# Energy storage: Wikis

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# Encyclopedia

Energy storage media are matter that store some form of energy that can be drawn upon at a later time to perform some useful operation. A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (eg. chemical, gravitational or electrical energy) or kinetic energy (eg. thermal energy). A wind up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to keep a clock chip in a computer running (electrically) even when the computer is turned off, and a hydroelectric dam stores power in a reservoir as gravitational potential energy. Ice storage tanks store ice (thermal energy) at night to meet peak demand for cooling. Fossil fuels such as coal and gasoline store ancient energy from sunlight. Even food (which is made by the same process as was fossil fuel) is a form of energy stored in chemical form.

## History

Energy storage as a natural process is as old as the universe itself - the energy present at the initial formation of the Universe has been stored in stars such as the Sun, and is now being used by humans directly (e.g. through solar heating), or indirectly (e.g. by growing crops or conversion into electricity in solar cells). Storing energy allows humans to balance the supply and demand of energy. Energy storage systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, biological, thermal and nuclear.

As a purposeful activity, energy storage has existed since pre-history, though it was often not explicitly recognized as such. An example of deliberate mechanical energy storage is the use of logs or boulders as defensive measures in ancient forts - the logs or boulders were collected at the top of a hill or wall, and the energy thus stored used to attack invaders who came within range.

A more recent application is the control of waterways to drive water mills for processing grain or powering machinery. Complex systems of reservoirs and dams were constructed to store and release water (and the potential energy it contained) when required.

Energy storage became a dominant factor in economic development with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural gas in the late 1800s. Unlike other common energy storage used in prior use, such as wood or coal, electricity has been used as it has been generated. It has not been stored on a major scale but that may soon change. In the U.S, the 2009 Stimulus plan is researching energy storage and how it may be used with the new plans for a Smart Grid.[1]. Electricity is transmitted in a closed circuit, and for essentially any practical purpose cannot be stored as electrical energy. This means that changes in demand could not be accommodated without either cutting supplies (as by brownouts or blackouts) or by storing the electric energy in another medium. Even renewable energy must be stored in order to make it reliable. Wind blows intermittently and so some form of storage is required to compensate for calm periods, and solar energy is not effective on cloudy days so stored energy must be available to compensate for the loss of sun energy.

An early solution to the problem of storing energy for electrical purposes was the development of the battery, an electrochemical storage device. It has been of limited use in electric power systems due to small capacity and high cost. A similar possible solution with the same type of problems is the capacitor. In the 1980s, a small number of manufacturers carefully researched thermal energy storage (TES) to meet the growing demand for air-conditioning during peak hours. Today a few companies continue to manufacture TES.[2] The most popular form of thermal energy storage for cooling is ice storage, since it can store more energy in less space than water storage and it is also cheaper than fuel cells & flywheels. Thermal storage has shifted gigawatts of power away from daytime peaks, cost-effectively, and is used in over 3,300 buildings in over 35 countries. It works by storing ice at night when electricity is cheap, and then using the ice to cool the air in the building the next day.

Chemical fuels have become the dominant form of energy storage, both in electrical generation and energy transportation. Chemical fuels in common use are processed coal, gasoline, diesel fuel, natural gas, liquefied petroleum gas (LPG), propane, butane, ethanol, biodiesel and hydrogen. All of these materials are readily converted to mechanical energy and then to electrical energy using heat engines (turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical power generation. Heat-engine-powered generators are nearly universal, ranging from small engines producing only a few kilowatts to utility-scale generators with ratings up to 800 megawatts.

Electrochemical devices called fuel cells were invented about the same time as the battery. However, for many reasons, fuel cells were not well-developed until the advent of manned spaceflight (the Gemini Program) when lightweight, non-thermal (and therefore efficient) sources of electricity were required in spacecraft. Fuel cell development has increased in recent years due to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity.

At this time, liquid hydrocarbon fuels are the dominant forms of energy storage for use in transportation. However, these produce greenhouse gases when used to power cars, trucks, trains, ships and aircraft. Carbon-free energy carriers, such as hydrogen, or carbon-neutral energy carriers, such as some forms of ethanol or biodiesel, are being sought in response to concerns about the possible consequences of greenhouse gas emissions.

