Conventional capacitors can provide some smoothing and storage of a fluctuating supply. We can learn from electric vehicles, where use of supercapacitors - sometimes called ultracapacitors or Electrochemical Double Layer Capacitors (EDLC) - to balance power delivery and store power is receiving increased attention. Like traditional electrolytic capacitors, supercapacitors rely on ionic effects to provide exceptionally large capacitance in a small volume and thin film versions are possible. However, leakage currents are relatively high, the charge being stored for weeks not years. The electrolyte ages.
Unlike batteries, supercapacitors do not rely on a chemical reaction so they last longer. Unfortunately, they usually have only 5% of the power storage of a battery, so any major use in energy storage rather than management of energy delivery has been elusive. Power balancing is more usual rather than replacing batteries themselves.
Recently, a souped up Saturn Vue car demonstrated outstanding performance thanks to power delivered as appropriate from batteries, a gasoline engine and a supercapacitor, the latter providing very rapid power delivery when needed. However, EEStor of Texas now seems to be changing the fundamentals of supercapacitors by employing barium titanate rather than an electrolyte. Its "Electrical Energy Storage Units" will go into production and the company already has a deal to supply energy units for mobile military applications.
Another envisaged application is the rapid charging of electric vehicles. Indeed, Zenn Motor Company in Toronto has signed a deal with EEstor to use the supercapacitors to replace batteries, according to the press. That means that the leakage of energy from such capacitors, which has prevented them storing energy for an extended period of time, has also been overcome. Such advances may benefit development of small versions for energy harvesting.
Indeed, Joel Schindall of Massachusetts Institute of Technology gets the necessary large surface area in an supercapacitor by using carbon nanotubes, each five nanometers across. He hopes this can push supercapacitors up from 5% to 50% of the storage capacity of a battery, including miniature ones.