Solar Charging System
|When I decided to take the plunge and get into solar, I prepared myself by purchasing and
intently reading Collyn Rivers book, Motorhome Electrics & Caravans Too! This publication gives a detailed
explanation of solar and lists the facts that you won't get from the solar shop down the road.
To start with - before buying anything, I needed to work out what current my system was using per day. With this knowledge I could work out how much solar I needed to become self sufficient. As we do not stay in the one place for more that 7 days max, we didn't need to be totally sufficient for weeks on end so our system had to be sized to take this into account.
The 12V equipment that we were using was the fridge, water pump and fluro light so it was relatively easy working out the daily current draw based on the current draw for each device. This was:
Next I needed to work out the duty cycle of the fridge. This varies with many factors such as type, ambient temp, usage pattern, how full it is and efficiency. With the Waeco, this is all listed in the manual so working out the duty cycle was simple:
Duty cycle at 20 °C (night) = 25%. Therefore 7.5A * 25% = 1.875 Amps / hour. For 12 hours =
Total Fridge = 54 Amps / day. Now these figures are actually worst case scenario which is perfect.
Add this to the other readings:
Fluro: 1 Amp * 4 Hrs = 4 Amps / day
We now know our total draw is 59.9 Amps per day worst case.
Our system has AGM batteries which can be discharged lower than Flooded batteries but you shouldn't really go below 50% capacity as recommended by the manufacturer. As I have 400Ahr, my limit is 200Ahr which, without any charging, is around 3.5 days. The panel I needed had to get me to 6 - 7 days so in essence had to put in half of what we were using each day. Another consideration in solar sizing are the number of Peak Sun Hours (PSH) in a day which determines how long a panel can be used for. This changes between the seasons and for the area we live in is around 6 PSH during summer and 3 during winter (average). We do not do much camping during winter and if we do, we don't stay in the one spot for more than a couple of days so I worked off the summer hours. As I needed to put back 30 Amps a day then based on 6 PSH I would need a panel that could generate at least 5 Amps an hour.
Another thing I had to consider was where to store the panel. I wanted to fit it to the pantry/stove box on the camper so was limited to a certain width panel. After looking around I found the only panel that would meet all my criteria was a Bifacial 90W + 60W panel. The claimed output of this panel was 5.29 Amps + 3.52 Amps = 8.81 Amps but this is claimed and in reality, you would not get anything close to this (a 120W panel for example has a peak current output of 7.1 Amps but in reality puts out around 5.5 - 6 Amps so therefore operates at around 75 - 80% of that claimed).
Below is a solar power calculator that will let you enter a few basic parameters about location and date/time and then give an estimate of how many amps one could expect from their panels at that time, or amp-hours that day. Here it is populated with parameters for our solar module and our current location North of Brisbane. Click the Calculate button now to see how much electricity our panels should produce today, or enter values corresponding to your own system and see what you get:
Some examples of latitude are:
A few notes about the calculator:
Bifacial PV modules were invented for use by the space industry where the rear side of module is able to produce additional energy from the Earths’ atmospheric reflection. Bifacial modules can produce between 10% and 40% more energy in comparison with monofacial modules with the same dimensions. Maximum gain is achieved with use of reflective or white colour objects behind the modules such as quartz gravel, sand, snow, water surface, white painted or galvanized iron roof. The modules consists of 72 monocrystalline solar cells laminated between high transparency, low iron 3mm tempered glass and EVA film. The laminate is supported by a clear anodized aluminium frame. Transparent Tedlar film is used on the backside laminate instead of a white opaque film. Junction box contains bypass diodes to prevent hot-spots if the module is partially shaded.
The 90/60 Bifacial compared to the output of a 120W panel generates around the same Amps (roundabouts) but when you factor in cost, then the Bifacial comes out on top (120W $1050 - Bifacial $770 on special). Our panel was purchased at Solar Panel Express near the Glass House Mountains North of Brisbane. They had a web special of $770 if you went to http://www.solarpanelxpress.com.au/ and ordered one. They were very helpful with advice and I would highly recommend them.
Obviously testing was required once I had purchased the panel to see if it would meet our needs. Over a weekend and in a
mixture of rain, overcast and perfect conditions I managed to generate a maximum of 5.8 Amps (64% of claimed) on a cloudless
sky with our duchess mirror 2 meters behind the panel directing perfect sunlight onto the reverse of the panel. Now this is
not practical out bush but using white card I could generate 5.1 Amps (57% of claimed) and with no reflective material I
could generate 4.8 Amps (54% of claimed) at best (this was utilising only the 90w side of the panel). I also noted that at
7.30 in the morning, when the sun had just popped up over the trees I was producing 2.9 Amps, during rain I could generate
around 0.2 Amps and when moderately overcast, I could generate between 1.9 - 2.4 Amps. For a moderately overcast day I could
produce 14.6 Amps for the day from the panel, changing its position every hour. On a sunny day with only small amounts of
cloud, I can produce 34.6 Amps for the day.
I hope to do further testing with different reflective backings. I believe a polished stainless steel backing would improve the 5.1 Amps I got with white card but not produce more current when using a mirror a couple of meters away at an angle.
Another thing I tried with my solar setup was to turn the 3 stage solar regulator into a 3 stage 240V charger. I knew it could be done and after finding Ray's Caravans, Campervans & Motorhomes page with his setup, proceeded to experiment. By substituting the solar panel for a power supply (this is all a solar panel is), you can turn your system into a very effective 240V charger at a fraction of the cost as a professional charger. All you need is something that can generate between 16V - 24V at full load and let the solar regulator do the rest. You can buy and modify a high current charger/power supply or build a power supply from scratch. The reason you need more than 16V is that flooded batteries require close to 15V from a 3 stage charger to be fully charged. If you use the 'Equalize' function on the smart charger then this is 1V higher than the Absorption voltage. If you do not require this feature then a 15V supply will suffice.
It must be made clear that you are working on 240V equipment. Please do not attempt to build your own power supply or do any work involving 240V without the necessary skills and knowledge as electrocution at this voltage can be fatal.
As I have a trade in electronics, I decided to build my own using a few components sourced from Jaycar for under $80. These included a 300VA 16.6A @ 18V toroidal transformer (part no. MT2132 cost $69.95) a bridge rectifier, 12V regulator, a couple of capacitors, fuse, switch and lead from my junk box. Next I gutted a computer power supply as this had a 240V connection and fan to keep everything cool and housed everything into a neat, professional looking package. I then fitted the transformer and other components and fired it up to see what it produced. With no load it has an output of 20V and at full load of 17A it sat at just over 18V which was perfect as it simulated a solar panel exactly.
When hooked up to the regulator however I blew the fuse in the power supply. The reason for this is that the solar regulator will suck as much current as it can from the source during 'Boost' mode until it reaches it max rating. This feature is different from manufacturer to manufacturer but Plasmatronics regulators for example have a programmable current limit which would be ideal in this situation. Mine however didn't which means I either needed a power supply that could generate more than 25A or have a current limiting circuit built into the power supply to protect it. I proceeded with the latter and designed a simple current regulator into the power supply so it maxed out at 15A.
The system works extremely well but I decided not to install it into the camper due to the very limited area I could fit it into and the fact that it did generate a bit of heat which is not good in a very confined space as a closed camper. Also I wanted a charger that would produce more that 20A so when I found the Durst charger I shelved the power supply project. I am glad I experimented with the idea though as the possibilities are endless with what can be done with a 3 stage solar regulator. If I had a Plasmatronics PL40 then I would have put more thought into the power supply design and made a 30-40A supply to be used when I had access to 240V instead of buying the Durst.