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CPE451 > Solar PV > Flashcards

Solar PV Flashcards

1
Q

What is solar photovoltaic energy conversion?

A

A one step conversion process that changes light energy directly into electrical energy.

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2
Q

What is happening inside a material when it heats up when exposed to sunlight?

A

Absorbed energy converted to kinetic energy of atoms and electrons i.e. the material heats up.

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3
Q

What is happening in a photochemical reactions when exposed to light? (e.g., photosynthesis)

A

Absorbed energy increases the potential energy of an electron.

This electron acts as a catalyst for the reaction.

This quantum energy conversion then results in a permanent increase in chemical potential.

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4
Q

What happens when luminescent materials are exposed to sunlight?

A

The energy is emitted again as a photon (re-emission).

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5
Q

What happens when photovoltaic materials are exposed to sunlight?

A

Absorbed energy increases the potential energy of an electron.

This increased potential energy is then used to provide electrical power.

This allows direct conversion of electromagnetic radiation into electrical energy.

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6
Q

What determines what wavelength and amount of electromagnetic radiation that can be absorbed by a material?

A

The energy levels/electronic properties of a material

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7
Q

What is the Pauli Exclusion Principle?

A

No two electrons can have the same four quantum numbers.

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8
Q

What does ‘s’ and ‘p’ describe in terms of the atomic structure of materials?

A

The sub-shell

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9
Q

What does ‘x’, ‘y’ and ‘z’ describe in terms of the atomic structure of materials?

A

The specific orbital

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10
Q

Describe what is meant by the energy levels in materials?

A

All atoms have discrete energy levels.

When atoms are combined to form molecules or crystals, these discrete energy levels split and merge to form a continuous energy band.

In a metal, all of the energy levels overlap to form a continuous energy band.

In semi-conductors and insulators, the energy levels do not overlap and have a band gap (conduction & valence band).

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11
Q

How do semi-conductors conduct electricity if they have a band gap?

A

A high voltage needs to be applied to excite electrons across the band gap between the conduction & valence bands.

At low voltages, there are no spaces available for electrons to ‘hop’ between.

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12
Q

What is the size of the band gap in metals, semi-metals, semi-conductors & insulators?

A

Metals - 0 eV
Semi-metals - 0-0.5 eV
Semi-conductors - 0.5-3 eV
Insulators - > 3 eV

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13
Q

What is an eV?

A

Electron volt

The amount of energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt.

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14
Q

What is meant by the relaxation process in a material?

A

When electrons, excited by photons, within a material quickly relax back to their ground state (i.e. where they came from).

This relaxation process usually results in heating of the material (phonons are produced).

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15
Q

Why are semi-conductors good for solar cells rather than metals?

A

Electrons in metals relax too quickly because they have more energy levels.

Semi-conductors have a wider range of band gap meaning it the process and time for excited electrons to relax is longer. This allows more electricity to be generated and less heat.

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16
Q

True or false: If the incoming photon has enough energy an electron can be emitted from the surface.

A

True - this is what Einstein won the Nobel prize for!

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17
Q

What happens if the energy of a photon is less than the energy gap?

A

It will not be absorbed or move from the ground state. Instead it will go straight through or be reflected (e.g. glass).

This is because there is no energy level in between (too big of an energy gap).

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18
Q

What happens if the energy of a photon is equal to the energy gap?

A

Absorption can occur.

If excited charges are not separated then spontaneous emission of a photon occurs in the opposite direction.

This instead releases energy out as light NOT electricity.

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19
Q

What happens if the energy of a photon is more than the energy gap?

A

Absorption occurs and fast relaxation to band edge (releasing heat).

Relaxation across the band gap is slower and similar to before, if excited charges are not separated then spontaneous
emission of a photon occurs in the opposite direction.

This instead releases energy out as light NOT electricity.

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20
Q

How can spontaneous emission be avoided?

A

By separating excited electrons and moving them within the energy bands. This is done by creating asymmetry within the device.

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21
Q

Summarise how photovoltaics work?

A
  1. Absorbed energy from the photon increases the potential energy of an electron.
  2. The fast thermal relaxation process needs to be slowed down. The presence of a band gap does this.
  3. The energy gap with a separation from the ground state by a gap > kBT (e.g. a semiconductor) is needed.
  4. Only photons with energy greater than the band gap energy are utilised.
  5. Charge separation is then required to extract the energy. Separation requires some asymmetry in the device
  6. The excited electrons are then fed into an external circuit so they can do useful work.
  7. The increased electronic potential energy of the excited electrons generates a potential difference (i.e. a voltage between the two electrodes of the device).
  8. This voltage can then be used to drive a current in an external circuit.
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22
Q

Why is built-in asymmetry important in photovoltaic devices? How is this done?

