Solar: 6-95W vs 2-275W? Boom Shadow

[KStepman]

I am about to install solar panels onto a hardtop Bimini:
78" fore and aft
90" across
My boom will cast a maximum shadow ~32" x ~6" across the forward part of the Bimini occupied by solar panels some of the time.
My objective is to minimize the solar energy loss resulting from my boom's shadow when falling on the solar panels.
Given the way modern, high density solar panels work, I believe that I have only two likely options:
To install 6- 20"x41" 95W solar panels, with 4- arrayed fore and aft across the forward part of the Bimini, with the remaining 2 across the aft part of the Bimini, producing a total 470W, or
To install 2- 40"x70" 275W solar panels lenthwise on each side of the Bimini.
With these solar panels wired in parallel, does anyone know which will produce the most energy when the shadow of the boom falls on the Bimini?

[noelex 77]

6x95=570w (not 470) if this is correct you have more watts, divided into more panels, with less shadowing so the 6 panels will obviously give better output.

[Sumner]

Originally Posted by KStepman

....To install 6- 20"x41" 95W solar panel...

Whose 95 watt panels did you get. We bought 80 watt panels from SolarBlvd.com and they are about 22 X 47, a fair amount larger. I think they are about $116.00 right now.

We ended up...



...with 4 across just behind the boom and 2 further back...



....that go out a little ways over the davits. The picture above is after the frame was mounted a couple months ago. We left the boat at the yard with just one panel in, not shown, in order to keep one battery up.

There is more here that I just posted today....

Endeavour 37 Interion Mods Index

Hopefully the rest of the install will follow in a couple days. We are using a MPPT controller and it is best to have similar panels with one of those. Which controller are you going to run?

Good luck,

Sum

[Azul]

Having just done almost exactly this job I think that you will be sorry if you try to mount six panels instead of two large ones both aesthetically and structurally and can avoid building a separate frame for the panels by tying the panels directly to the bimini frame which was a must in my application due to boom height. Bent stainless steel biminis are almost never symmetric, especially if anyone has ever moved any of the support clamps for example to tighten the fabric. I have two 145 W panels oriented longitudinally at a slight angle (like a slightly pitched shed roof) which took the place of my rear bimini fabric panel, and despite the several inch space between the two panels caused by the radar stanchion actually kept heavy rain out of the cockpit better than the sunbrella on the front portion of the bimini which leaked profusely during the numerous rainstorms of the last 10 days. If you have time to wait on the delivery of panels over 145 W which must be delivered by freight truck instead of UPS, even larger panels will work even better as sun/rain shades. OTOH, depending on the beam of your boat and layout of your bimini other considerations become potential damage to the panels by pilings when docking if they extend too far out and problems with attaching if the width exceeds the point where the bimini frame starts to arch down. Mine seem just right for the beam of my boat (10.5 feet.) If you are concerned about making the most power, I believe it would be much easier and more functional to set up lifting strakes to adjust the angle of the two panels to the sun ( I believe if you do a search someone on this forum had a nice setup to tilt panels on a bimini) than to divide the panes into six to minimize shading losses.

[KStepman]

Correction: 6x95=570 vs 2x275=550

I do not know yet the brand of the 95W panels. This was in a recent proposal from a vendor via email.

I do not appear to have the ability to upload an image of the hardtop Bimini I had constructed. It is constructed of 1/2" Starboard bolted fore and aft to 2" welded, stainless steel arches, and supported at the centerline, port and starboard by 1 1/2" welded, stainless steel tubes. Each solar panel is to be bolted to the starboard using 4" wide 'Z' brackets spaced 20" apart, or a track system.

The hardtop Bimini is 90" wide 30" short of the arch/Bimini width of 120" and the aft beam of 130," exactly for the purpose of clearing dock pilings in a heavy wind.

