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Garage_Hermit

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Reply with quote  #11 
well, I certainly would not like to start an argument on the issue, but based on one common airflow formula, namely

BTUH = M x sp.ht x delta T

I can't see how the fan *COULD* work, because M (air mass flow) is basically ZERO: to my admittedly inexperienced eye, the volume of that collector case looks to be around 50 cu.ft maximum (it is stated to be 5 square meters, and I guess it is around one foot deep).

So if 1 cubic foot of air at standard temperature and pressure (assuming average composition) weighs approximately 0.0807 lbs, then the mass of 50 cu.ft of air would be 4 pounds.

As that air gets hotter, its density will DECREASE, so even less efficient.

This is aside from another aspect, namely system effect:

Fans go with DUCTS, and in the present design, there is no duct...

http://www.captiveaire.com/MANUALS/AIRSYSTEMDESIGN/DESIGNAIRSYSTEMS.HTM#Types_of_Flow

Finally, as my last word on the issue [rolleyes] let's try comparing the proposed pneumatic system with an hydraulic system:  how well would the pump operate if all the valvework were CLOSED ?

Answer: no flow = no performance.

G_H

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paolometeo

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Reply with quote  #12 
Quote:
Originally Posted by Garage_Hermit
interesting approach,

however, where is the air intake and exhaust ?  i.e. how does the air circulate ?

G_H

The air is pushed through the radiator from the fan side, see the sketch. The fan produces a turbulent flow that involve all the box.
paolometeo

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Reply with quote  #13 
Quote:
Originally Posted by SolarInterested
The fan would also be circulating hot air against the glazing promoting heat loss. Your double wall 'hollow sheet' polycarbonate would be better than single wall in this respect but heat loss through the glazing will still occur. This is something that we try to avoid in hot air collectors with baffles to keep flow away from the glazing.

Of course, ARETHA losses some heat through the glaze. If we can have some building solutions that can avoid it, we can recover efficiency.
paolometeo

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Reply with quote  #14 
Quote:
Originally Posted by cwwilson721
As I see it, think of the box they have designed as a "solar oven", no in/out for the air, and using the fan to circulate the 'stagnant' air thru the radiator (as a air-to-water heat exchanger), and continuing the analogy, the radiator is the 'food' being cooked.

You have to take into account that air temperature in the box, must be higher than water temperature in the circuit, this for having positive transfer of heat. Only in that condition the fan starts, controlled by an Arduino system with some sensors and actuators.
Quote:

It will work. However, greater efficiency would be realized if:
  • The air had a intake and exhaust port. This would allow fresh, cooler air to enter and get heated, then exhausted. The exhaust hot air could even be utilized for more 'air' heat, or for drying of foods for preservation, or other uses. As it is currently setup, it's 'stagnant', or just getting super hot in the box, wasting alot of heat that could be used for other purposes. The higher the heat inside the box, the more that gets wasted thru glazing/etc. You also risk the possibility of melting the glazing from too much heat.
If I take fresh air from outside the box, I cannot reach the condition stated above.

Quote:

  • If you have intake on the 'bottom', and exhaust at the 'top', the fan would work better if the radiator was 'tilted', using the full 'depth' of the box. Would be best if tilted 'upwards', thus letting the radiator also absorb more sunlight, plus all the air in the box would flow thru the radiator. In this configuration, the fan would be best used as a "pusher" fan, blowing from the back of the radiator, to the front, then out the exhaust port. You would get *some* heat loss thru the glazing like this, but a lot less than if the box were left 'stagnant'. *Best* use of the current fan, however, would be to detach it from the radiator completely, and use it in the exhaust port. This would even out airflow, and lessen (to a degree) the heat losses of air/glazing contact. If you're NOT utilizing the 'waste' hot air, however, the fan could be kept as a pusher, because the loss of heat thru the glazing after it goes through the radiator wouldn't matter.


The main advantage to the current parts/design is that's it's cheap, made of locally available 'waste' materials, and fairly easy to build.

With the minor changes I've said above, which would require no additional parts/expense to implement, it could provide hot water, hot air, and be more efficient. HOWEVER, the intake and exhaust would have to be sized correctly and the fan speed adjusted to allow the air to get hot enough for the heat to be exchanged to the water in the radiator, before flowing through the radiator, then being exhausted out the exhaust port.

But, with all that being said, the current setup will work.


