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Energy & Environment > Solar Energy > Flashcards

Solar Energy Flashcards

1
Q

What are the two types of solar resources?

A

Solar photovoltaics

Solar thermal

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2
Q

What amount of solar intensity reaching the outer atmosphere (1370W/m2) reaches the earths surface?

A

950W/m2 approximately 70%

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3
Q

What is the radius of the earth?

A

6371km

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4
Q

What is the global average insolation ?

A

240W/m2 (a quarter of incident light intensity)

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5
Q

How do photovoltaic panels utilise solar radiation?

A

Incident photons absorbed

Releasing electrons and heat (electricity generated directly)

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6
Q

What wavelengths of light do photovoltaics use?

A

Mainly visible spectrum

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7
Q

How does passive solar, solar hot water and concentrating solar systems utilise solar radiation?

A

Absorption of thermal energy

Can store or use heat directly

An couple with a steam power block

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8
Q

What do some solar farms do in order to absorb the most sunlight through the day?

A

Single axis and dual axis tracking

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9
Q

Explain the band gap phenomenon between a conduction band and a valence band

A

It is the energy required to excite an electron from the valence band to the conduction band

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10
Q

How large would the band gap be in insulators,semiconductors and conductors respectively?

A

Large, moderate, non-existent

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11
Q

How do electrons get their energy to jump the band gap in photovoltaic cells?

A

Incident sunlight provides photons (energy)

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12
Q

How is PV efficiency determined by the materials it’s made from?

A

The material determines the band gap between valence and conduction bands

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13
Q

Why do we use semiconductors in PV?

A

Electrons need an actual gap to cross in order for power to be produced

Power = voltage x current (bandgap x no. of electrons)

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14
Q

What is bandgap measured in?

A

eV

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15
Q

What can bandgap vary between, depending on material?

A

0.5-2.5 eV

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16
Q

How is it that silicon, an almost ideal PV material is only able to convert 35% of incident radiation into useful energy?

A

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

17
Q

Describe the 1st generation of PV

A

Crystalline silicon - most common, based on silicon wafer

Moderate efficiency, high cost

18
Q

How does 2nd generation silicon wafer differ from the 1st?

A

Lower efficiency, but lower cost

19
Q

What is special about 3rd generation silicon wafers?

A

Multifunction cells which utilise different parts of the solar spectrum

High efficiency, cost effective, low material usage

20
Q

Why do we concentrate solar radiation?

A

To collect higher quality heat

21
Q

Why is there a limit to the concentration factor C of light?

A

There limit is there for practicality reasons, you need a material that can withstand high heat

22
Q

When concentrating solar power, what is Q. given as?

A

Q.=AcI - ArσTh^4

AcI - sunlight collecting area

ArσTh^4- radiated loss

23
Q

What can you do with the radiated losses produced from concentrating sunlight?

A

Connect it to a lower station and drive a steam cycle

24
Q

What is overall η efficiency equal to?

A

ηCarnot x ηabsorption

25
Q

If we want to store solar energy, how might we do it?

A

Through use of batteries for harnessed electricity

Through use of hot salt tanks for thermal energy

26
Q

The hot salt storage tank offers dispatchable energy, what does this mean?

A

It can offer energy on demand,by means of a steam cycle power system

27
Q

Why is solar energy becoming more cost effective?

A

It has a good capacity factor for storing energy due to the nature of sunlight being available some of the day and more so in summer than in winter

28
Q

How is a solar thermal energy performance governed?

A

Collection process, thermodynamics constraints

29
Q

For a flat plate, how much energy is lost due to reflection of the surface?

A

20%

30
Q

If the incident sunlight less intense, are the thermal losses of energy greater or lower?

A

Greater

31
Q

What are some kinds of thermal collectors in order of their efficiency

A

Evacuated tube
Flat plate
Air collector
Plastic absorber

Energy & Environment (13 decks)

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