The hardtop Bimini extends aft of the existing SS traveller arch of my Hunter 36, and extends up to my stern (over my swim platform), but not as far out as my davits, which are Atkins crane davits that can be rotated inboard when not in use, or to be used to lift the outboard off the stern rail and lowered onto the dinghy transom. I engineered the upper end of the Davit support tube to tie into the aft SS bimini arch for additional for and aft support of each Davit.

The aft edge of the hardtop Bimini is a small antenna farm (AIS, GPS, VHS, Sat Radio, WiFi. A leangth of Sunbrella extends from under the outer edge of the Starboard, between the fore and aft arches, following the radius bend of the arch, to shed water run off from the hardtop and to increase shade.

Short of a photo, I hope this helps visualize the structure upon which the solar panels will be installed.

The problem is still the disproportionate affect of shadow on photovoltaic power production. Opinions are all over the place from 100% loss of power on a panel, if even one PV cell was in shade (on older panels), on down, but certainly not to 0% on modern panels built to bypass effected modules on a panel. As you all probably know, for this reason individual modules are laid out on the long axis of most modern panels. Therefore, panels should be installed in this same orientation to expected shadows, such as a boom or mast, to minimize the number of module crossed by a shadow.

The strategy of using more, smaller solar panels wired in parallel, is even if the entire panel is crossed by the shadow of my boom, and I lose 100% from 1 of 6 panels, worst case is a transitory 17% loss. The strategy of using two large panels is lower cost but with an unlikely worst case of 50% loss, but a more likely 17% loss, or less, depending on how the PV modules are laid out on the panel. With the small panel option, regardless of how it's modules and circuitry are layed out my loss is limited to 100% of 1/6 and not 1/2 the array.

I am at the point of purchase, and wondered if anyone had experience with either choice?

[Sumner]

K. Stepman, I'd like to see a picture of your setup. It sounds very functional and I like that. If you can't post, but can e-mail pictures I'd post them for you if you would like.

Originally Posted by KStepman

.... Opinions are all over the place from 100% loss of power on a panel, if even one PV cell was in shade (on older panels), on down, but certainly not to 0% on modern panels built to bypass effected modules on a panel. As you all probably know, for this reason individual modules are laid out on the long axis of most modern panels. Therefore, panels should be installed in this same orientation to expected shadows, such as a boom or mast, to minimize the number of module crossed by a shadow.

The strategy of using more, smaller solar panels wired in parallel, is even if the entire panel is crossed by the shadow of my boom, and I lose 100% from 1 of 6 panels, worst case is a transitory 17% loss. The strategy of using two large panels is lower cost but with an unlikely worst case of 50% loss, but a more likely 17% loss, or less, depending on how the PV modules are laid out on the panel. With the small panel option, regardless of how it's modules and circuitry are layed out my loss is limited to 100% of 1/6 and not 1/2 the array.

I am at the point of purchase, and wondered if anyone had experience with either choice?

I think people try and over theroize a lot of this stuff. My thought is put the panels on and use them and what you get is what you get. There are always going to be good days and good situations with them and bad. We only have 200 watts...



Macgregor 26S Outside Mods page 33

...on our Mac and it (180 watts at the time) supplied almost all of our electrical needs for 7+ weeks out and that included a fridge and a lot of computer time. We ran the gen-set....



Macgregor 26S Outside Mods page 14

... I made 2-4 hours total in that time. I'm thinking that the 480 watts we have now on the Endeavour, with even a more efficient fridge, is going to be more than adequate and would think you 500+ watts should be also.

I like the 'more panel' deal like you also for the same reasons. Also stuff happens like when we lost a panel at Marco Island. I'd rather loose 80 our of 480 vs. say 240 out of 480.

Put the panels up, use a quality controller (preferably a MPPT) and enjoy all of the quiet electricity you will have. We also have our panels/frame inside the sides of the boat except for one little corner that I don't see being a problem with a piling.

