The capital cost of a flywheel storage system may be as high as $2000/kW, though costs should fall below this for standard modular units. However against this high cost must be balanced the fact that they are virtually maintenance-free, can be cycled indefinitely and are extremely predictable. The energy contained in the system can always be determined. In applications where medium capacity, short-term storage is required, flywheels offer one of the best solutions.
Capacitors are used extensively in electrical and electronic circuitry. In power networks they have been used to enhance system stability. More advanced capacitors are now being developed specifically for energy storage.
The classic capacitor comprises two parallel metal plates with an air gap between them. When a voltage is applied to the plate a positive charge collects on one plate and a negative charge on the other.
A number of different capacitor types exist. Those being considered for energy storage are called electrochemical capacitors. These utilise a solid electrode and an electrolyte. Charge collects at the interface between the two. These devices, sometimes called super capacitors or ultra capacitors can store a very large energy density, probably the highest of any storage device. They can respond in tens to hundreds of milliseconds and are most suited to short-term energy storage applications.
The technology is relatively new and there is little cost data available. Nor is lifetime or operational experience available, but static electrical devices of this type should show good long-term stability and should be relatively maintenance-free.
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.