Monday, 30 April 2012

Overproducing panels and overheating power conditioners

The display for the solar panels shows us how many minutes each day the electrical output is being limited by the power conditioners. There are two reasons it does this: one is that there is too much electricity coming from the solar panels, the other that the power conditioners are overheating. 

We have 9.12 kW of generating power on the roof, which means that the maximum theoretical generating power is 9.12 kW. In fact, the rating of each panel is 185 watts, but in fact their actual power output varies and all are above 185 watts, some up to 195. So in fact we have about 9.4 kW. Anyway, they told us we'd be fine with two 4 kW power conditioners. We believed them. Half of 9 is 4.5 and 4.5 is just a big 4, in the same what that 2 + 2 = 5 for large values of 2. Electronics comes across as this very exact precise science, but in fact it's as full of tolerances and variations as anything else. Carpentry can be a lot more precise. 

In the event, we've found that for a few hours per month the power conditioners are limiting the amount of electricity they put out. While they should probably be producing 8 or 9 kw, they are only sending 6 or 7 to the grid. It's very difficult to get an idea of how much this is, and I just realised that the display panel will only record a month of readings, so I only have records from 1st March. The display panel also has a "clear" button, which is usually the button for "select", and it does not ask you whether you are sure you want to clear the data. So it's very easy to lose it all. There is no way to automatically get the data from the display panel to a computer, so I've been copying it by hand.

Anyway, in March, we produced an average of 30 kWh per day, the solar panels were generating an average of 665 minutes (11 hours) and the output was being limited for an average of 20 minutes per day; a total of 6-and-a-half hours over the month. If we guess that it's limiting the output by 2 kW, then that's 13 kWh per month. That's over one percent of the total monthly production of around 1000 kWh. At 48 yen per kWh, that's a large beer.  

Last Saturday (20th April), I put an electric fan in the machine room, pointing at the power conditioners with a timer on the plug so that it comes on between about 11 am and 2 pm. I'm not sure of the effect, or how to measure it, without standing next to the panel all day for the next two years. I'm sure the amount of energy used by the fan will probably average out more or less the same as the amount of extra electricity it will allow to be generated. 

(In case anyone didn't get 2 + 2 = 5,

eg. 2.4 + 2.4 = 4.8

2.4 is a large value of 2, in a scenario where you're rounding to the nearest whole number. Similarly 4.8 is about 5, so 2 + 2 = 5.)