Solar power

8.4.12 Distinguish between a photovoltaic cell and a solar heating panel.


PV_type.jpgHeat_panel.jpg

A photovoltaic or PV cell, is a tiny cell which is often grouped together to form large panels of cells, which can create electricity from light. PV cells make use of semiconductors, something which has both qualities from conductors and insulators, in order to free electrons. While a solar heating panel heats water into a tank directly, without converting the solar energy into electricity.

How Photovoltaic Cells Work.

The most common material used for solar panels is silicon as it has 14 electrons. The 14 electrons are arranged in 3 different shells, with the last shell having 4 electrons. The atom will therefore constantly try to fill up its electron shell. However pure silicon turns out not to be very useful as all the silicon atoms are attached together with 4 other silicon atoms. To make it more useful, one can make the silicon impure, a process called "doping". The goal of doping silicon is to create two types of silicon, both of which are necessary in solar cells:
-N-type
-P-type

The N-type silicon is doped with phosphorus, as phosphorus has an abundance of free electrons. N-type silicon is a better conductor than pure silicon
The P-type silicon is doped with boron which has 3 electrons in its outer shell. Together they still need 1 electron to fill out their last electron shell, thus making P-type silicon positive.

Each type on their own, is hardly very useful, however when they get together, the free electrons from N-type will rush towards P-type. But not all the free electron spots in P-type gets filled, if they did it wouldn't be very useful. Rather an electrical field is formed, and a "junction" is formed where there is electrical equilibrium. As can be seen from the picture, when photons hit the electron-hole pairs close to the junction; it forces electrons to flow to the N-type rather than P-type, against the electrical field force. This frees up electron-holes in the P-type and N-type's free electrons rush back to the P-type, doing work along the way.

To put it into an Electricity context, the moving electrons provide our current, and the electrical field provides the voltage.


8.4.13 Outline reasons for seasonal and regional variations in the solar power incident per unit area of the Earth's surface.Solar_land_area.png



Pictured is the average watts per meters squared of the earth with 8% efficient PV cells. There are regional differences in the solar power available based on the intensity of the light, caused by Earth's angle to the sun. Therefore the equator has on average the highest solar power available because of the highest light intensity.
Because of the Earth's angle there are also seasonal intensity differences. The equator experiences nearly no seasonal change while countries farther away from the equator experience more drastic seasonal changes. Consider for example the poles. The North Pole has 24/7 sun light in the summer and 24/7 darkness in the winter. The South Pole has the opposite. The equator which has a nearly constant distance to the sun all year around, experience little seasonal change. The higher the latitude, the higher difference in seasonal sun light intensity variation.



8.4.14 Solve problems involving specific applications of photovoltaic cells and solar heating panels.


The latest solar panels can reach 40% efficiency and the sun's intensity can create a maximum of 300 watts per meter squared, near the equator. If 18TW is required to supply the Earth with energy, how large of an area would be required to supply it through these solar panels?

I= P/A
A= P/I

P= 18TW / 0.4 = 45 TW.

A = (45 x 10^12) / 300 = 1.5 x 10 ^11 square meters = 15 million hectars.


If the average solar power received on the balcony of a building is 350 watts per square meters, and that the area of a solar heating panel is 50 square meters. How efficient does it have to be a to boil 150kg of water which is at room temperature?
Assume room temperature in the boiling room is 30 degrees Celsius.

Q = mc(change in temperature)
Change in temperature = 100 - 30 = 70C
c = 4200 J/kg/K

Q = (150)(4200)(70) = 4.41 x 10^7 J = 12250 W

IA = 12250/efficiency
Efficiency = 12250/IA = 12250/ (350 x 50) = 07 = 70%