Its actually good practice for me, i want to teach science, so it helps me learn how to pitch the subject matter.
If you are serious, I hope you don't mind me tearing this to shreds, as per *my opinion*
. This kind of language may well do for people who are never going to hold a screwdriver, and therefore don't really need to know anything about it anyway, but let's not screw things up for those who might want more later!
(mini rant!!! ? - it's been a while since I've done one of them
. Must be a side-effect of my old physics teacher who was a stickler for accuracy - and getting things right!)
its quite simple really, big solar panels are made up of lots of little solar panels. The ones in the tops of solar garden lights have a voltage of about 2v and a current rating of about 10mA (in good sun)
will let you off there, if your not teaching basic English! It's.
the voltage is a measure of a power supplies ability to 'push' the current around the circuit. The current is a measure of how many electrons are moving and able to do work. Voltage is measured in volts (v), current in Amps (A), mA is milliamps, or thousanths of an amp, so 10mA is 0.01A
Well, voltage is defined as a measure of the amount of energy carried per unit of charge... [Eng]a power supply's[Eng] - measured in Volts (V) when using the SI system of units. Current is the rate of flow of charge - which may or may not be electrons.
power is measured in Watts (W) and in a DC circuit like this, is worked out by ohms law V x I = P, so these little panels are 2 x 0.01 = 0.02W, or 20mW
Power is the 'rate' of energy - and is watts in the SI system when using joules per second. Ohm's law {Eng!} is 'In a metallic conductor at a constant temperature, the potential difference across it is proportional to the current through it'. In the SI system this can be represented by the equation V=IR (still only in a metallic conductor at constant temperature) since the definitions of quantities are chosen to keep it simple. (This is nothing to do with power...). In an AC circuit the same law applies.
now, to be usefull the solar panel needs to give a voltage of 12v to charge a battery, so by wiring 6 of the 2v little panels in series (ie the + lead of one connects to the - lead of the next) we can make it a 12v panel (6x2=12). Trouble is, the current is still 10mA, even though we now have 6 panels. To increase the current, we wire in parallel (ie all + connect together, all - connect together). Lets say we want 100mA, we need 10 panels in parallel (10 x 10 =100), so what we end up with, is a grid comprising 10x6. ten lots of the 6 panel series wired units, wired in parallel.
OK, but practically speaking a 12V battery needs more like 13.9V to charge it - so add another little panel. Useful!
so a reasonable panel that can charge a battery (albeit only trickle charge at this current) needs at least 60 of the small panels. It starts becoming easy to see why big solar panels are expensive!
Ok, so then we need to charge the battery. Now, lets assume we have a good commercial solar panel, rated 12v 1A (12W!), a typical lead acid battery has a rating of 20Ahr. That means you can take 20A in an hour, or 1A for 20h, before its flat. We also ideally charge the battery at 1/10th its capacity, so if its capacity is 20Ahr, we charge it at 2A for 10hours. So our panel of 1A will take 20hour in good sunlight to completely charge the battery. But! lets say the battery is only used for a light, but lets say for simplicity the light takes 1A at 12v(ie a 12w bulb!), we have the light on for 1hour a day, so the battery goes down 1A in that day, so we only need to charge it for as long as it takes to replace that energy. If it takes 20hour to replace 20A, it will only take an hour to replace the 1A we have used. So one hour of sunlight a day keeps the battery charged.
Yuk. At first I thought it was a typo (generous
eh?) but then you do it twice more! How does a battery 'go down 1A' - is this a new measurment of distance? Has it sprung a leak? Certainly 1Ah of energy is used from the battery. and that 1Ah wants replacing, and at 1A it would take 20 hours to supply 20Ah of energy...
Of course this is very simplistic, ive assumed simple easy values and assumed good sunlight so the panel always gives its full rated output. In real life, the panel may often be giving lower output (poor sunlight/overcast days) or none at all for aprox half the day (ie at night). At other times the sunlight may be very strong and the panel exceed its ratings. This is where, if you have a big panel, a charge regulator becomes needed, to prevent overcharging the battery. As the sunlight increases in brightness, the voltage and current also increase. Its this increase in voltage above what the battery can handle that leads to overcharging. Current is more of an availability thing, the panel can supply 1A, but if nothing takes that 1A, it doesnt flow. Take as an example you mains electricity, your 100W bulb takes about 500mA, but your supply can give 100A! if all that was forced down your wires it would melt them very quickly!
