Unlike photovoltaics, which generate electricity directly from sunlight, concentrating solar power (CSP) technologies use heat to generate electricity in much the same way as a conventional thermal power station. A series of mirrors or parabolic troughs focus the sun’s rays on a central receiver, containing a mineral oil or molten salt. As this liquid heats up (reaching temperatures as high as 400-600 ºC), it passes through a heat exchanger and generates steam, which is then used to drive a turbine.
 

Several types of systems have been developed over the years, including parabolic troughs, solar towers, dish-Stirling systems and solar chimneys. Of these, the first two use variations on the theme described above. Solar towers employ fields of mirrors to reflect light onto a central receiver atop a tower, while parabolic troughs, as the name implies, use long fields of mirrors curved to reflect light on a central receiver, which runs along between them.

The Dish-Stirling systems could supply thousands of megawatts of renewable energy ( source: R. J. Montoya, Sandia National Laboratory)

The famous 344 MW parabolic trough array in California, USA, developed by LUZ Engineering between 1984 and 1992, was the first of its kind in the world, and is still the largest ever built. Altogether, LUZ built nine plants at this site until the company went out of business in the early 1990s. The troughs continue to function well however, and produce electricity reliably, with seven operated by FPL Energy and another two by Carlyle/Riverstone.

Similarly, the Solar tower is also a mature technology, with the first prototypes developed over 20 years ago in California. Solar One and Solar Two, as they were called, ran until 1989 and generated 38 GWh of electricity.

Dish-Stirling systems are slightly different and use a concave mirror to focus light on a Stirling-type external combustion engine, a special form of engine driven only by heat. At present there is an experimental array of six 25 kW dish-Stirling systems being tested at Sandia National Laboratory in Arizona, however the developer of this technology, Stirling Energy Systems, claims to have over 1000 MW of projects in the pipeline, some of which will be discussed below.

The fourth type of CSP application being considered is the solar chimney. In effect, solar chimneys work by capturing heat in a vast ‘greenhouse’. As the air heats up it rises to the centre of the greenhouse and rushes up a long narrow chimney driving turbines as it passes. The technology was first developed in Spain by a German engineering consultancy and has now being taken up in Australia, where there are said to be plans for several devices, each around 200 MW, although it is still unclear how realistic these are.

From a utility point of view, one of the most interesting things about CSP is that it can provide regular and predictable baseload power, often with capacity factors of over 60 per cent. One of the reasons for this (aside from the predictability of sunshine in some areas of the world) is that unlike electricity, heat can be stored relatively easily. By using molten salt storage liquids it is possible for CSP plants to run throughout the night - generating electricity using the excess heat they stored up during the day. While this is technically feasible, in practice not all of the current developers have decided this is economic, and many have opted for shorter storage periods, usually around an hour. This allows them to continue sending power to the grid in the event of a sudden change in weather, while giving the grid operators a full hour of warning and avoiding any non-compliance penalties.

After a 15-year hiatus, there are currently over 2600 MW of concentrating solar powering the pipeline, spread across the USA, Spain, North Africa and the Middle East. But by far the two most advanced markets however are in Spain and the USA, each of which will be looked at briefly below.

In 2004, Spain became the first country in the world to establish a dedicated feed-in tariff for concentrating solar power, and was recently described by Greenpeace as the ‘hottest’ place in the world for CSP. The boost provided by the feed-in tariff has been supported by further legislation allowing operators to use natural gas as back-up to keep CSP plants primed. This, together with an increasing demand for power in Spain’s growing economy, has caused a flurry of activity in the sector and there are now around 200 MW of CSP approved and up to 800 MW in the pipeline.

