Solar thermal energy
© Nuno Marques on Unsplash

Solar thermal energy

  • Energy and buildings

Solar thermal energy is an alternative to fossil fuels for heating. The solar panels that supply heat and hot water can be installed on a variety of infrastructures, such as houses, schools, hospitals or even on the ground. Some contexts are more suitable than others (sunshine, stability, local expertise).

Why is it important?

Energy is at the heart of the climate challenge – and the first solution. Fossil fuels, such as coal, oil and gas, are by far the biggest contributors to climate change, accounting for over 75% of global greenhouse gas emissions. To halve emissions by 2030, we must collectively end our dependence on fossil fuels and invest in alternative energy sources that are clean, accessible, affordable, sustainable and reliable. (1)

Solar thermal energy is a low-carbon renewable energy source. Its use emits few greenhouse gases compared with fossil fuels. Producing heat from solar thermal panels emits a similar proportion of CO2 to producing electricity from photovoltaic panels: around 44 gCO2e/kWh (Compared with 418 gCO2e/kWh for a gas-fired power plant). (2)

Moreover, solar energy has virtually unlimited potential since it is available everywhere. In contrast, fossil fuels such as oil, gas and coal are running out and are only produced by a handful of countries that hold the key to global energy supply and stability.

Like photovoltaic solar energy, solar thermal energy is set to grow significantly in the coming years. (3) Solar energy projects are on the increase, attracting the attention of both public and private funding bodies, who are showing increasing interest in financing them.

Solar thermal energy can be used to heat water or air. It is an alternative to gas or oil heating, or any other heating system that uses carbon-based electricity.

10x less

electricity produced from solar thermal panels emits 10 times less CO2e/kWh than electricity produced from gas-fired power stations. (4)

35 to 40 %

the average efficiency of thermal solar panels is rising thanks to technological advances.  (5)

5 to 10 years

in theory, a good-quality, well-maintained solar thermal installation will pay for itself in 5 to 10 years.  (6)

What is the solution?

Composition & Operation of the Solar Thermal System

A solar thermal energy production system converts the sun’s rays into heat. Using a thermal collector (thermal solar panels), the sun’s rays are captured by a highly inert material (copper in particular). The heat is then transferred to a heat-transfer fluid, which then transports the heat to a storage tank, for example, or to a central heating circuit. (7)

There are different types of thermal collectors/panels:

  • Flat plate glazed collectors: These are the most used. A black metal plate enables them to absorb the sun’s rays by being in direct contact with the heat transfer fluid.
  • Evacuated glass tube collectors: These consist of glass tubes placed side by side. The sun’s rays are absorbed by a black plate. They are called “evacuated” glass tubes because the residual air is then removed from the tubes to create better insulation and limit heat loss. They are more efficient, but also more expensive.
  • Unglazed collectors: Water-filled tubes made of black plastic or metal allow the collectors to absorb heat. This system is less efficient and deteriorates more quickly. (8)

Solar thermal energy has great potential and can be used in a wide variety of ways, whether in private households, the service sector or industry. (9)


Skills & Technical Expertise 

The design, installation and operation of solar systems requires a certain level of technical expertise, which can be brought in-house or outsourced depending on the size, needs and financial resources of the organisation. The equipment chosen must be robust and, as far as possible, repairable locally or regionally.

Internalising expertise within an organisation requires substantial, long-term financial resources (design, training, operation, maintenance). By limiting the number of intermediaries in this way, it is often easier to monitor the progress of projects, guarantee appropriate training for operators and ensure that installations are operating and maintained correctly.

Outsourcing expertise means relying on partners and subcontractors, from project design through to equipment end-of-life management. To ensure the success of projects, it is important to choose experienced and reliable service providers from the outset, to draw up long-term contracts providing for regular maintenance and servicing of facilities, and to ensure that there are sufficient funds to cover all costs over the lifetime of the facilities.

Solar thermal energy
© Markus Spiske on Unsplash

Point of attention

All energy production has a cost. Even if the source of energy – the Sun – is infinite and free, the production of heat from this source has a cost, limits and risks. The production of solar thermal panels requires metals and rare earth elements that are not accessible everywhere. These materials are often imported and transporting them has a significant environmental impact. Furthermore, the exploitation of the raw materials needed to manufacture lithium batteries is highly controversial in terms of respect for human rights (child labour) and the local pollution caused (Congo). Furthermore, all these devices have a limited lifespan, and special attention needs to be paid to their end-of-life, particularly in the case of electronic waste.

The installation of solar panels only becomes environmentally profitable if the installation has a long lifespan, in other words if the panels continue to produce energy for at least 10 years after their installation. If, due to lack of maintenance, the solar panels no longer produce much energy or stop working after just a few years, then the environmental and climatic impact of the solar installation is likely to be greater than simply connecting them to the local electricity grid. Therefore, installing solar panels should not be done lightly. (10)

Greenhouse gas emissions per 1 kWh of electricity produced

Nuclear → 6 g CO2-eq / kWh

Hydroelectric power → 12 g CO2-eq / kWh

Wind energy → 15 g CO2-eq / kWh

Solar energy PV and thermal → 44 g CO2-eq / kWh

Natural gas → 418g CO2-eq / kWh

Fuel oil → 730g CO2-eq / kWh

Coal → 1058g CO2-eq / kWh


Key actions

  • #1 Assess the needs

    Carry out an energy audit and regular monitoring of heating installations to gain a better understanding of consumption and suggest more appropriate equipment.