A

It enables charge separation so that the electrical energy to be extracted before the electrons relax by producing light or heat.

  1. in silicon solar cells a p-n junction results in a built in electric field. This is done by doping with 2 different elements to create a material with extra ‘holes’ and a material with extra electrons.
  2. in excitonic solar cells the electron donor-acceptor interfaces separate the e and h into different materials
  3. in perovskite solar cells the energy level alignment at the transport layer interfaces selectively separate the charges
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23
Q

What elements are popular to be used for pn junctions in Si solar cells?

A
  1. Phosphorous doping introduces an extra electron to the crystal structure (n-type doping)
  2. Boron doping leave the crystal lacking an electron (p-type doping)
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24
Q

How does a pn junction to create asymmetry?

A

The p-type generates electron ‘holes’ whereas the n-type doping element generates extra electrons.

This creates a depletion region which has an in-built electric field. This can separate photo generated charge carriers so that electrons are fed through the external circuit to generate electricity.

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25
Q

What is the most popular type of solar cell?

A

Silicon

26
Q

What other types of solar cell technologies are competing with Silicon?

A
  1. thin film
  2. Perovskites
  3. polycrystalline Si
  4. amorphous Si
  5. semiconducting polymer
  6. mono-crystalline semiconductors
  7. dye sensitised metal oxides
27
Q

What type of solar cell has the greatest efficiency?

A

GaAs - gallium arsenide

28
Q

Why have photovoltaics become more popular through history?

A

Initially they were high cost and low efficiency.

As prices dropped, they became popular for off-grid applications such as telecommunication stations, offshore oil rigs, navigational buoys and railroad crossings.

Research was stimulated by the oil crisis in the 70s. Today, global economies want to invest to enhance their energy security and low-carbon resources.

29
Q

What is open circuit voltage?

A

The voltage when no external load is connected. The load is said to have infinite resistance.

30
Q

What is the short circuit current?

A

When the terminals are connected directly together (or the LOAD is 0 Ω)

31
Q

What is the current & voltage determined by in an electrical circuit for a solar cell connected to a load?

A

The load resistance and the illumination

32
Q

Why is current density used rather than current?

A

Because current is dependent on the illuminated area so current density allows more accurate comparison.

33
Q

What does the short circuit current density tell us?

A

The likelihood that an incoming photon will deliver an electron to the external circuit

34
Q

What does the quantum efficiency of a solar cell depend on?

A
  1. Absorption coefficient of the solar cell material
  2. The efficiency of both charge separation and charge collection in the device
35
Q

True or false: For a good solar cell it is desirable for the QE to be high at wavelengths where the solar flux in high.

A

True

36
Q

Why do solar cells emit a much larger current in one direction (forward bias V>0) compared to the other (reverse bias V<0)?

A

Because of the asymmetry in the cell.

Because the current is mostly positive, the reverse bias is taken as I0 and J0 constants.

37
Q

True or false: The open circuit voltage occurs when the dark current density exactly matches the short circuit photocurrent density.

A

True

38
Q

When does a solar cell consume power?

A
  1. When the voltage is less than 0V
  2. When voltage > open circuit voltage
39
Q

What does the fill factor describe? Do you want a large or small value?

A

The ‘squareness’ of the curve. You want a large value

40
Q

What affects the fill factor and therefore, the efficiency of solar cells?

A

It is reduced by high resistance materials and leakage currents through the cell, sides of the device or the contacts.

We want to maximise fill factor

41
Q

What standard test conditions are used to compare solar cells?

A

AM (Air Mass) 1.5 (sun at 41.8° elevation)

42
Q

What would be the Air Mass (AM) when there are:
1. Zero atmospheres
2. The sun is directly overhead

A
  1. AM 0
  2. AM 1
43
Q

Why is AM 1.5 used as the standard test conditions?

A
  1. Because solar panels do not generally operate with the sun directly overhead.
  2. The solar intensity varies naturally depending upon the season, time of day, and location on the planet.
  3. Because the efficiency only depends on the band gap
44
Q

At standard test conditions (AM 1.5), what happens for higher and lower band gaps?

A

Large band gap - higher voltage = photons have more energy
Smaller band gap - higher current density = more photons can be absorbed

45
Q

What is the Shockley–Queisser limit?

