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Alternative Energy Storage Methods
Capacitors - The Electrostatic Battery
The use of capacitors for storing electrical energy predates the invention of the battery. Eighteenth century experimenters used Leyden jars as the source of their electrical power.
Capacitors store their energy in an electrostatic field rather than in chemical form. They consist of two electrodes (plates) of opposite polarity separated by a dielectric or electrolyte. The capacitor is charged by applying a voltage across the terminals which causes positive and negative charges to migrate to the surface of the electrode of opposite polarity.
The capacitance is a measure of the charge stored for a given electric potential between the electrodes. For a parallel plate capacitor the capacitance is proportional to the area of the plates and the permittivity (ε) of the dielectric separating them and inversely proportional to the distance between the electrodes. Thus:
C is the capacitance in farads (F)
A is the area of the electrodes measured in square metres (m2).
ε0 is the permittivity of free space (ε0 = 8.854x10-12 F/m)
εr is the dielectric constant (or relative permittivity) of the material between the plates, (for a vacuum εr=1)
d is the distance between the plates, measured in metres (m).
For high capacitance values, high permittivity dielectrics are needed and the area of the plates must be as high as possible while the separation between them should be as low as possible.
The energy stored is related to the charge at each interface, q (Coulombs) , and potential difference, V (Volts), between the electrodes. The energy, E (Joules), stored in a capacitor with capacitance C (Farads) is given by the following formula.
E = ½ q V = ½ CV2
See What can a Joule do? for an example.
Since capacitors store charge only on the surface of the electrode , rather than within the entire electrode, they tend to have lower energy storage capability and lower energy densities. The charge/discharge reaction is not limited by ionic conduction into the electrode bulk, so capacitors can be run at high rates and provide very high specific powers but only for a very short period. Typical numbers for capacitors and batteries are given below:
Capacitor / Battery Comparison
1 - 103
0.05 - 5
105 - 108
105 - 106
Typical Coulombic efficiency is around 90%
See also examples of the relative energy storage capacities of capacitors and batteries in the section on Short Circuits.
Since there is no chemical reactions are involved, the charge/discharge reactions can typically be cycled many more times than batteries (108 cycles per device have been achieved). For the same reason, capacitors don't require any special charging circuits and cells can be designed to accept very high voltages, although for very high capacities the working voltage is limited to a few volts.
Supercapacitors are simply capacitors employing plates with extremely high surface areas providing a high storage capacity. Maximizing the surface area of the electrodes within the available space means the thickness of the dielectric must be minimised. This in turn limits the maximum working voltage of the capacitor. For this reason, even though there is no fixed limit, set by the chemistry, on the working voltage of a capacitor as there is with batteries, for supercapacitors with a capacitance of over 1000 Farads or more the working voltage may be only a few volts.
For high voltage applications such as electric vehicles, a series chain of capacitors must be used to avoid exceeding the working voltage of individual capacitors and this reduces the effective capacity of the chain. For a series chain of N equal value capacitors the capacity is calculated from C=c/N where C is the capacitance of the chain and c is the capacitance of the individual capacitors. At the same time, the internal resistance of the chain is increased to R=rN, where r is the internal resistance of the capacitor, as more capacitors are added. This slows the charge-discharge rate and increases the losses.
Higher capacitances can be achieved by using parallel capacitors. In this case the capacitance of a group of N parallel capacitors is given by C=Nc. At the same time the resistance of the group is reduced and is given by R=r/N.
Capacitors are now used extensively as power back up for memory circuits and in conjunction with batteries to provide a power boost when needed. See Load sharing.
High power versions can provide high instantaneous power but they have limited capacity. See the Ragone Plot below. They are suitable for applications which require a short duration power boosts such as UPS systems which need fast take over of substantial electrical loads for a short period until back up power units, such as rotary generators or fuel cells, have switched on and reached their full output. Similarly they can be used to provide an instantaneous power boost in Electric and Hybrid vehicles.
Supercapacitors are however also ideal for absorbing the energy generated from regenerative braking in EVs and HEVs since they can accept very high instantaneous charge rates which would exceed the recommended maximum charge rate of the batteries. Used in conjunction with batteries the capacitors enable the full regenerative charge to be captured, avoiding the wasteful dumping of the excess charge which the batteries are unable to accommodate.
See more in the section on Capacitors and Supercapacitors.
