Glossary / Tutorials

Biomass means living organisms and those that recently died. It does not include those organisms that are already converted to fossil fuel. In energy generation, it refers to waste plants that are utilized to generate energy by combustion.

Geothermal energy refers to heat energy stored under the ground for millions of years through the earth formation. It utilizes a rich storage of un-utilized thermal energy that exists under the earth’s crust.

Hydro Electric power (HEP) is a major renewable energy source used all over the world today to produce electricity.  Falling water under high pressure has high kinetic energy.  Through magnetic induction, the generator converts the mechanical energy of the turbines to electricity.

The PN Junction

The PN Junction was invented by Russell of Bell laboratories in the USA. It refers to a junction between two semiconductors, that is, P-Type and N-type. Russell discovered that the two semiconductors have an interesting behavior at the junction that causes conduction in one direction only.

A P-type semiconductor has holes (absence of electron) as majority charge carriers. An Ntype semiconductor has electrons as majority charge carriers.

In the diagram given above, at the junction:

• Extra charges diffuse across to the opposite junctions such that the positive on the p-side gain negative charges and neutralize them.

• Similarly, the negatives at the N-side gain positive charges and neutralize them.

• This forms a margin (m) at either side where extra charge are depleted to make this region neutral and at a state of equilibrium. This region is referred to as a depletion layer and no charge from either side crosses.

• The depletion layer offers a potential barrier and thus requires external voltage to overcome it. This process is called biasing.

• To conduct, in forward biasing, the applied voltage should pump electrons (negative) from n-junction towards the p-side of the junction. Continuous flow of current guarantees a constant movement of electrons to fill holes, hence conduction across the depletion layer.

• Reversing the applied voltage, in a process called reverse biasing, causes holes and electrons to drift apart, increasing the depletion layer.

• An external load is connected to a solar cell with positive terminal connected to the N- side wafers and the negative terminal to the P- side wafers. A potential difference is created by photovoltaic effect.

The current obtained by electrons displaced by photons is not sufficient to give significant potential difference. The current is therefore contained to cause further collisions and release more electrons.

Photovoltaic Effect

A solar cell utilizes the concept of a p-n junction in capturing the solar energy. The following figure shows the fermi level of a semiconductor.

For a semiconductor to conduct, electrons must cross the energy gap from the valence band to the conduction band. These electrons require some energy to dislodge and move across the valence gap. In solar cells, photons emitted from the Sun provide the required energy to overcome the gap.

A photon incident on the surface could be absorbed, reflected, or transmitted. If it is reflected or transmitted, it does not help dislodge an electron and is thus wasted. Therefore, a photon must be absorbed to provide the energy required to dislodge and move electrons across the valence gap.

If E_ph is the energy of a photon and EG is the threshold energy to cross the energy gap, then the possible outcomes, when photon hits the surface of a semiconductor are −

• E_ph < E_G − In this case, the photon does not attain the threshold and will just pass through.

• E_ph = E_G − The photon has the exact threshold to dislodge an electron and create a hole electron pair.

• E_ph > E_G − The photon energy surpasses the threshold. This creates an electron-hole pair, though it is a waste, since the electron moves back down the energy gap.

Absorption of solar radiation

In most cases, absorption coefficient of the semiconductor is used to determine the efficiency of absorbing energy from Sun. Low coefficient means poor absorption. Therefore, how far a photon goes is a factor of both absorption coefficient (α) and wavelength of the radiation (λ).

Passive Solar − Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light-dispersing properties, and designing spaces that naturally circulate air.

Renewable energy is forms of energy that are naturally obtained from the environment and from sources that can be replenished naturally. These include solar energy, wind energy, geothermal energy, hydropower, and biomass.

The term renewable energy should not be confused with alternative energy, which describes sources of energy outside the regular forms like gasoline that are considered more environment-friendly or less harmful.

Solar energy is the energy obtained by capturing heat and light from the Sun. Energy from the Sun is referred to as solar energy. Technology has provided a number of ways to utilize this abundant resource. It is considered a green technology because it does not emit greenhouse gases. Solar energy is abundantly available and has been utilized since long both as electricity and as a source of heat.

Active Solar − Active solar techniques include the use of photovoltaic systems, concentrated solar power and solar water heating to harness the energy.  Active solar is directly consumed in activities such as drying clothes and warming of air.

Mono-crystalline solar cell is constructed from a single crystal cylinder sliced off to produce all the wafers in the array. The wafers are circular in shape.  It is characterized by a uniform blue color.

Known as malty-crystalline cells and are made by casting the silicon into a square mould. The resulting cast is then cut into a number of square wafers. The square block is made up of several crystals composed of arrays of blue variations.  They are typically cheaper and less efficient, yet less waste.

A amorphous form of silicon to make photovoltaic cells is a new technique that the experts are still researching to curb the challenges of the crystalline forms.  It is less durable, less efficient. The cells are flexible making them easy to mount on curves surfaces.

Since ancient times, wind was used to move the sails of the ships.  A turbine converts the kinetic energy of the wind to useful mechanical energy. This energy could be used in mechanical form or turn generator turbines and provide electricity.