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Old 12-23-13, 10:44 AM   #1
sunspot
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Default minimum solar thermal collector slope

Apologies if the answer is prominently posted in bold somewhere but I've searched to no avail so have to ask. What is considered the minimum slope to avoid stagnation? Are there ways to overcome stagnation if the minimum slope can't be achieved? Perhaps a lower set point on the collector inlet temp signal in combination with a tempering valve to bypass the thermal storage tank until operating temps are reached thinking out load a this point.

A recently started renovation gives me the opportunity to plumb for a future collector if it's feasible.

Thanks everyone.

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Old 12-23-13, 11:06 AM   #2
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First, I think we are talking about a solar hot water collector. Secondly, I believe you are talking about a thermo siphon system where the storage tank is located above the collector.

I think you are asking about the slope of the collector with a vertical collector allowing the best thermo siphon operation. You are correct that a flat collector, with a passive hot water collector plate does exhibit water flow stagnation.

Can you offer some additional details so we can understand better?

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Old 12-23-13, 11:21 AM   #3
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Avoiding stagnation can be done a few ways. First, is to angle the panels at 80-90 deg, next is to have panels with low terminal temps anyway. For example, most German panels (Viessmann for example) have highly selective absorbers such as "sunselect" or "bluetec". These panels can get up to 200C+ if left in full sun.

Thermo-Dynamics from N.S. has two different panels (as does Viessmann) with the highly selective surface or a painted surface. The difference in stagnation temps could be as much as 40C.

The other thing to think about is the insulation. The better the enclosure the higher the stagnation temp. Many people will decide to have a moderately insulated (lesser cost) case and more panel area to get more total heat out.

Most good controllers will limit the pump operation to collector temps below 130C as the pump is typically not approved for temps above that. After that, the collector will steam the antifreeze out of the panel which will then go into the expansion tank. If the panel temp can be limited to below 100C by angle and a large or multiple tanks, stagnation will be kept to a minimum. Good glycols, made for solar, are designed to take the high temps but standard glycols are not.
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Old 12-23-13, 05:01 PM   #4
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Thanks for the input so far. I see I was waaay short on detail.

I'd like to go with a drainback (similar to the BuilditSolar $1K) system with the water level in the tank ~ 12" below the bottom of the collector and roughly 40 ft away. This will be water only, no glycol. The thermal storage tank will be ~ 500 gallons. There will be other heat input to the tank the collector in question just an opportunistic addition if it's possible. The collector would be homebuilt of copper pipe/aluminum absorbers.
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Old 12-23-13, 05:34 PM   #5
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You don't really have to worry about stagnation with a drainback system. Fluid will not be in the collectors when the system is not on. Slope in a drainback is to ensure water can easily flow back to the tank so it is not left in the collectors to freeze.

I had a very similar situation with a long pipe run and antagonized on what slope I could get away with. It is commonly recommended to have a minimum of 1/4" drop per foot of run for drain back systems. At 40' , you'd only require 10" of drop to adhere to that.

There are a few sources that indicate you can get away with less. Dr. Ben suggests a slope of 1" over 20' Solar Water Heater Design (9 minutes into video 1.1).

This source even mentions completely level collector installations: Plumbing Engineer - Features: March 2011: Drainback Solar Hot Water Collection Innovations (see last paragraph). I would be careful with that though as construction tolerances could put you on the wrong side of level.
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Old 12-23-13, 05:50 PM   #6
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Should also have mentioned it's the angle of the collector itself I'm concerned about. Optimum is generally latitude plus 10 or 15 degrees to bias performance to the lower winter sun angle but in this case I'd be closer to 20 degrees above flat. If it would work summer DHW preheat would be taken care of.
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Old 12-23-13, 07:23 PM   #7
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Properly constructed panels of copper and aluminum should be fine with any tilt.

Selecting a tilt is more a matter of optimization of annual vs winter collection of energy. See graph 2 here: Optimizing Thermal Collector Tilt Angles | SolarPro Magazine for a general idea of the effect of collector tilt on annual production.

Panels at 20 degrees will experience more direct sunlight in the summer. Panels at 45 degrees will still experience direct sunshine, just later/earlier in the year.
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Old 01-07-14, 09:00 PM   #8
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After a few rounds of wrestling with the frozen Pex the tubes now reach to where the future solar thermal storage tank will sit. Hoooray!




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Old 01-08-14, 08:57 AM   #9
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Nice progress. Are you planning on just one panel? What other means of heating the tank will you be using? Any more details on the project? It sounds interesting.
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Old 01-08-14, 09:29 AM   #10
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Quote:
Originally Posted by Daox View Post
Nice progress. Are you planning on just one panel? What other means of heating the tank will you be using? Any more details on the project? It sounds interesting.
There'll be multiple panels and a woodstove contributing heat to the tank. I started the tank a while back. The plan is to fiberglass the inside (vinylester resin) and if all goes well (fingers crossed) get it in place this summer. It will have to be disassembled to get it through doors and reassembled in place.


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