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Complex (to me) heater project, 12v

Sommerfeldt

Dec 15, 2015
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Hi! It's my first post here, so please scald me relentlessly for anything I mess up. And I hope I'm in the right forum...

Basically, I only have an idea here, and I'm nowhere near realization yet, but I'm hoping you can help me out. The idea is, basically, a space heater for a small room, capable of running for ~3 hours or so, and raise the temperature in that room by about 4-8 degrees (C). Now, the physics of the whole thing is another matter, of course - I'm not sure that this will work at all, but I'm just looking to build a prototype here.

So, what I need is maybe a few recommendations on what kind of components to use, from people who obviously have oceans of experience more than I do (which is basically non-existent...).

I'm thinking my project will consist of this;

- A small, 12V cabinet cooling fan for a computer
- A time and temperature switch (I've no clue where to find such a thing)
- A small heating element (no idea what this should be)
- Battery, 12V, around 5,5Ah, maybe?
- A small solar panel to trickle charge the battery

All this in as small of an enclosure as possible - I'm thinking I'll install it in one of those sturdy carrying cases for electronics - the ones that come with the padding you can cut to size. I'm sure you get the idea.

So, if anyone has suggestions on the time/temp switch, heating element, battery and charging solution, that would be awesome - I feel like I'm wading through a jungle full of different stuff, trying to find a single gnat... :D

Thanks!

- S
 

Alec_t

Jul 7, 2015
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Welcome to EP!
Now, the physics of the whole thing is another matter, of course
The physics is all important and should be your starting point to see what's feasible.
1) How big is a 'small' room?
2) What percentage of the room is occupied, and by what? (The thermal capacity of the contents needs to be accounted for, since the contents also have to be warmed).
3) What are the 'U' values of the boundaries of the room?
4) Is there a door or window open/closed?
5) How quickly must the temperatue be raised by 4-8C, before the 3hr period begins?
6) What is the temperature outside the room?

I suspect you will be surprised by how much energy you'll need. A 12V 5.5Ah battery is likely to be woefully inadequate. The heating element is the least of your worries.
 

Sommerfeldt

Dec 15, 2015
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Hey!
Thanks for your reply. :) I know the physics of this thing is pretty much all important - right now I'm just focused on trying to get the "mechanics" of it together, so that I can actually test it in a "real world" environment. But as to your questions there:

1) Approx. 3,5 cubic meters of air.
2) Various materials, which is also why this one's pretty much impossible to calculate
3) No idea. Like 2), this one is very hard to calculate, and why a real world test is more viable (for me, who is not a physicist)
4) No open windows or doors - little to no air circulation.
5) The temperature doesn't need to be raised before the 3 hour period - 3 hour period is all inclusive, so to speak. :)
6) Temperature outside the room is highly variable, which is why I would need a temp switch as well as a timer. (and another factor for the calculations being pretty impossible)

I'm also thinking that something like this would only run during daylight hours, and that means that I would also be looking to use the solar charger for all it's worth.

- S
 

Alec_t

Jul 7, 2015
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Approx. 3,5 cubic meters of air.
That's not a room: that's a cupboard :).
Assuming 100% effective insulation (no heat loss at all from the room!), it would take about 3W of power to raise the temperature of just the air by 8C over three hours. The U-value of common building materials is around 2W-4W /m^2.
Some more questions then:
7) What are the approximate room dimensions,
8) What are the walls made of?
 
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Sommerfeldt

Dec 15, 2015
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You're right - I should probably have called it a container instead - a room sounds too large. :D
So, a 12v 5,5 Ah battery has 66Wh, right? That should probably be okay, then, even though 100% effective insulation is far from the case here...?

As to your questions;
7) Approx. container dimensions are 2.5 x 1.5 x 1.5 meters.
8) Walls/floor made of steel, wood floor and plastic/cloth lining.

- S
 

Alec_t

Jul 7, 2015
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Unless the steel walls are well insulated, e.g with many cm of fibreglass or expanded polystyrene, you are going to lose a lot of heat through them. My calcs show you'd need ~428W to maintain a 4C temperature difference between the inside and outside of the container, just to cover that heat loss through the steel walls, plus another ~42W for heat loss through a 2.5cm thick wood floor. And we still haven't accounted for the heating of stuff within the container.
So, a 12v 5,5 Ah battery has 66Wh, right?
Right. But if you used all of those 66Wh the battery would probably be fit only for garbage.
That should probably be okay, then
Nope.
 

Sommerfeldt

Dec 15, 2015
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Dang... I might need to scrap this project, me thinks.
Dude, thanks a bunch for doing the math on this - it's far above anything I've been able to even think about trying to attempt to do on my own. :D
I didn't even think to factor in that using the battery down to 0 would make it nothing but a can of goo at the end of it. :p

Thanks again!

- S
 

Alec_t

Jul 7, 2015
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You're welcome. Time now to think up your next project :).
 

cjdelphi

Oct 26, 2011
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A small oil/paraffin lamp, or heating an element, even a candle could possibly do what you ask...

If insulated well...
 
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