How Energy Storage Works

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By Nicholas Brown.

Table of Contents

  1. Definitions
  2. Understanding Energy Storage Units
  3. Energy-to-Weight Ratio/Gravimetric Energy Density
  4. Power-to-Weight Ratio
  5. Energy-to-Size Ratio/Volumetric Energy Density
  6. Energy Storage Efficiency
  7. Importance of Affordable Energy Storage to Power Plants
  8. Batteries
  9. State of Charge (SOC),  Depth of Discharge (DOD), and Their Importance
  10. Lithium-ion Batteries
  11. Extending the Life of Li-ion Batteries
  12. Lead-acid
  13. Using and Charging Lead-acid Batteries
  14. NiMH and NiCd Batteries
  15. Anticipated Battery Technology
  16. Sustainability of Lithium-Ion Batteries
  17. Fuel Cells
  18. Ultracapacitors/Supercapacitors
  19. Thermal Energy Storage
  20. FES (Flywheel Energy Storage)
  21. Pumped Hydroelectric Storage (Otherwise known as pumped hydro)
  22. CAES (Compressed Air Energy Storage)
  23. Power Plant Load Balancing
  24. Backup Generators

Definitions

Energy storage may be defined as the process of storing energy in any form for later use. An energy storage system may be defined as any medium which stores energy in any form such as chemical (batteries), thermal, mechanical (flywheel), electrical (capacitor), or another type of energy (in the form of compressed air, for instance) for use at another time.

Energy storage capacity is most frequently measured in Wh (Watt-hours).

A lithium-ion battery with an energy storage capacity of 1,000 watt-hours can supply 1,000 watts of power for a period of an hour or 1 watt for 1,000 hours. Some types of 1,000 Wh batteries cannot actually supply 1,000 watts for one hour without overheating and/or wasting energy.

Example: A tv has a power consumption of 100 watts, and you want to power it with the li-ion battery mentioned above. It could power the tv for a maximum of 10 hours. If the example tv was two hundred watts, then the battery could power it for only 5 hours.

Simplified analogy: Imagine a 1,000 litre tank as a battery. The tank has an outlet and an inlet for water, but a pump is required to pump the water out at a reasonable speed. The tank has to be refilled with water periodically as the battery has to be charged, and the pump can only pump water out at a rate of 100 litres per hour. Think of the water as energy, filling the tank as charging the battery, and the water being pumped out as electric power.

Batteries can discharge (produce) electricity at a certain rate that cannot be exceeded (otherwise they would overheat) unless their technology is improved.


Energy Density/Specific Energy/Energy-To-Weight Ratio

Energy-to-weight ratio is actually gravimetric energy density. This is defined as the amount of energy that an energy storage medium can store per kilogram of energy storage medium mass. In other words: Energy density/energy to weight ratio is the amount of energy that an energy storage system can store per kg of batteries, capacitors, or other energy storage mediums such as compressed air tanks (including the air), as well as pumped hydroelectric storage tanks (including the water).

This is not to be confused with power-to-weight ratio nor energy-to-size ratio. Please keep in mind that energy is the capacity of a physical system to do work and power is the rate at which work is done or the rate at which one form of energy is converted into another. Energy is storable, power is not. 

Also: Wh/kg is a measure of specifically gravimetric energy density.

This means that a 1 kg battery with an energy density of 100 Wh/kg, or a set of batteries like that with a combined weight that totals 1 kg would have a storage capacity of 100 Wh. This means that it could supply 1 watt for 100 hours, or 2 watts for 50 hours (yes, this is the same watt unit that you are accustomed to seeing on appliances).

This also enables you to determine how many kg or pounds of batteries you may need for a particular project. This matters to electric vehicles and anything else that is portable and needs to be lightweight. Lighter = more efficient. When an energy storage medium such as a lead-acid battery for example is referred to as "heavy", that is because of a low energy density.

I will use the Tesla Roadster battery bank as an example:

A battery bank/battery pack may be defined as a set of batteries which are connected to each other in either series or parallel to achieve a certain voltage and/or capacity.

Weight: 453 kilograms.

Capacity: 53,0000 Watt-hours.

