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Using wood as a heat store in a lean-to  RSS feed

 
Posts: 127
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Hi,

I am trying to store heat in a lean-to green house round the house to try and store heat to release at night and help stop heat loss from within the house itself (esp. the single-glassed windows).  

I am lugging home several bags of river stone and sand on the way home from church every week to make a gabion wall and rock stove and portable thermal mass, but I have also noticed that wood has quite a good heat-holding capacity when compared to sand and rock.
https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html


A google brought up this article:  
http://www.fridayoffcuts.com/dsp_article.cfm?id=307&aid=2612

On the other hand, it is also insulative, so how quickly it will give and release heat I am not sure.  But in theory, a good way to go would be to make a wood box that was a gabion wall on the outside (for aesthetics) - like this:
https://www.bing.com/th?id=OIP.QnXOrXPWkKYS5s2_qTi6VgHaJ3&w=135&h=180&c=7&o=5&dpr=1.45&pid=1.7

If we then put the wood inside, and then a wooden top to form a seat - if we pull in the heat from the top of the greenhouse to the bottom of the box with venting tube as a fan, it should help heat the wood, and also super dry it - a wood drier is another thing on our to-do list!

Anyone tried this yet?  Any other ideas or suggestions why this would not work?

Otherwise I will keep you updated 0 and if anyone lets me know how to attach pictures, I will do so.


 
gardener
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If we then put the wood inside, and then a wooden top to form a seat - if we pull in the heat from the top of the greenhouse to the bottom of the box with venting tube as a fan, it should help heat the wood, and also super dry it - a wood drier is another thing on our to-do list!

 If you are watering growing plants in a greenhouse, I think that the hot air at the top of the greenhouse will be humid.  The way I see it, this consistently warm air going by your wood, if it is moist air, is not going to dry your wood.  You wood is seeking equilibrium with the air in regards to humidity. Dry air will dry wood, wet air wont.   Or is this just a greenhouse type of a structure, but not used for growing plants?

At any rate, wood has some very interesting properties, but it is hard to classify all wood as the same.  Some wood has a much denser body compared to others.  Regardless of this, though, wood does indeed have some strange thermal capacity.  

There is something to the notion you are putting forward.  A log home builder friend of mine told me that heat coming up against a round log enters it very slowly, and follows the curve of the grain as much as penetrating to it's core.  It takes a long time to heat a log cabin if it has been left without heat for a time so that it is the same temperature as the winter cold outside, but once it is up to temperature it holds that temperature very well, and is quite easy to keep warm.  

I've heard the term thermal inertia used to describe this other relationship that you are referring to, but when I look up that term I get something different, so I'm not sure.  But that's what I think of it as.

So, in my thinking, there is something magical that happens when a body of material is both insulating and has thermal mass, that allows the sum of the two to be greater than expected.  The way I look at it, and I could be wrong, as I have not done scientific experiments with these things:  A volume of sand is similar in this capacity, since it has air pockets that solid stone or concrete does not, but it has some very dense material that acts as thermal mass.  The air in the sand volume does not flow, or move easily at all, it is just a bunch of little individual pockets of air, and heat does not transfer from sand grain to sand grain at the same speed that particles within a solid stone or concrete wall are transferring heat, and yet, if given time, the sand volume, and it's little air pockets do indeed heat up, transfering heat throughout all the sand grains, and once it does, again because of the air in the sand volume, it takes a long time for the heat to be released into the surrounding air.  I (probably wrongly) call this thermal inertia.    

That's my two cents.
 
Annie Hope
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Hi,

The main purpose of the lean-to is not to be a greenhouse for plants, it is mean to:
- Act as a cheap retro-fitting double glazing on the house, with thermal mass heating it as well
- To extend the area of the house that does have dry soil underneath (we have a pier and beam house that does not have plastic on the ground under it, so we are going to do this under the house, and also in the greenhouse area, and put stones top of the plastic to make a thermal mass.
- To act as a retro fitted fly screen in summer and shading of the west windows from afternoon sun.  We plan to high-power fan vent the greenhouse out, and can open the house windows then as well.  Otherwise we have a choice of flies or a hot house in summer.
- To expand the living areas of the house - e.g. put a large dinning table in the greenhouse area near the kitchen, and a court-yard retreat from the guest bedroom.

There will be plants in pots and the outer corners of the greenhouse, but we will also have heat-exchange ventilation installed as well, so that should also control humidity.
 
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I think it's a grand idea.
I would want a workshop in there,but it could be better as a living room.
This sounds more of a low mass solar space, and less of a greenhouse.
That part of it is below the inside living space is advantageous.
Heat that hits the wood should drive moisture from it, if that moist,heated air is circulated into the living area, it might actually improve the inside airs humidity.
You would want to filter it, but nothing extreme is needed.


You mentioned venting during the heat, will the greenhouse be glazed with ridged or flexible materials?
Ridged and you can stick reflective paper to the insides for summer.
We use this in our home, it really cuts solar gain.
Flexible and you can roll it up.
Either one could be covered with shade cloth.
Actually a Chinese greenhouse style of insulation on a roller would be good year round.
Roll it up to gain or release heat, down to retain or deflect.
Maybe use Mylar bubble wrap,or aluminized movers blankets.
Or just grape vines up the outside,for self adjusting shade.

Any chance you are building the rocket stove in the solar space?
That would be comfy ,add to your wood drying ability ,and excess heat could be siphoned off to the house.
 
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A very good " heat bank" in your area would be water.
Stored in drums, bottles etc, it will store enormous amounts of heat c.w. almost another product.
 
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