Energy storage technologies have existed for centuries but remain as one of the most difficult hurdles in the widespread use of renewable energy.

The most common commercial use of energy storage is in Uninterruptible Power Supplies (UPSs).

These units provide energy storage for only a few minutes. In the case of renewable power systems, energy storage is often needed in larger quantities. For wind, tens of minutes of storage is useful to smooth wind induced fluctuations and for solar power, hours of storage would provide 24-hour power.

What is it?
Storage technologies are simply a means of storing energy. There are many energy storage technologies on the market today, the most well known are batteries. Other types of energy storage devices available include super capacitors, hydrogen, flywheels, compressed air, thermal and pumped storage. A more detailed discussion on each of these technologies can be found in this Wikipedia article.

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How can they be used?
Energy storage devices can be used for a wide range of applications. From being the primary power source in watches, mobile phones, flashlights, laptops etc to being an auxiliary power supply in automobiles, power generation systems and data logging systems and finally the backup power in UPS systems and alarm clocks.

Batteries are best suited to sustained, medium level power output. For small systems, a battery can easily be incorporated into the generation scheme. All that needs to be noted is that batteries are inherently a DC source and so any system that requires an AC output will require an inverter. Large battery banks, such as those found in stand-alone power systems also require some form of shelter to protect them from the environment and to provide adequate ventilation. In areas that regularly experience extreme temperatures, climate control will also increase battery life and performance.

Flywheel systems are best suited to short term supply interruptions. This type of system utilises a large spinning mass, connected to a motor/generator and stores the energy as kinetic energy. Flywheel systems can output a lot of power in a short amount of time, but once their kinetic energy is spent, it must be replenished before they can do it again. In a similar manner, a compressor and a high-pressure air-powered turbine can be used to store and use compressed air to/from an air storage tank. Commercial UPS systems are available that use both of these technologies.

Pumped storage, also known as pumped hydropower, is a utility scale energy storage scheme. It can output a lot of power for long enough for the conventional generation to come up to speed and take on the load. One such scheme in North Wales can output 1,320MW after a response time of only 12 seconds.

Information on an Australian example at Wivenhoe Dam in Queensland can be found at the SEQWater website. This system is somewhat smaller but is still rated at 500MW.

Further information including international comparisons can be found in the Pumped Storage Wikipedia article.

Similarly superconducting magnetic energy storage systems can be used to supply energy to correct short-term voltage sags and momentary interruptions in utility scale generation schemes. Further information can be found on the Research Institute for Sustainable Energy web site.

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Where can they be used?
Energy storage systems can be used anywhere there is a need, though some are better suited to certain environmental conditions. Factors such as the physical space, climate, peak power demand and maximum energy demand will usually dictate what particular type of system is best suited.

Systems such as pumped hydropower are more geographically confined because they will require (unless already present) the formation of two lakes, vertically separated, and a hydroelectric power plant in between. Similarly constrained is large-scale compressed air energy storage, which requires the presence of suitable airtight caverns. Compressed air storage is also used for smaller scale systems (RAPS) where a steel cylinder is utilised to store the air. Other systems are geographically unconstrained; they can be as versatile as a small UPS (40cmx15cmx20cm), which can go just about anywhere, provided it is protected from the weather.

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Why are they used?
Energy is not always used and produced in equal quantities, and so some form of elasticity is required. The economic optimum size for a simple power system is where an energy resource meets the demand, but only just. In reality however there will always be uncontrollable power variations, that is, changes in the rate of energy demand and sometimes energy supply. That means power production will sometimes exceed demand, and at other times demand will exceed production. Where this is the case energy storage is very useful, provided that the cost of installing and using energy storage is less than the cost of installing larger amounts of power generation.

Each energy storage type (and sub-type) has its own advantages and disadvantages, and is best suited to a particular type of load and energy resource characteristics. Factors such as relative size, cost, weight, discharge time, capacity, maintenance, reliability and efficiency will greatly affect which system is most compatible.

The following table gives comparative properties between batteries, capacitors and fuel cells. Due to research and a high level of commercial competition, these figures are likely to change however.

A comparison of energy densities (kWh/kg) of some common storage media is given in the table below.

At present, the most cost effective utility scale energy storage system is pumped water storage. The most cost effective small-scale (RAPS) energy storage medium is lead-acid batteries that also have the advantage of being a stable and mature technology.

How does it work?
As there are so many technologies, this section will deal with two storage technologies that are more common in sustainable power systems. These two are batteries and flywheels.

Batteries
The most well know energy storage medium is batteries. Batteries store energy chemically in small cells. Each cell contains an anode material and a cathode material connected electrically using an electrolyte. The discharge reaction of a battery pushes electrons away from the anode towards the cathode material. If there is a conductor or load to connect the anode and cathode (terminals), then a current can flow and the chemical reaction progresses until complete. When this reaction is complete, the cell is discharged. Different battery types use different chemical reactions and hence use different materials for the anode and cathode as well as different electrolytes. Some of these reactions are not readily reversible such as those used in carbon-zinc batteries while others are. The reversible reaction is the characteristic required to recharge a battery.

Lead-acid type batteries are the most common in small to large power supplies. As an example, almost every motor vehicle in the world has one. Lead-acid batteries can further be divided into either liquid electrolyte (flooded cell) or non-liquid electrolyte (gel) batteries. One of the drawbacks of a flooded cell battery is that the water content of the electrolyte evaporates over time and hence they need regular maintenance of electrolyte levels. In addition the charging reaction involves the production of hydrogen gas that is very flammable. Gel cell lead-acid batteries on the other hand are often referred to as maintenance-free batteries. This is because they operate as a “sealed” battery and the moisture can’t escape (although a one-way valve does allow gases to escape if necessary). Gel cell batteries often have higher energy densities and can be operated in virtually any orientation. Gel cell batteries however are about twice the cost of flooded cell batteries. The following photo is of a gel battery bank used in a small stand-alone power system.

Flow Batteries
There are other types of batteries, in which the electrolyte fluid is pumped. These are referred to as flow batteries. There are four main types of flow batteries; Polysulphide Bromide (PSB), Vanadium Redox (VRB), Zinc Bromine (ZnBr), and Hydrogen Bromine (H-Br). These batteries work on the principle of having external tanks in which the electrolytes (catholyte and anolyte) are stored. The electrolytes are pumped through the cell and past a micro-porous membrane. The exact operation is kept secret as the technology is still emerging. The following table gives a comparison of the properties of different types of batteries.

For more information on the different types and operating parameters of batteries, visit the ThermoAnalytics web site. The Research Institute for Sustainable Energy web site also offers some useful points about batteries for stand-alone power systems in Australia.

Flywheels
Flywheel systems generally consist of a spinning mass, a motor-generator and a control system. The energy stored in a flywheel is proportional to the weight and to the speed squared. A flywheel will usually be connected to an electric motor/generator that can deposit or extract energy as required. When acting as a UPS, the flywheel control system monitors the grid voltage and/or frequency. If these parameters go out-of-range, then the kinetic energy stored in the flywheel is used to support the load until the grid is restored to normal.

Because flywheel velocity is more important than mass, modern flywheels are made from carbon fibre composites and spin at tens of thousands of rpm. Due to the advent of electromagnetic bearings and vacuum chambers, almost all friction and air resistance is removed and these flywheels run at efficiencies in excess of 90%.

A comparison of flywheel energy storage and other technologies can be found at the Research Institute for Sustainable Energy web site.

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