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 Energy research

Introduction to energy storage

What is energy storage?

  • Energy storage is needed to store electricity, heat and cold, which is produced at times of low demand and low generation cost and from intermittent energy sources such as wind and solar power. It is released at times of high demand and high generation cost or when there is no more generation capacity available.
  • Reliable and affordable energy storage is a prerequisite for using renewable energy in remote locations, for integration into the energy system and the development in a future decentralised energy supply system. Energy storage therefore has a pivotal role to play in the effort to combine a future, sustainable energy supply with the standard of technical services and products that we are accustomed to and need.
  • Energy storage is the most promising technology currently available to reduce fuel consumption in the transport sector. 

Diagram of energy sources, use of energy and energy storage

What are the current uses of energy storage technologies?

  • The decision to use an energy storage system depends both on the requirements of the application and the cost of competing solutions. In renewable energy systems, for instance, the use of fossil fuel based back-up generation and grid connection are competing solutions
  • Power stations, compressors, heating systems, etc. all have different performance characteristics as regards their response time to changing demand, their lead times for starting up or shutting down, and their most efficient points of operation.
  • Energy storage systems can usually be replaced by conventional energy generation. However, this can lead to an inefficient use of fossil fuels and a demand for investment in additional energy generators with high power output and fast response time.
  • The time required for energy generation from renewable sources, be they electricity or heat, cannot always be matched to the time of demand.
  • Energy storage systems are therefore an integral part of any renewable energy sources (RES) system.
  • Even when fuel-powered generation is used to cover periods of low RES generation, energy storage is required for economic reasons, as it is cheaper than the frequent use of a motor-driven generator.
  • Also, the stability of the electricity system and quality of the voltage supplied will be considerably higher when an energy storage system is used. The technical and economic optimum concerning the size of an electricity storage system needs to be defined in each case individually.
  • Conventional, commercially available lead-acid batteries have a very high-energy efficiency and all other technologies have to compete with this.
  • Batteries are the most expensive item in RES systems when the system's total lifetime costs are considered; and there are big variations in battery lifetime in different installations.
  • Excess electricity can always be stored cheaply in the form of heat and for a long time. However, the value of heat energy is much lower than the value of electricity.
  • In solar thermal systems for heating and cooling it is also necessary to store energy because heat generation depends on solar radiation for energy production. Overview of the technology

What are the different energy storage technologies?

  • Batteries
  • Flywheels
  • Reversible fuel cells
  • Electromagnetic
  • Compressed air
  • Super-capacitor
  • Pumped hydro storage

Different energy storage technologies coexist because their characteristics make them attractive to different applications. From a user point of view there are both technical and commercial criteria for selecting the most suitable technology.

What are the various aspects of the technologies and their applications?

Batteries and advanced batteries

Rechargeable batteries or accumulators are the oldest form of electricity storage and widely used. Batteries store electric energy in a chemical form. Their performance is linked in a complex manner to the materials used, the manufacturing processes and the operating conditions. Consequently, progress in battery technology is slow and the transfer of laboratory results into commercial applications is sometimes risky. Lithium ion and nickel-metal-hydride (NiMH) batteries are the only new battery technologies which have achieved significant market penetration in the last decade. Batteries can respond to changes in power demand within microseconds. Only super-capacitors equal such a response time. Batteries usually have very low standby losses and can have high energy efficiency, depending on the application and the details of the operation. Most batteries contain toxic materials, hence the ecological impact from uncontrolled disposal of batteries must always be considered.


Super-capacitors store electrical energy in the electric field between two electrodes. Ultra-capacitor, super-capacitor and electric double layer capacitor (EDLC) are also called electro-chemical capacitors working with chemical reactions or not like true capacitors. The fundamental design and electrical properties are those of conventional capacitors used throughout the electrical and electronics industry. EDLC uses electric double layer capacitance on both positive and negative electrodes.

Reversible fuel cell systems and redox flow batteries

Fuel cells convert hydrogen from a storage tank and oxygen from the air to water and generate a current from the electrochemical process. The electrochemical reaction itself is reversible. The fuel cells' energy capacity is determined by the size of the storage tanks for the active materials, and the power by the area of the electrodes and design of the reactor. Standby losses are low because the active materials are kept physically separate. Redox flow batteries are systems using materials other than hydrogen and oxygen. Their energy efficiency is higher than those of reversible fuel cells, but still below the energy efficiency of most batteries.

SMES (Super-conducting magnetic energy storage systems)

SMES store energy in the magnetic field of a coil made from special alloys. By cooling the conducting wires to - 269°C the resistance of the material to electrical current disappears, allowing it to conduct very high currents without electrical losses. When looking at the complete system, however, it is clear that there is considerable energy requirement for refrigeration. Also, the current has to flow through non-super-conducting components and solid-state switches, which cause resistive losses. Despite this, the overall efficiency in commercial applications is very high.


The energy is stored as kinetic energy in a rotating mass. The amount of energy stored increases with the square of the rotational speed, which is limited by the tensile strength of the material used.

Thermal storage (heat and cold)

Conventional heat and cold storage systems simply store excess energy in a large tank using the working medium at the temperature required for later use. Virtually every cooling and heating system has such storage tanks.

Compressed gas storage

Compressed air tanks are widely used in industry to provide a constant source of compressed air with uniform pressure in the range of 8-10 bar. There is renewed interest in compressed air storage for covering the demand of peak electricity or for small wind/hybrid applications, where the energy-to-power ratio of batteries is unsuitable, either because the energy content is very high but the power requirement low, or the energy through-put is very high compared to the energy content.

Pumped hydro storage

Pumped hydro storage is a conventional energy storage technology utilised by the electrical industry. Water in a basin at the top of a mountain is used to drive a generator in a reservoir at a lower level. When surplus energy is available, the water is pumped back up again. The power output and the cost efficiency of pumped hydro storage depends on the difference in height.   


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