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re: solar barn update
21 aug 2004
gary   wrote re:

>>>a glazing layer, a screen or shade cloth absorber, an insulated 
>>>partition with high and low vents, a chamber north of the partition 
>>>with water containers to store heat...

>> for overnight heat storage, you might do better just using the thermal
>> mass of the house... it has a lot more surface, so the air near the
>> collector glazing could be cooler, with less heat loss to the outdoors. 
>> if your house has 400 btu/h-f of conductance and 4k btu/f of capacitance, 
>> rc = c/g = 10 hours. if it's 70 f at dusk and 30 f outdoors, with no
>> internal heat gain... making it 60 f at dawn says rc = -16/ln(30/40)
>> = 56 hours, ie c = 56x400 = 22,246 btu/f, eg 2671 1-gallon jugs :-) 

or 4,345 hollow concrete blocks (a 19 foot cube :-) did you see the "water
chair" at the portland ases conf? it held about 30 gallons, as i recall.

>i have a simulation that includes the house (modeled as equivalent to
>6k lb of water), and the garage door collectors (with no storage).  i
>use the tmy2 weather data for billings to drive the simulation.

good... nrel data imply that january is the worst-case month for billings...
530 btu/h-ft^2 falls on the ground and 1020 btu/h-ft^2 falls on a south wall
on an average 22.8 f jan day with an average daily max of 31.8. the average
daytime temp is about 27.3. on a clear day, 700 btu falls on the ground and
1910 falls on a south wall.

>in the simulation, i let the garage door collector put heat into the
>house during the day until the house temperature gets up to 75f.  once 
>the house has reached 75f, i dump any additional heat from the garage 
>door collector -- this  happens fairly often in the shoulder months. 

it might be useful to let some thermal mass heat to a higher temp,
after the house is warm enough.

>at night i let the house temperature cool to 60f before turning the 
>furnace on -- this means i am getting 6000lb * 1btu/lb-f *(75f - 60f) 
>= 90k btu out of house as thermal storage?

sure, altho it would last longer if you could keep the house at
a constant 60 f all night. you can do this with ceiling mass...

>this is good for about 6 hrs -- 30f + (75f - 30f)e^(-6/16) = 61f?

yes, with no internal heat gain. what's your monthly electrical use?

>(i plan to check the 6k house mass (which i have no real justification 
>for :-) by measuring actual cool down rate and comparing it to 
>predicted with 6k mass and 400 btu/f-hr loss rate -- this should work?)

...including internal electrical use. why 400 btu/h-f? you might check that
with a kwh meter and an electric heater and thermometers on a mild day.
and try to estimate the house mass from the materials, eg 1/2 btu/f for
a square foot of 1/2" drywall. 

>i can add about 180 ft^2 more collector area to the south wall of the
>house, but it seems like there is not a lot of point to doing this
>without adding some new thermal storage that would allow the collected
>heat to be held over for the night.

more heat dumping...

>with its heat going to storage, the added collector might make another 
>110k btu of stored heat available on the night after a sunny day, 
>which would probably get me most of the rest of the way through a cold 
>night after a sunny day?

...24h(t-30)400 = 200k makes t = 51 f. brr.

>i have been thinking about some schemes for collector + storage:

>1) add 180 ft^2 of air collectors to the south wall of house, route
>the heated air to an insulated thermal storage enclosure (water?) in
>the conditioned crawl space that is right behind the collector wall,
>and draw heat from the storage at night.  this seems a bit complex,
>having ducts, fans, controllers, and two separate things to construct
>(collector and storage)?  but, maybe its more efficient than method 2?

storing heat in a compact box could be more efficient than storing it
in a wall. the solar collection efficiency depends on the mass/glazing 
surface ratio (10/1 is good) and the storage temperature. a low storage
temp raises collection and lowers storage efficiency. 

