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Old 07-29-2015, 10:59 PM   #1
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Solar Hot Water Queries/Concerns

Ok, so here's my plan:
I've got a 20 gallon electric hot water tank and im planning on adding a solar hot water panel to the roof of my bus. I want to use a solar-powered pump to pump the water through the panel while the sun's out. This is a kit I was looking at:
http://www.amazon.com/Complete-Panel...ot+water+panel

If I don't buy that, I might build my own version of the same design and concepts. I have a couple concerns though:

Imagine the sun is out on a warm day and the pump pumps the water through the panels until the tank is at it's max temp. Let's assume I have a t-stat to cut the pump off at this point. Will the water remaining up in the panels not boil and cause an overpressure/temp situation in my hot water system>? Or am I stupid and it would just drain back into the tank when the pump turns off? The system sits at 50psi from my water pump, but this doesn't override gravity if the system is closed....still if the water drains from the lines in the panels, what is it replaced by? air? from where?

Someone help me.

Also--anyone consider using a solar hot water heater to heat a working fluid and pump that through one of the original underseat heaters during cold sunny days? CO winters are full of those and it would be cool to have heat during the daytime this way...I could just paint the bus black in the winter, lol
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Old 07-29-2015, 11:48 PM   #2
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Join Date: Feb 2015
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We do have very sunny winters here....but they kill newbies, everyone stay away for CO! Hehe, just kidding....kind of.

Anyways, I work next door to a large car wash that gets all of its hot water from panels on the roof, I'll ask tomorrow what he does on a sunny day with no car wash action. Heck you probably know the area! Dayton and Arapahoe?
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Old 07-30-2015, 02:06 PM   #3
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Neat! That sounds nice. Check out builditsolar.com (if you haven't already) for lots of insight, advice, and ideas about solar water heating.

I would think the only concern is to prevent the heat collectors from "overheating." The threshold for that depends on the temperature limits of the materials. Some designs might use lower-temperature plastics etc and require protection. For one system it might be a cover over the collector to reflect away unwanted energy. Perhaps an adjustable rack could function similar to the cover by changing the collector to a less-optimal angle to limit its collection. For another system it might be a "dummy load" -- just some kind of radiator outside maybe with a fan to help dump the excess heat energy into the air. Other designs might be made so that even if they stagnate (no water circulation) in full sun they'll get blazing hot, but the materials and construction are designed to withstand the heat. Some means of draining the water out to prevent boiling (or a higher-temperature fluid) could also be needed, as well as pressure relief. In the case of a drain-back system, I would guess the tank has to be vented so that air can be let in at the top of the collector, which allows the water to drain back into the tank, where air must now be expelled to make room for the water. When the pump runs again the collector will be filled with water while the air in the collector is pushed back to the tank. I can think of a way to make a sealed drain-back system, but it seems complicated.
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Old 07-30-2015, 06:23 PM   #4
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Yeah, I think I need a sealed, drain back system. I would like the system to have as many passive safety/over temp/pressure protections as possible. I'd feel a lot better not having to rely on a dummy load cooled by a fan. There is a lot of time I'm away from the bus and if something blows, I dont want to come home to a wet bus!

I need a sealed, pressurized, drainback system. hmmm What are other people doing though? I imagine with the kit like the one I linked to above, there has to be some form of protection.
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Old 07-30-2015, 08:12 PM   #5
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I like hot water heating, and hate forced air.

I would love to see this done on a bus.

Please keep us in the loop, and take lots of pics.

Nat
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Old 07-30-2015, 08:29 PM   #6
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Alright, so... for drain-back, I'm thinking there has to be a way to admit air at the high point on the collector (after the circulator is shut down, of course) so that gravity can draw the water out and air into the collector. Later, when the air entry is closed and the circulator runs again, the air will be flushed out of the collector eventually into the water tank, and normally one would think that air would need to be vented out of the system -- either by "burping" a closed system, or by having the tank open to the atmosphere.

What if the tank were closed and only partially filled such that there's enough air held in the tank for filling the collector? A tube from this tank air space up to the top of the collector could allow air from the tank to rise into the collector while water from the collector falls back down into the tank. The system could remain sealed. This requires a third tube going to the collector, a valve, and a way to ensure that the air tube stays above water level inside the tank. Is a simple check valve good enough for controlling the drain-back? The pressure from the pump would have to close the valve so that water doesn't short-circuit around the collector instead of going through it.

What do you think? Go ahead and pick it apart for me. Remember too that this only addresses the boiling problem; the collector still has to be made to withstand the high temperatures it'll encounter when there's no water flow carrying the heat away.
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