Solar Heating During Winter?

[Nitack]

So, my wife and I are closing on our new house next week. For the first time I am going to have a large back yard that I can start putting some of my mad scientist notions to work. For those of you who who have not seen my Passive Electrical Generation thread, I am quite enamored with the possibilities presented by the behaviors of liquids and gasses at different temperatures/densities. So here is what I am kind of thinking and wondering if it is feasible.

 

Would a solar heat collector actually be able to supplement the heating of a house during a winter in Virginia, where the temperature does get into the 10-20 degree Fahrenheit range?

 

I actually thought about trying to create a universal system that could be switched based on the season from power generation to home heating, but have decided that it would require retooling the system each season. One system is being designed to shed heat at every possible point save the input, the other will have to be designed to retain as much heat as possible outside the collection point.

 

For my power generation project I decided to use a solar collector design that I saw at www.IWillTry.org, design here. I think the same design has similar potential for this project. The main question is, will my solar heat collector be able to collect enough heat during winter to actually make this feasible? The collector is going to be painted with a flat black paint, and then put into a greenhouse casing to get as much heat generation as possible. All other points in the system outside the house will be as insulated as possible. Before the winter I will be taking my caulking gun and insulation foam cans and seal up the house tight. One way valves will be used in the system to ensure that cold air is not feeding back into the system, and covers will be used during night time hours.

 

Perhaps this will just take some experimentation. Just wanted to get some impressions on the idea and if it is an unrealistic expectation that the solar collector could get reasonably hot enough during the winter.

[freeztar]

Sounds like a fun project. :hyper:

 

If they can do it in Canada, you can do it in Virginia:

http://www.cansia.ca/Content/Documents/Document.ashx?DocId=12271

 

Here's a great resource from the Dept. of Energy:

EERE Consumer's Guide: Solar Water Heaters

 

And this is awesome, build a solar water heater for $5:

Make a Solar Water Heater for Under $5 : TreeHugger

[Nitack]

I just re-read my whole post and think I completely missed in the whole thing to specify that I wanted to adapt my design for the ambient heating needs of the house. Air, not water. :hyper:

 

Thermosiphon - Appropedia: The sustainability wiki

[Zythryn]

Yes, the strength of the sun in winter is truly impressive. You should have no issues with power generation in the winter.

Our system in Minnesota works great in the winter (although it is a standard system as I am definately NOT a 'handy' person;)).

The solar cells themselves are actually much more efficient in the cold, although it doesn't sound like that will necessarily affect your setup.

I look forward to hearing more as your project progresses.

[freeztar]

Well, the hot water heater could double as a radiant air heater as well, with some modifications and some duct work.

 

The two most common designs for space heating seem to be roof structures that collect heat and distribute it throughout the house and radiant floor heating. There are lots of designs to be found here:

Solar space heating -- active and passive solar projects for space heating

[Nitack]

Well, the hot water heater could double as a radiant air heater as well, with some modifications and some duct work.

 

The two most common designs for space heating seem to be roof structures that collect heat and distribute it throughout the house and radiant floor heating. There are lots of designs to be found here:

Solar space heating -- active and passive solar projects for space heating

 

Why radiate? Why not just directly heat the air from the house and return it?

 

 

[freeztar]

Why radiate? Why not just directly heat the air from the house and return it?

Sure, but if this is a passive design, then how will you get the cold air to rise?

[Nitack]

air pressure :hyper:

 

Hotter air leaving will rise given the design, as it leaves and goes back into the house air will have to be drawn from the other end of the system to equalize the pressure. I just make sure that the intake is at the lowest point in the house.

[freeztar]

air pressure :hyper:

 

Hotter air leaving will rise given the design, as it leaves and goes back into the house air will have to be drawn from the other end of the system to equalize the pressure. I just make sure that the intake is at the lowest point in the house.

 

Yeah, it seems like that would work. The diagram threw me off as it seems to show the cold input up near the top. :D

[freeztar]

I was thinking about this more on the way home from work.

 

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. With your design, it would get really cold at night. With radiant floors, it would stay warmer longer (possibly through the night?). I saw a design once in Colorado where they had built these window box type structures along the wall of the south facing side of the house. The tops of the boxes were clear (plexiglass maybe?) and the sides were wood. Inside the "window boxes", they placed very large drums of water, painted black of course. The side of the box facing the house was left open. The tubs would heat up through the day and slowly release the heat into the ambient air. Just another idea.

 

It would be nice to see the breakdown of efficiencies with these different systems. Perhaps one of the physics wiz-kids will come along and help out with the equations and such. I'll try to look around a bit, but it usually takes me a long time to calculate these things. :(

[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]

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:

 

[math]\frac{1000Kg}{m^3}\times\frac{4184J}{KgC}=4,184,000 J/m^3C[/math]

 

For air:

 

[math]\frac{1.2Kg}{m^3}\times\frac{1020J}{KgC}=1,224 J/m^3C[/math]

 

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]

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]

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. :hihi:

 

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!