Some areas of the world (Washington and Oregon in the USA, and Wales in the United Kingdom are examples) have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up to the reservoirs, then letting the water fall through turbine generators to retrieve the energy when demand peaks.

Several other technologies have also been investigated, such as flywheels or compressed air storage in underground caverns.

Another method used at the Solar Project and the Solar Tres Power Tower uses molten salt to store solar power and then dispatch that power as needed. The system pumps molten salt through a tower heated by the sun's rays. Insulated containers store the hot salt and when needed, water is used to create steam that turn turbines to generate electricity.

## Grid energy storage

The upper reservoir (Llyn Stwlan) and dam of the Ffestiniog Pumped Storage Scheme in north Wales. The lower power station has four water turbines which generate 360 MW of electricity within 60 seconds of the need arising. The size of the dam can be judged from the car parked below.

Grid energy storage (or large-scale energy storage) lets energy producers send excess electricity over the electricity transmission grid to temporary electricity storage sites that become energy producers when electricity demand is greater. Grid energy storage is particularly important in matching supply and demand over a 24 hour period of time.

## Storage methods

### Hydrogen

Hydrogen is also being developed as an electrical power storage medium. Hydrogen is not a primary energy source, but a portable energy storage method, because it must first be manufactured by other energy sources in order to be used. However, as a storage medium, it may be a significant factor in using renewable energies. See hydrogen storage.

Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen are stored in underground caverns by ICI for many years without any difficulties[3]. The storage of large quantities of hydrogen underground can function as grid energy storage which is essential for the hydrogen economy. By using a turboexpander the electricity needs for compressed storage on 200 bar amounts to 2.1% of the energy content[4].

With intermittent renewables such as solar and wind, the output may be fed directly into an electricity grid. At penetrations below 20% of the grid demand, this does not severely change the economics; but beyond about 20% of the total demand, external storage will become important. If these sources are used for electricity to make hydrogen, then they can be utilized fully whenever they are available, opportunistically. Broadly speaking, it does not matter when they cut in or out, the hydrogen is simply stored and used as required. A community based pilot program using wind turbines and hydrogen generators is being undertaken from 2007 for five years in the remote community of Ramea, Newfoundland and Labrador.[5] A similar project has been going on since 2004 on Utsira, a small Norwegian island municipality.

Energy losses are involved in the hydrogen storage cycle of hydrogen production for vehicle applications with electrolysis of water, liquification or compression, and conversion back to electricity.[6 ] and the hydrogen storage cycle of production for the stationary fuel cell applications like microchp at 93 %[7] with biohydrogen or biological hydrogen production, and conversion to electricity.

About 50 kWh (180 MJ) of solar energy is required to produce a kilogram of hydrogen, so the cost of the electricity clearly is crucial, even for hydrogen uses other than storage for electrical generation. At $0.03/kWh, common off-peak high-voltage line rate in the U.S., this means hydrogen costs$1.50 a kilogram for the electricity, equivalent to \$1.50 a US gallon for gasoline if used in a fuel cell vehicle. Other costs would include the electrolyzer plant, hydrogen compressors or liquefaction, storage and transportation, which will be significant.

### Biofuels

Various biofuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to replace hydrocarbon fuels. Various chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal biomass, and organic wastes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. Examples are Fischer-Tropsch diesel, methanol, dimethyl ether, or syngas. This diesel source was used extensively in World War II in Germany, with limited access to crude oil supplies. Today South Africa produces most of country's diesel from coal for similar reasons.[8] A long term oil price above 35 USD may make such synthetic liquid fuels economical on a large scale (See coal). Some of the energy in the original source is lost in the conversion process. Historically, coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. And compressed natural gas is being used in special circumstances fuel, for instance in busses for some mass transit agencies.

