Goldsmiths’ 2013

Making a solar cell in a drinking straw

Introduction

In this experiment you can make a solar cell in a drinking straw. It makes use of the good light-absorbing properties of raspberry juice to capture electricity from sunlight.

Equipment

You will need following equipment:

Test tube rack and 3 test tubes;

A screw with about 5cm length thread, thin enough to fit into a drinking straw – used to make the coiled electrode;

A clear drinking straw;

25cm length of lead free solder wire;

6cm of plastic-coated copper wire stripped of about 1cm of plastic at both ends (or use wire-strippers/scissors to do the job);

Matches;

Tealight candle;

Heatproof mat;

Scissors;

Conical flask;

2 x 1ml plastic pipettes;

Retort stand and clamp;

Food bag tie / string;

Aluminium foil;

Tweezers;

Dish of raspberries;

Glass rod;

Bright light;

Small bottle of methylated spirits;

Small containers of copper sulphate, ferrous sulphate, zinc sulphate, sodium hydroxide pellets and ammonia solution;

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25 ml measuring cylinder;

Tissue;

Spoon or spatula;

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Pestle and mortar;

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Multimeter with probes;

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2 x crocodile clips with leads.

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Method

1. Bend the straw at its flexible hinge to make a U-shape.

2. Cut the longer arm to the same length as the shorter arm with a pair of scissors. You should now have a symmetrical U-shape and a long straight bit of straw.

3. Cut the long straight bit of straw into 3 equal parts by cutting at a 45° angle using scissors. These will then be used as scoops for the next steps (Picture 1). Keep everything for later

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Making the working electrode

4. Cut off a 1cm long piece of solder wire from the 25cm using the scissors. Keep it for the next stage.

5. Wrap the rest of the solder wire in between the grooves of the screw thread to make a coil that is about the same length as one of the arms of the U-shaped straw. Leave at least 2cm of wire not coiled to use as a handle. Twist the screw round to get it out of the coil when you have finished (Picture 2). You might need a little help from a screwdriver.

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6. Make sure the coil fits into the straw easily. If it doesn’t, stretching the coil lengthways slightly will make it thinner.

7. Measure out 10ml of methylated spirit into a measuring cylinder. Pour it into one of the test tubes. Add into this test tube 1 heaped spoonful (Picture 3) of zinc sulphate powder using a spoon-spatula and the 1cm length of solder wire you cut earlier.

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8. Measure out 10ml of methylated spirit into a measuring cylinder. Pour it into one of the test tubes.

9. Add into this test tube 1 heaped spoonful (Picture 3) of zinc sulphate powder using a spoon-spatula and the 1cm length of solder wire you cut earlier.

10. Use a plastic pipette to add in 2ml of ammonia solution.

11. Roll up a small bit of aluminium foil into a ball of about 1cm in diameter and drop into the test tube.

12. Finally put in the solder wire coil, so that it is fully submerged except for the uncoiled handle (Picture 4). Leave for 10 minutes.

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13. While you are waiting, measure out another 10ml methylated spirit using the measuring cylinder. Pour this into a clean test tube.

14. Add 1 heaped spoonful of zinc sulphate powder and clean the spoon with a tissue. Then add about 4 or 5 sodium hydroxide pellets.

15. Next, take the dish containing raspberries and mash them up using a glass rod or spoon-spatula. Pour into your third clean test tube

16. After 10 minutes are up, take out the coil using tweezers and gently wipe with a tissue to remove any large lumps of solid on it. Then put the coil into the second test tube and leave for 1 minute. Take out the coil, wipe with a tissue and wrap gently in a small piece of foil.

17. Place a tealight candle on a heatproof mat and light it with a match. Hold the foil-wrapped coil with the tweezers and wave it slowly side-to-side about 5cm over the flame for no longer than a minute (Picture 5). If smoke appears before this, the solder is about to melt so remove it from the heat early. Allow to cool.

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18. Unwrap the coil and put it into the test tube containing raspberry juice, leaving the handle above the level of the juice. Leave it here while you assemble the cell.

Assembling the cell

19. Push a small bit of tissue about the size of the end of your thumb into the hinge of the U-shaped straw. Use the screw to push it into place.

20. Put a few pellets of sodium hydroxide into the mortar and grind it up using the pestle into small pieces that will fit into the straw. (WARNING sodium hydroxide can be harmful and burn the skin) Use a straw scoop to put 1 or 2 scoops of sodium hydroxide into one arm of the straw so that it takes up about 0.5 cm.

21. Use another scoop to put about 3 scoopfuls (or 1cm) of copper sulphate into the straw on top of the sodium hydroxide.

22. Use the third clean scoop to put about 3 scoopfuls (or 1cm) of ferrous sulphate into the OTHER arm.

23. Attach the cell to a retort stand by letting the hinge rest on the base and loosely tying it with a food bag tie (or string) to the pole (see Picture 6).

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24. Remove the coil from the raspberry juice and wipe off the gunk with a tissue. Push the coil into the arm containing the ferrous sulphate.

25. Put one end of the blue plastic-coated copper wire into the arm containing the sodium hydroxide and copper sulphate.

26. Put some tap water into your conical flask.

27. Filling up the COIL arm first, use a pipette to add the water into both arms of the straw cell.

28. Attach one end of a crocodile clip wire to the solder coil handle and attach the other end to one of the multimeter probes.

29. Take the other crocodile clip wire and attach one end to the copper wire and the other end to the other multimeter probe (Picture 7).

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Measurement

30. Measure the voltage of the cell by turning the dial on the multimeter towards the left to 2000mV. If you do not get a reading check for air bubbles in the straw. The water needs to be all through the straw for it to work. Squeeze the straw gently to get rid of air bubbles.

31. Measure the current in the cell by turning the dial towards the right to 2000 µA.

32. Now shine a bright light (WARNING lights might be very hot) onto the cell and see if the current changes. Take the bright light away again and what happens? Switch the multimeter back to measure the voltage. Has that changed?

33. One single cell is not enough to power very much, but if we attach up cells in a series circuit we might be able to increase our output. Team up with a neighbouring pair and create a series circuit containing both cells. Measure the combined voltages and currents using a multimeter. Can it power an LCD display calculator?

Disposal

Wash up your mortar and pestle, spoon-spatulas, tweezers, test tubes, conical flask, raspberry dish and glass rod with plenty of water at the sink. Throw your rubbish in the bin (plastic pipettes, tissue etc)

Make sure your retort stand is packed away and all vials have their lids securely on. Place all the equipment back in the trays they came from. The straw cell itself can go in the bin.

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