Thermal solar system

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Front: solar panels on the roof of the University of Speyer ; behind photovoltaic system ; in the background roof- top systems on the GEWO houses in Speyer-West
Thermal solar system in Mörfelden-Walldorf

In solar thermal systems or solar thermal systems , a conversion takes place of solar radiation in heat . This is then made usable in process technology or building technology or used in thermal solar power plants to generate electricity .

The direct conversion of sunlight into electricity (electrical energy) - e.g. B. by means of solar cells - is called photovoltaics , the corresponding systems as photovoltaic systems.

Areas of application

System diagram of a solar circuit with solar station
Tichelmann system for solar collectors

Thermal solar systems are predominantly used in building services. The heat gained is mostly used to heat drinking water (dishwashing water , shower and bath water) and for the auxiliary heating of living spaces.

In the industrial sector, systems with mostly more than 20 m² collector surface are operated for the production of process heat in the temperature range up to 100 ° C or a little above, for example to accelerate biological and chemical processes in biomass processing or in the chemical industry or to heat / preheat air.

Solar thermal systems also include systems for solar air conditioning . Due to the high temperatures, they are comparable to the process systems.

On the other hand, they are used on an industrial scale in thermal solar power plants such as in Andasol . Most of these systems use concentrating collectors to focus the sun's rays on an absorber point or an absorber line in which temperatures from 390 ° C to over 1000 ° C can be reached. This heat is then either used as industrial process heat or converted into electricity using generators (solar thermal electricity generation). Since concentrating systems are dependent on direct sunlight, they are only used in sunny and dry regions (in Europe for example in southern Spain).

In the following, this article concentrates on the use of solar thermal energy for domestic hot water heating and heating backup, as this is (still) the most common and most widespread area of ​​application in Central Europe.


The thermal solar system consists of a collector, which converts the solar radiation into heat, a solar thermal store that stores the heat that is not immediately used, and the connecting solar circuit through which the heat is transported from the collector to the store. This consists of pipes, fittings and drive units, which ensure the proper operation of the system, as well as a controller, which switches the heat transport on and off (except for gravity systems ).

Collectors and absorbers

The solar collector is the part of the solar system that absorbs as much of the energy from sunlight as possible . The collector itself heats up and is protected by thermal insulation or a vacuum from being immediately re- emitted as thermal radiation . The absorbed heat should be transferred to the solar fluid in the solar circuit with as little loss as possible .

The term absorber refers to both the lines contained in the collectors, which are used to transport the heat transfer fluid, as well as independent components which absorb the solar energy directly without being additionally enclosed by glass or a housing, as is the case with a collector. The different types of absorbers are described in the articles solar collector and solar absorber  .

The most important designs are

  • Flat-plate collectors that are protected against heat loss with conventional insulation materials ( thermal insulation ). They pioneered the efficient use of solar energy. They have a long lifespan; some manufacturers should give a functional guarantee of 20 years.
  • Evacuated tube collectors , which are constructed in the same way as a thermos flask : The inner absorber tube containing the transport medium is enclosed by a second, outer glass tube. In order to reduce the energy loss of the heated liquid, the air is withdrawn from the space ( vacuum ). Due to the lower energy losses, vacuum collectors work more profitably than other types of construction despite the higher price, especially with high temperature differences of over 40 ° C between the outside air and the absorber. They are therefore used when the solar system is supposed to support the heating system and therefore the energy gain in winter is particularly important, as well as in the industrial sector, where process heat with temperatures of over 80 ° C is required. The efficiency of the evacuated tube collectors is better. The flat-plate collector has an advantage when the temperature difference is low. As a result of the better insulation, evacuated tube collectors defrost somewhat more slowly. This can be a disadvantage in regions with a lot of snow.

Flat-plate collectors are predominantly used in Europe. Vacuum collectors have a higher yield per square meter of absorber surface. However, the difference is reduced if the yield is related to the total area of ​​the collector, since the actual absorber tubes in tube collectors, due to the design, have a smaller proportion of the total area available. When used exclusively for heating domestic water in private detached houses, a vacuum tube collector only brings marginal profit advantages.

The performance data of the collectors in the Keymark certificates are used to predict the heat yield

So-called vacuum flat collectors are a mixed form . With these, the entire air space of the box-shaped flat collector is vacuumized. Due to their design, they tend to leak, so that air that penetrates them has to be extracted regularly with the aid of a vacuum pump .

