Reducing Energy Consumption, while Protecting the Environment - Energy and Climate: How to Achieve a Successful Energy Transition - page 60

Figure 5.4 Comparison between different energy storage systems
amounts to 12 kWh/kg, whereas in the case of storage of hydrogen at a
pressure of 700 bar, the stored energy density does not exceed 1.2 kWh/kg.
The electrochemical energy storage density providedby a battery is even
lower by an order of magnitude (20-120Wh/kg). In the case of mechani-
cal systems such as flywheels or compressed air storage, the stored energy
density is also comparatively small: 1-5Wh/kg for flywheels and 8Wh/kg
for air compressed at 200 bar. Still, electricity or mechanical energy
storage should not be considered as directly equivalent to the chemical
energy storage provided by a hydrocarbon. Ahydrocarbon delivers energy
in the form of heat, which has to be converted into mechanical and then
electrical energy with an efficiency substantially less than 1. Furthermore,
the energy is generated in an irreversible way and the system cannot be
reloaded.
Although the specific energy density is comparatively limited in most
cases and especially when mobile applications are considered, electro-
chemical battery storage is the most readily applicable. Extensive devel-
opment programmes are underway to further improve the different
types of batteries which can be used (lead-acid, nickel and lithium).
Nickel-metal hydride (NiMH) batteries are most commonly used in the
presently available hybrid cars. The specific energy storage density of these
batteries (55Wh/kg) is significantly higher than the storage density of lead
batteries (25Wh/kg). Lithiumbatteries (lithium-ion and lithiumpolymer)
are considered the most promising (70Wh/kg), but are comparatively