Solar Panel System

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Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Thu 30 Sep, 2010 22:50

(i.e. it would remain closed even with negative pressure relative to the atmosphere)


The panel manufacturers don't want the panels under vacuum, as that would tend to make the panels collapse. You could build stronger panels that could withstand the vacuum. That way you could build a closed loop system and get back the energy you had to put into the water to get it up to the roof. A closed loop system is only good up to about 32 feet, because the vacuum would cause the water to begin to cavitate (convert to gas) once you got above 32 feet.

The vacuum breaker does not necessarily need to close. It can be open, which will allow air into the return. Some people just live with the air bubbles and the resulting aeration. Although, it really does not save very much. It just reduces the pressure by a pound or so. To close the vacuum breaker, you could increase the flow rate and/or restrict the return with a valve, eyeball etc.

We don't try to make a closed loop where the water falling back down pulls the feed water up creating a syphon effect meaning the pump doesn't have to do any work. No No No. That would create negative pressure in the solar collectors and a negative pressure (a vacuum) can collapse pvc plumbing when it gets hot and hot it will get. http://www.h2otsun.com/pools/index.html


Located here, there tends to be more pressure on the vacuum breaker keeping it closed during operation and avoiding noisy air entering the system constantly. It will still open when solar shuts off and let air enter so the panels can drain, avoiding negative pressure (which could collapse them when combined with heat). The vacuum breaker breaks the vacuum. http://www.h2otsun.com/polypro/Powerpro.pdf


Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Thu 30 Sep, 2010 23:08

[Edit]
Your total head would be 6.5 psi static to get up to the panels + the dynamic head going to the panels + the dynamic head through the panels + 7 psi for the return trip from the panels to the pool + the dynamic head created by the water going through the bypass and back to the pool.


Your total head would be 6.5 psi static to get up to the panels + the dynamic head caused by the water going to the panels + the dynamic head caused by the water going through the panels + the pressure at the vacuum breaker (which should be close to zero). [End Edit]
chem geek
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Solar Panel System

Postby chem geek » Fri 01 Oct, 2010 01:30

The bottom line from needing positive pressure at the highest point of the panels is that if one wants to have really low pump energy costs, you shouldn't put your panels up very high. Unfortunately, the roof tends to be a great place to capture sunlight unless you've got a large piece of property with unobstructed sunlight.

I saved $700 per year in pump energy costs by changing pumps, but if I wanted to save a couple hundred dollars more, I was thinking that using larger piping would have done that (on initial install -- pretty expensive to do now), but now I see that I could only reduce friction losses on the feeder side to the panels (i.e. using larger diameter pipe), but on the return piping from the panels there's no point in using larger piping or wider eyeballs because you need the flow resistance to ensure enough pressure at the high point (even if done using a restrictor, there's no point in larger plumbing from the panels).
Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Fri 01 Oct, 2010 02:27

Here is a simplified example of a closed loop vs. a non-closed loop system:

23 feet of white 2-inch PVC rising vertically out of the water, then 100 feet of black 2-inch PVC going horizontally to act as the solar collector, and then 23 feet of white 2-inch PVC pipe back down into the water. Closed loop, no vacuum breaker. 48 gpm. In this case, you would have 146 feet of straight pipe and (2) 90 degree elbows 5.7 x 2 = 11.4 feet of straight pipe for a total of 157.4 feet of pipe. This would create 2.7 psi of head loss once the water filled the pipes. The problem with a closed loop system that could take the vacuum is that the panels would be much more expensive and probably less efficient. So, there is always a tradeoff.

However, if you add a vacuum breaker, you have to add 10 psi for the static head loss due to the water having to be pushed up to the top. Since the downward pipe contributes only 0.4 psi of head loss, you would have to add an additional 10.1 psi to the return line to achieve at least 0.5 psi at the vacuum breaker. The head loss would be 2.3 + 10 + (10.1 + 0.4 -10) = 12.8 psi.

For the return from solar, in this example, you could actually use a 1-inch PVC pipe and not lose any performance from an open loop system with a vacuum breaker because the 1-inch PVC would have a head loss of 10.7 psi, which would be just enough to close the vacuum breaker. The head loss would be 2.3 + 10 + (10.7-10) = 13.0 psi

I agree with you that you could only save on the trip to the panels and not from the panels to the pool (unless the head loss from the panels to the pool is more than the height of the vacuum breaker). Since you are not getting air in your system, I'm going to guess that they might have even added some type of restrictor to the return line as you have also guessed.

