also functions as a chimney, is built on suitable mountainsides, with a short vertical chimney on the mountaintop to accommodate the vertical axis air turbine. The results showed that solar chimney power plants at high latitudes may have satisfactory thermal performance.
History and progress
A chimney turbine was envisioned as a smoke jack, and illustrated 500 years ago by Leonardo da Vinci. An animal spitted above a fire or in an oven could be turned by a vertical axis turbine with four angled vanes in the chimney updraft. Alfred Rosling Bennett published the first patent describing a "Convection Mill" in 1896. Even if in the title of the patent and in the claims the word "Toy" clearly appears and even if in the overall description made inside the patent it is evident that the idea was to produce small devices, in page 3 at lines 49–54 Bennett envisions much larger devices for bigger scale applications. A model of this "convection mill", built in 1919 by Albert H. Holmes & Son (London) to demonstrate the phenomenon of convection currents, is on display in the Science Museum, London. In 1903, Isidoro Cabanyes, a colonel in the Spanish army, proposed a solar chimney power plant in the magazine La energía eléctrica. Another early description was published in 1931 by German author Hanns Günther. Beginning in 1975, Robert E. Lucier applied for patents on a solar chimney electric power generator; between 1978 and 1981 patents (since expired) were granted in Australia, Canada, Israel, and the US. In 1926 Prof Engineer Bernard Dubos proposed to the French Academy of Sciences the construction of a Solar Aero-Electric Power Plant in North Africa with its solar chimney on the slope of a large mountain. A mountainside updraft tower can also function as a vertical greenhouse. In 1956, Edgard Nazare, after observing several dust devils in the southern Sahara, filed his first patent in Algiers on the artificial cyclone generator. This patent was re-filed later in Paris
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