fuel calorific value
(kWh/kg) (MJ/l)
propane 13.8 25.4
petrol 13.0 34.7
diesel oil (DERV) 12.7 37.9
kerosene 12.8 37
heating oil 12.8 37.3
ethanol 8.2 23.4
methanol 5.5 18.0
bioethanol 21.6
coal 8.0
firewood 4.4
hydrogen 39.0
natural gas 14.85 0.04
battery type energy density
(Wh/kg)
(cycles)
NiMH 60–120 300–500
lithium-ion 110–160 300–500
lithium-ion-polymer 100–130 300–500
reusable alkaline 80 50
(a) (b)

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

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.

Figure 26.13. Some properties of storage systems and fuels. (a) Energy density (on a logarithmic scale) versus lifetime (number of cycles). (b) Energy density versus efficiency. The energy densities don’t include the masses of the energy systems’ containers, except in the case of “air” (compressed air storage). Taking into account the weight of a cryogenic tank for holding hydrogen, the energy density of hydrogen is reducedE 39 000Wh/kg to roughly 2400 Wh/kg.
Table 26.14. (a) Calorific values (energy densities, per kg and per litre) of some fuels (in kWh per kg and MJ per litre).
(b) Energy density of some batteries (in Wh per kg). 1 kWh = 1000Wh.
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