April 2018

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In contrast to the photovoltaic system, which converts solar energy into electricity, the solar thermal system converts solar energy into heat. The sun heats liquid or gas in the solar collectors on the roof. This heat is supplied to the heat exchanger in the house, which then heats water for the water supply or the heating system. An essential part of the system is a buffer memory, which bridges possible time delays between heat generation and utilization.

The solar system can be built solely for drinking water supply or support your own heating system. Particularly effective is the combination with heat pumps, in which you provide almost self-sufficient with heat. But the combination with the traditional gas or oil heating is energy and thus cost-saving.

Requirements of private solar thermal systems

A solar thermal system consists of the solar collectors on the roof, a storage tank, pump and control systems as well as the additionally connected heating system. Which types you use depends on the requirements of your building and the purpose of the operation.

It is suitable house and garage roofs, but also house facades and open spaces for installation of collectors. These should ideally be south facing. However, even southwest or southeast-facing collectors still produce a yield of over 85 percent of the maximum output.

The angle of inclination to the horizon depends concretely on the use of the heat gain. For the hot water supply, it is ideally between 30 to 50 °, for the heating support in the range of 45 to 70 °.

Unlike the collectors, the rule is not "the bigger the better". Because with increasing plant size, the losses increase. Already 5 m 2 of collector surface are enough to provide a 4-person household in the summer months with hot water.

The lifetime of a system is 20 to 25 years. Therefore, look carefully at your surroundings to exclude shady objects such as trees, bushes, neighboring houses and empty construction areas.

The end system, ie which radiators are used, is crucial for the construction planning. The existing heating system must be checked. If a traditional gas or oil heating system should give way in the course of the renovation of a heat pump, the environmental conditions must be checked. For geothermal heat pumps or groundwater exchangers, the geological conditions must be examined by an expert. More about that in our article on geothermal energy.

The combination of a solar system with the right heating system for you can bring a saving of 20 percent. For the pure hot water supply, the savings are even 60 percent compared to the traditional supply. As a result, the system is already profitable after 10 to 15 years.

construction work

Building permits are not necessary for roof and facade systems. However, if you are planning a superstructure on a flat roof, this should be clarified with the local building authority.

The required construction measures are based on the most suitable system for your home. A new building differs from a retrofit in terms of measures and costs.

The heating system, the storage and the collectors must be matched in size and performance to work efficiently. The system should therefore be calculated by a specialist.

Graphic of a solar thermal system

Graphic of a solar thermal system

Photo: ser_igor / istockphoto

solar collectors

On the roof or the suitable area, the solar collectors must be set up and connected, regardless of a new building or conversion of the system.

The most common solar collector type is the flat collector. He has a good price-performance ratio and a high resistance. This collector type works only in the low temperature range. They are mainly used to heat drinking water, or when the orientation of the area to the sun is particularly power intensive.

In contrast to the flat collector, the (vacuum) tube collector provides a higher temperature level. It can achieve high levels of performance even with low irradiation and is mainly used for heating support. However, it is more expensive than the flat collector.

Structural measures in the house

The heat must be routed into the house to the connected buffer and then distributed in the house. Ideal heating system is the surface heating (also floor heating). In addition, the house should be well insulated to have as little heat loss.

The buffer tank to be connected is a well-insulated hot water tank that can store heat for several days and deliver it specifically. The use of heat is therefore no longer directly linked to the solar radiation at the time of day, you will be flexible and independent in your consumption. The buffer uses the physical basis that cold water sinks down and warmer water strikes it. So you can always directly the water with the desired temperature are removed. The size of the memory depends on many factors and should be calculated by the professional. The costs amount to 800 to 1,200 euros plus installation costs.

So-called combi storage units are used if you design the solar system simultaneously for hot water preparation and heating support. This is called a tank-in-tank system. The drinking water is located in a stainless steel container and the buffer storage with the heating water around it. There is a heat exchange between the liquids, although both circuits are separated. The cost of such a combination storage are between 800 to 2,500 euros.

Costs and subsidies of solar thermal systems

Of course, the cost of the solar thermal system depends on many factors. Hot water systems with flat plate collectors are the cheapest option and cost around 3,500 to 8,500 euros including installation, less any funding. The heating support systems are much more expensive with 7,000 to 12,000 euros in total costs, but they receive higher subsidies.

