The Power of the Sun

Chem 51

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Introduction

In this experiment you will study the power output of a commercial silicon solar cell, 200-300 cm², in a load resistance R (see diagram). (In residential applications a load resistance might be a kitchen stove or washing machine)  With the cell facing the sun you will measure the voltage across several load resistances R with a voltmeter. The power output, appearing as heat in the resistor, is given by the equation P = V²/R. (P will be in watts if V is in volts and R is in ohms)

The greatest possible power output from the solar cell under given conditions (such as full sunshine, partially overcast, etc.), is obtained when the load resistance is “matched” to the solar cell and under these conditions. The optimum resistance can be found experimentally by performing the experiment with different load resistances and determining which delivers the most power to the load resistance.  You will be given 3 types of solar panels that each have different types of construction and thus have different maximum power outputs. Using a multimeter (voltmeter and resistance or ohmmeter) you can quickly assess the power output of each type of solar cell.

Measurements and Calculations

1. Measure and record the “open circuit voltage” of the cell. This is voltage across the solar cell with no load resistance attached. (Equivalent to an infinite load resistance.) To do this you touch the leads of the voltmeter (set for the 0-20 V range) to the leads on the solar panel. (Column 1)

2. Using different load resistances, measure and record the voltage for each. Use resistances ranging in value of 0.47 Ω, 1.0, 5.0, 10, 20, 50 and up to 100 Ω. (Resistance wire can be used for small resistances.) To do this you will connect the leads on the solar panel to a small wire or resistor, then measure the voltage (20 V range) between leads in the same way as before. You will also want to check the resistance with the multimeter by switching to resistance and checking the wires and resistors you use (0-100 ohm range). (Column 2)

3. Calculate in watts the power produced in the load resistance for each of the load resistance values you used. (P (Watts) = V²/R, Column 3)

4. Which load resistance produced the most power and what is that maximum power?

5. Estimate the front surface area of the solar cell by measuring the outside dimensions of the cell in square cm, and then convert the area to square meters.

6. By dividing the maximum power output, expressed in watts, by the area, expressed in square meters, you will obtain the power output of an identically made cell with an area of one square meter. (Watts/m² = Column 3 divided by area in m², put in Column 4)

7. The full power of the sun on a sunny day at the surface of the earth is on average about 1,000 watts/m². This value is called the “Standard Sun” by solar scientists and engineers. Using this value, what fraction of the power of the sun is your cell capturing? This fraction is called the efficiency of the cell. Efficiency = 100% x (your measurement in watts/m²)/(the theoretical 1000 watts/m²). (Column 5)

Data:

For each type of solar panel given, make a table with columns of volts, resistance, a calculated column of watts/cm², another column of watts/m² and the last column of efficiency.

Results:

The row with the highest efficiency gives the optimum load resistance for your solar panel.

Follow up questions

1. Calculate the area of a solar panel of a similar manufacture, which would produce enough power to run a 500 watt water pump in a remote village.

2. If the area of the roof of the Science Building is 20,000 ft², what is the power that could be produced if the roof were covered with silicon solar cells.

Panel Type: 1                                                   Surface Area: (cm2)

Panel Type: 2                                                   Surface Area:

Panel Type: 3                                                   Surface Area:

03/28/2007