How to connect 16 solar panels for best performance

[henningdalgaard]

Great site!

I have 11 panels (size 120x300 cm) on my roof connected in one big parallel group. I use them to heat my pool (55.000 liters). I just got 5 more. The question is how to connect them for best performance (I live in Denmark so I need to get the most out of them). You can see a picture here:

https://dl.dropboxusercontent.com/u/21941908/solardur.jpg

I read somewhere that it is better to divide a large number of panels into groups. But should the groups be connected in parallel or seriel? Or combined? My friend (who knows a lot about pools and solarheating) suggested this setup:

https://dl.dropboxusercontent.com/u/21941908/diagram.JPG

I thought 3 or 4 groups in parallel were better but my friend pointed out that it would be the same as one big group in parallel.

Any ideas are most welcome

[Robert1234]

I assume based on looking at the pic that these are solar collectors for heating water. Looking at the panels in the upper end of the picture, it looks like everying is in one big parallel stream as you say. If you hook them up like in your sketch you should (may) get a higher outlet temperature at the end of the string, but you will also get higher backpressure and probably decreased flow rate through the system. My tendency would be to keep backpressure at a minimum to minimize pumping power and potential leaks. Hooking everything up in parallel will give you either (1) increased flow at the same outlet temperature, or (2) increased temperature at the same flow.

The one advantage you have in segmenting the system into multiple parallel groups (even though everything is still in parallel as your friend says) is that you have the ability to take a small set of them off-line for maintainence should you have problems and/or desire to work on them.

Just my thoughts.

[Robert1234]

P.S. What brand / model solar heaters are those? Are they made of neoprene rubber?

[henningdalgaard]

P.S. What brand / model solar heaters are those? Are they made of neoprene rubber?

They are made of polycarbonate type Solardur from germany. They are very sturdy and should last for ever...

[Mike90250]

They are made of polycarbonate type Solardur from germany. They are very sturdy and should last for ever...

In about 5 years, they will start to yellow. They will also microscratch from dust blowing in the air. Polycarbonate is used in aircraft windows and bullet proof glass sandwiches, but it is soft and scratches easily. Plastic does not last in outdoor exposure - at least none that I know of.
Look at the headlight lenses on cars in a parking lot, some plastics stay clear, some get frosty.

[henningdalgaard]

they are used and I think some of them are almost 10 years old.
I was actually thinking about painting them to give them a protective layer. Should it be high gloss og low gloss?

[henningdalgaard]

sorry, yes I use them to heat my pool (55.000 liters)

[Robert1234]

With it being a pool install, yes... All parallel will maximize collection of heat.

[J.P.M.]

Great site!

I have 11 panels (size 120x300 cm) on my roof connected in one big parallel group. I use them to heat my pool (55.000 liters). I just got 5 more. The question is how to connect them for best performance (I live in Denmark so I need to get the most out of them). You can see a picture here:

https://dl.dropboxusercontent.com/u/21941908/solardur.jpg

I read somewhere that it is better to divide a large number of panels into groups. But should the groups be connected in parallel or seriel? Or combined? My friend (who knows a lot about pools and solarheating) suggested this setup:

https://dl.dropboxusercontent.com/u/21941908/diagram.JPG

I thought 3 or 4 groups in parallel were better but my friend pointed out that it would be the same as one big group in parallel.

Any ideas are most welcome

1.) Being so far north, is there any possibility of tilting them so they are facing south and less horizontal ? If so, they will collect more energy.

2.) Without knowing more about it, I'd plumb them in parallel. I doubt if trying to get to turbulent flow is worth it or possible.

3.) If you coat them, make it flat black, BUT CHECK COMPATABILITY FIRST. I've seen, but never used driveway sealer as a DIY recoat on pool water heating panels.

[henningdalgaard]

the ones already installed (11 panels) are tilted about 25 degrees and facing south. The remaining 5 are flat. But in the summertime we have sunlight untill 8-10 pm so it should be okay.

[J.P.M.]

With it being a pool install, yes... All parallel will maximize collection of heat.

Parallel flow will not, in and of itself maximize heat collection. It's not that simple.

[Robert1234]

Parallel flow will not, in and of itself maximize heat collection. It's not that simple.

With regards to the plumbing (and that is all that is being asked about), I respectfully disagree. You are mildly heating a relatively cold source and the flow is almost assuredly turbulent in the solar collector itself. Also with it being a pool install, the pump is most definitely centrifugal. Putting back pressure on a centrifugal pump reduces flow. Reduced flow reduces heat transfer coefficient in the panels (U). Reduced flow also causes a larger heat rise in water of the panels thereby reducing the driving force for heat exchange.

Heat Transfer = Flow * U * Area * (Temperature Difference between panels and water)

Parallel plumbing maximizes the Flow, turbulence maximizes the U, and parallel plumbing also maximizes the difference between the panel collector temp and the average temperature of the water in the panel being heated. You're not trying to make hot water, you are trying to put the most BTU's into the pool that you can.

Yes, it really is that simple - but I can make it sound more complicated if you want

[J.P.M.]

With regards to the plumbing (and that is all that is being asked about), I respectfully disagree. You are mildly heating a relatively cold source and the flow is almost assuredly turbulent in the solar collector itself. Also with it being a pool install, the pump is most definitely centrifugal. Putting back pressure on a centrifugal pump reduces flow. Reduced flow reduces heat transfer coefficient in the panels (U). Reduced flow also causes a larger heat rise in water of the panels thereby reducing the driving force for heat exchange.

Heat Transfer = Flow * U * Area * (Temperature Difference between panels and water)

Parallel plumbing maximizes the Flow, turbulence maximizes the U, and parallel plumbing also maximizes the difference between the panel collector temp and the average temperature of the water in the panel being heated. You're not trying to make hot water, you are trying to put the most BTU's into the pool that you can.

Yes, it really is that simple - but I can make it sound more complicated if you want

I guess we'll agree to disagree.

To only scratch the surface:

There is, IMO, no assurance of turbulent flow short of designing it that way. Putting back pressure on a centrifugal pump does indeed reduce the MASS flow rate of the pump. However, that may or may not reduce the overall heat transfer coefficient in the collectors, which may or may not increase efficiency. All that depends on the design. Lots of things are interconnected here.

For the same (centrifugal) pump, plumbing the collectors in simple parallel will probably result in increased MASS flowrate over a simple series flow arrangement due to lower head loss as a result of lower fluid velocity and shorter flow length.

For the same pump, series flow, depending on design and system parameters, including pump characteristics, will likely result in lower MASS flowrate, but just as likely, at least in a well designed system, increased FLUID velocity (but not in the same ratio as the parallel/series split ratio for a lot of reasons). Provided the flow is either turbulent or will get that way as a result of the increased fluid velocity, this increased FLUID velocity will probably increase the heat transfer film coefficient which affect the overall heat transfer coefficient, rather than decrease it as you state. How much is a matter of design and heat transfer analysis.

The decreased MASS flowrate and the increased heat transfer as a result of increased fluid velocity in series arrangement both tend to increase fluid temps. However (and this is, IMO, only a small part of where the complication begins), the increased fluid velocity will, depending again on design and collector specifics, TEND toward lowering the average collector "plate" temp., thus lowering heat loss to the environment and in the end TEND toward increasing efficiency.

All these things and lots of others interact with one another and the environment in synergistic ways making good design an often interesting set of choices/tradeoffs.

As a practical matter, what often happens is a system is reconfigured to series flow without knowledge or regard to what will happen. Pressure drop doubles or triples or more, mass flowrate drops by an order of magnitude or so, the increased fluid velocity, such as it may be is not enough to make up the difference by any increase in film coeff. and lower plate temp. So, fluid temps rise along with some increase in overall collector temp. leading to increased losses to the environment. Heat transfer is less and people thus think series flow is less efficient than parallel when the real reasons are more complicated than that but lie in poor (re)design.

That's not my idea of good design, which IMO, is usually a series of choices and compromises.

My engineering experience designing industrial, refinery and power plant piping, fluid flow and pressure vessel systems, including heat exchanger systems leads me to a different conclusion about how simple things are. To me the simpler, the better, the more efficient, the more reliable. Unfortunately or not, it has been my experience to find that in spite of my best efforts, things are almost never as simple as I'd like to make them.

[Robert1234]

....the increased fluid velocity will, depending again on design and collector specifics, TEND toward lowering the average collector "plate" temp....

Really??? You gotta take into account this is a very low tech solar heater design. With this type of collector plate design (i.e. flat plate collector with no cover sheet), the collector runs about the temperature of the water inside it. If you heat the water up above ambient (which I am SURE you will do if you pipe in series), the efficacy of the system just goes to heck in a heartbeat.

That is probably where we disagree - and why I simply stated "parallel"

[J.P.M.]

Really??? You gotta take into account this is a very low tech solar heater design. With this type of collector plate design (i.e. flat plate collector with no cover sheet), the collector runs about the temperature of the water inside it. If you heat the water up above ambient (which I am SURE you will do if you pipe in series), the efficacy of the system just goes to heck in a heartbeat.

That is probably where we disagree - and why I simply stated "parallel"

I believe I have been taking such things into account and dealing with them since 1975, in ways and using methods that are, IMO, and attempting to write respectfully and professionally, beyond your current level of understanding given what you've communicated thus far. Therefore, I see nothing further to be gained at my end by continuing this exchange. The last word is yours.

Respectfully and Sincerely