>2) do a collector with integrated storage, which might be a refined
>version of the one i am testing in the barn (described at top of this
>post and at the "test of integrated storage collector" link at 
>  the collected heat would be 
>removed from the storage chamber with a duct and fan that delivers 
>heated air to the house.  the storage containers might be 4 inch to 6 
>diameter lengths of plastic pipe a bit like the ones that steve baer 
>showed at the ases workshop?

sure. put vertical pipes inside walls, or make a heat storage tower beside
the house, 4'x8'x20' tall, with 200 4"x20' pipes (22k btu/f) inside. stand
on top of the tower and howl at full moons? :-)
>3) do a collector with integrated storage in which the storage 
>containers are located directly behind the collector glazing such that 
>the sun directly heats the containers.  at night automatically deploy 
>an insulated panel between the glazing and the water containers to 
>reduce night losses.

hard to implement. r-value? air leaks when closed? a fan moving air
through a hole in an insulated wall may be easier "movable insulation." 

>it seems like this would be more efficient as a 
>collector during the day than scheme 2, but the design and 
>construction of the movable (roll up?) insulated panel would be difficult.

moving lots of air can be efficient, with a large mass/glass surface ratio.
if it's 30 f outdoors and 225 btu/h enters a square foot of r1 glazing with
90% solar transmission and 20 cfm flows through a 100 f store with a 10:1
mass/glass area ratio, we have something like this, viewed in a fixed font:

  225 btu/h  tc              100 f
     ---     |   1/20  1/15  |
     ---     |
             |   what's tc, and the solar collection efficiency?
         1   |
30 f ---www--                         
                                   1/20+1/15 = 0.1166
  225 btu/h  tc          
     ---     |   0.1166 
|---|-->|--x-*---www--- 100 f   disconnecting the sun (at x) makes
     ---     |   <-- i
	     |    1                i = (100-30)/1.1166 = 62.9
              ---www--- 30 f
                                   tc = 30+62.9x1 = 92.7
  225 btu/h  tc          
     ---     |   0.1044            1/(1/1+1/0.1166) = 0.1044
|---|-->|----*---www--- 92.7 f

tc = 92.7 + 225x0.1044 = 116.2, so (116.2-30)/r1 = 86.2 btu/h flows through
the glazing to the outdoors, and the rest (225-86.2 = 138.8) goes to storage,
and the full-sun solar collection efficiency is 138.8/250, ie 56%. not bad.

>3) take a look at your suggestions on adding ceiling thermal mass.


>i am thinking that the amount of thermal storage for the new collector
>should be about equivalent to what the new collector can pick up
>in one sunny winter day.  there does not seem to be any point in
>trying to store more than this, because the loss rate of my house is
>high enough to use all of the stored heat on the night of the sunny day?

maybe it's time to try to convince your wife to let you push pieces of
foamboard into the windows in wintertime. or do a blower door test/
improvement program with a large exhaust fan in one window and your new
velocity stick in a slot in another window in the same room, for starters.
as you seal up the house, the slot air velocity should increase. 

>to store the heat from one sunny day i need about:
>heat to store = (1450 btu/day-ft^2)(40%)
>               = 600 btu/ft^2 of collector


>storage mass  = (600 btu/ft^2)/ (120f - 75f) (1btu/lb-f) =
>               = 13 lb/ft^2
>where 40% is kind of a target rate for getting incident solar energy
>into the water storage -- it has to account for collector losses and 
>other losses in getting heat transfered to the water -- reasonable?

if 1910 btu falls on 1 ft^2 of r1 south glazing with 90% solar transmission
over 6 hours on a clear 27 f jan day and the average collector temp is 100 f,
the net gain would be something like 0.9x1910-6h(100-27)/r1 = 1281 btu...

>and, 120f is max temp of stored water, and 75f in minimum useful temp 
>of stored water.
>and, 1450 btu/day-ft^2 is avg sunny day solar energy on vertical panel 
>in mid-winter at 45 lat.


>this would be 1.6 gal/ft^2, or 50 gal per 4x8 panel, or 320 gals for
>the full 200 ft^2.  thus the 40 one gallon jug 4x8 test collector.

i doan like those steenkeeng jugs.

   in our home solar heating system we used water as the thermal storage
   medium for an air-transfer unit, the water being contained in 1000
   one-gallon polyethylene bottles stacked so that air could flow between
   them. they worked satisfactorily until some desert pack rats invaded
   the storage bin, making nests of the insulation and chewing holes
   in the water bottles. 
                                 p 468, _applied solar energy_, by
                                 aden b. meinel and marjorie p. meinel
                                 addison-wesley, 1976

>i am also thinking about using the same system for domestic water
>preheat, since this would allow the collector to "earn its keep"
>during the summer (at the cost of some winter house heating performance).

you might preheat dhw in 5 btu/h-f-ft fin-tube pipe.