I agree that a better position of the radiator and a different geometry of the box may produce an increase in efficiency. It should be tested.
paolometeo

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Posts: 13
Reply with quote  #15 
Quote:
Originally Posted by Garage_Hermit
well, I certainly would not like to start an argument on the issue, but based on one common airflow formula, namely

BTUH = M x sp.ht x delta T

I can't see how the fan *COULD* work, because M (air mass flow) is basically ZERO: to my admittedly inexperienced eye, the volume of that collector case looks to be around 50 cu.ft maximum (it is stated to be 5 square meters, and I guess it is around one foot deep).

So if 1 cubic foot of air at standard temperature and pressure (assuming average composition) weighs approximately 0.0807 lbs, then the mass of 50 cu.ft of air would be 4 pounds.

As that air gets hotter, its density will DECREASE, so even less efficient.

This is aside from another aspect, namely system effect:

Fans go with DUCTS, and in the present design, there is no duct...

http://www.captiveaire.com/MANUALS/AIRSYSTEMDESIGN/DESIGNAIRSYSTEMS.HTM#Types_of_Flow

Finally, as my last word on the issue [rolleyes] let's try comparing the proposed pneumatic system with an hydraulic system:  how well would the pump operate if all the valvework were CLOSED ?

Answer: no flow = no performance.

G_H

OK, the air mass in the box is negligible but it is enough to curry energy from the black surface of the box to the water flowing in the radiator. As I stated above, fan and circulating pump  are switched on only when there is enough "delta" temperature between air and water. It is possible to put the fan in a duct, but you have to re-design all.
paolometeo

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Posts: 13
Reply with quote  #16 
Quote:
Originally Posted by stmbtwle
I think it would work. The way I see it the heated air flows up in the front of the collector, transfers heat to the radiator, then flows down the back of the collector to the bottom where the process is repeated. A large one might be easier/less expensive to build than a water-tube collector, if the radiator and fan can be had cheaply. I think the fan would improve air flow and heat transfer through the radiator, but I have some concerns about it's life in that hot environment. Might be better if the fan we're located at the bottom, in the cooler part of the collector. A black divider (corrugated roofing?) separating the front and back of the collector would control airflow and prevent mixing. Possibly a screen absorber could be added in the front part.


Hy Willie,
Let me post what my friend Franco, who built ARETHA with me, write down for your comment.

I'll start from the last mail:
- the first paragraph describe in a very good way the phenomena.
- about the second I can say: perfect. In fact, imagine a 100 m² panel: how much copper (sheet and pipes)? How much copper/silver for all the brazing operations?
- About the third:
we had some accidents due to blackout of supply, we reach also 150 °C, but the motor and the plastic air-screw are both in good conditions.
We state that a 5 m² is the largest panel with one radiator inside.
  
- For larger dimension we are studying a system with a forced circulation through the radiator (or a pack of radiators), outside the panel and through the “room” (between the glaze and the “roof”).
In fact we foresee a very thin (black painted) plywood as separator between the upper “room” (between then glazing and the “roof”) and a sort of “vault”, under the room. For this reason we think not necessary the screen absorber.
 
The different experiences on the three prototypes, give us to estimate the ratio between the active panel area and the power (at the wheels) of the radiator/s: between 20 kW/m², with a active ventilation and 40 kW/m², with low ventilation.
 

Franco GM     
paolometeo

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Reply with quote  #17 

This reply is directed to everybody taking part to this discussion. I try to summarize our (Paolo & Franco) opinion about ARETHA. Not to end the discussion but just for summarizing some points.
1.    The purpose of ARETHA is to produce hot water and to store it in a tank.

2.    ARETHA is simple to built because you can use materials easy to find everywhere in the world (polycarbonate can be replaced by glass).

3.    One car radiator of about 120 - 150 HP can be work for a 5-6 m2 panel. If you need more power, you can connect many panels to the same water circuit.

4.    Water temperature cannot rise beyond the thermal limit of the tubes and tank (generally made by plastic), that is 60 - 65 °C, enough for many applications.

5.    Power supply for pump and fan, if they are temperature-controlled, is negligible compared to the energy gain as heat.

6.    We are testing a prototype in Milan for computing the energy efficiency and we hope to write down some figures soon.

1.    7.    Now we want to built a new prototype in order to edit a tutorial for people willing to make one ARETHA by itself. For this reason we ask a money contribution by means of this crowdfunding campaign:

https://www.indiegogo.com/projects/aretha-project#home  . So, please, if you can share this campaign with other interested people we will be very happy!!  [smile]
Paolo
stmbtwle

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Reply with quote  #18 
All the fan does is force more air through the radiator/heat exchanger, and should I think improve performance over relying on a thermosiphon. 

