Welcome to the Solar Collector
Brainstorming and Development Page!


 

Home

Hot Air Collector

Hot Water Project 1

Hot Water & Space Heating

Solar Electric

Solar Construction 101

FAQs

Best Collectors

Simply Solar
Register Calendar Latest Topics
 
 
 


Reply
  Author   Comment   Page 2 of 8      Prev   1   2   3   4   5   Next   »
SolarInterested

Avatar / Picture

Spam Stomper
Registered:
Posts: 1,036
Reply with quote  #11 
For clarification Dan is quoting me quoting Scott. It's easy to get confused with quotes within quotes.
__________________
Both temperature rise and airflow are integral to comparing hot air collectors

Garage_Hermit

Avatar / Picture

Registered:
Posts: 2,240
Reply with quote  #12 
Hi, Scott,

I have done some simple process control drawings in the hope of persuading you that adding variable speed control isn't just adding unnecessary complexity and expense by introducing a bottleneck that reduces efficiency.

I hope you will be convinced with the technical arguments (-:

If not, I have got  couple of others (philospohical) up my sleeve, I guess U don't want boring with those now... Ha ha !

Happy reading,
Garage_Hermit

P.S. ANYBODY should feel free to comment, correct or destroy the attched as necessary !

 
Attached Files
pdf PROCESS-CONTROL_SCHEMATICS.pdf (76.18 KB, 28 views)


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

Scott Davis

Avatar / Picture

Super Moderator
Registered:
Posts: 697
Reply with quote  #13 
Hi Guys,

Dan wrote "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?"

In liquid collectors you should always be shooting for the lowest temperature rise possible.  The lower the collector temp, the more efficient it is.  it may seem counter intuitive, but you'll return more heat to your home at a 1 degree temperature rise at 10 gallons per minute than you will with a 9.5 degree rise at one gallon per minute.  That 1 degree at 10 GPM will raise your thermal storage tank temp more quickly.

SolarInterested wrote:  "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."

That may be an advantage.  It would be interesting to see how a lower continuous flow rate would compare to the panel cycling.  My panels typically only cycle once or twice first thing, so I don't suspect that would be significant either way.

I encourage you guys to continue experimenting.  I think I will bow out of this thread so you can focus on the construction details and see how it works out.  I think it is important for others who might come across it to understand that excellent systems can be built without the need for incorporating variable flow (and usually are) and it is questionable whether variable flow will provide any advantage in any case. 

Carry on!


__________________
Take care, Scott MD
Garage_Hermit

Avatar / Picture

Registered:
Posts: 2,240
Reply with quote  #14 
 
 
Hi, All,
 
 
I was just reading up on water collectors, in the Hydronic Section, and I think that the following thread is pertinent to the present discussion:

Solar Web Page > Forums > Solar Liquid (Hydronic) Collectors > PV pumping advantages

Guess the good news is that variable speed pumping looks like a prime option -- in fact, The Thing To Go For...

The reason is no so much to do with process control or system efficiency, as with Economics ! (they are probably all interconnected in any case !)

As far as I understand, the recommendation for water pumping, is to use a DC motor-pump unit, and take the power from a PV panel; below a certain sunlight, threshold, therefore: no pumping (at all).

Thus I guess that the inference is: for maximum efficiency, you "must" use PV, implying DC power, and brightness control as the primary control mode.

Therefore, by definition, any temperature-differential slaving as proposed by DRDean herein, would have to be *secondary* to the brighness slaving requirement (t° diff could only come into play during the "sufficient brighness" period).  This would presumably significantly hamper the temperature differential regime.

There is nonetheless a rider, namely, AC control *is* required in big head-loss applications (presumably because the system might be pumping (emptying) the collector back to store (drainback), and this takes place when the sun is down & thus requires an independent power source.   Therefore, in this case, the temperature-differential control mode would find its logical place. QED DRDean  !

(I am in effect looking at (= struggling to design" !) a large-volume hydronic system where the collector volume is as close to 100% of the storage tank volume as I can make it...  The idea would be to drain the collector back fully at night, into the storage tank.  Since I would only be using a small pump, this would take so much time, that I could not use PV to power a DC motor, therefore I would need an AC pump, off my mains, and I would effectively be looking for optimal drainback mode to benefit from every last bit of sunlight...).

Garage_Hermit

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

Avatar / Picture

Spam Stomper
Registered:
Posts: 1,036
Reply with quote  #15 
Quote:
Originally Posted by Garage_Hermit
Hi, All,


I was just reading up on water collectors, in the Hydronic Section, and I think that the following thread is pertinent to the present discussion:

Solar Web Page > Forums > Solar Liquid (Hydronic) Collectors > PV pumping advantages

http://simplysolar.supporttopics.com/post/PV-pumping-advantages-6090381

__________________
Both temperature rise and airflow are integral to comparing hot air collectors
Steve in Maine

Registered:
Posts: 5
Reply with quote  #16 
I think you can have both a simple control and a constant speed pump with little or no cycling.

SolarInterested wrote: "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."

Scott wrote: "That may be an advantage. It would be interesting to see how a lower continuous flow rate would compare to the panel cycling. My panels typically only cycle once or twice first thing, so I don't suspect that would be significant either way."

The problem that causes cycling and early panel shutdown is incorrect temp sensor placement. Say your controller is set to run the pump when the panel water is 5 degrees hotter than the tank. Here's what happens:
1. In full sun, the panel gets pretty hot and turns on the pump. As the tank water circulates through the panel it heats up. If the sensor is near the panel output, the water should be heated enough to stay above the 5 degree setpoint on the controller.

