Introduction
Aircrafts require some form of electrical power to operate navigation, taxi, landing, strobe lights and other electronic systems. The electrical system consist of a battery and an alternator (you will find a generator on older type aircraft) to power the system and recharge the battery. You will also find fuses and switches and lights for indication purposes.
Some electricity generators are used as backup power or other indication purposes. For example, a volt and/or ammeter are used for monitoring a change in voltage or current. However, this change in voltage or current could be brought about through the means of heat or light, which are electricity generators too. In a situation where primary power has failed, the aircraft storage batteries provide an emergency source of electrical power for operating electrical systems of an aircraft.
All of these are connected through several meters (kilometres in large aircraft) of wire and connectors, attached to the airframe with insulation materials.
In this report, I will be focusing on how these electricity generators are used in the various sections of the aircraft so as to understand the function they serve. I will be looking at four different methods of electricity generation. They are Magnetism and motion, Chemical Action, Light and Heat.

Method of Electricity Generation 1 (Magnetism and Motion)
It is important to note that magnetism itself does not create electricity. It requires motion relative to the magnetic field to produce electricity. Energy is required to drive the generators, which then makes use of the magnetic properties to create electricity. These are known as aircraft generators. Modern aircrafts however use alternators as they are lighter and have more capacity.
Aircraft generators/alternators help provide electrical power in an aircraft. The modern-day generator works on the principle of electromagnetic induction discovered by Michael Faraday in 1831-32. Faraday discovered that the flow of electric charges could be induced by moving an electrical conductor, such as a wire that contains electric charges, in a magnetic field. This movement creates a voltage difference between the two ends of the wire or electrical conductor, which in turn causes the electric charges to flow, thus generating electric current.
One major drawback to this method is that the aircraft engine RPM must be above 1200 for the generator to start charging a battery. It requires an energy input so as to produce a current.

http://www.experimentalaircraft.info/articles/aircraft-electrical-systems-6.php http://www.wisc-online.com/Objects/ViewObject.aspx?ID=DCE702 Figure 1 Figure 1.1
Representation of Magnetism Aircraft DC generator and electricity

Method of Electricity Generation 2 (Chemical Action)
Batteries are used in many day-to-day devices such as cellular phones, laptop computers, clocks, and cars. Batteries are composed of at least one electrochemical cell which is used for the storage and generation of electricity. Batteries play a key role in an aircraft as well. The aircraft storage battery provides a reserve source of electrical power for selected electrical systems.
Aircraft have two primary sources of electrical energy. The first is the generator, which converts mechanical energy into electrical energy. The second is the battery, which converts chemical energy into electrical energy. During normal aircraft operation, the generator maintains the battery in a charged state. Thus the generator is the main source and the battery is the auxiliary source.
There are two types of batteries, the Lead-Acid battery and the Nickel-Cadmium battery. Fundamentally, there is no difference between the lead-acid aircraft battery and the lead-acid automobile battery. Both have lead plates in a solution of sulphuric acid and water (electrolyte).
The nickel-cadmium battery gets its name from the composition of its plates, nickel oxide on the positive plate and metallic cadmium on the negative plates. The electrolyte consists of potassium hydroxide and water. Both operate on the same basic principles.
Electrodes (2 different metals) are immersed in a solution known as an electrolyte (Sulphuric acid). The chemical reaction between the 2 electrodes and the electrolyte produces a potential difference between the two electrodes and thus causes current to flow. This process produces electricity. http://en.wikipedia.org/wiki/Nickel%E2%80%93cadmium_battery_%28vented_cell_type%29 http://www.wisc-online.com/Objects/ViewObject.aspx?ID=DCE702 Figure 2 Figure 2.1
Chemical Action representation Turbine Starting Aircraft Battery
Method of Electricity Generation 3 (Light)

Photovoltaic (PV) cells found in solar panels convert the sun's energy (Photons) directly into electric energy. When photons are absorbed by the semiconductors found in PV cells, they knock of electrons from the semiconductors, creating free electrons. Thus usable current is produced.

Although this method of producing electricity in not popular in commercial aircraft due to its heavy dependency on light, it is still an excellent means to produce green energy. Some unmanned aircraft such as Helios, Centurion use this method to produce the required electricity.

The Solar Impulse is a one seater aircraft currently in development by the Swiss Federal Institutes of Technology. It uses Photovoltaic cells in conjunction with batteries to have longer sustained flight.

http://news.softpedia.com/news/Solar-Energy-to-Power-Up-Aircraft-98087.shtml http://en.wikipedia.org/wiki/Photovoltaics_in_transport Figure 3 Figure 3.1 Helios in flight Centurion in flight

Method of Electricity Generation 4 (Heat)

Thermocouples are used in different temperature-sensing applications in turbine engines. Thermocouples are based on the principle that when two dissimilar metals are joined, a predictable voltage will be generated that relates to the difference in temperature between the measuring junction and reference junction.

An electromagnetic field (EMF) is generated within the thermocouple when the ends are maintained at different temperatures. The magnitude of the EMF that is generated is proportional, and is related to the temperature difference between the two points, not just the temperature of the junction that is being measured. The EMF generated is then measured by a millivolt meter or potentiometer so as to determine temperature.

Temperature sensors that can monitor temperature directly on various locations in gas turbine engines are necessary for diagnosis of engine conditions during operation. Thus thermocouples are used.

http://en.wikipedia.org/wiki/Thermocouple http://www.omega.com/temperature/z/pdf/z021-032.pdf Figure 4 Figure 4.1 Thermocouple representation Thermocouple

Strengths and Weakness of the Methods

Magnetism and Motion
Strengths:
- Renewable electricity
- Common and relatively cheap

Weakness:
- Requires a significant energy input to get it started.
- Conversion of energy to electricity is done by generators not the magnet itself.

Chemical Action
Strengths:
- Inexpensive
- Reliable and last longer due to the high number charge discharge cycles
- Can have high discharge

Weakness:
- Heavy
- Can 'die' rapidly and unexpectedly. Internal resistance drops as temperature rises.
- Requires discharging for long life
- Toxic.
- Suffers from thermal runaway.

Light
Strengths:
- Sustainable
- It is a well proven technology
- Renewable Energy
- Categorised as green energy (pollution free)

Weakness:
- the manufacturing process requires large amounts of energy
- contains toxins that need to be disposed of carefully at the end of the life of the panel
- Solar energy is spread relatively thinly. If a photovoltaic generator is to produce much electricity (i.e. several megawatts) it has to cover a large area.
- Heavily dependent on sunlight (photons)

Heat
Strengths:
- Inexpensive
- Able to measure a huge range of temperature
- It is self-powered and does not rely on other systems for functioning.

Weakness:
- Electricity output is extremely low
- Extremely sensitive thus careful calibration and maintenance required for accurate temperature reading.

Conclusion

As we can see, even though there may be many ways to generate electricity, they are chosen specifically by the purpose they serve on the aircraft.
A thermocouple may be heavily dependent on heat and only produce a very small voltage, but this is extremely useful in measuring the temperature of various sections of an aircraft engine. Thus enabling diagnosis while operation. Likewise, Chemical action may be able to generate electricity for a given time, but with the help of generators, they are able to recharge and act as secondary power source for aircraft systems.