Large scale solar power plants addding to global energy needs

Paul Boughton

There are three technologies that make use of the sun’s energy: photovoltaic cellswhich generate electricity directly; thermalwhere an absorption media such as water or oil transfers the absorbed heat to a steam generator that drives a turbine; and solar towerswhich are effectively chimneys where rising hot air powers turbine generators.

One version of the thermal technology directs the heat from the sun onto a Stirling engine connected to a generator.

The Stirling engine is an external combustion engine. No combustion takes place internally so the gases used inside the cylinders never leave the engine. This means there are no exhaust valves that vent high-pressure gasesas in a gasoline or diesel engineand there are no explosions taking place. This makes Stirling engines very quiet.

The engine uses an external heat source to raise the temperature of a fixed amount of gas in the cylinderwhich increases the internal pressure and drives a piston.
A changeover valve releases the pressure and a second cylinder takes overreturning the first to its starting position.

External combustion engines are often less compact and less powerful than internal combustion engines but can be more efficient.

Thermal techniques use arrays of sun-tracking parabolic reflectorscalled heliostatsperhaps in the form of troughsto focus the sun’s rays on an absorber tube filled with oil or other fluid and mounted on a tower. The hot oil boils water to produce steamwhich is used to generate electricity via a turbine.

An early US demonstration plantSolar Onea 10MW facility in BarstowCaliforniaused water as the fluid that generated steam. The plant was later converted to Solar Twowhich used molten salt as the fluid. The hot salt could be storedthen used when needed later to boil water into steam to drive a turbine. Since 1985nine power plants in the Mojave Desert in Southern Californiacalled the Solar Electric Generating Systems (SEGS)have been in full commercial operation. They use parabolic trough technology to collect the sun’s energy.

The system generates steam at 400°C which drives turbines that output a total of 345MW of electricity for over half a million residentssaving over 2million barrels of oil a year. The plants range in size from 14 to 80MW and are located on three sites. Each plant sells power to the local utilitythe Southern California Edison Company. Solel Solar Systems took over the system some time ago and has since further advanced the technology.

Developments

Reflectors traditionally use aluminium or silver on the front or back surface of a thin glass or plastic substrate. Todayresearchers are developing new reflective materialssuch as advanced polymer filmsthat are less expensive to produce than glass. Thin versions are stretched across a drum and a partial vacuum draws the film into the required parabolic shape.

Over the past 15 years thermal efficiency has improved by 20percent as a result of continued research and development into selective coating technologies and improvements in the glass-to-metal welding of the collectorssays Solel Solar Systems.

The company claims world leadership in the development and manufacture of solar thermal collectors ranging from high temperature400°Cparabolic troughs to heating and cooling solutions for residential housing. It is set to build a US$1bn 500MW solar power station in Israel. It says one rule of thumb is that the basic foot print for a 10MW/hr solar thermal plant is 1km square.

What is claimed as the world’s largest solar photovoltaic system has just been commissioned. Bavaria Solarparkin MuehlhausenGermanycan generate 10MW of electricity from 57600 photovoltaic panels spread over three sites totalling 62 acres.

The solar electric system was designed and furnished by PowerLight Corporationusing its patented tracking system which follows the sun to make maximum use of its energy.

The solar power plant was built on fallow fields formerly used for agricultural purposes. But the area still provides ecological benefits as herds of moorland sheep graze the pastures and keep the grass short under the solar electric panels. Extensive vegetation areas were planted to enhance the integration of the project site into the surroundings.

In Australiasolar towers are being developed by EnviroMissiona newly-listed public company committed to establishing profitablelarge-scale renewable energy power stations. The company owns the exclusive licence to German-designed solar tower technology in Australia and its first project will focus on developing this revolutionary technology.

The company says the project is on track to get approval by the Australian government andon completionthe US$800million solar tower will be the tallest man-made structure in the world. It will generate enough electricity to supply 200000 typical Australian homes.

Towering heat

Solar towers use the sun’s radiation to create a constant thermal updraft of heated air to drive turbines that generate clean sustainable electricity. The radiation heats the body of air under a large collector canopy to 35°C above ambient.

The hot air rises up the towereffectively a chimneyat 15metres per secondpassing through large turbines positioned at the base of the tower to generate electricity. The scheme envisages using 32 pressure staged turbines each generating 6.25MW.

The technology has been tested with a successful small-scale pilot plant constructed in ManzanaresSpain as the result of collaboration between the Spanish Government and the German designersSchlaich Bergermann and Partner. It operated for seven years between 1982 and 1989and consistently generated 50kW output of green energy.

The collector zone will be roofed with a translucent material such as glass or polycarbonate and will be approximately 5km in diameter. The collector surface is raised several metres above the ground at the edgerising to a greater height at the base of the tower. This design facilitates an environment for air to be directed in vertical movement with minimum friction loss.

The ground under the collector can also be covered with heat absorbent tubing or similar material to further increase the plant’s ability to produce a higher power output over a 24-hour period.

The tower will be constructed from reinforced high strength concrete. It will be 1000m tall and 150m in diameter as the preferred proportions for maximum stability and thermal efficiency and can be built using conventional construction techniques. Its height compares with the television tower in TorontoCanadawhich is over 600m tall.

Analysis of the lifespan of a reinforced concrete tower in a dry climate is indicated at more than 50 years. This is because carbonisationthe usual reason for the deterioration of concretedoes not take place in low humidity regions.

Carbonisation is the process that causes concrete to lose its ability to protect its inner reinforced steel due to the gradual conversion from the surface inwards of calcium hydroxide in the cement into calcium carbonate as a result of the CO2 present in the atmosphere. This process is accelerated in the presence of excessive moisture.

Maintenance

The rate of air flow in the tower is sufficiently low to enable maintenance crews to work inside without shutting down the station.

Its efficiencythe conversion of heat into kinetic energyis determined by the difference between the temperature in the collector and the temperature of the air at the top of the tower. A 1°C drop in temperature over every 100m facilitates the necessary updraft.

The concept ensures effective operation even on cooler daysas it is primarily dependent upon the temperature differential between the air under the collector and air at the top of the tower. The low temperature differential needed for the tower’s updraft effect ensures the power station can continue to operate overnight. Heat storage can be achieved by closing shutters or doors at the base of the tower to trap the heated air until required.

There are no transport problems bringing fuel to a solar power sitewhich make the technology ideal for remote locations.

This also makes it ideal for powering water treatment plants and Acquasol says a 22MW/hr power plant can produce up to 40gigalitres of potable water per year from a range of undrinkable sources.

The company says solar thermal technology has large cost advantages over photovoltaicswind power and other renewable energy sources. It can produce high base load power as well as storing thermal energy that can generate electricity both at night and during periods of poor weather. Acquasol specialises in green powered water desalination.

The interest shown in large scale solar power is now reaching important levels and further developments will bring the concept into serious contention with other methods of generating electricity.

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