Ryan Carlyle
ALMOST ALL ELECTRICITY IS USED INSTANTLY AS ITS GENERATED.The basic problem is that electricity is a flow of energy which travels at nearly the speed of light. That makes storage very difficult. The electromagnetic field/wave that carries the energy moves so fast that it dissipates almost instantly if you dont use it. BY AND LARGE, POWER GRID OPERATORS SIMPLY RAMP POWER PLANTS UP AND DOWN TO PROVIDE EXACTLY AS MUCH ELECTRICITY AS IS NEEDED THROUGHOUT THE DAY.
This ramping capacity is provided by running a mix of low-cost "base load" plants at constant output, and then spinning up additional high-cost "peaker" plants when demand is high. Utilities do extensive demand forecasting and power grid modeling to make sure supply and demand perfectly match at all times. Its a delicate balancing act. The really fine control is handled by a mix of automatic and manual control systems. I talk about that a bit in Ryan Carlyles answer to What is the holding capacity of the US power grid? That is, within what margin of error must generation match up to consumption?
Image: DECCs Free resource supporting the development of CHP
The only way to physically store electricity directly is via superconducting rings, which are absurdly expensive and impractical. Superconductors have zero electrical resistance and therefore allow electricity to rotate in a closed circle without dissipating. Its unworkable for grid-scale storage, though. The superconductors have to be kept refrigerated to a cryogenic temperature very close to absolute zero. And the more energy you want to store, the larger the ring diameter has to be -- so they get impossibly large at high storage capacities.
For practical purposes you cant store the electricity itself, so it has to be converted into some other form. Here are is a list of the usual methods:
* Inductors store electricity in a magnetic field.
* Capacitors store electricity in an electrostatic charge.
* Batteries store electricity by using a redox reaction to convert it to galvanic potential energy (a form of chemical energy).
* Hydrogen synthesis stores electricity by splitting water into hydrogen and oxygen, which can be burned for heat or run through a fuel cell to get electricity directly (another form of chemical energy).
* Pumped-hydroelectric storage facilities use electricity to pump water uphill, then let it flow back down through a turbine to generate electricity later.
* Compressed-air storage facilities use electricity to compress air, then expand it through a turbine to generate electricity later.
* Flywheels use electricity to spin up a weighted disc, and then that angular momentum (rotational inertia) is used to drive a generator to generate electricity later.
* Thermal energy storage uses heat sinks like molten salts to store heat energy, then uses that energy to either generate electricity or provide heating later. Alternatively, electricity can be used to freeze water into ice, and then the ice can be used to provide air conditioning later. (This is a way of "time-shifting" electricity consumption moreso than "storing" it.)
* Various other schemes exist, such as driving large electric trains uphill and then using regenerative braking as they roll back down. Its really only limited by your imagination. The more exotic ideas tend to be less cost-effective, though.
All of these have significant limitations. The main issue is low energy/weight and energy/volume ratios, meaning they take up a lot of space and materials (and therefore money) to build. Each system has its own challenges:
* Inductors, capacitors, and batteries can only store direct current (DC), so additional equipment is necessary to use them for the alternating-current (AC) electric grid. Theyre also extremely expensive relative to the amount of energy they can store.
* Hydrogen is very corrosive to most common engineering metals (eg mild steel) so it requires expensive alloys for containment and transport.
* Pumped-hydroelectric and compressed-air storage require specific geography/geology and therefore are limited by the number of available construction sites.
* Flywheels cant store much energy, slowly lose energy to friction, and can occasionally explode when operated at high speeds.
* Thermal energy storage requires large, expensive systems of heat exchangers and heat pumps/engines, and slowly loses energy to the environment.
Another major issue is round-trip efficiency. You lose energy during both the storage and extraction phases. The total losses are usually more than half of the energy input (depending on the type of storage). The rest goes to waste heat.
ALMOST ALL (>99%) OF POWER-GRID-SCALE ENERGY STORAGE IN THE WORLD TODAY IS PUMPED-HYDROELECTRIC. Its safe and proven technology, with capacity only limited by how big of a reservoir you can find to dam, and how big of a lake near it you can drain. The use of natural terrain and water as the energy storage mechanism reduces the system cost to an affordable level. The efficiency can be quite high -- 70-85% if properly implemented. Developed regions like the US and the EU are pretty much maxed out on hydroelectric capacity due to lack of additional sites, but the developing world still has a lot of room for pumped-hydroelectric growth.
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There are also a small number of compressed-air facilities in operation, a few large battery banks, and a few small thermal systems. No one is using any other systems at grid scale yet.
Future proposals suggest connecting electric cars up to a smart-grid which will use the car batteries as distributed grid storage. But thats pretty far out in the future. Molten salt / flow batteries are another possibility if the costs can be reduced to a level where theyre cost-effective to deploy at large scale.
The simple fact is, its much cheaper to build and operate load-following power plants than it is to store electricity. No one has any credible suggestions for how to economically store more than, say, 5% of daily electricity use. Power grids will continue to require a high-wire balancing act for the foreseeable future.
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Grid energy storage
List of energy storage projects
