You can see them all over the country in fields and in the sea: wind turbines that produce energy. But only a few households use this technology to produce their own heat and electricity. Why?
Whenever the wind blows, you could use it to generate electricity and even heat for your own household. Because there are wind turbines for the roof or the garden - the small wind turbines. However, this only pays off at a few locations, because wind is not just wind.
How do small wind turbines work?
Small wind turbines or small wind turbines, as they are also known, are installed on roofs, but above all on masts in the garden or in the open field. The wind sets them in motion, and the energy produced from them is converted into direct current.
In the case of micro wind turbines, this results in 24 or 28 volts of direct current - which can be used, for example, for LEDs, battery charging or an electric paddock fence, explains Joachim Sroka. He is the second chairman of the Federal Association of Small Wind Turbines. The slightly larger systems are more interesting for households. They can be used to generate alternating current with 220 volts or high current with 400 volts.
"The basic principle of the horizontal-axial wind turbines is the same in the small and micro as well as the gigawatt class on the field," says the graduate engineer Sroka. But the size of the turbines differs enormously: "Small and micro wind turbines have rotor heads with a diameter of one to four meters." And Sroka advises mast heights between twelve and 24 meters. If the system is no more than ten meters high, you usually don't even need a building permit.
Can you use it to generate heat?
The electricity from small wind turbines can be used to operate heating rods in the buffer or hot water tank. Or to operate a heat pump.
What is the advantage of small wind turbines compared to other heating technologies?
Especially in the cold time of the year, the most energy can be generated with a wind turbine - i.e. when you also need heating. That's the problem with a solar thermal system: You would most likely need their energy from autumn to spring, but they produce around 80 percent of their annual yield from March to October.
What are the disadvantages of this type of energy generation?
The prices. "A wind turbine is a relatively high investment that only pays for itself over a long period of time," says Sroka. Interested parties have to reckon with 5500 euros per kW output. "If you want to supply an entire household - and initially only with electricity for your own needs, without heating - then you need a system with five kW in winter alone."
In addition, it may be that you need a building permit - and in many places the authorities are skeptical about this, reports Joachim Sroka, whose company sells and installs such systems. Because there were ideas of the gigantic wind turbines, which were previously known from fields, in the middle of the village.
"In addition, there is usually not enough space available in residential buildings to be able to install such a system," says Sroka. "Due to building law, you have to take shadows, sound insulation and clearance areas into account. There are currently very restrictive specifications here."
Nevertheless, there are options for private households: the low systems up to ten meters high, which do not require a permit. However, the main wind direction at your location must be barrier-free for these systems to be worthwhile. "But that is rarely the case," says Sroka. He therefore recommends mast heights of 12 meters or more. "Better even 24 meters" - but at this height the mast is very expensive due to the statics.
Which locations are suitable for the small wind turbines?
1. Free main wind direction
The wind needs a run-up: Even a house or a bush that is in the main wind direction of the system reduces energy production, according to the energy advice service of the Baden-Württemberg consumer advice center. And even a forest that starts 50 meters from the house can have a correspondingly unfavorable effect on the production of electricity.
2. Average wind force
And then, as a matter of principle, the wind in the region must be strong enough. According to Joachim Sroka, the average wind speed in Germany is two to eight meters per second. In the north, especially on the coasts, it is seven to 7.5 meters per second, inland only two to 4.5 meters per second. "I advise against choosing a location where the wind blows an average of less than three meters per second."
The German Wind Energy Association and the energy advisors from the consumer advice center also recommend checking the wind zone of the property during planning. "In general, proximity to the coast or an altitude is an advantage, but there must also be free space in front of and behind the wind turbine." That can make a significant difference: A good location delivers eight times the yield compared to a bad location with an average wind speed that is only half as high.
Lars Möller, energy expert at the consumer advice center in Lower Saxony, comes to a similar conclusion: "Small systems only make sense in the open area and then as high as possible and not on the roof and with sufficient constant wind, so usually almost never." For him, this even means that the systems are "not an option worth considering" for normal households. He would only address the topic in a very special environment, such as for farmers.
If I don't have a good spot in the garden: are there alternatives?
There are also small wind turbines for the roof, but the energy consultants at the Baden-Württemberg consumer advice center don't think these are a good idea. The building would ensure that the wind currents are turbulent and the yield is poor. And the vibrations of the rotors could be perceived as disturbing directly at the house.
But Sroka knows a solution: "There are initial ideas for sharing heating with larger systems." So several households in the neighborhood get together and get the heating current from a larger system. Advantage: The larger the system, the lower the system costs per kW output, which are then shared by several households.
Should I produce electricity or thermal heat with the wind turbine?
Expert Sroka recommends covering your own electricity needs as a priority for single-family homes, "because electricity costs are currently higher than heating costs". In the second step, he would cover the heating heat with surplus electricity production.
Either way, he advises a hybrid solution: "If you only rely on wind energy, it's not without risk. Because if you have a week's lull, you don't get any energy. But generally, lull is associated with nice weather." The photovoltaic system then takes over here and supplements the energy mix.
The energy consultants at the consumer advice center in Stuttgart also see it this way: A combination offers more independence, "in the summer mainly through photovoltaics, in winter through a lot of wind".
How do I recognize good small wind turbines?
"You can have a big problem if the inverter doesn't meet the DIN standard VDE-AR-4105," says Sroka. "There has recently been this technical regulation that requires very specific tolerances for the inverters. All power generators require this certification in order to connect the system to the power grid." In addition, Sroka advises clarifying whether all the documents required for the building permit are supplied with the system.
The consumer advice center NRW advises to critically evaluate the nominal output of the system specified by the manufacturer. The decisive factor is the performance at low wind speeds, not in rare strong winds.