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A renewable-powered future will need energy storage

Electric mountain - the reservoir at Ffestiniog power station, Wales. Arpingstone

The way we generate, transfer and use energy is changing, and our energy systems and infrastructure have come under increasing pressure to cope. Black-outs strike where we would expect reliable supplies, energy costs are rising, pushed up by fossil fuel prices and the expense of renewing ageing electricity infrastructure. As the proportion of energy generated from renewables like wind, wave and solar power rises, part of the solution to such intermittently generated energy is technology that can store the energy until it is needed.

Professor Richard Williams has explained why energy storage is needed, and how this was now being recognised by policymakers. Practically, the concept of “energy storage” varies considerably in how much energy can be stored for how long, and can be achieved through chemical, mechanical, thermal or electromagnetic methods.

The applications for energy storage have become apparent as nations’ energy systems undergo radical transitions. The reasons behind these changes are diverse: climate change mitigation policies have led to widespread use of renewables, in Europe especially. In rapidly expanding economies such as China, the priority is to meet rising demand.

In many developing countries, improving and expanding electrification is the goal. In the US, improving the resilience of the electricity grid to extreme weather events has become paramount, to prevent another hurricane Sandy knocking out much of New York City’s power grid, for example. All these examples require substantial change to energy systems, whether that’s the electricity grid, how energy is generated, or other elements. In each case, energy storage is an attractive technology that can reduce costs and improve efficiency.

The most developed form of energy storage is at the shortest timescales, such as for maintaining the quality of electricity supply within strict frequency standards. Flywheels - rotating discs - can absorb and release energy very quickly making them ideal. Losses are low; they can spin up and down countless times without affecting performance. The total energy flywheels can store is also low, so their use is limited to a matter of minutes – enough to provide the quick response required if there are rapid changes due to the interruption of energy generation - from a drop in wind, for example.

Storing energy over a number of hours is very competitive. The arbitrage of energy from times of over-supply (when the wind blows at night when there are few energy consumers, for example) to peak times can be provided by many technologies. Pumped hydro storage is where water is pumped to a high-level reservoir using off-peak electricity, which can then flow back downhill via a turbine to generate electricity. With four such facilities in the UK, including Dinorwig in Snowdonia, and may other s around the world, this is the current dominant technology: it is well-tested and reliable, and able to store and bring online vast quantities of energy, quickly. Of course, it’s limited by the availability of high lakes or reservoirs.

Large batteries, which store and release energy through electro-chemical reactions, have been widely demonstrated. They can operate over short time scales, but are able to hold larger amounts of energy. New battery chemistries such as Li-ion have been improved by their use in cutting-edge electric vehicles. But they are still expensive, and can degrade after rapid charge and discharge cycles.

When it comes to storing large amounts of energy, the need to add considerable capacity at low cost is key. Compressed air energy storage systems store energy by compressing air, in containers, or in very large volumes in underground caves or chambers. The technique either relies on the right geology, or suffers from low energy density that limits above ground capacity.

If this compressed air is instead liquefied, it vastly increases the energy density. Called cryogenic energy storage, this has been tested in the UK, building on mostly established industrial techniques. The efficiency can be increased by recycling the heat and cold that is released as the air is compressed or expanded. Rather than cooling, at the other end of the temperature scale is an experimental method of storing electricity by heating gravel beds using heat-pumps. With a heat engine to recover the energy and convert it to electricity when needed makes this a potentially efficient process.

Energy storage need not be on an industrial scale, it can also be stored in domestic properties. Heat, for example, can be stored for use as space or water heating. New materials whose physical properties change with temperature, called phase-change materials, can increase the amount of energy stored in a volume compared to hot water cylinders. By minimising their energy needs, better domestic energy storage means individuals can play a significant role in balancing the ebbing and flowing of energy demand nationwide.

Another important part of any future infrastructure is the capacity to transport energy over long distances for storage or immediate use. The concept of a supergrids linking remote renewable energy resources (for example, wind turbines in blustery Siberia) to areas where there is demand, such as cities perhaps hundreds or even thousands of miles away. For exmaple a supergrid connecting countries around the North Sea, across Asia and North Africa has been suggested. This is attractive in principle, but there are significant difficulties that will require new transmission technologies, including subsea connections. And that’s before one considers the task facing policymakers and regulators of integrating different energy infrastructures, regulatory systems and markets.

The challenge is to explore precisely how and where energy storage can create a more efficient energy system over the coming decades. Everyone in the sector will need to work together to develop the technology that delivers, and put in place a regulatory regime which turns the concept into a commercial opportunity for the private sector.

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