Subject: Re: Help with passive solar hot water storage

From: [email-masked] (Nick Pine)
Date: 29 May 1996 08:17:51 -0400

Gene A. Townsend <[email-masked]> wrote:
>[email-masked] (AndyM) wrote:
>>I have a 50 gal electric hot water heater in My Attick . I was thinking of
>>running about 120 feet of 1.5 " pvc pipe back and forth in the attick to
>>bost the hot water supply...

I wonder if you can somehow put the pipe under the water heater... Build a
shallow glazed box on the south wall of your house, with some pipe running
along the top edge, inside the box? Or put the pipe in a sunspace, or put
it near the peak of the attic, and use a small circulating pump... 

>>the attack maintains a temp of around 95-110 ( monitored useing a min-max
>>thermotere. at night it cools to around 75-90 depending on the outside
>>temp ( have an ovesized attack fan on a thermostat up there). My question is:
 
>>A) how long will it take for the water in the pipe to reach temp?
 
120' of 1.5" pipe has an area of about 47 ft^2, and the thin pipe wall has
a large thermal conductance (10?) compared to the still air film around it,
which has a thermal conductance of about 1.5 Btu/hr-F-ft^2. If the pipe
contains 10 gallons of water, by Gene's calculation, it will have an RC
time constant of 10gx8lb/g/1.5/47 = 1.13 hours. So if the water starts out 
at say, 60 F, and it sits in a 100 F attic for t hours, its temp T(t) will
follow T(t) = 100 - (100-60)exp(-t/1.14)). After 1 hour, it should have a 
temperature of T(t) = 100 - 40exp(-1/1.14) = 83.4 F. After 2, 93 F, after
3 hours, 97 F, etc.

>>B) has any one had any experience with this idea before is there any
>>thing i need to watch out for?

Watch out for freezing... But if your water heater plumbing hasn't ever
frozen in your attic, chances are the PVC pipe won't either. If you don't
use any hot water at all in your house, how long will it take this pipe to
freeze solid and burst and flood your house, when your attic is -1 F, the
record min temp in Columbia, SC? Delta T would be 33 F, with U about
1.5 Btu/hr-F, and the pipe has 47 ft^2 and there is 80 lb of water inside,
and it takes 144 Btu to freeze a pound of water. So... 144x80 = tx33x1.5x47,
ie t = 5 hours. But the pipe might not freeze evenly. A little heat tape
with a thermostat might not be a bad idea.

Another thing to watch out for is hydraulic short circuits. You don't want
the incoming cold water to somehow end up coming out of the shower instead
of the hot water that is inside the water heater. 

>The PVC pipe must go on the cold water side of the heater...

Seems like it might help a lot, if it's in a loop below the water heater,
so it can heat the water in the heater over and over, vs just heating the
incoming cold water in one pass.
 
>...now if only those pipes were painted black and put on the outside...

Good idea. Near the top of a shallow glazed box on the south wall? Say the
box were 12'wide x 16' tall x 3 1/2" thick, over an edgewise 2x4 frame with
no insulation around the edges, and the glazing were 4 $50 4' x 12' sheets
of Dynaglas corrugated polycarbonate plastic greenhouse roofing material,
with your 47 ft^2 of dark painted pipes in a 10' x 20" rectangle near the
top edge, and you had a $50 motorized damper behind them to allow some
warm air to flow into the house when you had enough hot water, but needed
some house heat (two thermostats), on an average January day in Columbia,
with 1140 Btu/ft^2/day of sun falling on the south wall, at an outdoor temp
of 55 F. If you kept the box at 130 F inside, you might collect 50K Btu/day
of hot water that way on a 6 hour day, while 6(130-55)192ft^2/R1 = 86K of
heat passed back out through the glazing, and you could put the rest of the
sun's heat, ie 0.9x1140x192 - 86K = 111K Btu/day into the house, saving
the heat equivalent of about a gallon of oil a day in the winter.

Now why not make that shallow box on the wall a bit bigger, say a lean-to
sunspace, 8' wide at the base, with a pebble floor and a railroad tie
foundation, and a picnic table, and use a bit more PVC pipe and keep the
sunspace temp at 80 F in January, to add some interesting daytime living
space and collect more heat for the house?

Nick

---

From: [email-masked] (Dan Settles)
Date: Wed, 29 May 1996 17:10:02 -0500

> the heat equivalent of about a gallon of oil a day in the winter.

Instead of running the pipe along the top of the box, how about running it
through a few of the water filled barrels?  If you used flexible plastic
tubing you could probably loop a couple hundred feet inside barrels.  Run
the tubing in an opening, coil it around the sides and run it back out
again.  You could hook up several of these in series and have a heat
exchanger that should heat the incoming cold water much faster than pipes
exposed to the air in the box. 

