27 May 2011

Solar cells


One of the decisions that we made early on when building our new boat was to have an array of solar cells on a spoiler at the rear. Well, I wanted them and had to convince the admiral, but anyway. The reason to choose for PV (Photo Voltaic) solar cells is that they work without needing any supervision when you are away from the boat, maintenance is limited to an irregular cleaning interval and most importantly on a sailboat, that they do not make any noise.

What we definitely did not want was flexible panels on top of the pilot house or deck. Not only are the flexible panels a lot less efficient by themselves, having multiple entry points for the cables makes for leaks, and they also heat up because they cannot shed their heat radiating downwards. Hot solar panels work slower. This then is a bad compromise: the warmer the climate the more likely you are that you are going to have a nice overhead sun but your panels will not be putting out as much. Just at the time that your fridge is working overtime! So no flexible panels for us.


As boat #1 had a quite nice antenna and PV spoiler at the very rear of the boat, we opted for the same manner of construction. Although the spoiler can be considered ugly, it keeps out of the way, provides for a good way of lifting the rowing dinghy. It turns out that it's also a half-decent bimini, although for that purpose it could even be larger.

Technically we used the best panels we could find. I did not want the total panel area to be wider than the boat or stick out from the rear; this way you can safely enter a lock without fear of endangering the panels. On the front side the panels are limited by the backstay. This gave us an an area of about 3,30 by 1 m. This enabled us to fit 6 panels rated at 100 Wp.

Given that we're (still) in the NW of Europe at 52° N and that Wp is a theoretical maximum, I was told that a fixed setup (on land) of 600 Wp would bring in about 500 kWh per year. On the downside, our boat's panels are not angled, and there is some shadow from backstay and antennas. Fortunately the sun tends to shine more at sea or near the sea. So I figured 25 % loss, e.g. 375 kWh per year or a 1 kWh per day average. Since the sun shines a lot more in the summer, figure about 70% more during summer, which happily coincides with our summer holiday!

In practice



So how did this work out? Very well. Last year we never used any shore power whilst cruising, but did not yet keep a full tally. Last winter when we were on board and the Kabola heater was running full blast (outside -7 C) we obviously did use shore power. Solar cells were not doing much then when the sun is above the horizon only a little and only for maybe 8 hours!

Back to today, and here are some snapshots from my monitoring app from this week -- end of May. This means the sun officially rises at about 6:30 and goes down at 21:30. In the first image you can see the current at which the house battery is being charged or discharged. The at-rest usage this week is 1,7 A (monitoring stuff, all mastervolt equipment on, winches and hydraulics powered but the fridge is off.)

These images span a week running from Friday morning to Friday morning. On Saturday and sunday I was on board, using all sorts of stuff including running the inverter for watching TV and some chores. I also did some upgrades to the monitoring software, so you see a 'glitch' in the data where the graph is interrupted. From Monday onwards no-one was aboard, and you can see that on Monday the sun was out, as the effective current into the batteries reached a maximum of 13.4 A. Combined with the load that means the solar array was pumping out 15.1 A.


You can also see that on Wednesday the batteries were reaching a completely full charge.

So you can see that certainly they are doing what they were supposed to do: deliver us the boat on a full charge in summer.

23 May 2011

Observing the electrical behaviour of a NMEA 2000 network

Earlier I wrote that I was going to investigate why Simrad/Navico is now advocating adding a Simnet Noise Filter to more complicated networks.

First I measured a small bench network using Simnet cable at home.

A screenshot of a simple network (one sender, one Actisense NGT-1 and about 5 m of cable in total, with no terminators. I was expecting swinging of the observed signal when it returns the bus to a non-driven state, and indeed the digital scope proves it worth and shows this to good effect. As you can see, the rising edge when the sender drives the bus the wave is pretty neat. The swinging happens when the sender stops driving the CAN output driver and the voltage returns to zero, with quite large oscillations around 0 V.


Adding a 68 Ohm resistor (which is the nearest equivalent of 2 x 120 Ohm terminators) results in the picture shown here at the right. As you can see the ringing is a lot less now.

Then I added 40 m of Simnet cable:
As was expected, this added a bit of capacitance to the cable: the rising flank of the pulses aren't straight up anymore, especially once the voltage reaches 2,5 V, and the ringing is back. I didn't expect that.

And finally I switched to a 39 Ohm terminator, which is what you end up when you install a 'Simnet Noise Filter' at the mast base which is what Simrad is telling me to do. We now get a bit of overshoot on the rising flank as well, and you can see the slower and reduced ringing because of the changed impedance.

Last weekend I finally had the chance to measure my own boat's onboard network, which has a what I'd call decent mid-level network with 13 active senders and about 60 m of cable. The added load of the devices and longer cable results in a cleaner waveform. Nothing really wrong here, but still there is a bit of overshoot on the rising signal.

The last image is of course with the filter installed. The effects seen in the test network were seen again. No overshoot or ringing; this is now completely gone. The rise and fall time is still completely acceptable.

The noise filter also has as an added benefit that the network will work more reliably when the cable going up the mast to the wind sensor is removed, either because accidentally or on purpose.

Conclusion

The Simnet Noise Filter seems to work and improve the signal behavior of the two networks that I tested it on, and I intend to keep it installed. It is recommended for large(r) Simnet networks.

It would be interesting to see the effect of such a network on other brands of NMEA 2000 cable.

Airmar DT800, DST800 and PB200 disappearing from the NMEA 2000 bus

Last year I had some issues on our own new boat with the NMEA 2000 network. Some devices, in particular the Airmar sensors, would suddenly drop off the bus as if they were no longer there. After about ten minutes or so they would magically reappear. Most often I would not be that patient and cycle the network power; this helped all the time, but was very annoying.

The problem happened a few times a day for the DST-200 depth/speed sensor and about once a day for the PB-200 multifunction sensor. I thought it was interesting that only the two Airmar devices had this issue whereas the 10+ Simrad/Lowrance/Mastervolt/Actisense devices on that same network had no such issues.

I can now report after 2 months of testing that this can be fixed by loading new firmware into the Airmar sensors. This can be done by downloading the Airmar Weathercaster software and then applying the firmware update through that. Note that this is the approved manner of doing this, even if you do not own a PB200 but only a depth/speed/temperature (DST800) or depth/temperature (DT800) sensor. I upgraded the DST800 to DST200_a1_1022 and the PB200 to PB200_a1_1611_a2_1611 after which the problems have been fixed. According to my friendly Simrad engineer, Airmar actually repaired the issue in software release 1021.

Link to the Airmar download page