Renewable energy storage systems

One of the great problems with renewable energy, as mentioned above, is transporting it in time or space. Since most renewable energy sources are periodic, storage for off-generation times is important, and storage for powering transportation is also a critical issue.

Hydrogen fuel cells

Hydrogen as a fuel has been touted lately as a solution in our energy dilemmas. However, the idea that hydrogen is a renewable energy source is a misunderstanding. Hydrogen is not an energy source, but a portable energy storage method, because it must be manufactured by other energy sources in order to be used. However, as a storage medium, it may be a significant factor in using renewable energies. It is widely seen as a possible fuel for hydrogen cars, if certain problems can be overcome economically. It may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without flames, in the same way the human body burns fuel. Making hydrogen requires either reforming natural gas (methane) with steam, or, for a renewable and more ecologic source, ''the electrolysis of water'' into hydrogen and oxygen. The former process has carbon dioxide as a by-product, which exacerbates (or at least does not improve) greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power, and both intermittent renewables and nuclear energy are considered here.

With intermittent renewables such as solar and wind, matching the output to grid demand is very difficult, and beyond about 20% of the total supply, apparently impossible. But if these sources are used for electricity to make hydrogen, then they can be utilized fully whenever they are available, opportunistically. Broadly speaking it does not matter when they cut in or out, the hydrogen is simply stored and used as required.

Nuclear advocates note that using nuclear power to manufacture hydrogen would help solve plant inefficiencies. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods and any not needed to meet civil demand being used to make hydrogen at other times. This would mean far better efficiency for the nuclear power plants.

About 50 kWh (1.8 MJ) is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial. ( At $0.10/kWh this means hydrogen costs $5 a kilogram for the electricity, equivalent to $5 a US gallon for gasoline if you use in a Fuel Cell vehicle, plus electrolyzer plant costs which will not be small.)

Other renewable energy storage systems

Sun, wind, tides and waves cannot be controlled to provide directly either reliably continuous base-load power, because of their periodic natures, or peak-load power when it is needed. In practical terms, without proper energy storage methods these sources are therefore limited to some twenty percent of the capacity of an electricity grid, and cannot directly be applied as economic substitutes for fossil fuels or nuclear power, however important they may become in particular areas with favorable conditions. If there were some way that large amounts of electricity from intermittent producers such as solar and wind could be stored efficiently, the contribution of these technologies to supplying base-load energy demand would be much greater.

Pumped water storage

Already in some places pumped-storage hydroelectricity|pumped storage is used to even out the daily generating load by pumping water to a high storage dam during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours this water can be used for hydroelectric generation. However, relatively few places have the scope for pumped-storage dams close to where the power is needed.

Battery storage

Many "off-the-grid" domestic systems rely on battery storage, but means of storing large amounts of electricity as such in giant batteries or by other means have not yet been put to general use. Batteries are generally expensive, have maintenance problems, and have limited lifespans. One possible technology for large-scale storage are large-scale flow batteries. NAS battery|NAS batteries could also be inexpensive to implement on a large scale and have been used for grid storage in Japan.

Electrical grid storage

One of the most important storage methods advocated by the renewable energy community is to rethink the whole way that we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Solar electric generation is a daylight process, whereas most homes have their peak energy requirements at night. Domestic solar generation can thus feed electricity into the grid during grid peaking times during the day, and domestic systems can then draw power from the grid during the night when overall grid loads are down. This results in using the power grid as a domestic energy storage system, and relies on 'net metering', where electrical companies can only charge for the amount of electricity used in the home that is in excess of the electricity generated and fed back into the grid. Many states now have net metering laws.

Today's peak-load equipment could also be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power when they were unable to. Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is now possible to predict wind generation output with 90% certainty 24 hours ahead. This means that it is possible to deploy other plants more effectively so that the economic value of that wind contribution is greatly increased.

Flywheel storage

Simple physics is the basis of this storage method. A heavy rotating disc is accelerated by an electric engine which acts as a generator on reversal, slowing down the disc and producing electricity.

Electricity is stored as the kinetic energy of the disc. Friction must be kept to a minimum to prolong the storage time. This is achieved by placing the flywheel in a vacuum and using magnetic bearings, making the method expensive. Larger flywheel speeds allow greater storage capacity but require ultra strong materials such as carbon nanotubes to resist the centrifugal forces (or rather, to provide centripetal forces).

Compressed air storage

Another method is to use excess electricity to compressed air|compress air, which is usually stored in an old mining|mine or some other kind of geological feature. And when electricity demand is high, use the compressed air to run an engine and generate electricity. Projects of this type have been tried fairly successfully in Alabama and in Germany.