> space and collect more heat for the house?

This sounds nice, but I can't picture how this would work.  I'd like to
have a collector that doubles as a sun space, but I don't want to sit in a
130 F room, even in January, and 80 F won't get the water hot enough.  Do
you mean the sunspace would be in addition to the shallow box?  So the
collector for the solar closet would be located at the back of the sun
space?

Dan Settles

---

From: Chad Berreau <[email-masked]>
Date: Wed, 29 May 1996 17:08:04 -0500

> exposed to the air in the box.

This barrel/coiling method sounds to be a great solution.  I would think 
that it would increase the rate of heat transfer to the water with less 
space.
 
> collector for the solar closet would be located at the back of the sun

Just make it a tall thin space.  The bottom portion would be where you 
would reside with your greenhouse plants.  Towards the top would be 
where the works would reside with all the "hot air".  To prevent 
overheating, vent excess hot air to adjoining internal spaces.  Or 
incorporate some shading measures designed only to shade during 
the hottest times of the day/year (i.e. a small treless(sp) with 
folage(sp) for summer, that is then bare in the winter.

The barrel concept could be used here.  Where the barrels were sort of 
suspended in the air, or on a cat walk.  with some dangling plants along 
the base of it and lights for night time use.  Sort of soften the solar 
ambience a bit.

'Just $.02 worth of info.'

-- 

Chad Berreau

---

From: [email-masked] (Nick Pine)
Date: 30 May 1996 03:33:56 -0400

>> with your 47 ft^2 of dark-painted pipes in a 10' x 20" rectangle near the

Oops, too many Btus...

197K Btu of sun gets into the sunspace, and
 86K leaks back out of the glazing (2 layers would help here), and 
 59K allegedly goes into the hot water, leaving
----
 61K for the house.

>through a few of the water filled barrels?

Water filled barrels? Where are the barrels? There's no solar closet here, 
altho there could be... This is just a sunspace with a bare water type solar
collector inside, that circulates its hot water through a conventional water
heater on the floor above by thermosyphoning, natural warm water convection...
If all goes well, the water heater heating element seldom turns on.

>If you used flexible plastic tubing you could probably loop a couple
>hundred feet inside barrels.  Run the tubing in an opening, coil it around
>the sides and run it back out again.  You could hook up several of these
>in series and have a heat exchanger that should heat the incoming cold water
>much faster than pipes exposed to the air in the box. 

True, but if we had a solar closet, it seems simpler to just put 10-20' of
fin-tube pipe near the ceiling, an air-water heat exchanger that works 24
hours a day, to heat water in a water heater upstairs by thermosyphoning
again. Seems to me it would be difficult to make water flow through all
that plastic tubing without a pump, and you might have evaporation at the
points where the tubing enters and leaves the drums. If the tubing is in
series with the water heater, which is inside the solar closet, the water
supply pressure could move the water through the plastic pipe, once, but 
it's more efficient to have the water continuously circulate through the
heat exchanger, to make a heat exchanger with a given surface area do more 
of a full-time job, rather than have the water only pass through it once.
 
>This sounds nice, but I can't picture how this would work.

The usual way is some sort of bare solar collectors inside a sunspace,
eg Zomeworks Big Fins, which are flat aluminum extrusions, about 4' long
and 8" wide, painted black on one side, that clip on to and are supported
by standard copper pipes running along the back side... Then there's the
standard water heater upstairs and a thermosyphoning water loop from the
Big Fins to heat it, when the Big Fins are warmer than the water in the
water heater. So in this usual case, the fins and pipes are, say 130 F,
with the sun shining on them, but the sunspace is only 80 F.

>I'd like to have a collector that doubles as a sun space, but I don't want
>to sit in a 130 F room, even in January, and 80 F won't get the water hot
>enough.

I see what you mean... If you put Big Fins or a bare solar panel in your
sunspace, IT would be 130 F with the sun shining on it, but your sunspace
could be 80 F, with some solar heat going out the glazing and some warm air
flowing into the house. The losses from the 130 F collector would be heating
your house, vs the outside world, which is a good thing... And you don't
need a big heavy expensive weatherproof insulated collector on the roof,
where you may break your neck trying to fix it, which is a good thing.

However, in this case we have 47 ft^2 of surface area, but only half faces
the sun, or a little more if we spread the pipes out to 24 or 36" vs 20"
so they shade each other less when the sun is not striking them head on...
and we want to collect a typical hot water heating load of 50K Btu/day...
How can we do that? The PVC pipes themselves, painted a dark color, might
absorb 20-30K Btu/day. How can we collect another 20-30K? Perhaps we can also
use them as an air-water heat exhanger... 47 ft^2 of pipe with 80 F water
inside can collect about (130F-80F)47x1.5 = 3750 Btu/hr of heat ie 22K Btu
per day, if the air around it is 130 F, even with no sun shining on the pipe.