Good luck and hope you can post some pictures,

Sum

[Mark Johnson]

I have found that disregarding shading, from a space efficient and financial point of view, fewer and larger wins out.

HOWEVER... From the point of view of minimizing the considerable losses from even small "dark" shadows, it is best to have panels as far removed from one another as possible, assuming that one panel will almost always be shadowed.

I hook them together, in parallel, with a Schottky "one way diode" in all 4 of the + lines. This prevents back flow between Sun lit panels and shaded panels. The built in "by-pass" diodes do not do this. They prevent backflow "within" the panel, not between them.

IMO, the < 1V. of loss from using these Schottky diodes, is trumped by the fact that unlike on a house's array, there is always a shaded panel. We have been 100% solar on Delphys for over 16 years, and it works great for us!

IF you can rig one large, (>110W) panel on the davits, or rack like I did, and have it where you can tilt it into the early and late "low" sun, it will almost double the panel's daily Ah output! We seldom need to do this, as we are usually charged back to 100% before noon, but on cloudy days, we do it effectively.

Over a year, a large panel mounted as I described, will produce FAR more than a wind generator. Also, if you do "both", (on a back tower or rack), then your production will usually go DOWN, not up. This is because the wind generator usually shades the more effective solar panel, for much of the day... Don't do both, too close together.

One last thing. JUST LIKE IN SHORESIDE HOMES...
EVERY $1000 spent on low energy consumptive versions, of EVERY single device on the boat, is by far the most effective. It would take at least twice as much space and money, if you go the "produce more energy" route, instead.

Even though we have ALL of the "conveniences" onboard, we only use about 35 or 40 Ah/day, and this allows us to charge back 100% daily. The result of THIS, is that batteries last well over 10 years, there is more reliability, and everything works "better" due to higher line V.

M.

Btw, I have charged my batteries back to 100% on all 3 of my cruising boats, going back to the mid 70s. I have always found that while hard dark shadows knock them down as much 80%, a dark, cloudy, & overcast day, only knocks them down 50%. By having twice as many panels as required on a good sunny day, I still get back to 100% by the end of the day. It just takes longer!

[SunDevil]

^^^That is correct.

Diodes are a very good thing to get, make sure they are installed the right way though.

Here is some more information and RV examples. Make sure you have a big enough battery for your power draw as well. Reducing the power draw is a good thing to do if you haven't done so already.
The RV Battery Charging Puzzle « HandyBob's Blog

[KStepman]

Thank you SunDevil,
I have two current Deka 8A8D AGM batteries as my house bank on my 2005 Hunter 36, 490AmpH capacity.

[Sumner]

Originally Posted by Mark Johnson

...I hook them together, in parallel, with a Schottky "one way diode" in all 4 of the + lines. This prevents back flow between Sun lit panels and shaded panels. The built in "by-pass" diodes do not do this. They prevent backflow "within" the panel, not between them.

If your panels are wired in parallel I don't see the reason for the additional diodes if the panels already have blocking diodes in them, which most do now. The panels we use have 'blocking diodes" built in that prevent back flow. They are not 'by-pass diodes' that I thought were used if the panels are wired in series to allow flow around a panel that is shaded. Here is a reference...

Blocking and By-Pass Diodes Used in Solar Panels

Our panels are all in parallel,

Sum

[noelex 77]

A lot of solar panels don't have blocking diodes installed. Most solar regulators will disconect the panel at night eliminating the need.
All (boat) solar panels will have bypass diodes.

A few posters have recommended blocking diodes between panels in a parallel array. IMHO you will generally loose more than you gain with these additional diodes.

If in doubt it is easy to measure the discharge your completely shaded panel produces. It is then possible to estimate if the adittional blocking diodes will reduce or increase the overall power output.

[KStepman]

Originally Posted by noelex 77

A lot of solar panels don't have blocking diodes installed. Most solar regulators will disconect the panel at night eliminating the need.
All (boat) solar panels will have bypass diodes.