In addition, costs for renovations around the heating system and energy renovations, such. B. by improving the house insulation, arise.

BAFA funding for solar thermal systems

The Federal Office of Economics and Export Control (BAFA) grants subsidies for both initial installations and extensions to solar thermal systems. The amount of production depends on the size of the collector surface and the type of collector. The exact conditions for the respective funding can be found here.

Water heating solar systems on an existing building are subsidized with 50 euros per square meter started and at least 500 euros per installation. The collector area must be between 3 to 40 m² and the water storage tank at least 200 liters.

The systems for heating support and the combined systems for hot water and heating are subsidized with 140 euros per square meter started or at least 2,000 euros per system. Here, separate conditions for collector types and areas are required.

The extension of an existing solar thermal system will be subsidized with 50 euros per additional square meter of collector surface installed.

Larger solar thermal systems on residential buildings with at least 3 residential units or non-residential buildings with at least 500 m² usable area will be supported by the innovation and additional funding.

In addition, BAFA promotes asset combinations. If you replace your oil or gas condensing boiler or install a heat pump, you will receive 500 euros extra per system combination.

KfW loan

KfW Bankengruppe promotes the renovation of private heating systems. In addition to biomass plants and heat pumps, this also includes solar thermal energy. The technical requirements are the same for the loan, as for the BAFA funding, since this is a cooperation project.

KfW is granting loans of EUR 50,000 per residential unit for the construction of solar thermal systems. The effective interest rate currently amounts to 1.31 percent with a term of 4 to 10 years at a fixed interest rate (2017 figures). The redemption-free start-up period is two years. Further information on the KfW loan can be found here.

Additional credit options

In addition to state subsidies, you receive comprehensive offers from various loan companies for your conversion and renovation measures, as a way to self-sufficiency. These renovation and modernization loans are awarded at very good interest rates, as the money paid out is tied to the purpose of the renovation. Inform and compare your options to keep your own contribution at the cost of acquisition low.

Which loan is suitable for your solar project depends on many factors. Depending on how long the term, how high the buzzer and what purpose the loan has, there are various offers. With a credit comparison, you can get an overview and find the best conditions for yourself.

Laws & taxes

Solar thermal systems are hardly tax specific. The costs can be deducted according to general tax law on the income tax, in so far as income from leases and leases arise.

If you have built a solar system to save energy measurably and permanently and you want to turn the investment over to your tenants in the form of a rent increase, this is possible. You are entitled to increase the annual rent by up to 11 percent of the construction costs. The tenant must be clearly informed. Of course, here are exceptions, for example, if a rent increase is contractually excluded or the rental property is a student or youth hostel.

The landlord is also obliged with construction and use of a solar thermal system to issue a heating bill.

For today's new buildings is the EEWärmeG (Renewable Energy Heat Act). This means that from 2009 onwards, all new buildings must receive at least 15 percent of the heat demand from solar thermal systems (or 50 percent from biomass / heat pumps or 30 percent from CHP plants). For the use of solar thermal energy, a flat collector area of ​​at least 0.04 m 2 per square meter usable area for one- and two-family houses, or of 0.03 m 2 per square meter usable area for larger residential buildings, must be fulfilled.

Advantages and disadvantages of private solar thermal systems

advantages disadvantage
  • Significant reduction in heating and hot water costs (up to 65 percent)
  • (Limited) independence from fossil fuels and their prices
  • No new building conditionally, in addition to existing systems possible
  • Increased efficiency through combination with other heating technologies
  • Fulfillment of the EEWärmeG for new buildings
  • Reduction of climate damaging CO 2 emissions
  • Modern systems achieve a basic supply even in winter
  • High subsidies and relatively short payback period (10 to 15 years)
  • Can even be paired with cooling system
  • By buffer memory consumption is not bound to solar times
  • Not every surface suitable
  • Space requirement in the house for storage and control systems
  • High energy consumption in collector production
  • Yield only in sunlight
  • Not self-sufficient, additional heating system needed
  • High investment costs

With your own solar thermal system, you can significantly reduce your annual operating costs for hot water and heating. As a self-contained hot water and heating provider they are not tied to fluctuating oil and gas prices. The investment costs of such investments are high, but are extensively supported by the state and credit companies.

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