Some real-world figures for solar PV

After two weeks on anchor, here are some real-world figures for a Solar PV system.

Some real-world figures for solar PV

Having recently spent two weeks on my boat and mostly on anchor, I have had a chance to really look at the efficiency of solar panels in a real-world situation.

Now, in fairness to large-scale generation systems I only have two 35w panels hooked up to a 12v lead acid 135amp/hr battery, but the fundamentals are still the same.

First, a few things to take into account: -

1) The panels are 35w flexible units

2) The panels are tied onto the bimini. One unit lies horizontally and the other at a slight angle (approximately 15 degrees)

3) The panels are wired via a two-into-one connector to a charge regulator into a 135amp/hr 12v battery. Amps monitored by a stand-alone battery monitor.

4) The boat on anchor swings through approximately 100 degrees and rotates according to the wind direction.

5) The boom and mast occassionally shade one or both panels. Back stays also cast a very narrow shadow depending on the sun position. This will affect the amperage produced.

Now, some basics. At 43N in midsummer, the sun is approximately 20 degrees from vertically overhead. At 1000w/m2 at the equator, this results in 660w/m2 incomming solar insolation assuming suitable angle of incidence. The two-panels cover a total area of 0.87m2 so theoretical maximum solar power is 575w. At a theoretical 15% efficiency (fairly average), this is 86w.

All of the above figures are theoretical and based on panels being optimumly angled to the incomming sunlight.

So, how did my panels do under real-world conditions?

Maximum amps observed was 2.8amps at 13:30hrs CEST (11:30GMT). At this time, the panels were "facing" West to North West, therefore the angled rearward panel was not optimally orientated. 2.8amps equates to 33.6w or 39% of the theoretical (15% efficiency) figure.

During the day, from 08:00 CEST to 19:00 CEST, with the panels facing W/NW, observed amperage rose from 0.6amps through 2.8amps back to 0.6amps. Plotted, this gives the bell curve shown.

Given thatat this time of year there are close to 16 hours of daylight and during the observation period it was clear, bright sunshine all day the maximum output (taken as over two amps) was for only seven hours. During these seven hours, 17.6amps were charged back into the battery bank, or 13% of total capacity. To replenish the 67amps of useable charge (50% capacity) would therefore take 3.8 days, assuming we were not drawing any power from the battery (the fridge alone takes 25amps per day, for example).

I hope this real-world observation will help in the understanding of how much solar power can actually be generated by PV. I agree that large-scale Solar PV with optimally angled panels will be able to achieve better efficiency, but given that a 20% efficient panel, at the optimal angle at 43N, can only produce a maximum of 115w per sq.m on a clear, sunny day for around seven hours in midsummer, one has to seriously question the maximum generation quotes given by Solar PV farms.


Topics: Renewables
Tags: bank | solar
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