Using a closed loop system would save money on pump costs but would increase panel cost and probably reduce efficiency. You could also reduce the flow rate to 3 gpm per panel to save on pumping costs, but at the expense of lost efficiency.

The way to optimize panel performance is to adjust the flow rate to the lower end of the recommended range and then adjust the return line until there is just enough restriction to maintain 0.5 to 3 psi at the top of the panels. Some systems have the vacuum breaker slightly lower than the top of the panels, and you can get closer to zero pressure at the top.
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Solar Panel System

Postby chem geek » Fri 01 Oct, 2010 19:57

Well, in my system I've got unusually high and unexplained losses even with solar off where if I run at 48 GPM I have 10.5 PSI read on my filter (maybe the gauge is wrong). I've got a roughly 50' 2" line going to the pool that then splits into 3 1.5" lines to each return where are each 3/4" eyeballs. The loss should only be around 0.9 PSI for the line to the pool, around 0.3 PSI for the individual return lines (looking at one of them at 1/3rd flow rate) and maybe 1 PSI for the eyeballs (looking at one of them). The filter loss (when clean, which it almost always is since it is oversized) is around 2.5 PSI. So that's 4.7 PSI with the rest (5.8 PSI) having to come from the gas heater and all the 90 degree elbows, valves and piping at the pad plus underestimation on my part. So something seems off in this situation.

With the solar on at 48 GPM, it's 24 PSI so that's a 13.5 PSI increment. This is far better explained, especially if there's any sort of flow restriction. The piping to the solar is 2". Let's call it 75 feet to get from the pump to the house and up to the roof. That's 2.7 PSI for dynamic head only. Then there are the very long runs of 2" pipe between solar panels and for the solar panels themselves (a 2-way trip since both in/out pipes are on the same side of the house). Say 120 feet so around 2.2 PSI. Going through one panel (according to their spec at 4 GPM) is around 0.9 PSI. The return piping from the solar back to the pad is another 2.7 PSI. So that's 8.5 PSI. Though there is no vacuum in the panels, I don't count the static head since it does cancel each other out up and down (it's just that one shouldn't go negative in the panels).

The height of the top of the highest solar panel may be more than 15 feet, possibly closer to 20 feet, though I'll have to measure that. So that's 6.5 to 8.6 PSI required in losses from the top of the panel to the returns, probably with a flow restrictor in there somewhere. If I add that to what it takes to get to that highest point I get around (2.7 + 2.2/2 + .9 + either 6.5 or 8.6) = 11.2 to 13.3 ([EDIT] MISTAKE! I'm only looking at the increment of the solar so should not include the full loss all the way to the pool, only back to the pad [END-EDIT]) so pretty much explains what I'm seeing in the solar on situation. Without a restrictor, the return loss is around (2.2/2 + 2.7 + 0.9 + 0.3 + 1) = 5.8 PSI so the flow restrictor (if any) is perhaps 2-4 PSI. [EDIT] I shouldn't count the loss all the way to the pool at this point since I'm only looking at the incremental loss due to solar so this incremental return loss is (2.2/2 + 2.7) = 3.8 PSI requiring the flow restrictor to be 2.7 to 4.8 PSI. [END-EDIT]
Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Sat 02 Oct, 2010 02:47

chem geek
Though there is no vacuum in the panels, I don't count the static head since it does cancel each other out up and down (it's just that one shouldn't go negative in the panels).

If there is no vacuum, they can't cancel each other out. You're still adding the static head; you're just doing it on the return side.

Think about this:

There is a pipe that rises 23 feet vertically out of the water, turns 90 degrees and runs 10 feet horizontally, and then turns 90 degrees back down 10 feet to the water. No vacuum relief. Once the pipe is full of water, the pressure in the top horizontal pipe is -10 psi (gauge) and the pressure at water level is zero psi (gauge) (static before moving the water). In this case, the rise and fall cancel each other out because the falling water "pulls" the rising water by the suction created from the water falling.

