sunspace thermal mass vs. solar collection efficiency SOLAR.KnowledgePublications.com

sunspace thermal mass vs. solar collection efficiency
30 may 1997
   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... 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, physics
     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 
     1995 edition, 88 pages, $50 
 
how does a sunspace's solar collection efficiency depend on its thermal mass
on a typical january day in philadelphia?

10 ag=1'sunspace glazing area (ft^2)
20 rvg=1'r-value of sunspace glazing (ft^2-f-h/btu)
30 tau=.9'solar glazing transmittance
40 vertsun=1000'average daily sun on vertical wall (btu/ft^2-day)
50 sunin=ag*tau*vertsun'daily solar input (btu/day)
60 ta=30'ambient temp (f)
70 rg=rvg/ag'thermal resistance of solar glazing (btu/h-f)
80 data 0.5,1,2,6.75,9,27,73,110
90 for i=1 to 8'read thermal capacitance (btu/f)
100 read c
110 tmin=ta+(70-ta)*exp(-18/c)'mass temperature at dawn (f)
120 tm=tmin'initialize mass temp (f)
130 eout=0'initialize useful house heating output
140 dt=1/6'time step for simulation (h)
150 for h = 0 to 23 step dt'simulate for a day
160 if h < 6 then ein=sunin*dt/6 else ein = 0'solar heat gain
170 ein=ein-(tm-ta)/rg*dt'subtract energy lost through glazing
180 tm=tm+ein/c'adjust thermal mass temp
190 if tm>70 then eout=eout+(tm-70)*c:tm=70'remove useful house heat 
200 next h
210 eff=int(100*tau*eout/(sunin)+.5)'daily solar collection efficiency
220 print c,tmin,eff
230 next i

thermal       sunspace       % solar 
capacity      temperature    collection  typical 
(btu/f-ft^2)  at dawn (f)    efficiency  construction

 0.5           30            66          1/2" drywall/ft^2 glazing
 1             30            65          1" drywall/ft^2 glazing
 2             30.00494      61
 6.75          32.77934      46          1 ft^2 of 3" concrete
 9             35.41341      41          1 ft^2 of 4" concrete
 27            50.53669      20          3 ft^2 of 4" concrete (psic)
 73            61.2589       7           vertical 55 gallon drum behind glass
 110           63.96203      3           horizontal 55 gallon drum   "    "
   
a quote from the acknowlegements section of the psic guidelines:  
 
   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