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Solar Panel System - Swimming Pool Help

Solar Panel System

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Solar Panel System

Postby akaaka50 » Wed 29 Sep, 2010 19:45

I have 12 (3'x10') Solar Panels on the roof of my garage and the Solar Panels works well when I have 2~3 days of Full Sunlight, but I feel that the pressure in the lines is too much (13 lbs.), which does Not give the Water adequate time to "slowly" run through the panels and "heat-up" --- could this be possible ? Location: Honolulu, Hawaii Pump Horsepower: 1 1/2 I am planning to install some ball valves in-line to the panels so I can reduce the line pressure and "slow" the water in the panels....

Aloha,

JT
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Solar Panel System

Postby chem geek » Wed 29 Sep, 2010 20:34

You WANT the water to run more quickly through the panels since that is more efficient. If the water runs through too slowly, it heats up too much and this higher temperature re-radiates heat back into the air, especially if there is any wind. Panels operate more efficiently when the flow rates are higher and the panels remain relatively cool. You more than make up for the lower outlet temperature by the higher flow rate. The rate of temperature rise in your pool is based on the amount of heat delivered to it and that is based on the temperature delta TIMES the flow rate.

So while a flow rate that is twice as fast would have half the temperature rise if there were no losses, the amount of heat delivered and therefore the rate of temperature rise in the pool is the same. However, in reality there ARE losses and these are proportional to the temperature difference between the water in the panel and the air temperature. This post has a table showing you how the efficiency of solar panels drops as the temperature delta between the water temp and the air temp gets larger (when the air is cooler than the water). So you WANT the water to not rise too much in temperature in the panel and the way to do that is to increase the flow rate. Heat is still absorbed by the panel and transferred to the water, but it results in a smaller temperature rise of water coming out of the panel, compensated by the faster flow rate, but with lower losses from re-radiating/conducting heat from the panels to the air.
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Solar Panel System

Postby Allen G Myerson » Wed 29 Sep, 2010 21:48

You want to make sure that you don't over pressurize the panels as that will reduce their life. You should consult the panel manufacturer's Installation and Operation guide to make sure that the flow rate is in the range specified.

The key is to keep flow rate in the correct range and the pressure at the vacuum release valve above zero but as close to zero as possible. Most people make the mistake of thinking that the water coming back down will offset the pressure needed to push the water up to the panels. This is not accurate because the system is not a closed loop; it has a vacuum breaker. A closed loop system would have the pressure needed to send the water up be offset by the water coming back down. However, the panels cannot be under vacuum because there is a vacuum breaker. For every 2.31 feet that the panels are above the pump, you should expect 1 psi of pressure increase.

The vacuum breaker is on the pipe going into the collector(s) even if the collector orientation is sideways. We're trying to have some restriction and head downstream of the VB so it doesn't open allowing air in during operation. Air makes noise and the VB will not last 30 years if it is oscillating constantly.

Then turn solar on and verify the pressure is no more than about 2 or 3 psi (At the panels) when solar is operating. Do the checks and then replace the vacuum breaker right away. Do not purposely add pressure. All you need is less than 1/2 psi (at the panels, not the pump/filter) to drive more than enough flow through solar.
http://www.h2otsun.com/ps/psgeonmanual.pdf
Allen G Myerson
 

Solar Panel System

Postby chem geek » Thu 30 Sep, 2010 02:04

Though it's true one does not want to exceed pressure ratings of components, it is highly unlikely that 13 PSI (30 feet of head) measured at the filter (where I presume it is being measured) is going to be a problem. As shown in this spec for FAFCO SunSaver panels, the normal operating pressure is up to 30 psi with a maximum intermittent pressure of 45 psi. Remember that these pressures are those at the first panel (later panels have lower pressure) and that you will have pressure loss from water flowing through the filter and then through the pipe to the solar system especially if on a roof (i.e. as was pointed out if there is a 15 foot rise to the panels on the roof, there will be a loss of 15/2.31 = 6.5 PSI).

This PDF file is an installation manual for Aquasol. It too can handle up to 30 psi and you may very well need higher than 1-3 psi at the relief valve in order to achieve the minimum or recommended flow rates. This is certainly the case in my system where I estimate the pressure is at least 5 if not 10 psi at the relief valve when the solar is on. Unfortunately, the 2" pipe isn't really large enough for the long 60' runs from the pump to the solar on the roof and the even longer runs across the roof for the multiple banks of solar panels so I'm wasting energy with dynamic head (pipe friction) losses.