In register shaped absorber pipes or if multiple solar absorber or -kollektoren in a common hydraulic system in parallel to be operated (for example, with a common circulation pump ), they need to Tichelmann be piped together to make a reasonably uniform flow through all the panels is ensured.

Solar storage

Pressureless 300 l solar storage tank

In order to be able to use the captured heat regardless of the current solar radiation, it must be stored. The storage capacity and heat losses are important quality parameters.

Thermal storage capacity

The storage capacity is proportional to the storage volume, the heat capacity of the storage medium and the usable temperature difference.

The main storage medium is water . Compared to other substances, water has a high specific heat capacity of 4.187 kJ / (kg · K). For example, a fully heated 500 liter hot water storage tank contains a usable amount of energy of at a temperature difference of around 45 K

between the inlet from the cold water pipe network and the storage tank. In reality, the amount of energy is smaller because there is always temperature stratification in a storage tank.

If a water storage tank is to be used for heating operation, it is advisable to use the highest possible storage tank temperature as well as low-temperature heating and the use of a heating mixer in order to achieve the greatest possible temperature difference.

A fully heated 800-liter storage tank with a storage temperature of 80 ° C and a flow temperature of 30 ° C for underfloor heating could then, for example

hold up.

Heat loss

A 300-liter storage tank that is common today has (depending on the make and manufacturer) z. B. a heat loss of approx. 1.9 kWh / day, a 600-liter storage tank with the same insulation standard approx. 2.4 kWh / day. If the storage volume is doubled, the heat loss only increases by approx. 30%. One reason for this is that the surface of a storage tank increases disproportionately with the volume.


Solar storage systems differ from conventional drinking water storage systems primarily in:

  • reinforced insulation; 10 cm and more (up to approx. 50 cm) are common, partly made of materials such as PU or PP foam with very low thermal conductivity (λ <0.04 W / (m K)), partly two-layered, but often only 5 cm mineral wool for conventional hot water storage tanks in central heating systems.
  • a tall and slim design of the water tank, which allows the development of different temperature layers (hot water above, cool water below)
  • a deep, large-area heat exchanger for the transfer of heat from the solar circuit.

Long-term storage

Central solar tank storage in a solar house complex consisting of 5 buildings with 16 residential units. The solar coverage of the complex is given as 65%.

For long-term storage in a seasonal heat store, e.g. from summer to winter, gravel is used in addition to water . The heat is brought in and out by means of air. However, water and solids are only suitable for such long-term storage if large volumes or masses are available.

An alternative is latent heat storage , these use the solid / liquid phase transition, e.g. B. paraffins , for heat storage and require a much smaller volume for the same amount of heat. With them, a large number of containers filled with paraffin are usually placed in a water tank.

Thermochemical heat accumulators use the heat conversion of reversible chemical reactions: When heat is supplied, the heat transfer medium used changes its chemical composition; when the conversion is initiated from the outside, most of the heat supplied is released again. In contrast to buffer and latent heat storage systems, thermochemical heat storage systems enable the almost loss-free storage of larger amounts of heat over longer periods of time. Therefore they are suitable for. B. as a seasonal storage for solar thermal applications in regions with high seasonal temperature differences.

Bivalent memory

Often, solar storage tanks are designed bivalent , that is, in addition to the heat exchanger of the solar circuit, they have a device for reheating by means of another energy source, e.g. B. a second heat exchanger in the upper storage area for connection to a conventional ( heating oil or natural gas ), heat pump or biomass boiler ( pellet or logs ). This reheating is always necessary when the sun does not provide enough energy to cover the hot water requirement (for example after several cold days with thick cloud cover). Alternatively, an electric heating rod can also be used; However, heating water with electricity is very inefficient in terms of energy and not very environmentally friendly.

Combined storage

In addition to pure drinking water storage tanks, there are also so-called combined storage tanks or tank-in-tank systems that also serve to support the heating system. The water from the central heating system flows through these containers , which is heated by solar energy in the lower area and reheated in the upper area from the boiler if required. Inside this heating water tank there is a second, significantly smaller container or a thick, coiled pipe through which the drinking water flows and - similar to a flow heater - is heated by the heating water. Such storage tanks have a significantly higher total volume than pure drinking water storage tanks (at least twice the volume); however, the amount of heated drinking water that is kept available is much lower (around 80 to 200 liters). Such systems are therefore also suitable for public buildings or boarding houses that have a high demand for hot water but do not want to use hot water tanks with more than 400 liters that require special protective measures against Legionella .

