Technology

Three quarters (232 MW) of the 307 MW of solar thermal electric capacity installed in the last decade around the world is in Spain, and more than 1,346 MW is under construction (around 400 MW of which will be commissioned during 2010) and 843MW at an advanced stage of development. This will result in an installed capacity of more than 2,400 MW in Spain by 2013.
The first commercial central tower plant supplying power to the grid was the PS10 plant (Sanlúcar la Mayor, Seville), which was commissioned in 2006. Since then, the solar thermal electric industry has not stopped growing in our country. All four solar thermal electric technologies are in use in Spain. The most widely developed at a commercial scale is the parabolic trough receiver, which makes up 93% of the 2,500 MW that will be rolled out by 2013. Central tower receivers account for close to 3%, as do parabolic disks that are normally connected to Sterling engines, while linear Fresnel receivers make up little more than 1% of all plants. Parabolic trough plants (PTP) use mirrored concentrators in the form of parabolic troughs, hence the name, to heat a fluid flowing through a tube in the concentrator’s line of fo- cus. The fluid can be heated in this way to a temperature of 400ºC and then used through a series of heat exchangers to generate steam and electricity. The fluid is normally thermal oil, although several projects are underway to replace oil for water to generate steam directly in the absorber tubes, thereby avoiding the need for heat exchangers. This will also permit the temperature of the steam to be increased and electricity generating efficiency boosted.
Central tower plants use a field of heliostats that focus the sun’s rays onto the top of the tower where a central receiver is used to transfer all this heat (which can reach 1,000ºC) to a fluid. In the plants in operation (PS10 and PS20), this fluid is water, while the Gemasolar plant, which has a heat storage system, uses molten salt. As with the PTPs, steam generated is used to drive a turbine to generate electricity using an alternator.
One of the main advantages of this technology is that it is possible to store the energy generated. This is already becoming a reality, since 30% of projects in the pipeline include storage. The Andasol-1 plant in Aldeire (Granada) is already capable of storing heat from the sun for seven and a half hours while the sky is cloudy or at night. Meanwhile, the Gemasolar central tower plant will have 15 hours of storage.
This proves that solar thermal electric power is manageable, programmable and capable of replacing thermal or nuclear power stations, while plants can be disconnected from the grid without wasting any energy. Another advantage of these plants is that they can operate in combination with other renewable sources such as biomass or biogas, and non-renewable sources such as natural gas – a possibility regulated by prevailing legislation.

No boundaries
The international reach of the sector is also noteworthy. Spanish solar thermal electric companies are conducting projects in the United States, Algeria, Morocco, Israel, the United Emirates, etc. These projects include spectacular arrays such as the 280-MW Solana plant in Arizona (US), which will start operating in 2011.
This does not mean that certain challenges do not still lie ahead, since improvements in the solar field and the industrialisation thereof are ongoing in Spain. Current research in the Spanish solar thermal electric sector is focusing on thermal storage, improving absorber tubes (a key component of parabolic trough plants), improving mirrors, and reducing cooling water demand, in order to ensure solar thermal electricity becomes ever more competitive.  

MILESTONES
• 1980s. The Plataforma Solar de Almería (PSA) opens as a leading international research centre, which belongs to the Centre for Technological, Environmental and Energy Research (CIEMAT).
• 2002. Spain introduces a feed-in tariff for solar thermal electricity. It is the first country in the world to do so.
• 2004. The Government approves a new regulation (RD 436/2004) establishing the first target for 2010, 200 MW, and increasing the feed-in tariff for this technology making it economically viable. The possibility of using gas (up to 12%) to maintain the temperature of the heat accumulator is also introduced.
• 2006. Commissioning of PS10 plant (Sanlúcar la Mayor, Seville), the first commercial solar thermal electric plant with a central tower in the world. PS20 opens a year later.
• 2007. Prevailing legislation is revised (Royal Decree 661/2007), establishing a new objective of 500 MW by 2010. The possibility of hybridisation with biomass and biogas is introduced.
• 2008. Andasol 1 (Granada), the first commercial parabolic trough plant in Europe, is commissioned.
• 2009. Council of Ministers agreement to establish a roadmap for the growth of solar thermal electric technologies to 2,400 MW by 2013.