Energy Storage

Energy storage is the storing of 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. Even food is a form of energy storage, chemical in this case.

History

Energy storage as a natural process is as old as the universe itself—the energy present at the initial creation 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) and indirectly (e.g., by growing crops or conversion into electricity in solar cells). 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 was 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 common energy storage in prior use, such as wood or coal, electricity must be used as it is generated and cannot be stored on anything other than a minor scale. Electricity is transmitted in a closed circuit, and for essentially any practical purpose it cannot be stored as electrical energy. Therefore, changes in demand could not be accommodated without either cutting supplies (e,g,, via brownouts or blackouts) or arranging for a storage technique.

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 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

• Chemical
  • Hydrogen
  • Biofuels
• Electrochemical
  • Batteries
  • Flow batteries
  • Fuel cells
• Electrical
  • Capacitor
  • Supercapacitor
  • Superconducting magnetic energy storage (SMES)
• Mechanical
  • Compressed air energy storage (CAES)
  • Flywheel energy storage
  • Hydraulic accumulator
  • Hydroelectric energy storage
  • Spring
• Thermal
  • Molten salt
  • Cryogenic liquid air or nitrogen
  • Seasonal thermal store
  • Solar pond
  • Hot bricks
  • Steam accumulator
  • Fireless locomotive

Hydrogen

Hydrogen is a chemical energy carrier, just like gasoline, ethanol, or natural gas. The unique characteristic of hydrogen is that it is the only carbon-free or zero-emission chemical energy carrier. Hydrogen is a widely used industrial chemical that can be produced from any primary energy source. Most of the world's production is by the thermal reformation of natural gas (methane) into hydrogen that is used immediately to refine petroleum into gasoline, diesel fuel, and other petrochemicals. The carbon dioxide produced by the reforming process is either captured and processed into liquid carbon dioxide or vented to the atmosphere. Because hydrogen is produced and distributed in such huge quantities, the technology needed to build infrastructure to serve wholesale and retail energy markets is proven, reliable, and commercially available.

Hydrogen can be used as a fuel for all types of internal and external combustion heat engines and turbines (with adjustments to compensate for the difference between, say, diesel fluid and hydrogen gas). Hydrogen-fueled heat engines can be optimized to operate at higher thermal efficiencies than traditional heat engines using traditional hydrocarbon fuels. The increased thermodynamic efficiency, and reduced pollution, would be beneficial, but they are not produced in quantity largely because hydrogen is not industrially available.

Sufficiently purified hydrogen can also be used to power electrochemical engines, such as the proton exchange membrane (PEM) fuel cell. Hydrogen fuel cells can be more efficient than hydrogen-fueled heat engines, and thus much more efficient than hydrocarbon fuel heat engines. They are also less polluting. Several companies are attempting to develop reliable, inexpensive PEM fuel cells. However, designs are not sufficiently developed to be routinely mass produced. The limited quantities available for purchase are handmade and much more expensive than conventional heat engines.

Hydrogen production in quantities sufficient to replace existing hydrocarbon fuels is not possible. Such production would require more energy than is currently being used, and would require large capital investment in hydrogen production plants. Because of the increased costs, hydrogen is not yet in widespread use. If the cost of greenhouse gas production is fully included into the market price of hydrocarbon fuels, hydrogen fuels may become more attractive commercially, providing clean, efficient power for our homes, businesses and vehicles.

Disadvantages of hydrogen include a low energy density per volume (even when highly compressed) compared to traditional hydrocarbon fuels. Also, for many hydrogen production methods, there is a significant loss of energy during the conversion. Some production methods—for instance, electrolytic generation from water—are more efficient.

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, and syngas. This diesel source was used extensively in World War II in Germany, when access to crude oil supplies was limited. Today South Africa produces most of the country's diesel from coal for similar reasons. A long-term oil price above $35 may make such synthetic liquid fuels economical on a large scale. 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. Compressed natural gas is being used as fuel in some special circumstances, for instance in buses 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. Another alternative for the energy is electricity or heat from solar energy or nuclear power. 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. If usable on a wide scale, this approach may help in the long term to avoid some of the deleterious effects of greenhouse gas emission.

Mechanical Storage

Energy can be stored in water pumped to a higher elevation, in compressed air, or in spinning flywheels, but mechanical methods of storing energy on a large scale are expensive and water-pumping systems require considerable capital investment. Several companies have done preliminary design work for vehicles using compressed air power.

Intermittent Power

Many renewable energy systems produce intermittent power. Other generators on the grid can be throttled to match varying production from renewable sources, but most of the existing throttling capacity is already committed to handling load variations. Further development of intermittent renewable power will require some combination of grid energy storage, demand response, and spot pricing. Intermittent energy sources are 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.

Nonintermittent renewable energy sources include hydroelectric power, geothermal power, solar thermal, tidal power, energy towers ocean thermal energy conversion, high-altitude airborne wind turbines, biofuel, and solar power satellites. Solar photovoltaics, although technically intermittent, produce electricity largely during peak periods (i.e., daylight), and hence do reduce the need for peak power generation, though somewhat unreliably in most areas since weather conditions interfere with terrestrially mounted solar cells.

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, electrically powered hydrogen production can be set to increase when electricity is being produced beyond current demand (and prices will be lowest), and conversely, hot water heaters can be automatically set to a lower temperature when demand is high and pricing is also high.