Thermal storage of cheap electricity for Flensburg’s district

I have published so many articles about energy storage here. This mainly includes battery storage, for example small batteries for private households or large battery parks in large halls to ensure grid stability. Of other technologies, power-to-gas technology is at best an issue. All other storage forms may be mentioned in a post, but then it’s quiet again.

water as an energy store

What about water as an energy store? We know pumped storage power plants, which are hardly worthwhile today, or the rock-heaving potential energy storage facility, which is fascinating but cannot really be imagined. There is also another possibility, the excess electricity can also be stored thermally. In Denmark, this form of storage has been in use for some time.

In line with the physical proximity to Denmark, there has been a thermal storage facility in Flensburg since January of this year to absorb excess electricity from the grid. The water reservoir contains 29 million liters of water, which is heated to almost 100 degrees Celsius in an electrode boiler from Stadtwerke Flensburg. From there, the hot water is routed to Flensburg’s district heating supply. It’s also worth it, because 98 percent of Flensburg’s 91,000 residents are connected to the district heating network.

Thanks to the new electrode boiler, the public utility company can run down the coal boiler that would otherwise be required on warm days to a minimum. In theory, a complete shutdown would even be possible on very warm days. With an output of 30,000 kilowatts, up to 29,000 cubic meters of hot water can be stored. In this way, the electrode boiler not only creates demand that can be used flexibly, but also replaces conventional energy supply and prevents CO2 emissions. “Cities combine energy consumption and therefore offer good conditions for supply via a district heating network,” explains Maik Render, Managing Director of Stadtwerke Flensburg. “A central task of the cities in the energy transition is to provide flexible consumption and thus an intelligent infrastructure alongside the changing supply of wind and solar power.”

Storage pays off with low exchange electricity prices

From an economic point of view, the electrode boiler always pays off when the public utility company can buy electricity cheaply on the electricity exchange. This is the case when significantly more electricity is produced in Germany than is required. With this oversupply, the power grid must be relieved in order to avoid overloading the power grid. In addition to the scheduled use on the power exchange, the electrode boiler is also intended to relieve the power transmission network by taking power from short-term load fluctuations.

The greater the surplus of electricity, the cheaper the prices. This can go so far that electricity is offered at negative prices, ie the municipal utilities receive money for taking electricity from the market. Around Christmas 2012, for example, the price of electricity was up to minus 200 euros per megawatt hour (MWh) due to a large oversupply. For the municipal utilities, the electrode boiler pays off from an electricity price of less than 10 euros per MWh. Normally the exchange price for electricity is around 50 euros per MWh, but at the moment it is closer to 40 euros. The current market data from the Leipzig electricity exchange show that, especially on weekends, the electricity price can drop below 10 euros per MWh for a few hours.

Claus Hartmann, project manager for the new electrode boiler at Stadtwerke Flensburg, is certain that electricity can now be purchased: “Before it was officially commissioned, we of course ran tests with the new boiler and used the first megawatt hours. We have also already produced the first hot district heating water. Everything went smoothly and the efficiency for hot water production was close to 100%. By purchasing the electricity that is already available, we effectively save on CO2 emissions, since no hard coal or alternative fuels have to be burned for the heat generated.”

The manager of the power plant, Dirk Roschek, adds: “On extremely warm summer days, the coal boiler that would otherwise be required in the power plant could even be switched off completely. And quite incidentally, with the electrode boiler, we have also built an additional small generation system for district heating, which further increases the security of supply in Flensburg.”

Stadtwerke Managing Director Maik Render is pleased about the nationwide attention to the project: “The combination of electrode boiler and heat storage, which from today’s perspective represents a sensible option for storing electricity, is to our knowledge the first nationwide. The presence of our energy transition minister, Dr. Habeck shows that the state government also appreciates this innovative project accordingly. We receive inquiries about this from all over Germany and have already presented the system on site to a number of competitions, including large ones.

In April, the city of Flensburg was awarded the title of “Energy Municipality” for this project. The Agency for Renewable Energies honors exemplary municipal energy projects with this title.

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