History (Electrolytic Capacitors)
Heat - The Thermal Battery
There are two basic types of direct conversion thermal batteries, the thermocouple converter based on or Seebeck effect and the AMTEC converter which uses an electrochemical heat engine. Both of these convert heat energy directly into electrical energy.
- Thermocouple Batteries
Based on the Seebeck effect, in a closed circuit made up from two dissimilar metals, an electrical potential is created between the two junction points when one junction is heated, usually by a gas burner, and the other kept cool. Since the late nineteenth century this technique has been used charge storage batteries and more recently to generate emergency power. The system is not energy efficient and is only suitable for low power applications. Modern gas powered batteries based on the Seebeck effect are still available today. They operate over a wide temperature range and are often used in conjunction with solar or wind powered batteries to provide remote or emergency power on dark, windless days. They are also used in spacecraft applications in RTG batteries. (See Nuclear Batteries below)
- (AMTEC) Batteries - Alkali Metal Thermal Electric Converter
Developed in the 1960s, the AMTEC converter is a heat engine which uses a high temperature metallic vapour working fluid. A solid electrolyte separates the electron flow from the ion flow which gives up its energy to the electrons passing through an external load. Energy conversion efficiency is about four times better than thermocouple batteries which makes them more suitable RTG batteries.
There is also a class of high temperature batteries based on conventional chemical or galvanic reactions and these are covered in a separate section on Thermal Batteries.
History See also Clamond's 1874 thermal battery.
Molten Salt - A Thermal Battery
The molten salt battery is designed to capture the thermal energy from thermal solar arrays and release it overnight when the Sun is not shining to generate steam for driving a conventional steam generator and so maintain the energy supply to consumers. To date several demonstrator plants have been constructed but the technology has not yet been widely adopted.
Solar concentrators focus the Sun's thermal radiation and use it directly or indirectly to heat a high thermal capacity salt with a suitable melting point such as potassium nitrate which melts at 370 °C (698 °F). The high temperature salt is stored in an insulated container until its energy is required. Various methods have been devised for extracting the heat. They depend on transferring the heat to water passing through a heat exchanger to create steam. See Solar Collectors and Solar Energy Storage for details of thermal energy capture, storage and release.
Molten salt batteries are suitable for bulk storage for grid applications but their round trip energy efficiency is only around 70%
Ice Battery - A Thermal Battery
The ice battery is a load shifting thermal energy storage application specifically designed for air conditioning systems which stores cooling energy at night, and delivers that energy the following day. It is a modification of a standard air conditioning system to extend its use to allow an alternative usage pattern to take advantage of lower off-peak electricity tariffs during the night.
It consists of a large thermal storage tank that attaches directly to a building’s existing air conditioning system. The compressor works at night rather than during the day using its cooling capacity to make ice which is used to deliver the cooling during the following day.
During its nightly "Ice Charging" mode, the charging system compresses the refrigerant and pumps it through a series of copper coils in a large insulated water tank where it evaporates, freezing the water. When the freezing is complete, the compressor is switched off and the ice is stored until needed. During the daytime, "Ice Cooling" mode, the refrigerant is cooled by the ice and a small, pump pushes it through a modified evaporator coil installed in the conventional air conditioning unit cooling the air.
A reduction on daytime energy consumption of 95% is claimed. Savings depend on ratio between peak and off-peak tariffs.
Opportunities exist for coordinating the timing of the energy load of multiple users on the supply network for optimum load balancing by means of the Smart Grid.
Springs - The Clockwork Battery
Energy is stored in spring which is wound up by a clockwork mechanism. When released, the spring is used to drive a dynamo which provides the electrical power. This is suitable only for low capacity and low power applications and limited by the short duration of the discharge. The discharge period can however be extended by using suitable gearing. The Trevor Bayliss wind-up radio is an example of this method. His clockwork battery produced 3 volts at 55-60 milliwatts giving 40 minutes of play for 20 seconds of winding.
The energy stored in a linear spring is given by the following formula
E = ½ Kx2
Where K is the spring constant (force required per unit extension) and x is the extension of the spring.
Flywheels - The Kinetic Battery
Energy storage in a flywheel is as old as the potters wheel. Slow speed flywheels, combined with opportunity charging at bus stops have been used since the 1950s for public transport applications, however they are very bulky and very heavy and this has limited their adoption.
The energy stored in a flywheel id given by the following formula
E = ½ Iω2
Where I is the moment of inertia of the flywheel (ability of an object to resist changes in its rotational velocity) and ω is its rotational velocity (radians/second).
