Newbie Here, so please excuse my inexperience and ignorance.
Here is what I am trying to accomplish: Keeping 4 - 1 gal. containers of water from freezing at my cabin. I want to do this because the only to get to my cabin in the winter is to snowshoe in about 3 miles, all going up So I want to reduce the weight of my pack. I am thinking of putting the water in a well insulated cooler with some kind of heat source. Here are the challenges:

1) My cabin is off Grid, the only source of power is a 18 watt solar panel charging a 12v car battery. The panel puts out 0.65 A in direct sunlight. About .006A at 3.46v on a cloudy day. (If my measurements are correct).
2) The average temperature during the winter is around 30F. with lows at night in the 20s.
3) The cabin is in the Central Cascades in WA. Sunlight islimited due to winter cloud cover and short days (as short as 8 hrs. on the winter solstice).
4) I was thinking of using a small 12v. LED bulb (an automobile tail light bulb) as a heat source. This bulb draws about .11 Amps. I think this draw will deplete the 12v battery given the amount of light available.

So, is it possible to do this another way using resistors, a thermostat, light sensor, etc.? Thank you...

-Bill

 

You could make far more efficient use of the scarce solar energy by simply converting it directly to heat and skipping the electronics. I'm thinking of a sort of greenhouse with the water in a black container to absorb the light. This is a common technique in actual greenhouses.

 

Yes, think of how warm your car gets inside when the sun shines, even on cold days.
You would want a well insulated container (say 6-8" of insulation) with the inside painted black and double layer glass on top facing the noon day sun in the winter.
If you could do this inside the cabin under a south facing window, that would be even better.

 

The only reason to make this an electrical problem is because you can't set your container in the sunlight.
Run the wires from the solar panel straight into the container and connect to just about any light bulb that won't burn out with 18 watts applied. Even if it doesn't make light, it will make heat at about 100% efficiency.

 

Of course, while you're heating the water the panel won't be able to charge the battery or do any other job. Best to buy an extra, larger panel for the water heating.

 

The only reason to make this an electrical problem is because you can't set your container in the sunlight.
Run the wires from the solar panel straight into the container and connect to just about any light bulb that won't burn out with 18 watts applied. Even if it doesn't make light, it will make heat at about 100% efficiency.

Hi #12,

OK, I tried running the wires directly from the solar panel without putting the 12v battery in the circuit, and trying a few different combinations. Here are the results of my testing...
1) Panel w/o the 12v controller: 21.5v
2) Panel w/ the controller: 18v

3) Panel w/o controller with 12v LED tailight bulb: 9.8v (bulb lit, but no substantial heat)
4) Panel w/ controller with 12v LED tailight bulb: 10.5v (bulb lit, but no substantial heat)

5) Panel w/o controller w/ 5w incandescent bulb: .09v (bulb not lit, no substantial heat)
6) Panel w/ controller w/ 5w incandescent bulb: 0.0v (bulb not lit)

I not sure how to interpret these results, or why the panel put out less/no voltage with the incandesant bulb. Does anyone think substituting a resistor(s) for the bulbs might give me more heat? Thanks,

-Bill

 

Your cabin needs heat, doesn't it? Eighteen watts is piddling. Have you considered burning wood to heat it and make water too? In my state, fallen trees can be used for heating a home, even if they are on state land.

Some back of envelope calculations for using propane show that a single 500 gal propane tank (2100# when filled to 500 gal) will keep your cabin nice and cozy for the entire Winter, produce approximately 1,562 liters of water from combustion and certainly keep a all of the jugs of water that you can stuff into the cabin from freezing at the same time. It is a bit harder to do the same calculation for wood. Because wood has more oxygen in it, the production of water produced per pound burned is less, but it is also a lot cheaper than propane and could be used exactly the same way. Plus, you would have the added advantage of smoke-scented water, which might have a secondary market for those who live in the city.

John

 

...........
I not sure how to interpret these results, or why the panel put out less/no voltage with the incandesant bulb. Does anyone think substituting a resistor(s) for the bulbs might give me more heat? Thanks,

The incandescent bulb has a lower resistance so the voltage is lower, since the panel is apparently putting out a low current.
For maximum power to the bulb you want the voltage across the bulb to be about half of the open circuit voltage from the panel, so it sounds like the LED bulb is absorbing near the maximum power from the panel.

No a resistor won't put out more heat than a bult.

If you want to use a solar panel for this purpose then it would seem you need a larger one that puts out more power, perhaps a 50w or larger.

 

no substantial heat)

The only thing that means is, "no substantial solar panel".
It doesn't matter whether the light bulb lights up. 100% of the energy you apply to the light bulb turns into heat. Even the light it makes turns into heat when it is absorbed by the inside of the box. Adding a controller only means some of your energy is dissipated by the controller. If the controller isn't in the box with the water, you are wasting the energy the controller uses.

Anything that uses the power from the solar panel will produce 3.413 BTUs per watt hour. That means incandescent bulbs, LEDs, resistors, room heaters, toaster ovens, wet earth worms...anything.

It looks to me like the 12V LED tail light bulb without a controller is the right one for this job.

 

You didn't hear us. Converting solar energy to electricity throws away 80-90% of the energy in the first step. Throw away the electronics and set your water in the sun. That'll give you 5-10X more heat for a given amount of solar exposure.

 

Converting solar energy to electricity throws away 80-90% of the energy in the first step.

That is a good point. Solar cells were at 6% the last time I looked. No doubt some of them are better now, but there is sure to be the worst dregs of old technology at The Dollar Store. Would you happen to know the latest in efficiency numbers?

I'm getting the idea that huge roof panels that are cheap would reduce the need for high efficiency, and somebody like Elon Musk is doing that lately.

 

@#12, Without looking it up I think the panels Musk will sell are about 22%, which is state of the art for commercial cells. They're amazingly higher in lab stuff, maybe 40%. But not necessarily practical.

 

Invest sweat and time into proper root cellar. Or bury water. Collect rain water in cistern.

 

A low-tech solution by today's standards, but how deep would you have to dig so that you can put the water below the frost line?

 

Four feet minimum where I live.

 

If your average winter temp is 30 degrees............3 ft deep is plenty. If you go east several hundred miles......say Montana.........where it gets to -40 degrees...................then go deeper.

30 degrees average winter temp is nothing. Very moderate winter. You should be able to store fruits and vegs in a cellar without any danger of freezing.

In cold climates..................put haystack on cellar. Compost can also heat a cellar.

 

2) The average temperature during the winter is around 30F. with lows at night in the 20s.

Yeah I missed that. I would think digging down a foot or so is all you'd need. A 'root cellar' could be really handy for a lot of things. Chilling beer comes to mind! Save your precious solar electricity for more interesting things.

 

So what's wrong with unfreezing your water once you get there or just melting snow?

I'm assuming your cabin is heatable once you get to it right? If not why?

 

Four feet minimum where I live.

15 feet in Leadville, Colorado which is at 10,000 feet.

 

The only heat I have is from a propane catalytic heater which does not exhaust outside. So I keep its use to a minimum while I am there in the winter. I was hoping for an eloquent solar panel solution. It looks like what I need to do is buy a bigger solar panel with more capacity. So I am going to go low tech, at least for this winter. I am going to dig a pit below the frost-line, put a 5 gal. jug into it and use a small pipe/manual pump to get the water up and out. I will remove the pump and bring it inside when I am finished so it doesn't freeze up.

Thank you all for all of your suggestions! It is great to be able to ask for help and have so many great minds respond!