The rendering above shows the proportions of a solar roof for the flying bridge (support is not yet shown, but will be required). The efficiency and cost per watt have gotten to the point where this may make sense. In the scheme above there would be eight 250 to 320 watt panels, depending on what we find next week. This is enough to generate a large chunk of our daily power usage at anchor. But is it worth the expense and hit on aesthetics?
Our approach to at anchor power has been working, and even with electric cooking, generator usage has been confined to evenings every second or third day, when the washing and drying were being done. Charging took place while we were powering, from the 150 amp Electrodyne alternators mounted on the engines. The 1600 amp hour battery bank would handle this for between three and five days, depending on the amount of computer time. Our eight kW genset was fine for the air con, and other AC loads, and when occasionally overwhelmed, would be assisted by the inverters adding the needed extra capacity.
But a couple of things have changed in the last year and a half. First, computer time has expanded and we are running a 27″ Imac with three external hard drives plus a second Mac at the same time. So, power consumption is up, now to 400 amps a day (at 24VDC). Second, we are sitting more and powering for shorter periods, which means the batteries are not being recharged as much by the engines. It has been necessary this year to run the generator a three hours every second or third day to keep the batteries feeling good. That in itself is acceptable. What is not is the dependence on this single piece of gear. We have the engines as a back up charging source, but we would not want to make a habit of using them this way.
Which brings us to the idea of solar panels. As the efficiency has increased and cost have come down they have become an interesting alternative. The issue is where to find the space required. In our case the roof of the pilot house makes an interesting possibility. The other potential location is an arch across the stern. Since we are working on a new awning and enclosure system for the pilot house, using the solar panels as a roof has merit.
There are three aspects to the decision. One is solar electrical production efficiency, a function of shading at various sun angles. A dense shadow line across a row of cells effectively shorts their output to zero. We are prepared to remove most of the antennae to facilitate output, but the radar and VHF need to remain.
The latest (beta) version of Rhino 3D has a shadow generator available without full rendering, so you can watch the shadows as the sun moves (or in this case the boat). The images above have six solar panels, but we are presently planning eight. This is a 1700 sun, from the stern quarter and directly aft. The radar antenna, all six feet (1.8m) of it, is in the worst position for shadowing. We are going to lose the better part of three panels in this situation.
Rotate the boat a few degrees and things get a lot better. Good friend and solar expert, Bob Williams, who is helping us with this, estimates that we will lose on average about ten to twelve percent of the total day’s output to shadowing. Lets assume 20% for fun. Using an MPPT controller increases output above panel ratings by 20 to 30%. We’ll call this gain and the shadowing loss even. We know we can source 250 watt panels (we are shooting for 320), so eight panels times 250 watts equals 2000 watts per hour. The solar day, i.e. average time of full output varies with latitude, time of year, and atmospherics. For temperate latitudes, we think we can assume 5.5 hours in normal climates. This works out to over 10,000 watt hours a day. To convert this to amp hours divide by your charging voltage, in our case 28, for 340 amp hours (for our 24 volt system) per day. This is between 70 and 90% of our daily consumption!
On the other hand, this is a large area in a potentially windy environment. The weight up high, an easy 250kg / 520 pounds, is way above the center of gravity so a knock on stability. Perhaps a five cell array on an arch aft makes more sense?
The decision will partly hinge on the new 320 watt panels and how soon we can get them. Stay tuned.