Flywheels store and release kinetic energy (mass in motion)  in this context, they apply to devices which may be used to store electrical energy by attaching a motor-generator to a high-speed rotating weight. 
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Sites 4
            
                                    Graphic comparisons of power, rotor speed, and energy.  Bibliography includes English and German titles.
                                    
                                  
                                
            
                                    Makes 2 and 6 kilowatt-hour flywheel (kinetic-electric) energy storage units.
                                    
                                  
                                
            
                                    article includes equation for kinetic energy
                                    
                                  
                                
            
                                    Markets PowerStore flywheel system.
                                    
                                  
                                
            
                                    Graphic comparisons of power, rotor speed, and energy.  Bibliography includes English and German titles.
                                    
                                  
                                
            
                                    Makes 2 and 6 kilowatt-hour flywheel (kinetic-electric) energy storage units.
                                    
                                  
                                
            
                                    Markets PowerStore flywheel system.
                                    
                                  
                                
            
                                    article includes equation for kinetic energy
                                    
                                  
                                