### Synthetic hydrocarbon fuel

Carbon dioxide in the atmosphere has been, experimentally, converted into hydrocarbon fuel with the help of energy from another source. To be useful industrially, the energy will probably have to come from sunlight using, perhaps, future artificial photosynthesis technology.[9][10] Another alternative for the energy is electricity or heat from solar energy or nuclear power.[11][12] Compared to hydrogen, many hydrocarbon fuels have the advantage of being immediately usable in existing engine technology and existing fuel distribution infrastructures. Manufacturing synthetic hydrocarbon fuel reduces the amount of carbon dioxide in the atmosphere until the fuel is burned, when the same amount of carbon dioxide returns to the atmosphere.

#### Methane

Methane is the simplest hydrocarbon with the molecular formula CH4. Methane could be produced from electricity of renewable energies. Methane can be stored more easily than hydrogen and the transportation, storage and combustion infrastructure are mature (pipelines, gasometers, power plants).

As hydrogen and oxygen are produced in the electrolysis of water,

2H2O → 2H2 + O2

hydrogen would then be reacted with carbon dioxide in Sabatier process, producing methane and water.

CO2 + 4H2 → CH4 + 2H2O

Methane would be stored and used to produce electricity later. Produced water would be recycled back to the electrolysis stage, reducing the need for new pure water. In the electrolysis stage oxygen would also be stored for methane combustion in a pure oxygen environment in an adjacent power plant, eliminating e.g. nitrogen oxides. In the combustion of methane, carbon dioxide and water are produced.

CH4 + 2O2 → CO2 + 2H2O

Produced carbon dioxide would be recycled back to boost the Sabatier process and water would be recycled back to the electrolysis stage. The carbon dioxide produced by methane combustion would be turned back to methane, thus producing no greenhouse gases. Methane production, storage and adjacent combustion would recycle all the reaction products, creating a cycle.

### Boron, silicon, and zinc

Boron,[13] silicon,[14] lithium, and zinc[15] have been proposed as energy storage solutions.

### Mechanical storage

Energy can be stored in water pumped to a higher elevation using pumped storage methods, in compressed air, or in spinning flywheels.

1 kg mass elevated to 1000m can store 9.81 kJ energy. This is equivalent with 1 kg mass accelerated to 140m/sec. 1 kg water's temperature can be elevated by 2.34 Celsius using the same amount of energy. Admittedly, this is a bit of an unfair comparison, but it makes it easy to see how it is possible to store more energy in 1 m3 of cheap rock or sand than 1m3 of Lead-acid battery, even if the battery is also moved to a higher elevation, not just charged.

Compressed air energy storage technology stores low cost off-peak energy, in the form of compressed air in an underground reservoir. The air is then released during peak load hours and heated with the exhaust heat of a standard combustion turbine. This heated air is converted to energy through expansion turbines to produce electricity. A CAES plant has been in existence in McIntosh, Alabama since 1991 and has run successfully. Other applications are possible. Walker Architects published the first CO2 gas CAES application, proposing the use of sequestered CO2 for Energy Storage on October 24 of 2008.

Several companies have done preliminary design work for vehicles using compressed air power.[16][17]

### Renewable energy storage

Many renewable energy systems produce intermittent power. In this case, energy storage becomes absolutely necessary to provide firm energy supplies using intermittent sources such as wind or solar power. Further development of renewable power will require some combination of grid energy storage, demand response, and spot pricing. Intermittent energy sources is limited to at most 20-30% of the electricity produced for the grid without such measures. If electricity distribution loss and costs are managed, then intermittent power production from many different sources could increase the overall reliability of the grid.

Non-intermittent renewable energy sources include hydroelectric power, geothermal power, solar thermal, tidal power, Energy tower, ocean thermal energy conversion, high altitude airborne wind turbines, biofuel, and solar power satellites. Solar photovoltaics, although technically intermittent, produce some electricity during peak periods (i.e., daylight), and hence do reduce the need for peak power generation. In general, peak demand periods for power in some locations do not correspond with peak availability of solar energy, which motivates producers to develop new and more effective methods of energy storage and recovery.

