Solar plant set to take heat

The world’s first utility-grade solar power plant with central tower and thermal molten salt storage technology is set for construction in Spain.

Its developers claim that its unique design will help capture and store heat energy from the sun to produce power at any time of the day.

Spanish engineering group SENER and Abu Dhabi renewable- energy initiative Masdar have formed Torresol Energy to build the Gemasolar facility in Seville.

The plant will consist of thousands of mirrors that reflect radiation from sun rays onto a collection tower. Inside the tower a series of pipes filled with molten salt will capture, store and transfer this solar heat energy for electricity production. Torresol Energy claims that Gemasolar will be the first commercial plant in the world to use this technology when it is operational in 2011.

According to Oihana Casas, a spokeswoman for Torresol Energy, the plant is expected to generate 110,000MW hours of power a year.

‘The plant can supply energy for 30,000 households,’ she said.

Torresol Energy also predicts that the solar-power plant will reduce carbon-dioxide emissions by more than 50,000 tonnes a year compared with a conventional coal- fired power plant.

The Gemasolar plant technology is based on solar thermal power generation. This technique is a less direct way of producing electricity compared with photovoltaics, which uses silicon semiconductors to convert light photons directly into electricity.

The Gemasolar plant will consist of 2,500 flat moveable-mirrors, called heliostats, that focus up to 95 per cent of solar radiation onto the top of a collection tower encircled with sun-receiver panels. Inside the tower, absorber tubes filled with molten salt will collect and trap the heat from the sun. Some of this molten salt will go to a heat exchanger that will transfer the heat to water and produce steam to move a turbine. A generator that is coupled to the turbine will produce energy that is sent out to an electrical transformer and the electrical network.

Another portion of the molten salt will transfer to a storage tank where excess heat is stored and used for operating the turbine during the night or when there is insufficient solar radiation.

Casas said SENER started working on this power-tower technology in 2001 when the company embarked upon the Solar Tres project, an experimental plant with central-tower technology and heliostats, in Andalusia, Spain.

Engineers at SENER used this plant and simulation software to size and optimise their commercial endeavour. Along the way the team came up with patented two-axes solar trackers to go into the heliostats. These trackers use structural, mechanical, electronic and control components to accurately point the heliostats’ reflective surface depending on the position of the sun. All of which optimises the plant’s electricity production.

Casas added that the main innovation for SENER was the thermal molten-salts storage system, which can reach temperatures of more than 500°C. Such a high temperature means steam will be more highly pressurised, which will optimise the turbine’s performance.

The molten salt held in the insulated thermal-storage vault can be used to generate power for up to 15 hours without sunlight.

Torresol Energy claims this means Gemasolar will produce much more power than other thermoelectric solar power plants.

‘In terms of performance and operation, the technology inherent within the Gemasolar plant will treble electricity production,’ said Casas. ‘This is because of the fact that the majority of thermoelectric plants being developed do not have a thermal-storage system, while Gemasolar has high- temperature heat storage that extends the normal operating period of these plants.’

Many commercial solar thermal plants consist of vast fields of single-axis parabolic-shaped reflectors that focus the sun’s radiation on a linear receiver located at the focus of the parabola. As the collector tracks the sun from east to west during the day, a heat-transfer fluid warms up and circulates through the receivers. The fluid returns to a series of heat exchangers at a central location, where it is used to generate steam, which is used in the same way as other solar thermal plants.

While parabolic-trough plants are effective at producing energy during the day, they must have a fossil-fuel-fired capability to supplement the solar output when there is low solar radiation. 

Despite these drawbacks, Casas said Torresol Energy is still interested in promoting parabolic- trough plants commercially in addition to central-tower receiver and heliostat technology.

‘Our researchers point out that central-tower receiver technology is more profitable in terms of efficiency than parabolic-trough technology,’ she added. ‘However, the central-tower receiver technology is newer and therefore more risky than parabolic-trough technology, which has been tested for years now.’

While the company is willing to consider more than one solar thermal technology, it does not plan to venture into other areas such as photovoltaics.

‘Torresol Energy and SENER are focused in obtaining electricity from solar energy on a large scale,’ said Casas, ‘And that kind of production can only be reached with solar thermal technology.’

The Torresol Energy company was set up in March 2008 and is 60 per cent owned by SENER and 40 per cent owned by Masdar, an initiative driven by the government-owned Abu Dhabi Future Energy Company.

The company recently secured €171m (£151m) to begin construction on Gemasolar through funding from Banco Popular, Banesto and the Instituto de Credito Oficial.

Siobhan Wagner