Subject: Hot water for storing heat

From: [email-masked] (Steve S)
Date: Mon, 19 Aug 1996 23:51:42 -0600

I've been thinking about storing heat (for home heating) with hot water.
The question is how. The 1500 gallon polyethelene stock tanks are cheap
($250-$300) but the manufacturer says they're only good to 120F, which
doesn't give you much useable heat storage. So, two questions...

1. What are some good high temp (say, 180F) tank options?

2. What would be the lowest temp that water is useful for heating,
assuming forced air heat and an in duct heat exchanger. Is 100F a good
number to calculate from?

Thanks,

Steve S

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From: [email-masked] (Nick Pine)
Date: 20 Aug 1996 08:56:41 -0400

Steve S <[email-masked]> wrote:

>($250-$300) but the manufacturer says they're only good to 120F...

I wonder who makes them, and what shape they are? I see a few in the
Jade Mountain catalog, but they are more expensive.

>1. What are some good high temp (say, 180F) tank options?

A concrete septic tank? A site-built ferrocement tank, as described in
S B Watt's book? A plywood box lined with a single piece of EPDM rubber?
Some plastic 55 gallon drums full of water?

>number to calculate from?

How much heat does your house need, and what would the water temperature be,
after a few days without sun?

The 2' x 2' Magic Aire SHW 2347 duct heat exchanger transfers 45K Btu/hour
between 125 F water and 68 F air at 1400 cfm, with a 0.1" H20 pressure drop.
A smaller water-air temperature difference would probably make proportionally
smaller heat flow, eg 100 F water and 68 F air might make 25K Btu/hour. 

Nick

Then again, you might store water in one of these...

               6" diam windmill        -----
                      ^
                    .   .
                  .   .   .               |
                .   .   .   .            3m
inner cone   ---->.       .   .           |
white inside.   .           .   .
          .   .    ------  -->.   .<-- 7 in
        .   .     | slot |      .   .
      .   .        ------         .   .<-- outer cone, black outside
................................................
        |            2.5m           |

Two 22 gauge Gl sheet cones were made and assembled as shown in the picture.

A 6 inch diameter fan windmill was fixed at the top of the outer cone as shown.
The solar heat (2.5 x 3/2 = 3.75 kW solar insolation) made the outer cone
66 C and the inner cone was 33 C. Four adjustable slots 4.5 in x 30 in were
made in the outer cone to obtain maximum flow rate of convection current to
turn the windmill. The temperature difference between inlets and outlet was
27 C. The following results were obtained:

    Free running, 2700 rpm      67 Watts, 970 rpm     92 Watts, 390 rpm

Mirrors were added to increase the insolation to about 5 kW. This produced
dT = 45-50 C. Now velocity increased to 13 m/sec (29 mph), 3 times the
previous figure. Output power was 250-300 W. This indicates an efficiency
of 250/5000 = 5%.

If we scale up to a (30 square meter = 20 kW average insolation) 300 square
foot dome house we should be able to see 250 x 20/5 = 1000 W, our expected
power output. Putting an insulating plastic film over the Gl sheet should
increase the output. 

Nick

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From: [email-masked] (Dan Settles)
Date: 20 Aug 1996 18:07:26 GMT

> Some plastic 55 gallon drums full of water?

Hi Nick,
I keep forgetting to ask about this, but why do you use 130 degree F as
your target temperature?  Is this the maximum safe temperature for the
plastic in the collectors or in the 55 gallon drums, or is there some
other element in the solar closet design that doesn't hold up well to
warmer temperatures?  

Good luck with the youth hostel project.  If I lived a couple hundred
miles closer I'd come over and volunteer some time.  I look forward to
hearing about your design and the results.

Dan Settles

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From: [email-masked] (Nick Pine)
Date: 21 Aug 1996 16:49:35 -0400

>Hi Nick,

Hi Dan :-)

>your target temperature?

Howard Reichmuth, PE, who more or less invented the Copper Cricket, the
Ecotope (Pragtree Farm) parabolic concentrating greenhouse, etc, told me
that it would be pretty tough to get any higher temp than that. Another
good solar engineer, Fred Dubin, PE, from NYC of all places, told me the
same thing.

Let's see. Full sun is about 300 Btu/ft^2/hr = 0.174x10^-8x(T+460)^4
for a non-selective surface. For a selective surface with emissivity E,
this would be
               300 Btu/ft^2/hr = 0.174x10^-8xEx(T+460)^4 

...so the surface temperature of a layer of dark paint in the sun with E = 1
would be 184 F, I think, if it were insulated on the shady side, and had no
cover in front, and the surroundings were cold, and the only heat loss were
by radiation.

I suppose the surface temp of a layer of glass would be about the same,
if it had a still airspace underneath, then the black paint. The glass would
just heat up like the black paint did, until the re-radiated IR power equalled
the solar power input, if the incoming solar power had no other place to go. 

If the surroundings were at temp Ta (F), eg 30 F, not absolute zero, and
the plate were a black body, this equation would become

      300 Btu/ft^2/hr + 0.174x10-8(Ta+460) = 0.174x10^-8(T+460)^4 or
      300 Btu/ft^2/hr + 0.174x10-8(30+460) = 0.174x10^-8(T+460)^4, or
      300 Btu/ft^2/hr + 100 Btu/hr         = 0.174x10^-8(232+460)^4,
                                                         ^^^
ie the plate temp would go up from 184 to 232 F, because the plate
would be heated by the surroudings as well as the sun.