  • #2 Rely on experts

    Developing, operating and maintaining a solar installation requires specific expertise, which should be drawn on by forging external partnerships with solid, experienced players, or by developing this expertise within the organisation itself. Contact specialist companies that offer technical support for installing solar thermal panels on buildings.

  • #3 Choose the right equipment

    The choice of solar thermal collectors/panels is crucial to controlling the profitability and environmental impact of the installation. Panels manufactured in China using carbon-based energy will often have a greater impact than those produced in Europe using low-carbon energy. Transport also has a significant carbon impact. The origin of the panels is therefore a parameter to be taken into account. The quality and performance of the panels are also factors to consider. Their efficiency can vary from 35 to 40%, depending on the technology. The profitability of the installation will be optimised with high-yield quality panels, even if the price will undoubtedly be higher.

  • #4 Position the installation correctly

    The positioning of solar installations is crucial to optimising heat production efficiency. Choose the location, orientation and inclination that benefit from the highest level of average daily sunshine. Avoid locations where the panels would be shaded all or part of the year (trees, neighbouring buildings).

  • #5 Maintain facilities regularly

    Cleaning solar panels over time is essential to maintain adequate electricity production and to ensure that the system does not malfunction. In the event of malfunctioning due to dirt, the environmental impact of the installation becomes very significant; the solution may be worse than the original problem.


To consider

Success stories

Bordeaux University Hospital goes solar

The Xavier Arnozan Hospital, part of the Bordeaux University Hospital, carried out major renovation work in 2016. They installed 60m2 of solar panels on the roof, which supply a hot water tank. This distributes domestic hot water throughout the building. These installations will save the building over €5,000 a year in energy costs. (13)

A Greek hospital installs solar thermal panels

In the town of Kalamata in Greece, the local hospital has invested heavily in solar thermal installations. In 2013, the hospital was equipped with 226 solar thermal panels covering a total surface area of 535m2 of solar collectors. The heat produced by this system is used to heat hot water and to heat the premises. Annual energy savings of around 1,615,411 KWh are achieved each year, corresponding to an annual economic benefit of more than €225,000 and a reduction of 546 tones of CO2 per year.  (14)

A solar-powered Romanian high school

In 2021, the Elie Radu secondary school in Ploiesti, Romania, will be investing heavily in solar installations, both photovoltaic and thermal. The school has installed 10 solar thermal collectors to supply a hot water tank and boilers with a capacity of 3,000 litres. Thanks to these installations, the school has reduced its energy consumption by more than 60%. (15)

Geneva flat blocks get thermal installations

In 2018, these flat blocks on chemin de mouille galant in Geneva were fitted with more than 300m2 of solar collectors. This installation provides hot water for all the buildings. This installation is one of the top 5 largest solar thermal installations in Switzerland. (16)

Tools and goods practices

  • Strategic thermal roadmap, ADEME, 2018 (FR)

    Find here
  • Energy management guide for healthcare buildings, PEEB, 2021 (FR)

    Find here
  • Training manual on solar water heating for Kenyan industry, UNDP, 2019

    Find here
  • Operational approach to the installation of solar thermal panels, French Ministry of Education (FR)

    Find here

To go further

  • Analysis of the environmental life cycle of electricity produced by photovoltaic systems, IEA PVPS, 2021

    Find here
  • The history of solar thermal energy, Energy Encyclopaedia, 2019 (FR)

    Find here


(1) United Nations, Reinforcing the ambition of renewable energies. – Read here

(2) Climate Selectra, what are the CO2 emissions by energy source, 2023 – Read here (in French)

(3) Inis iaea, Solar thermal energy around the world, 2012 – Read here (in French)

(4) Climate selectra, CO2 emissions by energy source, 2023 – Read here (in French)

(5) Nouvelr energie, Factors determining the performance of solar thermal panels,2023 – Read here (in French)

(6) Choisir, Is solar energy profitable, 2020 – Read here (in French)

(7) Engie, How thermal solar panels work, 2023 – Read here (in French)

(8) Engie, The different types of solar thermal panels, 2023 – Read here (in French)

(9) Swiss Federal Office of Energy, Solar energy, 2023. Read here

(10) Solar service, The importance of maintaining solar thermal panels. Read here 

(11) Trajectoires media, Decarbonised energies, 2023 – Read here (in French)

(12) Solar services, The importance of maintaining solar thermal panels. Read here

(13) Gironde, Bordeaux University Hospital goes solar, 2021 – Read here (in French)

(14) Andrianos, Solar thermal installation at Kalamata Hospital – Read here (in French)

(15) Balk green energy news, A Romanian high school on the way to a net-zero building, 2022. Read here

(16) Sunoptimo, Chemin du mouille galant in Geneva, 2018 – Read here (in French)