PV Panel Underside Temperature

OldGeekOldGeek Registered Users Posts: 2
Hi All!

First time poster. We (the royal we) are getting ready to do a solar system on our 48 foot Chris Craft. This installation is to be above the vinyl covered aluminum frame flybridge.

What temperatures develop on the underside of the PV panels? We'll have the panels mounted on a frame of 1" heavy wall SS tube and I need to know how far above the vinyl top this needs to be. Less is better though.

Thanks in advance.

Old Geek

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,800 admin
    Re: PV Panel Underside Temperature

    It is hard to say... We have always assumed that under the panels was very hot--but animals love to live under panels in some areas and are very happy. And my attic for my home is much cooler with panels on it vs no panels (so roof is cooler too vs under full sun with panels).

    More or less, if you want to keep the panels cool, you should have them 5-6 inches above the roof for good air circulation. For a boat, that kind of gap may not be acceptable.

    My guess is that the vinyl roofing will be cooler with panels installed over the roof in any configuration.

    If you wire three or more panels in parallel (or strings of panels in parallel), you need to use a combiner box with a fuse/breaker per string. If one panel gets shorted, the fuse/breaker protects against the other panels from feeding too much current into the shorted panel (check the data sheet for the correct fusing--Usually around 10-15 amp fuse for "typical" solar panel).

    And, lastly, but really firstly, we like to design the system for your loads.... Are you planning on using the panels to keep the boat's batteries charged when in storage (something like 1% of bank AH capacity... i.e., 200 AH would need a minimum of 2 amps rate of charge) or longer term occupation with minimum use of genset/engines (something like 5-13% of bank AH capacity is typical--but you may have limitations on how many panels you can install).

    Welcome to the forum "Old Geek".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: PV Panel Underside Temperature

    It's a difference between "with" and "without" air circulation. The panels will, as Bill said, block sun that would otherwise induce heat in the surface beneath. However, if the air below them is trapped or even restricted the heat build-up becomes greater.

    Here's an example in solar non-heating. The next stage in improving my own house, which has East-facing large windows that picked up solar from sun up 'til about 1:00 PM. They are thermal pain, but that doesn't stop the solar gain. So I'm installing smoked polycarbonate panels in front of the windows. Blocks some of the light, but a lot of the heat. The key being the free space beneath which prevents thermal build-up between the panels and the windows. It is just the same for PV's and any surface below them.
  • OldGeekOldGeek Registered Users Posts: 2
    Re: PV Panel Underside Temperature

    Thank you for your responses.Attachment not found.

    Here is poor photo of what I'm working with. The flybridge is covered in vinyl stretched over the 1" aluminum pipe frame.

    This is what got in response to this question from a PV supply house in California:

    " The temperatures that can develop underneath solar collectors can vary wildly but are based primarily on ambient temperature, sun exposure and air flow. Most NEC wiring calculations are based around either 75 degrees C or 90 degrees C which is 167 and 194 degrees F (respectively). Air flow beneath the collectors is vital to system performance as heat causes the voltage to sag and the overall production to be diminished. The worst case temperatures per NEC guidelines are unlikely to happen, but it's fair to assume it will get hot. Many people in mobile applications will use a set of Z brackets to flush mount collectors which will only leave about a half an inch between the panel and the mounting surface and while it's not ideal, the system still works well. You lose a little efficiency, but the risk of your panel blowing of the roof isn't as high as if you had it stilted up. Typically 3-5 inches is a great gap to shoot for (depending on the size of the array). For mobile applications, 5 inches is usually too much, but it depends on the racking solution. If you engineer it properly (i.e. if it's beefy enough), you can mount it anyway you choose. Shoot for 2 inches or so and you should be fine."


    2-3 inches above the vinyl covering I can live with.

    I am trying to use four (4) Suniva 310 or 315 panels because the are USA manufactured and are Tier 1 efficiency at about 16+%. But getting JUST 4 panel is proving to be problematic.

    We are planning to cruise our boat through the Bahamas and at least to the Turks and Caicos, but running the 8 kW genset for 5-6 hours a day is untenable. We have eight (8) GC2 batteries (Trojans new this year) for about 1000 Ahr. The base load on the boat is 240 Ahr, so I think 1200 Watts of solar will help even if it doesn't supply everything. Yes more batteries would be better and I will continue to consider this and where I can put them on the boat.

    I'm figuring 2 series of 2 parallel panels, all run via MC4 cables to a NEMA 4X box with individual breakers. Then running into the engine room via #1 AWG cables to a Midnight Classic 150 and then to a buss for the batteries and Trace 2012 Inverter/Charger.

    Hooking up all the PV components will be the easiest part of this project.

    Old Geek
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: PV Panel Underside Temperature

    More batteries will not really be a good idea. For one thing, having eight 6V on a 12V system means four parallel battery strings and that introduces the problem of equal current sharing. More batteries would be more parallel strings and more problems.

    For another, your 1200+ Watts of panel will produce probably 77 Amps on a 12 Volt system. This is about 8.5% peak charge rate, which is a good number where it is. So if the batteries can meet your load demands you're in good shape. :D
  • YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: PV Panel Underside Temperature
    OldGeek wrote: »
    Hi All!

    First time poster. We (the royal we) are getting ready to do a solar system on our 48 foot Chris Craft. This installation is to be above the vinyl covered aluminum frame flybridge.

    What temperatures develop on the underside of the PV panels? We'll have the panels mounted on a frame of 1" heavy wall SS tube and I need to know how far above the vinyl top this needs to be. Less is better though.

    Thanks in advance.

    Old Geek

    I have 7" standoffs on all my panels. Allows roof maintenance without removing racking, prevents obstruction of wind access across ridge vents where the flat panels are at (no inspections in Mesa AZ, i had panels mounted very close to the ridge, but still have easy walking access to every panels' junction boxes), and lots of space under the panels for air to carry heat away . Standoffs are a pain though. The 5/16 holes had to be bored to 3/8 on the swivel thingys (Unirac SolarMount Evolution). Lots of labor.
Sign In or Register to comment.