Solar movement: How solar heat is producing power on both sides of the Atlantic

From California to Spain, Israel to Algeria, new developments in concentrating solar power are promising to bring utility-scale solar power to the sunbelt regions of the world. By Alasdair Cameron, with Jackie Jones.

On both sides of the Atlantic, exciting developments are taking place in the extraordinary renaissance of concentrating solar thermal power. In Nevada, the 64 MW Solar One parabolic trough system is the first multi-megawatt power station of its kind to be built in over 15 years, while in Spain, the recently completed PS10 is the first commercial-scale solar tower in the world. Yet these two projects are just the beginning. Concentrating solar power (CSP) is emerging behind wind as a significant potential source of renewable wholesale generation capacity.

The dish-Stirling systems could supply thousands of megawatts of renewable energy randy j. montoya, sandia national laboratories

After a 15-year hiatus, there is currently over 2600 MW of concentrating solar thermal powering the pipeline, spread across the US, Spain, North Africa and the Middle East (see Table 1). But by far the two most advanced markets, however, are in Spain and the US. Spain’s Acciona reports over 1000 MW of solar thermal power under development in the short and medium term in the USA and Spain, while Iberdrola and EDP have developed significant pipelines of projects of more than 800 MW.

Concentrating solar thermal power

Unlike photovoltaics, which generate electricity directly from sunlight, concentrating solar 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 other thermal carrier. As this liquid heats up (reaching temperatures as high as 400ºC – 600ºC), it passes through a heat exchanger and generates steam, which is then used to drive a turbine.

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 fields of mirrors curved to reflect light on a central receiver which runs along between them. The famous 344 MW parabolic trough array in California, 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.

TABLE 1. Proposed solar thermal projects.a Source: Greenpeace 2005

In addition to parabolic troughs and solar ‘power’ towers, several types of CSP system have been developed over the years, including dish-Stirling systems and solar chimneys. Dish-Stirling systems 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 Laboratories in Arizona, US, but 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 concentrating solar thermal application being considered is the solar chimney. This works rather differently. 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 by Australian company Enviromission, which reports plans to develop devices of around 200 MW in Australia and Texas.

Reliable energy

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%. 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.

The state of the global market

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.

Power towers in Spain
It 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. The company is 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.

The 64 MW Nevada Solar One Plant went online in June ACCIONA

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 US 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 (see below).

Parabolic troughs in Spain
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 Andalucía. 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 10 parabolic trough projects totaling 500 MW, spread right across southern and central Spain.

The US
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, 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 1 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 first initiated by Solargenix Energy (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 expansion plans to allow heat storage (for generation of power at peak demand periods after sunset).

An aerial photograph of the recently completed PS10 plant in Spain SOLUCAR

As well as pursuing tried-and-tested trough technology, the US 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.

The new PS10 can generate electricity for several hours after the sun goes down SOLUCAR

As a first step towards developing these power plants, planning has begun for a 120 mile (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 for approval from the California Public Utilities Commission.


Given that they can provide carbon-free baseload energy, it is remarkable that concentrating solar thermal 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 of the US 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 and (until recently, wind), 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.

Alasdair Cameron is former Assistant Editor of Renewable Energy World. Jackie Jones is Editor of Renewable Energy World


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