[Greenbuilding] Power Vent for Solar Air Heater (conservation architect)
Gary Reysa
gary at builditsolar.com
Thu Oct 3 14:34:23 CDT 2013
Hi,
You might take a look at this collector:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
One of the things I like about it is that instead of seeing a big black
square where the collector is, you see a muted version of the siding behind
the collector.
The collector uses two layers of charcoal colored aluminum window insect
screen for the absorber. Its a flow through absorber with the inlet air
introduced on the glazing side of the screen. The room air flows through
the screen to get to the exit and picks up heat from the solar heated
screen.
We did some testing of various types solar air heating collectors, and this
very simple screen absorber collector was the most efficient of the bunch.
http://www.builditsolar.com/Experimental/AirColTesting/Index.htm
One simple way to measure how much of an impact tree shading will have on
solar thermal collection:
http://www.builditsolar.com/Experimental/TreeShade/TreeShade.htm
Of course, you have to wait for winter to do the test, which may be too
late for you.
Gary
On Thu, Oct 3, 2013 at 12:00 PM, <
greenbuilding-request at lists.bioenergylists.org> wrote:
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> 1. Power Vent for Solar Air Heater (conservation architect)
>
>
> ---------- Forwarded message ----------
> From: "conservation architect" <elitalking at rockbridge.net>
> To: <greenbuilding at lists.bioenergylists.org>
> Cc:
> Date: Thu, 3 Oct 2013 11:51:34 -0400
> Subject: [Greenbuilding] Power Vent for Solar Air Heater
> Laren
>
> Thanks for your substantive message on solar air heaters. I am backing
> off on the project because of the vulnerability of shading from trunks and
> tree branches. I do believe solar air heaters on houses with adequate
> sunshine is the low hanging fruit of renewable energy where the building is
> in full sunshine. I am going to save the information you have provided to
> apply to other projects for my clients. I think the using a clear glass
> which is permanent can allow the textures of the house surface to show,
> preserving conventional architectural style.
>
> My South wall has been retrofitted with 3” iso over the existing frame
> wall with 3/4” plywood furring on the exterior side with textured plywood
> and foe battens over that. Likely the space behind is fairly tight.
> However, no sealing effort has been done exterior to rigid insulation. If
> I was to develop into solar air collector, I would create some vent holes
> through the plywood that would allow greater convection and
> thermo-syphoning. If done so, would not the dark stained plywood provide
> an adequate heat absorption surface?
>
> Eli
>
> [Greenbuilding] Power Vent for Solar Air Heater
>
> LarenCorie LarenCorie at axilar.net
> Tue Oct 1 09:13:55 MDT 2013
>
> Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
>
> ------------------------------**------------------------------**
> --------------------
>
> 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
>
>
>
> Home base-LittleHouses YahooGroup
>
> http://groups.yahoo.com/group/**LittleHouses/<http://groups.yahoo.com/group/LittleHouses/>
>
>
>
> Founder-WoodGas - Power from wood
>
> http://groups.yahoo.com/group/**WoodGas<http://groups.yahoo.com/group/WoodGas>
>
>
>
> Founder-**RefrigeratorAlternatives YahooGroup
>
> http://groups.yahoo.com/group/**RefrigeratorAlternatives<http://groups.yahoo.com/group/RefrigeratorAlternatives>
>
>
>
>
>
>
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