However, if you put a vacuum relief at the top, there cannot be a vacuum, and the pressure at water level has to be +10 psi static when the pump pushes the water to the roof. Since the water falling back down cannot "pull" the rising water, its potential energy can only be used for the return trip to the pool. If the return trip generates only 3 psi of head loss, you lose the other 7 psi.

You only get back the potential energy if the return trip generates at least 10 psi, or more. In this case, you have to add an extra 7 psi to close the vacuum relief valve. Therefore, you do get to 10 psi, but only by adding 7 psi of extra restriction. You're still losing the 7 psi. Further, you don't need to add the 7 psi restriction. You can just allow the vacuum valve to stay open and suck in air.

To calculate the head loss for an open loop vs. a closed loop, you have to add the height of the panels, but you can subtract the head loss due to the dynamic head created from the panels to the pool (up to the amount of static head).

In your case, the rise and fall do cancel, but only due to the creation of additional head loss in the return from the panels to the pool due to the addition of some type of restriction. The math comes out the same, but I think that it's an important conceptual point to understand that the rise and fall do not cancel each other out in the same way that they do in a closed loop system.

chem geek
The height of the top of the highest solar panel may be more than 15 feet, possibly closer to 20 feet, though I'll have to measure that. So that's 6.5 to 8.6 PSI required in losses from the top of the panel to the returns, probably with a flow restrictor in there somewhere.

This is where you are adding the height of the panels.

chem geek
If I add that to what it takes to get to that highest point I get around (2.7 + 2.2/2 + .9 + either 6.5 or 8.6) = 11.2 to 13.3

You are adding the height (6.5 or 8.6 psi) to the dynamic head caused by the water going to the panels + the dynamic head caused by the water going through the panels

Allen G Myerson
Your total head would be 6.5 psi static to get up to the panels + the dynamic head caused by the water going to the panels + the dynamic head caused by the water going through the panels + the pressure at the vacuum breaker (which should be close to zero).


You can add the height going up or coming down, either way, it's still adding the same thing.

You do get some of the potential energy back from the water falling, but it is limited to the dynamic head loss from the panels to the pool, which should be much less than the height, which is why you need to add extra restriction if you want to close the vacuum relief valve, which you don't necessarily need to do.
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mas985
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Solar Panel System

Postby mas985 » Sat 02 Oct, 2010 11:03

Allen G Myerson wrote:If there is no vacuum, they can't cancel each other out. You're still adding the static head; you're just doing it on the return side.


Forgive me for butting in here but I would like to carify this statement, it seems contradictory and not quite correct. First of all pressure is relative and what a vacuum is defined as depends on what you define as 0 PSIR. At sea level, 14.7 PSIA is defined as 0 PSIR. So what is a vacuum at sea level may not be a vacuum on top of a mountain. The atmospheric pressure only determines the pressure that the vacuum release valve opens.

Second, not to be picky but techically ALL pool systems with or without a vacuum release valve are open loop systems since the pool itself is exposed to the air.

Next is the concept of “pulling” water. In the past, there has ben some debate on if water is pulled over a siphon or is pushed over a siphon. Most physicists now agree water is pushed over a siphon and Bernoulli’s equation fully explains the flow rate of water with pressure differentials and changes in height. Also, a siphon will still siphon if both tanks are under the same positive pressure and the top of the siphon is under positive pressure albeit less than the tanks. The pressure differentials are relative and it does not depend on a vacuum.

Next, absolute pressure is irrelavent for static head gain or loss, only the weight of the water matters which is dependent on the water column height. For a fully primed pipe, and it has to be fully primed, there is static head loss going up the pipe and static head gain going down the pipe both are equal but sum to zero and do not contribute to the total head. It doesn't matter if there is a VRV or not. Whenever there is an elevation change in a fully primed pipe, there is a change in static head. The static head comes from the weight of the water in the pipe so water pressure drops with elevation rise and visa versa. It doesn't matter if it is a closed loop system or open loop. As long as the vacuum release valve remains closed air is kept out of the pipe, both the open loop and close loop systems behave identically. If you were to measure the pressure along the two plumbing systems, they would have identical pressures.

