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Smart use of energy: photovoltaic experts on storage systems and heat pumps

There is no such thing as too much electricity - says Prof. Dr. Konrad Mertens. The photovoltaics expert in our electrical engineering and computer science department is committed to energy storage and heat pumps, and he clears up rumors.

Prof. Dr. Konrad Mertens teaches and researches in the Department of Electrical Engineering and Computer Science in the field of photovoltaics, sensor technology and fiber optic technology. (Photo: FH M√ľnster / Wilfried Gerharz)

Prof. Mertens, significantly more photovoltaic systems are currently being built than in recent years. Why is that?

The realization is gradually gaining ground that solar power systems are still worthwhile for homeowners. Generating one kilowatt hour of solar power on your own roof costs around 11 cents. If you use this energy for your own household, you have to draw less electricity from the grid, which saves a lot of money.


Every second photovoltaic system is now sold directly with a battery storage system. Is that also worthwhile?

There will always be moments when the photovoltaic system on the roof produces more electricity than is currently needed in the household. That is why it makes sense to simply store the excess electricity you generate yourself and use it, for example, in the evening or at night when the sun is not shining. So far, however, the solar batteries are still relatively expensive, so that the purchase for private individuals rarely pays off. However, it is a great feeling to cover almost 100 percent of your own electricity needs with environmentally friendly solar energy.


Could the electricity be stored in a battery storage system until next winter and only then used?

Battery storage systems are primarily short-term storage systems. You can store the electricity for one or two days, depending on the battery capacity. Of course, it would be best to also charge your electric car with solar power - I think in a few years the electric car will at least be commonplace as a second car. If you want to store solar power for a longer period of time, the still quite fresh power-to-liquid technology can be used in a few years. It turns excess electricity and carbon dioxide into liquid methanol. It can be easily transported, stored in a tank and thus used very well locally. When there is a need for electricity, the energy of the methanol is used to turn it into electrical energy.


If you don't want to buy an energy storage device yet, are there any alternatives to using the excess electricity?

There are: thanks to photovoltaics, heat pumps are becoming more and more fashionable. They convert electricity into heat and do it very efficiently: Depending on the model, they turn one kilowatt hour of electricity into three or four kilowatt hours of heat! Domestic water heat pumps that heat drinking water and are usually located in the basement are quite cheap. If you also want to use the solar power for heating, you use heating heat pumps, which are usually installed in the garden.


What are the possibilities in larger dimensions?

In order to be able to do without nuclear and coal-fired power plants, we need power-to-gas technology. It converts excess electricity into natural gas. There are losses, especially if you convert the methane gas back into electricity. But the losses are actually not bad - if the electricity comes from the renewable energies that are already available. Therefore, my appeal to all homeowners is: use the roofs that are already there as far as possible to generate solar energy! There is hardly any other technology that delivers clean energy in such a space-efficient manner and with so little environmental impact.


But what is preventing people from doing this?

There are some rumors that unfortunately persist - that photovoltaic systems are still far too expensive or that the fire brigade will not extinguish a house if solar modules are installed on it. Neither is true: Now is a good time to invest, the systems have become cheaper and cheaper and can be had from 6,000 euros, depending on the size of the roof. And the fire brigade teams also extinguish a house with a photovoltaic system that is burning. You have to treat the building in the same way as a house that is still connected to the power grid, for example, maintain higher safety distances when extinguishing.