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re: storing solar heat
14 feb 1997
greg burgin  wrote:

>nick pine  wrote: 

>> i like the idea of using sealed containers of water inside a small
>>insulated room... so sun-warmed air can heat the water... an 8x8x8'
>>room might contain 32 plastic 55 gallon drums full of water... when
>>the house needs heat, open a damper or turn on a small fan to this
>>solar closet to allow cooler house air to flow in at the bottom and
>>warmer closet air to flow back into the house from the top...  
>i find your idea of a such a room intriguing since i'm already considering
>such a room for a hydronic system to supplement a passive solar design. it
>sounds cheaper but i have a couple questions: can you give me some idea of
>the best way to use heated air to get heat to the "closet" without putting
>a lot of weight in the attic?

you might put this room on the ground, especially if you live in a place with
earthquakes. it might live in an inexpensive sunspace, or it might be part of
the house with its insulated and glazed south wall as a part of the insulated
wall between the house and the sunspace, looking like this from above:   

                 |                    |
                 |                    |
                 |                    |
                 |                    |
sun              |                    |
                 |    living space    |     this has the advantage that
                 |                    | 40' the solar closet can run a bit 
                 |                    |     warmer, since it lives in a 
         --------|                    |     warmer climate, and the losses
        |low-    |   8'               |     from most of its walls can
    12' |thermal |--------            |     provide a little heat for 
        |mass    | solar  |           |     the house.
        |sunspace| closet | 8'        |
        |        |        |           |
         -------- --------------------

>wouldn't it take a lot of hot air and collector area to heat the 2000
>gallons of water?

initially, yes, but the plan is that the sunspace provides all the heat the
house needs on an average january day, with some sun, by design, with the
closet only used for cloudy day house heat, as a sort of trickle-charged
heat battery, with only occasional cloudy-day loads. since the closet is
in the sunspace, closet losses during solar collection go into sunspace air
which heats the house on an average day.

>...1 btu per 1 lb water for 1 degree change in temp

that's how much heat is stored in the water...

>but air is a less efficient way to transfer heat.

true, but leaks and freezing and so on are less of a problem... 1 cfm of air
with a 1 f temperature difference transfers about 1 btu/hour of heat, and
1 m^3/sec of air with a temperature difference of 1 k transfers about 1 kw.

if the closet is fairly airtight and well-insulated and seldom used, it won't 
need much warm air to keep it charged up on an average day... the warm air
comes from the sun that shines through the solar closet glazing, btw, ie the
closet has its own separate glazing and low-thermal-mass air heater, inside
the sunspace. it's good to have at least 10 times more surface area for the
containers of water (eg about 25 ft^2 per 55 gallon drum) than the closet
glazing area, so full sun that falls on the closet glazing (~300 btu/h-ft^2
or 1 kw/m^2) can heat the water via the slowly moving (us r2/3 f-ft^2-h/btu,
metric 0.1 k-m^2/w) air film near the container surface with a temperature
difference of at most 300x2/3/10 = 20 f. a small fan can help lower the
air-water temperature difference by increasing the thermal conductivity of a
smooth container surface from about 1.5 btu/hr-f-ft^2 to 1.5 + v/5, where v
is in mph. a rougher surface like stucco works better, increasing as 2 + v/2.

>can such a room this size heat a 2500 sq ft house log house in asheville nc?

that depends on how well-insulated and airtight the house is, and how much
electrical power is used inside. say the 2-story house is a 30x40' rectangle
with an average us r-value of r for the 1,120 square feet of external walls
and 1,200 ft^2 of ceiling, and 0.5 air changes per hour, with an effective
heat conductance of about 1,120/r+1,200/r+0.5x19,200/55 = 2320/r+175 btu/h-f.

nrel says ashville's average outdoor temp is 35.7 f in january, so with very
frugal electrical energy use, the house needs about 24(68-35.7)(2320/r+175)
= 775(2320/r+175) btu/day to stay warm inside on an average january day, or
about 3900(2320/r+175) for 5 cloudy days in a row. suppose the closet begins
the cloudy period at 130 f, and it can provide useful heat for the house until
it reaches 80 f, and it has 32 55 gallon drums, ie about 16k pounds of water
inside, so it stores 16k(130-80) = 800k btu of useful heat. then to keep the
house warm for 5 cloudy days, we need 800k = 3900(2320/r+175) or us average
r74 outside walls and ceiling. that doesn't sound very practical. suppose the
house is more airtight, with 0.2 ach. then 800k = 3900(2320/r+70), so r = 17. 
that's better, but still impossible, if the living space has too many windows. 