>Do you mean the sunspace would be in addition to the shallow box? 

I had in mind that the black pipe manifold would be up near the top of the
lean-to sunspace. Since warm air rises, we can trap heat up there, stratified
hot air, much warmer than the rest of the sunspace. It would be a good idea
to put an extra layer of glazing across the top 4' of the 16' high x 12' wide
sunspace, on the inside, in front of the black pipes, here -  
.                                                           |
.  <--------------------------------------------------------
. .           sun
.
.   .   16'
.     
.     .
.  8'
.........

>So the collector for the solar closet would be located at the back of
>the sun space?

Yes, if there were one, but then we wouldn't need the black pipes, just a
little fin-tube pipe near the top of the closet, working 24 hours a day...

Nick

---

From: [email-masked] (Nick Pine)
Date: 30 May 1996 03:53:10 -0400

Chad Berreau  <[email-masked]> wrote:
 
>suspended in the air, or on a cat walk...

Might make for an interesting tension in the humans beneath, since 55 gallon
drums weigh about 500 pounds each :-) Is there a swimming pool like that in
Barcelona, with something huge suspended over the pool...?

How would we keep all the heat in the drums at night?
Seems like they need insulation around them at night.

I'd put 'em on the ground, so they don't fall on people's heads, inside a box
with insulation all round, with one side facing the sun, inside the sunspace,
and the other sides inside the house, perhaps. A solar closet, that stores
heat for the house for several days, and makes hot water with a fin-tube pipe.

So... the sun shines in through the sunspace glazing, then thru the 80 F
sunspace, then it shines on thru the solar closet glazing, then on thru an
air gap containing 130 F air (_not_ the same air as the 80 F sunspace air),
where it finally hits the outside of the dark-colored insulation of the
south wall of the solar closet, which warms the insulation, which heats the
130 F air, which air passes thru a one-way damper to heat the drums inside
the box, and the air returns about 5 F cooler from a hole at the bottom of
the box to this air heater, and runs around again.

At night, the air damper closes, the fan, if any, stops, the air stops
circulating in and out of the box, and the drums are insulated all round.

Nick

---

From: [email-masked] (Dan Settles)
Date: Thu, 30 May 1996 11:47:36 -0500

> If all goes well, the water heater heating element seldom turns on.

My mistake.  I was thinking of the solar closet idea.

> of a full-time job, rather than have the water only pass through it once.

Okay, why not put the water heater inside the solar closet instead of
upstairs?  The heater is now in the 130 F room, so you don't need to
circulate water through it in order to keep it warm. The tubing would be
connected to the cold water input, so the water pressure provides all the
pumping energy. 100 feet of 3/4 inch inner diameter polybutylene hot water
pipe from Home Depot costs $60 and would have a surface area of (¼ 7/8) x
100 x 12 = 3299 in^2 or 22.9 ft^2.  How does this compare with the fin
tube that you suggested?  I imagine the fin tube might make a more
efficient air-water heat exchanger, but maybe in a water-water heat
exchanger the plastic tubing might be a good, cheap alternative.  I agree
that evaporation may occur where the tubing enters and exits the barrel,
but I imagine that some type of gasket or pressure-type fitting could be
made to minimize the water loss. 

> with the sun shining on them, but the sunspace is only 80 F.

How much are the Big Fins?  Do you have phone number for the company?

---

From: [email-masked] (Nick Pine)
Date: 30 May 1996 17:32:53 -0400

>circulate water through it in order to keep it warm.

Right, but that way, the water needs to be heated on the way in, in a
single pass. After it's in the water heater, it won't gain much heat.

>100 feet of 3/4 inch inner diameter polybutylene hot water pipe from Home
>Depot costs $60 and would have a surface area of (¼ 7/8) x >100 x 12
>= 3299 in^2 or 22.9 ft^2. How does this compare with the fin tube that you
>suggested?

10' of fin-tube pipe gains about 50 Btu/hr per degree of air-water temperature
difference, and your pipe with 1/8" walls (?) might have a U-value of 10, with
a water-pipe-water interface, so it might gain 229 Btu/hr-F. If the incoming
water only passes through it once, at, say 3 gpm, and the pipe has a volume
of 1/2 gallon, the water will only be in the pipe for about (1/2)/3 = 1/6 min
or 1/360 hour, so if it starts out at 60 F, say, it will gain at most
1/360(130-60)x229 = 44 Btu, which will heat that 4 pounds of water to 70 F,
on the way into the water heater.

