Pumped Water:
Uses two reservoirs; pumps water up into one of them with excess energy and then use as traditional hydro electric when demand is high. Recovers 75% of energy stored, is currently most cost effective method, can come online in 15 seconds. There are 90GW energy stored around world, which represents 3% of world's current instantaneous generation capacity. There is talk of
using wind directly to move water up gradient.
Limited by geographical concerns. Need large bodies or large height for significant storage, 1000 kilograms of water (1 cubic meter) at the top of a 100 meter tower has a
potential energy of about 0.272 kW·h.
Batteries:
Discussed above, store energy in chemical reactions, most likely to be used in vehicle to grid setups.
Compressed Air:
Essentially consists of using electricity to force air into space, raising its pressure, and then releasing it from the high pressure vessel to power a turbine or compressed air engine. A portable system has similar weight and energy density to lead acid batteries, but less consistent voltage, although the vessels last longer and are less toxic. Large scale storage could be underwater or underground in old mines. good picture in (61). This is one of the more viable technologies out there currently, but is
limited due to geography.
Thermal:
- Water: Often used for air conditioning, use off peak energy to create ice that you run coolant through during the day to either augment chiller or replace it, usually reduce size of chiller by 40-50%, produce ice 16-18 hours a day, melt about 6 hours a day (partial storage), or can stick with larger chiller and shut off chiller during peak hours entirely (fully storage). Can also use ice as condensing medium for refrigerator coolant after it is produced at night. Combustion gas turbine air inlet cooling uses ice generated at night to cool intake gas to a combustion gas turbine to near freezing where is compresses more readily, increasing daytime generating capacity.
- Molten Salts: This involves using non-flammable, non-toxic salts at very high temperatures, so that they are liquid, to transfer heat. The system would consist of having a \"cool\" storage tank and a \"hot\" storage tank, both of which are at very high temperatures, so that the salt remains molten. The salt would pass from the cold tank through a solar thermal collector and into the hot tank, then from the hot tank probably to a heat transfer system where it creates steam to drive a turbine. One particular design from 2005 predicts a 99% yearly efficiency and uses as its salt a mixture of 60 percent sodium nitrate and 40 percent potassium-nitrate, commonly called saltpetre. It is already used in the chemical and metals industries as a heat-transport fluid. The salt melts at 430F (221 degrees Celsius). The system calls for the salt to be \"kept liquid at 550F (288 degrees Celsius) in an insulated \"cold\" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 1050F (566 degrees Celsius). It is then sent to a hot storage tank. This is so well insulated that the thermal energy can be usefully stored for up to a week. A 100-megawatt turbine would need tanks of about 30 feet (9m) tall and 80 feet (24m) in diameter to drive it for four hours by this design.\" The difficulty with these systems is finding a tank that is insulated well enough to prevent loses when the salt is over 1000 degrees F.
- Phase Changing Materials: These are any materials, including water, which are used to store energy through a phase change. When a material absorbs energy, it either increases in temperature and maintains phase or changes phase and maintains temperature. This second process requires much more energy than the first, so you can store more energy by turning ice into water than simply heating water, for example. High temperature systems using citric acid have been proposed, although again the difficulty is with finding a storage tank to hold the molten citric acid at its high temperature. The benefit with this system is that the material will stay a predictable temperature throughout the phase change. This concept is also used in some walls. These walls contain small capsules of a material that change phase at room temperature. During the day, the house will heat to room temperature, then the material will use the additional thermal energy to turn into a liquid as opposed to heat the house. At night, as the outside temperature drops, the liquid turns into a solid, releasing energy in the form of heat to the house, again helping to maintain room temperature. Insert chart showing energy vs. temperature
Flywheel:
Electric motor turns large wheel, which can then be slowed down and produce energy through a generator. Need to minimize friction, have special bearings, and made of materials strong enough to resist forces. They are unaffected by most things that bother other storage systems, and are made of benign materials, but their size and cost limits their usefulness in utility-scale storage.
Superconducting magnetic energy:
Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current electricity in a superconducting coil. A typical SMES system includes three parts: a superconducting coil, power conditioning system and cryogenically cooled refrigerator to get the superconductor material cooled to a low enough temperature that it acts as a superconductor. Once the system is charged, the current will not decay and the magnetic energy can be stored indefinitely until the coil is discharged. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct current or convert DC back to AC power, since the coil requires direct current but most energy is produced and consumed as alternating current. This conversion of energy accounts for about 2-3% energy loss in each direction, for about a 5% total loss, this makes it among the most efficient power storage techniques, if you exclude the energy used for refrigeration. Due to the energy requirements of refrigeration and the high cost of superconducting wire, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving power quality. If SMES were to be used for utilities it would be for storing energy to
correct swings in demand on an hourly basis. SMES has a short time delay, can offer large amounts of power over short period of time, short turn around time from storage to receipt (which is not necessarily an advantage in a grid-sized application), and is highly reliable because of its lack of moving parts. Typically used for grid stability and power quality. Can be quite expensive cost and energy wise because of the cooling of the superconducting material. It would be very large, about a 100 mile loop. Health concerns include losses of the liquid nitrogen used for the cooling of the coil and the possible effects of a large magnetic force on the human body and
the rest of the natural world. It is
currently a specialized storage medium, but could potentially become more mainstream in the future.
Hydrogen:
Hydrogen energy storage is just that, energy storage, not a means of creating energy. It involves using electrolysis to split water into H2 and O2 which then must be stored before being recombined to create energy. 50-60% total efficiency. Physorg.com website reports that the losses encountered with the hydrogen compression cycle are around 78%, while the hydrogen liquification cycle's losses are in the order of 81%, which combined make up the 50-60% total efficiency. About 50 kWh (180 MJ) is required to produce a kilogram of hydrogen by electrolysis, which means that hydrogen has good energy density by weight, but it is poor by volume, because the density of hydrogen is so small. Therefore, hydrogen storage requires large storage tanks.
Below are the current storage ideas: - High pressure storage, which is not suitable for bulk storage, but may be for a small fuel cell
- Natural geological cavities, but those don't always exist where needed
- Crygenic freezing to liquid nitrogen has large energy cost
- Metal hydride storage is being talked about a lot, but has not been proven on large scale
- Mixing with CO2 to make methane, which can then be mixed with natural gas, but only to a concentration of 10% so far given tests that have been done. This may turn out to be the most viable storage method.
- Mixing with any other hydrocarbon and using it as a fuel much as we do today, the only qualifier with this technology is that the hydrocarbon must be captured after it is combusted to be reused, making it a closed loop carbon cycle.
- MIT has developed a low-cost, low-toxicity fuel cell storage system for solar energy.
If the storage issue is resolved, however, it may make more sense to have hydrogen be the principle energy carrier as opposed to electricity to avoid needing to convert electricity to and from hydrogen.
This could also justify nuclear over solar or wind economy. Add more detail about this last statement . It has the possibility to be a
clean and flexible storage fuel in the future.
Super-capacitors:
Super-capacitors have become an option for the future since new materials enhanced their performance. They store energy as electricity on plates that are tightly wound into a cylinder and can provide energy quickly in large amounts. The energy is stored as a static charge, similar to the way charge builds up on your body as you walk across a carpet and is then released when you touch a metal doorknob.