Solar Energy Flashcards
What are the two types of solar resources?
Solar photovoltaics
Solar thermal
What amount of solar intensity reaching the outer atmosphere (1370W/m2) reaches the earths surface?
950W/m2 approximately 70%
What is the radius of the earth?
6371km
What is the global average insolation ?
240W/m2 (a quarter of incident light intensity)
How do photovoltaic panels utilise solar radiation?
Incident photons absorbed
Releasing electrons and heat (electricity generated directly)
What wavelengths of light do photovoltaics use?
Mainly visible spectrum
How does passive solar, solar hot water and concentrating solar systems utilise solar radiation?
Absorption of thermal energy
Can store or use heat directly
An couple with a steam power block
What do some solar farms do in order to absorb the most sunlight through the day?
Single axis and dual axis tracking
Explain the band gap phenomenon between a conduction band and a valence band
It is the energy required to excite an electron from the valence band to the conduction band
How large would the band gap be in insulators,semiconductors and conductors respectively?
Large, moderate, non-existent
How do electrons get their energy to jump the band gap in photovoltaic cells?
Incident sunlight provides photons (energy)
How is PV efficiency determined by the materials it’s made from?
The material determines the band gap between valence and conduction bands
Why do we use semiconductors in PV?
Electrons need an actual gap to cross in order for power to be produced
Power = voltage x current (bandgap x no. of electrons)
What is bandgap measured in?
eV
What can bandgap vary between, depending on material?
0.5-2.5 eV
How is it that silicon, an almost ideal PV material is only able to convert 35% of incident radiation into useful energy?
18% of radiation is too low energy to excite electrons across the band gap
Of the remaining 82%, many of it gets lost due to absorption via the environment, and that which arrives at silicon, silicon itself is not a perfectly efficient material, so further losses incur
Describe the 1st generation of PV
Crystalline silicon - most common, based on silicon wafer
Moderate efficiency, high cost
How does 2nd generation silicon wafer differ from the 1st?
Lower efficiency, but lower cost
What is special about 3rd generation silicon wafers?
Multifunction cells which utilise different parts of the solar spectrum
High efficiency, cost effective, low material usage
Why do we concentrate solar radiation?
To collect higher quality heat
Why is there a limit to the concentration factor C of light?
There limit is there for practicality reasons, you need a material that can withstand high heat
When concentrating solar power, what is Q. given as?
Q.=AcI - ArσTh^4
AcI - sunlight collecting area
ArσTh^4- radiated loss
What can you do with the radiated losses produced from concentrating sunlight?
Connect it to a lower station and drive a steam cycle
What is overall η efficiency equal to?
ηCarnot x ηabsorption
