Rev. April 20, 2014
Cruising farther means more nights on the road. Limited battery capacity flat means more nights in marinas. That's money, but more importantly, it means bending our plans to fit in marina stays, and I don't like that. We could go sans power, but when the mid-summer Chesapeake humidity hits and the wind fades, fans equal sleep.
We could run a Honda generator or use a wind generator. Too much noise. I like passive and silent, so panels it is. We have a substantial deck, but we use all of it, either lounging or sailing. Only 2 small areas on our hard top remain low-traffic, no more than 15 square feet.
We have 270 amp-hours (AH) in nameplate storage capacity from 3 x group 27 batteries, but realistically we can only use 140 AH without straining the batteries and shortening their life. We know from experience that we run about a 40-70 AH deficit each day, depending on use of lights, DVD, fans, and CPAP.
The available 15 feet square will fit 2 x 100-watt panels, or about 60 AH in real-world charging capability, if we consider shading and passing clouds. While this won't replace out entire deficit when we're energy hungry, it will stretch our battery life and may be enough if power is used responsibly and charging is supplemented with some coincidental engine time. For better or worse, this is our chosen compromise.
Price matters. We also needed a specific shape. I hunted for something cheaper with good customer ratings and came up with these:
They are of a simple design that appears to be well executed. I load tested them in the front yard, in the sun, and they were on the numbers. Nice wiring boxes; however, accessing an additional knockout required some very careful drilling (we padded and protected the back side of the cells with thin plywood over cloth), since smacking it firmly with a screw driver seemed unwise. Once inside, there are plenty of extra terminals.
(Note: to "yard test" the panels you will need a load close to the projected out put of the cells. Other wise, you will be so far from the maximum power point you will see a much lower value.Of course, without an MPPT controller in the circuit, this means running a bulb at 18 volts, so keep it brief!)
The adjustable feet used to compensate for the crown in the hard top may be handy any place a self-leveling or adjustable mounting is needed, just as threaded studs are used to mount and level engines, traffic signs, industrial equipment, and outdoor lighting poles; perhaps the only innovative part of this installation. they neatly address the recurring challenge every D-I-Y sailor faces who lives and hour from his boat but likes to tackle significant projects; how do I prefabricate as much as practical and insure it will fit easily and properly when I reach the dock? This was a true piece of cake.
The hard top is not flat; in fact, it domes asymmetrically about 3/4-inch over 3 feet. Additionally, we need to protect the panels from accidentally dropping of the boom and provide air flow under the panels (PV cells lose efficiency when hot--more on that later). I also dislike the idea of drilling holes in a foam core deck. The solution? A simple aluminum frame and 4 adjustable feet for leveling and load distribution.
Note: remember to degrease bolts and rods before embedding in epoxy. Residual thread cutting lubricant will interfere with the bond. A little sanding is good too. In this case, however, because the nuts tighten against each other, there is little torque on the rod.
Installed with nuts and washers above and below the aluminum frame, these provide a solid mount that easily accommodates the curve of the deck. In my case I embedded the cut end of the rod in the FRP block, but in some cases it may be desirable to leave the studs very long and then trim after test-fitting. To insure that the threads are not spoiled when the rod is cut, be certain to thread the nuts on first. After cutting the rod, camfer the cut end and then remove the nut to straighten the threads.
These are another excellent choice, though less "trick" looking. I have used these for interior equipment mounting, including bolting down an air conditioning unit. From Duckworks.
The frame is nothing more than two 2" x 2" x 1/8" aluminum rails bolted to pre-drilled holes in the underside of the panel frame. These rails are as stiff as a pine 2 x 6 and are high enough to keep the boom off the glass. The slightly elevated mount allows free air circulation under the panels for cooling and drying. To insure good alignment...
Note 4-20-2014: The boom has contacted this frame numerous times, occasionally with considerable force (me winching, not realizing the boom is resting on the frame) with no damage. I think the design is about right and would not go any lighter in this location.
- Drill the holes in the frame only slightly oversize (+ 1/32-inch) to insure good bolt alignment. Wobble the drill slightly to create an allowance for misalignment.
- Dry-run the installation, adjusting the legs to the correct height.
- Tape off the outline of each foot to minimize the epoxy mess. Sand the gelcoat surface.
- Bond with thickened epoxy (West System with fumed silica/Cabosil). Tighten the nuts slightly to insure the feet are aligned vertically with the frame. Remove tape before the epoxy cures.
- Be warned, the whole assembly will try to slowly slide off the deck. Temporary props and duct tape may be useful.
- Remove the masking tape before the epoxy is set. It become very difficult later.
- After final cure. remove the equipment and frame and re-drill the mounting holes about 1/32-inch larger to insure easy removal in the future. If there is significant crown on the deck, there may be some binding during this removal step because the studs will not be parallel to each other.
- Reinstall and tighten the nuts.
Pre-bonding adjustments were made.
A 15 amp charger with an LCD screen is mounted in the starboard hull equipment bay, behind the steering gear and galvanic isolator. Short wire runs, out of the weather, and easily accessible. PDQ did a nice job with the access panels.
Morning Star 15 Amp charger
What about MPPT (Maximum Power Point Tracking) chargers? Photovoltaic cells (PV) don't simply crank out 12 volts of electricity and magically charge batteries; they put out something between 23V and 0V at variable current. The maximum power point, where V x I = W is at a maximum, is generally at about 16.4V. An MPPT controller senses this and keeps the panel output in the sweet spot while providing the battery the voltage it requires for proper charging. That is assuming cool temperatures and a blue sky.