Solar buffer storage

Solar buffer storage tanks contain heating water - not drinking water. A typical example of this is the stratified charge storage tank . A solar buffer storage tank has i. d. Usually via a heat exchanger in the lower area of ​​the storage tank. The solar system heats the heating water. If the solar system does not achieve sufficiently high buffer storage temperatures, another conventional heat source (e.g. wood boiler, screw-in electric immersion heater, oil or gas heater) can directly re-heat the buffer water without the need to use a heat exchanger. Drinking water can be heated from the buffer tank using a fresh water station. A plate heat exchanger, dimensioned according to the hot water requirement, is used for this in connection with a regulated pump for the buffer water. A legionella infestation of the drinking water is practically excluded with the drinking water heating by buffer storage in connection with a fresh water station due to the small pipe volume.

Solar fluid

Solar fluid

In liquid-filled systems, the heat transfer fluid transports the heat from the generator to the consumer or store. In general, the solar fluid may evaporate under boundary conditions in hot periods, which in turn leads to stagnation of the collector.

Propylene glycol-water solution

The solar fluid is usually a water- propylene glycol solution that has a lower freezing point than water, thus protecting the system from frost damage. The boiling point of the solar fluid is much higher than that of pure water. Particularly in pressure systems, high temperatures (up to over 200 degrees Celsius ) and pressures in the solar circuit occur under boundary conditions in hot periods or when there is insufficient heat consumption . The pipe system and seals must be designed for this. If the solar fluid changes into the vapor phase when the temperature is too high, this leads to a system standstill and the stagnation temperature is reached; the pressure is then initially absorbed by the diaphragm expansion vessel (MAG) and when a limit is exceeded (usually 6  bar ), solar fluid is drained through the safety valve into a collecting container. The condition and change of the solar fluid is checked during maintenance, as the solution ages due to frequent unit changes. The mixtures used today are non-toxic and chemically relatively stable.

The higher the glycol concentration, the lower the temperatures the solar circuit can withstand without damage. A concentration of over 50% should, however, be avoided because the specific heat capacity of the mixture is reduced. The pump is also no longer reliably cooled. The viscosity of the mixture and thus the required pump work and power consumption increase. Overall, this reduces the efficiency of the system. In extreme cases, the pump may have difficulties starting. If the system is exposed to very low temperatures, a water ice mixture forms if there is a sufficient proportion of glycol , but this does not destroy the pipes. Heat pipes are not protected by the solar fluid. The frost resistance of heat pipes is approximately −30 ° C, depending on the manufacturer.

Pure water

There are systems that work directly with water (more precisely pure water ) as solar fluid. The degree of purity does not have to be particularly high. Normal drinking water or filtered rainwater is sufficient. In the case of tube collectors with direct flow through closed solar circuits, in which a residual amount of light hits the water, chemical additives are sometimes used that inhibit the formation of algae in the water. In the case of pure water systems, it is not absolutely necessary to have a heat exchanger between the solar circuit and the storage tank. This also makes it easier to integrate a solar system into existing heating systems. In winter it must be ensured that the collectors do not freeze. To do this, the outside temperature is monitored and, if necessary, warmer water is fed through the collector. The required energy (pump, hot water) can be offset against various savings, such as B. the better efficiency by not using an antifreeze additive. The higher heat capacity and lower viscosity of pure water therefore results in less pump work. Drain-back systems work in a similar way , in which the solar circuit is only automatically filled with water when the collectors are sufficiently warm and the storage tank is capable of receiving it. As soon as the automatic control switches off the pump, the water runs into an integrated collecting vessel. Under boundary conditions, in hot periods, lower temperatures occur in the solar circuit, since pure water has a lower boiling point than a propylene glycol water solution. It is therefore also possible to use lines, pumps and other components made of polypropylene, especially in pressureless systems.

Pipelines and thermal insulation

In the area of ​​single-family houses, copper pipes, externally galvanized carbon steel or stainless steel pipes of nominal sizes DN 15 to DN 25 or corrugated stainless steel pipes are generally used, and suitable composite pipes that are both temperature-resistant and chemically resistant can be used. Zinc must not be used anywhere in the pipe system if a glycol mixture is used.

