[Greenbuilding] Solar Water Heater architecture

[Keith Winston]

I would stay away from ethylene glycol. Toxic. Yuck. Also, how much less 
expensive? Also, I've never heard that ethylene glycol never needs 
replacement, but that may be because no one uses it anymore, so I don't 
hear anything about it. I've always assumed it degrades like propylene 
glycol in similar circumstances (i.e. system stagnation).

You can get good solar-quality propylene glycol for $12/gallon, and then 
you dilute it 1:1. Also, check how much that tank is going to cost you 
to make, at the current cost of copper. Unfortunately, if anything your 
estimate of the cost of a pre-made double-coil tank is probably also 
low. which tanks are you thinking of, the only ones I know that are 
readily available (sort of) are Vaughn (I hate their heat exchangers, 
though I like their tanks) and Stiebel-Eltron (expensive, standard 
glass-lined tanks, but well-made and good company). I can't remember any 
others right now (except some open-loop tanks like Trendsetter, or the 
one that Tarm sells). How big a tank are you talking about?

PV pumps often won't work too well in drainback systems, since they 
don't have much power, and a drainback system has to overcome the 
vertical head/friction of the system all day every day, whereas a closed 
system doesn't (the fluid coming down counterbalances the fluid going 
up: the pump only needs to overcome the pipe & fitting head/friction of 
the system).

As to your icemaker scenario, it depends a lot on your collectors, how 
well insulated they are and how much thermal mass they have. But it is a 
colorful scenario.

With respect to your humidity issue, I know you'll like this: to reduce 
vapor escaping from tanks in industry, they sometimes use small plastic 
balls floating on the surface. They can reduce surface area by 78% or 
more, depending on size(s) and relative density.

Warmly, Keith


Lawrence Lile wrote:
>  
> Getting ready to install a solar water heater in new construction, and have been agonizing about the arrangement. 
>  
> I have 3 used collectors, and El-Sid PV pump http://tinyurl.com/echz6, and an idea.  
>  
> The El-Sid PV pump is ingenious - it is a DC pump connected directly to a solar panel.  When the sun shines, it pumps.  
>  
> Friend #1 has installed drainback systems, and swears they are pretty straightforward to install.  We have hard freezes around here, so the consequences of the drainback system failing are severe.  I have read that drainback valves installed outdoors can ice up, then fail to work, then the collectors freeze and burst.  I have also read that PV pumps may not work well in drainback systems.  Picture this:  It is 20F below zero, your collectors are nice and dry because they drained back last night when the sun went down.  The sun comes up, nice and bright, but your collectors haven't warmed up and are still 20 below.  The PV pump turns on, and becomes a 3GPM icemaking machine.  Maybe the collectors would warm up later and thaw out, or maybe the whole thing would just ice up and bust. Also drainback systems with open tanks can be a constant humidity source in your house, which would be bad in the summertime.  I'm trying to avoid more complex controls, considering compex controls to be the achilles heel of the solar colelctor. 
>  
> Friend #2 swears by glycol systems.  They can't freeze, period. But Ethylene glycol is poisonous, and propylene glycol, while not poisonous, is expensive.  Glycol in the collectors means that the foolproof controls on the El-Sid pump will be able to function.   
>  
> Originally I had planned to have about 50 gallons of propylene glycol in a loop between a solar storage tank and the collectors, with a single wall heat exchanger (a length of soft copper tubing) immersed in the solar tank that preheats cold water to the domestiv water heater.  Simple!  But that's $800 worth of Propylene glycol, and it has to be replaced every 5-10 years.  Yow!  
>  
> So I am imagining a different arrangement - still relatively simple.  A smaller glycol loop from the collector to a tank heat exchanger (another loop of copper pipe in the tank)  Tank contains plain water.  Domestic water also passes through the tank, through another length of copper pipe.  This achieves a double wall heat exchanger, which allows the more dangerous but cheap ethylene glycol.  
>  
> Now you can buy a tank set up to do this, for another $800 or more.  I was going to fabricate the heat exchanger tank from scratch, since it is a relatively simple system.  
>  
> This problem has been solved a lot of times before.  What arrangements do you folks recommend? 
>  
> --Lawrence Lile
>  
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-- 
Keith Winston
Earth Sun Energy Systems
3927 Madison St.
Hyattsville, MD 20781
301-980-6325
keith at earthsunenergy.com
www.EarthSunEnergy.com