Power Generation, Density

Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other advanced electronic system of your choice. The electrical system consist of a battery and an alternator or generator on older aircraft. All of this is connected through several meters (kilometers/ miles in large aircraft) of wire.

All matter on Earth is made up from molecules and they basically consist of atoms. These atoms are made of electrons, protons and neutrons. And electricity is about the flow of free electrons attracted to protons and repelled by other electrons.

Our previous page discussed several ways to store energy in lead acid, lithium batteries and liquid fuel. The main difference between these are the amount of energy per weight or volume in such a storage system.

Energy Density

The definition of energy density is the amount of energy stored in a given system or per unit volume. I would like to expand that with the amount of energy generated per m³ volume of area for an energy generating system.

If we would create a short list from the highest to lowest amount of energy released by substance we get: nuclear, chemical, electrochemical and electrostatic. Examples are: nuclear power stations work by releasing the energy that hold nuclei together (which is a huge amount). Chemical energy is released by the fuel we use in our engines and food in our bodies. The last two are used by batteries and capacitors we have in our cars, airplanes and mobile devices.

For aircraft this means that we should use a source that has the highest amount of energy per weight, as this means more useful payload, lower stall speeds and higher range or lower fuel consumption for the amount of energy source we carry onboard to accomplish this objective.

Nuclear

Using a nuclear powerplant in an aircraft is not the best solution, it has been done in the past but safety regulations forbid that, common sense too. Land or ship based power plants have the highest energy density with regard to area use (m²) and availability (time).

For those of you who are afraid of radiation but love the sun (who doesn't) we have an image that shows that sunbathing is not so healthy compared to the amount of radiation that is considered safe for us humans. Hover your mouse over the image for a more readable picture.

Chemical

This form we see with the normal fuels we use in aircraft, JET A and A1, AVgas, MOgas and Diesel. These have the highest energy densities in liquids we know today. When burned completely they release heat, water and carbon dioxide which are common elements on this planet/ universe. For a fuel to burn effectively it needs to be chemically correct mixture (stoichiometric) with oxygen, which will not happen inside a fuel tank.

Electrochemical

Commonly found in batteries. These are mostly made up from lead-acid or a lithium based combination with a (rare Earth) metal. Their energy density is much lower (50 times) than that of liquid fuel. For portable devices it is a good solution as long as the amount of energy stored is limited. The reason being is that these devices contain everything that is needed (anode and cathode separated by a very thin dielectric) to create a huge current and fire or explosion when these batteries are overstressed electrically, mechanically or heat exposed.

Electrostatic

You will see this typical effect in all sorts and kinds of capacitors. These components charge via an electrostatic principle between two conductors separated by an isolator. You can compare it with walking on a carpet where you build up a static charge until you grab that metal door knob. These caps have very low energy densities and are used in electronic devices as energy buffer in power supplies, tuned LC circuits or as DC isolator between amplifier stages.

SuperCapacitors are used in regenerative braking systems, energy recovery and power supply buffering. A battery/ supercap combo is sometimes used as a starter combination for engines.

Overview

Below you will find a list of the most common energy sources and their densities:

This list is not complete by far but gives an idea of the materials and their densities to store energy in aircraft (except nuclear). The following conversions: 3.6 MJ equals 1 kWh is about 1.34 HPh, might help when comparing these to other sources. For a complete list, look at https://en.wikipedia.org/wiki/Energy_density, or click the next link for an image of energy vs mass for different sources.