Climate Energy Politics

Pumped storage plants do not fill the winter electricity gap

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The original article can be read as “Schlumpfs graphic 119” in the online Nebelspalter of 15 July 2024.

Two weeks ago, I showed here that the winter share of hydropower can be increased from 27 percent (natural inflows only) to 43 percent using all Swiss reservoirs (see here). As explained there, only storage power plants are responsible for this summer-winter storage process. I have not discussed the special category of pumped storage power plants because they do not play a role in seasonal storage. In the following, I will deal with the possible applications of such pumped storage power plants.

What is important:

– In a pumped storage plant, water can be pumped from a low-lying reservoir to a high-lying reservoir and released again when required.
– Because more electricity is required for pumping than can be generated from it, the possible applications are heavily dependent on the electricity exchange price.
– Pumped storage plants therefore pump in short-term cycles when electricity prices are low and turbinate when prices are high – there is no room for long-term seasonal storage.

A pumped storage power plant differs from a storage power plant in that it not only has access to an elevated reservoir, but also to a second, lower reservoir. These two reservoirs must be connected to each other by pressure pipes. The power plant that is connected to these pipes must also be equipped with power-driven pumps that enable the water to be pumped up. This enables the two reservoirs to be filled and emptied alternately.

Pumped storage shows losses of 25 percent

However, this means that a pumped storage plant can be used for actively controlled electricity storage – similar to a battery. Of course, this comes at a price: on average, 20 to 25 percent more electricity is needed to pump up water than can be recovered afterwards during turbining. Therefore, if the price difference between the pumped electricity and the electricity generated is higher than these losses, a pumped storage plant can be operated economically successfully.

Of the 17 existing pumped storage plants in Switzerland, Limmern in the canton of Glarus is the one with the highest maximum turbine output. The total output of 1000 megawatts (MW) is generated by four turbines of 250 MW each, which rotate at eight rotations per second. Each turbine can be switched individually to pumping or turbining. The next graphic shows a schematic overview of the Limmern pumped storage plant in Linthal, which is operated by Axpo (see here):

Sources: Axpo

Limmern went into operation in 2016 after seven years of construction at a cost of CHF 2.1 billion. The power plant, built 600 meters inside the mountain in a huge cavern, can pump water from Lake Limmern (left in the graphic) up into the 620-meter-higher Muttsee (above) and use it to produce electricity when needed. The Muttsee dam was significantly raised, increasing the storage volume of the lake from 9 to 23 million cubic meters of water. With a crest length of 1054 meters, the Muttsee dam is the longest and highest dam in Europe.

Limmern’s storage potential is only 0.026 TWh

The maximum possible power generation from a complete emptying of Lake Mutt is limited by the volume of the lake and the head. Based on the stated flow rate of 47 cubic meters of water per second per turbine (see here), it can be calculated that the lake will be empty after 34 hours if all turbines are operating at full capacity. With an installed capacity of 1000 MW, this results in an electricity volume of 34,000 megawatt hours (MWh) – minus 25 percent storage losses, this corresponds to a total storage potential of the Limmern power plant of 26,000 MWh or 0.026 terawatt hours (TWh).

This figure can be used to refute the false opinion that pumped storage can compensate for the winter electricity deficit from solar plants. As a reference figure, let’s take the calculation by Andreas Züttel and his colleagues from the ETH Lausanne, who assume a gap of 20 TWh by 2050 in their study (see here). If such a winter electricity deficit were to be covered with the storage potential of Limmern, 770 such plants would ultimately be required. This is of course a completely absurd idea, as we would have neither the space nor the money for it.

Pumped storage contributes nothing to seasonal storage

And it is also clear that this opinion is based on a completely wrong idea of pumped storage: it is assumed that pumping will only take place once (in summer) and turbining only once (in winter). However, this would mean that Limmern would be at a standstill for most of the year, which would be a disaster from an economic point of view. In reality, however, the operators of Limmern try to pump and turbinate as often as possible, as this allows cheap surplus electricity to be converted into valuable peak power that can be supplied when the electricity system requires it.

Imagine a beautiful summer’s day: During the day, when the sun is shining, Limmern pumps water into the Muttsee using cheap solar power. If the solar power then fails in the evening and consumption rises at the same time, it is replaced by valuable (i.e. expensive) Limmern electricity. This is how the pumped storage business model works: when there is a surplus of electricity with low prices, pumping takes place, and when there is a shortage of electricity with high prices, turbining takes place. And this is done at the shortest possible intervals so that the plant can be operated profitably.

Lake Mutt is filled and emptied every two weeks

In fact, the latest annual report from Kraftwerke Linth-Limmern (see here) shows that the Limmern pumped storage plant generated 1.42 TWh of electricity from Lake Mutt in the 2022/23 reporting year. Taking into account the calculated storage potential of 0.026 TWh, this theoretically means that the Muttsee was completely filled once and completely emptied once within two weeks on average. This confirms that pumped storage only makes sense if pumping and turbining alternate in relatively short cycles.

Conclusion: A pumped storage power plant can react very flexibly in the short term to differences in supply and demand in the electricity system because it can control both the inflow and outflow of a reservoir as required. However, this does nothing for the long-term storage of electricity from summer to winter: This is the sole responsibility of the storage power plants. However, only the outflow from their reservoirs can be controlled independently; the inflows from rain and meltwater are subject to the whims of nature.

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