Energy storage: a question of balance

Senior reporter

Developing viable energy storage technologies is one of the big engineering challenges when it comes to cutting carbon emissions, and yet the issue has received relatively little mainstream attention or support.

We desperately need energy storage to make renewable generation more practical and cost-effective, to remove the sight of wind turbines shut down during windy but off-peak times and reduce the need for backup fossil-fuel power when renewable energy runs short.

So the announcement that 12 projects have been put through to the first stage of the government’s energy storage competition is very welcome, even if the amount of money allocated at this feasibility stage (£500,000) is relatively small.

Those groups that reach the second, demonstration phase of the competition will be competing for a share of £17m ­– a more substantial sum but one that still requires plenty of private investment if these technologies are to be brought to a commercial level.

What’s perhaps most interesting about the winners list (and about the field of energy storage research in general) is the breadth of technologies identified as possible solutions. It’s both an opportunity and an obstacle: the UK is bursting with ideas of different ways to capture energy for different scenarios, but that means we need more funding to develop them.

In particular there are a huge number of battery technologies put forward as having potential for grid-scale storage, from lead acid to lithium ion to more unusual concepts. They’re costly but offer very high efficiencies, sometimes up to 95 per cent or more.

The one chosen to go forward in the DECC competition is for a vanadium redox flow battery. A group led by Wokingham firm REDT hopes to built a 1.2MWh system on the Isle of Gigha off the west coast of Scotland to store surplus energy from a wind turbine for use in the local network. The company claims the technology offers 75 to 80 per cent efficiency depending on duty cycle at a cost of £1.7m to £1.9m per MW, and says this is a lower cost over the 10 to 15-year life of the system than any other battery storage.

But several other groups in the competition are hoping to bring the cost of battery storage down by using existing batteries. The Aston University-led group plan to combine new lithium titanate batteries with old electric vehicle units to provide grid-balancing services: the second-hand batteries will provide most of the capacity while the new ones will cater the rare occasions when National Grid needs up to 30 minutes of extra power.

Yuasa Battery Europe, meanwhile plans to convert existing uninterruptible power supply (UPS) equipment used for backup in industry by replacing lead acid batteries with lithium ion ones, possibly pre-used from electric vehicles. The electricity doesn’t go back to the grid but it would help with balancing and would cost an estimated £600,000 per MW of capacity.

Other competition entries make use of batteries as part of an energy and money-saving domestic service, such as the Maslow system that Moixa Technology hopes to install in 750 homes. It stores energy from the grid overnight or from solar panels on the roof for use in a DC lighting and electronics system, but can also help with grid-level storage and balancing.

Alongside the batteries there are numerous other storage mediums being proposed. Liquid air storage might sound like it should be an experimental technology but could actually reach a commercial scale if it goes through to the next stage of the competition. Its efficiencies are below those of batteries but can be increased if waste heat (and cold energy) streams from power plants or other industrial sources are harnessed, and it uses a free medium (air) and well established liquefaction and evaporation technology. The firm developing it, Highview Power Storage, have even created a cryogenic system it says is the equivalent of a diesel generator powered by air.

Another alternative in the competition is storing energy by producing methane, which can then be burnt to produce electricity or used in industrial processes. Hydrogenics Europe wants to build the first power-to-gas plant in the UK, and the first power-to-methane plant based on biological methanation in the world at a waste water treatment plant. It says the efficiency is 58 per cent, rising to 78 per cent if heat from the process is recovered. The costs are currently £1.4m/MW but the company claims this could fall to £800,000/MW.

Using gas (air, methane or hydrogen) means you can effectively move the energy from where it is captured to where it is needed, and the technology is easily scalable and less location-dependant than pumped hydro storage or compressed air systems that pump into caves. But we’ve yet to see whether the efficiencies or costs can make the technology viable as a widespread commercial solution.

There’s also another more unusual concept amongst the competition winners: using gravel as a storage medium by carrying it to the top of a hill in buckets attached to what looks like a ski lift. It’s a messy idea (literally and perhaps metaphorically) but its US inventors Energy Cache claim it beats pumped hydro and compressed air for cost and performance and is also more easily sited. The company has yet to respond to The Engineer’s requests for more details but we’ll bring you a more in-depth report when we can.

Looking at the huge range of ideas for energy storage suggests two things: that not all of them will be successful but also that there probably won’t be a single winning technology. Just as with renewable energy generation, we’re likely to need multiple ways to store energy. Let’s hope the UK can provide the necessary support to turn our wealth of ideas into viable solutions and businesses.