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cwwilson721

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Posts: 303
Reply with quote  #11 
Myself, I can't use a passive thermosiphon system. I have no place to mount a tank higher than my collectors. So I had to go with an "active" system, as far as water goes.

I have some air thermal systems for daytime heat, that I'm still doing MAJOR tinkering with (mostly for heat control in daytime vs night time operation)

As I said, every situation is different.

Bradley

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Posts: 9
Reply with quote  #12 
Great response, I do get the idea and ramification of not planning the correct system from the right site. You have given me a lot to think about. But having said that if i have a closed loop passive system does that not alleviate a lot of those concerns you stated? I do know the real challenge of this systems is placement but outside of that what are your thoughts? BTW i live next door in Flowery Branch, GA 
Bradley

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Reply with quote  #13 
I guess my real question is from your experience does a passive system alleviate heat and stagnation concerns?
cwwilson721

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Reply with quote  #14 
You can still have stagnation events, whether the system is passive or active.

Anytime you have any type of a solar thermal collector, you stand a risk of a stagnation event happening (temperatures get way too high in the collector). The temps get too high, they are not lowered for whatever reason, and you, eventually, will have an event. 

What damage that results depends on the materials used to construct the entire system.

Ex: A metal-tube/fin, flat plate, glass-glazed, metal framed collector would be MUCH less vulnerable to a stagnation event causing damage to the collector itself. But, if you have CPVC piping carrying the liquid from the collector to the storage container, that portion can be affected, and damaged, while the collector itself is perfectly fine.

With that being said, collector positioning, materials selected, and type (passive or active) do play a part in avoiding any issues.

If, say, your collector is mounted vertical, it REALLY reduces the chance of an event happening, because the heat inside the collector can usually escape somewhat.. A passive system also helps, IF the storage temps are adequate (ex: If your storage tank is at 195F, good luck!), and the heat has somewhere to go, that would help avoid an event.

Remember my car example? Of the closed windows? That is 'stagnation'. A collector is a glass covered, insulated box that is made to collect heat and stop it from escaping, as compared to a car, that is not, so heat escapes rather more easily than a collector.

It mostly comes down to:

How much risk are you willing to take, as compared to cost/difficulty of the build?

It's all a balancing act.

None of this is to "scare you off". It's all just things to keep in mind as you design and build your system. Since you are also in the South, summer temps get a bit high. It's just something to keep in mind.
Bradley

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Posts: 9
Reply with quote  #15 
I do appreciate all of your insight it has been very educational. Thanks Again, Brad
netttech

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Posts: 720
Reply with quote  #16 
Bradley,
I know I'm answering this question late. Hopefully you will still see it.

I built a CPVC hot water system last year. Suggestion: make it with verticle runs, not horizonal.

I had a freeze-up last winter & my horizonal CPVC pipes overheated (180+) & sagged. There's no way to predict those kind of events & was lucky it occurred on a weekend when I was home. It didn't leak after thawing the pipes, but it sure sagged big time.

Personally, I'm not big on passive systems (hot water  or air). You only have a few hours per day of good sun, grab as much heat as possible.

Jeff
Central IL
stmbtwle

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Reply with quote  #17 
Would an active collector allow you to build smaller, too?
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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
netttech

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Posts: 720
Reply with quote  #18 
Smaller panel...no. You still need whatever size the panel needed to be to collect the amount of desired BTU's.

Using my active window panel as an  example, it's 2'W x 6'T x 8"D. Is it better to have a panel produce 125 degree hot air, flowing 10 cfm passsively or 100 degree hot air at 20 cfm?

You will pump more hot air/btu's with the active panel, in 1 hour than the passive panel.

(These temperature numbers are just examples & not exact.)

If the panel was half the size, it would be too small to sustain the same heated airflow being active.

Passively, the smaller panel may actually produce the same amount or slightly less heated air, but likely at the same airflow.

The key is the amount of heated air in the terms of CFM's.

Jeff
Central IL
cwwilson721

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Posts: 303
Reply with quote  #19 
Quote:
Originally Posted by netttech
Smaller panel...no. You still need whatever size the panel needed to be to collect the amount of desired BTU's.

Using my active window panel as an  example, it's 2'W x 6'T x 8"D. Is it better to have a panel produce 125 degree hot air, flowing 10 cfm passsively or 100 degree hot air at 20 cfm?

You will pump more hot air/btu's with the active panel, in 1 hour than the passive panel.

(These temperature numbers are just examples & not exact.)

If the panel was half the size, it would be too small to sustain the same heated airflow being active.

Passively, the smaller panel may actually produce the same amount or slightly less heated air, but likely at the same airflow.

The key is the amount of heated air in the terms of CFM's.

Jeff
Central IL

That also goes for hydronic systems, too.

The size of the panel determines the amount of BTUs you can harvest. Active or passive just describes the "delivery" of those BTUs.

In passive, there are no "outside" means of moving the BTUs.

Passive is "all-internal", i.e.: No fans, pumps/etc. you rely on heat convection to move your transfer medium (air or water)

Active=extra "costs" in terms of energy used. Those costs can be electrical, or cost-to-build.
Passive has no extra "costs", but you are limited by the design of the system. An example: Using a passive water collector, you need to have the tank above the collector, and the inlet for the supply water has to be lower than the hot water return, for passive thermosiphoning to work. Same with a passive air system. 

The major advantage of a active system is that you can move the heat gathered to any location, within reason.You can have a water collector on top of your roof, and the tank in the basement, 30 feet below it. But, you have to "pay" for the energy to pump the water/etc

Major advantage of a passive system is it just "works". You can have a water collector on the ground level, and the tank on a higher floor, for example.

There is no difference in the size of collector, tho. If you need 12000BTUs, it will require the same size panel, whether active or passive. 
stmbtwle

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Reply with quote  #20 
So the gain isn't in efficiency as mich as convenience?
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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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