If excess electricity from solar power plants – the plant here at Muttasi Dam with 4872 solar modules under construction in 2019 – was stored as heat, heat pumps could be relived in winter and electricity could be saved .
Gian Ehrenzeler / Keystone
Switzerland is investing heavily in solar power systems to address the impending winter power shortage. Seasonal heat storage can make a significant contribution to this.
Photovoltaics in Switzerland will be further promoted from January 2023, with a new one-time payment of up to 60 percent of the investment cost for solar power systems without self-consumption. That way, the federal government wants to counter any power shortages in the winter in the future.
The announcement is already taking effect. Until recently, Swiss energy companies have been relying exclusively on alpine solar power to generate electricity. According to a publication by the Swiss Federal Office of Energy (SFOE), systems in the Alps supply 50 percent more electricity per region in the winter months than systems in the lowlands. They are located above the fog cover and are therefore exposed to more solar radiation. In addition, the snow cover reflects sunlight.
Alpic is participating in Switzerland’s largest solar power plant above the village of Valais in Gondo. The Expo Mutsi produces a large amount of solar power at the dam. EWZ is currently building its second large photovoltaic system on a dam wall at Lago di Lei in Graubünden. The other is at the Albigna Dam in Burgel since 2020.
The State Council’s Environment Commission also wants to make progress in the expansion of solar energy. It has decided that solar systems will be mandatory for new buildings in general from 2024. Suitable surfaces of federal infrastructure systems should also be used to the best possible extent for the use of solar energy. These proposals are to be dealt with by the State Council in the autumn session of 2022.
converting electricity to heat and storing it
Another way to avoid the shortfall would be to significantly reduce electricity demand in winter – through seasonal heat storage. They are connected to the power system and thus to the entire energy system through heat pumps and photovoltaic systems. In summer, specifically, they absorb energy in the form of heat and release it again in winter. As a result, in the winter less energy, ie electricity has to be used for heating, would limit electric-powered heat pumps.
Gianfranco Guidati of the Energy Science Center at ETH Zurich estimates that seasonal heat storage could save up to four terawatt hours of electricity per year in the winter months.
However, seasonal heat storage has not yet been included in the federal government’s energy perspective. Prisca Wismer-Felder, a member of the National Council’s Environment Commission, is therefore pressing for this: “The administration and government need to assess the concrete potential of heat storage and make it available as a basis for political decisions,” she said. View”.
Othmar Reichmuth, a member of the Council of States’ Environment Commission, also calls for concrete steps to be taken in the media report: “It must be determined where and how exactly seasonal heat storage can be implemented and operated in Switzerland.” The president of the Swiss District Heating Association is convinced: “In the future of energy, thermal storage will play a central role.”
thermos flask principle
So far there are only a small number of individual projects in Switzerland, such as an underground storage facility in the Surstoffee field in Rotkreuz or a planned geothermal storage facility in Bern. However, some cities are now resorting to the tried-and-tested “thermos flask” theory as well.
Zurich, for example, plans to distribute huge silos throughout the city. It stores hot water that can be fed into district heating networks when there is a high heat demand, to cover short-term peaks in demand, for example in the morning when the heating is on and the shower is on.
“Urban planning aspects play a very important role in site selection and implementation,” says Tobias Nussbaum, spokesman for the Department of Waste Management and Recycling of the City of Zurich. They work closely with the Office of Urban Planning to ensure that the giant tanks can be designed in such a way that they fit as closely as possible into the city’s landscape. It would therefore be possible to cover the containers with glass fronts, green the façade or use the area for photovoltaics.
Although heat can be stored in “thermo flasks” for months due to their size, apart from aesthetics, such huge tanks cannot be built everywhere – sometimes there is not enough space, sometimes the building grounding occurs in unsuitable or retrofitting an existing building is often not profitable.
Boric acid instead of water as a storage medium
So scientists are experimenting with more efficient storage media than water that don’t require as much volume. Some rely on the conversion of heat into energetic chemical compounds that can be stored indefinitely and whose starting materials are inexpensive. “There are various chemical reactions that can be used for this purpose,” says Franz Winter from the Institute for Process Engineering, Environmental Technology and Technical Biological Sciences at the Vienna University of Technology.
Winter and his team have developed a boric acid reactor. Researchers create a suspension by mixing boric acid with oil that is heated in a container. The heat causes a chemical reaction in which boric acid is converted into boron oxide. This releases water, which escapes as steam. If water is added to the boron oxide storage medium again, the chemical reaction reverses and the stored heat is released again – a circuit closed over several cycles.
Winter estimates that the reactor filling may not need to be replaced for years. In addition, space is saved: due to the higher storage density of boron oxide, a reactor that is eight times smaller can store the same amount of energy as the corresponding water tank, according to Winter.
preheated floors in winter
Geothermal probes, on the other hand, would optically be completely imperceptible as they make use of the thermal potential of the ground. Especially in new construction, they often complement heating systems based on heat pumps, which support them in winter with relatively “warm” temperatures from the ground.
But geothermal probes can not only extract heat from the ground. Excess energy from solar power systems can also be sent back to the ground in the form of heat in the form of heat. Thus, it does not cool in the long run when heat is withdrawn from it to warm in cold weather, forming a circular system.
The EnergyUrban project for two districts in Vienna investigated whether soils heated by geothermal investigations could also be used as energy stores in existing urban structures. The test areas were a Grandzeit quarter with block perimeter development and a residential development for approximately 10,000 residents and 2,000 from the 1960s.
It turned out that there were enough solar sources as well as sufficient drilling areas for geothermal investigations to form an extensive network.
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