At that point, if the water heater is insulated, the 70 F water won't gain
much heat from the closet air. But say the water heater has no insulation...
Say it's a bare galvanized tank... (Which would make for a large backup water 
heating bill as the closet air temp drops to 80 F over 5 cloudy days.) If the
tank is bare, eg 6' tall x 1' diameter, it will have a volume of 4.7 ft^3,
holding 36 gallons or 292 lb of water, with a surface area of about 20 ft^2 
and a U-value of 1.5 Btu/hr-F, so the tank will have an RC time constant of
292/(20x1.5) = 9.7 hours, so heating that 70 F water to 110 F (a nice shower
temp) in the 130 F room will take time t, where 110=130-(130-70)exp(-t/9.7),
ie 0.846 = -60exp(), or exp() = -0.0141 or t = -9.7 ln(-0.0141) = 9.6 hours. 
Not exactly a fast recovery time for a water heater...

>I imagine the fin tube might make a more efficient air-water heat exchanger,

Well, the water can move through it more than once... How long would it take
36 gallons of water to heat from 60 to 110 F with the fin-tube pipe? RC =
292/50 = 5.84 hours, so t = -5.84ln(-0.0141) = 5.8 hours. Hmmm. Let's try
20'... 2.92 hours, so t = 2.9 hours... Still not very exciting. Let's try a
fan coil unit (overkill), e. g. the $139 all-copper 2' x 2' SHW 2347 duct
heat exchanger made by Magicaire, attached to the $318 4C860 Grainger fan
which moves 1840 cfm at 0.125" H20 of static pressure, and has an upper temp
limit of 311 F. This duct heat exchanger can transfer 45K Btu/hour between
125 F water and 68 F air at 1400 cfm, with a 0.1" H20 pressure drop, ie
789 Btu/hr-F. So now we have RC = 292/789 = 22 minutes and t = -22ln(0.0141)
= 22 minutes. But at that rate, we will run out of closet heat fast. We don't
need that much not water, on the average. So maybe we just need a bigger
tank, and, say 20' of fin-tube... or your plastic pipe setup with a small
circulator pump. But there's another bottleneck there, getting the heat
>from  the air into the drums. It helps if the drums are on top... 

>maybe in a water-water heat exchanger the plastic tubing might be a good,
>cheap alternative.  I agree that evaporation may occur where the tubing
>enters and exits the barrel, but I imagine that some type of gasket or
>pressure-type fitting could be made to minimize the water loss. 

Perhaps it's really just a matter of taste... Some fin-tube pipe with a
conventional water heater upstairs appeals to me...

>How much are the Big Fins?  Do you have phone number for the company?

Big Fins cost a lot, $4/ft^2? Because that's what Zomeworks (505) 242-5354
charges for them... And I'm not even sure they make 'em anymore... But they
work very well. It wouldn't be too hard to make something up like this on
one's own. A big plate with a good bond to a 3/4" copper pipe behind it,
and some dark paint, or maybe some Ovshinsky PVs glued to the front (or
electrodeposited on the front--hard to do at home.) Bare water-type solar
collector absorber plates would work well in a sunspace, but they would
probably need a pump, since the water passages tend to be small.

Or you could try making something attached to the north wall of the sunspace
with a single large piece of EPDM rubber, a bladder, folded at the bottom,
with silicone caulk and a 2x4 sandwich to seal the side edges and a piece of
dark painted sheet metal to keep it from bellying out too much to the south.
Sven Tjernagel has site-built about 700 large collectors like this over the
last 20 years for commercial uses, for hotels and car washes, each being up
to 10' tall x 50' long, with a materials cost of about $1/ft^2...

Nick

---

From: [email-masked] (AndyM)
Date: Thu, 30 May 1996 22:58:41 GMT

>> Nick
Andy Here again, to bring this back to reality . I live on the outer
banks of north Carolina . This area is prone to lots of hurricanes and
high winds , and if thats not a convincing argument about cutting
holes in my Roof.  The roof is 60-60 feet  above ground . All house
that are built here must be bult on piling  in case of costal
flooding. 
  This is my thinking on this project

store 20 gal of water in the pipes before its fed into the Electric
Hot water heater and the heater will only have to heat it from say
90-100 deg to the 130-140 the heater operates at. with the fact that
most hot water uses are less then 20 gal this should lead to x% of
savings in energy. I all ready have a shutdown timer on the heater ,
it kills the electric to the heater for a total of 10 hours a day
(energy saving of 10-20% per year).  Can any one quantify how much %
or $ this will save, Yes i know that i have to bypass the system for
the 4-5 months of winter or i will lose energy.

Thanks for all your input
AndyM

---

From: [email-masked] (Dan Settles)
Date: Fri, 31 May 1996 13:54:31 -0400

> single pass. After it's in the water heater, it won't gain much heat.