Due to the short of the stagnation condition resulting high temperatures should Lötfittinge in the run up to the store, as well as in the affected areas at a stagnation only retrace brazed be. When pressed connections are in these areas instead of the usual O-rings made of EPDM only those from FKM ( Viton are used).

As a rule, the strength of the thermal insulation of solar circuits is based on 100% thermal insulation according to the Energy Saving Ordinance . Many pipe insulation materials commonly used for heating installations cannot be used because the material must be able to withstand the stagnation temperature of the collector temporarily and permanently operating temperatures of at least 110 ° C. In the outdoor area, sheet metal-coated mineral wool shells and foamed EPDM are possible in order to avoid damage from UV radiation, weather and the effects of birds. Special airgel- based insulation (Spaceloft) is also offered, with 10 mm insulation corresponding to EPDM insulation with 40 mm.

Solar circuit, connections and fittings

Circulation pump

Heating pumps are usually used as circulation pumps, which are placed in the cold return line to protect against the high temperatures. Since the volume flow of the solar circuit is much smaller than that of a heating circuit, the heating pumps for small solar systems are often oversized. Solar pumps are often electronically controlled by the solar control system, and are also designed for small volume flows and are therefore energy-saving. Almost all small heating pumps that do not have their own electronics can be used for this, but also special pumps with electronics that allow PWM control with an additional control voltage from the solar electronics . So that defective pumps can be replaced without emptying the solar circuit, they should be installed between two gate valves. A backflow preventer in the return prevents the possible gravity circulation , one in the supply prevents backflow and thus cooling of the storage tank.

Management and control of the solar circuit

Volumeters are used to measure the flow rate, thermometers and manometers to control temperature and pressure.

Filling and treatment of the solar fluid

Temperature-resistant ventilators or air separators as well as fittings for flushing, filling and draining the solar system are required.

A dirt trap is not required. Since a filter increases the flow resistance, it should only be integrated into the circuit temporarily after the installation or modification of the system - for example via a switchable bypass. Special dirt and sludge separators with low flow resistance are suitable for permanent use.

Storage connection

In order to reduce heat losses in the connection pipes through internal circulation, the pipes should be arranged in the form of a thermosiphon convection brake - unless the storage tank connections are already designed in this form. Slightly different guidelines apply to drain-back systems.

Pressure maintenance

Printing systems

In the case of pressure systems, the safety devices include a diaphragm expansion tank (MAG) and a safety valve . The size of the MAG results from the expansion water volume plus the complete liquid vapor of the circuit. The discharge of the SV is intended to ensure that hot splash water does not pose a hazard. A lockable vent with a collecting section at the highest point of the system ensures that any air that has accumulated can be vented. This ensures that the heat can be continuously absorbed and transported by liquid-only and that the cycle is not interrupted.

Pressureless systems

Pressureless systems have an open expansion tank without a membrane at the highest point of the line system. There is no pressure relief valve and no additional ventilators. If water evaporates, it must be refilled, which is usually done automatically. Although the oxygen entry via the open pressureless system is low, all parts in the solar circuit must be made of corrosion-resistant materials.

Solar controller, solar station

Solar station with pump, safety group, controller, thermometers and air separator

A solar regulator consists of various regulation and control circuits . It processes set temperature values, measured temperature values ​​and measured temperature differences. Pumps and / or valves are switched depending on the set and the measured values. The temperatures are recorded in simple systems with two sensors (mostly platinum sensors of the "PT 1000" type = electrical resistance 1000 Ohm at 0 degrees Celsius) at the collector (flow) and in the storage tank; If the collector temperature is around 3–5  Kelvin above the storage tank temperature, the pump switches on; if the value falls below a limit, it switches off. When the temperature is recorded in the return from the heat storage, the heat energy gained can also be recorded for monitoring . Another sensor is occasionally required to determine the maximum storage tank temperature. More complex controls can also manage several collector fields with different orientations or irradiation and several storage tanks. An operating hours counter for profitability calculations is also usually integrated. Some controllers generate trend and plausibility values ​​from the measured values.

