These labs were part of the 2008 Energy Academy, where students built battery-powered toys that eventually were going to transform into solar-powered toys. Through these excersises, students can develop an understanding of PV panel efficiency, feasibility and better grasp the concept of energy production vs. energy usage.
How do we know how many panels we need for a particular situation?
- Trial and error? That’s ok for a small toy, but harder for a house or the whole country.
- Measurement and calculation. Let’s learn how to do that.
1. Measure the power used by our battery powered toys. (Or if their toy isn’t working, just use a motor and measure the power the motor takes.) Notice whether the power used is the same when the ‘wheels’ are spinning as when it is actually rolling on the ground or otherwise loaded down. Would you expect the motor to take more energy from the battery when it is working harder? Remember how to connect an ammeter to measure current.
2. Now let’s measure the power that a solar cell produces. But just like a person, a solar cell can put out a range of powers, depending on how much you are demanding from it. If you want to measure the maximum power a cell or a person can put out, you have to keep demanding more until they finally collapse. The power they were putting out right before they ‘collapsed’ should be their maximum power. Keep demanding more from a .5 volt cell by adding motors in parallel and also holding onto the motor shaft. Record V and I for each trial and plot the IV curve and calculate power for each level. What was the maximum power?
3. Compare max power to total available power from sun for same area to find efficiency. .5 volt panels are 2.5 cm x 5 cm, or 12.5 cm^2. Peak sun is 1000W/10,000 cm^2. 1000*12.5/10,000 = 1.25 W of total energy striking our panels. What fraction of that 1.25 W do we actually get as electricity? Does that tell you something about a solar perpetual motion machine?
4. To consider powering our toys, compare the max power you got from a panel to the amount your toy uses.
5. If you want to get more power you have to make a solar “array.” You have to add them in series and parallel. First in series to get the voltage you want, and then in parallel to get more current. The max power of the array will be the sum of the max power for each panel. Try it. How big of an array (how many panels) would you need to run your toy directly off solar? Show film of solar car driving down the street. Do you think this is running directly off solar? Why don’t we just have cars running around just on solar panels?
6. If you can’t or don’t want to make an array big enough to run stuff directly, you need storage. Like a bank account, you store up energy slowly (low watts) and then you can use it quickly (high watts) all at once later. The total energy (money) will be the same, but the spending rate will be higher than the saving rate. Let’s make solar battery chargers to charge up AA Nickel Metal Hydride batteries. Then we can sit our toy in the sun for a few hours and then use that stored energy to run it.