Assessing long-term PV performance in a low-energy home

Zero Energy Home Friends,

My daily PV output from a 6.56 kW PV system has averaged 22.7 kWh/day over the last two years.

kWh= AC kWh from inverter to grid

irrday= cumulative kWh/m2/day plane of array

pvptmp= array temperature (F) taken on back on one of the modules

obs= elapsed days (August 15th 2012 to August 15, 2014)

To illustrate, my PV output AC over the last two years. IRRDAY is daily cumulative irradiance on the plane of the PV array (kWh/m2). There are about 46 m2 of pv modules (32 ESA 205s) While my PV output looks to have gone down a bit this year, it is coming from lower irradiance primarily. We've had a rainy hot summer.

Now we increase the scrutiny: Here we plot the daily plane of array irradiance against the daily PV power production.

The plot symbols are the Julian dates over the two years (0-365) The mysterious days with high output, but lower irradiance (Julian dates 328 and 329 and 236-240) likely come from something like a well-aimed bird poop on the Licor pyranometer.

Ignoring those, there is a cloud of unusual points which start around Julian date 322 and get worse through the end of the year and also continue in early January. This is cause by shading of the extreme south end of the PV array in November - January by my avocado tree. The avos must be good because they are costing me 5-8 kWh a day during that time. As expected, the impact of the shading is worst around Julian date 340-358 (December 6th- December 24th)-- which is right at the solstice. Also, looking at the first plot, this shading has gotten worse in the last year. I think its time for some trimming. Finally, we can examine how PV performance varies with measured data over time to look for degradation. I created a dummy variable (0,1) for the shading that appears to take place from November 15th to roughly January 16th. (Julian dates 1-16 and 320-365).

The statistical model has an R-squared of 92%-- explains 92% of the variation. The shading is a very significant impact, although the daily solar irradiance is the largest factor (no surprise). PV array temperature has a negative influence (hotter array= worse performance). The variable iT is (irradiance * pvtemp) since solar irradiance is strongly correlated with array temperature in order for the influence of array temperature to properly show itself.

The elapsed days do show a negative course -0.0016 kWh lower each day-- but the term is small, equal to perhaps a kWh in two years. However, this could easily be affected by the growing avocado tree. In fact, a regression on the residual on the above regression showed that this is, in fact, the case. Most of the indicated "degradation" is almost certainly coming from a taller avocado tree in the second year as the shortfall in the prediction is coming during the shading period.

Lisa , just added a Ford Cmax Energi Plug-in to our loads so.... Time to trim! But the PV system appears to keep chugging along.

A further note: You can see most of these factors when you plot the PVoutput to grid against Julian Date and using the year as a plot symbol:

Highest output is in mid March on clear cool days. Output goes down over summer with its heat. And then, starting around November 15th (xlin), the output drops from array shading. This reaches a worse level around Julian date 342 (December 7th-- Pearl Harbor Day-- when it plummets as the output of much of the lower string is lost.

You can read much more than you likely want about the house here:

http://www.fsec.ucf.edu/en/publications/pdf/FSEC-RR-383-12.pdf

On-line data here:

http://infomonitors.com/dpr/

Views: 157

Tags: energy, home, low, photovoltaics, shading, solar, zero

Comment

You need to be a member of Home Energy Pros to add comments!

Join Home Energy Pros

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

Todd Collins replied to Carly@AirCycler's discussion The (Small) Cost of Ventilation Without Recovery
"I get it ... the article is focused on cost.  Cost is not why people put an ERV or HRV in a…"
1 hour ago
Green Training USA posted events
7 hours ago
Crosbey Archery posted a photo

Timber-Double-doors

This door is made and hand finished at Bradford workshops by skilled craftsmen.…
12 hours ago
Green Training USA updated an event
Thumbnail

BPI CEU's - Online Anytime at Online

December 4, 2014 to December 3, 2015
Get All 30 BPI CEUAvoid BPI's Written Exams for Certification Renewal…See More
12 hours ago
David I. Shepherd-Gaw added a discussion to the group Job Board
Thumbnail

JOB: Energy Efficiency Specialist @ Cowlitz PUD

Primarily a position with a residential focus in a great community. Opportunities to expand your…See More
17 hours ago
David I. Shepherd-Gaw joined Diane Chojnowski's group
Thumbnail

Job Board

This group is for posting jobs related to all aspects of the home performance industry including…See More
17 hours ago
John Wilkerson posted a discussion

Aeroseal Machine for Sale

Hello All, We have an Aeroseal SmartSeal system (w/ compressor and plenty of leftover…See More
18 hours ago
Rob Buchanan commented on Jose Macho's blog post Why Don't Homeowners Spend Money on Energy Saving Upgrades??? Blame "Cognitive Biases"
"I think this is an interesting comparison, but I would also want to know whether similar surveys…"
yesterday

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service