Storage solutions central to renewables

The future of renewable energies will rest on the right mix of storage

By Morand Fachot

As many countries try to increase the share of REs (renewable energies) in their portfolio for producing electricity, a major issue facing utilities is that of EES (electrical energy storage). Generally, electricity is consumed as it is produced; however, as the input from renewable sources is mostly intermittent and not always available when needed, EES is vital for enabling their integration in the overall energy mix. Different technologies are available or are under development to improve storage capacities for RE sources.

Upper and lower basin of Limberg II pumped storage plant, Austria (Photo: Voith)

Balancing needs

To balance increasing levels of intermittent RE generation from wind and solar systems, EES solutions are needed that use and store energy efficiently and help improve grid stability and flexibility.

The IEC strongly supports EES. The IEC MSB (Market Strategy Board) has published two White Papers, the first on EES, the second analysing the role of large-capacity EES systems that integrate large-capacity RE sources. Both White Papers stress the crucial importance of EES in future installations. There are a number of utility-scale storage solutions that can be classified loosely into three categories: mechanical, electrochemical and electrical.

Old is not out

EES is not recent: some storage solutions have been around for well over a century.

Pumped-storage hydropower was first used in Italy and Switzerland in the 1890s. It currently represents the largest and most flexible EES solution and is experiencing significant growth. Energy generated at low-demand periods is stored by pumping water into a higher reservoir. It can then be released at peak time to produce electricity.

Compressed air energy systems predate electricity and were initially installed in the late 19th century to deliver [compressed air] power to factories and homes. CAES (compressed air energy storage) was first used for utility-scale electricity storage in the late 1970s. Its use is similar to that of pumped storage. Air is compressed and stored in an underground reservoir during periods of excess power. It is then released, heated and expanded in an expansion turbine driving a generator to produce electricity at peak time.

Solid-state batteries, which convert stored chemical energy into electrical energy, are also a well-established storage solution. Modern battery systems have been able to benefit from advances in technology and materials to improve the capabilities of such systems.

But newer systems are in too

Flow batteries are a type of rechargeable battery that converts chemical energy into electricity; in some respects they are similar to fuel cells. In flow batteries, electroactive chemical components dissolved in liquids are separated by a membrane through which ions are exchanged to provide electrical current.

The principle of flywheels has been known for a long time; the devices were used in mechanical systems long before electricity was introduced. In EES, flywheels store electrical energy in the form of kinetic energy in a low-friction spinning mass (best operated in a vacuum) that is driven by a motor. When electricity is needed, the spinning mass drives a device similar to a turbine to produce electricity.

Thermal energy storage is used, notably in thermal solar plants, for storing excess energy during peak insolation periods in the form of molten salts or other materials. The stored heat can be used at times when sunlight is not available. Alternatively, excess electricity can be used during periods of low demand (night) to produce ice. This can be incorporated in buildings' cooling systems to reduce demand for energy during the day.

Chemical storage, in the form of hydrogen or SNG (synthetic natural gas) produced from excess electricity, is another form of storage. Both hydrogen and SNG can subsequently be used to produce electricity at peak time or for other applications such as transport.

Advantages and drawbacks

Each EES system presents characteristics that make it more or less suitable for different applications.

Pumping water: Pumped-storage hydropower currently accounts for more than 99% of installed storage capacity for electrical energy worldwide: around 127 GW (gigawatts), according to the EPRI (Electric Power Research Institute – the research arm of America’s power utilities) and Germany's Fraunhofer Institute. However, pumped storage can only be installed in places where water can be pumped into a higher reservoir. A new technology being developed, Gravity Power Module, uses similar principles, but as it stores water in underground shafts it is not constrained by the same specific geological features. Its developers claim that it has a small footprint and doesn't require the same high levels of investment or engineering work, making it suitable for application in many more locations.

Compressing air: In the only two CAES installations currently operating that use the so-called diabatic method, air is heated naturally when being compressed from atmospheric pressure to storage pressure. In these two installations this heat is mainly lost before air is pumped into the underground caverns. Another CAES system uses the so-called adiabatic method, which recovers the heat of compression. While this is much more efficient, it is still at the R&D (research and development) stage.

In another related process, called LAES, (liquid air energy storage) offpeak or excess electricity is used to power an air liquefier, which produces liquid air that is stored in a tank(s) at low pressure. Power is recovered when needed as the liquid air is pumped to high pressure, evaporated and heated. The high pressure gas drives a turbine to generate electricity.

Turning the wheel: Flywheels can capture energy from intermittent RE sources and deliver uninterrupted power to the grid. They can respond instantly to demand. The most efficient flywheels are made of carbon, can rotate at a higher speed than those made of steel, are low maintenance and have a long life. Flywheels are particularly well suited to a number of applications including power quality and reliability and frequency response. They are also used in hybrid sports cars and are being tested by a number of vehicle manufacturers (see article on energy harvesting in this e-tech)

Getting the right chemistry and right temperature: Secondary (rechargeable) batteries have been around for well over a century. They rely on different chemical bases. Beside the lead-acid type, the main types used for storage from RE sources are nickel-based NiCd and NiMH, as well as Li-ion and NaS (sodium sulphur). New chemistries and production methods have greatly improved the efficiency of secondary batteries. The main advantage of flow batteries, another electrochemical storage system, is that they can be recharged almost instantaneously by replacing the electrolyte liquid, which can subsequently be recovered and re-energized.

Thermal storage of the molten salt type is well suited to use in thermal solar plants and allows storage of large amounts of energy that can then be recovered to generate electricity as required.

Standards matter

IEC International Standards for certain mature EES systems, such as pumped hydro (developed by TC 4: Hydraulic turbines) or rechargeable batteries of various types (prepared by TC 21: Secondary cells and batteries) are already in existence. With the need for Standards for EES systems, the IEC created TC 120: EES (Electrical Energy Storage) Systems in 2012. The TC oversees the development of International Standards that address all different types of EES technologies taking a systems-based approach rather than focusing on individual energy storage devices.

EES systems will become essential technologies in achieving RE integration and Smart Grid expansion as well as achieving a more efficient and reliable electricity supply. IEC International Standards will be central to realizing these goals.

VH_Limberg2_Reservoir_Back_View_06_NE_38 Upper and lower basin of Limberg II pumped storage plant, Austria (Photo: Voith)
Highview's 350kW - 2.5MWh Pilot Plant 350 kW/2,5 MWh LAES (liquid air energy storage) pilot plant in Slough, UK (Photo: Highview Enterprises Ltd)
20mw_plant_7305_1920px Beacon Power 20 MW flywheel frequency regulation plant (Photo: US DOE)