On the demand side, demand response programs which send market pricing signals to consumers (or their equipment), can be a very effective way of managing variations in electricity production. For example, intelligent energy storage devices can be set to store energy when electricity is being produced beyond current demand (and prices are lowest), and conversely, and set to distribute energy when demand is high (and prices are highest.) This practice is called energy arbitrage.

### Thermal storage

Thermal storage is the temporary storage or removal of heat for later use. An example of thermal storage is the storage of solar heat energy during the day to be used at a later time for heating at night. In the HVAC/R field, this type of application using thermal storage for heating is less common than using thermal storage for cooling. An example of the storage of "cold" heat removal for later use is ice made during the cooler night time hours for use during the hot daylight hours. This ice storage is produced when electrical utility rates are lower. This is often referred to as "off-peak" cooling.

When used for the proper application with the appropriate design, off-peak cooling systems can lower energy costs. The U.S. Green Building Council has developed the Leadership in Energy Efficiency and Environmental Design (LEED) program to encourage the design of high-performance buildings that will help protect our environment. The increased levels of energy performance by utilizing off-peak cooling may qualify of credits toward LEED Certification.

The advantages of thermal storage are: Commercial electrical rates are lower at night. It takes less energy to make ice when it is cool at night. Source energy (energy from the power plant) is saved. A smaller, more efficient system can do the job of a much larger unit by running for more hours.

[18] More information on Thermal Storage See [19][20] [21]

# Study guide

Up to date as of January 14, 2010

### From Wikiversity

In this course we cover the basic physics behind energy storage, the important characteristics to consider when thinking about or discussing energy storage and then cover all the current technologies. This is followed by an examination and its uses in society including its benefits and leading on into the environmental impacts. The final section covers the use of energy storage in biological systems, demonstrating as always that nature usually gets there first.

## Introduction

Energy storage first became an issue with the introduction of electricity because there was no easy way to store electrical power. Therefore you had to use it when you produced it. By far the most successfull energy storage for electricity in significant quantities is via pumped storage. With this method you pump water up a hill to a lake and relase it when you want. However the number of sites is very limited.

As the era of cheap energy comes to a close due to Peak oil and to be followed by Peak gas, then the real costs in energy escalates and it is becoming more imperative to be able to store energy and cut down on wastage. This is especially the case with electrical power because if it is not used, it simply goes to waste. Up to now the abundant and easily accessible supplies of plentiful fossil fuels have effectively subsidized all other energy sources, thereby under valuing them and providing little incentive to use energy sparingly.

Further research, invention and improvement of energy storage methods and technologies can help make new energy sources such as renewable energy more practical and economical and energetically possible.

Energy storage is not just confined to large scale energy supply but there is also the whole array of battery technologies which are widespread and ubiquitous. These can be found in every single car and truck and in most consumer electronic products. Without battery technology, probably none of these would be possible.

A significant advance in energy storage technologies would most likely represent a significant technological shift and herald in a whole range of possibilities for society at large. Recognition of this fact may explain the massive interest in High Temperature Superconductors which appeared in the late 1980s. However progress has been relatively slow since then proving the point that major break throughs don't really happen overnight and any technology nearly always takes 20 or 30 years to mature.

## Basic Physics

Basic Physics for Energy Storage

From the Law of the Conservation of Energy in physics, we know that energy can be neither created or destroyed but it can be converted from one form into another. Overall though the energy is becoming more diluted or spread out as the universe expands. This can be best thought of in the way that the light from the sun is spread out into space and the way that heat dissipates into the environment.

The Second Law of Thermodynamics sets a theoretical limit on the amount of useful energy that can be extracted from any heat engine and the figure turns out to be around 66%.

However not all forms of energy conversion are subject to this. For example potential energy and kinetic energy are be converted in theory between each other with no losses.

With energy storage we are interested in what are the basic underlying forms that we can storage energy and given the physics what does this permit us to do and what limitations it imposes on us. The governing principles also affect how energy storage systems are designed and will also determine things like energy quantity, density and even ease of use.

The materials that are used in any energy storage device will also impact on what is possible or achievable. Depending on the method used, this could depend on any one or a combination of desnity, strength, electrical, magnetic and optical properties including numerous other physical parameters.

