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Solar updraft tower

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This article is about Solar Towers and Solar Chimneys and similar Solar power plants using the convective motion of heated air in a chimney for electric power generation. For various other meanings of the term "Solar Tower", including the astronomical instrument and other uses of the term, see solar tower (disambiguation). For the use of solar energy for ventilation, see Solar chimney.

The Solar updraft tower is a type of renewable-energy power plant based on the Solar chimney concept. There are presently no solar updraft towers. One was built in Spain, but has been decommissioned, and another is planned for Australia, but construction has not yet started.

Schematic of a Solar updraft tower








Background

The Solar updraft tower is a proposed kind of power station that harnesses solar energy by convection of heated air within a large chimney. For electric power generation a large greenhouse is used at the base rather than relying on heating of the chimney itself.

In 1903, Spanish Colonel Isidoro Cabanyes first proposed a solar chimney power plant in the magazine "La energía eléctrica" . One of the earliest descriptions of a solar chimney power plant was written in 1931 by a German author, Hanns Günther. Beginning in 1975, Robert E. Lucier applied for patents on a solar chimney electric power generator; between 1978 and 1981 these patents, since expired, were granted in the USA, Canada, Australia and Israel.

In 1982 a small scale working model of a solar chimney power plant was built under the direction of German engineer Jörg Schlaich in Manzanares, 150 km south of Madrid in Spain; the project was funded by the German Government. This pilot power plant operated successfully for approximately 8 years and was decommissioned in 1989. The chimney had a diameter of 10 metres and a height of 195 metres, with a collection area (greenhouse) of 46,000 m² (about 11 acres) obtaining a maximum power output of about 50 kW. During operation, optimization data was collected on a second-by-second basis.

Description

The technology is relatively simple. Inside a very large circular greenhouse (between 2 and 8 kilometres in diameter), air is heated by the sun and travels up a convection tower located at its center where it rises naturally, thereby driving wind turbines which generate electricity.

Either horizontal axis turbines can be installed on the ground in a circle around the foot of the tower as planned for the Australian project described below, or (as in Spain) a vertical axis turbine at the lower end of the chimney.

Apart from the intensity of the solar radiation the generating capacity of a Solar updraft power plant depends on two factors: the size of the collector area and chimney height. With a larger collector area more air is warmed up and flows up the chimney, while the pressure difference on the ground and therefore updraft increases with chimney height (stack effect). Therefore, an increase of the collector area and the chimney height both lead to a larger capacity of the power plant. The small test facility in Manzanares consistently generated 50 kW; it is this solar updraft tower which is the basis for the technology. This pilot plant had a collector diameter of 244 metres and a chimney height of 195 metres, and achieved an output of 50 kilowatts. For comparison, a single solar dish-Stirling engine installed at Sandia National Laboratories’ National Solar Thermal Test Facility produces as much as 25 kW, but its footprint is a hundred times smaller.

The advantages of the solar updraft tower are that the technology is relatively simple, and that it should required little maintenance. Moreover, the soil under the collector stores heat during the day and releases it again during the night, allowing the the system to operate for 24 hours a day. Water filled tubes may be placed under the collector to increase heat capacity as needed.

The main problem with the solar updraft tower concept is that airflow has generally only a small energy density. The reason for this it the relatively small difference in temperature between the highest and lowest temperatures and/or pressure gradients in the system. Carnot's theorem greatly restricts the efficiency of conversion in these circumstances. Furthermore, according to Betz' Law only 16/27 of the energy contained in air flow can be converted into rotational energy.

A solar updraft power plant needs to be large for it to be cost-effective because conversion efficiency increases with size, and therefore requires high investments upfront. According to model calculations an updraft power plant with an output of 200 MW would need a collector 7 kilometres in diameter (total area of about 38 km²) and a 1000 metre high chimney.
Because no data available to test these models on a large-scale updraft tower there remains uncertainty about the reliability of these calculations. The performance of a updraft tower may be degraded by factors such as atmospheric winds, or by drag induced by bracings used for supporting the chimney. On the other hand, a Solar updraft power plant located at high latitues such as in Canada may produce up to 85% of a similar plant at southern locations.

Solar Tower Buronga

File:SolarTower.jpg
Artist's impression of Solar Tower

Since 2001 EnviroMission has been planning to build a solar updraft tower, called the Solar Tower Buronga, at Buronga in Wentworth Shire in south-western New South Wales, Australia. The originally-announced design called for a 200 megawatt tower at a height of 1 km with a collector area of 7 km in diameter, and costing around AU$900 million, but these plans have not yet been realized.

