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DRDean

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Reply with quote  #1 

I am curious about something.  I had considered trying to adjust collector flow for my hydronic system via pump speed with an adjustable voltage switch (as in a dimmer switch).  My plan was to write a program for an Arduino controller with a loop inside a loop.  The first code loop would be for starting and stopping the pumps based on storage temp and collector temp. 

 

Once started, I thought a second program loop might be useful to optimize the flow rate for maximum temperature differential between return water after circulating through a collector and storage tank temperature.  The idea would be to use a controlled stepper motor on the variable voltage switch.  The temperature differential between return water and storage water would be constantly monitored and the speed of the pumps would be adjusted through the controller and stepper motor to keep the differential at the greatest value. 

 

In the end I did not take the time to learn the ins and outs of the Arduino (its still on my to do list) and just went with a simple preset controller that allowed me to start and stop pumps based on temp differential. 

 

Any thoughts regarding the idea of monitoring return water temps and variable speed pumping to get the greatest temperature gain on return water?  I would think the same thing could be done with the hot air collectors as well by controlling the speed of the blower.

Garage_Hermit

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Reply with quote  #2 
Hello, DRDEAN

You asked,

« Any thoughts regarding the idea of monitoring return water temps and variable speed pumping to get the greatest temperature gain on return water? »

I’m no expert in the field, but it sounds like a great idea to me !

Not just a great idea, but an absolute necessity, at least in the future when the technology has matured but more importantly when fuel prices are so high that nobody will be able to pass up on even an 0.05% efficiency improvement !

So I would say, right on, definitely, the principle is dead right for our applications.  In fact it exists already in at least one other application, and I am sure there must be many others…

I am thinking in particular of a modern diesel engine, where the engine coolant temperature (commonly referred to as « water temp ») is employed by the injection computer (along with plenty of other parameters), in determining injected fuel quantity -- for anybody interested, there's an excellent technical description HERE... http://www.volkspage.net/technik/ssp/ssp/SSP_222.pdf )
===============
Otherwise, given that proportional-differential control laws are used in lots of applications, a priori reasoning would suggest that there is no reason why they should not extend into the domain of SOLAR heating also, as you suggest.    Not only hydronic systems, like you mention, but also pneumatic collectors.

edited, 11 May 2014

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(1)  "Heat goes from hot to cold, there is no directional bias"
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Scott Davis

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Reply with quote  #3 
Hi Guys,

Generally speaking, slowing the returning fluid to create a higher temperature rise actually results in less heat collected, because the collector is running hotter, losing more heat to the surrounding environment than it would be with higher flow.  In other words, it is better to get a 5F rise at 10 gallons per minute than a 9F rise at 5 gallons per minute.  You'll collect more heat that way!

Simpler really is better, just shoot for a good flow and don't do anything to impede it.

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SolarInterested

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Reply with quote  #4 
Quote:
Originally Posted by Scott Davis
... Generally speaking, slowing the returning fluid to create a higher temperature rise actually results in less heat collected, because the collector is running hotter, losing more heat to the surrounding environment than it would be with higher flow. 
Yes so increasing the pump speed to lower the panel temperature would be more efficient, wouldn't it? I'm imagining the controller logic increasing pump speed to keep the panel below a set maximum temperature.

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Both temperature rise and airflow are integral to comparing hot air collectors
solardan1959

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Reply with quote  #5 
What is the recommended panel operating temp as a compromise for efficiency and water output temp?
All I ever see is 5 degree increase, which means a variable flow may be best for efficiency.
Is there a set range for panel/water output temp that I should shoot for?
Dan
SolarInterested

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Reply with quote  #6 
Quote:
Originally Posted by DRDean

I am curious about something.  I had considered trying to adjust collector flow for my hydronic system via pump speed with an adjustable voltage switch (as in a dimmer switch).  My plan was to write a program for an Arduino controller with a loop inside a loop.  The first code loop would be for starting and stopping the pumps based on storage temp and collector temp. 

 

Once started, I thought a second program loop might be useful to optimize the flow rate for maximum temperature differential between return water after circulating through a collector and storage tank temperature.  


I had been thinking about something similar. I picked up an Arduino to fool around with and learn to program. 

Quote:

The idea would be to use a controlled stepper motor on the variable voltage switch.
I'm still learning but I think the pump speed can be directly controlled by the Arduino through Pulse Width Modulation (PWM) which provides
variable speed control for motors

http://arduino.cc/en/Tutorial/SecretsOfArduinoPWM



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Garage_Hermit

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Reply with quote  #7 
Scott is 100% dead right, of course: slowing the pumping makes the collector run HOT, thus more losses back out thru the glazing, to warm the Brazilian rainforests (what's left of them...) even MORE... to the detriment of one's heat store  (house) !

