Solar Electric System Details
The solar electric system produces electrical power from solar power. The system consists
of an array of photovoltaic (PV) panels that convert photons from the sun into direct
current (DC) electricity. An inverter then converts this DC power into alternating current (AC) power which is then fed into the utility's power grid.
The system is owned by our nursery, McKenzie Valley Bamboo, and it is designed to provide sufficient power to operate the nursery's 3 hp irrigation pump. We usually need to irrigate from June through September and the best time of day to water bamboo is during the afternoon when it is hottest. Therfore our irrigation requirements closely match the time the PV system is producing the most power.
The solar array is a set of 24
Evergreen Solar ES-195
PV panels tilted at 14° (3/12 roof pitch) and facing at 177°
(almost due south). The array is rated at 4.7 kW.
Even though the array is mounted at a shallow angle, at this latitude (44°) output is calculated to be at 95% of the best case mounting angle.
grid-interactive inverter is a
SMA Sunnyboy SB5000US which is rated at
5 kW AC and has a 96% conversion efficiency. An optional RS-485 card is
plugged into the inverter which allows real-time data to be accessed remotely.
The inverter monitors the AC power grid and if it is alive and well, it will convert the DC power coming from the solar array into AC power. If the AC power grid goes down, the inverter stops converting power and will not resume until the AC power grid is up and stable for 5 minutes.
Below the big inverter box is the DC disconnect switch which disconnects the solar array from the inverter. The meter on the right reads total AC kWh produced. The inverter connects to the subpanel below the meter through a back-fed 2-pole circuit breaker.
The Sunnyboy inverter has a very intelligent interface that allows the device to be controlled and monitored remotely over its RS-485 serial link. The link runs at 1200 baud and uses a packet communications protocol which SMA has made available on its web site.
This picture shows how the panels are mounted on the roof. The system uses extruded
aluminum rails and aluminum mounting blocks which make for a fast installation.
The pole barn roof the array is mounted on was not designed to hold the weight of the array, so it was necessary to have a structural engineer evaluate the loading and come up with a fix if needed. In this case it was only necessary to add some 4x4s and mounting brackets to the underside of the rafters.
The system was designed and installed by Solar Assist which is located in Eugene, Oregon.
|The splice box on the left is used to connect together the panels. The 12 panels in each row are wired in series and then connected in parallel here. The combined power is then connected to the inverter through the conduit passing through the roof.||
The local utility requires a disconnect switch to be mounted within a few feet of the utility's power meter. There is no technical or safety reason to require this disconnect
with modern inverters, but our utility (EWEB) still required it in mid 2009.
The power meter records both the energy being consumed and the excess energy being generated. At the end of the month the two counters are read and we are billed for energy used and credited for excess energy produced through a net metering agreement with the utility.