If the plate had an emissivity of 0.7, we might have

      300 Btu/ft^2/hr + 100 Btu/hr = 0.174x10^-8x0.7(297+460)^4.
                                                     ^^^
If we moved some air under the glazing, and heated some drums full of 120 F
water inside a solar closet, with the solar closet air heater glazing exposed
to the inside of an 80 F sunspace, we would have an average glazing temp of
Ta = 100 F, and with a glazing emissivity of 1 (glass is 0.88), we might get
a useful heat output of approximately Q, where 

   300 + 0.174x10-8(80+460)^4 Btu/hr = 0.174x10^-8(100 F + 460)^4 + Q, or

   300 + 148 Btu/hr = 171 + Q, so Q = 267 Btu/hr,

and the 171 Btu/hr of heat lost from the closet glazing surface would go
into the sunspace air, which would heat the house. Remove the sunspace and
expose the solar closet air heater glazing to 30 F air, and the closet glazing
temp becomes (120+30)/2 = 75 F, so we get

   300 + 0.174x10-8(30+460)^4 Btu/hr = 0.174x10^-8(75 F + 460)^4 + Q, ie
  
   300 + 100 Btu/hr = 143 + Q, so Q = 257 Btu/hr,

and the 43 Btu/hr of net loss from the glazing is lost to the outside air...

At this lower temperature, the convective loss from the glazing may be larger
than radiation loss, especially since the outside air will be moving, vs the
fairly still sunspace air. Let's see: if the closet glazing is like a window,
with moving 30 F outside air on one side and still 120 F air inside, it will
have an R-value of about 1, so we might figure the convective loss as

   (120-30)1 ft^2/R1 = 90 Btu/hr, vs 143 - 100 = 43 Btu/hr, net.

At what temperature does convective loss equal radiative loss? 

              (T-30)1 ft^2/R1 = 0.174x10^-8((T+30)/2+460)^4 - 100

At T = 80 F         50        >             122             - 100          
      100           70   about the same     171             - 100
     
At about 100 F, in this case. So if the air heater temp is more than 100 F,
we might use the radiation formula, and if it's less, we might use the R1
arithmetic. Or we might combine them, in a big complicated quartic equation,
with U = 1.5 + V/5 as the U-value for the airfilm on each side of the glazing,
with moving air at V mph... Mathematicians might enjoy that.

Iron, copper and tin oxides are somewhat selective (E ~0.7), and darker
than aluminum oxide. I wonder how to make galvanized iron rust in less
than 20 years... :-) H. Tabor wrote about slightly more complicated ways
of making galvanized iron selective.

>Is this the maximum safe temperature for the
>plastic in the collectors or in the 55 gallon drums,

I don't know. I'd guess the max is higher. Depends on how high you stack 'em.
I did try filling up a plastic paint pail with a lid with 130 F water, and
jumping on it, when Demon [email-masked] told me that was stupid :-)

>or is there some other element in the solar closet design that doesn't
>hold up well to warmer temperatures?  

I think it's worth using a small fan to make closet collection more efficient,
and my favorite Grainger fan for this, the 4C688 560 cfm fan, 10" diameter,
36 Watts, about $60, has an upper temp limit of 149 F.

I'd use polycarbonate vs polyethylene glazing for the solar closet, because
it is almost as good as glass at blocking longwave IR, 1% vs 77% transmission
for 80 F IR... And putting some black window screen under the glazing reduces
IR loss.

>Good luck with the youth hostel project.

Thanks. The only thing that's holding it up at the moment is a possible
administrative nightmare. We are meeting with architect Eric on Sunday (who
recently installed all the single pane windows in the cabin), who will be
doing a lovely "rendering" for the project, as a part of the admin smoothing
process. What we build, of course, might be quite different :-)

>If I lived a couple hundred miles closer I'd come over and volunteer...

There's a place to stay :-)

Nick

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From: [email-masked] (Solardc)
Date: 4 Sep 1996 05:40:47 -0400

>1. What are some good high temp (say, 180F) tank options?

60 mil EPDM rubber in a box tank lined with urethane foam, then lined with
the EPDM.
>number to calculate from?
 Yep, just spec that when you order your coil. (I can get you one if you
want). Better yet, use under floor radiant heat. No wasted electricity on
that blower. Actually, Any water temp higher than your room temp will give
some heat. Don't forget DHW heating, which is a significant load (20
gal/person/day). Useful temp there starts from  what ever your ground
water temp is (= to avg year-round temp. )

Steve Pitney
Alternate Energy Inc
800 DCSOLAR Fax 888DCSOLAR
http://home.aol.com/solardc

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From: [email-masked] (BCooley102)
Date: 7 Sep 1996 18:31:53 -0400

much more thermal enegry can be stored in phase change than
heating/cooling
and the temp is more consistant. Remember, it takes 144 BTU to melt a lb.
if ice
@ 32deg F, and the same 144 BTU would raise a lb. of water @ 32deg F to
166deg.
A good phase change for storing heat for domistic use is parifin (wax) it
melts (fuses)
@ about 180deg F. So 1 lb of molten wax @ 180deg F will give up over 60
BTU and
still be 180deg F.  This means that if you calculate you need to store
60,000 BTU
of heat (as apposed to cool, like air conditioning) using water, you need
1000 lb
of water heated to 180deg (from 120deg) and the output temp to your
exchanger
would drop lineraly as you depleted the stored heat. the same 1000 lb of
parifin will
produce about the same amount of thermal storage but the output temp of
the transfer
will stay a constant 180deg. 

Bryan Clifford Cooley
If guns are outlawed only POLICE will have guns!
(Who has killed more? murderers or tyrants)