Solar

Question 1

Explain how the PV process works

Answer 1

A charge density gradient is set up in an n-p junction in Si. Solar photons are allowed to irradiate the material and excite electron-hole pairs to be created near the potential gradient.
The charged particles are swept across the potential (current)
The current is delivered at the operating voltage of the characteristic potential of the semi-conductor device

Question 2

Give four factors which explain why all the solar power radiating onto the pholtovotaic surface is not converted into electric power

Answer 2

Not all photons have enough energy to create an electron-hole pair (E_g)
Much of the energy of photons well above the threshold to create electron-hole pairs will simply heat up the device
Some photons are reflected from the surface
Some photons are absorbed by the current collection conductors

Question 3

PV Efficiency

Answer 3

All photons with energy EE_g the photon creates electron-hole pairs with energy E, but only E_g of energy is converted to electrical power; rest is heat
44% is the maximum upper limit of efficiency for Si
Expect 12 to 16%

Question 4

Manufacture of Si PV

Answer 4

Made by reducing Silica (SiO_2) with Carbon C
Carbothermic reduction at 1700C, so energy intensive it will take a PV cell two years of operation just to pay back the energy of manufacture
(SIO_2 + C –> Si + CO_2)
Still requires further processing (electrolysis in salt baths to get the desired purity of Si)