Solar thermal energy: Using solar energy for heating and hot water
Solar thermal energy – what is it?
A solar thermal system provides heat for drinking and shower water or to support the heating. Thermie is ancient Greek and means "warmth". This is made possible with the help of flat or tubular solar collectors on the roof, which contain black absorber surfaces. Solar thermal systems are not to be confused with photovoltaic systems. These generate electrical current from sunlight with solar cells that are combined in modules.
How a solar thermal system works
For more information, click on the digits.
Note: Pipes of the solar circuit are filled with a water-glycol mixture. The glycol serves as an antifreeze that prevents the water in the pipes from freezing on cold days. Similar antifreeze can also be found in the cooling water for car engines.
Graphic: Wagner Solar GmbH
Solar thermal energy: only for hot water or also for heating?
In the vast majority of cases, solar thermal energy supports the existing heating system. Small solar thermal systems are only used to heat drinking water. Depending on the size of the system and the level of hot water consumption, you can produce up to 70 percent of the energy required for hot water from sunlight with such a system. In a system for solar heating support, solar heat is also used for heating.
Typical sizes of a solar system for a single-family home:
| Kind | Hot water only | Hot water and heating |
|---|---|---|
| Collector size | 3 to 6 m² | 9 to 20 m² |
| Memory | 250 to 350 litres | 500 to 1,500 litres |
| Energy saving | approx. 400 kWh per m² collector | approx. 300 kWh per m² collector |
| Costs for retrofitting into an old building (as of February 2022) | approx. 6,000 to 10,000 euros | approx. 9,000 to 17,000 euros |
| Favourable conditions | Central hot water supply for at least 3 people | Heating system with low flow temperature, e.g. underfloor heating. Heat demand even in the transitional periods. |
Is my roof suitable for solar thermal energy?
Unshaded south-facing roofs are favorable. The orientation to the southeast and southwest also still brings a good yield. With an east or west orientation, the yield is reduced by about a quarter. Roofs with inclinations of 30 to 70 degrees are well suited.
Is solar thermal energy worth it?
A solar thermal system makes a lot of sense if you want to relieve the burden on the environment and make a contribution to the energy transition.
How much energy costs you can save through the system depends on the individual framework conditions such as energy prices, the consumption situation, the building characteristics and the building technology.
A 10 m² system saves up to 4,000 kWh of natural gas per year. These savings are offset by procurement costs, which you can reduce with the help of public subsidies. With rising energy costs, the result improves.
The operating costs are incurred by regular maintenance and inspection, during which the antifreeze is checked, among other things. In addition, there are costs for the pump electricity, reserves for minor repairs and possibly a higher premium for building insurance.
Solar thermal energy is also worthwhile as a complementary technology for heat pumps, because solar thermal systems supply almost all of the energy necessary for hot water preparation, especially in summer. The heat pump can be switched off during this time. This saves electricity and increases the annual performance factor of the heat pump, because it has to provide high temperatures of 60°C for hot water preparation. As a result, the efficiency or annual performance factor of a heat pump decreases in summer.
Solar thermal energy or photovoltaics: These are the advantages and disadvantages
There is competition for space, especially in the case of small roof areas: The roof area can be used either for a photovoltaic system to generate electricity or for a thermal system for heat generation.
There are a few things to be said for photovoltaics:
- The technique is simpler.
- Power lines are usually easier to lay than insulated pipes filled with liquid.
- Surplus solar energy, e.g. during the holiday season or when the children are out of the house later, can be sold.
- Overheating is not possible in summer (but the problem is controlled by correctly designed thermal systems).
- A PV system can be easily expanded.
- It can supply part of the heat pump electricity.
- It contributes to environmental protection because fossil fuels are saved.
- In the event of sharply rising energy prices or inflation, the cost-saving advantage increases. (For photovoltaic systems, on the other hand, the feed-in tariff remains fixed.)
- The energy yield per square metre is higher than with photovoltaics.
- Vacuum tube collectors (especially for heating support) can also be attached to the wall of the house.
If you have enough space and money, you can of course also implement both: hot water systems require only a small amount of space. An alternative is the installation of a PVT system.
PVT: Photovoltaics and solar thermal energy combined
In the meantime, collectors are also available on the market that produce electricity and heat from sunlight at the same time. These collectors are called PVT collectors. They offer several advantages compared to solar thermal systems or photovoltaic systems, especially when combined with a brine-water heat pump.
