ENERGY STORAGE

selected from: http://www.nrel.gov/analysis/power_databook

battery
"Electrode plates of chemically reactive materials sit in a bath of electrolyte, which facilitates the transfer of electrically charged ions. The negative electrode gives up electrons during the discharge cycle, this supplies a flow of electricity to any system attached to it. During the charging cycle, the flow of electrons is reversed as energy is sent into the battery. Batteries only store direct current, so a transformer is necessary to convert battery power into AC power."

Lead acid is the most common and reliable; other types: nickel-cadmium, lithium ion, nickel metal hydride, zinc bromine, sodium bromine, lithium polymer, sodium sulfur are under development; largest system to date for lead acid is 20 mw

Lead acid batteries give off hydrogen gas when charging, and have to be ventilated; lead acid batteries can be recycled

flywheel
Flywheels store kinetic energy in a rotating mass
Low speed flywheels are less expensive, metallic; high speed flywheels spin at very high speeds inside vacuum chambers. Their composite rotors are made of carbon-fiber materials; they can fly apart, and have to be encased in heavy protection; superconducting electromagnetic bearings can virtually eliminate energy losses through friction

superconducting magnets
"SMES systems store energy in a magnetic field created by the flow of direct current in a coil of superconducting material that has been cryogenically cooled. SMES systems provide rapid response to either charge or discharge, and their available energy is independent of their discharge rate. SMES systems have a high cycle life and, as a result, are suitable for applications that require constant, full cycling and a continuous mode of operation. Micro-SMES devices in the range of 1 to 10 MW are available commercially for power quality applications."

pumped hydropower
"Pumped hydro facilities use off-peak electricity to pump water from a lower reservoir into one at a higher elevation. When the water stored in the upper reservoir is released, it is passed through hydraulic turbines to generate electricity."

supercapacitors
"With characteristics of both batteries and capacitors, supercapacitors (also called electrochemical capacitors or ultracapacitors) could be used by utilities to regulate power quality. .. charge is stored electrostatically in polarized liquid layers between an ionically conducting electrolyte and a conducting electrode. No chemical reactions occur, charge and discharge based entirely on physical properties."

direct to the grid
No need for storage! But who benefits? Either the health club gets compensated, or the utility gets free power?