For one and two-family houses, the minimum equipment is offered in a compact unit, which is called a solar controller , compact station or solar station , depending on the make . It is slightly larger than a shoebox and surrounded by thermal insulation, in which the four connections (flow and return to the collector or storage tank), usually two thermometers, the pump, a manometer , the safety valve with blow-off line, the connection for the The diaphragm expansion tank and the regulator with its power supply are located. These compact units, mostly with an integrated air separator, save space and are easy to install.

Commissioning and maintenance

Once the system has been completed, it is commissioned , for which it must be subjected to a leak test and a flushing process. In the case of pressure systems, a pressure test is carried out with 1.5 times the maximum operating pressure, which results from the static system height of 0.1 bar per meter and 0.5 bar as the distance to the response pressure of the safety valve. Rinsing the system removes dirt residues and ensures a trouble-free flow. Since it is rinsed with water, this should be done in the frost-free time. Depending on the system design, residual water could not freeze. The collector system is filled - depending on the absorber manufacturer and system type - with ready-made mixtures or pure water that can be added to the algae protection. Mixtures and additives or treated water can be pumped into the system via a filling hose and a filling pump. Then the operating pressure must be applied to the MAG and the system flow set. Complete escape of the air is important in order to maintain the circuit and avoid operating noises. The oxygen in the air causes the antifreeze to oxidize more quickly. Allowing the pump to idle for a long period of time should be avoided as this could damage the pump. Maintenance of the pressure must be carried out annually and the system pressure must be restored. The solar fluid concentration must be checked every two years. The measurement is carried out with a spindle hydrometer and a pH value measurement, which must be above 7 (slightly basic ). If the mixture is more acidic, the entire solar fluid may have to be replaced. The soiling of the collector cover usually does not play a major role and leads to a maximum loss of performance of 2 to 10%. Special cleaning of the collectors is not necessary.

Construction types and system technology

Roof assembly of two evacuated tube collectors on a single-family house in southern Germany
A vacuum tube collector
Flat-plate collector system for hot water preparation and heating support
simply constructed thermosiphon systems on a high-rise building

The construction types of solar systems can be differentiated according to various criteria.

In the field of building technology, the intended use

  • Systems for heating drinking water and
  • Systems to support space heating

differentiate (see also below).

A distinction is made according to the type of collector used

  • Systems with flat-plate collectors
  • Systems with vacuum tube collectors
  • Systems with air-filled collectors

It is also possible to differentiate between storage technology ; there is a multitude of different developments here. These mostly focus on optimizing the temperature stratification in the storage tank or on the implementation of extraction strategies that avoid disturbing the stratification. The aim is to maintain a consistently high temperature in the upper storage area, where the heat is extracted, and a lower temperature in the lower storage area compared to the collector temperature, where the heat is supplied from the collectors, so that the system can run continuously.

A distinction can be made according to the system technology as such

Gravity systems work entirely without a pumping station. Their cycle is driven solely by the heating in the collectors: The water heated in the collector is specifically lighter, rises and collects in the storage tank, which is typically located above the collector. When it cools down, it sinks to the bottom of the storage tank and flows back to the collector through the return pipe.

The distinction between "high flow" and "low flow" refers to the throughput volume in relation to the collector surface area per unit time. High flow means that around 30 to 50 liters per hour and square meter of collector surface are converted, with low flow it is 10 to 20.Low flow can thus both a very slow circulation in the solar circuit and a fast passage with an overall very low volume in the Designate solar circle.

Most of the smaller systems used today are high-flow systems that can be operated with normal heating pumps ( circulation pumps ). They are able to dissipate large amounts of heat from the collector at a low to medium temperature level.

The technological advantage of low-flow systems is based on the fact that they create higher temperature differences between the collector and the storage tank and that they also remain in operation. As a result, the collector efficiency drops somewhat, but at the same time they can produce heat at a higher temperature level when there is less solar radiation and, since there is no longer any need for post-heating with average solar radiation, they can achieve somewhat higher annual average coverage. Compared to high-flow systems with the same area, cheaper piping, smaller heat exchangers and weaker pumps can be used. Because of these advantages, large systems are usually operated in low flow . Systems with very narrow pipe cross-sections can only be operated as low-flow systems, otherwise the flow resistance increases too much. Narrow pipe cross-sections are desirable within the absorber so that the collector heats up quickly and the least possible amount of water is displaced by the steam when it overheats.

