[Greenbuilding] [BULK] Solar thermal with sand storage

[Keith Winston]

Hmm. I don't quite have the attention to sort out all the implications 
of your design. Especially since, at the end of your counterflow heat 
exchanger, there's a (dry? wet?) sand/bottle storage bed that we don't 
really know how it will work... I'm a little concerned that you'll bleed 
the high temps off your solar loop, and then temper your potable water 
downward as it runs through a (colder) storage bed, and end up with poor 
heat stratification (which is the big question about the sand bed in my 
mind anyway: to what extent does stratification occur? It's a bit 
complicated, depending I think on bottle layout as well as sand 
saturation, etc).

Even if you don't join, you might want to search the Solarheat archives, 
since there've been a few discussions about sand beds there. Though 
Niko's subsequent response was quite interesting/thorough in that 
regard. One comment I alluded to earlier is, water has apparently fairly 
poor conduction, but that's ignoring convection, which changes 
everything. In a non-liquid or gaseous material, there's no convection, 
and conduction is most of the story.

This is a site I often use for thermal conductivity questions, they've 
got a good list:

http://www.hukseflux.com/thermal%20conductivity/thermal.htm I like this 
because it includes volumetric heat capacity and diffusivity.

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html is 
also good for thermal conductivity.

Water = .6 W/mK
Sand = .35
Saturated Sand = 2.7

For comparison (when we start talking about heat exchangers, and 
especially metal vs. plastic heat exchangers):
PVC = .16
Polytheylene = .4
Polypropylene = .1
Copper = 400

You get the idea. In case it's not obvious, that means that it would 
take 4000 times as much surface area of Polypropylene (very 
approximately) to equal the heat transfer of copper, all other things 
being equal.

Anyway, that's just me riffing on plastic coils for heat exchange...

Keith



Speireag Alden wrote:
> On 2008, Jan 28, at 22:24, Keith Winston wrote:
>
>   
>> The problem is heat transfer in and out. Coils of plastic/copper/??
>> That's where wet sand changes things, since conductivity is extremely
>> improved, and in fact the flow through the sand can utlized for
>> exchange. But then you have to have a watertight tank... Actual
>> conductivity of dry sand isn't so great, and you don't get convective
>> loops like in water. But it's an interested idea, it would be
>> interesting to experiment with it.
>>     
>
>      I'm thinking multiple runs of PEX, plumbed such that it's a sort  
> of contra-flow heat exchanger.  The line from the solar collector  
> runs one way, and the line from the cold water supply toward the hot  
> water heater runs the other way, and they're taped together.  You  
> could even split the line from the solar collector into two or three  
> lines and surround the domestic water line.
>
>      So the cold-to-hot water line would circulate around the  
> perimeter to get an initial charge, and then move in between three  
> other lines and coil with them for awhile, and then off to the heater  
> to bring the water up to correct temperature, if it's not there already.
>
>      What do you think?
>
>   
>> For those interested, this kind of gory solar talk is all we do on the
>> solarheat group at yahoo. You can join if you want...
>>     
>
>      If I join another group, or add anything to my activities in the  
> face of all the house building which is waiting for me to do it, my  
> loving and patient spouse would have good reason to be very upset...
>
> -Speireag.
>
>
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