PV Gap

Alma Said:

make a light go using solar power?

We Answered:

You need to work out how much power you need in watt hours per day. That is how big a light bulb and how many hours it must run for, based on a 24 hour period. A light bulb can provide the heat, its output is only a few percent light, the rest heat. For this approach I would use a 12/24V battery (to store the energy) and 12/24V bulbs to provide the energy you want.

The amount of heat required depends on the thermal characteristics of the container. That is the insulation, and its thermal mass (how much heat it takes to change its temperature). A given heat input will eventually find some balance at some particular temperature.

Once you know the watt hours (for a 24W bulb it is 24W x n hours) you can determine the size of the battery and solar panel. Lets say 10h @ 24W so 240Wh. WIth a 12V battery/bulb the current is 24W/12V = 2 Amps. The battery needs to provide at least 2A for 10h = 20Ah to provide this. A typical car battery is just big enough, though a deep cycle battery is better suited. It needs to be 30Ah or more for a bit of a margin. Bigger is definitely better.

Solar panel size...
The number of Ah taken from the battery is 240Wh/12V or 20Ah. The amount required to recharge is 20Ah x 1.4 so 28Ah. Lets call that 30Ah to replace every day. For your region there is a figure called effective full sun hours per day (search on the internet). This is 5 hours in better places (tropics) and less for temperate zones because the sun does not get high enough in the sky. During that 5 h we must produce 30Ah so we need a panel that can provide 6A while charging the battery. This is roughly a 12V panel with S/C current of 7 Amps. That will be rated around 150W. A big and expensive panel.

Cloudy days...
So far no allowance for cloudy/foggy/dusty/smoggy days. You need a bigger battery to hold over and a bigger panel (or more of them) to catch up on the recharge in a reasonable time. This depends on the weather where you are. If you decided to just have two batteries and panels, connect them in series, making a 24V system. It will work better with smaller wires and less losses than a parallel 12V system. The charger and the light bulbs will be 24V too of course.

Charging...
You will need a solar charger (12V or 24V x 10A or so) between the solar panel and the battery. There needs to be an appropriate fuse between the battery and the cable to the lamps, more than 2A in our example, but not more than the wire rating in Amps. The wire to the lamps should be big enough to minimise voltage drops (less than 1V total for 2 wires), and a minimum size that can carry 2A easily even when in the sun on a hot day. Cable suppliers have tables for these ratings.

Another idea..
You could provide heat in a different way, by heating water in an insulated tank during the day and circulating it by night. This could be more of a do it yourself project, using black poly pipe for the heat. I think you may need a pump, but may be able to do without batteries by thermo-syphoning at night, use the pump in the day?? You will have to research that.