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Alternative Energy Storage Methods

 

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A brief diversion

Several non chemical energy storage techniques have been developed over the years, mostly for very high power applications and while all of them have been used in practical systems, apart from capacitors, there has been slow take up of the ideas up to now. Some examples are given here.

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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.

 

Capacitance

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=ε0εr A

             d

where

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.

 

Energy Storage

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:

 

Corner

Capacitor / Battery Comparison

Corner

Device

Energy density
Wh/L

Power density
W/L

Cycle life
Cycles

Discharge time
Seconds

Batteries

50-250

150

1 - 103

> 1000

Capacitors

0.05 - 5

105 - 108

105 - 106

<1

 

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 metalic 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

 

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.

History

 

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

Corner Energy Storage Characteristic Resulting Benefits Corner

5 to 10+ times greater specific energy

Lower mass
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
Deterministic state-of-charge Improved operability

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.

 

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.

 

History

 

Compressed air - The Pneumatic Battery

Compressed Air Energy Storage (CAES) uses pressurized air as the energy storage medium. An electric motor-driven compressor is used to pressurize the storage reservoir using off-peak energy and air is released from the reservoir through a turbine during on-peak hours 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 be stored in pressurized tanks for small systems.

 

Small systems have also been used in demonstrator hybrid cars.

History

 

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 about 30% so that only 70% 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 generating 1320 MW of power.

History

 

Superconducting Magnetic Energy Storage (SMES) - The Magnetic Battery

Superconducting magnetic energy storage systems store energy in the field of a large magnetic coil with direct current flowing. It can be converted back to AC electric current as needed. Low temperature SMES cooled by liquid helium is commercially available. High temperature SMES cooled by liquid nitrogen is still in the development stage and may become a viable commercial energy storage source in the future.

SMES systems are large and generally used for short durations, such as utility switching events.

History

 

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

 

History

 

Comparisons

The Ragone plot shows the energy storage and power handling capacity of some alternative storage techniques.

Ragone Plot

 

See more Ragone Plots in the Performance section

See also History 100 Battery Types

 

 

 

 

 

 

 

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