MINIATURE SOLAR SYSTEM
There are 4 major concepts to a miniature solar system design to keep in mind:1) Load requirements, 2) Solar Cell Basics, 3) Light Source, 4) Increasing assets.
1. Load requirements. What is the power requirement of your system under use? Either find this in the information of the devices or written on the devices, or attach a AA battery to the device - and if this will run the device, measure the current draw with a common voltmeter. Multiply the current times the voltage to get the required power.
2. Solar Cell Basics: The standard silicon solar cell junction is 0.5V, with the current varying by the size of the cell. While the current of the cell varies greatly in relation to the amount of light available, the voltage is much more stable - usually any reasonable amount of light will produce close to the rated voltage. For this reason, very small systems will usually consist of great numbers of tiny cells connected in series. The key is that the power equation counts current and voltage equally, while the construction of the device props up the voltage - power equals current times voltage.
The voltages of the cells add if attached in series (positive terminal of one to negative of the next cell). Currents add if attached in parallel (positive terminals attached together, negative terminals attached together). One can add both currents and voltages if there are enough cells by attaching parallel strings in series. A couple of very important things to remember are:
a) solar cells are current source devices - that is, you cannot create a larger current by short circuiting the system (caution, this is not true if you have a battery in the system - a battery can supply a great deal of current, and can electrocute and burn you). The only current that is produced in the solar electric system comes from light (strictly speaking they are photoelectric cells - not requiring the sun - though they produce most electricity over a wide spectrum of the visible light range - they can run on other light sources.)
b) Solar cells are current limiting devices - if you put a tiny cell in series with a large cell, you will only pass the current of the tiny cell through the string. You want to match the current of the cells in the string, as much as possible, to avoid wasting power.
3. Light Source: You will want to consider what your light source is, and where it is located in relation to the system. The major concepts to consider here are:
a) There is a concept known as "the inverse square law" that governs the behavior of light. That is, as you double the distance to the source, the intensity of the light is only 1 quarter as great. So you want to minimize the distance to the light source. You will also want to have the light hit the panels at a right angle if possible, to minimize the reflective losses from the glass panel cover plate.
b) Matching spectrum of light produced to the spectrum sensitivity of the cell - here we are assuming that the cell is a simple silicon solar cell, such as you can buy here, and most hobby cells. These solar cells are sensitive to the entire spectrum of visible light, plus some of the UV range and infrared range. So if you are going to run something from a light bulb, you are only going to get a small portion of the possible electric current production from it. If possible, a panel on a window sill, or a tiny pole mount with a long cord is much better than a panel on the dollhouse roof far away from the window.
4) Increasing assets: If you don't have enough light to run your system, you can do a couple of things:
a)You can build in a battery back up and charge this battery closer to the light source, or in the direct sun, and then use both the battery and the cell as a power source.
b)You can increase your available light. You can do this by adding a closer light, by increasing the spectrum of the bulb, or by adding one or several mirrors to direct extra light sources onto the solar cell. The current output of the panel directly corresponds to the light focused onto the cell. The only hitch here, is heat produced by sunlight - heat will lower the voltage of the cell, thus lowering the power output. And you guessed it, you can cool the cell to increase the voltage, thus increasing the power output.
Building the System: Now you have the basic knowlege to build the system. There are just a few additional concepts to add:
1. Wiring size: The larger the wiring, the less resistance in the wire, and therefore the smaller the power loss in the wiring. This means that if you have to run a wire a long distance, you want to go with a large wire diameter - for a miniature system this might be size#10 or #8 copper wire.
2. Solar Panels allow current in the opposite direction in the dark: Therefore, if you have a battery in your system, you must either disconnect in the dark, or insert a diode to prevent current direction reversal (or drain your batteries in the dark!)
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