For a little more on MPPT charging, Wikipedia is always a quick source: Wiki on MPPT chargers
If there are clouds or haze, or if the panel is heated above it's rated temperature, the maximum power point will shift left, to lower voltage. On the Chesapeake in mid-summer heat can easily shift the MPP from 16.5V to 15V. What is the actual required charging voltage? That varies with the state of charge; when near full charge 14.5V is a very good match, but when first charging a 50% discharged battery we may need only 12.2V and some efficiency will be lost, perhaps 16%. MPPT is at it's best when first charging deeply discharged batteries.
What does a power curve look like? The below table and graph are for a 1.1 watt panel, but you can easily scale it to fit your application. All 36 cell panels will have very similar voltages and power curves, regardless of wattage. This graph assumes 75F ambient temperature, a high sun and a right angle orientation, and NO shading or cloud cover.
How much does heat lower efficiency? the output voltage is lowered by about 0.041V/F, or about 1V lower on a scorching day than on the cool spring day when you did your installation (assuming you allowed good ventilation to the underside--it could be worse). In other words when it hits 95F, the MPP voltage will be about 16.4 - 1V = 15.4V. Since out electric load is maximum on a hazy 95F day, this is the most relevant MPP. (Some will argue that cool weather charging is more critical, since the days are shorter, nights longer--more lights--and the sun lower, but it is running the fans in the summer heat that always does us in. Plan according to your reality.)
Shading due to haze lowers the output amperage but does not significantly lower the MPP until severe (evening or heavy clouds). Spot shading (a sail or even a shroud) can be devastating, depending on whether it takes a portion of a column (small amperage drop) or a row (small voltage drop that effectively shuts the panel down). However, the reason we did not put a panel under the boom was not shading (if we want zero shading we take the boom far to the side); it was because we walk there when furling the sail and wanted to leave one free impact zone where we wouldn't worry over sailcovers and ropes and even loungers.
For some detail on panel output corrections: PV cell output
Series vs. Parallel Wiring
This has been debated to death on the web. When panels are wired in series an MPPT controller can deliver slightly more power during periods of low light; simply put, the voltage can stay at usable levels longer and resistance losses can be a bit less. However, if any shading occurs, the drop in output is much larger than it is in parallel installations, where only the blocked cell is depowered, not the train. For boats where some shading is likely, parallel wiring is more practical. For a terrestrial roof top installation, series wiring and higher voltages can be explored.
And I didn't even analyze the shading loss owed to seagull poop. It turns out we can't really see much difference when we clean the panels; I guess we're disgusted long before it is functionally too bad.
I've probably given up 5-10% in charging capacity on a typical day by using a simple controller. I suspect for most people, larger panels are a better investment at this scale, but it could go either way; for a project over 200 watts, choose an MPPT controller.
I expected hiding the wires to be a battle, but pulling the wire took less than 30 minutes. Unique to the PDQ 32, but here it is:
- The panels are connected to each other above the deck by hiding the wire (2 x 12 awg) in wire duct. The stuff is intended for hiding phone cables, is strongly self adhesive, and would probably fit 3 x 12 awg or 2 x 10 awg wires. It snaps open to the side, should you need to service the cable.
- From the panel to the deck above the helm light is only ~ 2 1/2 inches; wire loom covers this.
- From the light to an existing hole in the stainless hardtop support is only 2 inches. Again, a bit of wire loom covers this plus the existing wires nicely.
- The wire runs down inside the support, forward across the aft cabin ceiling above the liner to the steering gear/instrument cluster access, and down to the charger, below the galvanic isolator.
- From the charger to the main bus bars.
- In-line 15 amp fuse between the bus and charger.
With all materials (of course, the might-need stash coughed-up some bits an pieces, including the wire and FRP), about $700.00. I struggled with the decision, but if it becomes a 20-year investment and saves a few nights a year charging batteries for $1.50/foot + electric + taxes, then I come out ahead in only 2 1/2 years. Not too bad and better than my 401-K in the best of times. Installation only took 8 hours, including making the feet and frame at home, so not too bad. Time spent puzzling it through? We don't count that.
- The birds love to poop on the boat, but unless the situation is awful, the amperage drop is only a few percent.
- I've seen panels installed under impressive crash bar protection, but this amounts to permanent shade.
- I've unplugged from shore power as there doesn't seem much use in running both (if I had a large sump pump I might reconsider this, but I have none). Moreover, both systems have charging algorithms the float and equalize, and they can easily confuse each other. One less thing to disconnect when day sailing, an advantage I had not foreseen.
- The panels easily manage any day sailing usage (1-8 amps) before we even return to the dock.
- While sailing one panel is often shaded, while the other is almost always clear. When anchored, both are easily exposed by swinging the boom off and securing forward.
- Summer charging has been great, but cloudy fall weather has been troublesome: shorter days, more clouds, and longer nights.
- Typical output, with no shading: 9 amps at 14 volts on a sunny day, 0.8-2.5 amps in heavy clouds. The most I have seen was 9.6 amps, and by Fall 7.4 amps is good, given the lower sun angle. The reality is that without angling the panels toward the sun you will always see somewhat less than the rated capacity. While sailing everything depends on the course; both panels may be shaded, or neither.
Follow-up, March 25, 2012: Amperage on sunny day still tops out at 8 amps, about the same as day-one when corrected for sun angle.
April 20, 2014: Still charging at the original rate. No sign of water intrusion.
August 11, 2016: Still charging at capacity.
After 5 years.