|diesel oil (DERV)||12.7||37.9|
|battery type||energy density
A flywheel system designed for energy storage in a racing car can store
400 kJ (0.1 kWh) of energy and weighs 24 kg (p126). That’s an energy den-
sity of 4.6 Wh per kg.
High-speed flywheels made of composite materials have energy densi-
ties up to 100 Wh/kg.
Supercapacitors are used to store small amounts of electrical energy (up to
1 kWh) where many cycles of operation are required, and charging must
be completed quickly. For example, supercapacitors are favoured over
batteries for regenerative braking in vehicles that do many stops and starts.
You can buy supercapacitors with an energy density of 6 Wh/kg.
A US company, EEStor, claims to be able to make much better super-
capacitors, using barium titanate, with an energy density of 280 Wh/kg.