Losses, Effiency Limits

Question 1

What is the difference between following upper efficiency limits:

1. Carnot
2. Landsberg
3. Black body limit

Answer 1

1. carnot -> 95%
2. 33%
3. 85.4%

Question 2

What could the idea solar energy conversion device look like to achieve an effiency of h>86%

Answer 2

carnot cycle

definition in GN

To achieve effiency limits; the main requirement is no entropy generation during the transmission, absorption or conversion of the sunlight.

Question 3

What does the Shockley-Queisser (SQ) efficiency limit tell us? What
assumptions are involved?

Answer 3

The SQ effiency limit considers the single junction solar cell as a black body that obeys fundamental characteristics of a semiconductor.

1. no parasitic, optical losses
2. perfect transmission Eph < Eg (no absorption within bandgap, defect states)
3. Perfect absorption
4. EQE = 1 for E > Eg
5. Plancktian black-body radiation at the absorber temperature

Question 4

What is the ultimate limit for any energy conversion process?
Describe the derivation of this efficiency limit.

Answer 4

Eg(Si) = 1.1eV

Question 5

Explain the two key losses related to spectral mismatch.

Answer 5

parasitic and optical losses

Question 6

Why is radiative recombination considered for the SQ limit?

Answer 6

As the perfect solar cell has no non-radiative losses. It only exhibits radiative recombination

Question 7

What is the concept behind “detailed balance”?

Answer 7

Detailed balance provides a technique to calculate the maximum effiency of photovoltaic devices.

Equation in GN

Question 8

What’s entropy?

Answer 8

Provides a measure of the amount of thermal energy that cannot be used to do work

Question 9

What’s clausius?

Answer 9

Entropy expressed as heat divided by temperature

Transfer of a small amount of heat causes a larger change in entropy in a cold boy rather than in a hot body

Question 10

For which bandgap range is the maximum PCE according to the SQ limit
larger than 30%

Answer 10

1.1eV