Thermal solar energy

Solar thermal energy is a renewable energy production technology that converts solar radiation into usable heat. It is a renewable, environmentally-friendly alternative to conventional energy sources such as gas and electricity.

How it works

The solar thermal system is based on solar collectors that absorb thesun’s energy and transform it into heat. There are different types of collector, such as flat-plate glazed collectors and concentrating collectors. The heat-transfer fluid circulates through these collectors and heats up by absorbing heat. This thermal energy is then stored in a storage tank and used to heat domestic hot water, heat buildings or generate electricity.

To understand how solar thermal energy works, it’s important to understand the various components that make up a solar thermal system and their role in converting solar energy into heat.

Solar thermal collectors

Solar thermal collectors are the key components of a solar thermal system. They are responsible for capturing and converting solar radiation into heat. There are several types of solar thermal collectors, the most common of which are glazed flat-plate collectors and evacuated tube collectors.

  • Flat-plate glazed collectors: This type of collector consists of a frame, an absorber plate (usually metal) and a selective coating that absorbs solar radiation and minimizes heat loss. The collector is covered with transparent glass that allows solar radiation to penetrate while preventing heat loss through convection and radiation.
  • Evacuated tube collectors: These collectors consist of evacuated glass tubes containing a metal absorber and a selective coating. The absence of air in the vacuum reduces heat loss by convection, making this type of collector more efficient, especially on cold or cloudy days.

Heat transfer system

The heat transfer system is responsible for transporting the heat captured by solar collectors to the point of use or storage. In most solar thermal systems, a heat transfer fluid (usually water or a mixture of water and antifreeze) circulates through the collectors and absorbs the heat. The heated heat transfer fluid is then transported to a heat exchanger, where it transfers the heat to the domestic hot water, heating or energy storage system.

Thermal energy storage

Thermal energy storage is a crucial aspect of solar thermal systems, enabling captured energy to be used even when the sun isn’t shining. The most common thermal energy storage systems use insulated hot water cylinders or tanks to store energy in the form of sensible heat.

More advanced technologies, such as phase-change materials, are also being developed. These materials store heat by changing phase (for example, from solid to liquid) and can store large quantities of thermal energy in a small volume.

Regulation and control systems

Regulation and control systems are essential for the efficient operation and management of solar thermal systems. They monitor and control the temperature, pressure and flow rate of the heat transfer fluid, and regulate fluid circulation between the solar collectors, the heat exchanger and the storage system. Control systems can also be used to optimize energy production by adjusting the orientation of solar collectors according to the position of the sun, or by modulating the operation of circulation pumps.

Integration into existing systems

Solar thermal energy can be integrated into existing heating and domestic hot water systems. In the case of heating, solar thermal energy can be used to preheat water for the boiler or underfloor heating system, thus reducing energy consumption of fossil fuels or electricity. For domestic hot water production, solar thermal energy can be used in combination with electric, gas or heat pump water heaters, reducing energy consumption and greenhouse gas emissions.

Solar thermal power plants

There are mainly three types of solar thermal power plant, differing in the way they capture and concentrate solar energy. These are as follows:

  1. In parabolic concentrator power plants (CSP), parabolic mirrors are used to concentrate sunlight on a receiver tube located at the focal point of the parabola. The heat transfer fluid circulating in the receiver tube is heated by the concentrated light, reaching high temperatures (up to 400°C). The heat is then used to produce steam, which powers a turbine and generator to generate electricity.
  2. Solar tower power plants use a large number of flat mirrors (heliostats) to concentrate sunlight on a receiver located at the top of a tower. The heliostats follow the path of the sun throughout the day, adjusting their orientation to maximize the concentration of light on the receiver. The heat transfer fluid in the receiver, often molten salt, is heated to very high temperatures (up to 1000°C). The heat is then used to produce steam, which powers a turbine and generator to generate electricity.
  3. Linear Fresnel power plants use flat, angled mirrors to concentrate sunlight on a receiver tube located above the mirrors. Linear Fresnel mirrors are less expensive to manufacture and install than parabolic mirrors, but they are generally less efficient at concentrating solar energy. As with other types of solar thermal power plants, the heat captured is used to produce steam, which powers a turbine and generator to generate electricity.

Advantages and disadvantages of solar thermal energy


  • Renewable, clean energy: Solar thermal energy is a renewable, non-polluting energy source that helps reduce dependence on fossil fuels and cut greenhouse gas emissions.
  • Energy efficiency: Solar thermal collectors are generally more efficient than photovoltaic solar panels in converting solar energy into useful energy, particularly for heating and hot water applications.
  • Lower energy costs: Solar thermal energy reduces energy costs by providing a free source of energy for heating and hot water production.
  • Improved energy autonomy: Solar thermal systems can be installed on buildings or isolated sites, improving energy autonomy and reducing dependence on the electricity grid.


