What it shows
A solar cell behaves like a current source up to a knee and then like a diode. Its power peaks at one load, the maximum power point, and the fill factor shows how close the curve is to a perfect rectangle.
The science
In light, a solar cell gives a current nearly independent of voltage (short-circuit current Isc) until the voltage approaches the open-circuit voltage Voc, when the current drops rapidly. The power P = V I is zero at both ends and peaks at the maximum power point (Vmp, Imp). The fill factor FF = Vmp Imp / (Voc Isc) says how square the I-V curve is; good silicon cells give 0.7 to 0.8. Isc is proportional to light intensity, while Voc changes only slowly (logarithmically), and falls as the cell gets hot.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small solar panel of about 6 V and 100 to 200 mA (from a garden light or toy shop)
- A 100 ohm rheostat or a set of resistors from 5 ohm to 1 kohm
- Two multimeters (voltage and current)
- Crocodile clips and wires
- A bright sunny spot at noon, or a bright lamp held at a fixed distance
- A cardboard tube or cloth for shading
- Graph paper
Safety
For this project: An adult must supervise work outdoors in the heat. Do not touch hot lamps; use only the low voltage panel.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the panel facing the Sun or the lamp at a steady angle and note the light conditions.
Measure Voc with the multimeter across the open panel and Isc with the ammeter directly across it for a second only.
Connect the rheostat in series with the ammeter across the panel, and the voltmeter across the panel.
Vary the rheostat from the highest to the lowest resistance, recording the voltage and the current at about ten settings.
Take extra readings near the knee of the curve where the current changes fastest.
Plot I against V, and then calculate P = V I for each reading and plot P against V.
Mark the maximum power point and calculate the fill factor and the load resistance Vmp / Imp at which the power peaks.
Shade half the panel with cardboard and repeat the readings to see how Isc falls.
What you should see
The I-V curve is flat and then falls sharply, the power curve has a single peak, and the fill factor is typically 0.6 to 0.75. Shading half the panel roughly halves the short-circuit current.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the maximum power point?
The voltage and current at which the cell gives the highest output power.
2What is fill factor?
The ratio of the maximum power to the product of Voc and Isc.
3How does light intensity affect the current?
The short-circuit current is proportional to the intensity of the light.
4Why does the output fall when the panel gets hot?
The open-circuit voltage drops with rising temperature, so the power drops.
Take it further
Measure the efficiency by dividing the maximum power by the light power on the panel’s area, taking sunlight as about 1000 W per square metre at noon.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology