Thermal mass in a rental unit

[jnik]

My house has lots and lots of west-facing windows, so even in winter it gets nice and toasty in the afternoon. I'd like to hold on to that heat. Since I'm renting I can't exactly lay in another slab of concrete, so I'm looking for cheap, non-permanent ways to get more thermal mass. I've been thinking in terms of saving milk jugs and filling them with water (using the water on the lawn in the spring), but I suspect I'd need a LOT of milk jugs. I also looked into substances which melt in the low 60s--the freeze/thaw transition being a lot more energetic than simple temperature changes--but they're all expensive and mostly very nastily toxic.

Ideas? Cinderblocks from Craigslist maybe?

[strider3700]

water is the cheapest and most effective way to go.   look for some plastic 55 gallon drums.  paint them black, park them in a sunny spot and fill them up.  Just remember if you're not on the ground floor that's going to be a lot of weight.

Milk jugs will break down. I wouldn't trust them not to leak quite quickly if exposed to the sun. 

[AlexK]

I was thinking of doing the 55 gallon poly drums thing in my house too but my wife for some strange reason doesn't think it would look good.

[frugal_engineer]

Water is the best material you will be able to use.  The container's contribution will be negligible.  The metric you want is specific heat capacity (http://en.wikipedia.org/wiki/Heat_capacity).  Notice the material table that water has the highest heat capacity of any material that is liquid at room temperature with the exception of ammonia.  Not sure you'd want many gallons of pure ammonia in your living room.  Water also has the highest heat capacity by volume of any material on the list.

In short: you can't do better than water for heat storage at home.

[jnik]

In short: you can't do better than water for heat storage at home.

Sticking with Wikipedia as a source, water's specific heat is 4.2 J/gK, but the heat of fusion for acetic acid (which, in pure form, melts around 62 F) is 192 J/g! So bringing pure acetic acid from 61F to 62F stores as much energy as bringing the same mass of water from 33F to 115F! (47C change). Shame about the flammability/causticity.

I agree water's probably the best by-the-numbers without phase change, but there are convenience issues and the phase change is so tempting. Assume a temperature range from 60F to 75F. That's 8.3C, so 35J/g vs. 192J/g for AA. DMSO is 184 J/g, so slightly over 5x the storage capacity of water.

Of course water costs me 4.2c/kgal, or 1.16x10^-6 c/g. DMSO is $12 for 4oz, or 10.6c/g. A factor of 5 in heat storage, a factor of 10 million in cost. I'm not even going to think about the acetic acid given the hazmat fees...where's those milk jugs again?

Thinking supply side: window is about 100"x40", or 2.6 square metres. Best-case around here is maybe 60 degree angle between the Sun and the window orientation, maybe 5 hours of good exposure so 1000W/m^2 * cos(45)  * 2.6m^2 * 5 * 3600 = 33087600 J to store, or 260 gallons of water at 35J/g, weighing around a ton and costing two cents. The 55 gallon drum is going to be the expensive part.

[Another Reader]

"...weighing about a ton..." 

Ummm....the most expensive part is likely to be the floor damage and the subsequent eviction by a very angry landlord.

[needmyfi]

55 gal drums are 10 bucks each! Check craigslist maybe.  Dh bought mine from a guy who had a servicemaster franchise.  I use them for thermal inertia in my cold frame.  Don 't forget to close some heavy drapes over the window at night.

[frugal_engineer]

water is only ~8.5lbf/gal. so 55 gal drum is ~470lbf.  just put it in the corner or near the wall and not in the center of the floor joist span and you'll likely be fine.  probably not so fine if you do 5 drums or something.

[TomTX]

My house has lots and lots of west-facing windows, so even in winter it gets nice and toasty in the afternoon. I'd like to hold on to that heat. Since I'm renting I can't exactly lay in another slab of concrete, so I'm looking for cheap, non-permanent ways to get more thermal mass. I've been thinking in terms of saving milk jugs and filling them with water (using the water on the lawn in the spring), but I suspect I'd need a LOT of milk jugs. I also looked into substances which melt in the low 60s--the freeze/thaw transition being a lot more energetic than simple temperature changes--but they're all expensive and mostly very nastily toxic.

Pick a wax/oil/blend with the right melting range, but you can pick up cheap. For example, pure coconut oil melts around 76F - probably too expensive for your use, but it's a non-toxic example.

[Gerard]

I don't have the science background to contribute to the choice-of-filler conversation, but would the shape/size of the containers maybe matter more? Presumably it would be better for you if your device released the heat back into the house in the hours between sunset and bedtime, when it'll do you the most good (because you're likely to be home and active). Would that make smaller containers better? Particular shapes? Maybe 2l soda bottles, like here:
http://eere.buildinggreen.com/overview.cfm?projectid=271

Also, to look at this from a different angle for a minute, would you benefit even more from rearranging your life so that you can be home during winter afternoons, and just basking in that free natural heat?