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re: piping to collectors
9 oct 2000
bert menkveld  wrote:

>hi nick,

hi bert,

>>...you might collect 0.5x1000btu/dayx64ft^2 = 32k btu of heat at 50%
>>efficiency on an average december day, enough for 22 minutes of 110 f
>>3 gpm showers, heating water from 50 f...
>
>...our total hot water usage (at about 125 f) is about 29 us gallons
>per day (laundry, bathing, washing dishes, ...).

that's 29x8(125-50) = 17.4k btu. maybe you should add more collectors
and thermal storage, or build a concentrating cabana.

>>>i'm wondering what kind of pipe or tubing i could use...

>>you might look at swan's pm5875 160 f 100 psi 5/8" reinforced garden hose,
>>$23.95 for 50' from grainger as stock #1p609 or $34.95 for 75' as #2p098.
 
>how about "pex" tubing?  it seems to be readily available here, and not
>very expensive.

probably costs more than that garden hose, and skinnier too. and that
hose is still garden hose (with an unconditional lifetime guarantee :-)
with standard connectors on the ends, if you decide to rearrange things,
take the system apart, move, give away all your possessions so you can
more easily enter heaven than a camel can eye a needle, etc. 

>>...why would you need more than 5 psi with glycol?
>
>right, i expect the system to operate at low pressure, although i'm not
>clear how much pressure i'll get just from the length of tubing involved.

about 0.43 psi per foot of height, plus a smidge for dynamic pressure.

>>...2" (r10) styrofoam all round would make the heat loss something like
>>40'(140f-40f)1ft^2/r10 = 4k btu/h with 140 f water and 40 f soil, 24k
>>btu over 6 hours. you might lose 3/4 of the solar heat to the piping...
 
>oy, i hope you're being pessimistic here!  i don't quite follow your
>numbers.  i like to work in metric, but let me see if i can manage with
>these british units... 

i was thinking the square "conduit" might be 4" square in cross-section, 
with an effective area of about 1 square foot per foot of pipe length.
looks like i overestimated that area.
 
>circumference of a 1/2" pipe is 1.57", or 0.13 ft.  hence, one foot of pipe
>has a surface area of 0.13 ft^2.  then 80 ft has an area of 10.5 ft^2.

but real insulation has thickness, eg us r5 per inch for styrofoam.
so we might think of the first 10.5 ft^2 as having no insulation,
in series with the resistance of a larger 1" us r5 cylinder with
0.5" id, 1.5" od, 1" mean diameter, 15.7 ft^2 mean area, and r5/15.7
= 0.318 f-h/btu resistance, in series with a larger cylinder, etc. 

the continuous formula for the resistance uses the "log mean area"...
r = ln(r2/r1)/(2pikl), eg ln(5"/1")/(2pi/60x80') = 0.192 f-h/btu for a
1" pipe surrounded by 4" of r60/foot (k = 1/60 for a foot of thickness)
styrofoam. (how can this be less than the 0.318 above? maybe the shell
size increment was too large.)

>rate of heat loss is then:
>
>(140f - 40f) * 10.5 ft^2 / r10 = 105 btu/h

or maybe (140f-40f)/0.192 = 521 btu/h, or maybe half that, if both pipes
are inside a slightly larger insulated box. 

>in actual fact, i measured the real distance, and it turns out it will be
>about 85 ft from tank to collectors, so the rate of heat loss is doubled.

ok...

>i'm also thinking that the styrofoam "box" might have trouble over time
>with sagging and moisture.

well, who cares if it sags, as long as it doesn't break? styrofoam is
fairly strong, esp. in thick pieces connected with long deck screws.
it doesn't absorb much moisture, and the heat of the pipes will keep
the inside dry, as long as it is above the water table.

>now i'm thinking of using a 3" or 4" abs pipe as a "conduit" for the pair
>of pipes, with standard pipe insulation around the pipes.  the pipe
>insulation only has an r value of 2.3,

per inch?

>i figure the rate of heat loss is still barely over 10% of the heat
>collected in full sun... does anyone see any problems with this idea?

hmmm. fuzzy pipe insulation might absorb rainwater, and it could be hard
to keep the pipes in the middle, and if the pipe insulation filled the
entire 4" conduit, the resistance of an 85' length with r2.3/inch might
be r27.6ln(4"/1")/(2pi85') = 0.072 f-h/btu with (140f-40f)/0.072 = 1,396
btu/h of heat loss, 8,375 over 6 hours, altho the soil has some thermal
resistance as well, and rain, which carries away the heat. 

>>can this be a batch/draindown process, with intermittent flow and no
>>glycol, and a single hose that slopes down to the house? you might pump
>>55 gallons of 50 f water out to the collectors over 10 minutes at dawn,
>>then let it thermosyphon to 50+32kbtu/(55x8) = 123 f by afternoon, then
>>let it run back to the house over 10 minutes at sunset.
 
>an interesting idea.  you mean there should be a storage tank above the
>collectors, as well as one in the house?

maybe, a drain-down version of those caribbean water heaters with a
sloped flat plate and an insulated cylindrical tank above.

>unfortunately, the collectors are downhill from the house, so i can't
>do this.

it might still work, with a pressurized system or a pump near the
collectors. or you might unshade the south side of the house, cut down
the trees or change the curb appeal and put this thing in a sunspace.

nick




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