What is a solar thermal heat pump: operation, selection criteria and price

The solar thermal heat pump is the combination of two technologies that respect the environment without compromising the efficiency and thermal comfort of the occupants: the heat pump and solar energy.

In this practical guide, we propose to explore in detail the solar thermal heat pump: how it works, the types of solar panels, the advantages and disadvantages, the criteria of choice, the installation process, the aids and the feedback from users.

What is a solar thermal heat pump?

The solar thermal heat pump, sometimes called solar heat pump or solar PAC, is a heating and cooling system that combines the action of solar thermal panels (STP) and an air-to-water (or water-to-water) heat pump.

ObjectiveTo ensure the thermal comfort of the occupants while optimizing energy consumption and reducing greenhouse gas (GHG) emissions through the use of a renewable and free energy source.

Solar thermal heat pump: how does it work?

The solarothermal heat pump combines the mechanism of the classic heat pump and solar energy. Its operation can be summarized in 7 steps: 

  1. The cycle begins with the capture of solar energy. Thermal solar panels are installed on the roof or in an open space exposed to the sun. Made of absorbing materials, these panels will convert solar energy into heat that will then be transferred to a heat transfer fluid (water or a water-antifreeze mixture), inside the panel's tubes;
  2. Heated, the heat transfer fluid circulates in an exchanger. The heat is then transferred to another fluid that heats the air or water in the building; 
  3. The water heated by the exchanger is distributed inside via a heating or cooling circuit, usually a radiator, underfloor heating, fan coil units, etc. ; 
  4. Excess solar thermal energy can be stored in a thermal tank for later use when there is insufficient sunlight; 
  5. In this system, the role of the heat pump is to take over when solar thermal energy is not sufficient to meet the needs of the occupants. The heat pump extracts heat from the outside air, the ground or water, depending on its type, to transfer it inside (heating), or extracts heat from inside to reject it outside (cooling).

To ensure the proper functioning of the system, a control and regulation system is set up to monitor the room temperature and the energy needs of the building.

PAC solarothermique : which solar panels to use ?

The solarothermal heat pump uses thermal solar panels that absorb the heat of the sun's rays to transfer it to a heat transfer fluid that circulates in the system. The solar thermal heat pump should not be confused with systems of the "heat pump - photovoltaic panels" type. In this case, the photovoltaic panels supply the heat pump and other appliances in the home with electricity, via an inverter that transforms direct current into alternating current.

Finally, we note that a solar thermal heat pump can use hybrid panels, insofar as they combine the action of thermal solar panels (rear side) and photovoltaic solar panels (front side).

Type of solar panelOperating principleBenefitsDisadvantages
Thermal solar panelsSolar thermal panels absorb heat from the sun's rays and transfer it to a heat transfer fluid that circulates through the system.High energy efficiency for heat production.
Less expensive than photovoltaic and hybrid panels.
Possibility of storing thermal energy for later use.
Do not produce electricity directly.
Performance is highly dependent on weather conditions and sunlight.
Hybrid solar panels (PVT)Hybrid solar panels combine photovoltaic and thermal technologies in a single panel. 
They produce electricity and heat simultaneously, allowing a heat pump to be powered while generating electricity for other uses.
Combined production of electricity and heat.
Higher energy efficiency than photovoltaic or thermal panels alone.
Optimal use of the roof space.
Higher installation cost than solar thermal and photovoltaic panels alone.
Increased complexity of the system requiring more rigorous maintenance.

Solar thermal heat pump: advantages and disadvantages

The advantages are numerous: energy savings and low greenhouse gas emissions, especially if the heat pump is coupled with hybrid solar panels. Indeed, this combination uses a renewable and free energy source to heat or cool.

The solar thermal heat pump is also very versatile, as it can be used for heating, cooling and domestic hot water (DHW) production. Finally, the solar thermal heat pump is a proven and reliable equipment with a good durability.

As for the disadvantages, the yield fluctuates according to the season, but the autonomy remains very interesting, close to 80 %... provided that the installation is carried out according to the rules of the art and that the house is impeccably insulated (RE 2020).

We can also deplore the high initial cost of the installation, with an average of 13 000 € without counting the installation. However, this investment is alleviated by the aid and financing mechanisms as well as the energy savings made in the long term.

What criteria should I use to choose my solar thermal heat pump?

The choice of each element of your solar thermal system (heat pump - solar panels - control system) requires certain prior knowledge.

