Solar Thermal Electricity (STE), also known as Concentrating Solar Power, is a technology that produces electricity by using mirrors to concentrate direct-beam solar irradiance to heat a liquid, solid or gas that is then used in a down- stream process for electricity generation.
We have known the principles of concentrating solar radiation to create high temperatures and convert it to electricity for more than a century but have only been exploiting it commercially since the mid-1980s. The first large-scale solar thermal power stations were built in the California Mojave desert. In a very short time, the technology has demonstrated huge technological and economic promise. It has one major advantage – a massive renewable resource, the sun, and very few down- sides. For regions with similar sun regimes to California, STE offers the same opportunity as the large offshore wind farms in Europe.
Generation of bulk solar thermal electricity from solar thermal power plants is one of the technologies best suited to mitigating climate change in an affordable way by reducing the consumption of fossil fuels. Unlike photovoltaic technology, STE offers significant advantages from a system perspective, thanks to its built-in thermal storage capabilities. Solar thermal power plants can operate either by storing heat or in combination with fossil fuel power plants, providing firm and dis- patchable power available at the request of power grid operators, especially when demand peaks in the late afternoon, in the evening or early morning, or even when the sun isn’t shining.
The main benefit of STE systems is in replacing the power generated by fossil fuels, and reducing greenhouse gas emissions which cause climate change. Each square metre of STE concentrator surface, for example, is enough to avoid 200 to 300 kilograms of CO2 each year, depending on its configuration. Typical STE power plants are made up of hundreds of concentrators arranged in arrays. The life-cycle assessment of the components and the land surface impacts of STE systems indicate that it takes around five months to ‘payback’ the energy that is used to manufacture and install the equipment. Considering the plants last at least 30 years with minimum performance losses, this is an excellent ratio. In addition, most of the STE solar field components are made from common materials that can be recycled and used again.
The cost of solar thermal power is going down. Experience in US shows today’s generation costs are about 12 US cents/kWh for solar generated electricity at sites with very good solar radiation. The US Department of Energy’s SunShot Initative predicts on-going costs as low as 6 US cents/kWh. STE technology development is on a steep learning curve, and the factors that will further reduce costs are technological improvements, mass production, economies of scale and improved operation. Concentrating solar power is becoming competitive with conventional, fossil fuelled peak and mid-load power stations. One of the benefits of adding more STE to the grid is that it can help stabilise electricity costs, mitigating fossil fuel price volatility and the impact of carbon pricing when it takes effect.
Hybrid plants combine concentrated solar power and fossil fuels. Some, which make use of special finance schemes, can already deliver competitively-priced electricity. For small-scale, off-grid solar power generation, such as on islands or in rural hinterlands of developing countries, STE is a compelling alternative to diesel engine generators, which are noisy, dirty and expensive to run.
Several factors are increasing the economic viability of STE projects, including reform of the electricity sector, rising demand for ‘green power’, and the development of global carbon markets for pollution-free power generation. Direct support schemes also provide a strong boost, like feed-in laws or renewable portfolio standards for renew- able power in some countries. Last but not least, increasing fossil fuel prices will also help bring the price of STE in line with the cost of conventional power generation.
The 3 most common technologies available are Parabolic Troughs, Central Receivers, and Linear Fresnel Reflectors. These technologies concentrate the direct solar radiation from the sun to a heat transfer fluid that carries the heat to a steam circuit and activates a turbine, thus creating electricity. The STE technology has the great advantage to be dispatchable and be easily integrated to the grid at any time thanks to a storage system and hybridisation systems that could be integrated in the power plant.
Parabolic Trough technology consists of rows or loops of parabolic trough-shaped mirror reflectors that are used to collect the solar radiation and concentrate it onto a thermally efficient receiver tube placed in the trough’s focal line. The fluid is heated up to approximately 400°C by the sun’s concentrated rays and then pumped through a series of heat exchangers to produce superheated steam. The steam is converted to electrical energy in a conventional steam turbine generator, which can either be part of a conventional steam cycle or integrated into a combined steam and gas turbine cycle. This fluid can also be used to heat a storage system consisting of two tanks of molten salt.
A circular array of heliostats (large mirrors with sun-tracking motion) concentrates sunlight on to a central receiver mounted at the top of a tower. A heat- transfer medium in this central receiver absorbs the highly concentrated radiation reflected by the heliostats and converts it into thermal energy that is used to generate superheated steam for the turbine. To date, the heat transfer media demonstrated include water/steam, molten salts, liquid sodium and air. If pressurised gas or air is used at very high temperatures of about 1,000°C or more as the heat transfer medium, the gas or air can be used to directly replace natural gas in a gas turbine. This application makes use of the excellent efficiency (60% and more) of modern gas and steam combined cycles.
An array of nearly flat reflectors concentrates solar radiation onto elevated inverted linear receivers. Water flows through the receivers and is converted into steam. This system is linear-concentrating, similar to a parabolic trough, with the advantages of low costs for structural support and reflectors, fixed fluid joints, a receiver separated from the reflector system, and long focal lengths which allows the use of flat mirrors. The technology is seen as a potentially lower- cost alternative to trough technology for the production of solar process heat and steam.
A range of technologies are used to concentrate and collect sunlight and to turn it into medium- to high temperature heat. This heat is then used to create electricity in a conventional way, i.e., run a turbine. Solar heat collected during the day can also be stored in liquid or solid media such as molten salts, steam, ceramics, concrete or phase-changing salt mixtures. At night, the heat can also be extracted from the storage medium to keep the turbine running. Solar thermal power plants work well to supply the summer peak loads in regions with significant cooling demand, such as Spain and California. With thermal energy storage systems, they operate longer and even provide baseload power. For example, in Chile the 110 MW Atacama STE plant with 17.5 hours of thermal storage, is capable of providing clean electricity 24 hours a day every day of the year.