Siemens Gamesa, the wind turbine manufacturer, began building a 30-megawatt-hour precursor to a gigawatt-scale thermal energy storage system this month. The Future Energy System, first announced last year, is expected to come on-line in early 2019. 

It will use industry-standard heaters and fans, powered when there is a surplus of power coming into the grid, to heat 1,000 tons of rock up a temperature of 600 degrees Celsius. When needed, the heat will be used to drive a 1.5-megawatt steam turbine, feeding electricity back to the grid.

The plant, being built on a site owned by aluminum smelting giant Trimet in the Altenwerder container terminal quarter of Hamburg, in northern Germany, is expected to store enough thermal energy to deliver electricity for up to 24 hours. 

The round-trip efficiency of the system will be around 25 percent, potentially rising to 50 percent if the technology is scaled up to triple-digit-megawatt levels. 

At that scale, Siemens Gamesa expects the technology to deliver stored energy at a cost of less than €0.10 ($0.12) per kilowatt-hour. 

The company would not discuss costs for the plant it is developing at the moment, but the project has been funded to the tune of €27 million ($32 million) by the German Federal Ministry for Economic Affairs and Energy. 

Siemens Gamesa has been working on the concept for three years, the company said in a press release. “The focus of Siemens Gamesa's R&D activities was on the insulated container to house the rock fill, which is the virtual battery and the core innovation,” it said. 

The company had investigated passive and active methods of insulation, said Till Barmeier, Siemens energy storage program manager. He declined to confirm which was being used in the Altenwerder project. 

The research led to an optimized shape of the rock fill container.

“Its round-bodied shape will have a decreasing diameter at both ends, where the inflow and the outflow openings are positioned," according to the release. "The ferroconcrete giant will have a content of 800 cubic meters of rock fill…with a meter-thick layer of thermal insulation.”

At Altenwerder, Siemens Gamesa is working with local utility Hamburg Energie to evaluate the technology’s commercial prospects in energy markets and Technical University Hamburg-Harburg to model the thermodynamic characteristics of the technology. If the first stage proves successful, the next step would be a commercial-scale plant that could potentially deliver electricity for several days.

Siemens Gamesa was looking to develop the Future Energy System (FES) as a more cost-effective alternative to batteries for large-scale energy storage, Barmeier said. 

“The thing with batteries is they do not allow for upscaling with sufficient economies of scale,” he said. “When you go from a 10-megawatt to a 20-megawatt battery double the cost. In the case we’re looking at, you do have economies of scale: If you double the power, you won’t have to double the price.”

The scalability could extend to long-duration storage, too.

“The biggest advantage that this system will have is the size and duration of the storage compared to electro-chemical batteries," said Hong Durandal, business analyst with MAKE Consulting. “Thermal storage would be able to store large chunks of renewable energy when it is being produced in excess and discharge it when it's needed without worrying too much about the degradation of the system.”

Thermal storage beats batteries for renewable energy time-shifting and capacity-firming applications that require more than 4 hours of continuous discharge, he said.

“We will see in the near future how the capex plays out,” he commented.  

In the meantime, Trimet is working on its own "virtual battery" project in a smelter near the FES site. 

Completely unconnected to the FES project, Trimet’s €36 million ($39 million) two-year industrial-scale pilot aims to allow power use across 120 electrolysis cells to be dialed up or down by 25 percent in either direction, for up to several hours.

The smelter is planning to use a technology called the EnPot Shell Heat Exchanger, which it tested in 2014. 

Image credit: Peter Hindmarsh / Flickr