shade-tolerant collectors?

Hi Folks,

I'm new here. I am shopping for components to add solar battery charging capability to my pickup truck camper. I think I have everything figured out except which collector(s) to get. I'm thinking of about 150-200 watts, 12 volts, probably 2 panels.

Because this is a camper and can move around, passing under trees, shading of the collector may be the biggest issue. I know that for some collectors, when one cell is shaded, the output of the whole array drops to that of the shaded cell. And, some collectors have a couple of zones and bypass diodes, so that only half the collector is disabled by a single shade cell. But, I have also heard of collectors with a diode in each cell (or some other mechanism) that prevents this behavior.

Can anyone direct me to a source these? I started looking at what's available, but there are an overwhelming number of possibilities.

Thanks in advance for any help...

- Bernard


  • BB.BB. Super Moderators, Administrators Posts: 28,172 admin
    Welcome to forum Bernard.

    In general, solar electric panels do not like shading. As you have seen/read, any shading can easily stop a panel from generating any useful amount of power.

    The best answer, is to install panels with no or a minimum of shading. Especially between ~9am and 3pm when you have the strongest sun.

    For basic shading issues... Years ago we had one person here that mounted all is panels in "landscape" (on their "sides") in the same orientation. For his panels, the two parallel cells strings ran lengthwise down the panel. When he would get sticky snow, it would only cover the bottom string of cells, and his array would still provide 1/2 current until the rest of the snow melted.

    There are two major types of charge controllers. The simpiler PWM type (pulse width modulation) controller (less expensive).

    These controllers the solar array Vmp voltage needs to be matched to the battery bank. If you have a 12 volt battery bank, then (typically) you would need 1x Vmp~18 volt panel string, then put multiple panels in parallel. If one panel was shaded, the other panels would produce full power. The bypass diodes are mostly irrelevant in this configuration.

    Another type of charge controller is the MPPT type (maximum power point tracking). You could put the same 1x panel Vmp~18 volt in series, then parallel them--And get the same result as the PWM controller.

    Or you can put, for example, 4x 18 Volt Vmp panels in series with a 12-48 volt battery bank. If one cell/panel gets blocked, then that cell/panel goes "high resistance" (no current wants to flow through a "dark cell"). Obviously, we need current to flow through a dark panel to get current/energy from the other series panels (plus, too high of voltage can ruin the non-conducting cell). So bypass diodes around a "group" of cells will let the current go "around" the dark cells--Generally, (if I recall correctly), there are 2 to 3 bypass diodes per group of cells (roughly 9-10 volt maximum drop or so).

    So, even with a bypass diode, you still lose a lot of voltage becasue of the dark cell(s). Power = Voltage * Current -- so if you drop 1/3 to 1/2 of the panel voltage, then you drop (roughly) 1/3 to 1/2 of the power from that panel/string.

    And if you have several strings in parallel--Then the "low voltage"/"dark" string may drop out from producing any useful current at all in the presence of the other strings that are not shaded.

    Which is better with shading problems, MPPT or PWM? It is usually not that simple. Usually PWM controllers are used on smaller solar arrays/systems and MPPT are used on larger systems. Rarely, do we get to make a choice on best output.

    For Grid Tied systems (not off grid), there are "micro inverter" type systems--These put one AC inverter per solar panel--So, that a shaded panel only affects its own output. Central inverters have the same issues as large MPPT controllers. A single shaded panel can take out an entire "string" of solar panels until the shade "goes away".

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • scrubjaysnestscrubjaysnest Solar Expert Posts: 174 ✭✭✭
    Bill, covered most of the points to consider. Since we are from FL the first thing we look for in a camp site is shade. For that reason we made our panels portable. By that I mean we use 3/4 inch pvc stands to support the panels with 8 awg extension wire limited to 25 feet so the panels can usually be placed in the sun. With RV's everything is a trade off since we want to keep weight down. If I were to do it over I think two to three panels in the 130 watt range with maybe something like the Morningstar Pro 30 for a controller. Again keeping portability in mind. We have found that full sun in summer for about 4 hours gives us plenty for our light usage, no TV, no high wattage devices like drip coffee makers, or microwaves. Winter full sun conditions our system doesn't make the 5% minimum for our batteries so it takes a lot longer to get to 100% SOC with most likely a reduction in battery life. Adding a 100 watt panel should fix that.
  • ShadowcatcherShadowcatcher Solar Expert Posts: 228 ✭✭✭
    We have a 185W 60 cell high voltage panel meant for a grid tie system (my mistake), it required an MPPT controller to use the voltage. Morningstar has a white page concerning the advantages. Our experience is that it provides usable current from sun up to sun down and with the panel shaded. We spent eight days on the north shore of Lake Superior in a camp site with maybe one hour of no shade on the panel. With a 150AH AGM battery and the main load being the WAECO refrigerator LED lights fan... at the end of eight days we were at 64% SOC. Even shaded I typically see ~35V.
  • littleharbor2littleharbor2 Solar Expert Posts: 1,108 ✭✭✭✭
    Shadowcatcher said: Even shaded I typically see ~35V.
      Solar modules will produce near rated voltage in very low light situations. The important numbers are amps. Amperage produced is heavily affected by the intensity of the sun hitting the module. Low light/ shaded situations will produce very low amounts of usable amperage.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • Dave AngeliniDave Angelini Solar Expert Posts: 4,135 ✭✭✭✭✭
    I saw some of these on a boat in the marina in santa cruz last week.

    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

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