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Siemens tries to enter the ETES market amid round-trip efficiency questions

Everyone has got very excited by the claim this week from Siemens Gamesa, that is has a new form of storage, one well known, but mostly seen as impractical, based on heating rock – a form of Electrothermal Energy Etorage system or ETES. It has opened a pilot plant in Hamburg-Altenwerder which it says can store 130 MWh of energy for up to one week, and says it will gradually raise this above 1 GWh.

Before we get too excited, recent white papers from Siemens Gamesa show that this technology is fine if you want to go from heat to heat, where it is able to retain 98% of heat. But the round trip from electricity to heat back to electricity only outputs 45% of the electricity you started with. Conventional batteries such as Lithium Ion and Vanadium Flow are closer to and above 90%.

The only real considerations then are the two other main criteria – that you can build this with 80% off-the-shelf equipment, and that it can slot directly into a closed coal plant and use the existing steam turbine. Consequently, it costs very little.

That immediately jolted our memory – someone else has been chasing this same consideration for a while now. Back in March, we carried a story also about Germany having a 3-year plan to convert coal plants to molten salt storage, pitched by the German Aerospace Center (DLR), also offering a pilot using existing storage tanks to store molten salt.

Siemens Gamesa has tried to suggest it has a three-level business plan built around this, one where it offers 1 GWh storage for large industrials which it calls ETES: Base; and another which attaches to a fossil fuel or energy intensive industrial plant, with much of the idea being around buying cheap power at the wrong time (night time), and storing it until the industrial unit is at full speed, it calls this ETES: Add; and finally a future for thermal fossil fuel power stations, becoming CO2 free energy storage plants, which minimizes the negative effects of plant closure with the furnace being replaced by ETES, while steam cycle and operational processes remain in place.

All of these products suggest the idea of buying spare wholesale electricity from renewables, and using it to heat stones, which hold the energy until prices are higher. How that makes a profit when you are throwing away 55% of the output, is anyone’s guess. There may be some upsides here for using it for industrial heating, perhaps?

It seems that everyone is looking to somehow create value at “stranded” coal plant assets, and previous offerings have positioned this to include solar panels throughout the outside of the plant, or at nearby coal mines, to offer some generation capability alongside it. It could be that there is some kind of bounty from the German government to save the locations of its coal plants, and reduce the value of assets which are essentially stranded, and the cost of putting them back into general purpose usage. Paying for a refit around renewable energy would retain the location of some jobs, and reduce the bill for closing them, which in turn is usually held in escrow by the plant for that purpose, so it may release cash out of a coal plant balance sheet.

This follows a clear trend of building renewables energy near or next to existing power sources, so that the cabling infrastructure that is already in place can be used to export the energy to the grid. For instance, power companies are adding solar panels which float on top of reservoirs, on dams made to produce constant hydro power, and can use pumped storage to store the solar energy, and the existing grid connection to use the solar power.

If this becomes a straight fight between thermal stone and molten salt, our understanding is that around 70% of the energy is harvestable as electricity using off the shelf equipment with molten salt, so we need to talk to Siemens Gamesa and see how this new process compares.

Other substances can hold a lot more heat, such as water, but stone and even concrete holds a lot less, but because they can survive being heated to seriously high temperatures, they can retain more energy per overall volumetric capacity. So, stone may make these stores physically smaller.

This initial Siemens ETES heat storage facility contains around 1,000 tons of volcanic rock for energy storage, fed with electrical energy converted into hot air by means of a resistance heater and a blower that heats rock to 750°C. When demand peaks, ETES uses a steam turbine for the re-electrification of the stored energy.

The aim of the pilot is really to provide evidence that it works for things like grid stability and renewables’ 4-hour offset. So, Siemens Gamesa plans to use the storage in commercial projects and scale up capacity to several GWh in the near future.

“Decoupling generation and consumption of fluctuating renewable energy via storage is an essential contribution to implementing the energy system transformation. We therefore need cost-effective, efficient and scalable energy storage systems,” says Andreas Feicht, State Secretary at the Federal Ministry of Economics and Energy.

Professionals we have spoken to in the past, have shrugged and said this type of technology has been understood for years, but is too inefficient for long term storage, and profitability will really depend upon the underlying service pricing.

Siemens Gamesa has been helped by The Institute for Engineering Thermodynamics at Hamburg University of Technology and the local utility company Hamburg Energie, who will actively sell the stored energy in the electricity market.

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