Combination Thermal Shade and Collector

Updated 1/17/06 -- see 2nd test below -- much better performance.

Any comments or suggestions on this scheme would be appreciated.

This concept comes out of a news group discussion on insulating window shades. It was pointed out that using insulating shades on windows might damage the window by overheating it if the shade were left down during intense sun. Nick Pine suggested that if the heat behind the insulating shade could be collected it 1) would not damage the window, and 2) would help heat the house.

This would be one simple way to collect the heat behind the insulating shade. In this scheme the window shade acts like a thermosyphon solar air collector when the sun is shining on the window. When the sun is not shining (e.g. nighttime) the shade automatically goes back to just insulating.

I decided to try this on our 5 ft by 5 ft south facing bathroom window, which seemed like a good candidate, since the shade stays down all the time. Its not such a good choice in that its very light colored (reflective), and it has huge clearances on the edges -- but, this could be fixed with a new shade.

The following changes were made to convert the shade into a thermosyphon collector: -- see picture below

The top of the shade is lowered a couple inches so that a slot is formed between the top support of the shade, and the top of the window frame. This slot allows air heated behind the window to flow into the room.
A spacer is added at the bottom of the window frame on the lower sill. This space is about 3 inches high, and is mostly open. The bottom bar of the window shade rests on top of the spacer when the shade is closed. The openings in the spacer allow cool air to flow into the space behind the window shade, where it is heated by the sun, and rises out of the top slot.
Plastic film back-draft dampers are installed in the openings in the lower spacer (2) that act like a check valve to allow air to flow into the space behind the shade when the sun is shining, but prevent flow out of the bottom slot as would happen at night when the air cooled by the window glass would sink and flow out the openings in the spacer.

So, when the sun is out, sun shines on the back of the shade and heats it, this heats the air between the window and the shade. The heated air rises out the top slot, which pulls cool air in the bottom slot. At night, the air near the window cools and would like to exit out the bottom slot, but the plastic film back-draft dampers prevent this -- or so the theory goes.

These are the nominal pros and cons as I see them now:

Pros:

The shade can act as both a collector (day) and an insulating shade (night) with no intervention.
The temperature in the cavity between the shade and window is reduced, which may protect the window from overheating damage?
Not too ugly or odd?
Simple to build.
Allows solar collection during those times when the shade is down for reasons of privacy, glare, laziness, or vacation.
For rooms that would tend to overheat from solar gain if the shade were left open all day, thermal mass (water tubes?) could be placed just above the hot air vent at the top of the shade to store heat during the day and release it later.

Con:

You would probably get better collection efficiency by just opening the shade during the day -- so this thing only has advantage as a collector if you know the shade is going to be down a substantial part of the day (for either glare reduction, privacy, or other reasons)
Does not look as nice as the shade alone.

Geometry:

Window: Width = 58 inches by Height = 52 inches; Gross Area = 20.9 ft^2

Upper slot = 2 inches high by 51.5 inches wide = 103 in^2 = 0.715 ft^2

Lower slot = 2.6 inches high by 55.5 inches wide = 144.3 in^2 = 1 ft^2

Depth behind shade to glazing aprox 3 inches.

Comments:

An even simpler version would just have the slot along the top. During the day you could open the shade a couple inches, and it would make a thermosyphon collector, and at night you could close the shade and it would insulate. This would be simpler and probably work better, but does require that you take some action twice a day.

Open Issues:

Does it collect enough to be worth the bother? <-- probably yes -- see below

Do the two slots degrade the insulating performance
enough to make it not worth it, or even a net negative? <-- I don't think so

Do the back-draft dampers really work well enough? <-- they appear to work pretty well

How much better would it work with: <-- much better -- see below

- Tighter fit on the edges (the prototype is awful)

- Darker color on back of shade

Initial Performance: ( see 1/17/06 update below for revised performance)

This is just a first try, and its a bit surprising that it does do something even with the light colored shade and the poor edge fit.

The sun was in and out during the day (as indicated by the deep cuts in the temperature and sun intensity curves).

From top to bottom the temperatures are:

red -- temperature in the collector cavity with sensor exposed to direct sun

green -- temperature at the outlet vent on top

blue -- temperature at the inlet vent (room temp)

blue -- outdoor ambient temperature

During the times when the sun was out, the exit air velocity was about 40 fpm. This was only enough to slightly open the back draft dampers -- I suspect that a lot of air was actually entering along the edges.

So, in its current state, it achieves about a 17F temperature rise, and 40 fpm exit velocity. My workshop thermosyphon collector during the same period was doing a 40+F temperature rise, and 110 fpm.

window shade collector thermal performance

This is the sun intensity over the same time period.

Window shade collector sun intensity

I think that with a better fit on the edges and a darker color on the back of the shade , that the collection efficiency could be improved by a lot.

One way to get the better edge fit would be to use the "energy tracks" from Symphony shades.

Symphony shades energy track for insulating shades

The edge track is installed on the vertical edges of the window frame. The edge track engages a slot or groove in the edges of the pleated shades, so that the air has a harder time leaking between the edges of the shade and the window frame.

