2010:
The year 2010 had a late wet Spring and an early and snowy Fall. This is beginning to look like the new normal for our area with more overcast days than in the earlier decade. I ran out of credits in late December and had to start paying for power in January of 2011. Still quite good. Nothing of exception to note, it just plugged away and did what it could.
2011 (as of Sept 26, 2011)
The 2011 year was eventful in a couple ways. First, we had a very wet early spring and then an early monsoon like time in July/August just when I should have been gathering my most credits. This can be clearly seen in the Yearly graph display for 2011. This has taken down my total credits quite a bit.
The second issue was related to the inverter failing. Starting at ~05/05/2011 I noticed that there were odd clipped data in the Daily graphs. Every so often the input power would drop almost to zero, then the next 10 minute sample would be way up again. This was not attributable to clouds or anything I could detect. On the 18th of July I let the folks at Namaste know about this. After some back and forth about trying to catch it in the act they thought it might be that the AC line side was fluctuating and causing the inverter to trip out and wait for 3 minutes before starting again. So they came up to try to widen the window of sensitivity for that. While they were there they could see that it was just building up the DC power, then shutting down, in a repeating cycle and determined that the inverter was bad. SunPower who re-brands the Xantrex inverter replaced it under warrantee. However, the Xantrex brand was no longer available from them. In the selection process of available replacements Stephen Kane determined that the only one that would also do data logging was the Fronius inverter with the addition of a data card for $200. He was able to convince SunPower to cover this cost since the Xantrex already had that built in.
The inverter was replaced on August 14th. If you look at the Monthly Data you can see that the data logging stops then . Looking at the Daily data you can see that it resumes September 1, 2011. It took that long to get the Daily Logger software working again. The Totals Logger software took even longer and was not working until September 23, 2011
Here are the issues:
- The fronius card uses a primitive interface. While the Xantrex worked with simple string commands to the serial port and returned string values for the answer, the Fronius operates in an old binary/Hexadecimal data mode. It is necessary to load up a byte array with hex string values using the Data Card Manual to determine how to encode them. You have to also calculate your own Hex checksum. Then use the perl "pack" command to make the string array into Hex/binary series of bytes and send that off to the serial port. It gets worse. The returned value is in the same packed hex binary format and the the three bytes with the actual data is of the form of two value bytes, High-Byte and Low-Byte, followed by a hex number that is a representation of the exponent for a power-of-10. To figure out the exponent value you need to use a perl hash table with the hex values and their comparable exponent values as a "decoder-ring". Then the High/Low bytes need to be unpacked and combined into a single value and multiplied by 10 to the exponent value.
- But it is worse that that ("He's dead Jim!") in that the Fronius has a command for AC power out in Watts but no command for DC power in in watts. So you have to query via this arcane process for the DC current Now and DC Volts Now and multiply them together to get DC Power Now (P=IE).
- So all that get me the Daily Logging of AC and DC values. For the Daily Totals you would think it would be similar but you would be wrong. Even though the manual clearly says that the card is powered from the AC side so that it is accessible 24 hours a day, it fails to mention that the inverter where the data is stored is powered from the DC side and disappears when the sun goes down. Stupid but there it is. The solution was to make one script that gets the Daily AC Total Kwh and the Time On Line Today values, again with the process mentioned above, and save these values off to a file. I have this start running with a Linux cron job that fires up ~5:00pm and run till ~8:00pm logging every 10 minutes. This covers the general range of when the sun goes down throughout the year. Each time it runs it opens the file and overwrites the data so that there is always only one set of the latest values in the file. When the sun goes down the values returned are either zero or perhaps some UnderFlow of the exponent. So I test for that in the subroutine that does the Write/Read to the card and set the return value to zero in those cases. If the returned value is zero from the subroutine then I don't open or write to the file. So the final value in the file has the Daily Total data +-10 minutes. Then, at ~9:00pm I have another script run by the cron process that opens the data file, reads the values and puts them into the MySQL database.
There is still another little irritant with the Fronius version of the Totals data. The data card returns only integer values where as the Xantrex would return decimal values. So, for example, where the Xantrex might have returned 21.999Kwh for the day the Fronius reports only 21Kwh. The High/Low data bytes could have returned more digits and then been multiplied by a power of 10 in the exponent to provide more detail, but it doesn't. It returns only the simple integer value.
So, the net result is that it takes a lot more lines of code to do the same thing as I was doing with the Xantrex inverter. For comparison, the original Totals logger file for the Xantrex, including blank lines and comments, was 62 lines long. The equivalent of the two scripts for the Fronius is 245 lines. What a crude system, and they actually charge you an additional $200 for it. If you are planning on data logging to a Linux machine you might take this into consideration when selecting inverters and check out exactly how the various companies deliver their data.
To be fair, I have been in contact with the Fronius Tech and the person has been very helpful and within the limits defined by the card/inverter itself. He admits that the card is old tech and was originally designed for their technicians. The thrust of their data loging these days is in Windows and they do sell various other devices for internet logging or taking data from an array of panel-mounted mini-inverters that are becoming more popular. I may just be trying to do something that is now considered "old tech'. Still, it was a lot of extra work just to log simple data from the inverter.
All that having finally been done it does appear that the inverter itself is working well. Unlike the Xantrex that used passive cooling with large fined heat radiator the Fronius seems to use an active system with internal fans. This will probably eat some power but should be rather minor in the scale of things. I'm not sure how I would know if the fans go bad though, something to ponder.