Power of the sun

Using solar power to create electricity is a fantastic idea but it’s not very efficient. At work I have investigated the possibility of installing solar panels on warehouse roofs many times over the years and can never manage to justify the expense. Admittedly, the payback time has improved over the years but nevertheless it still stands at around eight years in the best location. The problem is that the payback depends on government subsidies and as the panels become cheaper to buy and their efficiency improves, so the subsidies reduce and quite rightly so. After all, that subsidy is funded by everyone who pays an electricity bill with a feed in tariff applied to it. Most solar panels today have an efficiency rating of around 19% and the best (and most expensive) can achieve 26 – 28% so there is still a long way to go in their development. Whether we like it or not, the most efficient way to make electricity is in a giant gas fired power station.

On a narrowboat, electricity is produced by an alternator bolted to the engine and although it’s a reasonably efficient process when travelling, it’s a costly business to run the engine when just moored up. Until this year, our boating has consisted mainly of weekends and holidays so we would normally be travelling most days and therefore charging the batteries as we went. Now of course we are just about to embark on a six month long journey and travelling every day is not part of our plan. The idea of running the engine every day or two to charge the batteries didn’t really appeal so I decided to investigate the practicalities of a solar installation.

As usual, there seemed to be a lot of information but much of it unclear or contradictory. In the end, I think that I got to where I wanted to be in terms of sourcing the right equipment and installing it. I got a lot of good information from the Bimble Solar website although in the end only bought mounting brackets from there. The brackets are made from stainless steel and allow the panels to be tilted towards the sun if required. They also mean that the panels can go over the top of things like mushroom vents rather than require a roof area completely devoid of obstructions. An added bonus is that the panels get better airflow around them which will keep them a little cooler than they might otherwise become. Solar panels lose efficiency as they heat up which is a bit of a drawback for something which needs to be exposed to the sun to work!

Bimble mount with security chain.

Mounting brackets from Bimble Solar.

The largest components are the panels themselves and I elected to go for two standard size domestic panels which I bought from Edmondson Electrical in Rugby. These panels are approximately 1m by 1.6m so on Caxton, two of them fit end to end in the roof space between the two houdini hatches. My research had led me to believe that mono crystalline panels were better than poly crystalline but I couldn’t really find out why. My first visit to Edmondson’s changed my mind and this is why. Mono crystalline cells are created using a process not to dissimilar to how you grew crystals as a child at school or at home. The silicon grows until it is large enough to be machined but there is a lot of wasted material due to the odd shapes that are created. The process for making poly crystalline cells involves melting the silicon and pouring it into a mould before letting it cool down and then machining it. This is a quicker process and less material is wasted because the solidified silicon is uniform in shape. In the early development of the solar cells, mono crystalline cells were much more efficient than their poly crystalline cousins. Today there is very little, if anything, in it. Cosmetically, the mono crystalline panels are black which makes them more appealing when bolted to the roof of a house. Poly crystalline panels have a bluish hue but that is of no consequence when mounted horizontally on the roof of a brightly painted boat. The panels that I bought are rated at 260 watts each with a voltage of 32v.

Solar Panels

The two panels fit neatly on Caxton’s roof.

The next piece of equipment and arguably the most important piece is the controller. This converts the relatively high voltage from the panels into someting that can be used for charging a twelve volt battery bank. There are two types: PWM and MPPT, here there seems to be no doubt or contradiction, MPPT is the way to go. More expensive but it is in effect a three stage intelligent charger. Bimble sell Epever 40 amp units for just over £200 and they seem to be a reputable make. I discovered that they are made in China and that I could buy direct through Aliexpress for £160 so that is what I did. The unit arrived ten days later followed by an invoice from TNT. The covering letter said that they had paid VAT on my behalf for the import and that was £15. Their admin charge was £20 and the VAT on that was £4. All in all I ended up saving around £4 so my advice would be to just buy from Bimble Solar.

The rest of the bits required I bought from various ebay sellers but I’ll cover those in the next section which describes the installation.

Caxton has a full height cupboard which houses the inverter as well as most of the other electrical switch gear and this is just inside the rear of the cabin area. The first part of the installation was to mount the MPPT controller in the bottom of the cupboard which was straightforward enough, four screws hold it in place. I then connected the unit to the battery bank by wiring it through 16mm2 cable to the heavy duty input/output cables on the inverter. I included a 40 amp fuse to protect the cable but since the controller is rated at 40A and the cable is rated at 70A, I don’t expect to change it any time soon. The next thing was to wire the input of the controller to an isolator switch which would be connected to the solar panels. It is important to get an isolator for DC  circuits rather than the more common AC circuits. I got mine from ebay rated at 25 amps. The panels have a total maximum rating of 520 W at around 70V when wired in series so the maximum current can only be 520/70 or 7.5A. After that I cut a hole in the wall panel and installed the small monitoring unit which displays the performance of the panels. Finally, I connected the optional temperature sensor before threading it through the bulkhead and into the battery box where it monitors the temperature of the battery bank.

The Epever MPPT controller and the DC isolator for the solar panels sit inside a cupboard.

With the control end of the installation complete it was time to mount the panels on the roof. Bimble sell a mounting kit comprising of two drills, a tap to thread the holes and some weatherproof mastic. I bought the kit but it was a waste of money. Perhaps the steel used on Caxton is special but I suspect not, the drills were only capable of managing two holes so I ended up buying more from Screwfix. The tap broke in one of the holes so I had to buy a proper tap and die set to complete the job. All in all I drilled ten holes, four brackets with two each and another two into the cupboard for the cables. The mastic provided a seal under the brackets and around the bolts as well as around the plastic box sitting over the cable entry point. Next I drilled the edges of the solar panel frames and then mounted them on to their support brackets. All that remained was to crimp plugs to the ends of the cables that I had bought from ebay and feed the cables through some conduit and into the electrical cuboard before connecting the cables to the isolator switch. The penultimate action was to connect the cables to the panels and then finally to turn the isolator switch on. Electricity flowed immediately and all that remained was to set the controller up to charge the batteries properly. This was done by disconnecting the small monitoring panel and then connecting the controller to my laptop and then running the Epever software. After that I reconnected the monitoring panel and started to observe the activity.

The monitoring panel.

The question is, do they work? In the limited scope of testing, the answer is yes. They managed to keep the batteries at 100% for two weeks when we had mostly overcast days. I then ran the fridge and freezer for a week and still the panels kept the batteries at 99%. I used the laptop to monitor the state of charge over a 24 hour period and saw that after sunset the battery bank eventually dropped to 95% but by early next evening it had recovered to 99%. Bearing in mind that it is still early April, I think that this is an encouraging result. It will be interesting to see how well the system copes once we move aboard full time and increase our power usage.

I reckon that I spent just under £600 installing this system and although the prime motive was that of convenience rather than cost saving, there is a decent payback time. Caxton’s beta 43 engine consumes approximately 1.5 litres of diesel an hour and at today’s price of around £1.00 per litre it’s easy to see that by not having to run the engine for 400 hours we will save the installation cost. We won’t break even in the first year but we will at some point in 2018. This doesn’t take account of the fact that there will be less wear on the engine or the cost of servicing the engine less frequently but those are also beneficial factors. I’m hopeful that when we return to the marina next winter the panels will still produce enough energy to keep the batteries topped up without having to rely on being plugged into the mains supply.

Watch this space for regular updates.


May 2017
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