A few posters have recommended blocking diodes between panels in a parallel array. IMHO you will generally loose more than you gain with these additional diodes.

If in doubt it is easy to measure the discharge your completely shaded panel produces. It is then possible to estimate if the adittional blocking diodes will reduce or increase the overall power output.

You last paragraph makes objective sense.
I have yet to come across solar panels made for boats.
The awareness for the need of blocking diodes on the panels that I select is something I read, but failed to put on my requirements. Thanks you for reminding me that the down panel could create a draw from the other panels.
It was my understanding that good controllers have blocking circuits so the battery could not move current back to the panels.

[senormechanico]

Originally Posted by Sumner

If your panels are wired in parallel I don't see the reason for the additional diodes if the panels already have blocking diodes in them, which most do now. The panels we use have 'blocking diodes" built in that prevent back flow. They are not 'by-pass diodes' that I thought were used if the panels are wired in series to allow flow around a panel that is shaded. Here is a reference...

Blocking and By-Pass Diodes Used in Solar Panels

Our panels are all in parallel,

Sum

We have a couple of Aurinco flexible panels which do not have blocking diodes.
The documentation made a good point about it.
They state that more power is lost to the even small drop across a diode during charging than is lost due to back current at night.

I haven't measured back current with mine, but did measure back current a couple of years ago with my 120 watt rigid panel.

It was only about 20 milliamps at 12 volts. That's how much an LED draws.
In other words, not worth mentioning.

[noelex 77]

Originally Posted by senormechanico

It was only about 20 milliamps at 12 volts. That's how much an LED draws.
In other words, not worth mentioning.

Solar panels do vary a bit, but that is a very typical figure and that is for a fully shaded panel at night. To install a blocking dione at a cost of 0.3 v or so to eliminate the draw of a shaded panel (which will be less than this) just does not make sense IMHO.
If your controler does not shut down the panel at night there is reason to fit a blocking diode, but not for the very small loss associated with a shaded panel.

[KStepman]

Sumner has been assisting me in getting images of the Hardtop Bimini, Boom and the Shadow that fall on it. So, here goes...
http://purplesagetradingpost.com/sum...-1-smaller.jpg
http://purplesagetradingpost.com/sum...-2-smaller.jpg
Hopefully, you will be able to see two images: One, the top of my Hartop Bimini of 1/2" Starboard, 90" port to starboard, 78" for and aft, and the ~32" x ~6" Boom Shadow that, when at anchor, will fall on or near the centerline of the Bimini, in a space allowed between my Solar Panels. And, when sailing close hauled, can be expected to shadow one or more modules on one solar panel, but not an entire panel.

The other photo is of how I engineered the Bimini support. My Hunter already had a heavy, twin, 2" tubular, ss arch for the traveller. What I did was have fabricated a reverse facing arch welded onto the existing one. With three 1.5" ss tubes welded between the two arches, together supports the Starboard top.

The aft facing arch also serves the purpose of added support for the tops of my rotating davits, that also serve as cranes. Now, with a longer support tube on my davits, and supported at three points, the dinghy may be higher off the water.

For those interested further, you will note a small antenna farm on the aft end of the Bimini: two VHF, one WiFi, a GPS, and one for XM radio. The GPS and one VHS is for my AIS transponder. The other VHS is a spare.

Regarding the decision to install six smaller or two large solar panels, I have decided that the two will work as I expect. Meaning that by leaving a space between the panels at the centerline, when at anchor, with my topping lift eased, there will rarely be a shadow on either solar panel. When there is, the panels that I have chosen will each have 60 cell modules running lengthwise (fore and aft) on the panel, each with bypass diodes. Thus any shadow from the boom will affect only one, and at most six modules, for only a 2% to 10% loss in energy. At least that is the theory. We shall see if reality agrees.

Thank you all for your help. Now let's see if the pictures link.