Matched flow systems, in which the pump output is regulated over a wide range, are currently the exception. Like a high-flow system, they have to be equipped with expensive technical equipment, so that their advantage over this is only slight.

Outside of Central Europe, thermosiphon systems are often in use, primarily in warmer regions. Thermosiphon systems with tube collectors can, however, be operated down to −30 ° C without frost protection and often still supply warm water even at very low temperatures with diffuse and indirect solar radiation. Frost protection is primarily to be provided for the pipe system. Thermosiphon systems often have an open circuit : in the simplest systems, drinking water flows directly through the collectors, which is then tapped from the storage tank as hot water. The slightly more complex variant uses a pressureless storage tank with an integrated smooth-tube heat exchanger that can withstand normal line pressure.

Drain-back systems , which provide for complete emptying of the collectors in the event of extreme temperatures or system shutdowns, are an exception . These can be operated with pure water. However, they are also mostly operated as closed circuits that transfer the heat to the domestic water via heat exchangers.

Overheating of the system

If the collector surface is designed large enough to generate the required amount of hot water in winter and, if necessary, to contribute to the heating of the building, the liquid circulating in the system can be heated to above boiling point by the much stronger solar radiation in summer. The standstill of the system due to evaporating solar fluid is known as stagnation .

The stagnation of the system must be taken into account in the design:

  • The volume of liquid contained in the collector is kept as small as possible in order to limit the amount of vapor generated.
  • The materials in the system parts through which steam flows when overheating are selected so that they can withstand the high temperature.
  • The membrane expansion tank is designed to be large enough to completely absorb the amount of water displaced by the steam.
  • The discharge line of the overpressure valve is designed in such a way that the boiling water that escapes at high pressure in the event of damage does not pose a risk.
  • If there is a risk of overheating, the solar circuit pump is switched off, whereupon the amount of liquid contained in the collectors flows back into a storage tank. See drain-back system

Since a high thermal load acts on the system components in the event of stagnation and solar energy can temporarily no longer be obtained, various concepts have been developed to prevent the solar fluid from boiling:

  • The system is to be shaded in order to limit solar radiation.
  • A cooling system is provided, which can be switched into the solar circuit if there is a risk of overheating. For example:
    • A storage system that is large enough to absorb all of the thermal energy. So-called seasonal storage has enough capacity to store all of the excess thermal energy until winter.
    • A cooling register which is cooled by the outside air flowing through.
    • A swimming pool.
    • A cooling circuit laid in the ground or in the basement that transfers the heat to the subsurface. Depending on the groundwater and soil conditions, the excess heat energy is ideally stored until winter and is then available to heat the building.
  • The system is designed to be pressure-resistant in order to raise the boiling point of the solar fluid above the highest temperature occurring in the collector.

Typical system sizes

Most of the systems in use today are systems for heating drinking water in 1-family or 2-family houses. The aim when designing the solar system is to achieve full coverage in summer so that the normal heating system can remain completely switched off. Due to the strong seasonal differences, however, a system that can cover more than 90% of the demand in winter would have to be designed so large that enormous heat surpluses would arise in summer that could not be used. Since the system switches off as soon as a preset target temperature is reached in the solar storage tank, such systems would often stand still in summer. But when no more heat is dissipated, the collectors heat up so that the solar fluid they contain turns into steam. If, in this situation, the storage tank cools down due to high consumption, this can lead to the paradoxical situation that conventional reheating has to be carried out in summer because the system can only be put back into operation after the collectors have cooled down at night.

A typical system size in Germany and Austria is designed for a four-person household, has a 300-liter solar tank and a collector area between 4 and 5 m². The next larger system with a 400 liter solar tank and a collector area between 6 and 8 m² can supply up to six people with normal water consumption with an annual solar coverage of around 70%.

In the Netherlands , most systems are designed to be around a third smaller; There are also systems with 150 or 200 liter solar tanks, which then usually only achieve a coverage ratio of less than 60% on an annual average.

In Austria there are also systems with larger drinking water reservoirs. This is rather unusual in Germany. The latter is also related to the fact that the so-called " Legionella Ordinance" of the German Association of Gas and Water Sectors prescribes special measures for regular sterilization of the drinking water system from a storage tank size of more than 400 liters . Although this guideline does not apply to single-family homes, people often refrain from installing larger storage tanks due to health concerns.

