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Solar design Challenge - off-grid workshop

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Posted by appsol on November 9, 2009, 9:55 pm
I'm building an off-grid workshop, and want it to be solar heated or
at least solar tempered. The building site has reasonable south
exposure. I would like to arrive on a random winter morning and find
the inside temperature at or above 10deg C. Additional heat would come
from a wood stove.

The problem is how to get enough thermal mass !

I want a wood floor, because it's nicer to work with, and I want a
crawl space underneath. I'm tempted to try a large box of rocks under
the floor, but am worried about the number of PV panels required to
move the hot air down to the crawl space.

Since the building may be un-occupied for weeks on end, I'd like to
avoid the use of large amounts water that might cause problems with a
hard freeze.

Some specs.
- 45.5N, 65.9W
- 4700 heating degree days (C), 1950 hours of bright sunshine
- 22'x30', with the long axis facing south, single story with a loft
- there will be a 1.5 story 10'x6' air lock entry/sunspace in the
center of the south wall
- walls will be R30 or greater, shallow foundation w/ R20 horizontal &
- roof insulation R40 or greater

It might be interesting to build air-heaters for the south wall or
roof space.
There is also a possibility of building a brick wall in the center

I'd be interested in any suggestions you might offer !


Posted by Josepi on November 9, 2009, 10:09 pm
Water has a much larger specific heat / thermal mass, is easier to control
and manipulate and keep clean.

If the water tanks ar mounted in the ground with enough mass and proper
thermal sensing and controls are used it will never freeze. You may have to
stop using the heat from it at a minimum temperature though. depending where
you are located, geographically you may not get any sun in December and/or
January. A glycol solution would be best in order to guarantee no freezing.
Back-up heat and water may be the best so you can forget about it once set

Posted by Morris Dovey on November 9, 2009, 10:35 pm
 appsol wrote:

Ok, by way of encouragement, here's a completely solar heated shop that
hasn't been cooler than 18C since construction:


If you click on the photo, you can have a look at the construction used.

Hm. That's only a problem if you make it a problem.

How about a 22'x30' box? I think your fondness for the wood floor is, in
this context, in conflict with your desire to store heat reasonably. An
insulated 6" slab would provide an excellent thermal flywheel, and a 12V
ceiling fan would be adequate to deliver the heat to the floor, prevent
stratification, and improve comfort levels year-round.

Cushioned mats would provide even more comfort than wood in areas where
you'll work standing.

Solar heating doesn't require occupancy to work. :)

Within walking distance of Darlings Island Bike Shop?

That all sounds good to me.

If you put 'em in the wall, snow reflection will boost your thermal
input considerably. If you put 'em on the roof, you'll lose that boost
(and they won't do much for you if they become snow-covered), and you'll
need to expend energy to bring the heat down to where you want it. I
suggest sticking to panels in the south wall.

I'd suggest postponing the woodstove until the sun goes out. If you
build carefully and insulate as you've described, you won't need the
wood stove.

There's a gentleman over in your area who builds solar heating panels -
I don't know anything about him or about how well his panels perform,
but I'd probably be worthwhile to track him down and get his suggestions.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by schooner on November 10, 2009, 10:53 am


You might want to contact Wayne.  His new shop has solar panels as well as a
heat pump for heating.  Located over in NS.

Posted by Ecnerwal on November 10, 2009, 7:04 pm
Wood floor can be over a concrete slab - but concrete (and rocks, etc)
is actually a poor thermal storage as opposed to water, so little point
in it. Still, for simplicity of getting a regular construction crew to
build it, concrete slab with radiant heat tubing (filled with
antifreeze) is probably the one least likely to confuse the workers. Set
it down a bit and put your wood floor over it.

However, if you are putting a wood floor on top, and insulating under,
and are willing to involve the space/excavation/insulation (or need to
for foundation purposes anyway) an insulated block of dirt in the
crawlspace with radiant heat tubing and antifreeze is probably the most
cost-effective method (dirt is much cheaper than concrete, and stores
heat pretty much the same) IMHO. Run plenty of large bore (less pumping
head loss) PEX tubing through it and you have a heat storage system.
Pumping water is usually far less power/bother than the sort of
foolishness you have to go through to get air/mass coupling with ducts
and fans. Air collectors still make plenty of sense for getting heat
while the sun shines, and can also be coupled to the water system for
transporting heat to storage.

"Better storage" (more compact, mostly - but also adding the possibility
of leaks, freezes, etc) would (IMHO) start to involve actual insulated
water storage - be it barrels of water you blow air over (more power, I
think, from my research, as with other air/mass schemes) or a large
insulated tank you pump water through. Large volume water storage is
inefficient to put antifreeze in - you can use antifreeze in collector
loops and park the storage where it would not normally freeze anyway
(underground) and/or monitor storage tank temperature. If temperature
drops too low (sort of in order):

sound an alarm
have it make a call
have it fire up some sort of backup heat
have it open a big drain valve

FWIW - I built a large workshop. I thought it was going to be offgrid.
By the time I had collected enough money to put in a power system barely
adequate to power it offgrid, I had more money collected than it
required to place it on-grid. You situation may vary, but the price of
not overly dependable 50 amp off-grid service actually turned out to be
~$000 more than 400 amp on-grid service.

If, as I suspect from your location, you get long periods of cloudy
weather when it is also cold, too much dependence on solar schemes
worked out for places like Arizona (where a week without sun is all but
unheard of) can bite you. For short-term thermal mass, a very simple
approach is double drywall - if mudded and taped on both layers, it also
helps with fire resistance - but it only stores heat for a few hours.
Too much thermal mass in the space can bite you if you come in when it's
cold and try to heat with the woodstove - the cold mass keeps the space
from warming. Isolating your mass with insulation can help with that
problem, and also allows the mass to be heated much hotter than you ever
want the space to be when the sun does shine.

Cats, coffee, chocolate...vices to live by

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