Where Do You Put Renewable Energy Until You Need It?

The cost of renewable energy continues to fall, and more and more of our electricity comes from renewables and low-carbon sources. On one day in June the renewable electricity supply peaked at over half the UK’s demand, and National Grid reported that across the summer 52% of electricity generation was low-carbon.

My energy colleagues tell me that to continue to decarbonise the electricity supply, we are going to need a lot more energy storage to buffer the load. How much renewable energy can be incorporated into a supply network depends on a lot of factors like renewable mix, size of the grid, number of interconnects to other grids, and so on. But more renewables inexorably means more storage and smarter grids. National Grid in the UK estimate that they will need to quadruple grid storage by 2030.

‘Electric Mountain’

Storing energy conveniently when you have spare capacity, to release at periods of peak demand, has been a long term goal in energy supply. In 1984 the Dinorwic power station opened in North Wales. A pumped-hydro scheme, it is delightfully simple. A lake at the top of a mountain and another at the bottom. When there is spare energy, you pump water from the bottom lake to the top. When you need the energy you allow the water to run back down the mountain through the pumps, which now act as generators. The six main inlet valves that control the flow are enormous and Dinorwic can go from a standing start to full power in under 16 seconds. It is an impressive facility, and under the name of Electric Mountain a major tourist attraction.

Unfortunately, pumped-hydro schemes need the right sort of geography, and are large civil engineering projects that cannot be built quickly. They are not the solution to ramping up our storage capacity.

The other type of storage that we hear about all the time is battery storage; typically lithium-ion batteries. This is a well established technology that is easy to scale from domestic to grid-scale applications. Tesla has created a lot of excitement, but there are smaller UK based businesses such as Powervault and Moixa working in the same space.

But batteries are not the solution to all problems. They are good for quick response with limited demand, but less suitable for longer and larger power gaps. There are also concerns about the material intensity of lithium ion battery systems, the lifetime of the battery, and problems with recycling.

Fortunately, almost every possible way of converting electricity into another, storable, form of energy is being worked on. Large and small systems, fast and slow energy release, high and lower energy densities.

Rude mechanicals

For example, the pumped-hydro system stores energy mechanically, as mass raised through a gravitational field. There are other mechanical storage options.

Gravitricity plan to use heavy weights in disused mineshafts, using spare power to raise them, and recovering the energy by allowing them to drop down the shaft like the weights in a grandfather clock.

Another option is to store the energy in a rotating flywheel. As the energy stored goes up as the square of the rotation speed, spinning the flywheel fast gives very high energy densities. Flywheel systems have a long life and can delivery energy extremely quickly. Not really suitable for grid scale storage, companies like Flybrid are finding applications in uninterruptible power supplies, smoothing demand for conventional power generation, and in stop-start vehicles like buses.

Compressing a gas can store energy like a spring, using it to spin a turbine when the pressure is released. People are looking at large scale systems where the compressed gas is stored in exhausted gas fields or salt caverns. Where there is no suitable void, one idea is to cut a 250m wide cylinder of rock and raise it like a piston by pumping the gas underneath it.

If you compress air enough you can liquefy it. Now it can be stored at low pressure in standard insulated vessels, and allowed to turn back into a gas to drive a generator. The great benefit of this method is that most of the equipment is standard in the process industries, so it can be deployed almost anywhere. This is the approach that Highview Power are taking.

The heat is on

Heat is another good way to store electrical energy, especially if that is the ultimate form of energy that you want. After all, 44% of UK energy consumption is for heat.

On a small scale we are used to storing heat in hot water tanks and storage radiators, but you can do it on a much bigger scale. Concentrated solar power plants often store heat in molten salts; allowing them to continue to generate electricity when it is dark. Sunamp heat batteries use phase change materials to store heat with four times the energy density of water. They can efficiently soak up excess renewable energy and provide space or hot water heating when the renewables are not enough.

The problem with most grid-scale storage solutions is that they are expensive compared to pumped-hydro. One interesting approach that could match this baseline cost is pumped heat energy storage. Energy is used on a pair of heat stores to drive one to a high temperature and the other to a low temperature with a novel heat engine. These stores can be used for heating and cooling directly, or the system can be reversed to generate electricity again. A grid scale prototype is being developed at the Sir Joseph Swan Centre for Energy Research, Newcastle University.

Pushing chemical reactions

Finally we can use chemistry. Making and breaking chemical bonds, changing chemical state and generally pushing chemical reactions in one direction to store energy and in another to release energy. This is how photosynthesis turns solar radiation into energy stores that can be used by living organisms, and this is how batteries work.

There is plenty of effort going into new battery chemistries to reduce cost, increase lifetime, and to increase energy density. But the simplest chemical reaction being exploited at the moment is to produce hydrogen from water by electrolysis using spare electricity. Hydrogen is useful stuff. You can burn it directly as a fuel, or you can use it in a fuel cell to generate electricity.

H2GO Power are using a novel nano-technology storage material to make a safe and handleable form of energy. One of their developments is a complete containerised system that stores excess energy from renewables as hydrogen and generates electricity on demand. This is targetting off-grid uses and critical applications, such as hospitals, in locations where energy security is not guaranteed.

ITM Power focuses on the efficient generation of hydrogen  using polymer electrolyte membrane (PEM) electrolysis. The gas produced can be directly injected into the natural gas network and burned alongside the methane; taking advantage of the existing distribution and use infrastructure. Alternatively, it can be used as a vehicle fuel.

The internet of energy

With more distributed generation of electricity and more storage of  all sizes and in all locations, we need different ways of managing our electricity network. Jeremy Rifkin talks a lot about the internet of energy, peer to peer trading, in “The Third Industrial Revolution”, and companies like Upside Energy are starting to make it happen. Upside has developed a virtual energy store that aggregates the energy stored in systems people and businesses already own through a cloud-based service. By aggregating hundreds or thousands of local systems they can add significant storage and resilience to the grid; making these fragmented resources a strength, not a weakness.

“Let a hundred flowers blossom”

Two things are clear; just about every potential form of energy storage is being actively explored, and reliable energy grids will require multiple storage technologies of different capacities, power densities, capital cost and response times. Which specific solutions will become fully commercial we don’t yet know, but it is going to be an exciting innovation space for a good few years.