Energy Density: 53000 / 453 = 117 Watt-hours per kilogram.

Power Density/Specific Power/Power-To-Weight Ratio

This is defined as power output per kg of mass (per kg of the energy storage medium. for example: per kg of batteries). It can be calculated by dividing power output by weight. Power density is measured in W/kg (watts per kg). It is how many watts of power a 1kg battery can produce.

Power is the rate at which work is done and energy is the capacity of a physical system to do work. The physical system can be a battery. Work is not necessarily mechanical.


Volumetric Energy Density

Volumetric energy density, other wise known as energy-to-size ratio, is how much energy can be stored in an energy storage medium per litre of that medium. It is measured in Wh/Litre or Wh/L, because the litre is a measure of volume. It is not to be confused with weight or mass which is measured in kilograms.

Therefore, an example li-ion battery with a volumetric energy density of 200 Wh/L that has a volume of one litre can store 200 Wh of energy. This also means that the example battery above can supply 1 watt for 200 hours, or if you want, you could draw 2 watts from it for 100 hours.

When someone refers to an energy storage medium such as lead-acid batteries for example as "big". That means that they have a low/poor volumetric energy density.


Energy Storage Efficiency

Energy storage systems also need to be as efficient as possible, because if you want a certain amount of power from a generator setup, such as 1000 watts for example, then the lower the efficiency of the energy storage system, the less power will be available, and the more powerful the generator would have to be to compensate for energy losses. If the energy storage system in this case is 50% efficient (which is the lowest for lead acid batteries), then only 500 watts would be available from this example setup:

1,000 watt 12 volt DC natural gas-powered generator.

50% efficient lead acid battery.

If the energy storage system was a 90% efficient lithium-ion battery, then the power available would be 900 watts, because 0.9 * 1000 = 900.

To obtain 1,000 watts from the lead acid setup you would need a 2,000 watt generator.

To obtain 1,000 watts from the lithium-ion battery setup mentioned you would need a  1,100 watt generator because the battery wastes 100 watts.


Affordable Energy Storage is Very Important to Power Plants

Some electricity sources, such as solar and wind powered generators produce electricity intermittently or of varying intensity. There are a variety of ways to counter this, some of which include the use of one or more of the following:

  • Peaking power plants/generators usually fueled by diesel, gasoline, or natural gas.
  • Pumped hydroelectric storage.
  • Compressed air energy storage or CAES.
  • Thermal energy storage.
  • Batteries.
  • Supercapacitors.

Apart from that, all other power plants except hydroelectric benefit from energy storage due to the fact that almost all power plants are not dispatchable (because they utilize steam turbines). This means that they cannot be turned on and off on exactly when necessary and the amount of power that they produce is not practically adjustable.

Therefore they have to almost match electricity demand throughout most of the day and when demand peaks (during the afternoon is one time that it peaks), combustion engine powered peaking power plants fueled by gasoline, diesel, and natural gas are started to augment electricity supply to prevent blackouts.

Combustion engine powered power plants are inefficient and as a result of that they are not the most economical way to generate electricity. Gasoline and diesel are among the most expensive sources of electricity.

At night, electricity demand is significantly lower and, due to the fact that most power plants are not adjustable, they generate more electricity than necessary. In other words, electricity demand drops below supply, and the additional electricity that is not used is wasted. When electricity is wasted, customers have to pay for that, someone has to.

The steam power plants that I referred to which are usually powered by coal, natural gas and nuclear reactors are classified as thermal power plants. Their fuels are used to generate heat which boils water or another fluid such as mineral oil to produce steam which turns a steam turbine.


Battery Technology

Lithium-ion cell (battery). Image obtained with thanks from Argonne National Laboratory on Flickr.

Batteries are a frequently chosen energy storage medium for a variety of applications, including electric and hybrid electric vehicles, portable electronics, generators, and power grid load balancing. They store chemical energy which can be converted into electrical energy via chemical reactions, therefore, batteries do not store electricity, they generate it.

