Western Sun Systems: Williams Residence

The Williams residence is located on a mountaintop more than a mile from the nearest utility connection point. The client considered bringing utility power to the site, but aesthetic issues associated with overhead lines and the high cost of running the lines underground made an off-grid PV system preferable.

The design load included a groundsource heat pump for space and water heating, which significantly increased the system requirements. The final design utilizes ac coupling between SMA Sunny Boy and Sunny Island inverters and a backup generator. A Multicluster Box aggregates the Sunny Boy, Sunny Island and backup generator ac outputs and allows all three sources to provide power to the residence simultaneously.

Exide Absolyte VRLA batteries were specified due to their low maintenance requirements and because they can be installed horizontally and stacked vertically, which reduces the battery bank’s footprint. VRLA batteries require moreprecise charging than their flooded counterparts. The sophistication of the Sunny Island battery charging was a big selling point given the large financial investment in storage.

The array, battery and inverter shed and backup generator are located 700 feet from the house to minimize noise and improve the project’s aesthetics. To overcome the ac voltage drop, the system’s 208 Vac 3-phase output was stepped up to 480 Vac 3-phase and stepped down to split-phase 240 Vac at the house. The step-up transformers created an inrush current issue that SMA helped Western Sun Systems solve.

The client wanted the PV array mounted flush to the roof of the battery and inverter shed, which was completed prior to the PV installation. However, the roof was not large enough for the required array. To solve this issue, Western Sun engineered a combination roofand ground-mount system designed for a 120-mph wind load. The lower section of the array is ground mounted, while the upper section is secured to the building’s roof. This approach required custom roof brackets compatible with the Unirac U-LA ground-mount system. The mounting rails are installed to allow for some movement between the roof-mounted and ground-mounted array sections.

Another challenge came from the local fire department, which required a single switch to disconnect the entire system. Western Sun developed a relaydriven disconnect strategy, controlled by one of the Sunny Island inverters. When the main disconnect switch is physically opened, the relay in the master Sunny Island inverter opens as well, disconnecting all the inverters in the system.

“The owners and their general contractor, Kevin Raph, helped make this project a very enjoyable and successful endeavor. We’re very happy with the support we received from the SMA staff. Proper battery management is a key component of off-grid systems, and we are very confident with SMA’s ac-coupled technology.”

Jim Dunn, Jr., Western Sun Systems

Overview

DESIGNERS: Jim Dunn, Sr., Western Sun Systems, westernsunsystems.com; Phil Boutelle, Real Goods Solar, realgoodssolar.com
LEAD INSTALLER: Jim Dunn, Jr., Western Sun Systems
DATE COMMISSIONED: December 2010
INSTALLATION TIME FRAME: 60 days
LOCATION: Carmel Valley, CA, 36.3°N
SOLAR RESOURCE: 5.5 kWh/m2/day
HIGH/LOW DESIGN TEMPERATURES: per Solar ABCs solar reference map: 77°F/30°F
ARRAY CAPACITY: 39.5 kW
ANNUAL AC PRODUCTION: 65 MWh

Equipment Specifications

MODULES: 168 SolarWorld Sunmodule SW 235 mono black, 235 W STC, +5/-0, 7.77 Imp, 30.3 Vmp, 8.19 Isc, 37.5 Voc
STRING INVERTERS: Sunny Tower-US with six SMA Sunny Boy 6000-US inverters; 6.0 kW, 600 Vdc maximum input, 250–480 Vdc MPPT range; 36 kWac output total; string inverters ac coupled to 3-phase, 208 Vac SMA Sunny Island system
BATTERY-BASED INVERTERS: Nine SMA Sunny Island 5048-US, 5.0 kW, 48 Vdc nominal input, 120 Vac output; system configured for 3-phase, 208 Vac output; Sunny Island inverters are grouped in three clusters of three, with one inverter in each cluster providing one leg of the 3-phase output; 45 kWac output total
INVERTER INTERFACE: groSolar MCB-12 Multicluster Box (The Multicluster product is currently manufactured by SMA. At the time of installation, groSolar was manufacturing and listing the product per SMA’s design specifications.)
BATTERIES: 48 Exide Absolyte GP 3-100G23 AGM VRLA, 6 Vdc, 1,260 Ah at 20-hour rate; two battery strings per three-inverter cluster, 2,520 Ah at 48 Vdc nominal; 7,560 Ah total
BACKUP GENERATOR: Kohler 80 kVA propane with noise-attenuating enclosure
ARRAY: 14 modules per string (3,290 W, 7.77 Imp, 424.2 Vmp, 8.19 Isc, 525 Voc), two source circuits per inverter (6,580 W, 15.54 Imp, 424.2 Vmp, 16.38 Isc, 525 Voc); 39.5 kW total
ARRAY INSTALLATION: Continuous roof/ground-mounted array, Unirac U-LA racking with custom-engineered brackets for roof-mounted array section, 180° azimuth, 22° tilt
ARRAY STRING COMBINERS: SMA Combi-Switch, four 15 A fuses per switch, three switches total
SYSTEM MONITORING: SMA Sunny WebBox and eGauge monitoring; Web- Box monitors Sunny Boy and Sunny Island inverters and weather data from an SMA sensor box; eGauge monitors ac output of Sunny Boy and Sunny Island inverters and backup generator

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