Since some forms of renewable energy do not produce consistent power twenty-four hours a day it is important to develop methods of energy storage that are in themselves ecological or do not cause more harm in their production and disposal than the old forms of electrical generation that renewable energy technology is replacing. While there are great transitional applications for lithium-ion, nickel-metal hydride, and other electrochemical batteries, these may not be the best long term solution since the metals that go into their production are not in themselves entirely renewable. Some other interesting methods of energy storage are listed below.
Chemical Energy Storage
Electrical power can be used to produce Hydrogen Gas which can then be used to power hydrogen fuel cells with water as the only product required during production and the only byproduct output during consumption.
Biofuels and Methane Gas can be considered a type of energy storage. If renewable energy is the power used in the processing of these fuels then the energy that is embodied in them can be considered entirely renewable.
Mechanical Energy Storage
Compression of gas or water can be used during times of peak capacity. Then the pressure can be released during times of lower production to augment power. This can be accomplished in very large quantities with the use of underground cavities.
Mechanical Flywheels can be used to store energy. They consist of heavy weighted round cylinders which are designed to rotate on a central axis with as little friction as possible. Electrical energy can be put into the flywheel causing it to rotate up to very high rotations per minute speeds. Then when energy is required at a later time, the rotational energy is extracted from the flywheel, slowing its rotation down.
Hydroelectrical Pumped Storage uses conventional dammed hydroelectrical technology to fill a reservoir with water while there is access to electrical power. The water is then released to power turbines when electricity is required.
Thermal Storage
Storage of energy in the form of heat. Some concentrated solar power plants use liquid sodium or other thermal storage mediums that store heat for long periods and allow for the operation of steam turbines for many hours after the sun has set. Alternatively, energy can be stored by cooled liquids during off-peak hours (and when temperatures are cooler) that can then be used for air conditioning purposes in the daytime.
Grid Storage
With a large enough, well interconnected, and technologically sophisticated enough power grid, intermittent energy generation can be managed within limits by allowing peak electrical generation capacity in some locations to make up for lower capacity in other locations.
Demand Side Management
While not really a type of energy storage, if end use of electricity can be managed through the application of smart technologies so as to even the peaks and valleys that exist in usage or time them to correspond with the peaks and valleys of intermittent energy production, then the problem of intermittency is reduced. Smart technology would allow end users to monitor in real time the amount of energy available in the grid at any point in time and plan their use patterns accordingly. Appliance control technologies can be designed to perform this function automatically (you load your washing machine and press the start button, but it instead informs you that it will be waiting for two hours before beginning its operation because it senses a demand peak at the moment).