# Solar panel output at sea

Discussion in 'Boat Design' started by floating, May 6, 2015.

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### floatingJunior Member

I need to estimate the power output from vertically mounted solar panels on an ocean buoy. I have the downward solar radiation [W/m^2], sun angle, and panel area [m^2], so I can estimate the output due to the sun shining directly on the panel. But reflected/scattered radiation from the water must be a pretty big contribution. Would love a pointer to a formula to calculate this, or a head's up that it's not a big contribution. Note these are film solar panels.

Last edited: May 7, 2015
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### portacruiseSenior Member

Have you considered buying the smallest flex panel, wrapping/taping it around a buoy shape and measuring output while upright in water? Some older panel formulations do relatively better in ambient light, yet are not as efficient in full sun compared to state of the art.

PC

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### gonzoSenior Member

One thing you can be sure of, is that the panels will only be vertical momentarily. There will be a contribution from the reflection on the water in some conditions, but will mostly be a flicker of varying intensity. Why are the panels vertical, are the buoys in a very high latitude?

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### floatingJunior Member

Portacruise: We do need to put these panels on the buoy (or at least a dock piling) to measure their output, for example is efficiency really 20% as vendor says, and does salt/gunk accumulation in the ocean environment reduce output? These are solar film panels, as you suggest (have edited original post to say that).

Gonzo: Panels are mounted vertical because that's where they can be fitted onto the buoy. Good point about the buoy's motion affecting panel orientation. Most interested in solar panel output when things are dead calm in summer, when buoy & panels should be fairly vertical.

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### portacruiseSenior Member

Vendor ratings are typically absolute maximum measurements at absolutely ideal conditions. I would expect ocean environment to reduce output due to fine salt coating. Wouldn't be much water reflection with sun directly overhead which is normally peak and I don't expect much gain at other angles since it is only partial reflection, not like a mirror, even in dead calm. It may be that a much smaller high efficiency cell mounted on top horizontally will produce more output, even with considerably less surface area.

FWIW.

PC

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### philSweetSenior Member

Floating, you have a rather daunting task. You have to model the output of each individual cell with respect to its buoy position, time of day, sea state, buoy orientation, buoy motion, and insolescence; and then model the cell connections and derive a model of instantaneous module performance as a function of load and integrate that over a day to get performance. This is not for the faint of heart. There is an old Sandia National Labs document floating around that has a bunch of empirical formula for modelling module performance given odd cell orientations and fluctuating conditions. Modern controller electronics will also play a big role in getting something like this to work. There is no easy way to do this. This is why a whole crew of engineers worked on the problem at Sandia and in Hawaii for a year.

Normally, the two things you can do to have some hope at predicting output of a solar power system are to keep the cells in a module coplanar and equally irradiated, and keep the panel steady or tracking the sun. It is very difficult to predict overall system efficiency if you don't do this. Each module that has a different orientation should have a separate isolated feed to the controller. Modern MPPT controllers will maximize the power generation from each input and combine them.

So if the point is to get predictable power, you want flat, mostly horizontal panels. Four panels each tilted 5 degrees from flat can be combined in series in opposite pairs and run to a two input controller. That should be plenty interesting to model.

This one looks about right. Most of the pics I looked at did show a lot more angle to the panels that what I would expect, and I don't know why. Probably for self cleaning or just to have the panels not extend beyond the footprint of the buoy.

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### floatingJunior Member

phiSweet: To reduce the task, I am making a lot of simplifying assumptions. I have a hourly timeseries of downward radiation and sun position, so I can estimate power output from direct radiation on the panels. I am ignoring buoy motions because solar panel output is most important in dead calm for this particular project. Solar panel efficiency is reduced to account for salt/grime.
But the big question: how can I estimate additional solar panel output due to scattering/reflection off ocean waves? Presently I am neglecting this, which may be justified for the 'dead calm' scenario that is the focus. I did find 'Reflection from Capillary Waves' in the book 'Introduction to Remote Sensing' by Seelye Martin, and hope to get something to use from there. Definitely a specialist area but I suspect there are sensible rules of thumb out there (I just don't know them).

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### gonzoSenior Member

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