- You will not have to make a decision about whether solar thermal or photovoltaics will be installed on the roof (no competition for space between electricity and heat).
- PVT systems work more efficiently per square metre than a solar thermal system or a photovoltaic system of comparable size.
- Some PVT collectors also make the outside air available as a heat source for a brine-water heat pump.
- A PVT heat pump system works much quieter than an air-to-water heat pump.
The cost of PVT collectors is in the range of €2,500 per kWp. The costs for the heat pumps are added to this. However, unlike a photovoltaic system, PVT systems can be subsidized in part by the Federal Funding for Efficient Buildings (BEG).
In addition, the surplus electricity yield can be sold and is then remunerated in accordance with the Renewable Energy Sources Act (EEG). As a result, and through the self-consumption of the electricity produced in the house, the PVT collectors are partly self-financing.
Solar thermal energy: What do I have to pay attention to during planning and construction?
It is important to estimate in advance how much heat you will consume in the first place. Based on these figures, it is better to plan the system a little too small than too large. If you have a gas heating system, you will get good reference values if you regularly read the gas meter reading - especially in summer.
Vacuum tube collectors take up a little less space and can reach slightly higher temperatures. However, vacuum collectors are also more expensive. Which collector type is suitable for you often depends on where you want to install the collectors, how much space is available and what structural conditions are available.
Take this opportunity to check whether the roof is sufficiently insulated, otherwise roof insulation may be more sensible and important as a first step.
In the case of a flat roof of an existing building, you should have it clarified whether your roof is suitable for retrofitting with solar thermal energy (or also with solar power). You should have the statics and raintightness of the roof carefully checked.
The pipes from the roof to the attic should be as short as possible and as well insulated as possible. The thermal insulation of the pipes must also be able to withstand high temperatures of over 100 degrees Celsius and be weather- and UV-resistant outdoors. The storage tanks and the storage connections are also very well insulated.
Regardless of a solar system, you can save energy by optimising the distribution of hot water and heating.
It is important to regularly record and monitor the yields, i.e. the amount of heat produced by the solar thermal system. Therefore, you should also have a heat meter installed in the solar circuit of the system.
Connecting a washing machine or dishwasher to hot water: More tricky than often thought
Many consumers hope to be able to operate the dishwasher or washing machine inexpensively with the water heated by the sun "free of charge". However, there are a number of things you need to consider here. For example, it is possible that the water valve in the machine opens, then cooled water flows in from the pipe and the valve closes again before any appreciable warm water has flowed in. The water volumes are low with modern machines anyway.
In the 30-degree washing program, the laundry should be protected from too hot water that comes directly from the thermal system. This is possible with a special ballast for temperature limiting, but it must be set correctly for each wash cycle.
Such a ballast can be omitted if you have an expensive washing machine with its own hot water connection. The whole thing is a little easier with dishwashers, but their "cold water connection" must also be approved for the high temperatures.
Important: According to the new requirements of the Federal Funding for Efficient Buildings (BEG), the funding application for heating replacement can only be submitted after the conclusion of a supply and service contract. However, this must contain an exit clause in the event that the funding is not approved.
Tips:
- If you need a new heating system or a new hot water tank anyway, you should think about whether you want to integrate a solar thermal system into your system right away. The costs are then lower than with a subsequent installation.
- If your roof needs to be reroofed, you can save the installation costs for a solar thermal system if you have it installed at the same time.
- The system should not be larger than necessary. This is because the profitability of a solar thermal system depends decisively on whether the heat demand and the size of the system are in the right proportion to each other.
What to do if the solar system is not running optimally?
Don't have enough hot water in the summer months? The system is making noise? She's getting too hot? Water and steam are emanating? The returns of the investment are decreasing? Then your system is obviously not running optimally. In most cases, long-lasting problems are due to errors in planning or installation as well as a lack of maintenance. Air in the solar circuit can also cause disruptions in the operating process.
Where the fault lies and how it can be rectified must be clarified by experts on site. To do this, you should request the help of a specialist solar company or installers. Further information on the costs and benefits, possible applications and requirements of a solar thermal system can be found in the guide "Heating" and in the guide "Electricity and Heat: Ways to an Energy Self-Sufficient House".