Systems that are supposed to provide space heating support in addition to heating drinking water (shower and bathing water) require buffer storage tanks that are designed with a capacity of at least 700 liters in the area of ​​single-family residential buildings; However, this is not drinking water, but heating water that only circulates in the closed circuit of the heating system. The corresponding collector area can be set between 9 and 12 m². Combination tank systems with a total buffer capacity of approx. 1000 liters (including up to approx. 200 liters of drinking water in an inner tank) and a collector area of ​​12 to 15 m² achieve good performance values. In addition to solar coverage of the annual drinking water heating requirement of approx. 60–70%, such systems in low- energy houses can provide up to a quarter of the annual heating energy requirement.

The differences between the locations (annual radiation), the orientation or inclination of the collector surface (reduces or increases the yield), the domestic hot water requirement and the building's heat requirement and, ultimately, the quality of the solar systems (efficiency of the collectors, insulation quality of the solar storage tank, intelligence of the solar controller ), however, significantly influence the required size. Over-dimensioning hardly brings additional annual yields. Exceptions are steep and shadow-free collectors oriented exactly south. In this way, more winter sun can be captured and overheating in summer can be avoided. Overheating in summer and the risk of system downtime due to stagnation can be reduced by using excess heat for other purposes, such as drying the cellar or heating a swimming pool .


Approx. 61% of the energy consumption of a private household is accounted for by the total heating energy requirement (8% drinking water heating, 53% heating energy requirement), approx. 31% for motor vehicles and 8% for electricity.

Systems for heating drinking water

Today's solar thermal systems are primarily used to heat drinking water, in this case they can cover an annual average of 55% to 60% of the heating energy for heating drinking water, which corresponds to approx. 8% of this total heating energy requirement or about 5% of the total energy requirement. The useful life of such a system is given as 20 to 25 years.

The energy consumption of a model family for heating drinking water (shower and bath water) is around 420 liters of heating oil (or 420 cubic meters of natural gas). A solar thermal system can save around 55% to 60% of this, which corresponds to an annual saving of approx. 250 liters of heating oil and at a heating oil price of approx. € 0.6 / l (as of August 2017) to an annual saving of approx. 150 € leads.

Furthermore, a solar system can save electricity if the hot water is also used for the washing machine and dishwasher.

The acquisition costs of a solar thermal hot water system for a four-person household are between 4800 € (flat plate collector) and 8800 € ( vacuum collector ) including transport and assembly , depending on the technology and required effort . If the assembly is not carried out by specialists but by the buyer, the cost of the system itself is between € 2880 and € 6850.

The operating costs are essentially the electricity costs for the solar pump and the maintenance costs, which vary greatly depending on the system installer. Dismantling and possible disposal costs as a result of a modernization of the plant may be added. Depending on the property, savings can often be credited to the solar system by eliminating sweeping by the chimney sweep in summer, extending service intervals on the boiler due to the absence of short-term loads in summer, and extending the service life of the boiler and chimney.

Systems for heating support (solar systems with return flow increase)

In spring in particular, there is high solar radiation (in mid-April it is about as high as at the end of August) and low outside temperatures occur together. Solar thermal systems are therefore increasingly being used, which, in addition to heating drinking water, also provide thermal support for space heating water in the transitional periods (spring and autumn). These so-called "combined systems" are significantly larger and therefore also more expensive than systems for heating drinking water only.

The costs and income fluctuate significantly more than with pure drinking water systems, as the temperature levels of the heating systems (flow temperature 35 ° C for underfloor heating, 75 ° C for older systems), heated area and specific heat demand from 0 to 300 kWh / (a ​​· m²) each may vary according to house. In the case of an old building that has not been renovated, prior thermal insulation, making it windproof and possibly replacing windows and doors is advisable.

Systems are currently common in Europe that save around 15% to 45% of the annual heating energy of a single-family house. Typical storage tank sizes to be used for thermal heating water storage tanks are around 1000 l per 100 m² of heated living space.



