On several previous occasions Euan Mearns has fulminated about this photo he took of roof-mounted solar PV panels in Aberdeen. If you’re going to do something as dumb as installing solar panels at latitude 57N, he argues, at least point them south. Don’t point them east.
So why are these panels pointing east?
Because that’s the way the roof points.
One would think that in Aberdeen, where solar PV load factors are less than 10% to begin with, PV panels would be pointed in the direction that gives maximum output, which according to NREL’s PVwatts calculator (which allows for local weather conditions as well as sun angles) is ten degrees west of south tilted 35-40 degrees south, an orientation which allows the panels to generate 27% more kWh/year than they will when pointed east. But few rooftop PV panels in Aberdeen or anywhere else for that matter are optimally aligned. Most are bolted flat onto the roof regardless of which way it points.
In this post we will take a photographic tour of some of the outcomes.
This array in the Southwest US is typical of many rooftop installations. The homeowners wanted a larger system than one roof segment would accommodate, so they bolted the panels flat onto two roof segments angled at 90 degrees to each other. Assuming that one of the segments points south then the other will be operating at only about 80% efficiency:
Next is a photo of a subdivision in Japan, taken I would guess around the time Japanese utilities imposed a moratorium on new solar hookups to avoid destabilizing the grid. The roofs are plastered with panels pointing ninety degrees apart and the only house with panels that aren’t bolted flat to the roof is the third from the left in the first row:
In this aerial view of houses in Germany every panel also lays flat on the roof:
As do most PV panels in the UK. (These houses are in Nottingham. Panel tilts don’t change but azimuths vary gradually through about 90 degrees. This will smooth out solar generation during the day but still won’t make the sun shine at night.)
Now for some individual cases. This pyramidal rooftop array in Japan makes use of all the available roof space by pointing panels east, south, west and north. According to PVwatts the north-facing panels will generate only about 60% as much power as the south-facing ones, all other things being equal.
Somewhat more efficient is the Atlantic Health Training Center in New Jersey, which at least omits the north-facing panels:
But note the flat panels on the roofs at left. Owners of buildings with large flat roofs commonly lay their panels flat even though it cuts output. According to PVwatts these panels on the roof of the US Department of Energy are 13% less efficient than correctly-oriented panels. And DOE has a website extolling the need for energy efficiency:
The huge solar array on the roof of the Goodyear-Dunlop plant in Phillipsburg, Germany is similarly afflicted:
And then there are the artistic but equally inefficient PV panel arrays, such as the bulge on top of London City Hall:
The roof of the Vatican, which needed specially-shaped panels to make everything fit:
The National Stadium in Taiwan, which “with the assistance of construction geniuses from Japan, US and R.O.C. … has become a proud milestone in green architecture application.” It has a load factor of 12.5%, about 5% less than optimally-oriented panels at this latitude would have:
And the “Photovoltaic Wall” in Barcelona, Spain. The curve is presumably there to relieve the monotony:
The Barcelona example shows us that people don’t stop at bolting PV panels to the roof. They bolt them to the wall too, usually suffering even larger efficiency losses in the process (the estimates below are again all from PVwatts). I can’t get a number for Barcelona because I don’t know which way the panels are pointing, but the panels on the south wall of the Solar Building in Anchorage, Alaska generate only 78% as much power as they would if they were angled skywards at 45 degrees:
The 15kW of south-southwest-facing thin-film panels implanted in the wall of City Hall in Flagstaff, Arizona, generate only 71% of the power they would generate if they were optimally aligned:
Or at least they would if it weren’t for the tree the city has planted in front of them:
As well as being spectacularly ugly the vertical panels on the wall of Colorado Court apartments in Santa Monica, California are 31% less efficient than properly-aligned panels:
And then there’s the CIS Tower in Manchester, which is festooned with over seven thousand 80-watt PV panels on its south (actually south 25 west), east and west sides. Relative to the optimum panel angle the south-facing panels are 70% efficient, the east-facing panels 63% efficient and the west-facing panels 59% efficient, giving an overall efficiency of 64%. With solar load factors averaging around 10% at this latitude we would therefore expect that this £5.5 million, 575.5kW system to operate at a load factor of only about 6%.
But it doesn’t even achieve that. According to Wikipedia and other sources the load factor is less than 4% (575.5kW capacity, average generation 20-21kW), a number low enough to rank the CIS Tower among the world’s most inefficient electric power generating plants. Yet in 2006 the UK Department of Trade and Industry still chose it as one of the “10 best green energy projects” of 2005.
It would nevertheless be a mistake to think that all PV panels are bolted onto rooftops or walls with a cavalier disregard for orientation. Some people get it right:
And those who are motivated to optimize occupant comfort will usually make sure the panels are angled in the right direction:
For those interested in how much difference PV panel azimuth and tilt makes I put together the following table from the PVWatts calculator. It shows the percentage of generation obtained at different panel azimuths and tilts relative to the maximum of 100% at azimuth = 180 degrees (south) and tilt = 30-40 degrees relative to the horizontal. The numbers are for Gatwick Airport (PVwatts insists on a specific location) but they are broadly applicable at all mid-to-high latitudes.