In late 2006, Spain unveiled the first commercial solar tower in the world - an 11 MW facility called PS 10 near Sevilla, which was developed by Solucar, the solar arm of engineering firm Abengoa. PS 10 is the first of Spain’s new CSP projects, but there are many more. Two other power towers are being planned by Solucar, the 20 MW PS 20 and AZ 20 projects, the first of which began construction in October 2006. Unlike some of its competitors, none of Solucar’s tower projects (or its trough projects - see below) uses a salt storage system, with the company instead focusing on technical reliability and lower temperature water storage. This should allow the towers to generate electricity for up to an hour at half load, avoiding penalties for failing to meet grid obligations.

An aerial photograph of the recently completed PS 10 plant in Spain (source: Solucar)

In addition to the three Solucar plants, there is a fourth power tower being planned for southern Spain. The 17 MW Solar Tres (Solar Three) project, being developed by Spanish company Sener, is a continuation of the technology used in Solar Two in California, and will employ molten salt technology similar to that used in the USA demonstration project. This will give Solar Tres a 16-hour back-up facility and the ability to generate electricity 24 hours a day. Indeed, such will be its reliability that the developers claim it will produce as much electricity per year as the 50 MW power trough systems currently being planned.

As well as leading the way with a new generation of power tower projects, Spain is host to a large number of potential parabolic trough developments. The first three are being developed by ACS Cobra and Solar Millennium in Andalucia. Of these, Andasol I and II will have a capacity of 50 MW, a 510 000 m2 solar field and up to seven hours of thermal storage. Andasol III will also be 50 MW, but will have a larger solar field (620 000 m2) and up to 12 hours of heat storage. The first of these projects - Andasol I - is due to be completed by the end of 2007. Power tower developer Solucar also has a pipeline of some 300 MW of parabolic trough projects, made up of six 50 MW arrays located around the town of Sanlucar el Mayor. These projects could be just the start though, as Iberdrola, one of the world’s largest developers of renewable energy, has announced a string of nine to ten parabolic trough projects totaling 500 MW spread right across southern and central Spain.

With is clear skies and huge energy demands, the Southwest of the United States is ideal for the deployment of CSP, and it was here that the technology was first developed, and, along with Spain, where it is staging its comeback.

In April 2006, Arizona became host to the first megawatt-class commercial parabolic trough generator to be built anywhere in the world in 15 years - a one MW facility in Saguaro in Arizona. The $6 million plant covers an area of 9300 m2, and is located 50 km north of the city of Tucson. This new facility was developed by Solargenix (now a subsidiary of Acciona Energy) and the Arizona Public Service (APS).

More recently, to the east in Nevada, Acciona has just finished working with the US Department of Energy and the National Renewable Energy Laboratory to build the 64 MW Solar One project just outside Boulder City. Assuming that these plants function well, the developers are considering major expansion plans, totaling several hundred megawatts.

The USA is also pioneering the commercial use of dish-Stirling systems. Although the largest existing array is a 150 kW test facility run by SES in Arizona, the company has recently signed a number of power purchase agreements with utilities Southern California Edison and San Diego G&E for over 1800 MW of power from two fields - SES Solar 1 and SES Solar 2. If built, these fields could make SES the largest generator of solar electricity in the world.

As a first step towards developing these power plants, planning has begun for a 200 km grid extension from San Diego to Imperial Valley. Called the Sunrise Powerlink, the plan received the approval of the California Independent System Operator (ISO), and is now waiting approval from the California Public Utilities Commission.

Given that they can provide carbon-free baseload energy, it is remarkable that CSP technologies, many of which are well proven, have not been more extensively deployed. They can be constructed on a utility-scale, fitting well into the current energy use models and it is estimated that a solar dish park of 26 000 km2 in the southwest USA could provide enough electricity for the whole country.

So why has this not begun to happen? One of the reasons, say industry commentators, is that unlike solar CSP has been forced to compete directly with mainstream fossil-based generation, with its well-established subsidies and grandfathered costs. It is only in the last few years as global energy prices have soared, fears about security have grown and climate change has become an undeniable urgency that serious interest in this exciting technology has again emerged. Still, CSP has proven itself successful and reliable and looks likely to play an important role in the future of world energy.