Solar Energy

Question 1

What do pyranometer, pyrheliometer, shaded pyranometer and integrated pyranometer do?

Answer 1

pyranometer - measures total solar irradiance on a planar surface
pyrheliometer - measures direct beam solar radiation
shaded pyranometer - measures diffuse solar irradiance
integrated pyranometer - measures both total and diffuse

Question 2

Describe photovoltaics and the physics behind them?

Answer 2

• Overall, make use of the photoelectric effect where a photon striking an atom in a semi-conductor material can liberate an electron, these liberated electrons flow in an external circuit giving rise to an electric current
  Physics:
• Energy of a photon is related to its frequency: E = h*v
• Energy needs to be greater than the band gap energy (Eg, difference between valence and conduction bands) to create electron-hole pair
• This is limiting factor of PV generation as only a fraction of solar radiation can be used (Stockley-Quiesser limit = 33.7%)
• Each photon with energy greater than the band gap can only create one electron-hole pair so excess energy is released as heat

Question 3

Describe a PV cell?

Answer 3

• Essentially made up of two dissimilar semi-conducting materials
  
  • P-type (electron deficit)
  • N-type (electron surplus)
• Metal contacts are added to allow the liberated electrons to flow round an external circuit
• So combination of doped silicon and metal contacts form the PV cell, then combined with glass cover, substrate and power circuitry to form a panel

Question 4

How can PV performance be improved?

Answer 4

• tilt the solar panels depending on position of sun in the sky
• use power point tracking which optimises the power yield from the PV as the current total and T vary with time

Question 5

Solar declination, what is it?

Answer 5

Arises from the earths axis being tilted in relation to the earth-solar-plane and the relative tilt of the earth in relation to the sun over the course of a year

Question 6

What are the following quantities:
Idn -
Idh -
Ifh -
Igh -
rg -

Idβ -
Isβ -
Irβ -

Answer 6

Idn - direct normal
Idh - direct horizontal
Ifh - diffuse horizontal
Igh - global horizonal
rg - gound reflectivity

Idβ - direct radiation
Ifβ - diffuse radiation
Irβ - ground reflected radiation