2. In less sun, partial clouds or early morning/late afternoon, the heat that the circulating water picks up isn't enough to increase temperature above the 5 degree controller setpoint, so the pump shuts off. Then the panel drains, the sensor heats up, pump turns on. It does this for awhile and finally settles down once the sun moves out of this range.

One way to fix this is to use a variable speed pump. You can buy one (expensive), make a homebrew controller (complex) or use a solar panel and DC pump. The solar panel and DC pump is appealing, but how do you size the solar panel? You want it large enough so that the pump doesn't let the panel get too hot, but small enough so that the pump shuts down at the right times.

There is an easier way. This entire issue is caused by where the sensor is placed, measuring panel water temperature. This is the wrong place for the sensor. What you should care about is that the panel fin temperature is hotter than the tank water.

Put the panel sensor on the fin, not the pipe. If the panel fin temperature is warmer than the tank water, circulating the water through the panel will increase the temperature of the water, even if only 1 degree. The fin temperature will respond mostly to the sun, not to the circulating water temp. Any time the fin temp is greater than the tank, you want to run the pump. If the water circulating through the panel (with the pump at full speed) only picks up 1 degree, that's still useful heat.

Using the fin temp sense, when the sun rises first thing in the morning, the panel warms up a bit...30F, 40F, 60F. When it gets above tank temp, the pump turns on and stays on.

You do still want to set a delta temp in the controller. This will cover issues like sensor mismatch (say the panel sensor is off by +1 degree compared to the tank sensor) or heat loss between the tank and the panel (in my case a run of about 80 feet).

Credit where credit due: this idea comes from John in PA (zero pass hot air panels). He suggested this as a cure to the excessive cycling I was seeing. It works like a charm. In my case, I just added a 3" square of black flashing with the sensor glued to the back of it, and attached that to a spot on the fins. Note that it doesn't matter where on the panel your sensor goes now, the temp will read the same.

I also have a hot air panel on my shop, which had a lot of cycling during startup. In an air panel there generally isn't anywhere that the airflow isn't happening, besides being tough to retrofit while mounted on the roof. Instead, I made a small 12" square insulated and glazed panel for the sensor (but with no airflow), and attached it to the side of the main air panel.

Steve
SolarInterested

Avatar / Picture

Spam Stomper
Registered:
Posts: 1,036
Reply with quote  #17 
Thanks Steve. There's nothing like the voice of experience.
__________________
Both temperature rise and airflow are integral to comparing hot air collectors
Steve in Maine

Registered:
Posts: 5
Reply with quote  #18 
One more bit on motor speed control. Only some types of motor can be speed controlled by a dimmer switch. The little bit of looking I did found Taco inline pumps have a split phase motor. These motors run at a speed determined by the incoming line frequency. If you use a dimmer on them, the amps go up and the speed stays pretty much the same. Once they get low enough in voltage they will lose torque, but this is not a good way to do speed control (it is a good way to fry your motor).

Speed control for split phase and induction motors is done with a power supply that has variable frequency output. This is lots more complex and expensive. If you want variable speed, either buy a pump designed for it (with a built in controller) or use a DC motor.

Steve
Garage_Hermit

Avatar / Picture

Registered:
Posts: 2,240
Reply with quote  #19 

To those interested in the furtherance of this topic, I can recommend a look at this paper:

http://www.ewp.rpi.edu/hartford/users/papers/engr/ernesto/changj2/EP/supporting_matls/topics/flowcontrol_solar_coll.pdf

in which the author discusses "Optimal Strategies for Maximizing Performance in Open-loop Solar Collection Systems".

Despite quickly getting heavily formulated (out of necessity !), it is worth soldiering on, particularly as the discussion gets textually rich and interesting on page 4318, Chapter 6, "Aspects of Controller Design".

The conclusion is that performance increases are "largely conditioned" by the control of exergy, in particular, temperature and flow rates.  More especially, the author proposes employing the water mass-flow rate in the collector as the control parameter.  To this end, he looks at various control strategies, and determines that even ON-OFF switching or proportional control of pumps will not allow optimal performance...

He in fact proposes an advanced "level-3" control strategy, to regulate the *outlet temperature of the collector* by suitably adjusting the working fluid flow...  So in this sense, he is promulgating "tuning" the collector... and his propositions are interesting to say the least !

G_H


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

Avatar / Picture

Registered:
Posts: 78
Reply with quote  #20 
Hey Hermit. I read your system requirements for your automated variable speed pumping system. I have extensive programming experience with Allen Bradley, S5 an S7 PLC programming. The code would not be that hard BUT (you knew there was going to be a but) the PLC with the associated I/O cards would not be accessible to the average builder ( read cha-Ching ). However what you propose COULD be done with relay logic. If someone that already has an up and running collector with baseline data and a set of desired NEW performance data they wish the same collector have, I might be able to come up with something. I would experiment on my own but the project has been put on hold because I have to replace my 120 yr old roof and, as you can guess, it's gonna set me back a metric butt-ton to get that done. I do like the idea of varying pump speed to the process for maximum efficiency (PID) and what is being proposed here is used commonly on industrial processes even if for very small gains as it is cost effective. If we want to vary speed by temperature directly, a PLC is not needed. This can be used:

http://m.ebay.com/itm/131001963104?nav=SEARCH


Sent from Ken's iPhone
As long as the pump doesn't draw more than 1.5 A it can drive the pump directly. If the pump draws more. We can use the output of the above to be the input to a larger traditional speed controller that uses a "pot" to manually adjust the speed.
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.

 

web statistics