[Greenbuilding] solar hot water

[Doug Kalmer]

It's drainback, draindown systems have largely been abandoned because the valves that were supposed to allow household water to drain out of the panels typically failed, and the panels plumbing froze and broke. I agree completely that solar water heating is cost effective, especially DIY. My PV pumped closed loop glycol system has been producing most of our hot water for 20 years now, built from an early 80's collector, locally built heat exchanger tank, and a Zomeworks matched PV panel and pump. http://www.builditsolar.com/Projects/WaterHeating/DougsSolarWater.htm Since I sized panel to tank to usage well, no heat dump is needed. An easy way to dump excess heat, assuming the typical situation where the storage tank is below the collector, is to use a "vacation bypass valve". Closed loop systems in the configuration I described above have a check valve to prevent nightime thermosyphoning, losing tank heat to the night sky. Just plumb in a full flow valve around the check valve, and the need for solenoids and running heated water down a drain is eliminated. Doug

www.builditsolar.com/Projects/SolarHomes/Doug/DougsProjects.htm


http://www.youtube.com/user/sundug69


Date: Fri, 7 Jan 2011 11:42:52 -0600
From: Lawrence Lile <LLile at projsolco.com>
To: "Environmentally-preferable design, construction,  building
       elements"       <greenbuilding at lists.bioenergylists.org>
Subject: Re: [Greenbuilding] one more pleasure (or not) of heating
       with wood
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Lynelle,

Greetings from Missouri, where we are treated to a month of clouds in a typical November.  Solar pre-heat can be one of the most cost-effective solar installations if done right, even here.

Good solar panels can boil water!  I've seen this happen, it is real.  They can definitely produce higher temperature water than you need under the right conditions.

There are two ways to get rid of excess heat in the summer, if you don't have the luxury of a hot tub:

1.  Don't make excess heat.  Use a drainback (or is it draindown?  I can never keep these two terms straight) system that leaves the collectors dry when heat is not required. The water drains into a tank when not needed.   This uses a more complex controller, but eliminates need for antifreeze in your collectors, even in Ontario.  This is really a very straightforward solution.  If I built another solar water heater this is what I would do.

2. Have a heat-dump circuit.  In a glycol (antifreeze) collector, the controls can be really simple - a little pump run by a solar cell.  Size the system so that in the summer it matches your typical usage, and it won't overheat under normal conditions.  But if it does overheat, then you will need a solenoid valve and a little control that dumps domestic hot water on the hot side of your solar tank straight down the drain.  If your system is sized correctly, this will almost never be used, so it is not a big waste. Dumping 10 gallons will cool off your solar storage tank a lot, and the heat was free anyway. I've wasted more than ten gallons trying to wash my dog.

3. Throw a tarp over the collectors when you are going on vacation, as I did before I installed the heat dump.  There is no good reason to do this except for desperation born of poor planning.

There are arguments pro and con for each system.

You always need a tempering valve after a solar water preheater to eliminate any risk of sending scalding water to someone in a shower.

If you can generate all of your hot water in the summer, and then 30% or 50% in the winter, you probably have a system that has the optimum payback.


--Lawrence Lile, PE, LEED AP, CEM

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