Since it is often not possible to achieve economic efficiency with constant oil and gas prices, the BAFA funded the construction of solar systems in Germany . As part of the budget approval, funding by the CDU / CSU / FDP coalition was initially discontinued. Since July 12, 2010, solar thermal systems for heating support have been subsidized with reduced funding rates. Solar systems for hot water are only subsidized in connection with a complete heating renovation. The funding of systems in new buildings has been completely canceled, as this was regulated in Germany in the Renewable Energies Heat Act. The current funding framework for a solar thermal system by BAFA is published on the website. The federal states and in some cases also the cities and municipalities or the local energy suppliers offer further funding opportunities. The Kreditanstalt für Wiederaufbau promotes solar thermal systems with a collector area greater than 40 m² through a loan with a residual debt discharge of 30%. Not all forms of funding can be freely combined.


In Austria, the responsibility for promoting the construction of solar systems for single-family houses is the responsibility of the federal states. As a result, the non-repayable subsidy amounts for solar systems for hot water generation vary from € 0 (Lower Austria) to € 1,700 (Upper Austria, Burgenland), the subsidy for heating-supporting systems from € 0 (Lower Austria) to € 3,325 (Vorarlberg) . In addition, some municipalities are also promoting the construction of solar systems.

Solar system and monument protection

Solar systems and monument protection are in a tense relationship, since solar systems on the roof usually interfere with the substance of the building and / or its visual effect. Since resource conservation and sustainability are part of the legal mandate of monument protection and monument preservation, there have been efforts of monument preservation for many years (as of 2010) to find sensible solutions. In order to erect solar systems on a listed building, it is often necessary to deal intensively with project and solution proposals for the integration of solar modules. In case of doubt, a judicial clarification may be necessary. In the last few years (as of 2012) the tendency of the jurisprudence - depending on the concrete aspects - is no longer unreservedly friendly to monument protection.

Historical precursors

The idea of ​​“capturing” the sun's rays in order to use their heat in a targeted manner is old. For centuries, inventors have been concerned with capturing solar energy, particularly with the use of burning glasses .

The Swiss naturalist Horace Bénédict de Saussure built a “simple solar collector” in the 18th century, which consisted of a wooden box with a black bottom and was covered with glass. Its solar panel absorbed the heat of the sun, and Saussure said it reached temperatures close to 90 ° C in its box.

In 1936, the magazine Die Woche reported on a roasting oven developed in California that worked with the rays of the sun focused through a lens. The editors did not give solar energy any great future prospects, but conceded that under optimal solar radiation "a lens radiation area of ​​one square meter should produce a power output of 1 1/2 hp [and] solar machines are more profitable than fired steam machines".


Web links

Commons : Solar thermal system  - collection of images, videos and audio files

Individual evidence

  1. Some comparisons with diagrams - English text
  2. - Yield comparison tube vs. Flat-plate collector for a thermal solar system that supports the heating
  3. - Yield comparison tubes vs. Flat plate collector for drinking water solar system
  4. Certificates online ( Memento from March 17, 2014 in the Internet Archive )
  5. Warm water from the sun tank . In: Main network . July 5, 2013. Retrieved May 10, 2014.
  6. Viega requires the use of FKM only for systems with vacuum collectors , see the Sanpress instructions for use ( Memento of February 23, 2018 in the Internet Archive )
  7. Jörn Scheuren: , dissertation at the University of Kassel, 2008
  8. a b c d Stiftung Warentest: A technique for heating . In: test. No. 4, 2002.
  9. Tomke Lisa Menger: Article: Old substance meets new energy - Do solar systems affect monuments? In: EnergieAgentur.NRW, October 1, 2018, accessed on July 4, 2020 .
  10. Ulrike Roggenbuck, Ruth Klawun, Roswitha Kaiser: Worksheet 37: Solar systems and monument protection. Information from the Association of State Monument Preservators, prepared in spring 2010 by the construction technology working group. In: Association of State Monument Preservators in the Federal Republic of Germany, 2010, accessed on July 4, 2020 .
  11. Annette Stoppelkamp: Monument protection can be in harmony with renewable energies. Don't let that stop you from using the roof or the facade with solar energy! In: Solarenergie-Förderverein Deutschland eV (SFV) , June 24, 2020, accessed on July 4, 2020 .
  12. ^ Stefan Pützenbacher: Renewable energies vs. Monument protection. Does monument protection law conflict with environmental protection? In: Publics (Richard Boorberg Verlag), February 15, 2012, accessed on July 4, 2020 (German).
  13. The week . Issue 21, May 20, 1936, p. 23.