That's true.  With the insulation on the heater, it will gain heat from
the room very slowly, but if I have the water heater temperature set at
129 F, the heating elements should never have to turn on to maintain that
temperature after it reaches 129 F.  How much energy does a typical hot
water heater use just to keep the water hot until we are ready to use it
vs heating the incoming cold water?  Also, the conventional heater outside
the solar closet will heat the air in the house.  In the winter, this
wouldn't be a problem, but this would be a liability in the summer,
especially in warmer areas like the south. I'm not sure how significant
these factors would be, but it would be interesting to find out.

> on the way into the water heater.
Actually, the 100 feet of 3/4 inch inner diameter tubing would have a
volume of 2 gallons, so the water would be in the tube for 2/3 min or
2/180 hour with a gain of 
2/180 (130-60) x 229 = 178 Btu.  Unfortunately, those 178 Btu's now have
to heat 16 pounds of water, so that doesn't really change anything does
it?  It looks like I'd have to increase the length of the pipe in order to
get any greater increase in water temperature.  So, if I put in 500 feet
of tubing, the water would be in the pipe 5 times longer, and I'd have 10
gallons or 80 lb of water. So I'd gain at most, 10/180 (130-60) x (229 x
5) = 4452 Btu.  Is that correct?  That should heat the water to 110 F, and
in addition I'd have 10 gallons at 130 F.   However, his would now cost me
$300 and the resistance of the extra length of pipe would probably start
to slow down the flow rate.  Not to mention dealing with 500 feet of
pipe.  

> Not exactly a fast recovery time for a water heater...
How about using two tanks, the first being the above mention galvinized
tank and the second being a conventional hot water heater?  It's not quite
as interesting as a fin tube, but it would give you plenty of hot water
during sunny days, and it would cost you less than a conventional heater
after those 5 cloudy days.  You'd still have your incoming cold water
preheated to 80 F before entering the conventional heater.  

> conventional water heater upstairs appeals to me...

I agree.  I'll have to think about his a bit.
> to 10' tall x 50' long, with a materials cost of about $1/ft^2...
I haven't heard of this before.  So how does the water move through this
device?  Does he just let cold water  flow into one end, and run out the
other end after absorbing heat for 50 feet?

Dan

---

From: [email-masked] (Nick Pine)
Date: 31 May 1996 17:59:33 -0400

Dan Settles <[email-masked]> wrote:
>> >...why not put the water heater inside the solar closet instead of
>vs heating the incoming cold water?

Perhaps you could figure that out, Dan. Suppose the water heater has 2" of
fiberglass around it (~R8), and it's 6' tall and 2' diameter, containing
130 F water, sitting in a 70 F room, and you use 50K Btu of hot water
per day at 110 F, heated from 60 F.

>Also, the conventional heater outside the solar closet will heat the air
>in the house.  In the winter, this wouldn't be a problem, but this would
>be a liability in the summer, especially in warmer areas like the south.
>I'm not sure how significant these factors would be, but it would be
>interesting to find out.

Agreed :-)

>>10' of fin-tube pipe gains about 50 Btu/hr per degree of air-water
>>temperature difference, and your pipe with 1/8" walls (?) might have
>>a U-value of 10, with a water-pipe-water interface, so it might gain
>>229 Btu/hr-F. If the incoming water only passes through it once at,
>>say 3 gpm, and the pipe has a volume of 1/2 gallon, the water will
>>only be in the pipe for about (1/2)/3 = 1/6 min or 1/360 hour, so if
>>it starts out at 60 F, say, it will gain at most 1/360(130-60)x229
>>= 44 Btu, which will heat that 4 pounds of water to 70 F, on the way
>>into the water heater.
 
>volume of 2 gallons...

I goofed. Pi((3/4)/2)^2/144x100 = 0.307 ft^3, ie 2.4 gallons... 

>so the water would be in the tube for 2/3 min or 2/180 hour

2.4/3 = 0.79 min or 0.013 hr...

>with a gain of 2/180 (130-60) x 229 = 178 Btu.

gaining at most 0.013 x (130-60) x 229 = 212 Btu...
(the water heats up a bit as it travels through the heat exchanger, so
the warmer water at the end doesn't see as big a temperature difference.)

>Unfortunately, those 178 Btu's now have to heat 16 pounds of water, so
>that doesn't really change anything does it?

I guess not, in the sense of the 70 F water going into the water heater.

>It looks like I'd have to increase the length of the pipe in order to
>5) = 4452 Btu.  Is that correct?

Not quite, altho it goes in that direction. The first 100' of pipe gets us to
70 F, at most, but the water entering the second 100' of pipe is at 70 F, not
60 F, so the temp difference is 130-70, not 130-60, in that pipe, so it gains
less heat in the second 100', and so on. This is a heat exchanger problem with
a solution with an exponential in it, and you can find the solution on page
3.4 of the 1993 American Society of Heating, Refrigeration and Air
Conditioning Engineer's Handbook of Fundamentals... 

