Solar Power

Question 1

How does the photovoltaic process work?

Answer 1

• Charge density gradient is set up in a semi-conductor composite (e.g. an n-p junction in Si).
• Energetic photons displace electrons in the semi-conductor structure and moves them from valency to conduction states.
• The electrons find themselves in an electric field and so flow in the field, resulting in a current.

Question 2

Why is not all of the solar power radiating converted into electrical power?

Answer 2

• Not all photons have enough energy to make an electron-hole pair (23% for Si)
• Some have too much energy (above the threshold), these simply heat up the device
• Some photons are reflected (4%)
• Some photons are absorbed by the current collection conductors

Question 3

How is the voltage from a PV panel increased to the 240V mains supply?

Answer 3

• Voltage is increased by connecting the PV cells in series.
• DC current is fed to an inverter that converts it to AC.
• Inverters also monitor the grid to ensure that the voltage, frequency and phase angle are synchronised.
• Pulse width modulation is used to achieve the desired DC to AC and frequency.

Question 4

Which two forms of energy sources are not directly or indirectly related to the Sun?

Answer 4

Tidal (mostly Moon)

Geothermal

Question 5

What determines the thermal effectivity of a solar thermal collector?

Answer 5

• High absorptivity
• Low emissivity
• Good concentration ability
• Good view factor (sun tracking)

Question 6

How does the quality/quantity of power from PV panels compare to thermal collectors?

Answer 6

Although PV panels often generate less power than thermal heat harvesting, the PV power is high quality (electric) and can be used for many processes.

The low grade thermal energy from thermal collectors can be converted to mechanical however this is inefficient, and normally results in a lower power output.

Question 7

What happens to current, voltage and power when the temperature of a PV panel is increased?

Answer 7

Current - Increases due to a greater free electron density and the corresponding current density

Voltage - Decreases due to a reduction in the potential gradients at the p-n and n-p junctions

Power - Decreases due to the reduction in V being greater than the increase in I

Question 8

What are the units of solar current flux and how do you calculate it?

Answer 8

A/m^2

Calculate the number of photons required per m^2 and then multiply by 1.6x10^-19

1 Coulomb per second = 1 Amp