3.4 - Energy Density of Fuels
- 1Understanding the concept of energy density
- 2Comparing energy densities of different fuels
- 3Calculating the volume of fuel required for energy generation
- 4Applying energy density to practical scenarios
What is Energy Density?
Energy density refers to the amount of energy that can be released from a unit volume of a fuel. It quantifies the energy content of a fuel per cubic metre (m³) and is typically measured in gigajoules per cubic metre (GJ m-3).
The extraction of fossil fuels often involves arduous and hazardous work, yet these fuels continue to be widely used. The reason lies in their high energy density, which makes them an attractive source of energy despite the associated risks and efforts.
Comparing energy densities of fuels
Different fuels possess varying energy densities, ranging from low values for gases like hydrogen to extremely high values for nuclear fuels like uranium. The table below illustrates the energy densities of some common fuels:
| Fuel | Energy Density (GJ m) |
|---|---|
| Uranium (nuclear fission) | 1.3 × 10⁹ |
| Coal | 20 - 80 |
| Diesel | 37 |
| Gasoline (Petrol) | 35 |
| Natural Gas | 0.036 |
| Hydrogen | 0.01 |
Notice the wide range of energy density values across different fuel types. Fuels with higher energy densities can release more energy per unit volume, making them more efficient and compact for energy generation.
Calculating the volume of fuel required
To determine the volume of fuel needed for a specific energy requirement, we can use the following formula:
volume = $\frac{\text{energy}}{\text{energy density}}$
Where:
volume is measured in m3
energy is measured in J
energy density is measured in GJ m-3
Worked example - Calculating the volume of fuel required
A fossil-fuel power station burns coal with an energy density of 42 GJ m-3. The station has an efficiency of 25 % and generates 1200 MW of useful electrical power.
Calculate the volume of coal burnt every minute.
Step 1: Calculate the fuel energy required per second.
Fuel energy required = $\frac{\text{power output}}{\text{efficiency}}$
Fuel energy required = $\frac{1,200\text{ } ×\text{ }10⁶}{0.25}$ = 4.8 × 10⁹ J s-1
Step 2: Calculate the volume of coal required per second
Volume of coal = $\frac{\text{fuel energy required}}{\text{\text{energy density}}}$
Volume of coal = $\frac{4.8\text{ } × \text{ }10⁹}{42\text{ } × \text{ }10⁹}$ = 0.114 m3 s-1
Step 3: Calculate the volume of coal burnt every minute.
Volume of coal = volume of coal per second × 60
Volume of coal = 0.114 × 60 = 6.9 m3 min-1
Therefore, the power station burns 6.9 m³ of coal every minute.