Batteries are usually charged by using a power supply which basically uses a step down transformer to lower the voltage of mains electricity from 110-130, 240, or more volts down to a much lower voltage such as 3, 4, 6, 12, or 24 volts AC (especially 12), other devices to prepare the current for battery charging purposes by converting the 3,4,6,12, or 24 volts AC mentioned into 3,4,6,12, or 24 volts DC. It is rectifiers that are used to convert DC to AC which is then supplied to the battery.

AC current cannot charge batteries because it changes direction. The DC current actually powers a chemical process that charges the battery (this is simply known as charging).

Explanation 2: When you supply a battery with current, the current drives a chemical process which generates electricity when reversed, so you are not "filling" the battery with anything, just preparing it to generate electricity, after you prepare (charge) it, you connect the battery to whatever you want to power it with, and the reverse of the charging process takes place. This is known as discharging.

To learn more about individual types of batteries, including how they are charged, scroll down or check the table of contents above.

Advantages and disadvantages of using batteries in electric vehicles:

Heavy weight, high cost, undesirable lifespan (this isn't the case with all battery technologies, thin-film lithium batteries can be cycled 40,000 times) and slow charge/discharge time are common battery issues. It can take hours to charge and discharge batteries, and they are costly. One of their qualities is that they can be recharged with a variety of electricity sources.

Battery technologies are improving, and new ones are being born. Some new batteries can be charged in five minutes, such as the Toshiba SuperCharge battery.

Brief Glossary:

Discharge: This is the process of using the batteries to generate electricity and supply it to what is called a load. A load can be a fan or lightbulb, for example.

Ah = Amp-hours of energy storage capacity.

Wh = Watt-hours of energy storage capacity.

W/Watts = Rate at which power is supplied. Wattage = Voltage or V * Current.

A/Amps = Rate at which current is supplied.

mA = Milliamps.

mAh = Milliamp-hours.


State of Charge (SOC),  Depth of Discharge (DOD), and Their Importance

State of charge is defined as how much a battery is charged, or how "full" it is, as some would say. If a battery is half-charged, then the state of charge is 50%. This is very commonly written as 50% SOC. If fully charged, that is written as 100% SOC. If charged to 10% of its energy storage capacity, that is written as 10% SOC. I gave you all of those examples and interpretations to make this topic as clear as possible to you.

Depth of Discharge is the inverse of state of charge. It is how much a battery is discharged, or how much you "drained it" rather than charged. If a battery is 50% discharged, or half "empty", then it's depth of discharge is 50%. This is very commonly written as 50% DOD.

If a battery is fully discharged ("empty") then the DOD is 100%, and the state of charge (SOC) is 0%. If you have drained the battery by 30% (leaving 70% of its energy in it), that is a 30% DOD, or 70% SOC.

Depth of discharge is important to li-ion and NiCd batteries. The greater the usual depth of discharge of a li-ion battery, the shorter it's lifespan is going to be. In other words, if you normally discharge a lithium-ion battery fully, it will go bad quickly. NiCd batteries are the opposite, they need to be discharged fully before you can recharge them.


Lithium-ion (Li-ion)

Lithium-ion batteries have the lowest environmental impact, the best energy-to-weight ratio, and the longest lifespan, as well as the largest amount of cycles (which is 1000-2000 cycles) of the three most common types of batteries used for electric and hybrid electric vehicles. There are many different lithium-ion battery chemistries, some of which are very different from each other as well, these include: Lithium Iron Phosphate, Lithium Manganese, Lithium Polymer, Lithium Sulfur, and more. Read the introduction to batteries above before this section.

Lithium-ion batteries are charged by supplying them with electric current which drives the lithium-ions in the battery from the cathode to the anode. When you use these batteries to supply anything with electricity, they reverse this process and generate electricity which then powers your laptop, phone, etc. This is called discharging. If you haven't already guessed, in this case, the lithium-ions move from the anode back to the cathode. Lithium-ion batteries do not store electricity, they generate it.

The main drawback of lithium-ion batteries is their high price. Lithium-ion batteries self degrade with or without use and will eventually die. This is why people only get a few hundred cycles out of the 1,000-2,000 mentioned above. They financially pay you back more if you normally cycle your batteries more frequently (in other words, they pay for themselves faster if you normally use your batteries more heavily than the average person).