the r17 house needs 24(68-35.7)(2320/17+70) = 160k btu/day to stay warm. 
nrel says a south wall in ashville gets an average of 1180 btu/ft^2 of sun
with an average daily max outdoor air temp of 46.5 f, so a square foot of
single-glazed r1 sunspace with 90% solar transmission would gain 1180x0.9
= 1062 btu/day and lose about 6h(68-46.5)1ft^2/r1 = 129 btu over a 6 hour
january day, for a net gain of 933 btu/ft^2/day, if the sunspace were 68 f
during the day. so the house needs 160k/933 = 171 ft^2 of sunspace, say a 
16' high x 12' wide x 12' deep lean-to sunspace with r20 endwalls, and an
8x8' sliding glass door to the solar closet, with an air gap behind that,
then dark-colored window screen, then r20 insulation, then the 8x8' room
with r20 walls and ceiling.

a sunspace with a roof or transparent r1 endwalls would need to be wider,
to gather more sun and make up for greater losses, but it might be less
expensive than one with insulated endwalls, if it's made with 5 cent/ft^2
polyethylene greenhouse film or very clear replex flat polycarbonate plastic
(available for $250 per 49"x50' roll from rimol greenhouse systems at
(603) 425-6563) over 2x6s or standard curved galvanized steel greenhouse
pipes on 4' centers, with pipe ground stakes or some pressure treated 2x4s
spiked to the ground with 4' of rebar as the foundation, and a gravel floor
over plastic film. 

let's check the closet mass/glass area ratio: 32x25ft^2/(8x8') = 12.5. fine.
to check the steady-state closet temperature t in january, figure it gains
8x8x0.9x0.9x1180 = 61k btu/day and loses

         6h(t-68)64ft^2/r1 from the south wall during the day, 
      + 18h(t-36)64ft^2/r20 from the south wall at night,
      + 24h(t-68)4x64ft^2/r20 from the other walls continuously, ie

61k = (t-68)(384+307) + 57.6(t-36) = 748.6t - 49062, or t = 147 f, but
it probably won't get much warmer than 130 f without a selective surface. 

the 2-story sunspace probably won't need a fan to keep the house warm on
an average day, but the solar closet will probably need an internal fan.
if it loses 18h(130-36)64ft^2/r20 + 24h(130-68)4x64ft^2/r20 = 24k btu
each night, it needs to gain 4k btu/h during a 6 hour day. budgeting an
air temperature difference budget of say, 10 f, it needs about a 400 cfm
fan, eg grainger's $60 36 watt 560 cfm 4c688 10" circular equipment cooling
fan, which has a temperature rating of 149 f.

if the house has significant thermal mass (which helps the house and closet
to stay warm overnight) we might size the closet discharge airflow based on
ashrae's 97.5% winter design temperature of 14 f for ashville. at 14 f, the
r17 house needs about (68-14)(2320/17+70) = 11k btu/hr of heat, which might
come from replacing 70 f air with 130 f air at a rate of 11k/(130-70) = 183
cfm. so, this house might not need a closet discharge fan in cold sunny
weather, just a thermostat and a large motorized insulated damper. 

here's an empirical formula for airflow in a chimney:

       cfm = 16.6 av square_root(h(tu-tl)) where
                  av is the area of top and bottom vent holes in ft^2,
				 h is the chimney height in feet, and 
                                   tu and tl are top and bottom temps (f.)

considering this 8' tall closet as a chimney, 

       183 = 16.6 av square_root(8'(130-70),

means av = 0.5, so a 1' x 6" hole near the bottom of the closet and equal-
sized damper near the top might allow this airflow by natural convection. 

after 5 cloudy days at an average outdoor temperature of 35.7 f, when the
water temperature is 80 f, the 68 f house needs about (68-35.7)(2320/17+70)
= 6,600 btu/hr of heat, which might come from about 550 cfm of airflow, so
it looks like it may need a fan at that time, or maybe more attic insulation. 


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

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:
web site: 

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