Storing electricity to fill up with renewable energy

Achieving a balance between electricity supply and demand

This is the challenge posed by renewable energies. At a time when the share of wind and solar energy is growing, mastering storage appears essential. From collective or individual solutions already in place to ongoing research, where do we stand? Inventory.

This is one of the most hackneyed arguments of opponents of wind or solar energy: intermittency. Subject to weather conditions or the presence of sunlight, these renewable energies (RE) are not always in line with electricity demand. However, the objectives set by the State leave no ambiguity. With 40% of electricity production coming from renewable energies by 2030, the energy transition will significantly accelerate. In this context, the increase in storage capacity, as well as innovations, are necessary to support the rise of renewable energy.

Producing is storing

Thermodynamic solar power plants have a major advantage. They allow electricity to continue to be produced after dark. Several technologies exist, but the principle remains the same: Concentrate the sun's rays using a mirror to heat a heat transfer fluid (oil or molten salts). This fluid can then be stored and transformed into steam to turn a turbine. This way, electricity production can be extended by 16 hours. The plants can therefore operate day and night with maximum sunshine. This is why they are currently located in California, southern Spain, and Morocco. The fact remains that the cost per kWh is more expensive than photovoltaics.

Powerline

If electricity cannot be stored sustainably and efficiently, mature solutions exist. This is the case with pumped storage power stations (PSHPs). The principle is simple and allows large quantities of electrical energy to be stored in the form of... water. Located in basins at different altitudes, the water can be pumped or released (turbinated). Pumping uses surplus electricity production. Conversely, turbines inject electricity to meet peak consumption. This is the most widely used storage solution in the world. Half of the more than 400 PSHPs in operation are in Europe. On an individual level, another well-known solution is hot water tanks. Combined with photovoltaic panels, they can then release energy later in the form of heat.

In batteries

Stationary and decentralized, batteries are another storage method, albeit with lower intensity. They can meet the needs of photovoltaic self-consumption of individuals or be deployed in microgrids. Only their energy density (number of kWh per kg or liter) still represents a barrier. If the 20th century was built on oil, it is because of an energy density 35 times higher than that of a lithium battery. And even if this rare and expensive metal were replaced by sodium ion, the environmental impacts would be far from satisfactory. Furthermore, no battery offers the energy density required for massive storage on the scale required by the energy transition.

The H2 Hour

Could hope then come from hydrogen? "Power to Gas" transforms surplus electricity from renewable sources into hydrogen. To do this, the electric current passes through water to break down its molecules and separate the oxygen from the hydrogen. It is the latter that can be stored and transported in natural gas networks. Far from having the efficiency of STEPs, "Power to Gas" has the immense advantage of being able to be deployed anywhere, unlike hydraulic storage. Projects have recently been launched. Engie is developing solutions for housing and mobility in Dunkirk, and GRTgaz is building an industrial demonstrator Jupiter 1000 in Fos-sur-Mer.

Energy from Compressed Air

Much less well-known is compressed air storage. This involves compressing excess electricity production. Injected under pressure into a reservoir, it is then released by a turbine to meet electrical energy needs. However, compression causes the air to heat up, which must be cooled. The same goes for decompression; the air must be heated before being put through the turbine. To avoid the energy losses caused by these steps, a solution is currently being developed. While compressed air storage remains technically complex to implement, it offers the advantage of large storage capacities, which explains the boom in projects worldwide.

In summary

While storage solutions exist, and many factors argue for their development, the question of cost constantly arises. Certainly, batteries could be the most obvious way to match supply and demand on the electricity grid. However, many technical, regulatory, and economic constraints are holding back their development. Shouldn't we further reduce environmental impacts? Limit self-discharge? One thing is certain: for most of the technologies mentioned, significant research and development efforts must be pursued to achieve economically viable solutions. The recent rise in oil prices and forecasts of crude oil prices to 2025[1] could, however, help unlock the investments needed for the energy transition.

Cyrille Arnoux, Web Editorial Manager

[1] Deloitte Oil and Gas Price Forecast, June 30, 2018.