Unnecessarily simplistic and often wrong! You have the good idea of illustrating a 12W bulb on a 12V system taking 1A, and therefore using 1Ah in an hour. Keep hold of this basic idea (energy) and that covers all the variability of night, shade, etc. since your solar panel just needs to produce 1Ah of energy to recharge the battery! This can be the peak output of the panel for 1 hour, or half-power for 2 hours etc.
Increasing light does not necessarily increase the output power, voltage or current of PV panels. They have an 'open circuit' voltage rating which is the maximum voltage they can produce (some light under no load). This will be reached quite quickly even in dim light since it is a function of the physics of photons and matter. It may be affected by shading of parts of the panel (and cloudiness). Panels also have a rated 'short circuit' current - what they will deliver if you connect the + and - together in full light. The current will increase with increasing light upto near this level, but not go above it. The rated power is derived from the optimum particular balance of voltage and current the panel is designed to deliver. Thus a panel may be Voc=44V, Isc=5.12A, yet be rated at 180W since its peak output is at Vmppt=37V Imppt=4.86 (mppt=maximum power point)
It is not the excess voltage/current of the panel caused by excess light that causes the need for a charge regulator - it is simply the balance between energy put in against energy taken out. If your panel cannot produce enough energy, your battery will eventually go flat. So you need a panel that can produce more than you use! The charge regulator simply disposes of any surplus energy from the panel when the battery is charged. It should also increase the efficiency of the panel by letting the panel run at its Vmppt and Imppt - and transforming these to a lower ideal battery charging voltage at slightly higher current. (12V nominal PV's are often 17Vmppt, and connecting them directly to a 12-13.8V battery wastes that surplus energy potential.)
Of course, if your light or whatever is a lower wattage on your solar setup, say a 5W 12v bulb, the current it takes in use is lower, the battery is less drained, and so it takes less time to restore it to full capacity.
If it is a 5W bulb, it doesn't matter what voltage it is - it will take 5W. Turn it on for an hour and it has used 5Wh! If the PV panel puts out 5W for 1 hour (or 2.5W for 2h etc.) this will be 5Wh and cover the energy used.
This is why, except very rarely, a solar panel never directly powers anything, its output will tend to fluctuate, is not available at night, and is often too low. Its like drinking water, we get a constant supply from the tap despite the fact that rainfall and riverflow are not constant, due to the storage of water in reservoirs. The battery is your reservoir. And just as water systems have sluiches and valves to keep the reservoir form getting too full, thats what a charge regulator does!
Nothing before the 'this is why' is the reason! Why not just a plain statement of 'A solar panel rarely... because...' ?
OK, so, what panels are there? well, theres three basic technologies -
Amorphous silicon - brown, like in calculators, not very efficient, not good on overcast days, needs good light. But the cheapest!
Work better on overcast days
polycrystalline and monocrystalline silicon - both blue colour, more efficient and able to keep a good output in poor light, disadvantage is they are more expensive.
Not very good on overcast days Polycrystaline are blue, monocrystaline are charcoal.
4th type is 'hybrid' - expensive but good in both overcast conditions and strong light - and expensive, of course!
My panel is made of a pair of 6"x6" (roughly) amorphous silicon panels.
The other essential is the Diode. This is a little semiconductor eletronic device (like a black cylinder with two wires sticking out) and acts as a one way valve to the flow of current. Each panel has one of these in its + lead, and it stops current flowing back through the panel from the battery or from any other panels its wired to, when the light is poor (which can damage the panel). They are cheep ( a 1N4001 diode is rated at 1A and cost about 3p if that) and small (3mm long ish), but have a slight disadvantage, the lose 0.6v across them. So a 2v panel will be a 1.4v panel on the other side of its protection diode. Because of this, a panel built of lots of smaller ones needs a few extra small panels to account for the loss. But thats preferable to breaking the panel!
'cheep? ' Am I back on the poultry pages???
Congratulations on saying 'voltage accross'. A parrot probably would 'voom' if you put 4 million volts across it!
for best results from a panel, it would follow the sun around, but such tracking is very expensive. For a simple setup, just get it facing south, at an angle where it will pick up most of the sunlight, say between 30-50degrees (45deg is good 
) mines at about 50.
[Eng]mine's[Eng]
There is only a few percentage points of loss at SSE or SSW orientation. 40deg is probably better, especially with mono/poly which will really gobble up the late spring to early autumn sunshine on 23rd August each year.
there we go, a quick introduction to the basic concepts of solar electricity, and a physics lesson thrown in free, gratis, AND fer nowt!, what more could ya want? 
And don't take offense, please, but the physics lesson was a bit dodgy. Glad I didn't pay for it!
. If you want to teach the stuff, teach it right from day one.
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February 03, 2009, 17:12