Efficiency of lithium-ion batteries: Upwards of 80%.

A few examples of li-ion battery-powered vehicles are:

  • Upcoming Toyota Prius plug-in hybrid.
  • Upcoming Chevrolet Volt.
  • Tesla Roadster.
  • Tesla Model S.
  • Fisker Karma.

Extending the Life of Li-ion Batteries

Older nickel-cadmium battery technologies required their owners to fully discharge (use them until they are dead) them before recharging them, however, lithium-ion batteries are the opposite. Their lifespan (as in, how long they last before they malfunction and need replacement) decreases as you discharge them more. [Source]

Therefore, waiting until they are almost out of energy actually shortens their lifespan, and they last longest when you keep them topped up. Please read the section just above this one to learn more: State of Charge (SOC),  Depth of Discharge (DOD), and Their Importance.

One of the main advantages of such a practice is that you won't have to worry about your phone or laptop dying as much.

Where convenience is concerned, just plug your chargers into a small power strip and leave them, plug them into your phones and notebook computers every evening, and let them charge briefly, you can turn off the power strip and leave everything plugged in until the next time you need the phone or laptop. They don't consume power when the strip is turned off because it physically disconnects them from the power outlet.

A guide to capacity loss.

Alternative Lithium-ion Chemistries

  1. Lithium sulfur.
  2. Lithium-air (energy density of up to 11,140 Wh/kg/11.14 kWh per kg or for every 2.2 pounds of these).
  3. Lithium-polymer (this has become common)
  4. Thin-film lithium.

Lead-Acid

Lead-acid Battery (AGM type) - Image Obtained from TruckPR on Flickr. Click the image to view a larger version of it.

Lead-acid batteries can be cycled a few hundred times, it varies significantly, and Nickel Metal Hydride batteries can be cycled up to 1000 times. Nickel Metal Hydride and Lead acid batteries have a significantly higher environmental impact than lithium-ion batteries, which have a low environmental impact. Lithium ion batteries are the batteries of choice for many electric vehicles now, except for the Toyota Prius which uses Nickel Metal Hydride batteries.

There are other batteries such as lithium sulfur and thin-film lithium batteries which are currently not being mass-produced, but, they might be one day. One advantage that lead acid batteries have over lithium-ion batteries is a significantly lower price tag, but, their lifespan is significantly shorter. Deep-cycle lead acid batteries are commonly used to store electricity generated by residential solar panel setups because of their low price tag.

Lead-acid batteries are the heaviest of the three main types of batteries used for electric vehicles due to their low energy density. Nickel metal hydride batteries are sometimes much lighter than lead acid batteries, but still much heavier than lithium-ion batteries.

According to a U.S Navy study, lead acid battery life can be extended to 8-10 years using pulse charging. According to Navy scientists (the source page is missing): Excessive sulfation takes place on the plates of lead acid batteries and charging them with pulses of DC current can break up the sulfation and return it to the electrolyte where it belongs so that it can be cycled again.

Efficiency of lead-acid batteries: 50-80%.

Using Lead-acid Batteries As A Power Source

If a lead-acid battery is rated at 7 Ah (usually printed on the front of it), then you will achieve the greatest efficiency if you draw 7 / 10 (remember that this is Ah rating divided by 10) which is 0.7 amps, or 700 mA from the battery. This means that you would draw 700 mA (0.7 A or Amps) from the battery for 7 hours. Source.

Another way to calculate this is by multiplying 0.1 b the AH rating. The amount of current you would charge the battery with is equal to 10% of the Ah rating on the battery.

You can supply up to Ah / 3 rating if you want to, and sometimes you will have to, but efficiency is not the best. Ah / 3 in the case of this 7 Ah example battery is 2.3 amps. It is not going to be able to supply that much current for 3 hours due to wasted energy, though.

The calculation above is equivalent to Ah / Number of hours you want to power something with it. It tells you how much current the battery could supply for 3 hours. Remember that this is only a guideline and the inefficiency of these batteries limits their "life" to less than the 3 hours mentioned.

