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two views on sunspace design
2 apr 1996
   it is hard to think of any other system that supplies so much heat
   (to an existing house) at such low cost...

   one could shorten the warm-up time of the enclosure and increase
   the amount of heat delivered to the rooms by making the enclosure
   virtually massless--by greatly reducing its dynamic thermal capacity.
   this can be done by spreading a 2-inch-thick layer of lightweight
   insulation on the floor and north wall of the enclosure and then
   installing a thin black sheet over the insulation. then, practically
   no heat is delivered to the massive components of floor or wall;
   practically all of the heat is promptly transferred to the air.
   and since the thermal capacity of the 100 or 200 lb. of air in
   the room is equal to that of one fourth as great a mass of water
   (about 25 to 50 lb. of water), the air will heat up very rapidly.
   i estimate that its temperature will rise about 40 f. degrees in about
   two minutes, after the sun comes out from behind a heavy cloud cover.
   at the end of the day, little heat will be "left on base" in the 
   collector floor or north wall and, accordingly, the enclosure will
   cool off very rapidly.

     new inventions in low cost solar heating--
     100 daring schemes tried and untried
     by william a. shurcliff, phd
     brick house publishing, 1979, 293 pages, $12

   a sunspace has extensive south-facing glass, so sufficient thermal mass
   is important. without it, the sunspace is liable to be uncomfortably hot
   during the day, and too cold for plants or people at night.

   however, the temperature in the sunspace can vary more than in the
   house itself, so about three square feet of four inch thick thermal
   mass for each square foot of sunspace glazing should be adequate...

   the sunspace floor is a good location for thermal mass. the mass floors
   should be dark in color. no more than 15-25% of the floor slab should be
   covered with rugs or plants... another good location for thermal mass
   is the common wall (the wall separating the sunspace from the rest of
   the house)... water in various types of containers is another form of
   energy storage often used in sunspaces.

	     passive solar design guidelines--
	     guidelines for homebuilders
	     for philadelphia, pennsylvania
	     passive solar industries council
	     national renewable energy laboratory
	     charles eley associates 
	     current edition, 88 pages, $50 

so, which is the most energy-efficient sunspace in a partly cloudy climate
like philadelphia?

shurcliff's plastic film sunspace, wearing the green uniform in this contest,
might cost about $2/ft^2 and begin an average december day at 36 f, and like
the psic sunspace, it would receive about 1000 btu/ft^2/day over the average
day. let's assume that both sunspaces have a perfectly insulated wall between
them and the house, to avoid the thermal disaster of a poorly insulated trombe
wall in a partly cloudy climate. (trombe walls can't even get to the starting
line in this race.) and that there is no air infiltration from the outside in
either case. shurcliff's sunspace air would be circulated through the house
with some dampers or fans, keeping the sunspace at 80 f, say, while the house
remains at 70 f. with single glazing, about 900 btu/ft^2 of sun might enter
the sunspace during the day, and the amount of heat lost through a square foot
of shurcliff's sunspace over a typical 6 hour december day would be about
6 hr (80-36)/r1 = 264 btu/ft^2/day, for a net gain of 636 btu/ft^2, ie his
$2/ft^2 sunspace would be about 64% efficient, as a solar collector. as
an auxiliary living space, it could be heated up instantly on some starry
night for a party, by moving some warm air from the house into the sunspace.

a psic sunspace, wearing the brown uniform, would perform better with double
glazing. he might cost $10/ft^2. say his thermal mass is poured concrete,
4" thick, with an official psic heat capacity of 8.8 btu/ft^2, and say it
absorbs 100% of the sun that falls on it, vs the official psic solar
absorptance of bare concrete of 0.65 (table k, page 57.) then about 800
btu/ft^2/day of sun will enter the double glazing and be absorbed by the
concrete, and the concrete surface will warm up the sunspace air, and that
warm air can be used to heat the house when the sunspace temperature is more
than 80 f. suppose the concrete loses no heat at all to the soil below (i'm
giving quite a few handicaps to the psic sunspace in this efficiency race.)
suppose the psic sunspace starts out the day at temperature t, and the
concrete charges up in the sun to a maximum temperature of t + dt, and
returns to temperature t at dawn, on an average day in december. how can we
calculate t and dt? we have an equivalent electrical circuit that looks
something like this:

