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Putting thermal energy storage on the rocks

Putting thermal energy storage on the rocks

Methods of generating renewable energy are becoming more productive. Whether it’s a more efficient solar cell or a larger rotor blade, developers are continually coming up with more efficient methods of harnessing natural resources. While many companies are devoting their efforts to refining existing technologies, there is an issue with renewables which has yet to be definitively resolved: energy storage. That is, how do we store the excess energy generated by renewable resources?

The storage situation

One of the advantages of utilising oil or gas in energy generation is that they are ‘on-demand’ energy sources. When a driver wants to make a trip in his vehicle, he can be sure it will start if there is fuel in the tank. Oil also boasts a high energy density, meaning that a relatively small amount of refined crude oil can produce a comparatively large quantity of energy. However, as the world seeks to decarbonise, it must also develop equally dispatchable methods of powering our way of life.

Lithium ion batteries — with their impressive energy density and long lifetimes — have been touted as the favoured method for combatting the intermittency of renewables, especially solar. Tesla famously utilises the batteries in its home-use Powerwall and utility-scale Powerpack energy storage solutions. The lifetime of these batteries, and equivalent products manufactured by Panasonic, are reportedly limited to roughly 500 ‘deep cycles’. This is the measure of the number of times a battery can deplete from full charge to 20 percent power before its performance deteriorates. 

While lithium ion batteries might be suitable for use in smartphones with an anticipated life of two years, they may not be the large-scale storage ‘silver bullet’ that the renewable industry is seeking. This doesn’t mean that battery solutions should be abandoned altogether, but companies in the sector are already trialling new and innovative ways of capturing energy for later use.

The Siemens solution

In October of this year, Germany’s Siemens, the largest engineering firm in Europe, debuted a thermal storage concept for wind power which promises low costs and reliability. The company has partnered with Technical University Hamburg Harburg (TUHH) and utility Hamburg Energie to test the viability of what the company calls its Future Energy Solution (FES). The system will reportedly deliver energy at below US $111 per megawatt hour.

The technology combines existing steam turbine technology with packed-bed heat storage. The system works by utilising excess wind energy to power an industrial-scale heater which then blows air heated to 600 degrees celsius across a bed of rocks inside an insulated container. When power is required, the heat stored in the rocks provides steam to a conventional turbine capable of funneling electricity to the grid. The system is being designed to work with wind energy, but could theoretically accommodate other forms of renewable generation, too.    

With the help of scientists for the TUHH Institute for Thermofluid Dynamics, Siemens is currently researching how to make the process of charging and discharging the FES more efficient. The firm and its venture partners are assembling at 25-cubic-metre model of the storage tank in Hamburg’s Bergedorf borough. As it stands, the mock-up environment only tests the thermal requirements for the storage process with no reverse current generated. Researchers are planning to test the system’s energy conversion potential in spring of next year. According to a statement issued in late September, the company is also constructing a full-size FES capable of storing some 36 megawatt hours of energy in a container with 2,000 cubic metres of rock. Siemens predicts that the solution will generate up to 1.5 megawatts (MW) of electricity for up to 24 hours a day. 

"The technology of our FES store deliberately uses mainly tried and trusted technology," said Till Barmeier, Siemens' Project Manager in a statement. "Because we are working here with tested thermal components and a series-ready steam turbine, we will be able to offer a practical solution within a few years. Our complete experimental system will be operational in just around 15 months."

Barmier has said that Siemens is going to develop a commercial pilot project of between 30 and 40MW in 2019. It is in the process of identifying potential customers. Further down the line, the firm anticipates that the FES would have a capacity that could exceed 100 megawatts and be able to store 48 hours’ worth of energy for up to seven days.

No single answer

Just as no single form of renewable energy generation will be capable of meeting all demand, no storage solution will universally house excess energy. It’s likely that a mix of technologies — from solid-state batteries to pumped hydropower — will act as storage facilities. Siemens’ FES admittedly breaks new ground in terms of cost and efficiency, but it will be one of many available options for capturing energy to be dispatched as-needed.

 

Read the January 2017 issue of Energy Digital magazine

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