In reality it's really not much different than any other air collector connected to say your house.   The air is heated in the collector, pumped to the house where it gives up its heat and then is returned to the collector.  The air doesn't "go" anywhere, but it DOES circulate.  In this unit the heat is given up to the radiator and then returns to the main part of the collector where it is heated again.  It's the same concept without the ducts.   If set up right it might be more efficient than a ducted air collector, and a lot less expensive than a large water-tube collector.  It WILL work, and if it's cheap enough you can live with a lower "efficiency".  Just build a bigger box.  The expensive part is the radiator/fan assembly.

I have a small 2x4 single-solar-screen collector I built for a lark and an aluminum oil cooler (something like a radiator).  I think I'll stick 'em together and see what happens.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Budwyzer

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Reply with quote  #19 
Quote:
Originally Posted by stmbtwle
All the fan does is force more air through the radiator/heat exchanger, and should I think improve performance over relying on a thermosiphon. 

In reality it's really not much different than any other air collector connected to say your house.   The air is heated in the collector, pumped to the house where it gives up its heat and then is returned to the collector.  The air doesn't "go" anywhere, but it DOES circulate.  In this unit the heat is given up to the radiator and then returns to the main part of the collector where it is heated again.  It's the same concept without the ducts.   If set up right it might be more efficient than a ducted air collector, and a lot less expensive than a large water-tube collector.  It WILL work, and if it's cheap enough you can live with a lower "efficiency".  Just build a bigger box.  The expensive part is the radiator/fan assembly.

I have a small 2x4 single-solar-screen collector I built for a lark and an aluminum oil cooler (something like a radiator).  I think I'll stick 'em together and see what happens.


I have to disagree here, and agree with solardan.  

The fan seems like it would be more detrimental in this setup.  The reason fans are needed in a hot air collector is because the medium for thermal distribution between the collector and the home is the air.

This setup is circulating water, and has the pumps to do so. Bringing in cold water and removing heat from the collector to heat the water.

All any fans will do inside the collector will do as Dan suggests and blow air around and against the glazing. The same effect that putting extra fans inside of a hot air collector would have. 


The radiator is being used in reverse in this system. Normally hot fluid is being cycled into the radiator and a fan is used to blow cooler air across it and remove the heat. In this system cooler liquid is being ran inside the heated radiator. No need for a fan, the heat in the box will find its way to that metal.



This idea seems really cool and I would like to know how well it works out.  Right now my heater and my water heater are the only two things I have running on natural gas in my house and if I could cut them both off that would be great! Especially since I cut my gas line running power to my shed, in order to have a place to build my air heater this summer.  [frown] 
stmbtwle

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Reply with quote  #20 
We'll find out soon enough, I'm in the process of building a small one.  It'll be simple enough to turn the fan on/off while the pump continues running, and see if there's any difference in output.

I still think the fan will help.  The more air moves through the radiator, the more the heat transfer should be.  Relying on convection alone will IMO have the same effect as relying on convection alone with nearly any other collector. 

I have some reservations on the unit's efficiency though, simply because of the losses in the heat exchanger (never 100%) and the losses through the glazing due to it's having to run at such a high temperature.   My guess it'll need  to run at near 150F (maybe more) to get usable hot water.  However the object of the system is COST efficiency, not thermal efficiency, and if it achieves that aim then it works.

My test procedure will be as follows (when I get it together): Set up the water-side to run through a 5-gal bucket.  Record start temperatures of ambient air, collector air near the top, and water.   Allow the collector to stagnate till it reaches 150 or thereabouts.  Record temps.   Start pump but not fans to investigate thermosiphon mode.  The air temperature in the collector should drop as heat is  extracted  by the water.   See if the collector temperature stabilizes.  Record temp.  Start fans.  See if temperature drops further (this will tell us if the fans are helping), and if it stabilizes.  Record.   Run for a while and record temps.  See if water temperature stabilizes.  Record.   End of test.

This is not an efficiency test, it's a proof-of-concept test.  If it works at all, it works; efficiency is for someone else. 

The starting configuration is something like the original diagram.  Fans and radiator at the top of the collector, and  no divider or screen.   I'll set up the fans so that the "cooler" air from the radiator runs downward past the glazing (in theory at least) and returns up the (presumably hotter) backplate/absorber.  IF the fans survive the original test (they're computer fans and may not) and it looks like a modification is in order, I'll install a screen absorber (I have one already) down the center with the fan at the bottom and the radiator at the top, sort of a zero-pass setup. 

I may try a solid metal divider/absorber, this will prevent short-circuiting of the air through the open box or through the screen.  May even try a screen absorber with the divider.  IMO this would be the most efficient from a thermal standpoint, but bear in mind that the more complicated the critter is the more expensive it will be, and that's not the goal.

Input/ideas/suggestions are welcome.

__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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