Don't forget that you have to transfer the heat into the drums that 
contain the pipe...

>preheated to 80 F before entering the conventional heater.  

Sounds a little better...

>other end after absorbing heat for 50 feet?

Sven used to run a length of PVC pipe with some holes in it along the top
edge, and let the water trickle down inside the rubber bladder and collect
the water at the bottom edge, but that way, there can be hot spots, and his
slope was limited to about 45 degrees... He didn't use any sheet metal on
the front, and his EPDM rubber wore out and had to be replaced after about
7 years. I think it's probably better to use some sheet metal on the front,
and fill the bladder solid with a thin sheet of water, to make the EPDM
last forever and allow a more vertical collector orientation to maximize
winter gain. 

Nick

---

From: [email-masked]
Date: Sat, 01 Jun 1996 10:24:53 -0400

> Thanks for all your input

OK guys, you want cheap?, Get the cheapest bare steel tank you can find.
Paint it black, set it in the yard (in the sun) and pipe the potable
supply into the bottom. Draw the hot out of the top to become cold feed
into the main dhw. Bingo, solar dhw. Drain exposed piping if you're
worried about freeze damage, or oversize them and insulate like crazy. In
the Outer Banks, if you use hot water every day, you make the system flow
at least once a day, you shouldn't have to worry about freezing. 

You want to increase the output? Enclose the tank, insulate the north
side, and glaze the south and top. More? Instead of a tank use oversized
pipe, like 4" to increase surface area exposed per gallon of water,
instead of paint, use a selective surface coating, keep the pipe as close
together as possible and either reflect the sunlight between pipes to the
pipes or attach metal fins with the same selective surface. The tighter
the mechanical bond between fin and pipe, the better the heat transfer.
This is called a batch heater and it works very well.

Dan

---

From: [email-masked] (Dan Doty)
Date: 2 Jun 96 00:02:05 -0700

Dear Nick:

I have been heating my hot tub with 200 ft. of black plastic pipe 
running back and forth across my black roof on the north west side of 
my house for 2 weeks. The temp comming off the roof at the hotest 
part of the day is 122 degrease at 3:00 in the afternoon. This is to 
hot for the hot tub at this time of the year. The best place to fine 
info about this is Home Power Mag. or there web site at 
http://www.homepower.com/hp or E-mail at [email-masked]

Dan
@ [email-masked]

---

From: dale <[email-masked]>
Date: Sun, 02 Jun 1996 06:51:26 +0100

Where are you located? 
And how much water per hour is passing through your pipe?

Also, what is the temperature at noon?, or perhaps 6:00pm or 
on a cloudy day?
This would be helpful for those of us who cannot get home 
power magazine.

Finally, What is the pipe made of? Can you say anything about 
the safety and durability of black plastic pipe? 

dale

---

From: [email-masked] (Nick Pine)
Date: 2 Jun 1996 01:23:01 -0400

>Dear Nick:

Hello Dan...

>my house for 2 weeks.

The northwest side... I wonder if you have a southeast roof you could use.

>The temp comming off the roof at the hotest part of the day is 122
>degrease at 3:00 in the afternoon.

Sounds like it works pretty well, in this season. It would be fun to try to
keep the tub warm enough to use in January. Seems like that wouldn't be too
difficult, if it had a good cover or moved to some place with a hot spring...
A small solar collector/water store built into the south wall of the base?

>This is to hot for the hot tub at this time of the year.

People like hot tubs around 104 F. There's a pretty narrow range... 100 feels
cool to me, and 106 feels very hot... It's odd that there's all that water,
but it isn't very useful for storing heat for a few cloudy days, because the
usable temperature range is so small. We might start with a $600 6' wide x
5' deep x 12' long 1,500 gal septic tank... If it drops from 106 to 104 F in
5 days, it needs 104 = 36 + (106-36)exp(-5x24/RC) ==> RC = -120 hr/ln(68/70)
= 4140 hours, ie 6 months. Wow. The septic tank has 324 ft^2 of surface and
12K pounds of water, so the R-value of the surface would have to be
324x4140/12K = 112, eg 22" of Styrofoam...

On the other hand, if it had 6" of Styrofoam or a strawbale wall around it,
it would lose about 5x24(106-36)324/R30 = 91K Btu over 5 days, which might
be supplied by 91K/50 = 1,800 pounds of water cooling from 154 to 104 F.
Another water wall on the south side, 12' long x 5' high x 6" thick? Where
I live, that south wall would get about 60K Btu/day of sun in December, of
which about 6hr(154-36)60ft^2/R2 = 21K would go back out through 2 layers of
glazing, on an average day, leaving more than enough to keep the tub warm...