In order to maximize the efficiency of these batteries and actually enjoy as much as the rated storage capacity, you would use them to supply Ah / 20. Which is 7 / 20 in the case of the example battery mentioned above. 7 / 20 = 0.35 Amps, or 350 mA.

Charging Lead-acid Batteries

When charging a lead acid battery, the amount of current that you would supply to it is 10% of the Ah capacity printed on the front of the battery, which is either Ah divided by 10, or 0.1 * Ah. Source.

If you wanted to charge the same 7 Ah battery mentioned in the example in the section above this, then you would supply it with 0.1 * 7 Ah or 7 Ah / 10 which equals 0.7 Amps. 0.7 amps is 10% of the 7 Ah rating. It is best to charge 12 volt lead acid batteries at 13.8 volts. Please stay well below 15 volts. You need to connect a voltmeter to the battery terminals while charging it to ensure that the voltage is less than 14.8 but more than 13.8 volts.


Nickel-Metal Hydride (NiMH)/Nickel-Cadmium (NiCd)

Nickel metal hydride and nickel-cadmium batteries are between lead acid and lithium-ion batteries performance wise, but their lifespan in some cases can exceed that of both, but it is normally significantly longer than that of lead-acid batteries. Nickel Cadmium batteries do have a disadvantage though, which is that they suffer from the memory effect and have to be charged and discharged fully each time. This is not recommended for lithium-ion nor lead -acid batteries.

Like the others, nickel based batteries are expensive, but they are also more toxic than lithium-ion batteries. Things have been changing though. Lithium ion batteries could become cheaper than nickel batteries in the future. Lithium ion batteries are not much more expensive anymore and their cost has been decreasing.

Energy Density of NiCd batteries: 30-60 Wh/kg.

Efficiency of Nickel-Cadmium Batteries: 70-90%.


Anticipated Battery Technology

There was a lithium-ion technology breakthrough and the new batteries can be charged in less than 20 seconds, they are also smaller, cheaper, lighter, and more efficient. This technology is expected to take a few years to become mainstream, but it is a significant breakthrough.

The fast discharge time means that electric vehicle manufacturers won't have to use the excessively large, heavy, and costly batteries to compensate for their slow discharge rate anymore, permitting the manufacture of faster, lighter, more cost-effective, and more efficient electric vehicles. Environmental impact and lifespan depend on the type of battery used, and how it is treated.

Lithium sulfur batteries are another interesting type which have a high energy density of 400-600 watt hours per kg, but cannot be cycled many times.

Silicon Nanowire Li-ion (lithium-ion) Battery: This is a battery which has a silicon nanowire anode which has ten times the surface area of the conventional type of anode which is made of carbon, and that silicon nanowire anode increases the energy density by ten times, which is over 1,000 Wh/kg. This makes an electric vehicle with an extremely long range possible, a range so long that basically everyone could complete all of their daily transportation after charging overnight at home.

Lithium-air Batteries: This type of battery can have an energy density of up to 5,000 Wh/kg, which is much greater than all other energy storage systems available today. The main advantage of such a high energy density is that it provides vehicles with very long range. IBM, MIT and other organizations are currently working on this technology.


Lithium Reserves, Sustainability and Sources

Lithium makes up less than 3% of the mass of lithium-ion batteries (1.5% to 3%). Source - PDF. Lithium can be extracted from multiple sources, including lithium brine (lithium chloride), seawater (although in low concentrations), it is also found in practically all igneous rocks, as well as minerals such as: lepidolite, petalite, amblygonite, and spodumene (this is the most common mineral that lithium is extracted from).

Lithium content per kWh if lithium-ion batteries were 1.5% lithium by weight: 0.211 kg. If it was 3% lithium by weight: 0.422 kg.

Lithium content per MWh if lithium-ion batteries were 1.5% lithium by weight: 211 kg. If it was 3% lithium by weight: 422 kg.

If you are interested in learning about lithium reserves, you can read about that here: USGS Mineral Information.


Fuel Cell Technology

Honda Fuel Cell Stack - Obtained from joezero5 on Flickr. Click the image to see a larger version of it.

