Jump to content

Calculating Head Loss In Roof Mounted Solar Systems.


Recommended Posts

When the water is pumped to the top of a building or other structure, it gains potential energy. This potential energy can only be used for the return trip to the pool. Therefore, it does not provide any advantage to reduce the return piping head to less than the head provided by the height of the building.

The best you can do in a solar installation is: Suction head loss + head loss from pump to building + height of building + Panels.

The equation for head loss for a solar system is:

Suction head loss + head loss from pump to building + height of building + Panels + X

X = Head loss in the return to the pool – height of building. X cannot be negative due to the use of a vacuum breaker.

If the head loss in the return to the pool is less than the head provided by the height of the building, the water in the return will try to go too fast and the vacuum breaker will have to open. This will allow air into the system, which could cause problems such as noise and increasing pH from aeration.

Ideally, the head loss in the return from the building to the pool will be as close as possible to the height of the building without going over. Actually, you will want to have the return head loss to be slightly higher than the height of the building to maintain a low, but positive, pressure in the panels. Therefore, the head should be designed to be slightly less than the height of the building and increased as needed to the proper pressure by the use of a restrictor.

Example: Solar panels on a roof 12 foot above the water level in the pool located 200-foot away requiring 60 gpm. Suction pipe is 50 feet total length.

The suction should be limited to 3 psi or 6.93 feet of head, and a velocity of less than 8 feet per second.

A 1.610 I.D pipe would have a head loss of 4.7 psi and a velocity of 9.46 feet per second.

A 2.067 I.D pipe would have a head loss of 1.4 psi (3.205 feet of water column) and a velocity of 5.74 feet per second.

Therefore, a 2-inch pipe would be the correct choice for the suction.

The pressure piping should be limited to 4 psi (9.24 feet of head), and a velocity of 11 feet per second.

For the pipe from the pump to the building, a 2.067-inch I.D pipe would have a head loss of 5.6 psi (12.821 feet of head), and a velocity of 5.74 feet per second. This is too inefficient. Not acceptable.

A 2.469-inch I.D pipe would have a head loss of 2.3 psi (5.4 feet of head) and a velocity of 4.02 feet per second. This is an acceptable pipe.

A 3.068-inch I.D pipe would have a head loss of 0.8 psi (1.877 feet of head) and a velocity of 2.61 feet per second. This may be the preferable pipe if the 3.523 feet of head difference from the 2.469-inch pipe to the 3.068-inch pipe makes a significant difference in the choice of pump or the energy cost.

Costs can be calculated by various methods, including net present value, to compare the cost of going with a larger pipe vs. lower operating costs in the future.

The best choice for the pipe from the pump to the building would probably be the 2.469-inch I.D pipe.

Assume one foot of head for the solar panels

If you used 2-inch PVC for the 200-foot return from the roof to the pool at 60 gpm, the head loss would be 12.821 feet. Since the building is 12 foot tall, there would be no benefit in using a larger pipe.

Therefore, the total head would be

3.205 + 5.4 + 1.0 + 12.821 = 22.426 feet of head (9.71 psi)

If you used a 2.469-inch I.D pipe for the 200-foot return from the roof to the pool at 60 gpm, the head loss would be 5.4 feet of head and a velocity of 4.02 feet per second.

In this example the calculation for the head loss would be:

3.205 + 5.4 +12 + 1.0 + 0 = 21.605

This example is overly simplified and assumes equivalent straight lengths of pipe. Fittings add significant head loss to a pipe. For example, a 90-degree, 2-inch fitting is equivalent to 5.7 feet of straight pipe. I also included the length of the pipes going up the building in the pipe length. An appropriate "safety factor" should be added to allow for differences in theoretical design and "as built" reality.

For the PVC plumbing on the roof, I recommend the use schedule 80 CPVC as shown here:

________________________________________________________________

http://www.charlottepipe.com/Default.aspx?...mp;type=PVCCPVC

"CPVC Schedule 80 pipe and Schedule 80 fittings system is intended for pressure applications where the operating temperature will not exceed 200 ° F."