The scale of the design has recently been downsized to a 50 MW tower with a height of 400 m and a cost of AU$250 million. . EnviroMission has applied for support from the Federal Government's Low Emissions Technology Demonstration Fund (LETDF) to help finance the project.

There has been an interest from China, Texas, and Spain for the Solar Tower, but no concrete plans to built one.

Evaluation

The Solar updraft tower is part of the solar thermal group of solar conversion technologies. There are several other designs that work in a similar way. The first is the solar trough design and another is the solar dish/stirling design. Of these technologies the solar dish/stirling has the highest energy efficiency (the current record is a conversion efficiency of 30% of solar energy). Solar trough plants have been built with efficiencies of about 20%.
The Concentrated Solar Power (CSP) Plant using the parabolic trough principle called the SEGS system, in California in the United States, produces 330 MW, and it is currently the largest solar thermal energy system in operation. Furthermore, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. Stirling Energy Systems announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricity.

The gross conversion efficiencies (taking into account that the solar dishes or troughs occupy only a fraction of the total area of the power plant) are determined by net generating capacity over the solar energy that falls on the total area of the solar plant. The 500-megawatt (MW) SCE/SES plant would extract about 2.75% of the solar power (1 kW/m²; see Solar power for a discussion) that impinges on its 4,500-acres (18.2 km²). For the 50MW AndaSol Power Plant that is being built in Spain (total area of 1,300×1,500 m = 1.95 km²) gross conversion efficiency comes out at 2.6% For the Solar Tower (original design: 200 MW, total area 38 km²) it appears that it would extract about 0.5% of the solar power (1 kW/m²) that falls on the area it covers.

The low efficiency of the Solar Tower is somewhat balanced by the rather low investment cost per m² of aperture. Even so, it has been pointed out that a Solar Tower is an expensive way of generating electricity as compared to a conventional wind farm.

Solar updraft towers will contribute to reducing greenhouse gas emissions by producing sustainable electricity. To replace a typical 2000 MW black coal-fired power station, about 10 solar chimneys of the capacity of the originally proposed test plant would need to be built (depending on scale and capacity). This would also abate more than 14 million tonnes of greenhouse gases from entering the atmosphere annually.

So an area of 380 km² (this is about the size of the Isle of Wight) would be required to replace one coal-fired power station. To replace all 28,000 MW of coal-fired electricity generating capacity in Australia a total of 140 solar updraft towers would have to be built, and 5320 km² covered under glass; an area 50% larger than Long Island or Mallorca. This makes this technology useful only for some countries and lands, with large desert areas.

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 labor to be used for its construction and maintenance.

Competition

The Vortex engine, proposed by Louis Michaud, is an idea very similar to the solar chimney but replacing the physical chimney by a vortex of twisting air. However it is not absolutely clear that this could be made workable (for instance, wind might disrupt the vortex). Another approach, Floating Solar Chimney Technology, proposes to keep a light-weight chimney aloft by lifting balloon rings filled with a lighter than air gas. Or the chimney could be constructed up a mountain side.

An alternative of the solar updraft tower is the energy tower proposed by Dan Zaslavsky. The "energy tower" is driven by spraying water at the top of the tower; evaporation of water causes a downdraft by cooling the air thereby increasing its density, driving windturbines at the bottom of the tower. The "energy tower" requires a hot arid climate, and large quantities of water. Saline seawater may be used for this purpose, since fresh water is a scarce commodity in arid areas. Though significant amount of energy (about 40-50% of the windturbine generated output) is consumed by pumping the liquid water to the top, the major advantage of this downdraft scheme is that it avoids large area solar collectors needed to drive an updraft tower. Dry air is continuously sucked into the tower at the top, without need for a large area collector.

The largest solar power station in Australia is the 400 kW array at White Cliffs, New South Wales. The White Cliffs Solar Power Station was originally built using solar dish steam boiler technology, but has now been upgraded to photovoltaic to obtain almost twice the electric output from the same dishes. Other significant solar arrays include the 220 kW array on the Anangu Pitjantjatjara Lands in South Australia, the 200 kW array at Queen Victoria Market in Melbourne and the 160 kW array at Kogerah Town Square in Sydney. Numerous smaller arrays have been established, mainly in remote areas where solar power is cost-competitive with diesel power.

See also

References

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