Where things get tricky is when the pressure is insufficient to keep the vacuum release valve closed and air gets sucked into the return pipe. At this point the system effectively breaks into two pieces that are somewhat independent. The first part is the pump overcoming the the static head loss going up the pipe as well as the dynamic head loss in the pipe to the vacuum release valve. At this point the pressure is 0 PSIR and the pump only needs to over come the pressure to this point. After this point, the system behaves as a gravity fed plumbing system. Water falls down the return pipe through the air and the return pipe may still be partially filled creating some head gain. This water height then determines the flow rate through the remaining part of the plumbing.

So in summary, static head gain and loss can occur with or without a vacuum release valve and the plumbing system behaves identically as long as the vacuum release valve stays closed.
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chem geek
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Solar Panel System

Postby chem geek » Sat 02 Oct, 2010 13:07

When I wrote I wasn't counting static head (because it netted to 0 up and down), I was specifically talking about the calculation I was doing to account for the incremental head loss (pressure difference seen at the filter's pressure gauge with vs. without solar) going from the filter to the panels and back to the pad (I corrected my earlier post where I underestimated the flow restrictor amount). I did not use any static head in that analysis (the 2nd paragraph in this post) nor should I unless, as Mark points out, the vacuum relief valve was open and I assumed it was not.

The later calculations (in the third paragraph) where I did count static head were only to figure out the minimum pressure needed (again, as measured at the filter gauge) so that I would not have less than zero net pressure (relative to air pressure -- I called this a vacuum, but technically it's just a pressure lower than atmospheric pressure). Essentially, I was just trying to figure out the implied loss from a flow restrictor. If I calculated the overall head loss with that assumption, then I don't use any static head calculation: 2.7 + 2.2/2 + 0.9 + 2.2/2 + 2.7 + ( 2.7 to 4.8 )) = 11.2 to 13.3 PSI which is reasonably close to the 13.5 PSI increment I measured for the solar given my underestimation (not counting the numerous 90 degree elbows, T-junctions and associated extra pipe length on the roof, etc.). There is no static head in this calculation (i.e. it cancels out up and down so I did not include it), but that's because I assume the vacuum relief valve is closed due to sufficient pressure helped by a flow restrictor.

The interesting thing is that my unusually high head loss even with the solar off may imply that the flow restrictor was put in the general piping line to the pool, not just in the loop that included the solar. I'll see if I can find out where the flow restrictor is in my system. Maybe it was in the main line for the solar off situation to give the 1 HP pump more resistance to limit the flow rates somewhat. If that is the case, then now with my variable speed pump I can have that flow restrictor moved to return side of the solar loop instead so that my solar off energy costs can be further reduced.
Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Sat 02 Oct, 2010 20:05

mas985 wrote:
Allen G Myerson wrote:If there is no vacuum, they can't cancel each other out. You're still adding the static head; you're just doing it on the return side.


Forgive me for butting in here but I would like to clarify this statement, it seems contradictory and not quite correct. First of all pressure is relative and what a vacuum is defined as depends on what you define as 0 PSIR. At sea level, 14.7 PSIA is defined as 0 PSIR. So what is a vacuum at sea level may not be a vacuum on top of a mountain. The atmospheric pressure only determines the pressure that the vacuum release valve opens.

I don't think that the statement is at all contradictory, or incorrect. I think that it is obvious that I was referring to local gauge pressure, where vacuum means a pressure less than 0 gauge, which is 14.7 psi absolute. In a closed loop system that is less than 32 feet high, the static height up and down cancel due to the vacuum created by the falling water and the siphon effect..
mas985
Second, not to be picky but technically ALL pool systems with or without a vacuum release valve are open loop systems since the pool itself is exposed to the air.

This is not correct. As long as the loop is airtight, it is closed. There is definitely a difference between a closed loop and one that has a vacuum relief.

mas985
Next is the concept of “pulling” water. In the past, there has been some debate on if water is pulled over a siphon or is pushed over a siphon. Most physicists now agree water is pushed over a siphon and Bernoulli’s equation fully explains the flow rate of water with pressure differentials and changes in height. Also, a siphon will still siphon if both tanks are under the same positive pressure and the top of the siphon is under positive pressure albeit less than the tanks. The pressure differentials are relative and it does not depend on a vacuum.