  • High initial cost: Installing a solar thermal system requires a substantial initial investment, although costs can be amortized over time through energy savings.
  • Sunshine variability: The performance of solar thermal systems depends on sunshine, which can vary according to season, weather and latitude.
  • Maintenance: Solar thermal systems require regular maintenance to ensure optimal operation and a long service life.
  • Space requirements: Installing solar thermal collectors requires available space, usually on the roof or close to buildings, which can be a challenge in densely populated urban areas.

Solar thermal applications

Domestic hot water (DHW) production

One of the most common applications of solar thermal energy is the production of domestic hot water for households and commercial buildings. Solar thermal collectors absorb solar energy and transfer it to a heat transfer fluid, which then heats water stored in a tank. This hot water can be used for everyday needs such as showering, cooking and washing.

Heating buildings

Solar thermal systems can be used to complement or replace traditional heating systems in residential and commercial buildings. Solar thermal systems can be integrated with central heating systems, such as boilers or heat pumps, to preheat water and reduce energy consumption. Underfloor heating systems and radiators can also be powered by solar thermal energy.

Pool heating

Solar thermal collectors are often used to heat swimming pools, extending the swimming season and reducing the energy costs associated with water heating. In this case, the heat transfer fluid circulates through the solar collectors and transfers the heat to the pool water via a heat exchanger.

Solar air conditioning

Solar air conditioning is a less common but promising application of solar thermal energy. Solar air-conditioning systems use the heat produced by solar thermal collectors to power absorption or adsorption refrigeration machines, which then cool indoor air. This type of system reduces electrical energy consumption for air conditioning, especially during periods of high demand, such as summer.

Steam and electricity generation

In large-scale solar thermal power plants, solar energy is used to produce high-pressure, high-temperature steam, which can be used to power steam turbines and generate electricity. Solar thermal power plants typically use parabolic mirrors or heliostats to concentrate solar radiation on a central receiver, where a heat-transfer fluid is heated to high temperatures. The steam produced is then used to drive a steam turbine and an electrical generator.

Industrial processes

Solar thermal energy can be used to power industrial processes requiring heat, such as pasteurization, sterilization, drying and concentration. By integrating solar thermal collectors into industrial facilities, companies can reduce their energy consumption and dependence on fossil fuels, while cutting their greenhouse gas emissions.

Solar thermal desalination

Solar thermal desalination is a promising application for providing drinking water in arid regions and areas facing water shortages. Solar thermal desalination systems use solar energy to heat salt water, causing pure water to evaporate, which is then condensed and collected. Solar thermal desalination technologies include solar distillation, solar reverse osmosis and multiple evaporation processes.

Solar thermal projects and companies

A number of projects and companies have contributed to the development of solar thermal energy in France and Europe. These include

  • The PSD (Pôle Solaire de Démonstration) project in France, aimed at developing and demonstrating advanced solar thermal technologies.
  • BRGM, a French company working to improve solar thermal collectors and storage systems.
  • Manufacturers of solar thermal collectors, such as the French company Thermor.

Future prospects and innovations

The solar thermal energy market is constantly evolving, and technological innovations are helping to improve the efficiency and profitability of this renewable energy source. Future developments and current innovations include :

  • Improved materials and manufacturing techniques for solar thermal collectors, which will increase their efficiency and reduce production costs.
  • The development of new thermal energy storage systems, such as phase-change materials, which offer higher energy density and greater storage capacity.
  • The integration of solar thermal energy with other renewable technologies, such as geothermal or wind power, to create more efficient and reliable hybrid energy systems.
  • The expansion of large-scale solar thermal power plants for electricity generation, particularly in sun-rich regions such as southern Europe, the Middle East and North Africa.

The development of solar thermal energy worldwide

hese are some of the main countries using solar thermal energy and the most important power plants in these countries:

United States

The United States is a world leader in the development of solar thermal energy. The largest solar thermal power plant in the USA is the Ivanpah plant, located in California, with a capacity of 392 megawatts (MW). This solar tower power plant uses more than 170,000 heliostats to concentrate solar energy on three receiver towers.


Spain is another leader in solar thermal power, with several plants in operation. The Andasol solar power plant in Andalusia is the largest parabolic concentrator plant in Europe, with a total capacity of 150 MW divided between three 50 MW units.


Morocco has invested significantly in renewable energies, particularly solar thermal. The Noor solar power plant, located in Ouarzazate, is one of the world’s largest solar thermal power plants, with a planned capacity of 580 MW when completed. It will comprise several units using different solar thermal technologies, including parabolic concentrators and solar towers.

South Africa

South Africa has also invested in solar thermal power to diversify its energy sources. The KaXu solar power plant, located in the Northern Cape province, is the country’s first solar thermal plant, with a capacity of 100 MW. It uses parabolic concentrators to capture solar energy.


China has also developed solar thermal projects to meet its growing demand for clean energy. The Delingha solar power plant, located in Qinghai province, is one of the country’s leading solar thermal power plants, with a capacity of 50 MW. It uses parabolic concentrator technology.