Solar Power

Question 1

When does a solar panel experience maximum voltage? (draw graph too)

Answer 1

Peak voltage occurs when the system is in open circuit e.g. I = 0

Question 2

What is the v open circuit value?

Answer 2

0.590V

Question 3

What is the best and typical efficiencies of PV panels?

Answer 3

Best = 21.5%
Typical = 15%

Question 4

Provide three reasons why all solar radiation doesn’t turn into electricity? (6)

Answer 4

• Photon energy less than Eg: Photon energy must be > Eg PLUS Δφ
• Photon energy very high and percentage turned to heat. Easily makes an electron but remaining heat is lost
• Recombination of electron - hole pairs
• electrical resistance within the cell
• lost harvesting area due to the need to place conduction bars on the surface of the cell
• refection of light from the surface

Question 5

What’s the difference between thermal and direct solar?

Answer 5

Photovoltaic provides direct conversion from solar (photon) irradiation to electricity with no moving mechanical parts, but it is currently very expensive. Thermal harvesting collects heat from the incident solar flux; the collection of low grade heat is relatively inexpensive

Question 6

Pros/Cons of thermal solar?

Answer 6

Pros:

• low grade heat is relatively inexpensive
• good areas with cheap land and lots of sun

Cons:

• need cheap retail space
• efficiency loss when turing into electricty
• if not large, need many stirling engines

Question 7

Why are some photons not strong enough to generate electricity?

Answer 7

the energy content of the light (the photons) must be equal or larger than that required to get a vacancy electron within the photovoltaic material to become a conduction electron. Thus the solar flux with wavelengths longer than a certain value cannot generate electron hole pairs and is simply lost

Question 8

What are donor and acceptor impurities?

Answer 8

Impurities are elements added to the base semiconductor which have more or less electrons on their outermost shell than that of the base material. Taking Si as an example; Si has four electrons on the outermost shell of the atom. The crystalline structure of Si is obtained by covalent bonds holing each Si atom in place with f our adjacent Si atoms. A ‘donor’ impurity will have 5 electrons on its outmost shell. There will now be a ‘spare’ electron, even though the composite is electrically neutral. This loose electron is only weakly bound and finds it relatively easy to move around the crystalline structure (eg P). Materials doped with donor impurities are known as n-type.

An acceptor impurity will have 3 electrons in its
outmost shell. This will appear as a ‘hole’ for adjacent electrons to occupy (eg Al). Materials doped with acceptor impurities are known as p-type.

Question 9

What are diffusion and field currents? (2)

Answer 9

• Diffusion currents are those which result from an electron rich zone being close to an electron poor one, electrons diffuse from the rich to the poor zones giving rise to the diffusion current. Once the electrons have diffused, they will leave behind an electrically positive zone and have created a negatively charged zone in their new location.
• This electric field encourages electrons to flow in the opposite direction to that of diffusion. This is known as the field current

Question 10

What is the energy gap between covalent and conduction electrons? (3)

Answer 10

• In non-conducting poly-crystalline ceramic-like structures, electrons are held in position by valence bonds.
• In conducting metals, the outmost electrons are in a conduction zone and are free to move about. In semiconductor materials, most of the electrons are held in position by strong valence bonds, but there are a few that, due to thermal excitation have managed to move from the valence to the conduction band.
• The energy required to excite a valence electron to become a conduction one is the energy gap between the covalent and conduction electrons (1.12eV for Si)

Question 11

What happens to the efficiency of PV panel as the temperatures increases and why? (5)

Answer 11

• increase in the number of thermal generated electron
  hole pairs in both the n and p side of the device.
• Close to the junction, the field will push the electrons on the p-side across to the n-side and vice versa for the holes - field current will increase with an increase in temperature.
• Results in a temporary increase in the number
  of electrons and hence the diffusion current will increase.
• Provided there are no externally applied potential, the diffusion and field current will be equal, giving a zero net current across the junction.
• However, the potential at the junction Δφ will decrease.
• The electrical power generated during solar irradiation is I x V. Increasing the temperature will
  increase I, but will reduce V by more (reducing overall efficiency )

Question 12

What is the typical flux falling on the planet?

Answer 12

1kw/m^2

Question 13

What reduces the efficiency of a solar panel? provide a breakdown of the efficiency of different aspects and the overall final efficiency

Answer 13

• 77% can generate electricity (but 30% ends up as heat)
• 47% efficient
• void factor = 65%
• Refelection = 96%
• Recombination of hole pairs = 90%

Total efficiency = 0.47 x 0.63 x 0.96 x 0.9 = 24 - 25%

Question 14

What is the equation for energy in a photon?

Answer 14

E = hv

Question 15

The equation for the velocity of a photon?

Answer 15

v = c/ lamda

Question 16

why does the solar flux reduce from 1.37 kw/m^2 to 1kw/m^2?

Answer 16

scattering and absorption getting to the earth’s surface

Question 17

what is the estimated utilisation factor of solar?

Answer 17

0.2 of day producing

Question 18

what does e_b equal?

Answer 18

1.12 eV moves electrons from the low level to the high level where it can now move - amount Work we need to do to pull the electron out

Question 19

n type and p type (draw diagrams with Silicon)

Answer 19

n type - phosphours (gives electron)

p type - aluminium (makes a hole)

Question 20

what does adding impurities do to the silicon?

Answer 20

increases the electo conductivity of the material

Question 21

what happens when you combine n-type and p-type?

Answer 21

• stay electrically neutral but the electrons want to diffuse
• diffused electons result in a positive area left behind
• stop diffusion when the potential pulls it back

Question 22

how could we heat up a solar panel? (keep it warm)

Answer 22

Maintaining the cell at a higher temperature is relatively easier as the cell will heat up in the sun (not all photons entering the cell are converted into electricity; the rest will tend to as heat) simply insulating the back and side of the cell will naturally increase its temperature above the surrounding environment, provided it is in the sun.

Question 23

how could we cool a solar panel?

Answer 23

Maintaining the cell below the environment temperature is more difficult. Using conventional refrigeration will work, but will also be expensive in energy use. An alternative is to engineer some form of evaporative cooling (simply blowing dry air over a dry cell will tend to maintain its temperature close to the environment temperature; it will NOT reduce it below the environment, however, blowing air over a damp porous membrane held on the back of the cell will reduce the temperature below the environment temperature).