__________________________________________________________________

Now, in a closed loop system with no vacuum breaker, you get back up to the first 32 feet of building height when the water goes back down. This is because the falling water creates a suction that can be used by the water going up. Any height of the building over 32 feet can only be used for the return trip.

Therefore, the head loss for a solar installation is:

Suction head loss + head loss from pump to building + height of building over 32 feet + Panels + Y

Y = Head loss in the return to the pool – height of building over 32 feet.

Y cannot be negative.

The 32 feet is calculated by multiplying atmospheric pressure of about 14 psi by 2.31 feet per psi. Anything more than 32 feet would not be possible; and the water would begin to cavitate.

Link to comment
Share on other sites

  • 3 weeks later...
...This will allow air into the system, which could cause problems such as noise and increasing pH from aeration.

Since the bubbles from the aeration are large I would not expect a significant increase in pH.

The pressure piping should be limited to 4 psi (9.24 feet of head), and a velocity of 11 feet per second.

11 fps exceeds the max of 10 fps pvc experts recommend and exceeds the max 7 fps for an efficient system.

For the pipe from the pump to the building, a 2.067-inch I.D pipe would have a head loss of 5.6 psi (12.821 feet of head), and a velocity of 5.74 feet per second. This is too inefficient. Not acceptable.

Why is this "too inefficient"?

Costs can be calculated by various methods, including net present value, to compare the cost of going with a larger pipe vs. lower operating costs in the future.

Operating costs add up over time, cost of pipe is one time only. This is often overlooked...

This example is overly simplified... oh yea?? lost me long ago...

For the PVC plumbing on the roof, I recommend the use schedule 80 CPVC...

The system just became cost-prohibitive. A properly designed system works just fine with sch 40 pvc. There is no reason for the temperature to exceed 110° and should be much lower (in a typical pool system).

The easy way to calculate pipe size is to design for max 7 fps, if it is a long run increase pipe size for reduced head loss. Determine design flow using 3 to 5 gpm per panel (mfr has specs).

If calc gives pipe size between two standard sizes, pick larger size. Estimate total system head then you can pick your pump.

Link to comment
Share on other sites

"Since the bubbles from the aeration are large I would not expect a significant increase in pH." - billp

Aeration should be controllable by the pool owner. Creating enough backpressure to stop the air intake would not significantly increase total dynamic head.

____________________________________________________________

"11 fps exceeds the max of 10 fps pvc experts recommend and exceeds the max 7 fps for an efficient system." - billp

Yes, I had meant to clarify that in a post about proper plumbing design. The velocities that I gave in this post were for maximum velocities in short straight pipe with no 90 degree bends. They were not the appropriate velocities to use in this post. They are not velocities that should be used for system design.

I suggest that suction be designed for a total suction head loss of 1 to 4 psi. (2.31 to 9.24 feet)(2.04 to 8.14 inches of mercury), and a velocity of less than 7 feet per second if there are no 90-degree fittings. The velocity should be limited to less than 6 feet per second if there will be any 90-degree fittings.

I suggest that pressure pipe be designed for 2 to 8 psi (4.62 to 18.48 feet) and a velocity of less than 8 feet per second if there are no 90-degree fittings and less than 7 feet per second if there will be any 90-degree fittings. The plumbing should be designed to keep head loss to less than 10 feet wherever possible.

Obviously, the lower velocities and head loss will provide for a more efficient system. The system designer will have to take in many factors such as initial cost of construction, including labor and materials, and long term costs such as energy and maintenance costs. Appropriate Risk Management factors should also be considered and properly valued. Risks such as major repairs, unexpected system upgrades, remodeling etc.

___________________________________________________________________

"Why is this "too inefficient"?" = billp

It is too inefficient because the total dynamic head would end up being too large. Total dynamic head translates into increased power consumption.

____________________________________________________________________

"Operating costs add up over time, cost of pipe is one time only. This is often overlooked..." = billp

That is one of the points I was trying to make.

____________________________________________________________________

"A properly designed system works just fine with sch 40 pvc. There is no reason for the temperature to exceed 110° and should be much lower (in a typical pool system)." – billp

Temperatures on some roofs can go well in excess of 120 F. At high temperatures, PVC can shrink and the fittings can come loose.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...