The moment of inertia is given by
I = kMr 2
Where M is the mass of the flywheel, r its radius and k is its inertial constant.
k depends on the shape of the rotating object. For a
flywheel loaded at rim such as a bicycle wheel or hollow cylinder rotating on its axis, k = 1, for a solid disk of uniform thickness or a solid cylinder, k = ½.
Modern super flywheels store kinetic energy in a high speed rotating drum which forms the rotor of a motor generator. When surplus electrical energy is available it is used to speed up the drum. When the energy is needed the drum provides it by driving the generator. Modern high energy flywheels use composite rotors made with carbon-fiber materials. The rotors have a very high strength-to-density ratio, and rotate at speeds up to 100,000 rpm. in a vacuum chamber to minimize aerodynamic losses. The use of superconducting electromagnetic bearings can virtually eliminate energy losses through friction.
The magnitude of the engineering challenge should not be underestimated. A 1 foot diameter flywheel, one foot in length, weighing 23 pounds spinning at 100,000 rpm will store 3 kWh of energy. However at this rotational speed the surface speed at the rim of the flywheel will be 3570 mph. or 4.8 times the speed of sound and the centrifugal force on particles at the rim is equivalent to 1.7 million G. The tensile strength of material used for the flywheel rim must be over 500,000 psi to stop the rotor from flying apart.
Flywheels are preferred over conventional batteries in many aerospace applications because of the following benefits
Flywheel vs Battery Energy Storage
||Energy Storage Characteristic
5 to 10+ times greater specific energy
|Long life (15 yr.) Unaffected by number of charge/discharge cycles
||Reduced logistics, maintenance, life cycle costs and enhanced vehicle integration
|85-95% round-trip efficiency
||More usable power, lower thermal loads, compared with < 70-80% for battery system
|High charge/discharge rates & no taper charge required
||Peak load capability, 5-10% smaller solar array
Inherent bus regulation and power shunt capability
Fewer regulators needed
Advanced flywheels are used for protecting against interruptions to the national electricity grid.
The flywheel provides power during period between the loss of utility supplied power and either the return of utility power or the start of a sufficient back-up power system (i.e., diesel generator). Flywheels can discharge at 100 kilowatts (kW) for 15 seconds and recharge immediately at the same rate, providing 1-30 seconds of ride-through time. Back-up generators are typically online within 5-20 seconds.
A flywheel storage plant for grid power storage with a capacity of 5MWh, providing a power output of 20 MW for over 15 minutes has been installed at a Beacon Power plant in new York and other large installations are in the pipeline.
Flywheels have also been proposed as a power booster for electric vehicles. Speeds of 100,000 rpm have been used to achieve very high power densities, however containment of the high speed rotor in case of accident or mechanical failure would require a massive enclosure negating any power density advantages. The huge gyroscopic forces of these high speed flywheels are an added complication. Practicalities have so far prevented the large scale adoption of flywheels for portable applications.
Compressed Air - The Pneumatic Battery
Compressed Air Energy Storage (CAES) uses pressurised air as the energy storage medium. An electric motor-driven compressor is used to pressurize the storage reservoir using off-peak energy. During peak times the air is released from the reservoir and mixed with natural gas to drive a gas turbine generator to produce electrical energy. 1 m3 of cavern space can store 5 kWh of energy and minimum pressures are about 1200 psi.
Ideal locations for large compressed air energy storage reservoirs are aquifers (water bearing rock formations), depleted oil and gas wells, conventional mines in hard rock, and hydraulically mined salt caverns. Facilities are sized in the range of several hundred megawatts. Air can also be stored in pressurized tanks for small systems.
A diabatic process is one that occurs with an exchange of heat between a system and its surroundings. (Diabatic from the Greek "passable"). This is typical of conventional CAES systems in which the heat generated by the compression process is lost to the atmosphere reducing the round trip efficiency of the storage process. In the reverse process, as the air expands on its release from storage, its temperature falls causing a further loss of efficiency. The magnitude of expansion envisioned in large CAES expander systems is such that the outflow of the turbine would be nearly cryogenic in nature, making the design of the turbines problematic so the air must be substantially re-heated prior to expansion in the turbine to power the generator.
Round trip efficiency is between 40% and 45% and about 10% more if heat recovery systems are used.
An adiabatic process is one that occurs without transfer of heat between a system and its surroundings. (Adiabatic from the Greek "not passable"). Adiabatic CAES attempts to capture and store the heat energy generated by the compression process as well as the pressure energy, returning it to the air when the air is expanded. The theoretical round trip efficiency approaches 100% but so far no practical systems have been developed.
