Add Solar or not - A Year of Solar on MV VIKING STAR in the Pacific Northwest

http://mvvikingstar.blogspot.com/2013/06/a-year-with-solar-or-is-it-wait-until.html

Friday, June 14, 2013

A Year with Solar, or is it: “Wait until he figures out what a monumental waste of money that was”. . . . .

Less than a month after installing our two large Solar Panels on Viking Star I overhead a passerby make the above statement to his companion.  I did reply to him that he was a bit presumptive with his judgment and that modeling indicated they would have a ROI of less than 5 years.  I can tell you he just discounted me as one of those ‘fantasy’ types.  Well, here we are – a bit over 1 year on this system and I think it would be good to look back and see how well they actually worked.   Was installing Solar a great idea, or as so elegantly put a ‘monumental waste of money’?

Before deciding to place the panels aboard, one of the greatest difficulties I had was getting actual DATA from folks on how their panels performed.  I got things from ‘All We need’ to ‘Great’, to ‘Fills my batteries by 11am’.  (Monumental guy did not have any panels, so I am not considering his input).  But without exception:  all the ‘observations’ were gut feelings.  No real data.

Being a (at one time, before crossing to the dark world of Marketing) Engineer – this just would not do.  So I set out to model the panels, you can read more about that here: 
           mvvikingstar.blogspot.com/2012/10/solar-panels-on-boat-modeling-and.html

- - - snip from above url - - -

Solar Panels on the Boat - Modeling and Performance

One of the challenges I had while deciding about installing solar panels on Viking Star was getting an idea of what to expect.  Asking around got a WIDE range of answers, from: “All I need”, to “Charges the batteries by 11am!”, to “Will not work at all in the rainy PNW”, to even “What a monumental waste of money solar is”.   Needless to say - no consensus.  More troubling was, without exception, none of the "inputs" were based on  ACTUAL data.   Even those who claimed ‘All I need’ were not able to tell me what their needs were – rendering their input useless to me.

I just could not go down a path blind.  I needed SOME idea on what to expect.  And so the old engineer in me emerged.  Solar system output is really a factor of four things:

1. Expected solar radiation from the sun  (time of year, location, and cloud cover)
2. Size and efficiency of panels and system
3. Orientation of panels relative to the Sun
4. Placement of the panels relative to obstructions (shading)

(BTW, the following is the Long Path I took.  To see a shortcut, skip to the end of this blog post!)

Solar Radiation:  I located a local shore-based installation who had been tracking their results and who would send me their actual energy produced data.  Using this I was able to extract the solar radiation per month in this location covering not only seasonal variations (Summer vs.Winter), but also average cloud cover over a year period.

Size and Efficiency of panels and System:  This is easy.  Viking Star has two 240w panels  so a 480w system.  Completing the system is the MPPT controller I selected which has a conversion efficiency of 95%. Need to throw in some additional losses for the wiring voltage drop.

Orientation relative to the Sun:  Here is a point where installations on a boat differ greatly from shore-based ones.  While most shore installations are pointed due south and tilted towards the Sun (often at a compromise angle between peak output for summer vs. winter) panels of a boat are often placed flat.  (or some very small angle). Mounting flat is easiest.  While using a more complex mounting system and being able to point the panels towards the Sun can give significant output gains  – it also requires adjustments throughout the day as the Sun moves and as the boat swings around anchor.  (Shore-based installations do not need to deal with this swinging - hence they often just use fixed tilt mounts pointing due south)  I chose to mount Viking Star’s panels flat for two reasons: The panels are rather large - a tiltable mounting system would have to be very robust to securely handle them.  Plus I am just lazy.  I do not see myself climbing out several times a day ‘repositioning’ the panels as we swing around the anchor…

How much do we lose by mounting flat?  That depends on the time of year, as well as the latitude of the boat.   The actual formula is: 

   Loss% = 1 - SIN(90 – Latitude + Solar Declination + Panel Tilt)  

(Yes, those Celestial navigation skills come into play!).   Putting this all together we get  for 48 North:

	Sun Declination (Degrees)	Solar Elevation Angle (at 48N)	Loss due to  Flat Panels
Jan	-22	20	66%
Feb	-13	29	52%
Mar	-3	39	37%
Apr	10	52	21%
May	18	60	13%
Jun	23	65	9%
Jul	21	63	11%
Aug	14	56	17%
Sep	3	45	29%
Oct	-8	34	44%
Nov	-18	24	59%
Dec	-23	19	67%

During peak cruising months (April – September) the loss is relatively small  – 10-20%.  But this quickly increases as we get into winter where 60-70% of the potential output is ’lost’ due to mounting the panels flat. Combined with the much shorter days means during the winter we can expect little output from the solar system. Happily those are also the months we tend to be in port with shore power readily available.

