Concentrating solar thermal power, also known as solar thermal electricity (STE), is on a strong growth trajectory, according to the Renewables Global Status Report 2018, published by REN21, an international non-profit association which is based at the United Nations Environment Programme (UNEP) in Paris. While Spain and the USA have the largest installed capacity, according to the report, a new STE Plant came on line in South Africa in 2017 and facilities are being built in other countries around the world.
CSP technologies use reflective material such as mirrors to concentrate the sun’s heat. That heat then drives steam or gas turbines to produce electricity. Most new STE plants incorporate some form of thermal energy storage (TES). This means that they can store heat during the day and convert it into electricity at night time. This is one of the important selling points for STE together with its high degree of production reliability. STE plants are built in sun-drenched areas, from Andalusia in the south of Spain to the edge of the Sahara desert in Morocco, guaranteeing predictable production levels and ensuring grid stability.
Four different CSP technologies are used in the various STE plants installed around the world:
- Parabolic trough - a mirror-based parabolic solar collector which reflects light so that it heats a tube containing a fluid to a high temperature. The heated liquid is then used in a heat exchanger to produce steam which then goes into a steam turbine driving an electric generator to produce power.
- Solar power tower – a series of flat movable mirrors called heliostats focus the sun’s rays upon a collector tower with a receiver situated at its top. This device transfers the sun’s heat to a fluid which produces steam and generates power in the same way as above.
- Linear Fresnel – flat mirror stripes concentrate the sunlight onto a receiver containing tubes located above them. These reflectors use the Fresnel lens effect (a concentrating mirror with a large aperture and short focal length). Fresnel is the name of the French physicist who invented that type of lens. Power is generated in the same way as above.
- Dish – the devices look similar to TV satellite dishes, but they are made of curved mirrors and lenses. Solar thermal energy is concentrated onto a single receiver point situated above the reflector dish. The dish can be connected either to a steam engine or to a Stirling engine. A Stirling engine operates by cyclic compression and expansion of air or gas at different temperatures leading to a conversion of heat energy to mechanical work which then drives a generator.
New IEC publications
While various fluids (water, synthetic oil…) have been used to produce steam, new installations are increasingly employing molten salts. The heated salts generate steam and are easy to store, decoupling energy collection from energy generation, which is crucial for TES.
According to SolarPACES, the international network of researchers and industry experts for the development of CSP and solar chemistry technology under the umbrella of the International Energy Agency (IEA), the most widespread CSP technology used in STE plants around the world is parabolic trough, followed by solar tower. The number of linear Fresnel plants is increasing but remains low in comparison. In the network’s global listing, there are only two dish/engine projects.
IEC Technical Committee (TC) 117: Solar thermal electric plants, is currently preparing the IEC 62862 series of Standards which will specify the different requirements for these various STE technologies. The TC has recently issued its first publications. You can find more information about some of these publications in the article Solar thermal first, published in the January 2018 issue of e-tech.
A large amount of research and development work is focusing on technology which can lower the costs of STE and make it more efficient. Its drawback is that it is more expensive to generate than electricity produced by other forms of renewable energy sources, such as solar photovoltaic (PV) systems. Research projects include an US lab’s high-temperature falling particle receiver, in which sand-like ceramic particles are heated as they fall through a beam of highly concentrated sunlight focused by an array of mirrors. The falling-particle receiver can process heat at significantly higher temperatures than existing CSP technologies (up to 1000 °C and higher), which can increase power cycle efficiencies and reduce costs. Like molten salt, the particles can be stored conveniently.
Alternative salts are also being considered, for instance chloride. The chemical compound can reach higher temperatures than the sodium/potassium nitrate used today. IEC TC 117 is monitoring these technology breakthroughs closely as standardization is expected to play an essential role in achieving greater competitiveness for STE.