Energy Density

Quantity of energy stored by unit of volume (or mass)

What is energy density ?

In physics, energy density refers to the quantity of energy in a material per unit of volume. We also talk about power density.

In the measurement system adopted by the international scientific community, the unit of energy density currently used is joule per cubic metre (J/m³) equivalent to the Pascal (Pa).

Energy density is a key notion in the storage of energies and fuels.

The characteristics of energy density

In terms of energy storage, there are 2 types of energy density:

1. mass energy density, that is to say the quantity of energy stored by unit of mass: this is expressed in watt-hours per kilogram (Wh/kg): 1 Wh/kg corresponds to 3600 J/kg ;

2. volumetric energy density, that is to say the quantity of energy stored by unit of volume: this is expressed in watt-hours per litre (Wh/L) or watt-hours per cubic metre (Wh/m3). The unit of volumetric density expressed in Wh/L is generally used to measure the volumetric energy density of combustible liquids (such as fuels), in contrast to solid combustibles such as wood or coal.

Thanks to these measurements, it is possible to compare the performances of different energy generation processes:

· the combustion of petrol hydrocarbons (13.1 kWh/kg) or diesel hydrocarbons (12.6 kWh/kg) and fuels such as hydrogen (34.1 kWh/kg) ;

· an electrochemical reaction produced within lead batteries (0.027 7 kWh/kg) or lithium-ion batteries (0.5 kWh/kg)

· the use of capacitors (0.001 kWh/kg) or supercapacitors (0.155 kWh/kg) to capture kinetic energy and store it in electrostatic form to be subsequently restored...

For a means of transport, the choice of energy source to be used is in line with the energy density to be deployed, and varies according to the yield from the powertrain.

Examples and application

Energy density is a key indicator for the electrochemical storage of electricity in batteries.

In fact, the 3 main measurement indicators for an electric battery are:

· the voltage in volts (V) indicating the potential difference measured between the 2 terminals (+/-) ;

· the capacity can be expressed in Coulombs (C) or Ampere hours (Ah) corresponding to the quantity of electrical charge able to be stored by the battery ;

· the energy density of the battery by volume or mass.

There is research on the development of technologies which would allow the weight and volume of the batteries to be reduced as far as possible. This is a key point on the production of electric vehicles. The challenge ? Obtaining high-performance batteries offering good autonomy, while being as small and light as possible.

Already used in Li-ion batteries, lithium is currently at the heart of new promising research. American and Japanese researchers have managed to develop a lithium-air accumulator battery able to achieve an energy density greater than 500 Wh/kg.

The energy density of batteries in numbers

There are currently various types of batteries on the market, lead, NiCd, NiMH, Li-ion batteries... As their energy density is a function of the technology used, they are chosen to meet energy needs.

As an example, here is the energy mass given for some batteries used in transport vehicles:

· 0.027 7 kWh/kg for lead batteries used to start combustion engine vehicles ;

· 0.2 kWh for lithium-ion batteries, even 0.5 kWh/kg for lithium-sulfur batteries in electric vehicles.

Regulatory framework

Not applicable