Monday, May 9, 2011

Solar panel design and installation

In a typically thorough treatment, Drew from Sail Delmarva takes us through his analysis, decision-making process, design and construction for installation of solar panels on his PDQ catamaran.

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.
The Choices
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 expereince 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 80 watt panels, or about 50 AH in real-world charging results. 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.

The Panels
Price matters. We also needed a specific shape. I hunted for something cheaper with good customer ratings and came up with these:
UL Solar
80 Watt Panels

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; accessing and additional knockout required some very careful drilling, since smacking it firmly with a screw driver seemed unwise. Once inside, there are plenty of extra terminals.

The Mounting
The hard top is not flat; in fact, it domes asymmetrically about 3/4-inch over 3 feet. Additional, we need to protect the panels from accidental 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.

The feet are 2" x 1/2" prelaminated FRP with 1/4" x 1 1/2" stainless threaded rods tapped and epoxied in place. This material holds threads very well, and even without the epoxy, any thickness over 1/2" will break the bolt before stripping.

Installed with nuts and washers above and below, these provide a solid mount that easily accommodates the curve of the deck.

The frame is nothing more than two  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. This mounting allows free air circulation under the panels, for cooling and drying.

Pre-bonding adjustments were made.
The Charger
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 temerature, the maximum power point will shift. 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 12.5V and some efficiency will be lost, perhaps 15%. 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.

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 and the sun lower, but it is running the fans in the 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.

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 larger project, choose and MPPT controller.

Wire Routing
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 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.
Total wire run, about 15 feet each side.

Total Cost
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.

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