Unless you are well-versed in the subject, we warmly recommend that you call on a professional who will be able to analyze your needs and advise you on the most appropriate equipment. However, here are 6 essential criteria, given as an indication, to make an enlightened choice:

  1. Type of solar panel Refer to the table above to decide on the type of solar panel to couple to your solar thermal system;
  2. Type of heat transfer fluid Some systems use water, others use a mixture of water and glycol (antifreeze). If you live in a region where temperatures regularly drop below freezing, we recommend an antifreeze heat transfer fluid to prevent the pipes from freezing;
  3. Type of heat pump Choose between the heat pump air - water, air - air, water - water and soil - water depending on the heat source available (air, water, soil) and the desired use (heating, cooling, production of domestic hot water). Note: the air - water heat pump is suitable for hot or temperate regions, while the geothermal heat pump will give you better performance in cold regions;
  4. Coefficient of performance The COP is an indicator of the energy efficiency of a heat pump. The higher the COP, the more efficient the heat pump is at converting electrical energy into thermal energy. Look for a heat pump with a high COP to maximize energy savings (and take advantage of state aid);
  5. Regulation and control system It must allow to monitor and adjust the temperature according to the weather conditions and the heating or cooling needs;
  6. Compatibility with other heating systems If you already have a heating system (boiler, radiators, underfloor heating), check that the solar thermal heat pump is compatible with this equipment;
  7. The guarantee It is generally very long for the solar panel (up to 25 years), and 5 to 10 years for the heat pump.

How do I size my solar thermal heat pump?

Only a qualified professional will be able to answer you with precision... which does not prevent you from having an idea of the order of magnitude. Remember the following principle: very roughly, one square meter of solar thermal panels will heat an area of 10 m². Let's assume that an average solar thermal panel is 2 m² (in reality, it can go from 1.5 to 4 or 5 m², with an average of 2 m²): 

Living space (m²)Panel surface solar thermal (m²)Number of solar thermal panels (2 m²/panel)
5052.5 (rounded to 3)
757,53.75 (rounded to 4)
12512,56.25 (rounded to 6)
150157.5 (rounded to 8)

What you need to know about installing a solar thermal heat pump

The installation of the solar thermal heat pump must be carried out by a qualified professional, with the necessary certifications and expertise. In France, use a professional Recognized as a guarantor of the Environment (RGE).

Installation of the solar thermal heat pump

It will start with a site assessment to determine the optimal location of the solar panels and heat pump. The solar panels should face south and be angled to maximize exposure to the sun. The heat pump should be installed in an accessible location for easy maintenance and repair.

The solar thermal heat pump can be used to complement an existing heating system, such as a gas or oil boiler. The installer must ensure that the two systems are properly integrated and operate optimally. For the sizing, your installer will have to take into account the living space, the insulation of the building, the climate of your region and the heating needs.

He will then proceed with the installation: solar panels, heat pump, hot water tank and other components, according to the manufacturer's specifications and local regulations. Once the installation is complete, he will proceed with the commissioning and the usual checks. The installation usually takes about a week (it's a lot of work!).

In general, solar thermal heat pumps require little maintenance. However, you should plan regular checks to ensure the performance and longevity of the system and avoid major breakdowns that threaten the profitability of your investment. Your installer will provide you with service and maintenance instructions and will inform you about the frequency of checks.

Solar thermal heat pump: installation cost and life span 

As explained above, the cost of a solar thermal heat pump is around €13,000, plus the cost of installation which is between €4,000 and €6,000. Annual maintenance (mandatory in some countries such as France) ranges from €150 to €300 per year.

As for the lifespan, the solar thermal panels can exceed 30 years, against 15 to 20 years for the PAC.

PAC solarothermal: you are entitled to aid to finance your project

Several EU countries offer support and financing schemes to encourage households to switch to clean energy, in line with the 2050 carbon neutrality target. Let's take France as an example: 

  • The energy bonus, amounting to between €5,000 and €10,000 for the most modest households to finance renovation work or to replace a heating system as part of the energy transition;
  • MaPrimeRénov', a scheme for owner-occupiers and landlords (and co-ownerships under certain conditions) to finance energy improvement work in housing: insulation, change of heating system, installation of heat pumps or mechanical ventilation, etc. The amount varies according to the income level of the households, the type of work carried out and the post work energy performance. For the PAC solarothermal, the amount of the premium can go up to 10 000 €;
  • Eco-Loan at 0% interest: this is an interest-free loan of up to €30,000;
  • Aid from local authorities, depending on the region and the commune.

Finally, note that the French government applies a reduced rate of VAT (10 %) on solar thermal heat pump equipment.

Opinions and testimonials: what do users think of the solar thermal heat pump?

It is not easy to give a comprehensive overview of the opinions and testimonials of users of solar thermal heat pumps, as experiences can vary depending on the model, manufacturer, quality of the installation and climatic conditions in the region. 

However, here are some common points found in the feedback from users of solar thermal collectors:

On the plus side:

  1. Energy savings achieved by using solar energy to heat the house and produce hot water, with reduced energy bills and greater independence from energy suppliers;
  2. Ecology Many users are satisfied with the positive environmental impact of their solar thermal heat pump. By using solar energy, they reduce their consumption of fossil fuels and contribute to the fight against climate change ... even if the manufacturing process of solar panels is pinned by some users who consider it not very environmentally friendly;
  3. Reliability Users generally report that their solar thermal collectors are reliable and require little maintenance. If properly designed and installed, the systems last for decades.

For the negative points:

  1. High initial costBut the aid and energy savings over time accelerate the payback;
  2. Dependence on sunlight Some users report inadequate performance during harsh winters. They usually have to resort to a supplementary heating system to meet their energy needs;
  3. Complexity of the installation.