                             ts sunspace temperature
                             |
                      r2     |            s 
       36 f ---------wwww-----------------|-------------------- 70 f
       outdoors      glazing |            open switch to heat   house 
                             |
                             w
                             w  r0.5 concrete - sunspace air resistance 
               800 btu/ft^2  w
                   per day   |
            |      ---       |
          | | ----|-->|------|--tc concrete temperature
            |      ---       |
               sun current   w
                   source    w  r0.4 concrete bulk thermal resistance
                             w
                             |
                          ------- 8.8 btu/f thermal mass of concrete
			  -------           
                             |
                             |
                            ---
                             -

let's simplify this, by assuming the thermal mass of the concrete is infinite,
vs 8.8 btu/ft^2/f. lots of concrete, or a water wall, or something that has
so much thermal mass that the temperature inside the sunspace never changes
at all from day to night or day to day over a long string of average december
days, with some sun. this is an optimal sunspace with more than "adequate" or
"sufficient" thermal mass by official psic standard guidelines. let's also
assume that the two small resistors have a value of zero, ie let's assume the
the r0.4 bulk thermal resistance of the concrete, that makes the surface heat
up more than the inside, while the sun is warming it up, and makes it harder
to get heat out of the inside of the concrete and into the sunspace air, and
the r0.5 concrete-sunspace air resistance, are both r0 conductors. what will
tc be in that simplified case?

the sun shines into the sunspace during the day and adds 800 btu to our
concrete capacitor, and over 24 hours, 24(tc-36)1ft^2/r2 = 12 tc - 432 btu
flow out of the capacitor. if ein = eout (providing no heat for the attached
house), then tc = (800+432)/12 = 103 f. pretty nice, but this sunspace is
not providing any heat for the house, just keeping itself warm on an average
day, and losing lots of heat on a cloudy day. suppose we allow some heat to
flow from the sunspace into the house, ie close the switch, ie turn on the
fan or open the damper between the sunspace and the house often enough to
limit the maximum sunspace temp to 80 f instead of 103 f. then the heat loss
to the outside world over the course of a day is 24(80-36)1 ft^2/r2 = 528 btu,
and the rest of the heat that enters the double glazing, ie 800 - 528 = 272
btu/ft^2/day goes into heating the house, so the solar collection efficiency
of this $10/ft^2 sunspace in terms of useful heat provided for the attached
house is 27%. as an auxiliary living space, the temperature of this sunspace
is largely out of our control. it takes a long time and a lot of house heat to
warm it up on an evening or cloudy day, and after we leave the space, it stays
warm for a long time, giving up precious house heat to the outside world.

how curious that by carefully following the current official guidelines of
the passive solar industries council, we can reduce the performance of
shurcliff's low-thermal-mass sunspace from 64% to 27%, while increasing the
price from $2/ft^2 to $10/ft^2, unimproving the cost-effectiveness of the
sunspace by a factor of 12, even with all these psic-slanted assumptions... 

here's a quote from the acknowlegements section of the psic guidelines:  

   _passive solar design strategies: guidelines for home builders_ represents
   over three years of effort by a unique group of organizations and
   individuals. the challenge of creating an effective design tool that
   could be customized for the specific needs of builders in cities and towns
   all over the u. s. called for the talents and experience of specialists
   in many different areas of expertise.

   _passive solar design strategies_ is based on research sponsored by the
   united states department of energy (doe) solar buildings program, and
   carried out by the los alamos national laboratory, the national renewable
   energy laboratory (nrel)... and the florida solar energy center (fsec.) 

   the national association of home builders (nahb) standing committee on
   energy has provided invaluable advice and assistance during the
   development of the guidelines.

   valuable information was drawn from the 14 country international energy
   agency (iea) solar heating and cooling program, task vii on passive and
   hybrid solar low energy buildings...

   although all the members of psic, especially the technical committee,
   contributed to the financial and technical support of the guidelines,
   several contributed far beyond the call of duty. stephen szoke, director
   of national accounts, national concrete masonry association, chairman of
   psic's board of directors during the development of the guildlines; and
   james tann, brick institute of america, region 4, chairman of psic's
   technical committee during the development of these guidelines... 
   gave unstintingly of their time, their expertise, and their enthusiasm. 

nick




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