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mattie

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Reply with quote  #21 
It was part of my understanding that the fan speed dictated the heat exchange rate,another factor would be the flow speed of the water away from the heat sink fins to increase the temperature difference and boost the efficiency of the heat exchange .
The point in the first place was if space was a concern, also you could simply increase the numbers of PC cooling units like in a modular system.
Regards Mattie

stmbtwle

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Reply with quote  #22 
Yes no doubt enough additional cooling units could do the job. They would also increase the space required so we might well be back to square one.

I have thought that increasing air through the heat exchanger might help, but as the same fan circulates air through the collector and across the glazing, it might be counterproductive. Possibly a bypass?


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Willie, Tampa Bay
Garage_Hermit

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Reply with quote  #23 
er... just a thought: the French for "heat sink" is ... "radiateur"  (in many applications, at least...)

guess this demonstrates that there's often two ways of looking at the same problem [redface]

G_H
aka
ronny descartes
("heat sink, therefore I am...")

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(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
stmbtwle

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Reply with quote  #24 
There always is. A radiator is just a heat exchanger, and they work both ways. You can transfer heat from water to air, or from air to water. The only constant is that the heat always flows from hot to cold (downhill).

"Heat sink....." I love it!

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Willie, Tampa Bay
mattie

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Reply with quote  #25 
Ha ha GH good one,maybe you should consider a flat Plato collector to go with that [smile]
Garage_Hermit

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Reply with quote  #26 
Mattie, there U R Virgil on the ridiculous [biggrin]

G_H

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(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
mattie

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Reply with quote  #27 
My honest answer here Stmbtwle is that i need to look at the Aretha concept more before i can give a proper opinion.
Is there an issue where the collector size sets a limit on the amount of available energy ,relating to how quickly the air will heat?(Sorry im not as savvy on air based collectors).
Seems like you have to match fan rotation to suit this? Which in turn seems to impose limits on increasing the fan speed and air flow across the heat sink fins.
Unless of course the air heats really quickly and this is not a design concern.

When you say bypass do you have a sketch to illustrate your idea that will help me picture it in my head more clearly?

Regards Mattie
stmbtwle

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Reply with quote  #28 
Mattie...
There are several threads here on the Aretha, both by Paolometeo (in air collectors) and myself.   While Paolo is very cost oriented and has so far taken the "black box" approach, I feel that a metal absorber plate is better and like the dual pass.   As cost is ALWAYS a factor, who knows which is better in the end?  Anyway, it's easy enough to build a "black box", and put an absorber in it later if desired. I did.

I don't see where size is a limitation, as long as you can get the heat out of the system and into storage.   Storage is the key, and is probably a bigger investment than the collector itself.  However I don't think "maximum" size has been researched.

Generally I feel the more air one can pump through a heat exchanger the better the heat transfer, but it has been my feeling that increasing the air flow through the collector itself may increase heat loss through the glazing.  So my feeling is that a bypass that would allow one to increase air flow through the HEX without increasing it through the rest of the collector, might help.  Again, I haven't researched it. 

The holidays, cloudy weather, and my health have conspired to bring my experiments to a standstill.  When/if I will resume them I don't know. 


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Willie, Tampa Bay
stmbtwle

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Reply with quote  #29 
Re fan speed:

Air heats up pretty fast.  The way understand it, in a regular air collector, how fast depends on how much energy is hitting the collector and how long the air is in the collector.  So the faster the fan runs the less time the the air is in the collector, so the less it will heat up.  Lower temperatures are more efficient, BUT you need to achieve a usable air temperature.  Slowing the fan speed allows the air to stay in the collector longer, and get hotter.  Increasing the fan speed lowers the temperature. 

Arethas are a bit different, the air never leaves the collector.  So increasing fan speed just means you're moving the already hot air around faster.  It doesn't make much difference whether the fan is off or running flat out, without water flow the collector will get really hot really fast.  What controls the air temperature in an Aretha is not the air speed but the WATER speed, as the heat in the air is transferred to the water in the heat exchanger.  The faster you can pump this heat out (via water), the cooler the collector will run.  In my experience the air temperature is pretty much tied to the water temperature.   Because of the difference in specific heat, you really don't need much in the way of water flow.  My latest Aretha has a water flow of about 1 gallon/minute, and I only got about a 1F temperature rise (at 4gpm I couldn't measure it).  But like the air, the water is being recirculated.  That 1F every pass through the collector adds up.  Over time the water will heat up, and the air temperature in the collector will rise accordingly.

The heart of the beast is the heat exchanger.  Here I think is where higher air flow makes a difference, at least I know that reducing fan speed DOES tend to reduce the heat transferred to the water.  However Paolo seemed to think that the air flow against the glazing was reducing performance.  There is probably a compromise in there somewhere.

There's a lot of research yet to be done and a lot of room for improvement, but they DO work.

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Willie, Tampa Bay
Garage_Hermit

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Reply with quote  #30 
Nice summary, Willie !

and read with great interest !

I suspect that what comes out of this, is the need for variable speed fan AND pump...

This implies a central CONTROLLER, which would have a man in the loop, with a THERMOSTAT knob to turn...

From reading Paolo's stuff, he has gone this road already...

In the earlier post, you are right to insist on the need for STORAGE... and right again to state that it is the expensive element...

In my book, the storage would be determined/sized FIRST, according to the heating/cooling demand...

The collector is then built LAST, to meet the heating requirement.

So the LOAD takes priority (because it is fixed) (the load is a constant) and the generator (the collector) is sized according to it. (After all, this is how one would design any other engine (for a locomotive or a truck... or an aircraft...).

========
One other thing that strikes me, is that ARETHA is an air-cycle machine.
In this respect, it is REVERSIBLE...

precisely as you said at Number 25 above:

Quote:
A radiator is just a heat exchanger, and they work both ways. You can transfer heat from water to air, or from air to water. The only constant is that the heat always flows from hot to cold (downhill)


translation: (like any other solar collector, in principle...), it can be run at NIGHT, to provide COOLING...

just a thought !

G_H


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(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
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