[Greenbuilding] Power Vent for Solar Air Heater

[LarenCorie]

> From: "conservation architect" <elitalking at rockbridge.net>

> I am proposing to use 6mil poly that we purchased for a hoop house
> a few years ago.  This is unused off the same roll.  I called the 
> supplier
> and learned it has a solar transmittance of 92%.

Hi Eli;

 That sounds like the greenhouse grade, that has a life of around
four years, more in overcast climates.     A couple of years ago,
on a moment's whim, I grabbed a roll of standard 6mil, and took
an hour or so to tack it over the South wall of my log home, in a
lean-to configuration. I didn't even bother to insulate the ground,
more than to just pile the extra polyethylene on it.    I ran some
insulated flex-duct from the opposite end of the collector, through
a small window, with an 8" table fan taped to the end.  When my
basic digital indoor outdoor thermometer said that it was hotter
in the lean-to, than in the room, I would open the window, pull
the fan and duct through, and turn on the fan.  I calculated the
payback, against electric resistance heat (though I heat with
wood) as something around (IIRC) nine sunny days.   This
stuff can be real simple. However, to optimize it, for heating
under difficult conditions, does not allow that kind of free
wheeling casualness.

> I had assumed all the insulating value was similar to a single layer
> of glass where I thought all the insulating value was the air film on
> each side.

Glass blocks IR. Air films do not. So, glass provides a little more.

> Does solid glass material actually have insulating value?

  Everything has some. Glass has very little. Virtually all of its
insulation value comes from its surface air films, and blocking
of infrared.  If optical clarity is not required, TwinWall poly-
-carbonate has good insulation value, and better Solar trans-
-mittance than two layers of glass. It is also easier to work with.

> In any case, it does trap and convert light to heat in the hoop
> house that I am sure is not very tight.  My proposed installation
> would be much tighter.

That is relatively important, since collector are usually pressurized,
one way or the other.    You can think of the fan/pressurization as
having a few potential advantages over passive air flow:       1) to
reduce the size of the ducting and holes through the wall,     2) to
reduce the collector thickness, and   3) to bring warm air down,
below cooler air. However, for point three, most people do not
understand that it is a good thing, for a collector to bring hot air
into a room, up near the ceiling. Introducing the Solar heated air
near the floor will cause it to rapidly overheat the room. Instead,
bring the hot air in near the ceiling, where it can heat the thermal
mass of the ceiling and upper walls. There will still be heat radiating
down into the living space.  With high temperature air near the
ceiling, wall vents can also be used to allow heated air to spread
to adjacent rooms. By venting the Solar heated air in near the
ceiling, you can have a collector area that is around 10% of the
heated floor area, without annoying temperature swings.  Older
homes, with thick plaster can handle larger collector areas. You
might even add a little thermal mass to the ceiling.  Interior wall
stud cavities, with high/low vents and a small fan can be used
to bring warm air down when needed (thermostat if desired)/
There are simple, larger heat storage that could be used if
needed, and a heat pump water heater, located where the
Solar heated air will vent to it, can turn your basic Solar air
heater, into a simple, freeze proof, Solar water heater, for
less than standard roof mounted systems, and it can work
better in Winter, and also air condition/dehumidify in Summer. 

>  I have dark wood siding that should
> absorb light to convert to heat.

  The log walls of my home are dark like that, and they made a
good absorber.  However, there is a lot more to an efficient air
heater than just a dark surface, and a glazing. For high efficiency
it is critical to address the heat losses, back through the glazing,
which generally means reducing the radiant temperature of the
surfaces that see the glazing. That usually means more creative
tricks than just a simple hot wall. You need a heat exchanger.
A layered, or mesh absorber, with air flow from the glazing
toward the interior of the collector, will reduce convection,
conduction, and radiation heat transfers to the glazing.  So,
if you can keep the glazing and the surfaces it sees, fairly
close to inlet air (room) temperature, the collector will be
about as efficient as a window is during direct sunshine,
without the heat losses from the other 80-95% of the time.
This is particularly important in overcast climates. Simple
(also relatively low cost) Solar air heaters made overcast
Michigan the #2 Solar space heating state in the nation,
during the Solar craze of the 1980s.  You can not drive
around northern Michigan without still seeing houses with
30 year old Solar air heaters covering their South walls.
Typically, there will still be about one per thousand
people, even though the fans failed many years ago, and
there is no company fixing them.    Heating contractors  
charge ridiculous fees, and siding contractors offer to
tear them off houses for free, if they get paid for the siding
that replaces them.    So, many, perfectly good collectors
have been lost, to the failure of an inexpensive fan. Cover
a wall with such collectors, and you have a lot of heat
available, usually in excess of 100,000BTU/sqft per heating
season, less in warm climates, and more in cold and sunny
climates, like Colorado..

