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Sunspace efficiency

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Posted by Gary on September 8, 2004, 1:33 am
 
Hi,

I have about 22 lineal feet of south facing wall that I plan to either
use for a solar air collector, or for a sunspace.  The main objective
of either scheme would be to provide heat for the house  The sunspace
would have some additional benefits like a seating area and/or season
extender for plants.

My question is how would a dedicated air collector's efficiency
compare with a sunspace in terms of providing heat to the house?

The sunspace would be about 22 ft wide, about 8 ft deep, and would
have 7 to 8 ft tall vertical glazing facing nearly south.  The
dedicated collector would be similar, except that the depth would be 4
to 8 inches rather than 8 ft.
Air heated by the sunspace would be picked up by a duct along the
sunspace ceiling that would use a duct fan to distribute air to the
main living area of the house (which is a ways from the sunspace).
The dedicated collector would use low velocity forced air circulation
with plenum at the top and bottom, and a duct with fan to push heated
air to the living space.
Either one might also direct heated air to thermal storage in the
crawl space just behind the sunspace wall.
I am located in SW Montana (lat 46).

My initial thought was that the dedicated air collector would be
significantly more efficient, but on trying to quantify this, I'm not
so sure -- the factors that I looked at in comparing the two are
listed below -- am I missing something?
Would there be any significant difference in efficiency between the
two schemes?

Collecting sun:
Both schemes would have the same glazed area, and same type of
glazing, so there is no difference here.  About 80 to 90% of the sun
gets through the glazing either way.
No advantage to either scheme?

Absorbing sun:
The dedicated collector would have a metal screen, or expanded lath,
or some such dark material to absorb the sun, and would probably
absorb about 95% of the incident sunlight.
The sunspace could have dark back wall (which would be the house wall)
to absorb sun.  Or, it could have a vertical dark shade cloth screen
(say) a foot behind the glazing.  It seems like this should also
absorb about 95% of the radiation.
No advantage to either scheme?

Losses out the glazing:
For a given collector temperature,
it seems like the sunspace might be better here, in that the air
velocity next to the glazing would be less, and so convection losses
would be less.
Radiation losses should be  similar?
The sunspace might have the advantage that I could add a layer of film
glazing inside the glass during the winter season(e.g. greenhouse poly).
Slight advantage to sunspace?
(but see next item)

Transferring heat from absorber to air:
It seems like the dedicated collector could be more efficient here in
that the air velocity next to the absorber would be greater, and
transfer more heat for a given absorber temperature. So, for the same
heat transfer the dedicated collector absorber temperature would be
lower, and this would tend to reduce losses.
Not sure how much this is worth?
Some advantage to dedicated collector?

Sidewall and roof losses:
It seems like the dedicated collector would have an advantage here in
that the area of the sidewalls and "roof" are much smaller.
But, with good insulation, it seems like this loss could be made quite
small compared to the glazing losses?
For example, if wall + ceiling area = 1.8 glazed area, and Rglaze = 1,
and Rceiling&wall = 22, then the ceiling + wall loss is (1.8/22) = 8%
of glazing loss.
Slight advantage to dedicated collector?

Delivering heat to house:
I don't see why either one would have any advantage here?


All in all, I'm not seeing any reason to think that one might do a
significantly better job than the other in producing heat for the
house?  Anyone have any ideas on this?
Has anyone seen test or analysis on this?

Thanks Gary


---
I should mention that the sunspace is phase 3 of a 3 or 4 phase
program to try to cut my heat bills in half for THIS winter:

Phase 1 is reducing house heat loss by cutting infiltration and adding
insulation -- this is coming along OK -- I've found some pretty
amazing gaps in my furnace  ducts that run through unconditioned areas
-- I have gone through a  gallon and a half of duct mastic :-)  Also
sealing wiring other attic penetrations, sealing can lights, ...  I
plan to add insulation in attic over the current loose fill.  Also
plan to add greenhouse bubble sheet insulation over some of the larger
windows.

Phase 2 is to build the "solar garage doors" that have been discussed
here before.  This adds a set of glazed doors just outside of my
regular garage door, so that by rolling up the regular door I can
effectively use the garage as a sunspace to collect solar energy, and
then distribute it to the house.  This is coming along OK -- I have
all the materials in hand, and one of the 4 doors completed just to
see how well it all fits and looks.  This will add about 130 ft^2 of
glazed collector space.

Phase 3 is the sunspace and related storage.

Phase 4 (next year?) would be to use the south facing roof for more
collector area if needed.

Posted by nicksanspam on September 8, 2004, 8:56 am
 


How about a 22' quarter-cylinder with an 8' radius? This would collect
overhead as well as south sun. A sunspace with an insulated ceiling
could store heat in the ceiling...


With higher air velocity near the glazing?


I like the latter.


Make it transparent? :-)

Nick


Posted by Gary on September 8, 2004, 10:09 pm
 nicksanspam@ece.villanova.edu wrote:

The storage in the ceiling sounds interesting, but how do you keep the
heat from getting lost rapidly out the front of GH as soon as the sun
goes down?
I remember that Steve Baer showing an experimental solar shed with
ceiling louvers -- is that what you have in mind?


I guess it would be better to use the style in which air circulates
only behind the solid absorber sheet, and at fairly high velocity?

We have come around to the position that we would rather build a
sunspace than a dedicated collector because of the side benefits of
the sunspace.  We would be willing to take some hit in performance
compared to a dedicated collector, but not a large hit -- any thoughts
on this?
In mid-winter, the sunspace would be operated strictly as collector
for the house (i.e. no heat storage in sunspace, and no attempt to
keep the sunspace warm enough for plants at night).


Because it keeps the floor from absorbing sun and storing heat?
or?


It could be the 2nd floor sunspace :-)

Gary


Posted by nicksanspam on September 9, 2004, 12:14 am
 

Overhead insulation and an insulated lip in front that extends below
the ceiling store.


It makes the space more usable for humans and allows storing ceiling heat.

Nick


Posted by Anthony Matonak on September 8, 2004, 3:50 pm
 Gary wrote:
...

I'm no expert in these things but you could try some calculations or
simulations and figure out how they perform. The dedicated air heater
could be angled for the winter sun and will have less thermal mass.
This means it'll heat up faster in the morning and it won't absorb
heat that will later be released at night. I would think that this
would make it more efficient at heating the house. The sunspace may
be less efficient but it would have other benefits which you may feel
outweigh this inefficiency.

My suggestion, cover the roof (and some of the roof of the sunspace)
with dedicated air heaters and optimize the sunspace for the other
things.

Anthony

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