### Types of Energy Storage

#### Potential Energy

This is the energy of an object due to its position in force field such as a gravity field or electric field. Here, we will only consider a gravitional field as the case of an electric potential is discussed later in the section on electrical energy.

As anyone knows to raise an object higher takes energy and when it is allowed to fall, it release that energy into the form of moment or kinetic energy. A classic example is a marble released from the top of a smooth frictionaless bowl. It will roll that to the bottom increasing its kinetic energy all the way and roll up the other side to almost a step. In a frictionaless environment this activity should continue indefinitely. It is evident that there is a smooth switch over from 100% potential energy to kinetic energy and back.

The equation for the potential energy of an object is actually quite simple. It is

$U_g = mgh \,$

where

m is the mass of the object
g is the standard gravity (approximately 9.8 m/s2 at the earth's surface)
h is the height to which the object is raised, relative to a given reference level (such as the earth's surface).

If we are interested in energy storage, then both the quantity of mass and the height are important. There is nothing we can do about g because it is effectively constant near the surface of the Earth.

#### Kinetic Energy

This is the energy inherent in a moving object and is described by the famous equation:

$E_k = \frac{1}{2}mv^2$

where E is the Kinetic Energy often donated as Eke, m is the mass of the object and v is the velocity (or speed).

It is immediately obvious as the velocity is increased the embodied energy rises rapidly. Harnessing the kinetic energy of an object travelling in a straight line is difficult. For example think of trying to capture the energy in a bulletin and successfully converting 100% of that energy to say electrical power. Kinetic energy also applies to rotating objects and the governing equation is slightly different. The capture and conversion of that energy though is a lot easier.

##### Rotational Kinetic Energy

The equation for the rotational kinetic energy is of the same form of the above except it is slightly different. It is:

$E_{rotation} = \frac{1}{2} I \omega^2$

where

I is the moment of Inertia given by I = mr2 where m is the mass and r is the radius.
ω is the angular velocity given by v/r where v is the rotational velocity and r is the radius about which the object is rotating.

This is just a simplified explanation because for example in reality where you might have a rotating cylinder, to correctly calculate the moment of inertia, you have to effectively sum up the mass times the radius2 over all parts of the radius from the centre out to the edge.

The key point to note though is that the faster your object rotates the more energy it has and since the moment of inertia is dependent on the radius2, then as the mass is moved further away from the axis of rotation, then the quicker the energy rises. Think of a block of concrete rapidly rotating on the end of a long metal arm and it will be clear and this has a lot of kinetic rotational energy

#### Electrical Charge

Energy stored as electric charge is really a form of potential energy, except it's electrical potential energy. The presence of a charge sets up a voltage potential difference which can be used at some point to attract electrons and thereby make current flow. When storing energy in this way, one needs to accumulate charge on a surface and then keep it insulated or isolated so that the charge does not leak away.

A good example of a device for storing energy as electric charge is a capacitor. This is a common circuit component very widely used in circuit designs of all types. Typical capacitors however store tiny amounts of energy. Capacitance is measured in farads and your average capacitor is rated anywhere from a few microFarad (µF) to as low as a few nanoFarads (nF) or even a picoFarad (pF).

A capacitor can be thought of as two parallel plates separated by an insulator. The equation for capacitance is:

$C = \epsilon \frac{A}{d}$

where

C is the capacitance in farads, F
ε is the permittivity of the insulator used (or ε0 for a vacuum)
A is the area of each plate, measured in square metres
d is the separation between the plates, measured in metres

Therefore it is immediately obvious that to increase the capacitance, the area needs to be increased while the distance separating the plates decreased.

The equation for the energy stored in a capacitor is:

$E_{stored} = \frac{1}{2} C V^2 = \frac{1}{2} \epsilon \frac{A}{d} V^2$ .

where

E is the energy measured in joules
C is the capacitance, measured in farads
V is the voltage measured in volts

To increase the energy stored, the fastest way to do this is by increasing the voltage. Unfortunately the material properties get in the way of increasing this indefinitely and if raised too high, the device will suffer voltage breakdown. Early capacitors would have used air gaps between the plates, but the dielectric value for many insulators is actually higher than that of air. Therefore modern capacitors uses novel materials with high dielectric constants to enable them to use higher voltages.

