Operating electrical grid systems in cities and communities can be a balancing act. Grid operators constantly endeavour to maintain a delicate equilibrium between the available supply of electricity and the demand of connected users. Both factors tend to fluctuate wildly, and often the amount of energy produced and the amount needed are out of synch.
Maintaining grid balance is an even greater issue when renewable energy sources such as wind and solar power are part of the energy mix. By their very nature, these types of energy – called “variable resources” in the industry – are subject to changes in weather: on a particularly sunny or windy day, they may yield excess amounts of electricity, only to fall short when conditions change.
In order to accommodate these imbalances – and to integrate renewable energy sources more smoothly – grid operators rely on electricity storage solutions. As an alternative to simply raising the proportion of conventional, non-renewable energy sources when supplies run low, stored electricity keeps electrical grids balanced while maintaining a light environmental footprint.
Energy storage is also a key issue in current policies to make greater use of renewable energy sources. The European Commission's 2020 action plan of covering 20% of EU electricity demand through sustainable resources also poses challenges in terms of electrical grid stability.
In practice, only a handful of electricity storage technologies are suitable for grid-balancing operations on a mass scale. Other technologies, although promising, are still being developed for commercial applications. Here is an overview:
Currently the most mature technology, hydropower storage relies on storing water in reservoirs at different levels of elevation. When the electrical grid needs electricity, water is released from the more elevated reservoir, powering a turbine generator on the way down. This principle yields high outputs at dams holding natural bodies of water. It can also be applied in pumped hydropower storage (PHS) facilities, where water can be pumped into the upper reservoir as needed.
Compressed air energy storage (CAES). Frequently used for commercial applications, CAES systems store compressed air in underground geological formations, for instance disused coal mines. When the grid needs electricity, the air is decompressed to power a gas turbine for electricity generation.
Similar to CAES systems, hydrogen energy storage relies on keeping deposits of hydrogen in underground cavities. The hydrogen is then turned into electricity with a turboexpander; an efficient operation that only consumes 2.1% of the stored energy content.
Many energy applications produce heat, which can be stored for energy generation at a later point in time. Storage media may include hot water tanks or molten salt for capturing residual heat from solar energy panels. Stored heat is used to generate electricity or, funnelled directly through pipes, for residential heating.
Current battery solutions mostly serve to stabilise power distribution networks short-term, as opposed to providing substitute electricity for prolonged periods of time. In Puerto Rico, a battery system stabilises the local grid with a capacity of 20 MW for up to 15 minutes. Promising future technologies include "flow" batteries based on zinc-bromine components as well as lithium-ion polymer and sodium-ion battery solutions.
An efficient technology with promising future applications, SMES conserves energy in a magnetic field created by a cryogenically cooled superconducting coil. Because the coil is cooled below its critical superconducting temperature, the electricity will only be conducted when operators push a button.
According to the European Commission, pumped hydropower storage (PHS) currently represents almost 99% of the world's electricity storage capacity. Around the world, governments are investing heavily to add water power to their energy mix: in May 2013, the Chinese government green-lighted a new hydroelectric dam, which will be 313 metres high, on the Dadu River in the Sichuan province.
Europe currently leads the way in PHS. The three global market leaders in hydropower equipment technology, controlling 50% of the worldwide market, are based in the EU.
The greatest concentration of facilities among European countries is in France, Germany, Italy and Spain.
Some of the world's biggest hydropower plants are located in Europe, including plants in Dinowig, Wales, and Isère, France - both capable of churning out over 1 800 MW of electricity respectively. The Commission now acknowledges the enormous potential for adding pumped hydropower storage technology to existing facilities across the EU.
Looking ahead, the growing share of renewable energies will also necessitate growth in electricity storage capacities. Renewable energy is already the fastest-growing segment of the global power market and the International Energy Agency (IEA) expects 40% growth over the next five years, despite difficult economic conditions.
By the year 2018, renewable energies will represent 25% of all energy generation worldwide, says the IEA, up 20% on 2011.
The natural fluctuations of renewable energy sources will also require more stored electricity to maintain balance in electrical grids: according to the IEA, global electricity storage capacities from 189 GW to 305 GW will be needed by the year 2050 to mitigate grid imbalances attributable to variable energy resources.
Ultimately, making this growth in storage capacities a reality throughout Europe - and beyond - is a task for inventors, grid operators and policymakers alike.