THEREFORE, when the collector starts getting hot, the sensor SPEEDS UP the fan !  Thus the Collector cools down, and the heat energy makes it successfully into the store (house walls..., tank etc.).  When the fan is taking TOO MUCH HEAT, the collector temp drops so MUCH that the differential temperature between the collector and the house (store) is so low that if the collector got any LOWER, the heat would flow back from the STORE (house...) INTO the collector - bad scenario (-:

THEREFORE the controller tells the fan motor (PUMP) to cut it out, which it does, so therefore less heat is taken out of the COLLECTOR, and the collector RISES its temperature once again, until the CONTROLLER sas, WOW, we are starting to get a little HOT in here, again, Chaps, put the fan (pump...) BACK ON !

And so on and so forth.

This is exactly the same logic employed in a sewage processing plant, where the effluent tanks start getting so full that they would overflow down the road (or into the river), therefore the LEVEL controller turns on the PUMP, (or if the pump was already ON, makes it work FASTER...) so that the pump pulls faster to get the effluent out of that tank before it goes over...

I believe it is called proportional differential control...
it relies on several essential notions, namely SENSING (using sensors), and control signals (to drive pumps, valves, motors), and above all, FEEDBACK SIGNALS and is generally controlled by a programmable logic controller.
SO IMO, DJDEAN is bang on the mark, he is simply ahead of his time for the moment !)
Needs severe encouragement, in my view !

Garage_Hermit



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Scott Davis

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Reply with quote  #8 
Hi Guys,

SolarInterested wrote:

"Yes so increasing the pump speed to lower the panel temperature would be more efficient, wouldn't it? I'm imagining the controller logic increasing pump speed to keep the panel below a set maximum temperature."

The flip side of that statement is that when you throttle your pump back, you are reducing the flow rate through your panel to less than what your system is capable of producing.

Even in marginal sunlight, I'm not able to envision any set of conditions where it is advantageous to let our panels run even one degree hotter than necessary.  For every degree that our panels are hotter than ambient, a percentage of the heat inside the panel is lost through the sides, back and glazing.  The greater the spread of collector temperature to ambient, the more loss that occurs.

Keeping in mind that the measure of total heat returned to the house is flow rate times temperature rise, can anyone help me understand why adding variable speed control isn't just adding unnecessary complexity and expense by introducing a bottleneck that reduces efficiency?

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SolarInterested

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Reply with quote  #9 
Scott thanks for creating a new section for this. It's new to us but as Garage_Hermit has mentioned "proportional differential control" is used in other applications so might be useful here too.

Quote:
Originally Posted by Scott Davis
... is that when you throttle your pump back, you are reducing the flow rate through your panel to less than what your system is capable of producing.

True but it normally would only throttle back when a fixed speed pump would be shutdown so you'd gain extra heat input early in the morning and late in the afternoon. Think of it sort of like the systems where the pump is hooked directly to a solar panel. The pump runs slower on either side of the peak sunshine period.

Quote:
Even in marginal sunlight, I'm not able to envision any set of conditions where it is advantageous to let our panels run even one degree hotter than necessary.

True but a variable speed pump could be used to increase pump rates when necessary to keep the panel cooler assuming the water coming from the tank is still cooler than the panel. This might require a higher volume pump is chosen when designing the system

Quote:
...  For every degree that our panels are hotter than ambient, a percentage of the heat inside the panel is lost through the sides, back and glazing.  The greater the spread of collector temperature to ambient, the more loss that occurs.

No argument.

Quote:
... Keeping in mind that the measure of total heat returned to the house is flow rate times temperature rise, can anyone help me understand why adding variable speed control isn't just adding unnecessary complexity and expense by introducing a bottleneck that reduces efficiency?

I don't know that it would be adding any extra expense as micro controllers like the Arduino are cheap and might not be any more than a differential controller. I believe existing pumps DC pumps could be used but that needs to be verified. The programming logic is more complex but that only has to be developed once.

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Both temperature rise and airflow are integral to comparing hot air collectors
solardan1959

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Reply with quote  #10 
Solar interested wrote:
"Even in marginal sunlight, I'm not able to envision any set of conditions where it is advantageous to let our panels run even one degree hotter than necessary. For every degree that our panels are hotter than ambient, a percentage of the heat inside the panel is lost through the sides, back and glazing. The greater the spread of collector temperature to ambient, the more loss that occurs."

   In theory this is great but...  What if the outside ambient is 6 degrees?  Should I be shooting for a 10 degree output to try and be more efficient? (I guess he did say "hotter than necessary")
  Yesterday my long small screen collector and newer solar screen collector were kicking out 144 and 114 degrees but could not melt the ice and snow on the bottom half of the collector with single wall glazing after several hours.  Was I losing heat to the outside with a 138 degree difference between the panel temp and the outside air?  I would have to assume yes but apparently not enough to overcome the cold.
   As I asked in an earlier post, yes we do want these to be efficient but we need them to be hot enough to provide a useful product.  What is a good level to shoot for the output of a water collector? To me focusing on our heat requirement first then how we keep from losing that heat to the outside environment is more important than shooting for the most complicated and efficient.  Though once we reach that first goal the second one is certainly worthy of pursuit.
Dan
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