Power sources: fuel cells, solar cells, heat, vibration and fusion

June 15/22, 2005
Batteries, fuel cells and solar cells each mimic one of the functions plants perform to gain energy from the sun. Plants capture photons and use their energy to break water molecule bonds to release the energy tied up in them. They liberate electrons to drive chemical reactions that turn carbon dioxide into various carbohydrates.

Batteries, fuel cells and solar cells use free electrons to generate a flow of electricity.

Solar cells transfer the energy contained in photons directly to electrons to cause a flow of electricity.

Fuel cells break chemical bonds to gain electrons.

Batteries are similar, but cyclic - to charge they draw energy to form chemical bonds that store energy, then break those bonds to release the stored energy when it is needed.

Breaking chemical bonds

Batteries generate electricity when two solutions of ions chemically react to transfer electrons from one to the other. Ions are atoms that carry a charge because they have more or fewer negatively-charged electrons than positively-charged protons. Instead of a direct transfer, however, the electrons leave the batteries via one electrode - the cathode - and return through another - the anode. This provides a current of electrons to a device connected to the battery. The anode is generally made from a more conductive material than the cathode.

Fuel cells are similar, but their fuel is replenishable and they cannot be recharged. In fuel cells, the cathode and anode are separated by a membrane. The electrodes are catalysts; the cathode strips electrons from hydrogen, leaving hydrogen ions in the membrane. The anode combines oxygen and electrons with the hydrogen ions to produce water. The electrons flow in a circuit from the cathode to the anode.

Electricity from light

Different types of atoms contain different numbers of electrons arranged in layers around a nucleus of protons and neutrons. One way to picture electrons is in orbit around the atomic nucleus, similar to planets orbiting the sun. Electrons jump to a higher orbit, however, when they gain energy.

Different materials also contain different bandgaps; a material’s bandgap is the energy needed to push an electron from the first orbit, or valence band, to a conduction band where electrons can flow from one atom to the next.

Solar cells consist of two layers of a semiconductor; one carries positive charges and the other negative charge.

Sunlight is absorbed by the positive layer, and the photons’ energy is imparted to the semiconductor’s atoms. Electrons in the energized atoms jump from the valence band to the conduction band. From there the electrons are drawn to the negative semiconductor layer and passed to an electrical circuit.

Electricity from heat and vibrations

In theory, it is possible to extract energy from any environment where two areas consistently hold different amounts of energy.

Thermovoltaics extract energy from environments where there are large temperature differences, and piezoelectric materials do the same where there are differences in mechanical energy.

Thermovoltaic methods use the temperature differences between a heat source like a car engine or even a warm body and the outside air.

Given the right materials, electrons will flow from an area of high temperature to an area of low-temperature or vice versa. Thermoelectric generators consist of a pair metals or semiconductors, one negative and one positive, that produce flows in opposite directions. Connecting the cold ends of the pair to a circuit generates electricity because electrons in the negative material flow to the cold end and onto the circuit, where they are are drawn to the cold end of the positive material.

Piezoelectric crystals generate electric charges on their surfaces when they are subjected to mechanical stress. These charges can produce a small electric current when connected to a circuit.

Some watches are powered by the motion of the people wearing them. And some downhill skis contain light-emitting diodes powered by the vibration of the skis.

Energy from Atoms - Fusion

Nuclear fusion is the reaction that powers the sun and hydrogen bombs. The challenge to using fusion as an energy source is controlling it.

It takes a great deal of energy to force a pair of atoms together, but once they come close enough, they fuse to form a new type of atom; the reaction gives off a tremendous amount of energy.

A controlled fusion reaction involves fuel that is compressed to become very dense, a great deal of ignition energy, and a lot of tricky timing. The reaction must also be contained, which is difficult because it takes place at 100,000,000 degrees Celsius, which is hot enough to burn through any material.

Scientists are working on controlling fusion reactions fueled by hydrogen isotopes. Hydrogen is the simplest atom, and normally consists of one proton and one electron. Hydrogen isotopes are denser than plain hydrogen because they also contain one or more neutrons.

The reaction is similar to the internal combustion engines used in cars, where fuel is compressed and a spark ignites the fuel. When successfully sparked, the fuel explodes, releasing energy.

The compressed fusion fuel, or plasma, is held in place by lasers, and the reaction is sparked by a laser. The laser energizes the plasma because as photons travel through it, they slow down, transferring heat to the plasma.

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