Of course water cannot be "pulled". It is just a conceptual device that can be used to make things easier to explain. When someone sucks water through a straw, they are just lowering the pressure at the top of the straw enough for the ambient atmospheric pressure to push the water up the straw. If there is a vacuum relief valve, then a siphon cannot work. If there is no vacuum relief, then a siphon can work. This is the difference between an open loop and a closed loop.

mas985
As long as the vacuum release valve remains closed air is kept out of the pipe, both the open loop and close loop systems behave identically.

This is the key point. If the vacuum relief valve remains closed, then the return head has to be greater than the static head and the static head will cancel out. However, if one decides not to worry about closing the vacuum relief, then the static head does not cancel. The potential energy of the water coming from the roof can only be used for the trip from the panels to the pool. If the head loss of the plumbing from the panels to the pool is less than the height of the top of the panels, then the difference is lost and the static head does not cancel.

mas985
Where things get tricky is when the pressure is insufficient to keep the vacuum release valve closed and air gets sucked into the return pipe. At this point the system effectively breaks into two pieces that are somewhat independent. The first part is the pump overcoming the static head loss going up the pipe as well as the dynamic head loss in the pipe to the vacuum release valve. At this point the pressure is 0 PSIR and the pump only needs to overcome the pressure to this point. After this point, the system behaves as a gravity fed plumbing system. Water falls down the return pipe through the air and the return pipe may still be partially filled creating some head gain.

This agrees with what I have been saying.

mas985
This water height then determines the flow rate through the remaining part of the plumbing.

The flow rate, in gallons of water per minute, remains the same. It has to be the same as what is supplied. If the vacuum relief valve is open, you will get a turbulent mix of water and air. The water height will determine the velocity of the water air mix, but the flow rate remains the same.

mas985
So in summary, static head gain and loss can occur with or without a vacuum release valve and the plumbing system behaves identically as long as the vacuum release valve stays closed.

This agrees with what I have been saying. My point is what happens when the vacuum relief opens due to the return trip having less head loss than the height of the panels.

For example:

Solar off: Going from out of the filter to the pool, you have the equivalent of 100 feet of straight 2-inch PVC. Flow rate 48 gpm. The head loss is 4.24 feet.

Solar on, with a vacuum relief valve, and allowing the vacuum relief to remain open:
Going from out of the filter to the panels, you have the equivalent of 100 feet of straight 2-inch PVC. Flow rate 48 gpm. Head loss through the panels = 2 feet. Height to the top of the panels = 20 feet. From the panels back to the filter pad you have the equivalent of 100 feet of straight 2-inch PVC. And then, going to the pool, you have the equivalent of 100 feet of straight 2-inch PVC.

With the solar on, you have 4.24 feet of dynamic head to the panels + 2 feet of dynamic head through the panels + 20 feet of static head, which equals 26.24 feet of total head pressure that the pump has to push against.

From the panels back to the pool, you have the equivalent of 4.24 feet + 4.24 feet = 8.48 feet of head. Since this is less than the static head, you don't have to count it. Therefore, of the 20 feet of static head potential energy, you only use 8.48 feet and the rest (11.52 feet) is lost. This is a case where the static head going up was only partially offset by the static head coming down.
Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Sat 02 Oct, 2010 22:54

The situation can be made clearer if we use larger diameter pipes to reduce the total dynamic head loss, and take it out of the equation. Of course, it would take an infinitely large pipe to reduce the total dynamic head loss to zero.

However, if we decide that any total dynamic head loss of less than 1 foot can be considered to be negligible, then we could use all 4-inch PVC and the total dynamic head loss would be 0.5 feet. (48 gpm going through 300 foot of 4-inch diameter PVC.

In the case of an open loop, you are left with the static head due to the height of the panels and the dynamic head loss due to the water going through the panels. In my earlier example, this would be 20 feet + 2 feet for a total head loss of 22 feet. This is the theoretical design limit for an open loop.

In the case of a closed loop (less than 32 feet high), the static head automatically cancels due to the siphon effect, and you are left with only the dynamic head loss due to the water going through the panels. This would be 2 feet. This is the theoretical design limit for a closed loop.

The difference in theoretical design limits is the height of the panels. The height of the panels has to be added to an open loop and it does not automatically cancel, whereas for a closed loop, it does automatically cancel.

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