True South Solar: Johnson-Torrence Residence

Bonnie Johnson and Paul Torrence represent ideal candidates for a residential grid-tied system. The retired couple has a goal of achieving net zero energy use and prior to the PV installation had invested in energy-efficient household appliances. The installed 6.58 kW array is sized to generate approximately 75% of their annual electricity. When a planned domestic solar water heating system is installed, their overall energy use will be close to net zero.

A DPW Solar Multi-Pole Mount system was specified to provide adequate ground clearance and to minimize the number of ground penetrations required, reducing installation time and cost compared to other ground-mount options that were considered. DPW Solar provided engineering assistance for the footing specifications and associated racking components. Dirt work was subbed out to a local skid steer operator with a 24-inch auger.

DPW Solar’s redesigned Multi-Pole Mounts, released in 2010, have improved setscrews in the rail-to-pipe brackets that strengthen the mounting system. The installed Sharp NU-235F3 modules feature two cross rails that add to the array’s overall rigidity and resistance to wind loading.

Wildfires are a real concern in this part of the country. To protect against this threat, the ground underneath the array was surfaced with weed barrier and crushed rock extending 4 feet beyond the southern edge.

“I’ve seen what a hot, fastburning grass fire can do to ground-mounted PV arrays that are installed without adequate ground clearance or surface preparation—it melts aluminum racking and modules wholesale. In our location, it’s worth it to take every reasonable precaution to minimize the risk of potential array damage due to wildfire.”

Eric Hansen, True South Solar

Overview

DESIGNER: Jacob Wood, design and sales manager, True South Solar, truesouthsolar.net
LEAD INSTALLER: Eric Hansen, general manager, True South Solar
DATE COMMISSIONED: August 2010
INSTALLATION TIME FRAME: 5 days
LOCATION: Williams, OR, 42°N
SOLAR RESOURCE: 4.5 kWh/m2/day
RECORD LOW/AVERAGE HIGH TEMPERATURE: –3°F/90°F
ARRAY CAPACITY: 6.58 kW STC
ANNUAL AC PRODUCTION: 8,705 kWh

Equipment Specifications

MODULES: 28 Sharp NU-235F3, 235 W STC, +10%/–5%, 7.81 Imp, 30.1 Vmp, 8.5 Isc, 37.0 Voc
INVERTER: SMA SB 7000US, 7 kW, 600 Vdc maximum input, 250–480 Vdc MPPT range, 240 Vac output
ARRAY: 14 modules per source circuit (3,290 W, 7.81 Imp, 421.4 Vmp, 8.5 Isc, 518.0 Voc) with two circuits total (6,580 W, 15.62 Imp, 421.4 Vmp, 17.0 Isc, 518.0 Voc)
ARRAY INSTALLATION: DPW Solar Multi-Pole Mounts on 4-inch Schedule 40 pipe, 210° azimuth, 30° tilt
ARRAY COMBINER: OutBack FWPV-8, 15 A fuses

 

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