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Water: The Ideal Thermal Mass  RSS feed

 
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Fascinating blog post here from an architectural company:
https://www.bdmd.com/water-the-ideal-thermal-mass/

The final graph on the blog is the most important. Water is far better than any other (common) material at storing heat.

An interesting idea mentioned in the blog is that of "roof ponds" where a building has a pond built into the roof to absorb and release solar heat.
 
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Interesting data, analysis and comparison, but there's more to it....

The last paragraph is the most important.  "...water can quickly damage a building...".  Methods of water storage seem quite important. 

Anyway, for now going with their assumption that the thermal-mass is monolithic, a useful parameter here is "Thermal diffusivity".  The diffusion constant is in units of distance squared per unit time.  For adobe-brick it is 0.27 mm^2 / s.  There is an explanation here.  The useful "thermal depth" into a material depends on the relevant time duration in your application for heat conduction.  If the time duration is 1 hour, then the thermal depth of adobe is 55 mm (2") , (so about the half-width of a brick).  Whereas if the time is 24 hours, then the thermal depth is 768 mm (30"), closer to an earth-berm sized depth.  If it's a half-year time-scale, then you have 34 ft to work with. 

So for that reason the final graph could be a bit misleading.  We assumed the mass is monolithic when it need not be.  Typically the number of channels within the mass can be increased when a design needs faster heat transfer.  There is a similar effect with water convection. 

Also they've assumed identical temperature differences for all the materials, but while it's true many of these common materials (brick, concrete, soil, air, etc.) only have 1/4 the specific-heat of water, they also have say 4 times the useful temperature range.  So in applications with a wide temperature range, like a rocket mass heater for example, a brick can get much hotter than water, so it can store as much heat or more than water, at least when the temperature swing is available. 

Also the heat required to heat gypsum up to 200 deg.C (400 deg.F) is quite high, because you have to boil off some of the water in the calcium sulfate hydrate.  If you then use that dehydrated calcium sulfate as a desiccant then it can absorb a lot of heat.  So, there are plenty of details to consider here, lol. 

 
Matt Coston
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Mike Phillipps wrote:"...water can quickly damage a building..."


Mike Phillipps wrote:a brick can get much hotter than water, so it can store as much heat or more than water


Some very good points you've raised there.

I'd like to get your opinion on the following: Temperature gradient is an important part of heat exchange. Water can be pumped, so it means you can force "cool" water through pipes to maintain a very high heat gradient against hot gas in a chimney. You can also pump water downwards to overcome any effect of convection in the pipes. That way you can have your water in contraflow to the chimney gasses which improves efficiency further (which I believe is how it's done in industrially made heat exchangers).

One more advantage of water I'd like to add is that because it can be pumped, your thermal mass can be stored elsewhere and circulated to where you need it, which means you can potentially make the storage reservoir much larger (than a solid mass).

Below is the video that made me start to look into this in the first place. This guy built "pillars" of water to absorb solar energy.



 
Mike Phillipps
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Matt, true, that works.  The main problems are that 1) cooling a chimney can condense tar and water causing creosote and corrosion.  Something like a rocket stove or rocket mass heater help by getting a ceramic riser hot enough to crack tars in order to burn the tar gas (aka smoke).  2) Also the water pipes need surface area from fins of suitable spacing or coils in order to get better heat transfer with air.  Yes, "counterflow heat exchangers" are the best.

Water is fricking fantastic.............except when it leaks, freezes, corrodes metal(s), conducts electricity or grows algae in a solar collector, haha.    Yeah, other than all that it's wonderful. 

I just had a pipe freeze and it's awful.  I'm a huge fan of "heat pipes".  Now those things are amazing!  You discover how heat-pipes work when you try to re-solder your split frozen pipe but find you can't even melt the solder with a blowtorch because there's a single drop of water inside!  haha.  working like a steam engine to pull all the heat away.  oh well...

edit: Why do you think floods are the #1 natural disaster?  Water is destructive.  There was someone on here who wanted to put a RMH in a mobile home but it couldn't handle the weight load.  Could I recommend putting water storage over an OSB floor?  hell no.  Have you seen what water does to OSB?  At first one thinks the urea formaldehyde glue or whatever sheds water...then before long it turns to wet cardboard

If only there was some kind of sponge to soak up the water....  http://en.wikipedia.org/wiki/Superabsorbent_polymer

-Mike
 
Been there. Done that. Went back for more. But this time, I took this tiny ad with me:
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