>  Behind siding is a 3/4" air space over continuous 3" rigid insulation.

  You need high temperature insulation (isocyanurate) between the
collector interior and siding. The interior of a collector, even vertical,
can stagnate at quite high temperatures especially if they are oriented
very much East or West of due South.

> The reason I thought it might produce a significant amount of heat
> is that I feel the heat at my South facing windows.

It definitely can produce a lot of heat, and do it during the heating
season.

>  The intention is to only circulate air during the productive times

That control is the advantage of air heater. It allows them to be
oversized.

> which you advise to cut on fan circulating air into room at 90F
> and cut off at 80F.

     You can get thermal snap-switches that turn on and off, usually
with a 20F differential/hysteresis.  Generally, the fan should pull the
collector air temperature down at least 10F, usually more. If it does
not your fan is probably too small, or you have a flow restriction, or
poor heat exchange (depending on where you are sensing your
collector temperature)  While there is a general rule-of-thumb, of
about 2CFM (air flow) per sqft of glazing, maximizing collector
efficiency might require close to double that.  Of course, all these
things can bring on other issues (noise, physical size of the fan, etc).
I have been designing and building these things for the past 35 years,
and the realities of each situation can be as challenging as the theory
behind it all.

>  At night it will get cold fast with no thermal mass  and warm up
> quickly when sun returns.

Another advantage that Solar air heaters provide (over hydronic
systems), especially in overcast climates is their ability to heat up
in as little as about two minutes when the sun peeks through holes
in the clouds. It may not be a lot of heat, during those times, but
those are the most critical times, when a house, or heat storage,
needs heat, the most.

> This would be outside the thermal envelope and therefore not
> subtract heat from the house.

Yes.....That is why we refer to it as "Isolated Gain" and the
collector's heat losses are also isolated, so the temperature
swings are too. The house can be temperature stable, and
super-insulated, while the air heater, be it a panel or a low
mass sunspace, can go through wide temperature swings.

> This is the advantage over passive windows are thermally
> weak areas in thermal envelope.

Yep ;O)   However, you can build a walk-in Solar air heater
(low mass sunspace) over south facing windows and doors,
to create a sort of "Solar Yard", a modified climate area, that
can produce a far milder environment for windows and door
to be exposed to. Similarly to how an insulation umbrella can
raise the ground temperature around a house, to match that
of a location several hundred miles further South, a sunspace
can produce a similar modification, for the air against the
South wall of the house.

> The big unknown is how much do the tree branches diminish
> the solar gains.

  Shaded glazing has as much heat loss as unshaded glazing
(keeping it simple).   If the shading is clustered, you might
compartmentalize the collector, into multiple smaller panels,
so that when one area is shaded, it does not destroy the
efficiency of the others.  In a retrofit situation it can often
be easier to pre-build, multiple panels, then mount them
on the wall, rather than building a big one on the wall. 
Small collectors are also easy to vent through the wall,
to and from individual rooms, and can use small fans
that will fit within the wall thickness. This also can solve
the problem of heat distribution between rooms.  Solar
air heaters are usually the most economical, fastest
payback, form of Solar heating.

 -Laren Corie-
 Natural Solar Building Design and
 Solar Heating/Natural Cooling/Energy
 Efficiency Consultation Since 1975
 www.ThermalAttic.com  (many new
 photos and pages, coming soon)

Read my Solar house design articles in:
-Energy Self-Sufficiency Newsletter-
www.essnmag.com

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