Wednesday, January 5, 2011

Solar Design

Our roof slopes in two directions: a southwest facing slope that gets most sun during the afternoon and a northeast facing slope that gets most sun in the morning. The roof spine runs on approximately a north/south axis. Our neighbor on the south side has a hedge of large redwood trees along the property line. In summer, the trees only shade the south side of the roof, but by late October, the sun is far enough down in the sky that half the roof gets almost no sun at all for most of the day. On the winter equinox, the northeast slope on the north side gets only 4 hours of sun in the morning, the southwest slope gets a bit more in the afternoon. In the summer, both sides get lots of sun for the whole day. Overall, our solar resource is adequate if not overwhelming, but these are the rough parameters we have to work with.

Our system has a total of 30 panels, and we need to optimize the amount of power the system will generate, given the constraints of our roof. The directional and large scale shading constraints above are one set. Another set is stuff already on the roof. This includes:
  • A solar powered attic fan to keep the attic cool,
  • Various vents from the kitchen and laundry room fans on the southwest slope,
  • The 2 panel solar thermal hot water system on the northeast slope we had installed last year,
  • Shading from the two dormers, one on either slope of the roof,
  • The skylights, which are actually along the spine running from the north side to central roof but do project slightly down the two slopes.
The current system consists of 18 panels on the southwest facing slope, but some of that space won't be usable with the new system. The new system will be slightly down from the ridge line since there are new building codes requiring a slight offset from the ridge so a fireman can chop his/her way into the house through the ridge in case of a fire. In addition, REC has calculated using their software system that panels which are currently in the bottom row on the southwest slope will be shaded too heavily by the west side neighbor's trees. These trees are pruned down heavily usually on an annual basis, but they grow back quickly and if we miss a year, like we did this year, they do end up shading more of the roof. So REC has figured that only 16 panels can go on the southwest slope where the current system is, two less than currently.

On the northeast facing slope, the solar thermal panels take up prime real estate on the top of the slope near the ridge. Having the solar thermal panels there is better than having PV, since this is the area that only gets 4 hours of sun in winter and the solar thermal panels won't heat up much if they don't get any sun. REC doesn't want to put any panels on top of the feed lines leading from the solar thermal panels to the house, so that leaves just the space on the bottom of the slope below the solar thermal panels. Space for 2 rows of 5 is available below the solar thermal panels, 10 panels in all.

That leaves 4 panels that still need placement. Since PG&E pays the best rates for power during summer afternoons, it makes the most sense to place the panels on the southwest slope. Given that, though, we want to optimize the amount of electricity generated regardless of the tariff, since we are constructing the system primarily to offset our carbon footprint.

So the first design REC proposed was with the 4 panels strung out along the south side of the southwest slope (the panels are outlined in magenta):

According to their calculations, the array on the southwest side has 80% annual sun and 20% shade while the array on the northeast side has 87% annual sun and 13% shade.

I asked them if they could do better, so they proposed two additional configurations. One has the 4 panels on the south side of the southwest slope, but stacked instead of strung out:
The other has the 4 panels on the dormer roof on the southwest slope:
REC doesn't have any calculations on top of the dormer yet, but they gave me estimates of the improvement. The stacked configuration would increase annual power production by 50 kwh/year while the dormer configuration would increase power production by 250 kwh/year. This amounts to around .6% for the stacked configuration and around 3% for the dormer configuration, figuring on 8722 kwh/year from the original REC calculation. 3% isn't much but it is something, so unless we can come up with some other configuration that does better, I will probably go with the dormer.

Tomorrow REC will remove the old solar panels and at that time they'll take a shade measurement on the dormer to get an exact value, so we should know shortly. Once the final design has been decided upon, REC will send it to the city and they'll take 2-3 weeks to consider sign off, provided there are no  problems. Then comes the installation. After the installation, PG&E must certify the system (they don't have to install a dual ported meter in our case, we already have one) which should take a day or two (or three, PG&E has already taken 2 months with our request for a 200 amp service upgrade) and then we can turn on our new system.

1 comment:

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