In practice high capacitance is achieved by rolling up the plates or sheets into cylindrical and other shapes. More recent designs have tended towards porous type structures which are able to achieve huge surface areas in tiny volumes.

#### Magnetic Energy

Any electric current will always create a magnetic field around it, and an changing magnetic field can induce an electric current in any nearby electrical conductor. Thus a wire carrying electrical current will also have a tiny magnetic field associated with it. By coiling the wire around in a cylindrical shape or like a coiled spring, the strength of this magnetic field can be increased and also made to self induct. This is the basis of electrical inductors which are common electrical components found in many types of circuits particulary those used for radio receivers and transmitters.

The energy stored in a magnetic field is:

$E=\frac {1}{2}\cdot L\cdot I^2$

Where

E = energy measured in joules
L = inductance measured in henries, which are units of magnetism.
I = current measured in amperes

The problem with magnetic energy and the electro magnets created by passing current through inductors is that current must be continually added in. This is because your typical copper wire is not completely resistance free, although it is very small. If one were to loop back both ends of an inductor after injecting lots of current into the wire, the current would soon die away and all that energy would be wasted.

Superconductors, unlike normal conductors have essentially zero electrical resistance and is of the order of 10-24 ohms (or even lower and in the idealized scenario outlined above, the current would continue to circulate in the loop for millions of years. Clearly then superconductors are the saviours for devising a means to store energy in magnetic fields.

The problem with superconductors is that they have to be cooled to extremely low temperatures, making them expensive and quite impractical. An additional and important problem is that each type of superconductor has a threshold magnetic field in which it can reside. As the magnetic field strength is increased, this serves to increase the temperature until suddenly it is back above the critical threshold between a superconductor and a normal conductor and thereby limiting the capacity of a given superconductor system.

Overall though from the equation above, it can be seen that the most important parameter for raising the amount of energy stored is the strength of the magnetic field.

#### Chemical Energy

Chemical energy is the most diverse of the various energy storage mechanisms and it is the energy stored in setting up certain higher energy chemical bonds. A single atom can actually have one of its orbiting electrons raised temporarily to a higher energy level, but typically it will fall back to the lower energy level very rapidly. In fact this is the basis of flouresence.

In this discussion we only consider the chemical bonds between atoms as these are far more stable and long lasting. For the chemical energy to be released there must be a new state for the molecule to fall energetically down to. For example the methane molecule given by the chemical formula below can combine with oxygen to form carbon dioxide and water. Actually you need two molecules of each to get a perfect burn and for everything to balance.

2 CH4  + 2 O2 → 2CO2 + 2 H2O

Looking at chemical energy from physical first principles it is really a form of electrical potential energy because each of the various electrons in the atoms of the molecule are at a higher potential energy and will release energy by falling to a lower potential energy.

Luckily for us all the molecules around do not fall automatically to these lower states and this is for a variety of reasons, the main one being that in nearly all cases you need to first raise the energy of the molecule to get it to go to the lower state and form different types of molecules. This is known as the energy of activation. If that was not the case then everything flammable would immediately burst into flames. As we know to burn paper or wood, we first must bring it up to the ignition or activation temperature.

Because of the enormous number of possible types of molecules that can be created, this gives rises to a huge range of molecules capable of storing energy due to their inherent chemical properties. For example some typical fuels that store considerable energy are ethanol, methane, butane, ethane, gasoline and hydrogen.

Explosives also store lots of energy except they release it rather quickly.

To create an energy storage system that makes use of chemical energy, you need a mechanism to pump energy into the system and raise the energy content. Splitting water molecules by electrolysis into their consitutent elements of hydrogen and oxygen requires energy. When you recombine them again through burning, you get back that energy again. Since no system is perfect it will always be less than 100% efficient, so you will lose a bit of energy in the process. A good rule of thumb is that the more chemical steps the lower the overall system efficiency.