A

The optimum efficiency for a single junction solar cell under AM 1.5 standard test conditions.

Efficiency, η, has a maximum of ~33% at an Eg ~1.34eV

46
Q

What are the highest efficiency solar cells? What makes them superior?

A

Multi-junction concentrator devices.

They have multiple absorbing layers and each layer absorbs a different wavelength of light so more of the solar spectrum is used.

47
Q

Why do solar cells do not reach their optimum performance?

A
  1. Incomplete absorption of available light - photons are shaded, reflected or pass through the cell.
  2. Recombination of the photo generated carriers -
    defects can trap excited carriers such that they recombine before they get to the electrodes. The defects tend to occur at surfaces, near the electrodes or near the interface.
  3. Voltage drop due to series resistance - any resistance within the materials results in some of the carrier potential energy being lost getting to the electrodes.
48
Q

What factors should be considered in a photovoltaic to optimise efficiency?

A
  1. Energy Gap of the semiconductor material used
  2. Light Absorption – depends upon material and thickness
  3. Charge Separation – how efficiently can this be done
  4. Transport losses – are there losses as the charges transport
  5. Load resistance – is the load matched to the photovoltaic?
49
Q

What is the benefit of choosing materials such as GaAs, InP, perovskites or polymers rather than Silicon?

A

They are much cheaper.

Perovskites absorb at much smaller thicknesses

50
Q

How can light absorption be improved?

A

By increasing the thickness of the material. However, this can cause problems for charge-transport and recombination of charge carriers.

51
Q

How is asymmetry created in polymer solar cells?

A

Through an electron density gradient between 2 materials.

52
Q

How can charge separation be improved?

A

Creating a large area junction to maximise amount of solar energy produced. However, doing this can reduce the material quality, which holds all of the electrical properties.

53
Q

How are transport losses minimised?

A
  1. High electron & hole mobility
  2. The absorbing material should also make good contacts with the conducting electrodes attached to the device.
54
Q

How can load resistance be optimised?

A

The load resistance should be chosen to match the optimum operating point of the solar cell array.

For applications that require voltages more that 1 volt several cells may be used in series to form a module and an array.

55
Q

What impact does defects & impurities have on the solar cell?

A
  1. Carrier recombination
  2. Enable the excited electrons to relax back to the valence band without going around the external circuit.
  3. Reduce the conductivity of the device materials
56
Q

Why are series and parallel connections of solar cells used?

A

PARALLEL connections are used to achieve higher current
SERIES connections achieve higher voltages

57
Q

Why are bypasses and blocking diodes used?

A

To prevent complete power loss within each module should one cell fail.

58
Q

Why are power regulation and charge storage mechanisms used in solar modules or arrays?

A

Because illumination is often too variable for efficient operation day and night

59
Q

Why is an inverter required in solar modules or arrays?

A
  1. To feed into the grid an inverter is required to convert the DC supply to AC at the correct frequency.
  2. Minimise losses.
60
Q

What are the advantages of solar voltaic power systems?

A
  1. Very large free energy resource
  2. High power density (global mean of 170 W/m²) compared to other renewable energies.
  3. Pollution-free during use. Production wastes and emissions are manageable and end-of-use recycling technologies are under development.
  4. Can operate for many years with little maintenance or intervention after the initial set-up.
  5. Therefore operating costs are extremely low compared to existing power technologies.
  6. Efficiencies are rising while mass-production costs falling rapidly
  7. Solar electric generation is economically competitive now. Previously it was only economically superior where grid connection or fuel transport was difficult, costly or impossible. Solar is now one of the cheapest ways to generate power.
  8. There is still some room for improvement on costs and efficiencies because compared to fossil and nuclear energy, very little research money has been invested in the development of solar cells.
  9. Experimental high efficiency multi-junction solar cells for concentrating photovoltaic applications already have efficiencies of > 40% and conventional panels > 20%
61
Q

What are the disadvantages of solar voltaic power systems?

A
  1. Almost all the cost associated with a photovoltaic system is capital installation costs. For domestic systems the high investment can be lost if you move home.
  2. Solar electricity is perceived to be expensive – it is no longer expensive.
  3. The amount of power generated is variable depending upon the insolation levels. Therefore, storage or a complementary power system is required.
  4. Solar cells produce DC which must be converted to AC for use existing distribution systems. The inverter incurs an energy loss of 4-12%.
  5. The lifetime of the inverters is limited to about 10-15 years

CPE451 (7 decks)

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