Wikipedia:Mediation Cabal/Cases/2006-07-12 Solar Updraft Tower/Version1

The Solar updraft tower is a type of renewable-energy power plant based on the Solar chimney concept. Air is heated in a wide greenhouse; convection causes the air to rise inside a tall chimney. The moving air spins turbines that produce electricity. There are no Solar updraft towers in operation at present. A research prototype operated in Spain in the 1980s.

Description
The Solar updraft tower is a proposed kind of power station that harnesses solar energy by convection of heated air within a large chimney. Inside a very large circular greenhouse (up to 8 kilometers in diameter), air is heated by the sun and travels up a chimney 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 proposed for utility-scale power plants, or (as in the prototype 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 volume of air is warmed up to flow 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 soil underneath the collector stores heat during the day and releases it again during the night, allowing the system to operate for 24 hours a day. Water filled hoses may be placed under the collector to increase heat storage capacity.

History
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.

Design
Image:SolarTower.jpg In 1982 a small scale working model of a solar updraft power plant was built in Manzanares, Ciudad Real, 150 km south of Madrid in Spain, under the direction of German engineer Jörg Schlaich. This pilot plant, funded by the German Government, operated for approximately 8 years until 1989. The chimney had a diameter of 10 meters and a height of 195 meters, with a collector (greenhouse) diameter of 244 meters and an area of 46,000 m² (about 11 acres). The power plant achieved a maximum generated electricity output of about 50 kW. During operation, optimization data was collected on a second-by-second basis; these data are the basis for the design of a utility-scale Solar updraft power plant.

According to model calculations based upon the data obtained at Manzanares it has been estimated that an updraft power plant with an output of 200 MW would need a collector 7 kilometers in diameter (total area of about 38 km²) and a 1000-meter high chimney. These model calculations indicate that the solar to electricity conversion efficiency improves with size; therefore, a solar updraft power plant needs to be large for it to be cost-effective. EnviroMission proposes to construct a 50 MW power station with a 400 m high chimney for Australia.

Because no data are available to test these models on a large-scale updraft power plant there remains uncertainty about the reliability of these calculations. The performance of an updraft tower may be degraded by factors such as atmospheric winds, or by drag induced by bracings used for supporting the chimney.

Advantages
Like other sources of renewable energy a Solar updraft power plant would contribute to reducing CO2 greenhouse gas emissions by producing sustainable electricity.

One of the strengths of the Solar updraft power plant is that the system can be configured to operate for 24 hours a day.

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.

It has been suggested that a Solar updraft power plant located at high latitudes such as in Canada may produce up to 85% of a similar plant at southern locations.

Solar to electricity conversion efficiency
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 50kW Mantazares plant achieved a conversion efficiency of 0.53%, but SBP believe that this could be increased to 1.3% in a large and improved 100MW unit. 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 than the Mantazares pilot plant. The low efficiency of the Solar Tower is somewhat balanced by the rather low investment cost per m² of aperture.

One reason for the low conversion efficiency of the solar updraft tower concept is 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, with an upper Carnot cycle extractable efficiency of about 15% (assuming $$T_H$$ ≈ 80°C and $$T_C$$ ≈ 20°C). Other losses contribute to account for the measured conversion efficiency of 0.53%, such as reflection of light off the top of the canopy, heat loss through the roof of the collector, drag in the chimney, and turbine losses.

Solar updraft towers would impact a significant area of land to generate as much electricity as is produced by modern power stations using other technology. Because of the low conversion efficiency the proposed Solar updraft power plant in Australia would take a 5x larger area of land than the Solar thermal systems planned for California or Spain to generate the same amount of electrical power. The 500-megawatt (MW) SCE/SES plant planned for California covers 4,500-acres or 18.2 km² (or 36.4 m²/kW) and the 50MW AndaSol Power Plant that is being built in Spain has a total area of 1.95 km² (39 m²/kW), as compared to 38 km² for the 200MW Solar Tower design (190 m²/kW).

Competition
Over the past several years there has been substantial investments and commitments towards Solar thermal power plants of different design. 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 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, primarily in remote areas where solar power is cost-competitive with diesel power.

Related and adapted ideas
The Vortex engine proposal is similar to the solar chimney but replaces the physical chimney by a vortex of twisting air.

Another approach, Floating Solar Chimney Technology, proposes to keep a lightweight chimney aloft by lifting balloon rings filled with a lighter than air gas.

Another proposed design would construct the chimney up a mountainside, rather than a freestanding structure in the centre of the solar collector.

The inverse of the solar updraft tower is the energy tower, which 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 wind turbines at the bottom of the tower. This design does not require a large solar collector, but does consume up to 50% of the generated energy operating the water pumps.