Note: Natural gas is largely made up of methane. There will be traces amounts of other gases usually present.

#### Thermal Energy

In its simplest form thermal energy is the energy that has been put into an object or liquid to make it warm or hot. In its application to energy storage one is usually heating something and keeping it at that temperature to save the effort of doing it later and use energy at that time that may be unavailable. An example is the thick walls of a building heating up in the sun and releasing that heat later on during the night when that energy is not available.

In terms of physics though, thermal energy represents the vibrational energy of all the molecules of your substance or object. In a liquid and a gas it also includes the convectional flows. These vibrations will damp down and the object will cool if there is somewhere for the energy to escape too. An object can release its thermal energy by transferring to through contact to other objects and by radiating out infrared radiations. The infrared comes about because the electron orbitals and the molecules themselves are vibrating and rotating. They can move to lower energy levels resulting in less vibration by releasing infrared photons -i.e. infrared radiation.

If you can seal off the object and perfectly insulate it, then you can store that thermal energy indefinitely. In practice this is very hard to do. Storing hot tea in a flask for 3 or 4 hours is good enough for most applications involving hot tea because one is unlikely to want to store it for days or weeks. For energy storage applications, the better the insulation and the quantity of thermal energy stored then the more uses and applications can be made.

When performing engineering analysis of heat flows[[Heat_Transfer]b:Heat_Transfer] there are three primary mechanisms typically considered.

• Radiation - All mass radiates heat based upon its internal absolute temperature. This is called Blackbody Radiation and is calculated precisely by a simple mathematical formula. Since heat radiated is proportional to the fourth power of the absolute temperature warmer mass radiates a lot more energy than colder mass and the net flow of energy is always from the warmer mass to colder mass.
• Conduction - Heat flows through continuous solid materials by molecular vibrations being transferred between constituent atoms and molecules in the solid. This can be calculated from the temperature potential difference or range just as in electric circuits. Heat always flows from warmer areas to colder areas.
• Convection - In liquids and gases heat is carried to or from an immersed solid or energy source or sink by the gas or liquid expanding or contracting from the temperature change. The difference in density then results in the liquid or gas moving from original position due to different gravity forces upon differing temperature areas in the fluid. As the gas moves it carries heat with it which is easily calculated from the temperature and the heat capacity of the fluid. An extreme example of convection is boiling where the newly created steam moves from bottom of boiling water pan in contact with heat source to the surface of the fluid ... carrying all the heat from the heat source through the fluid (which remains at constant 212 deg Fahrenheit) to the atmosphere.

## Technologies

In this chapter all of the main energy storage technologies are outlined and explained in terms of the basic physics that they are taking advantage of. For consideration of their usage see the next chapter.

### Compressed Air Storage

This technology, sometimes referred to by the longer title Compressed Air Energy Storage (CAES) uses the difference in pressure to effectively raise the potential energy of air to store energy. Since air is not a very dense substance large quantities of it must be compressed and typically this means storing the compressed air in a sealed underground cavern.

As of 2007, there were in fact only two such installation but with others under construction. The first was the Huntorf plant, located in North Germany built in 1978 and this can produce 300 MW for 2 hours, whilst the second is the McIntosh plant in Alabama, USA. This plant stores Natural Gas with the air and burns this off when it is released to generate additional power over and above that due to the pressure difference allow.

### Dams

A hydroelectric dam is using the exact same physics as a pumped storage hydro scheme to generate power which is by using the high pressure flow of the water to turn the turbines of a large electric motor. The difference of course is that we rely on nature to effectively pump the water back up behind the dam in the form of rain (and snow) in the catchment area of the river. In both cases though the storage of water behind the dam represents stored potential energy

### Batteries

The above new page should cover at least these battery types.

