Solar Heating During Winter?

[Zythryn]

I believe Freeztar is correct for the water vs direct heating of air issue. Water will hold onto that heat better than directly heating the air would.

Another passive solar heating method I saw was to have a whole bunch of south facing windows allowing sunlight to warm a floor made of thick granite.
The granite would absorb the heat during the day and release it slowly during the night.

[modest]

Originally Posted by freeztar

I think using water (or some liquid that holds heat well) is a better way to go all around. Water can retain heat much longer than air.

Not only is the specific heat capacity of water four times higher than air:

Specific Heat Capacity Table

The density of water is also much higher than air.

For water:



For air:



So, a one degree temperature change in water represents 4,722 times more energy than a one degree temp. change in air (of the same volume). This makes water much better at storing heat.

Also, water is a better conductor of heat - it will absorb it and release it more readily. All around better I would think - though I know nothing of natural heating systems.

I suppose you might run into some trouble if the water froze. Maybe some antifreeze in the mix?

~modest

[freeztar]

Originally Posted by modest

Not only is the specific heat capacity of water four times higher than air:

Specific Heat Capacity Table

Thanks Mod-est!

According to that table, H2 would be ideal. Unfortunately, it would be expensive to contain and then there's the obvious fire hazard if leaks occurred. The same issue of containment applies to He, not to mention controlling the pressure. So it does indeed seem that water is best. Would adding salt or some other chemical make water more efficient in this regard?

I suppose you might run into some trouble if the water froze. Maybe some antifreeze in the mix?

Some of the solar water-heater designs that I was looking at earlier used antifreeze (not in the mix - straight antifreeze) for circulating the heat from the collector into the tank and back. I assumed that it was to prevent steam buildup and possible rupture. I wouldn't think a system like this would freeze, except maybe at night and without active flow. I'll have to look into this some more.

[modest]

Originally Posted by freeztar

According to that table, H2 would be ideal. Unfortunately, it would be expensive to contain and then there's the obvious fire hazard if leaks occurred.

And also, It has an incredibly low density which that table doesn't take into account. I looked for a table that lists volumetric heat capacity, but couldn't find one. That's why I converted from density in my last post. As the system we're talking about can only hold a set volume regardless of mass, we should have to consider density along with heat capasity.

If a substance has a lighter molar mass, then each gram of it has more atoms or molecules available to store heat energy. This is why hydrogen—the lightest substance there is—has such a high specific heat capacity on a gram basis; one gram of it contains a relatively great many[quantify] molecules. If specific heat capacity is measured on a molar basis (uppercase C), the differences between substances is less pronounced and hydrogen’s molar heat capacity is quite unremarkable.

-specific heat

~modest

[freeztar]

Thanks Modest.

So, it's back to water...or not...

At this site, it is posted that the antifreeze is not the auto type, but rather polypropylene glycol (PPG).

Antifreeze

I'm having no luck finding the specific heat of PPG.

[Serapis J.]

I have been experimenting with this application for some time now. I have found that water is an excellent heat transfer medium (relative to air). It will readily absorb radiant heat and release it to a thermal storage medium (such as a subfloor gravel bed.) In a simple circulating loop this works fine when temps are above freezing, however shortly after sundown when your circ. pump shuts down any water in the portion of the line outside will freeze if not drained out. One alternative is to use ethylene glycol as a transfer medium rather than water, or if you would rather not run antifreeze under your floor or through your walls, you could utilize a second heat transfer loop of EC/h2o where an insulated transfer tank kept outside, will circulate water through the interior loop and EC through the collector loop.

None of these methods however, approaches the effiency and promise of hydrated salts as a latent heat bank. Na+, Mg+,Li+, sulfate or nitrate will collect heat as a liquid while the sun shines and then release it's latent heat as it solidifies as temps drop to freezing. These PCMs (phase change materials) are being utilized for both heating and cooling applications and if purchased in raw form and mixed DIY are realitively cheap. There are many companies clamoring for patents of proprietarty blends with specific properties (and collecting a handsome return for their R&D!) but these compounds are inexpensive and not particularly hazardous with proper handling and storage. There are some challenges to be overcome involving the settling and separating of the matrix over time but I am really jazzed about the possiblities. As the heat is released by the salt bed through the night, your circ. pump will continue to pumping heat into your home.

I am limited by my residence (renting) as to just how much I can experiment with this but if any of you have the means and inclination I would love to hear your experience!