Hydrogen fuel cells are another way to store energy for future use. They use hydrogen and oxygen to generate DC (direct current) electricity via an electrochemical process which involves the oxidation of the fuel (hydrogen) using the oxygen which is fed to the cathode, and the hydrogen is fed to the anode. Fuel cells have had a problem with the high cost of the platinum that they contain, but, significantly cheaper electrode replacements such as carbon nanotubes have been found.

Hydrogen fuel cells only need hydrogen and oxygen to generate electricity (and in some cases, they may use other fuels), and the fuel can be extracted from water via electrolysis. During electrolysis, two electrodes are placed into water and electricity is passed through them, then hydrogen and oxygen bubble up at the cathode and anode respectively. You can even extract your own hydrogen and oxygen, the process is simple, although energy intensive. There are many other types of fuel cells, including phosphoric acid, alkaline, molten carbonate, direct-methanol, solid oxide, and polymer exchange membrane fuel cells. The average fuel cell stack is 50% efficient, and can be up to 70% efficient. An example of a fuel cell powered vehicle is the Honda FCX Clarity.


Supercapacitors/Ultracapacitor Technology

3,000 Farad Supercapacitor. Image obtained with thanks from Maxwell Technologies on Flickr. Click the image to see a larger version of it.

A supercapacitor or an EDLC (Electric Double Layer Capacitor) is an energy storage device which is capable of being charged and discharged very quickly. A capacitor is an energy storage device which stores an electric charge between two conductors which are separated by a dielectric (a dielectric is a non-conducting substance, such as glass, air, ceramic material). They literally store energy between two metal plates.

They are also more efficient than batteries. Supercapacitors have hundreds of thousands to millions of charge and discharge cycles, unlike conventional batteries which have 100 to over 2000 charge cycles. They have high charge and discharge efficiencies so they don't waste much energy, but they do self-discharge fairly quickly. They are also very safe and environmentally sound.

Low-cost ultracapacitors could also help with one of the major energy storage problems that off-grid solar and wind powered homes have, which is the cost to replace the batteries due to their short lifespan. They would also be good for electric vehicles because of their long lifespan and their ability to discharge energy very quickly (usually in seconds) to provide short bursts of power, provided that they are affordable, but they are currently cost prohibitive. Their energy densities are low (and their power densities are high), but they are improving.

A high power density means that a small ultracapacitor can produce a large amount of power in a short period of time and a high energy density means that a small ultracapacitor can store a large amount of energy.


Thermal Energy Storage Technology

Energy may be stored in the form of heat and utilized in a variety of ways, some of which include:

  • Powering solid-state thermoelectric modules by taking advantage of the Seebeck effect. When one side of a thermoelectric module is heated (with heat from the sun, generators, engines, or heat from the earth), and the other side is kept cool, a potential difference (voltage) occurs and that causes electric current to flow.
  • Heating a liquid to produce steam which would be used to turn turbines.
  • Heating a Stirling engine. A stirling engine is a mechanical device which converts heat energy into mechanical energy when one cylinder is heated, and the other is kept cool.

Thermal energy is often stored in insulated tanks filled with a storage medium such as molten salt in the case of solar thermal power plants. The insulation inhibits heat transfer, therefore, it helps to trap heat inside those tanks. The ability of these tanks to prevent heat from escaping is a very important factor that affects how economical and efficient they are over extended periods of time, because wasted heat is wasted energy.

Flywheel Energy Storage (FES) Technology

Flywheel. Image obtained with thanks from NASA.

Energy may be stored as rotational energy by using a motor (such as an electric one which would also act as an alternator) to accelerate a flywheel up to a high speed (for example, 41,000 RPM). The inertia of the flywheel causes it to keep spinning until friction, drag, and gravity slow it down.

While the flywheel is spinning, it turns a generator that in turn produces electricity. As the flywheel slows down, it generates less electricity. One challenge associated with flywheels is to maintain their speed. This is often achieved by reducing aerodynamic drag on the flywheels, by operating it inside a vacuum, which in this case is a case that contains very little air.

Flywheel energy storage systems do not suffer from the memory effect that some (especially older) battery technology does, and they can help to stabilize the supply of power. Flywheels have many applications due to their heavy weight, which causes inertia (which is the resistance of a body to change its velocity).