#### Vanadium redox battery

For the moment, see *Vanadium_redox_battery (wiktionary | wikipedia | wikibooks)

### Fuels

The above new page should cover at least these sub categories of fuel below

#### Oils and Gasoline

Fossil hydrocarbons are ideal. Though neither clean nor safe nor sustainable, they were previously cheap and possess a very high energy density. Natural gas and hydrogen powered vehicles both suffer from the same weaknesses. Both are very explosive gases that require a great deal of energy to compress into a portable form. While both provide clean energy through a fuel cell, neither are very energy dense even after compression to a liquid. Natural gas is currently cheap but unsustainable and hydrogen is sustainable though not particularly cheap.

Biodiesel is safe, cheap and fairly clean for a combustion fuel, very energy dense and thus quite portable. Unfortunately biodiesel is not sustainable since the fuel actually comes from processed biomass.

A synthetic hydrocarbon is essentially a fuel like ethanol or methane (natural gas), but one that has been chemically synthesized and not mined. Since liquid hydrocarbon fuels are the most energy dense and easy to transport of the new storage ideas and since they have a well established infrastructure already in place, they seem to be the best solution. Unfortunately liquid hydrocarbons like LNG, methanol and ethanol are explosive and toxic.

The ideal energy source would be clean, efficient, portable, sustainable and safe. If we intend to synthesize a high energy density hydrocarbon, why not produce a non-toxic substance. The class of hydrocarbons comprises not only explosive fuels (alkanes), but fats and sugars too. If we synthesize a non-toxic, even edible fuel, then all environmental factors may essentially be waived.

#### Hydrogen

Hydrogen is often touted as the next great energy source. However hydrogen is not a fuel at least on Earth, where it is not found in its free form. Most hydrogen is bound up in the water molecules (H2O) and to free it requires adding energy to break the chemical bonds. It is only then that hydrogen can be burnt with oxygen back into water and thereby release the energy again as it falls to lower energy state again.

Any number of techniques can be used to split water ranging from electrical hydrolysis to illuminating water or steam with ultraviolet light. Heating to very high temperatures can also work. In all cases though it is crucial that the water splitting process is carried out efficiently so that total overall system efficiency is kept high.

The storage of hydrogen itself presents problems because the molecule itself is very small and it can easily diffuse through thin metal structures. Additionally hydrogen remains in the gaseous state down to very low temperatures and to store sufficient quantities, it needs to be both cooled and kept at high pressures. Therefore it is necessary to use fairly dense and thick metal casings.

Lastly when hydrogen burns the flame is invisible making it even more hazardous.

Currently hydrogen's main energy storage usage today is as a liquid fuel for rockets such as the Space Shuttle, but for everyday usage it is still quite impractical and expensive

### Supercapactors

These are often also known as ultracapacitors.

### Thermal Energy

There are a number of technologies, some still experimental that are used for thermal energy storage. In some cases this method can be used as a proxy for storing electricity.

The criteria to consider are sizing, costing and effectiveness and one needs to take these into consideration when selecting a particular technology.

#### Graphite

There was a lot of work done with high purity graphite in the nuclear industry and it is quite a good material especially for high temperatures where it exhibits increasing heat capacity with temperature.

This form of graphite sometimes known as crystalline graphite is relatively expensive but then carbon itself is very abundant. In a renewable energy setting, it would be best used if place at the focus point of high temperature solar concentrating systems such as ones using arrays of mirrors.

#### Hot Rocks

Rocks can be used for storing low grade heat such as solar thermal energy absorbed during the day and for releasing that for heating during the night. A good example of this in action can be witnessed in any of the large cathedrals in Europe which were built out of huge blocks of rock often tens of feet or more thick. During the day these buildings remain cool even in the hot sun, and at night remain relatively warm even though temperatures can have dropped significantly outside.

In recent years there have been numerous buildings, mainly houses, built that have a bed of rocks in the basement which release the heat at night. Some of these system used semi-active or passive means to direct the flow of warm air over these rocks during the day to capture the heat.

The chief advantage of using rocks for thermal energy storage is that they are abundant and cheap.

## Research Areas

Areas of Research in Energy Storage

This section covers the current research areas in energy storage

## Energy Storage in Biological Systems

This covers energy storage in Glucose, Starch, Fats and ATP.