It's also true that you should have the flow rate in the manufacturer's recommended range. For the FAFCO panels I linked to, the recommended flow rate is 4 GPM per panel so with 12 in parallel (same number as in my system) that's 48 GPM overall flow rate, though the panel allows for a 3-8 GPM per panel range (the 8 GPM is not practical with that many panels as the pressures would be to high at that flow rate due to dynamic head from piping, filter, eyeballs, etc.).
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Solar Panel System

Postby Allen G Myerson » Thu 30 Sep, 2010 06:28

The pressure at the vacuum relief valve is largely going to be determined by the return line from the panels to the pool. Since it is all downhill, there should be very little pressure. In fact, in a properly designed system, you will probably need to add a restrictor to increase the return pressure enough to close the vacuum valve. If the pressure at your vacuum valve is 5 to 10 psi, that would indicate that the plumbing from the panels back to the pool is too small.

Assuming 15 foot panel height, that would mean return head of 6.5 psi at 0 psi at the vacuum valve. That would be equivalent to 48 gpm going through about 300 feet of straight 2-inch PVC pipe. So, I wouldn't think that your pressure at your vacuum valve would be as high as 5 to 10 psi.

As this Fafco installation manual shows, the rated operating pressure at 28°C (82.4 F) = 2.40 bar (34.8 psi), and at 90°C (194 F) = 0.34 bar (4.93 psi). This shows that the pressure rating drops off significantly as the temperature goes up.

There will be times, such as upon startup, that the panels will be well above 120 degrees F. It is my experience that most equipment does not do well at ay pressure above 1/2 of its rated operating pressure. And, this is for Fafco panels, which are probably better quality than some of the other brands out there.

I think that the best way is to measure the flow rate with a flow-meter to make sure that it is within the specified range and then to increase the return resistance until the vacuum valve closes. This way, you get the required flow at the minimum operating pressure.

If it spikes more than 5 psi, it's too much. Contact Hot Sun. The bottom line is DO NOT PRESSURIZE SOLAR PANELS! The lower the pressure on the solar panels the better.
http://www.h2otsun.com/ps/psgeonmanual.pdf
Allen G Myerson
 

Solar Panel System

Postby Allen G Myerson » Thu 30 Sep, 2010 06:50

That would be equivalent to 48 gpm going through about 300 feet of straight 2-inch PVC pipe.

Actually, it's closer to about 380 feet of straight 2-inch PVC pipe. Every 1 psi above 0 at the vacuum valve would be equivalent to adding another 58 feet of straight 2-inch PVC pipe.

PSI at................................Feet of 2-inch PVC
vacuum valve
.....0...........................................380
.....1...........................................438
.....2...........................................496
.....3...........................................554
.....4...........................................612
.....5...........................................670
Allen G Myerson
 

Solar Panel System

Postby chem geek » Thu 30 Sep, 2010 11:40

I understand your calculations though they ignore the resistance to flow from the returns themselves but that isn't huge: for the 3/4" eyeballs at 3 returns (so 16 GPM each, roughly, and counting only one since they are in parallel) that's only about 1.2 PSI (using K=2). You are right that I am probably overestimating the effect, but my runs are quite long. I believe the pressure relief valve is at the top of a two bank high set of panels in the middle of the roof -- there is another pair of them before those and one panel after. The distance from that top panel to the returns in the pool is probably at least 20' + 35' + 35' + 50' + 60' + 50' = 250 feet (maybe a lot more than that; there are lots of 90 degree elbows and I'm probably under-estimating lengths a lot) through 2" pipe (both the headers on the panels and the pipe connecting them and to/from the pump and the main one to/from the pool until split into 3 1.5", one for each return).

On the other hand, the top panel where the pressure relief valve is likely to be is higher than 15' from the ground so perhaps you are right that there is a flow restrictor somewhere. That would explain why my PSI seems higher than it should be based on straight calculations of piping, fixtures, filter, etc. though it's higher than expected even when solar is off and I wouldn't expect the restriction to be outside the solar loop (unless they made a mistake).

I have an IntelliFlo VF pump that I set to 48 GPM when solar is on (24 PSI read at the filter) and originally I had a Jandy 1 HP HHP pump (1.65 SF) that I believe was at around 55 GPM or so when the solar was on (around 28 PSI read at the filter, though with my IntelliFlo 54 GPM is 29.5 PSI at the filter so perhaps the Jandy was around 52 GPM) though that flow rate is just a rough estimate based on pump curves.

Based on what you are saying, I should be able to lower the flow rate on my pump and at some point I should see the vacuum relief valve open (unless the flow restrictor were dynamic to have more restriction at lower flow rates). What happens then? Do I get air seen in my returns? If they had set up the pressure at the relief valve to be close to zero originally, then I must be very close to that point now and lowering the flow rate should start pulling in air, right? I lowered the flow rate somewhat when I got the IntelliFlo because 1) it doesn't lower the efficiency of the panels very much (82% to 80%) and 2) greatly improves the energy savings (48 GPM with the IntelliFlo VF is 1500 Watts in my system with the solar on, 54 GPM is 2050 Watts, 42 GPM is 1180 Watts). I believe my older single-speed pump was more efficient at the higher flow rates than the IntelliFlo at the same flow rate, but I've cut my overall pump electricity costs in half with the IntelliFlo because it can be run at much lower speed with the solar off at high efficiency (26 GPM @ 275 Watts) and also saves as indicated when running at 48 GPM vs. 55 GPM.

