Solar Thermal Electricity (STE), also known as Concentrating Solar Power (CSP), 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.
Generation of bulk solar thermal electricity from CSP 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 dispatchable 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.
Solar thermal power requires direct sunlight, called ‘beam radiation’ or Direct Normal Irradiation (DNI). This is the sunlight which is not deviated by clouds, fumes or dust in the atmosphere and that reaches the earth’s surface in parallel beams for concentration. Suit- able sites must get a lot of this direct sun – at least 2,000 kWh of sunlight radiation per square metre annually. The best sites receive more than 2,800 kWh/m2/year.
Typical regions for concentrating solar are those that lack atmospheric humidity, dust and fumes. They include steppes, bush, savannas, semi-deserts and true deserts, ideally located within 40 degrees of latitude north or south. The most promising areas of the world include the south- western United States, Central and South America, North and Southern Africa, the Mediterranean countries of Europe, the Near and Middle East, Iran and the desert plains of India, Pakistan, the former Soviet Union, China and Australia.
In these regions, one square kilometre of land can generate as much as 100–130 GWh of solar electricity per year using solar thermal technology. This corresponds to the power produced by a 50 MW conventional coal- or gas-fired mid- load power plant. Over the total life cycle of a solar thermal power system, its output would be equivalent to the energy contained in more than5 million barrels of oil.
Like conventional power plants, solar thermal power plants need cooling at the so-called “cold” end of the steam turbine cycle. This can be achieved through evaporative (wet) cooling where water is available or through dry cooling (with air), both of which are conventional technologies. Dry cooling requires higher investment and eventually leads to 5%–10% higher costs compared to wet cooling. Hybrid cooling options exist that can optimise performance for the site conditions and are under further development.
Water consumption for use in wet cooling in the Spanish plants has proven to be half of the water needs per hectare, as compared with the consumption of agricultural crops, like corn or cotton in Andalucia, Spain. Also, STE uses 200 times less water than a coal power plant to produce the same amount of electricity, according to IRENA’s soon-to-be-released regional report.
The huge solar power potential in the “Sun Belt” regions of the world often far exceeds local demand. This creates the potential for excess solar electricity to be exported to regions with a high demand for power but a less favourable solar irradiance. In particular, southern European and North African countries could harvest the sun for export to northern European countries in the medium and long-term. Of course, for any new development, local demand must be met first.
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. There are four main types of commercial STE technologies: parabolic troughs and linear Fresnel systems, which are line concentrating, and central receivers and parabolic dishes which are point concentrating central receiver systems, also known as solar towers. Dishes with Stirling motors are not an appropriate technology for utility scale applications and therefore we will only refer to solar towers when talking about central receiver systems.
Since the last update on STE technologies in 2009, on-going progress has been made in the use of STE technologies outside of the electricity sector, namely solar fuels, process heat and desalination. Advances have also been made in storage systems for this technology.
A full list of the plants operating, in construction can be found in this map.
Dispatchability is the ability of a power producing facility to provide electricity on demand. Dispatchable power plants, for example, can be turned on and off and adjust their power output on demand. Conventional power stations, like fossil fuel plants, are dispatchable but produce, among other things, CO2 emissions. STE plants, however, which produce electricity in a manner similar to conventional power stations, i.e., by driving a steam turbine, are also dispatchable.
Dispatchability is one of the characteristics that makes STE a favoured option among other renewable energy technologies. All solar thermal power plants can store heat energy for short periods of time and thus have a “buffering” capacity that allows them to smooth electricity production considerably and eliminates the short-time variations that non-dispatchable technologies exhibit during cloudy days.
What’s more, thanks to thermal storage systems and the possibility of hybridisation,4 solar thermal power plants can follow the demand curve with high capacity factors delivering electricity reliably and according to plan. Thermal storage systems also allow STE to provide power in the absence of direct solar radiation, such that periods of solar generation and demand need not coincide.
For this case, the solar thermal power plant supplies electricity when needed to help meet peak demand.
Firmness and dispatchability are the main benefits of STE. STE and other renewable energy technologies, such as PV and wind, can thus be combined in an energy system to balance supply. In this way, STE can replace fossil fuel power plants and contribute to a 100% renewable energy supply as one of the renewable technologies capable of following the demand curve and ensuring a 24/7 secure supply. STE plants can also contribute to the stability of the system, i.e., maintaining voltage and frequency within required ranges, and allowing further penetration and integration of intermittent sources without the need for fossil fuel back-up.