>The best place to fine info about this is Home Power Mag or there web site at 

I remember a solar heater described their using lots of
black tubing in flat spirals on a roof...
Worked pretty well, as I recall.
And cheap, vs store-bought,
rooftop, solar
collectors.

Nick

---

From: [email-masked] (Dan Settles)
Date: Tue, 04 Jun 1996 16:48:50 -0500

Path:
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From: [email-masked] (Nick Pine)
Newsgroups:
alt.energy.renewable,sci.energy,alt.solar.thermal,alt.architecture.alternative
Subject: Re: Help with passive solar hot water storage
Date: 31 May 1996 17:59:33 -0400
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Okay Nick, 

I think you gave me more information than I need, or I don't quite
understand the calculations yet.  So, the water heater will have a surface
area of:
2 x (¼ r^2) + 2¼r x h = 2 x (3.14) + 2 x 3.14 x 6 = 44 ft^2  

In order to maintain the temperature at 130 F in a 70 F room it would take:

44ft^2 x (130-70F) x 24 hr /R8 = 7920 Btu

That would be 15.8% of the total if the 50K Btu figure is your total hot
water expenditure.  

Agreed :-)

     Yes, I thought there would be a change in the rate of heat transfer,
but as you can tell I have not yet mastered the required formulas.  I
guess I should try to find a good book to read more it this.  Have any
other suggestions?

     I guess your point is that heat transfer from the air in the solar
closet to the water will be slower than from the air to the BIg Fins?

Nick

     By the way, I received the copy of your paper last night.  Thanks
very much.
Dan

---

From: [email-masked] (Nick Pine)
Date: 5 Jun 1996 09:51:57 -0400

Dan Settles <[email-masked]> wrote:

>>How much energy does a typical water heater use just to keep the water hot
>>until we are ready to use it vs heating the incoming cold water?
 
>per day at 110 F, heated from 60 F.

Hmmm. Is there too much information here?
 
>So, the water heater will have a surface area of:

That's what I get...

>44ft^2 x (130-70F) x 24 hr /R8 = 7920 Btu

Looks good.

>That would be 15.8% of the total if the 50K Btu figure is your total hot

I agree. It looks like we didn't need to know that the water started out
at 60 F, or that we mixed the 130 F water with cold water to take 110 F
showers. The 50K Btu/day was enough, assuming (reasonably) that the water
in the heater was almost always 130 F, because it had a powerful heater.

Suppose we take a 10 minute, 110 F shower using water at 3 gallons per minute.
That will use 30 gallons of water, and a fraction f of the shower water will
be 130 F water from the water heater, where 110 = 130f + 60(1-f), so
f = (110-60)/(130-60) = 0.714, so 30x0.714 = 21.4 gallons or 171 pounds of
60 F water will flow into the water heater, which will take 171(130-60)
= 11,970 Btu of heat to reach 130 F, and a typical electric water heater
with a 5 kW element, ie 17,065 Btu/hour, will need 11,970/17065 = 0.7 hours
or 42 minutes to heat that water, which is a fairly small fraction of the 
6'x3.14(1^2) = 18.8 ft^3 or 151 gallons of water in the heater, most of which
will be at 130 F during that time. Heating the water will take 0.7x5kW
= 3.5 kWh, ie 35 cents for that shower at 10 cents/kWh.

>>I'm not sure how significant these factors would be, but it would be

You found 15.8% of the heat was used to keep the water warm, vs heating it, so
0.158x50kWh/24hr = 329 Watts or 1122 Btu/hr is the average power added to the
house over a summer day, ie 27K Btu, that might need to be removed from the
house by a 5K Btu/hr AC running 5 hours a day (PV-powered, of course :-) But
the plumbing system might have a lot to do with this. If the cold water
meanders through lots of pipe in warm parts of the house on the way to the
water heater, it may absorb a lot of heat from the house on the way there.
On the other hand, the shower water may cool a lot within the house on the way
out, via the drain plumbing. Charlie Wing did a TV program in which he built
a tempering tank for incoming cold water along a New England basement ceiling,
starting with a 10 or 20' length of 4-6" PVC pipe, and adding on fittings at
each end to connect it to 3/4" pipe...

>>>>= 3299 in^2 or 22.9 ft^2. How does this compare with the fin tube that you

[stuff snipped, on solar closet air-water heat exchangers] 

>>>229 Btu/hr-F, in one pass...

>a solution with an exponential in it, which you can find on page 3.4 of the
>1993 American Society of Heating, Refrigeration and Air Conditioning
>Engineer's Handbook of Fundamentals (the "ASHRAE HOF.")
 