Combining this loss table with the observed Solar Radiation derived from the shore based installation's production data,  I came up with the following modeled output for our system:

	Ah/Day (480w system)	Normalized (100W system)
Jan	11	2
Feb	38	8
Mar	66	14
Apr	110	23
May	149	31
Jun	153	32
Jul	161	33
Aug	142	30
Sep	99	21
Oct	50	10
Nov	17	4
Dec	16	3

And here you can see the actual results so far:

We have been tracking ahead of the predicted performance, but then this summer was one for the records books (literally).  Side note:  Those BIG dips to 50 or below the 1st month are the result of not attaching the remote voltage sensor to the MPPT controller.  See more here: http://mvvikingstar.blogspot.com/2012/06/controller-alternator-interaction.html

An easier way (the Shortcut)

If you too want to model output of solar panels you can follow the steps above (and I can send you .xls files with all the formulas, though you will need to locate someone near your cruising grounds that has reliable data from which you can start the assessment).   OR, you can just go to this site (which of course I found AFTER going through all the above): http://gisatnrel.nrel.gov/PVWatts_Viewer/index.html

It not only has a large database of solar radiation throughout the world, but will also do the tilt (orientation) adjustments for you!  On the map click the location you are interested in, and then select the ‘Send to PVWatts’ button.   That will transfer the Solar Radiation data for your selected location into the Tilt calculator where you can enter your system size as well as tilt angle.  A few notes:

• PVWatts will not work for systems under 1,000w in size.  A workaround for this is to multiply your expected system size by 10 and enter that .  For example: Viking Star’s 480w system is entered as 4.8 kW (4,800 watts)  in the program.  
• ‘DC to AC Derate Factor’ is the efficiency of the rest of the system.  I found using 0.85 worked well to account for the MPPT controller efficiency and voltage drop across wires.
• ‘Array Tilt’ -  Enter 0 if the panels are flat or leave it with the default latitude for a good approximation if you plan to tilt the panels.   You can get even better results by changing the mounting to 1-axis or 2-axis tracking (depending on your mounting system - and desire to reposition things throughout the day), but I did not play with those.
• Press the Calculate button to get predicted results throughout the year.
• Looking at the AC Energy (kWh) column, divide these numbers by 4 to back out the 10x we did in step one and make a rough conversion into Ah per day. (for a 12v system.  If you have a 24v system, divide by 2 for a ruff idea of 24v AH's)

This web page predicts 162Ah / day expected output from Viking Star's system in June.  A bit more than the 153Ah I modeled, but our actuals aboard Viking Star seem to be running ahead of modeled results.

The final issue is shading.  Here you need to get creative and try to minimize it.  The MPPT controller, combined with built-in bypass diodes in the panels will help a LOT with shading, but still shading will reduce output dramatically   During the peak summer months I noticed about a 30Ah reduction per day if we were docked pointing North as opposed to South, this I expect was due to the shadow from our mast when docked pointing north.  Just one more ‘consideration’ when pulling into dock now. . .

BTW:  There is a good argument for using a separate a MPPT controller for each panel installed on a boat, to help work around this partial shading situation.  Given that MPPT controllers are becoming more and more common, and the prices are dropping, doing so would be a good consideration for future installations.  But if you do use a single controller, like on Viking Star, make sure to wire the panels in SERIES to maximize output during partially shaded conditions.  And note this is a dramatic departure from what was recommended a few years ago (Parallel).  It is due to both the usage of the MPPT controller and increased presence of zoning / internal bypass diodes on modern large panels.  If connected parallel - a shaded panel will drop out entry  while if a modern panel is connected in series the partly shaded panel will lose the shaded segments, but the unshaded segments will continue to contribute (due to internal panel zoning and by-pass diodes).

Overall we are happy with the system.  It accomplished it's key goal: reduction of generator time.  It is only this last month we needed to start using the Generator, and that was just for covering the drying cycle during laundry.  Though as expected -  as we are heading into the latter part of October we are finding we need to put time on the generator, varying from day to day.

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To get actual data - the charge controller I purchased has data logging capabilities.   But before we get much further, let’s make sure some ground rules are established:

• We cruise in the Pacific Northwest- if you are somewhere else, likely your results will be different.  (See the Modeling post to learn more).
• Our  ‘Cruising’ season is March through the end of October.  During this time we are very rarely in port, and even more rarely have access to low cost shore power.  

That last point is a critical one, as it means -- almost without exception -- every AH we consume we need to make on the boat.  Either via the large 270A  Leece Neville alternator attached to our main engine, or via the Kubota Dc generator -  OR  via the Solar Panels.  And this, to be fair, is huge.  It is VERY EXPENSIVE to generate electricity on a boat.  I estimate it costs $1.14 / 100Ah running the DC Generator – direct costs only (Oil and Fuel).  That does NOT include any amortization for the original cost of the Generator, or depreciation for its replacement.  Every 100Ah costs over a buck….   And with us away from the dock so long, costs add up very quickly!


OK, on to the results.  Here is a graph of the actual daily AH log as provided from the Solar Regulator.  The thin purple line is the actual daily results – you can see cloudy days and sunny ones.  The black line is a 15 day rolling average - to try and smooth things out a little.  Finally, the thick blue line is what the modeler predicted our output would be.

1 year actual results
(Click on image for a larger version)

You can see the modeler did a rather good job of predicting.  In fact, over the year the Actuals were 2% greater than the Modeler predicted.  Looking behind the scenes in more detail:  Over our cruising season (March through the end of October) we produced an average of 124.7Ah/day.  We saved 245 hours of run-time on the generator and that results in a raw ROI of 5.5 years.   Figuring in the Federal Tax Credit, it drops to 3.9 years.

So, for a boat cruising in the rainy Pacific Northwest, the Solar Panels have a ROI of under 4 years.  And best of all?   I do not need to listen to the Generator during most of the time as a result!   In fact, we are back into the time of year where we likely will not need to use the Generator at all until the end of September.  And then only when we do Laundry.

Posted by Viking Star at 3:29 PM