>i realize (and the simulation reminds me) that even in mt, not all 
>days are sunny, and, in the end, this setup will save less than 50% on 
>my heat bill...

it might save more...

>> if you heat a ceiling capacitance c near the air heater outlet to 100 f
>> by dusk and keep the house 50 at night, the house needs 16(50-30)400
>> = 128k btu over 16 hours, and c = 128k/(100-50) = 2560 btu/f, eg 307
>> ceiling jugs or a 30" diam x 36' greenhouse poly film duct filled with
>> 4" of water, over some welded-wire fence or 1/2" plywood with a layer
>> of foil underneath.
>i'll think about how i could accomplish something like this.  i have 
>to say that having a lot of water over my head in poly film ducts is a 
>bit unsettling.

you could get used to it.

>i guess the poly ducts could be over the ceiling sheet rock with insulation
>over them?  perhaps with a safety container of heavy poly sheeting?

that's one way. it might be simpler to put them on a plywood platform under
the sheetrock, over a poly film leak prevention layer draped over new 2x4s,
like this, viewed in a fixed font:

   |  |     <-- ~2'-->         |  |  poly film duct    |  |  new 2x4
   |  |                        |  |  poly film layer   |  | <-- tube?
 ------------------------------------1/2" plywood------------   

>i'm trying to think how i sell my wife on the alum foil ceiling :-)

you might say it makes the room brighter or attenuates alien signals or
fullfils an interior decorating fantasy. you might look for a low-e paint.
you might put fluorescent tubes above the plywood perimeter. 

>>>just to get a feel for how well this might work i am converting one 4
>>>ft bays of the barn collector to this configuration...

>i used the gallon jugs in this little test collector because they
>were only 50 cents each (including the distilled water :-), so it
>was a cheap/easy way to get an feel for how well this kind of 
>collector might work. i have it set up in the barn now and i am 
>recording temperatures at the top and bottom vents and top, middle, 
>and lowest row of jugs, and ambient.  i have only got about one day of 
>data, which looks roughly like this around mid afternoon:
>	ambient: 	     72f
>	top vent:  	    142f
>	bottom vent:         93f
>	top row bottles:    112f
>	mid row bottles:     94f
>	bot row bottles:     81f
>	air velocity in vent: 30fpm
>i plan to refine the setup a bit (fix heat leaks etc), and get a few 
>more days data, and also check on cool down rate at night, but 
>tentatively, it seems like more air velocity would lower the air 
>temperature some, and reduce collector losses, and would also transfer 
>heat to the water more rapidly?

maybe, but increasing the air velocity near the glazing could increase
the loss to the outdoors. that airflow path wants to be large, to keep
the velocity down.

>as the water heats up, the air temperature would come up, and heat the
>water to a higher temperature, but at a lower efficiency?


>judging by the strong vertical stratification in jug temperatures, and
>the relatively low lower vent temperature, i am inclined to say that
>heat transfer from the air to the bottles is fairly efficient?

you might calculate that. you might enjoy a way to measure solar intensity, 
eg a $134 daystar ( meter or a small glazed box with
a thermometer.

>it seems to me that a refined version of this integrated storage wall 
>collector could be an interesting idea for new construction for folks 
>planning houses having quite a bit of south wall, but who don't care 
>for the acres of windows and bare concrete floors approach??

maybe. i'm fonder of the ceiling mass idea, which allows setbacks,
using a slow ceiling fan.

>it would look pretty much like a normal wall, only 6 inches or so thicker
>with some nice looking vents?  windows could be mixed into the wall for 
>light and views.  i think that its overall performance would be better 
>than lots of window area with thermal mass  in the floor, because the 
>window losses are so high at night?

and insulated mass can be higher than 70 f, with better room temp control. 

>i've also noticed that the low winter sun, and strong reflection off
>snow cover, results in a lot of glare, and that people tend to draw
>the curtains to cut the glare -- which somewhat defeats the heat
>collection in floor mass.

that seems fine, for ceiling mass.

>as an aside, i have been working on reducing the heat loss of the
>house.  i am using the book "insulate and weatherize", bruce harley,
>taunton press as a guide.  i find this book to be very helpful and
>highly recommend it.

thanks for the recommendation.


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