Better pic here: http://symphonyshades.com/comfortracks.html

We have used these on some of our windows for several years and like them.

1/17/06 Performance Update With Improvements: (much better)

I had another go at measuring the performance after making these changes:

To darken the absorber surface, I taped a sheet of black "weed" cloth to the back side of the shade. Weed cloth is a landscaping material that you put down on the ground to prevent weeds from growing. Its about like a fine mesh shade cloth -- its black, but you can see quite a bit of light through it.
I'm not particularly recommended weed cloth -- I just had it on hand -- I think it smells a bit when heated.
Closed up the gaps along the edges of the shade with spacers. They are still not very tight, but much better than they were.

With the weed cloth in place, not as much light comes through the window to light the room, but its still enough to make the room pretty bright when sun is on the window.

The performance with the changes is much better:

The temperature rise from the lower vent to the upper vent went from 17F to 37F
The exit velocity went from 40 fpm to 80 fpm.

About a factor of 4 improvement! The logger temperature and sun plots are below.

Day Output:

The output for the day was about:

Qout for day = (106 BTU/hr-ft^2)(6 hrs/day)(18.7 ft^2) = 12,000 BTU

or, about (12000 BTU) ($2/gal)/((92000BTU/gal)(0.7 efic) )= $0.37 per day, maybe $40 a heating season

The 106 BTU output per hour per sqft is from the calculation just below.

Compare the Collector/Shade to a Known Good Air Collector:

Output per square foot:

Qshade = (Tout - Tinlet)(AventOut)(VventOut)(density)(SpecificHeat of Air)/(Acollector)

= (37F)(0.715ft^2)(80ft/min)(60 min/hr)(0.065lb/ft^3)(0.24 BTU/lb-F) /(20.9ft^2) = 95 BTU/hr-ft^2

Output of workshop collector under same conditions:

Qws = (56F)(140fpm)(0.5ft^2)(60 min/hr)(0.065 lb/ft^3) (0.24 BTU/lb-F)/(16 ft^2) = 229 BTU/hr-ft^2

So, its about half as efficient as the my workshop collector, which I believe to be a good one -- not too bad?

This may be unfair to the window collector, in that the shop collector has a taller (7 ft ) thermal stack driving the flow.

Compare the Collector/Shade to a Direct Gain Window:

How does the output of the Collector/Shade compare to just pulling the shade up during the times when the sun is shining and letting the sun shine into the room where it is absorbed by room surfaces?

In rough terms: Assume that the sun power input is 250 BTU/hr-ft^2, and 80% transmittance through the glazing, and that the double pane window glass is R2, then the net energy collected if the shade is up is:

Qwindow = (solar gain) - (window heat loss)

Qwindow = (250 BTU/hr-ft^2)(0.8 trans) - (1 ft^2)(70F - 33F) / (R2) =

= 200 BTU/ft^2 - 54 BTU/ft^2 = 146 BTU/ft^2

So, maybe the collector/shade does not do as good a job as just opening the shade to let the sun in. Not too surprising. But, there are windows and times that you don't want the shade open, and the collector shade still does a pretty good job of collecting energy then.

All of these calculations have large tolerances associated with them -- they are just intended to give a rough idea what's going on, and as a guide for further changes.

Collector temperatures with improvements -- full sun

(note that the plot title is wrong -- left over from an old plot -- the data is OK)

Sun intensities to go with temp chart above

During times of full sun, the vent velocity was 80 ft/min (as measured by the Kestrel wind meter and the Dwyer Vane Velocity meter).

Potential Further Improvements:

More changes might be made to improve the collector/shade:

The gaps along the edges of the shade could be closed up tighter.
A shade with "Energy Tracks" could be used to seal the edges better (see Energy Tack above)
The back draft damper geometry is not ideal -- this might be improved (see below)
A flow divider configuration (as proposed by Pine/Delaney) might improve performance.
??

The back draft dampers (in the bottom slot) swing toward the window, and tend to obstruct part of the flow channel between the shade and the window. This restricts the flow to some degree and is not desirable. Maybe there is a better back draft damper configuration?

Construction Pictures:

The construction is very easy -- took a little over an hour.

Construction of the insulating shade and solar collector

The lower spacer, and applying the back-draft damper plastic to the spacer (Costco garbage bags).

The plastic is just stapled to the spacer. I stapled it down first, and then trimmed it to size with a razor knife.

Construction of the insulating shade and solar collector

Upper spacer to create vent slot along top of shade. Lower spacer and back-draft dampers in position.

The shade snaps into the metal clip.

Each upper spacer has a single drywall screw into the window frame. The screw is counter sunk 3/4 of the way into the spacer.

Instrumentation:

Dwyer $25 air velocity meter

Onset Computer data logger temperaturr probes

The $25 Dwyer air velocity meter. Onset logger temperature probe.

I really like this little meter.

Onset Computer pendant light/sun intensity logger

The new Onset Computer pendant sun intensity logger.

Logs sun intensity and temperature for days -- waterproof -- $60.

Not intended for precise measurements, but gives a reasonable idea of sun intensity for not much money or hassle.

The Onset 4 channel logger (the little white box), and readout from same.

Gary 1/13/06, 1/17/06