Solar Tower Energy
The solar updraft tower is a proposed type of renewable-energy power plant. It combines three old and proven technologies: the chimney effect, the greenhouse effect, and the wind turbine. Air is heated by sunshine and contained in a very large greenhouse-like structure around the base of a tall chimney, and the resulting convection causes rising airflow to rise through the updraft tower. The air current from the greenhouse up the chimney drives turbines, which produce electricity. A successful research prototype operated in Spain in the 1980s, and many modelling studies have been published as to optimization, scale, and economic feasibility.
The generating ability of a solar updraft power plant depends primarily on two factors: the size of the collector area and chimney height. With a larger collector area, a greater volume of air is warmed to flow up the chimney; collector areas as large as 7 km in diameter have been considered. With a larger chimney height, the pressure difference increases the stack effect; chimneys as tall as 1000 m have been considered.
Heat can be stored inside the collector area greenhouse to be used to warm the air later on. Water, with its relatively high specific heat capacity, can be filled in tubes placed under the collector increasing the energy storage as needed.
Turbines can be installed in a ring around the base of the tower, with a horizontal axis, as planned for the 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.
Carbon dioxide is emitted only negligibly while operating, but is emitted more significantly during manufacture of its construction materials, particularly cement. Net energy payback is estimated to be 2–3 years.
A solar updraft tower power station would consume a significant area of land if it were designed to generate as much electricity as is produced by modern power stations using conventional technology. Construction would be most likely in hot areas with large amounts of very low-value land, such as deserts, or otherwise degraded land.
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 its construction and maintenance.
In 1903, Catalan Colonel of the spanish army 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 Australia, Canada, Israel,and the USA.
Prototype in Spain
In 1982 a small-scale experimental model of a solar chimney power plant was built under the direction of German engineer Jörg Schlaich in Manzanares, Ciudad Real, 150 km south of Madrid, Spain; the project was funded by the German government.
The chimney had a height of 195 metres and a diameter of 10 metres with a collection area (greenhouse) of 46,000 m² (about 11 acres, or 244 m diameter) obtaining a maximum power output of about 50 kW. However, this was an experimental setup that was not intended for power generation. Instead, different materials were used for testing such as single or double glazing or plastic (which turned out not to be durable enough), and one section was used as an actual greenhouse, growing plants under the glass. During its operation, optimization data was collected on a second-by-second basis with 180 sensors measuring inside and outside temperature, humidity and wind speed.
For the choice of materials, it was taken into consideration that such an inefficient but cheap plant would be ideal for third world countries with lots of space – the method is inefficient for land use but very efficient economically because of the low operating cost. So cheap materials were used on purpose to see how they would perform, such as a chimney built with iron plating only 1.25 mm thin and held up with guy ropes. For a commercial plant, a reinforced concrete tower would be a better choice.
This pilot power plant operated for approximately eight years but the chimney guy rods were not protected against corrosion and not expected to last longer than the intended test period of three years. So, not surprisingly, after eight years they had rusted through and broke in a storm, causing the tower to fall over. The plant was decommissioned in 1989.
Based on the test results, it was estimated that a 100 MW plant would require a 1000 m tower and a greenhouse of 20 km2. Because the costs lie mainly in construction and not in operation (free ‘fuel’, little maintenance and only 7 personnel), the cost per energy is largely determined by interest rates and years of operation, varying from 5 eurocent per kWh for 4% and 20 years to 15 eurocent per kWh for 12% and 40 years.
EnviroMission has since 2001 proposed to build a solar updraft tower power generating station known as Solar Tower Buronga at a location near Buronga, New South Wales. Technical details of the project are difficult to obtain and the present status of the project is uncertain.
On 18 March 2007, the company board announced a merger with the US-based SolarMission Technologies, Inc.,but the relationship was terminated on November 1, 2007.
Conversion rate of solar energy to electrical energy
The solar updraft tower has power conversion rate considerably lower than many other designs in the (high temperature) solar thermal group of collectors. The low conversion rate of the Solar Tower is balanced to some extent by the low investment cost per square metre of solar collection.
According to model calculations, a simple 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. One 200MW power station will provide enough electricity for around 200,000 typical households and will abate over 900,000 tons of greenhouse producing gases from entering the environment annually. The 38 km² collecting area is expected to extract about 0.5 percent, or 5 W/m² of 1 kW/m², of the solar power that falls upon it. Note that in comparison, concentrating thermal or photovoltaic solar power plants have an efficiency ranging from 20-40%. Because no data is available to test these models on a large-scale updraft tower there remains uncertainty about the reliability of these calculations.
The performance of an updraft tower may be degraded by factors such as atmospheric winds, by drag induced by bracings used for supporting the chimney, and by reflection off the top of the greenhouse canopy.
Location is also a factor. A Solar updraft power plant located at high latitudes such as in Canada, could produce up to 85 per cent of the output of a similar plant located closer to the equator, but only if the collection area is sloped significantly southward.
It is possible to combine the land use of a solar updraft tower with other uses, in order to make it more cost effective, and in some cases, to increase its total power output. Examples are the positioning of solar collectors or Photovoltaics underneath the updraft tower collector. This could be combined with agricultural use.
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