[Greenbuilding] Solar Water Heater architecture

Sun Aug 13 23:19:31 CDT 2006

I put together a system like this a few years ago on a small farmstead which
had no power and a failure prone windmill for water. What I've found is that
there are (or were) no 12VDC differential controllers so a 12 volt pump
directly connected to a PV module was the easiest control available. My
client already owned a polyethylene tank so that was to be the heat storage
tank. I used propylen glycol from AAA Solar, diluted 1:1, in a loop of 1/2"
flex copper via a Hartel pump.

A few observations from this experience:

 1. At first I was concerned about getting the tank above the 140F (for
short periods) the tank is rated at, so I wired the PV to pump loop through
an electric water heater thermistat. The contactors are way big enough to
handle the DC current, it's normally closed and opens on rise when the water
temp hits the adjustable limit. A melt down could be a problem if you used
collectors in series or had too many sq ft of collector surface feeding a
too small tank.
     The first water that has a chance to sit in the heat exchange tubing
for a fair period, will be fairly warm, but once that water has cleared the
tubes a second wave of water comes through that is arriving at the
temperature of the ground water, heating up only as high as the heat
exchanger allows in transit...not cold but also not hot.  I thought that
once the water in the tank (250 gals) got hot, it would be like getting the
Queen Mary going...there would be a huge thermal inertia. However it's
gotten into the 90's, and still not hot enough to provide a delta T of more
than 40F. This would be all fine if my client was satisfied with a quick
shower but not if she wants to soak in a hot tub full. It would still be
worth while as a preheater.

2. The PV match is not perfect. In the early morning and late evening, the
pump will get going with the heat not yet arriving to the collector. My
solution is simple aluminum "blinders" so the the module will only see the
sun during the hotter part of the day. It may also be beneficial to face the
PV slightly west so it pumps also when the ambient air temp is higher.

3. I'm not sure what the difference is between ethynol and ethylene glycol.
Why can't we just use Everclear?  In some older cars (mine) ethynol works on
the gaskets and may do so on the pump gaskets. But at least it is made for
imbibing, in small amounts.

4. As someone mentioned, with a small 12 VDC  centrifugal pump, the lift is
balanced against the weight of the water returning back down to the tank.
Water is a polar molicule so it acts as a continuous chain, pushing up and
pulling on the downward journey. This is the only portion you have to be
careful to have no air bubbles in the line. You break the chain, and the
little pumps would have to do all the work pushing up hill. But if you used
a small positive displacement diaphragm pump, you could get by without being
overly concerned with small leaks that would leave a pressured system out of
operation. Yes, it would lead to a single walled heat exchanger but you
could pump directly out of the storage tank. A little ShurFlo would operate
for years on a $28 diaphragm. I would plumb the tube arriving at the
collector to the bottom of the collector so the entire collector was full
before it drained back out the top. Otherwise the fluid would tend to
trickle into the collector top, not effectively filling the collector
tubing.

Bill Dorsett
Sunwrights PV
Manhattan, KS

----- Original Message -----
From: Corwyn <corwyn at midcoast.com>
To: Lawrence Lile <LLile at projsolco.com>
Cc: <greenbuilding at listserv.repp.org>
Sent: Sunday, August 13, 2006 9:45 PM
Subject: Re: [Greenbuilding] Solar Water Heater architecture

>
> On Aug 13, 2006, at 08:22, Lawrence Lile wrote:
> > Getting ready to install a solar water heater in new construction, and
> > have been agonizing about the arrangement.
>
> Lawrence,
>
> Here is my current plan (not done yet so take with a grain of salt).
> Water panels on the standing seam roof, mounted with S-5 connectors.
> Propylene glycol (with water, and neon colorant (to detect leaks)) in a
> loop to the tank.  Probably an El-Sid or similar PV pump.  Large site
> built 500 gallon non-pressurized tank in the basement (possibly wood
> lined with glass).  Tank holds plain water (with a different highly
> visible colorant).  I am hoping that I will be able to detect a leak
> from solar loop to tank, or tank to water supply.  More coils of copper
> tube to pre-heat DHW, and heat the thermal mass (possibly also a hot
> tub to deal with summer excess).  The solar loop should only have a few
> gallons, so that much glycol shouldn't break the bank.  Wouldn't catch
> me with ethylene glycol anywhere near my drinking water.
>
> Open questions for me remain:
> Can I get dyes that can be detected for both kinds of leaks?
> Best method of building the tank (and ensuring that failures don't
> result in a flooded basement?
> Control systems?  I would like to be able to control movement of heat
> based on tank temperature, house temperature, time of day, outdoor
> temperature, season, occupancy level (and probably some others I can't
> think of right now).
> Cost?
>
> Thank You Kindly,
>
> Corwyn
>
> --
> Corwyn
> Kermit didn't know the half of it...
> http://www.greenfret.com/
> corwyn at greenfret.com
>
>
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