Saturday, 24 May 2014

Solar Panels

When first researching solar panels I identified there are three main types and in order of efficiency they are:
  • Amorphous
  • Polycrystalline
  • Monocrystalline
With the latter being the most efficient.
Additionally, it's not recommended to purchase a panel with a black frame or black backing. Apparently black can cause a heat build-up which has the effect of degrading a panel's output.  The same can occur if the panel is fixed directly to the steel roof of the boat. 
Solar panels are made up of a matrix of silicon wafers (cells). What many people don't realise (I didn't) is that cells are manufactured in four grades. 
Class A. Cells with absolutely no defects whatsoever. No saw marks on the wafer,edge chips, P or N dopant fade, microcracks, Al paste smear or large % of wafer thickness deviation.
Class B.  These are cells that have saw marks still in the wafers (cells) that were not properly polished, have thickness deviation and colour deviation.
Class C.  Chips,breaks and microcracks. Breaks cannot be more than 1/4 of the cells total surface.
Class D.  3/4 broken cells which in effect are just pieces of wafer. Pieces are just that, pieces. They are not graded and are considered scrap for remelt or sale to micro solar appliance producers (China,Taiwan,Hong Kong) for laser cutting and use in walkway lights,etc.
There are only a small number of silicon wafer manufacturers and they tend to keep all their Class A cells to make their own panels. They usually sell their Class B, C, D to other panel manufacturers who do not manufacture their own cells. Often these panel manufacturers classify or advertise their panels as being made from Class A cells (despite being Class B or C) as they are the highest quality they can obtain.
Even more panel sellers don't advertise or inform potential purchasers of the grade of cell used in the panel. It's very much Caveat Emptor.
So the whole process of purchasing solar panels can be rather fraught with potential hazards.
In our case we have two 100 Watt monocrystalline panels connected in parallel.  This gives us a potential 200 Watts of electricity.  The panels can produce more than 12 Volts.  To regulate (control) the output from the panels a solar controller is required.  Controllers for such a small system are usually either PWM or MPPT.  A simple way of explaining the difference between the two would be to describe the PWM as momentarily stopping the transmission of electricity to the batteries when the voltage gets too high.  It might be likened to a switch which is being rapidly turned on and off to prevent the panels damaging the batteries by delivering too higher voltage.  The MPPT controller converts the additional high voltage to amperes.  So a MPPT controller allows the panel to deliver more useable electricity.  This is probably why a MPPT controller is more expensive.
Our MPPT controller delivers electricity from the panels to the domestic battery bank at 14.6V.  We have a potential 200 Watts of generating capacity so the maximum amperes should be 13.7 amps (200÷14.6).  Assume 95% efficiency and you have a theoretical capacity of 13 amps.  However on a good day we might get 10 amps from the panels.  This could probably be improved by aligning the panels to the sun (and constantly moving them to track the sun).  I have considered doing this, but the cost of installing a swivel and tilt solar panel mounting system to gain an additional 3 amps doesn’t look that financially viable.  The positive news I take from this is my eBay panels appear to be made from either Class A or B cells.  It must have been my lucky day!
If it were possible to produce the additional 3 amps for 50% of the year at an average of 8 hours daily the panels would generate an additional 4380Ah annually (((365÷2)x8)x3).  However I suspect this is wildly optimistic.  The boat alternators produce approximately 180 amps which means we wouldn’t need to run the engine for 24 hours each year to achieve 4380Ah.  Not much of a saving on such an optimistic figure! 
Where a solar panel array is useful is during the final two stages of charging the battery bank.  During the Absorption stage our batteries accept a maximum current of 10-20 amps whilst during the Float stage they accept 3-10 amps.  It therefore makes sense to use the solar panels to complete and maintain the final stages of recharging the batteries,` rather than add wear to the engine by running it to produce such a low charging requirement.
My Conclusion
If we wanted to increase our solar power production it would be cheaper to buy a third panel rather than purchase a swivel and tilt mounting system for the existing setup.








8 comments :

Jacquie said...

To Tom and Jan,

Thank you for the solar panel information, I found it a real help as it was so technical and informative, and has helped me to reach my decision on fitting them, and as to whom with.

Once again, many thanks

Jacquie.

Tom and Jan said...

Jacquie,
You're welcome but please don't take my opinion and conclusion a fact. It's only my opinion!

Working Narrow Boat Hadar said...

Some very interesting and useful info on solar panels there Tom. We only have one 120watt panel, and at times it has produced 9.5amps max. During the summer it lays flat on the roof, but during the winter I have a pair of extension legs to lift it as the sun is lower, but we do not try to follow the sun, not worth it in my book. Last summer I monitored it's effectiveness. As we do not move at weekends, we used to run our built-in generator twice a day for an hour each occasion, so over a weekend that was 4 hours, at 1.2litres of diesel and hour means we were using approx 5 litres each weekend. This one panel meant that we did not run the generator at all last summer, so saved enough money last summer to cover half the cost of the panel. As we have decided not to move this year so I can recover fully from my operations, we are on a shoreline, but at the start of last month we have been turning the shoreline off during the day and running solely on solar power, our next electric bill should be a lot less than during the winter.

Tom and Jan said...

Hi Keith

If you are getting 9+ amps then it sounds like you have a panel with A or B class cells. It would have been sufficient to keep the fridge and freezer going plus trickle charge the battery bank. I'm guessing that unlike us; you don't have a damned big 32" colour TV :-)
We hope you're back and running on all cylinders shortly. You've certainly had a rough time recently!

Halfie said...

If your MPPT controller can cope with the voltage, I understand that you get better efficiency if you connect the panels in series rather than parallel. That's what I've done with mine. It must certainly help in low-light conditions.

Tom and Jan said...

MPPT controller aside. What led you to believe series was better than parallel? The only advantage I can think of is the diameter of the cables wouldn't need to be as big!

Halfie said...

The MPPT controller is an essential part of the set-up, but I have to confess that I don't know how it works. However, if the amount of light is such that each panel produces only, say, 11V, then in a parallel-connected system the controller has only 11V to play with - insufficient to provide any charge. But with the panels in series the controller sees 22V - plenty for charging. My (limited) understanding is that the MPPT controller can cope with higher input voltages than other controllers, and that it can channel most of the available power to the batteries it is charging.

I'm going to have to find out more about MPPT ...

Tom and Jan said...

Halfie

Power (Watts) is Volts X Amps.

A 12V solar panel can sometimes produce up to 30V but this would damage the battery. A PWM controller basically does some high speed ON/OFF withthe power to reduce the voltage to an acceptable battery level. A MPPT controller uses some smart electronics and attempts to convert surplus Volts to Amps.

Over simplified explanation

Panel produces 30V and 2A = 60watts
With a PWM controller the volts get reduced to 14.8V which means powers is 29.6W (14.8 x 2)
The MPPT converts the surplus Volts to amps to give 60W (14.8V x 4.05A)

Having two solar panels in series would mean 60V and 2A (=120W)
Having them in parallel would mean 30V and 4A (=120W)

The advantage of series is the higher voltage (ie pressure) which means smaller cables could be used. This is why the 12V cables from your battery to your inverter are massive and the 240V cable to your toaster is much smaller.

Of course all of the above might be plausible bluster :-)