>but as you can tell I have not yet mastered the required formulas.

The ASHRAE HOF procedure in this case, with 100' of pipe sitting in 130 F
water, might go like something like this:

1. Define the heat exchanger effectiveness as E = (Tco-Tci)/(Thi-Tci) where 
   Tco is the cold water outlet temp, to be determined,
   Tci is the cold water inlet temp, eg 60 F, and 
   Thi is the hot water inlet temp, eg 130 F. 

   This formula applies to the case where the cold fluid changes temperature
   a lot more than the hot fluid does, ie "Cc = Cmin."

   So, E = (Tco-60)/(130-60), so if the "effectiveness" were 1 (for some
   reason they don't call this "efficiency"), the cold water would emerge
   at 130 F. In reality, (perhaps :-) Tco = 60 + (130-60)E.

2. Find NTU, the "Number of Heat Exchanger Transfer Units":

   A substep: find the "heat capacity flow rate" for the cold water.
   At 3 gpm, this would be Cc= 3 gpm x 8lb/gal x 60min/hr = 1440 Btu/hr-F.

   NTU = AxUavg/Cmin = 22.9 ft^2xU10(?)/1440 = 0.159.

   (Adding more pipe raises this.) 

3. Find the capacity rate ratio, Z = Cmin/Cmax. In this case, I'd say the
   hot water has an infinite capacity rate, compared to the cold, so Z = 0,
   and in that case, according to the book, E = 1-exp(-NTU) = 0.147, so the
   cold water outlet temp would be Tco = 60 + (130-60)(0.147) = 70.29 F,
   almost the same as what we calculated with R-value arithmetic.

I wonder how this changes with 500' of pipe...

I think I did this right, although some ASHRAE engineer might find a mistake.
There are other formulas in the HOF for different situations, on page 3.4.

>I guess I should try to find a good book to read more it this.  Have any
>other suggestions?
 
A good high school or college physics book would help. One that talks about
heatflow. "Ohm's law for heatflow," aka Newton's law of cooling, or "the
fundamental law of heat conduction" is cleverly disguised with calculus
on page 552 of Halliday and Resnick's _Physics, Part I_, 1966. An older
high school physics book might use arithmetic. Check out a used bookstore.
Some old technical books are a lot easier to read than new ones. Cheaper too.
Basic heatflow physics descriptions haven't changed much in hundreds of years,
and the physics itself hasn't changed in millions of years. 

Take a look at the ASHRAE HOF, and try to understand it in small doses :-)
It's a fairly unbiased collection of practical heating knowedge gathered over
many years by a dedicated non-profit group of HVAC engineers without too 
many axes to grind, except to search for truth and systems that work, who
have been doing the practical work of figuring out how much heat houses need
in the winter, by adding up areas divided by thermal resistances of various
wall and window and roof materials, and multiplying by temp differences. This
R-value arithmetic really does work, altho the rules are sometimes confusing.
The ASHRAE folk tend to gravitate towards active mechanical systems with lots
of pumps and pipes and electricity, but R-values can be used to design high-
performance solar houses. R-values work, for predicting heatflows within the
reasonable temperature ranges where they have been measured.

Another good and less technical book is Charlie Wing's _From the Walls In_,
which reproduces some ASHRAE tables, and has lots of other good info inside
about beam strenths, etc. It's out of print, unfortunately.

Another in-print book is _HVAC Contracting_, a $24.50 Craftsman book by
Robert and William Dries, which has 30 pages on heat load estimation, and
lots of other practical stuff, like how to run a business.

And you might look at Duffie and Beckman's _Solar Engineering of Thermal
Processes_, 2nd ed, again in small doses, looking more at the graphs and
arithmetic than all the complicated calculus.

>closet to the water will be slower than from the air to the BIg Fins?

No, I just meant that if we put 500' of pipe in some of the drums, we may
find that the more significant bottleneck for heatflow becomes the path for
getting the heat from the hot air into the drumwater, via the limited drum 
surface surrounding the 500' of pipe. Say we used 4 drums, with a surface
of 100 ft^2 and an approximate U-value of 1.5. Then we have 150 Btu/hr-F,
which is a big resistor, electrically speaking, 75 times bigger, and in series
with our 500x22.9 Btu/hr-F for the pipe... So that would be the limiting
factor for heatflow, even if we could fit 10 miles of pipe inside the drums
to make the water-water heat exchanger almost perfect. One nice thing is
that the drum water has significant thermal mass, so its temperature won't
change much during a shower, so we can calculate the heatflow into the drums
over 24 hours, and not worry much about how that happens in the short term.

>By the way, I received the copy of your paper last night.  Thanks very much.

You're welcome. Perhaps you will build a bigger sunspace and solar closet :-)

Nick