Kachadorian's air-core slab consists of hollow concrete blocks forming ducts
under the floor. Kachadorian claims that air in these ducts would be warmed
by the heat stored in the thermals mass of the hollow concrete blocks,
create air movemnet and allow warmed air to float into the room above via
duct openings in the floor.
I have always wondered would this design be improved if hydronic pipes were
above the blocks in a cement screed. Any heat lost beneath the heated
screed would heat the blocks and hence air in the ducts underneath. This
must create air flow out of the ducts into the room above. Sort of
re-claiming heat that may be lost.
I'm not familiar with the details of Kachadorian's design but it seems to
me that a good bit of insulation under the floor, hydronic pipes on top
of that, and flooring on top of the pipes, would be the most sensible
arrangement. Why let heat get lost beneath anything? If the flooring is
more or less directly on top of the pipes then the floor itself would
heat the air and create air flow directly in the room. Cleaning a floor
certainly would be easier than cleaning air ducts in blocks under it.
Problem is that the floor isnt a very good source of
lower grade heat. Its got its real downsides even
with electrical heating embedded in the concrete floor.
Even without carpet, its not ideal and worse with carpet.
And the problem is that how warm the room feels is mostly
due to the surface temperature of the walls and the air temp,
and its non trivial to get those up to reasonable levels with
any form of heated floor. Even with an electrically heated
floor, you can get a situation where the floor is hotter than
is comfortable while the room still feels too cold.
Corse another way of looking at it is to not
attempt to do all the heating using solar, but
to use that to minimise the heating fuel costs.
Still got some real downsides with just a uniform floor
surface that the solar heat is being supplied thru tho.
Wrong again Rod. A hydronic floor with lots of surface and a low water-air
thermal resistance in an airtight house with lots of insulation works fine.
Ignoring the R1 radiation conductance, the min floor temp required to keep
a house with a 200 Btu/h-F conductance and a 2400 ft^2 floor 70 F on a 30 F
day with a U1.5 slow-moving airfilm conductance is 70+I/(2400x1.5) = 72.2 F.
I know of a few installations of PVs where the underside of the PVs is
ducting with air florced through. A heat pump collects the heat from the
panels and house exhaust air. It wasn't that brilliant. I think it could
have been better thought out.
How much heat do PV cells emit on the back and front of the cells?
You could think of it this way. Typical PV panels are around 12%
efficient at converting sunlight into electricity. This means
that the other 88% is converted into heat. Clearly they are much
better at making heat than electricity.
I think that has a lot to do with whether there is an antireflective coating
on the surface or not. Silicon is very shiny in the optical, (and I think
the reflectivity is a function of doping), where most of the energy comes
in, so, especially if the sun is at a substantial angle to the array, a lot
of the energy is reflected from the top surface. What gets in and is not
absorbed or reflected from the top surface metalization (around 10-20% I
think for typical cells) passes through the active cell and what isn't
absorbed there then passes through the rest of the bulk silicon used as a
handle. The back side metalization would be under all this heavily doped
handle. Very thin cells would be very different from the currently more
common thick cells. I am not sure about the ones that are epi deposited on
material other than silicon. If they are thrown down on a metal contact they
are probably very reflective.
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