Small systems have also been used in demonstrator hybrid cars.
Pumped Storage - The Hydraulic Battery
Pumped storage hydroelectricity is another, relatively simple method of storing and producing large amounts of electricity to supply high peak demands. At times of low electrical demand, excess electrical capacity is used to pump water into an elevated reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine, generating electricity . The round trip efficiency loss is around 15% to 25% so that only 75% to 85% of the electrical energy used to pump the water into the elevated reservoir can be regained in this process. Some facilities use abandoned mines as the lower reservoir, but many use the natural height difference between two natural bodies of water or artificial reservoirs. Many pumped storage plants have been installed throughout the world. Dinorwig in Wales is an example with a capacity of 9 GW generating 1800 MW of power.
Superconducting Magnetic Energy Storage (SMES) - The Magnetic Battery
Superconducting magnetic energy storage systems store energy in the field created by direct current flowing in a large magnetic coil cryogenically cooled to a temperature below its superconducting critical temperature when an electrical currents flow without resistance or loss of energy. It can be converted back to AC electric current as needed. Low temperature SMES cooled by liquid helium (Boiling point -268.6 °C (- 451.48 °F) )are commercially available. Recent advance with high temperature SMES cooled by liquid nitrogen (Boiling point -195.79 °C (-320 °F) ) are also becoming available.
SMES systems are physically large. They currently have a high cycle-life and power
density, but low energy density and high cost that make them best suited for supplying short
bursts of electricity into an energy system such as such as the electricity grid where they are used for frequency regulation and power quality management. Superconductors currently have the highest round-
trip efficiency of any storage device, but they are costly to manufacture and maintain.
Radioisotope Thermoelectric Generators (RTG) - The Nuclear Battery
Radioisotope Thermoelectric Generators (RTGs) were designed for space applications and for providing power to remote installations such as lighthouses. Developed in 1959 by the Atomic Energy Commission at Los Alamos and introduced in 1961, these primary batteries are essentially nuclear powered heat generators which use energy emitted by the natural decay of radioactive isotopes of Plutonium (Pu-238) to provide the heat which in turn is used to generate electric power in a thermoelectric generator made from an array of thermocouples. Because the electric energy is created indirectly using the intermediate thermoelectric process the overall conversion efficiency is only about 4%, however the energy density of the radioactive source is thousands of times greater than Lithium Ion batteries. The technology provides long life batteries which never need recharging. Early batteries are still operational after over 25 years.
See more about nuclear energy content.
The direct conversion of nuclear energy into electricity is being developed for low power consumer applications. See Betavoltaic Batteries
Hydrogen Storage (Electrolysers with Fuel Cells)
Electricity can be converted into hydrogen by electrolysis of water. The hydrogen can be then stored and eventually re-electrified in fuel cells. The round trip efficiency today is as low as 30 to 40%. Despite this low efficiency the interest in hydrogen energy storage is growing due to its high storage capacity.
Typical applications are capturing the intermittent electrical energy from solar and wind generators by electrolysis and storing it in the form of hydrogen then using a fuel cell to convert it back to electrical energy in a controlled manner when it is required
Water can be split into its component parts, hydrogen and oxygen, using Proton Exchange Membrane (PEM) technology. When a DC voltage is applied to the electrolyser, water molecules are oxidised at the anode releasing oxygen and electrons leaving protons (H+ hydrogen ions) in the remaining water. The protons pass through the PEM to the cathode where they meet electrons from the external circuit, reducing the protons to hydrogen gas which is released at the cathode.
Small amounts of hydrogen (up to a few MWh) can be stored in pressurized vessels at 100~300 bar or liquefied at 20.3K (-423 deg F). Alternatively, solid metal hydrides or nanotubes can store hydrogen with a very high density. Very large amounts of hydrogen can be stored in man made underground salt caverns of up to 500,000 m3 at 200 bar (2,900 psi), corresponding to a storage capacity of 167 GWh hydrogen (100 GWh electricity).
Hydrogen can be re-electrified in fuel cells with efficiencies up to 60%, or alternatively burned in combined cycle gas power plant.
See more details on the page about Hydrogen Fuelled Electricity Generation.
Storage Capacity and Power Handling Comparisons
The Ragone plot shows the energy storage and power handling capacity of some alternative storage techniques.
See more Ragone Plots in the Performance section
See also History 100 Battery Types