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heat exchanger options

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Posted by EricY on June 19, 2007, 5:12 pm
 
I've been building my solar hot water heating system and am stuck with
deciding on how to transfer heat from my drainback tank to the solar storage
tank for DHW.

Some info on the system:
192 feet^2 of collectors
Old 50 gallon water heater tank used for drainback tank (not sure how much
fluid I'll run in this yet - at least 20 gallons probably)
Another 50 gallon water heater for heat storage - this is connected to the
water supply
Water supply flows through the water storage tank, then on to the actual
water heater for the house, acting as a pre-heater

In the winter I will be using the system to supply heat for the radiant
heating in my shop floor (4" slab) which explains the fairly large amount of
collector area.  When used for heating I will dump as much heat as possible
into the slab since it can hold considerably more heat than any tank scheme
I've been able to design - thus the fairly small heat storage tank.

I'm considering two options:

Option 1) external homemade heat exchanger.  The high-head collector pump
would draw water from the drainback tank, send to the collectors and return
to the tank passing through one side of the exchanger.  Another pump would
circulate pressurized water from the storage tank through the other side of
the exchanger.  The exchanger would consist of 1/2" copper inside of 3/4"
copper in a counter flow design.  Probably 2x 3 ft sections in parallel to
slow the fluid for better heat transfer (is this better than one long
section?).

Option 2) cut a hole in the top of the drain back water heater tank and drop
a 25ft copper coil through which the heat storage tank fluid would be
circulated.  The tank is an older Rheemglas tank that has a porcelain enamel
lining.  Can I even cut this without damaging the entire tank?  I got the
tank for free so no huge loss there, but it's a good tank so I'd prefer not
to ruin it.

Thanks for any suggnestions!

Eric



Posted by Loren Amelang on June 19, 2007, 7:22 pm
 
On Tue, 19 Jun 2007 13:12:51 -0400, "EricY"

...

That doesn't sound like much area. With enough fluid velocity on both
sides it might sort-of work, but then you're going to spend a lot of
energy running pumps.


That sounds more like it. Cutting the tank will for sure mess up the
edges of the protective coating, but since it will hold only the same
"dead" water with no fresh oxygen being added, that shouldn't reduce
its life much.

However...  Consider Option 3:

Forget the drainback tank, run propylene glycol, make a loop of the
collectors, a small external exchanger leading to your storage tank,
then your underfloor tubing, and back to the collectors. That way the
floor heat doesn't need to cross any exchanger. You could even arrange
bypass valves to take one or the other sink out of the loop. Or, if
you turn off the little pump that circulates your tank water through
your exchanger, all the heat goes to the floor anyway. Plus you don't
need the high-head pump to lift water out of the drainback tank (BTDT
and hated listening to it).

The "solar" antifreeze market was a ripoff jungle last time I looked,
but around here you can buy Prestone "LowTox" propylene glycol at
AutoZone stores. I've had no problem using it for solar systems.

Loren

Posted by EricY on June 19, 2007, 8:41 pm
 

I agree - it's not a lot of area.  I wasn't not sure what to expect with
performance with such a setup.  The idea was to run both pumps off the diff
controller until the storage tank reached temperature.  I also considered
using 3 tubes - 1", 3/4", and 3/8", but by the time you get all the fittings
for such a contraption you may as well buy a commerical exchanger.




I seriously considered option 3 when I started designing this thing.  I
steered away from the pressurized collector loop for several reasons:
1) Fear of overheating in the summer when I didn't need the heat
(unwarranted?).  It gets hot here - right now it's 98F at 4:39PM.
2) Dealing with maintenance of the glycol (doesn't it need to be flushed
periodically?)
3) Refilling of the system if I need to reconfigure - I can just dump the
water as opposed to draining and refilling the glycol  Again, maybe
unwarranted but I like to tinker so

As for the floor running through the heat exhanger, I am planning on sending
fluid directly from the slab to the collectors since this would likely be
the coldest part of the system.  Fluid would return to the slab via the
drainback tank, but through a tempering valve to prevent the slab water temp
from getting too hot.  In the summar a bypass assembly will take the slab
out of the loop.

The panels are on my detached shop (insulated pipe runs have been installed
to and from the house for DHW) that only gets used on weekends and at night
so the noise doesn't bother me.  I've got it running now just heating the
drainback tank to see what kind of heat it will generate and so far the
noise isn't bad.  I put a horizontal section of pipe in the return just
before the fluid enters the tank and that helped reduced the noise.

Thanks very much for your advice and insight - I might go ahead with the
coil approach.  It would certainly be quicker and easier than sweating
together the heat exchanger!

Eric



Posted by Loren Amelang on June 20, 2007, 6:19 pm
 On Tue, 19 Jun 2007 16:41:50 -0400, "EricY"



Ah. A serious advantage of drainback. Provided all of the water does
drain back by gravity. The problem of no circulation through a
pressurized system on a hot day is serious enough that I have a
redundant circulator on an independent thermostat ready to take over
if the main pump fails. I keep imagining an opaque emergency cover
that would be released to roll down over the collectors if they began
to overheat, but haven't built one yet.

My solution to the summer/winter difference is to aim the collectors
at the winter sun. In summer, they produce about the same amount of
heat, since they are so far off-axis.


In a car it does, but mainly to ensure a fresh supply of corrosion
inhibitors for the aluminum parts. The glycol itself lasts pretty much
forever, and with the right tool you can measure the state of the
corrosion inhibitors and add more as needed. In an all copper solar
system they aren't that important.


Yes, refilling a pressurized system can be a pain.


Clever. I didn't get that from your first post. So the exchanger is
only for the DHW...  

Loren

Posted by Eric Yancey on June 21, 2007, 1:27 am
 

Thanks for the info!

Yes, the exchanger is only for the DHW.  I expect to get the greatest
payback on heating my shop.  I calculate that the collectors can raise the
temperate of the 24x32 slab a few degrees on a good day of heating, so
hopefully with a good week of sun the shop will be reasonably warm.  No
miracles expected here, just putting a dent in the bill will be a big plus!
I'm not in the shop that much and there is a good deal of latency heating
that much concreate (about a day or so to bring to 65F ambient temp).  The
latency will still be high but I'll basically be getting free heat so I can
keep it heated continuously without worrying about the electric bill.  Plus
with the mass of the concrete hopefully it can coast through the cloudy
days.  That's the theory anyway - we'll see how it works in practice!

Eric



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