SOLAR FACILITY DESIGN A BRIEF PRIMER PV MODULES, STRINGS, AND ARRAYS A generic description of fixed ground mounted solar photovoltaic (PV) cells and how they can interconnect individually into an electrical infrastructure is provided below. The processes and component equipment used in PV plants starts with a hand sized solar or photovoltaic cell which converts light into DC electricity by the photovoltaic effect. Photovoltaic cells are interconnected silicon based cells connected in series creating an additive voltage. Solar cells are electrically wired together to produce a PV module. A PV module encases the stringed solar cells in an all-weather protective housing. It is generally made of layers which can include a transparent glass face, anti-reflective layer, solar cells, metal or synthetic back sheet, an electrical junction box with wiring connections and a frame. PV modules are commonly 3.5 feet wide by 6.5 feet long by 2-3 inches thick in size. PV modules can be built to order; therefore electric output depends upon the specifications required by a developer. Several PV modules can be electrically connected together in series and/or parallel to achieve the desired electric output, and then bolted together into a racking system to make a row or string (Figure 1). Figure 1. Relationship among a PV module, string, and array. SUPPORT STRUCTURES Racking refers to the support structure that the PV modules are affixed to that allows them to be properly positioned for maximum capture of the sun s solar energy. The PV 1
module arrays would be oriented along a north-south axis and mounted on sets of galvanized steel racking to form a row or string (Figure 2). Figure 2. Example of an assembled racking system. The project would use a combination of galvanized I-beams, tubular steel posts, and channel steel rails. I-beam/tubular steel posts would be driven into the soil using a pile/vibratory/rotary driving technique similar to that used to install freeway guardrails. If the subsoil on the Project does not have excessive rocks, posts will be installed without the need for concrete foundations. I-beam/tubular steel posts are driven 12 to 18 feet into the ground with about 5-6 feet exposed as in the example in Figure 3. Technicians, using stepladders or working from the ground mount PV modules onto the racking system. Each row will consist of about 60 PV modules mounted on the racking system. Each row will have an opening in the middle to allow access for operation and maintenance activities. Each row or string will be approximately 200 feet long, 8 to 12 feet wide, and approximately 20-30 feet apart (Figure 4). A schematic of a typical 5 MW solar farm layout is shown in Figure 5. 2
Figure 3. Driven pier and lower racking system, grounded Figure 4. Row or string example 3
Figure 4. Example of a typical 5 MW solar farm layout ELECTRICAL COLLECTIONSYSTEM The system will collect the direct current electricity (DC) electricity generated by PV modules within its array to combiner boxes similar to what is shown in Figure 5. Combiner boxes are protected by fuses or breakers and are about two feet by two feet square and affixed to the racking system. Cables from the combiner boxes are placed underground in trenches one to two feet wide, at a depth defined by the National Electrical Code (typically two to three feet deep). Trenches are backfilled with screened native soil after cable installation. Cables from 4
combiner boxes connect to an inverter which converts DC electricity to AC electricity. Single inverter installations will be installed on a central inverter skid which is supported by driven or concrete piers. If multiple, smaller inverters, are used, they may instead be installed on the racking system. The electricity from all of the inverters and step-up transformers will be collected via underground cables at intermediate voltage to the solar project substation. The solar project substation will transform the electric voltage from the intermediate level of 34.5kV to the interconnection voltage (typically 69kV to 115kV). The electricity will be taken from the solar project substation into the grid via an overhead high voltage transmission line (HVTL). Figure 5. Collection system example: (A) array; (B) combiner; (C) inverters under roof; (D) metering; (E) meteorological monitoring and data communication hub; (F) transformer 2015 Mario Monesterio OPERATIONS AND MAINTENANCE BUILDING The O&M building for the project will be a single level, all weather, wooden, or metal frame building approximately 3,000 to 5,000 square feet. It will house the equipment to operate and maintain the facilities. It will provide access and storage for project operations and maintenance. The location of the O&M building will be determined as design of the facilities progresses. 5
AUTOMATED FACILITY CONTROL AND MONITORING SYSTEM An integrated project wide supervisory and data acquisition (SCADA) system will be installed to cover each facility. It will have the capability to be remotely operated with real-time control over most operational functions. Operations at an individual facility will be performed from time to time as required for certain component tests, resets and troubleshooting activities. One or more meteorological stations up to 15 feet tall will be installed at the facility. This will provide an additional data source to the SCADA system. 6