some sunspace heatflow basics SOLAR.KnowledgePublications.com

some sunspace heatflow basics
19 nov 1996
meanwhile, in connecticut...

>>>what is a lean-to sunspace?              .
>>                                      .        .
>>a simple glazed wall over the     .                .
>>  south side of an existing     g  .    house     .
>>  house, like this:            g   .              .
>>                              g ss .              .
>>                    ------------------------------------

>to contain heat, the lean-to sunspace would need side walls...

yes, but if it's only warm during the day, the sidewalls don't have to
have much insulation. they could be a single layer of polyethylene film,
like the south wall.

>would this look a bit like 1/2 an a-frame...

sure.

>in other words, like a pup tent...

the added-on part would look like half a pup tent, sliced lengthwise through
the top, perhaps between lengthwise cub scouts, by a man with only 3 teeth and
a hideous laugh and a large chainsaw, by the light of a silvery full moon.

>>>i was thinking about glassing in my porch and using it like
>>>a big solar heater.

>>sounds good, with no little thermal mass inside it, so it can get cold at
>>night. and plastic is cheaper and much easier to install than glass...

>why not put a thermal mass in it?

you can get more solar heat out of it for the house if you don't.

>wouldn't that mass absorb heat during the day, and release it slowly
>over the night? 

yes, but most of that stored heat would go back outside through the low
thermal resistance of the solar glazing at night. better to keep it inside
the house at night, behind an insulated wall.

>are you telling me that it is better to transfer the heat into the house
>while the sun is out and seal off the sunroom during the evening?

yes. let it get cold at night. keep the thermal mass in the house.

>if that is the case, i would be well served to put a pair of fans, one
>pushing and one drawing when i get home from work to 'harvest' the hot air?

you would be better served to do this during the day while the sun is shining,
since this sunspace does not store heat, so it gets cold quickly at night.
i'd use 2 thermostats in series with the fans, a heating thermostat in the
house and a cooling thermostat in the sunspace, to turn on the fans when the
following things are both true: the sunspace is warmer than, say 90 f, and
the house is cooler than say, 75 f. a couple of grainger's dpst $16.55 5e266
thermostats might do this. 

and one fan is probably enough, a $12 20" window fan in an opening at the top,
with a dry cleaner bag over the inside, hinged at the top, to keep house air
from flowing out into the sunspace at night. you could have an open window near
the bottom of the other side of the sunspace with a similar one-way damper
that only allowed house air to flow into the sunspace.

larger or more fans raise the sunspace solar collection efficiency: moving
1 cfm of air with a temperature difference of 1 f transfers about 1 btu/hr
of heat. full sun is about 300 btu/hr-ft^2 (or about 400 btu/hr-ft^2 with
some sort of ground reflector to the south.) a sunspace with 100 ft^2 of r1
glazing in full sun receives about 300x100 = 30,000 (30k) btu/hr of peak heat.
or the net heat equivalent of about 1-2 gallons of oil per square foot of
vertical south glazing per winter. 

looking at fan size extremes... 

if the sunspace were completely sealed off, with no fan, it would heat up
until the thermal loss through the glazing equals the solar input. suppose 
it's 37.5 f outdoors (the average daily max temp in hartford, ct in december,
where an average 820 btu/ft^2/day fall on a south wall) and house wall behind
the sunspace and the endwalls of the sunspace are perfectly insulating. then 
the sunspace temp t adjusts itself until 30,000 btu/hr = (t-37.5)100ft^2/r1,
or t = 37.5 + 30000/100 = 337.5 f, theoretically. radiation losses limit t to
something like 130 f, but linear arithmetic works well at low temperatures.
the solar collection efficiency of the sunspace, defined as the heat provided
to the house divided by the solar input, is zero, because the sunspace is hot,
but it doesn't heat the attached house at all.

if the fan were huge, and it moved 68 f house air at a very large rate into
the sunspace, the sunspace would be very close to 68 f, and air would return
to the house at a temperature very close to 68 f. the thermal loss through the
glazing would be (68-37.5)100ft^2/r1 = 3050 btu/hr, and the house would gather
30,000 - 3050 = 26950 btu/hr of heat (about 8 kw) and the sunspace collection
would be 26950/30,000 = 90%, but the huge fan would make the house drafty
and consume a lot of electrical power.

if a typical window fan moved 1000 cfm of 68 f house air into the sunspace
with 100 ft^2 of r1 solar glazing, in full sun, and all the sun went into
heating the air, the air would return to the house at 68 + 30000/1000 = 98 f.
but some of that solar heat is lost through the glazing: if the sunspace has
temperature t, the glazing loss is (t-37.5)100ft^2/r1, and the fan carries
away (t-68)q of heat for the house, so if the solar energy that flows into
the sunspace equals the solar energy that flows out of the sunspace with no
heat storage in the sunspace), 30,000 btu = (t-37.5)100ft^2/r1 + (t-68)q.

if q = 1000 cfm, 30000 = 100t - 3750 + 1000t - 68000, so 101750 = 1100t,
or t = 92.5 f, and the glazing loss is (92.5-37.5)100 = 5500 btu/hr,
and the house gets the rest of the 30k, ie 30k - 5500 = 24500 btu/hr,
and the solar collection efficiency of the sunspace is 24500/30k = 82%. 
not bad, for a solar collector. one reason this is so good is that the
back of the collector is the house wall, not outdoor air. and it's nice
to be able to put a chair in your solar collector and read a book on a 
sunny winter day.

here's a more general formula:

   t = (i+taag/rg+68q)/(ag/rg+q) where
	i is the total solar input in btu/hr, eg 300 ag, perhaps reduced
	by a sunspace glazing transmission of less than 100%,
	  ta is the ambient temperature, eg 37.5 f in hartford
	    ag is the sunspace glazing area, eg 100 ft^2
	       rg is the sunspace glazing r-value, eg r1, and
		    q is the fan cfm, eg 1000 cfm.

the sunspace solar collection efficiency is approximately (i-(t-ta)ag/rg)/i.

this can be improved by adding a mesh absorber near the back wall of the
sunspace, eg some greenhouse shadecloth, which would allow the sunspace to
fill with 68 f house air, which would the moves sideways through the absorber,
horizontally from south to north, before being sucked back into the house
by a window fan behind the absorber near the top of the sunspace. 

this can also work well by natural convection, eg with a 2 story sunspace
and a motorized damper in series with 2 thermostats...  and you might put
some bare water heating panels or pvs inside the sunspace, too.

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email: nick@ece.vill.edu

computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. solar closet paper: http://leia.ursinus.edu/~physics/solar.html
web site: http://www.ece.vill.edu/~nick