I wonder where the flow restrictor, if any, is in my system. Where do they normally put it?

This brings up an interesting dilemma with solar panels on a roof. If they are on a roof without extremely long pipe runs or other restrictions, then they require flow restrictors which would increase pump energy costs substantially if there are a lot of solar panels so that high flow rates are required. Is there any other way to solve this problem with the pressure relief valve? It seems to me that a self-draining valve should be designed to be triggered to open based on flow rate (i.e. only close the valve when water flow "filling up" the panels is detected flowing towards the highest point) and not as currently designed based on pressure alone though obviously that's a much more complicated mechanism.
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Solar Panel System

Postby Allen G Myerson » Thu 30 Sep, 2010 17:57

The key thing to understand is that 1 psi at 15 feet up will generate the same flow as 7.5 psi at ground level. If we assume that the pump can supply more flow than the panels need, then you would just send the necessary flow to the panels and allow the rest to go through a bypass and back to the pool.

For example, if you had a pump that could send more than 48 gpm to your panels, you would use a flowmeter and close a valve until you achieved 48 gpm, and the rest would be sent directly to the pool.

If we assume that the return pipe from the panels to the pool was equivalent to 300 feet of straight 2-inch PVC (using 2.067 I.D, and a coefficient of 145), it would only generate 5.2 psi. You would have to add (6.5 + 0.5 - 5.2) = 1.8 psi of restriction to the return pipe from the panels to the pool to generate a positive pressure of 0.5 psi at the vacuum valve.
http://www.engineeringtoolbox.com/hazen ... d_797.html

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.

Based on what you are saying, I should be able to lower the flow rate on my pump and at some point I should see the vacuum relief valve open (unless the flow restrictor were dynamic to have more restriction at lower flow rates). What happens then? Do I get air seen in my returns? If they had set up the pressure at the relief valve to be close to zero originally, then I must be very close to that point now and lowering the flow rate should start pulling in air, right?


Yes, at some point, lowering the flow rate should cause the vacuum valve to open and you would begin to have air being sucked into the water stream. The only way to tell for sure would be to put a pressure gauge in the line near the vacuum valve at the same height to see what the pressure is.
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Solar Panel System

Postby akaaka50 » Thu 30 Sep, 2010 18:27

Thanks for All your Information.....

Aloha,

JT


chem geek wrote:You WANT the water to run more quickly through the panels since that is more efficient. If the water runs through too slowly, it heats up too much and this higher temperature re-radiates heat back into the air, especially if there is any wind. Panels operate more efficiently when the flow rates are higher and the panels remain relatively cool. You more than make up for the lower outlet temperature by the higher flow rate. The rate of temperature rise in your pool is based on the amount of heat delivered to it and that is based on the temperature delta TIMES the flow rate.

So while a flow rate that is twice as fast would have half the temperature rise if there were no losses, the amount of heat delivered and therefore the rate of temperature rise in the pool is the same. However, in reality there ARE losses and these are proportional to the temperature difference between the water in the panel and the air temperature. This post has a table showing you how the efficiency of solar panels drops as the temperature delta between the water temp and the air temp gets larger (when the air is cooler than the water). So you WANT the water to not rise too much in temperature in the panel and the way to do that is to increase the flow rate. Heat is still absorbed by the panel and transferred to the water, but it results in a smaller temperature rise of water coming out of the panel, compensated by the faster flow rate, but with lower losses from re-radiating/conducting heat from the panels to the air.
akaaka50
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Solar Panel System

Postby chem geek » Thu 30 Sep, 2010 21:11

(akaaka50, your welcome. Glad I could help. Sorry to hijack the thread with this other discussion, but it is still relevant for you as well as something new I've learned.)

Allen,

OK thanks. I understand. There isn't any bypass in my system, but there may very well be a flow restrictor. Do you know where they normally put that in such a system? I would assume it would be somewhere in the piping after the vacuum release valve. I suppose I can call SolarCraft that did the installation to find out.

I'll also experiment to see at what GPM I start to see air coming through the returns, just for curiosity.

People with really high roof or 2-story houses must have a lot of pump energy wasted to create sufficient positive pressure so high up. I'm really surprised someone hasn't come up with a relief valve not based on pressure (i.e. it would remain closed even with negative pressure relative to the atmosphere) and would instead get triggered by water flow through the pipe (i.e. would only open when water flow stopped as it would when the valve to the solar loop switched off).

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