Chapter 2 Introduction to Solar Cells

Question 1

What is the flux from the sun being absorbed by the absorber?

Answer 1

I_E,use = omega/pi * sigma * Aa * (Ts ^4 -Ta^4)

Question 2

What is the absorbing efficiency and how can it be improved?

Answer 2

efficiency = 1 - (Ta^4 / Ts^4) the efficiency can be improved by keeping the temperature of the absorber as cold as possible

Question 3

Of which two components would an ideal solar heat engine consist of?

Answer 3

black absorber + carnot engine

Question 4

What´s the maximum efficiency of solar energy conversion?

Answer 4

absorber efficiency * carnot efficiency efficiency_max = (1 - (Ta^4 / Ts^4)) * (1 - T0/Ta)

Question 5

How is photon energy converted into electrical energy? Explain the semiconductor

Answer 5

A semiconductor consists of two bands with a gap in between. When an incoming photon has higher energy than the gap, it excites the electrons and frees them from their bonds - a charge is generated. Next, the electric field sweeps out the electrons, they move from full to empty band and therefore the charge is separated. Then, the charge is collected when electrons deposit their energy in the external load and the circuit is complete.

Question 6

What are technical limitations of solar PV?

Answer 6

1. Light has to come into solar cell (avoid reflections on the surface) 2. Optical losses: not all photons are absorbed 3. Impurities 4. Electrotechnical limitations

Question 7

Name 3 possibilities to further increase the efficiency of solar cells

Answer 7

1. Tandem cells: two PV cell semiconductors on top of each other, so that not-absorbed photons by one go to the other 2. Concentrator PV cells (CPV): use reflectors and filters to increase the light 3. Thermo-photovoltaic conversion: allow only light from a certain spectrum to reach the solar cell through a filter