Some devices, such as video cassette recorders, take advantage of their inertia by using them to keep the speed of the VCR's gears and wheels constant. They also have a higher power density than batteries and can be charged within seconds and discharged within seconds, unlike most batteries.

Reliability and Waste: Flywheels tend to be much more reliable than batteries and are more able to withstand frequent charging than batteries and are not constructed using toxic materials like certain (older) battery technologies

Such toxic technologies include lead acid and nickel cadmium, resulting in a much greener life cycle (this is because batteries have to be thrown away more often, and hence, more batteries end up in landfills, causing more pollution).

Another advantage of flywheels is that their efficiency can be as much as 90%.


Pumped Hydroelectric Storage/Pumped Hydro Technology

Pumped hydroelectric energy storage technology normally uses the electricity provided by any type of generator to power a water pump which then pumps water from one water reservoir upwards into another such as a tank or anything large enough that water can be stored in, a mountaintop is an example.

The tank which the water is pumped into and stored in is to be above the tank that it releases water into. People sometimes come up with creative ways to store energy without wasting space such as the Germans.

The environmental concern mentioned above is not an issue if holes are excavated underground for use as reservoirs because they enable pumped hydroelectric storage plants to be completely isolated.

This type of energy storage does not store actual energy but potential energy, meaning that it stores water in a tank which can be released through a hydroelectric turbine to turn it so that it generates electricity.Notice the emphasis of "can be" because that is what potential means.

One disadvantage of relying on pumped hydroelectric storage is that such projects are difficult to approve due to environmental considerations. One environmental issue associated with this energy storage system is that water levels fluctuate with electricity demand and that can disturb the local marine ecosystem.

It is a low tech and simple alternative to batteries with a very high efficiency, low maintenance cost, and a very high up front (capital) cost.

Other advantages include the ability to adjust power output very quickly and one of the lowest overall costs per kWh of energy that they store of all large grid-scale energy storage methods. Notice the emphasis on "large grid-scale". This type of plant can also store energy for more than half a year which is the longest of all energy storage systems.

It isn't necessarily cost competitive with other energy storage methods on a small scale. Batteries are currently dominant for small-scale energy storage for portable electronics and the storage of energy for individual homes.

It is the most common form of grid-scale energy storage in use today (2011). Grid scale means that it stores energy to supplement power shortfalls in the electricity grid.


Compressed Air Energy Storage (CAES) Technology

An air compressor may be used to force air into storage tanks so that it can be released from the tanks at a controlled rate to turn a turbine which would generate electricity. According to the University Of Oregon website, compressed air energy storage systems can have an energy density of 2,000 watt-hours per kilogram.


Load Balancing and How Power Plants Benefit From Economical and Efficient Energy Storage

Energy storage systems are sometimes used for load balancing. Load balancing is the process of storing excess electricity generated during off-peak hours especially (in the night) and using it to help power plants to meet electricity demand during the day by jointly supplying electricity with the existing generators (example: steam or wind turbines).

Without load balancing, power plants such as coal, nuclear, natural gas (steam), and geothermal generate too much electricity at night when electricity demand is lower, and just about enough, or too little during the day.

The result is that more electricity is wasted at night. Power plants of that type are not load-following power plants, their power output cannot be adjusted much.

Gasoline powered generators' efficiency varies with speed, and they can benefit from economical and efficient energy storage by operating at their most efficient speed at all times and charging cheap, efficient batteries, which would then supply just enough electricity to meet demand, and at all times.

Wind turbines generate electricity dependent on the speed of the wind. Wind speed patterns do not consistently follow electricity demand patterns, and the wind blows intermittently, therefore, energy storage helps wind power plants by enabling them to supply exactly the right amount of electricity demanded and at all times.


Backup Generators

All types of generators can be used with other backup generators which would be switched on when there is a power shortage caused by a malfunction, low wind speeds, cloudy weather, or even just because the main generator needs maintenance.

This is to enable the power plant to provide a constant supply of power all the time, and is a common alternative to batteries. Backup generators are normally combustion engines that are fueled with gasoline, diesel, biofuels, propane, or natural gas.