Thoughts on thermal mass in a camper


Old 03-22-10, 01:12 AM
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Thoughts on thermal mass in a camper

While I have recently insulated my camper van and have the ability to run air conditioning (last summer a generator, this summer I'm going for a 24v batter/solar panel/inverter solution), I am interested in using thermal mass as a form of passive cooling. I camp primarily during the day in Northern Virginia (working nights) The temperatures swing wildly between 6AM and noon.

From what I've read, water is excellent for thermal mass applications.

My general thought is to fill up some 5-gallon jugs and store them inside the van. As the temperature rises, they should absorb some level of heat and then radiate it out at night as the external temperature drops. Increasing ventilation at night would assist in having that heat dissipate.

I was wanting to get some opinions on how effective this could be and how many gallons of water would be needed to affect the van's environment.
Old 03-23-10, 10:32 PM
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Some futher numbers

I have been clicking around and think I am closer to a way to work this out. I have been writing this all down on notepaper, and I decided to talk it out on here to let others work out my logical fallacies.

Some numbers (all units are English unless otherwise noted):
  • Between 32-122 F, air weights .078 - .066 lb/ft^3
  • 1 gallon of water = 8.34lbs

Specific heat:
  • 1 BTU raises 1 lb of water 1 degree F
  • 0.24 BTU raises 1lb of air 1 degree F

In my situation I am dealing with a volume of 250ft^3 of air being raised approximately 25F over a period of 6 hours (60F - 85F; 65F - 90F between 6am and noon).

I know that 250ft^3 is 20lbs of air, so to raise 20lbs of air 1 degree requires 83 BTUs. So to raise the air 25 degrees would require 2,075 BTUs.

In my confused mind, I think that means I need enough thermal mass to absorb 2075 BTUs.

We know that 1 BTU raises 1lb of water 1 degree. So 2075 BTUs will raise 1lb of water 2075 degrees (though it would boil away at 212F). Since I have never seen water boiling in a car, I have to assume that at some point, water will stop absorbing thermal energy. At this point in my searching, I don't know what that point it.

Soldiering on:
  • 2,075 BTUs into 1lb of water = 2075F
  • 2,075 BTUs into 8.345lbs (1 gal) = 248F
  • 2,075 BTUs into 80lbs (10gal) = 25F
  • 2,075 BTUs into 160lbs (20gal) = 12F
  • 2,075 BTUs into 320lbs (40gal) = 6.5F

I am not sure how the above table helps me to my question.

From an observational standpoint, water will remain cooler longer than the air, since it takes more thermal energy to heat up water than it does to heat up air. Going with this idea, perhaps it's a ratio. For ever lb of air, I need .24 lbs of water to keep temperatures equal But 20lbs of air means I need 4.8lbs of water or less than a gallon of water. So that doesn't make sense. If I go the other way, 20/.24 = 1041bs (125 gallons). That actually makes more sense to me (though I would hate to have to put 125 gallons in the van).
If 125 gallons is the correct amount, perhaps a smaller amount would go a long way to stabilizing the internal temperature.

Beyond my ramblings on determining the correct, number, there is also the issue of dissipating the heat afterwards. Once the sun fades away and the air starts to cool, the stored heat within the water will begin to dissipate. Since I am no longer inside the van at this point, I'm not too concerned about it radiating heat into van, but I wonder if the heat will fully dissipate by sunrise.

Home-based systems will in fact pump water into the shade during the day and then out of the building to let the heat dissipate quicker.

I would appreciate any input other DIYers may have.
Old 03-24-10, 09:43 AM
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passive heat

No more room than you have for insulation in a RV I think passive heating would be like p###### in the ocean. I just spent a cold few months in Fla. and the sun would warm the unit up quite nicely during the day but when the sun went down within 20 minutes you needed heat. Good luck. RW
Old 03-26-10, 07:44 PM
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I think while not overly familiar with passive cooling like a radiator you would need to move the water out to cool it or remove the heat my thoughts on this is like a heat pump system that moves heat to the ground and vice versa when looking for warmth.I don't think storing the heat inside in the form of hot/warm water without radiating it out to someplace would work for a temp. reduction inside during the time you will be in it.I'm basing this idea on the principal of air conditioning where heat is removed you are not cooling the air but that it (the heat) gets dissipated to the air via the condenser and the result is cooler temp of output.
Good Luck
Old 03-28-10, 08:12 AM
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I would have doubts about the heat/cold retention of water where I would pick it for something like that, but maybe it comes up because of the cost/convenience of it.

When I saw this a couple of things came to mind. When home solar units came in heat sinks were made from salt blocks, I believe. Then inside the house in areas of direct sunlight you decorate with bricks for the flooring material. This is probably still used. So the bricks/salt blocks were good for the heating side of things and I'd assume for cooling. I know the weight is a consideration but just an idea.

Also for a completely passive system wood exposed on the walls is considered to be a good heat sink. You may be able to incorporate something along those lines into your trailer.
Old 04-20-10, 03:31 AM
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To much water required I think...

CampAtWork I ran across the following information at the following url: Prevent temperature swings in sunrooms with fiberglass heat storage tubes for solarium cooling and solar heat storage

Your calculations yield similar results as shown in the chart above.

According to the site info 47 gal. of water @ ~400lbs. will sink, with a 20 deg increase, 7,800 BTU.

The site above uses a fiberglass tube of water. The fiberglass aides in the heat transfer process where as other materials may insulate and inhibit the process.

While there is no doubt the water would help do what you want to do, the problem is the amount of water needed to be effective.

I have a camper and it has a 23gal fresh water tank. It would be great if I could use that to keep the camper cool.

- Phil

Last edited by OntheRoadAgain; 04-20-10 at 03:37 AM. Reason: adding a little more.
Old 04-20-10, 02:33 PM
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This morning I thought more about this. During the late summer here in California in the Central Valley it often gets to 110 F during the day. Last Year I was able to keep my camper 8' long and about 6' wide, about 7' tall inside, comfortable with a 5,500 BTU window air-conditioner. Granted a little more cooling would have been nice maybe 7,000 - 8,000 BTU?

Now that would work out to about 17 BTU/sqft. [5500/(8x6x7)].

Using 5500 BTU/hr over say 8hrs that would be 44,000 BTU. According to that chart I uploaded last night that would require about 260 Gal of water.

Here is a thought. This may not be legal from a health perspective but I have often thought of using the city water supply to provide the required heat sink capability.

My idea was to connect from my home to my neighbors using a reversible circulation pump part of the time I would draw water through my meter and pump it back through his, then reverse the process. with a 50% duty cycle there would be Zero net water used but, that would provide me with a constant flow of water cooled to ground temp. In my case that is ~55 deg water with virtually unlimited cooling capacity.....

Hooking up in a campground to a couple of faucets would be easy enough. Depending on the campground plumbing you may be able to get the desired cooling without stashing 260 gal or more of water??

I would make a condenser with copper tubing put fins on it, pump the water through it and a blower.

Circulation pumps are usually about 1/3hp so they don't consume much power. And a couple of muffin fans and there yea be a low energy air-conditioner.

Hey I may just do this myself!

If anyone tries this before I do let me know how it works!



Last edited by GregH; 04-20-10 at 03:11 PM. Reason: Email address removed.......not allowed

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