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Green Collar Careers - Solar Electric Overview

Component Design Review

• Solar PV Panels
• Solar Panel Mounting Racks/Rails & Clamps
• Tracking Systems
• Connectors & MC Cables
• Combiner Boxes and Lightning Arrestors
• DC Disconnect
• Inverter
• AC Panel
• kWh Meter
• System Diagram
• Battery Bank (Off Grid)
• DC Charge Controller (Off Grid)

Solar PV Panels have a footprint of about 40" x 60" and are in the 200 Watt DC range.  Consisting of an array of 50-60 individual polycrystalline silicon cells, a metal backing/frame and a non-reflective glass cover these panels typically weigh about 40 Lbs each. There are about a dozen key manufacturers and each has similar sizes and specifications, but not exactly the same.  Some use higher voltages and lower currents and may have slightly different foot prints.  In most cases mounting provisions and power cables will use the same hardware.

Mounting Rails are used to support solar PV panels.  These rails are usually an aluminum extrusion with a clamp that fits in a set of slots or flanges depending on the rail.  Clamps attach to the solar panels and rails attach to the structure.  Rails can be attached with screws, threaded rods and several other ways.  There are hundreds of specialized extruded rails designed for modular frame structures, and each have some special benefit.

Tracking Systems can boost the production by as much 1/3 over that of a static mounted system.  Azimuth axis tracks the sun through the day from sun up to sunset giving the largest gain.  Elevation axis tracks the sun through the year as the elevation changes with the season.  Tracking systems are generally much more costly then just adding extra panels so they are rarely used when cost benefits are the primary concern with system design.

Connectors and cables manufactured since 2008 must conform to the new NEC code requires the shrouded bayonet style latching connectors, as shown in the picture. Solarline MC4 and Tyco Solarlok are the primary connectors in use since 2008.  Some older stock panels have the SL1/MC3 type connectors and some have the new SL2/MC4 latching type. While it is possible to make these cables in the field, its better to buy them already to length.  Standard lengths are 15, 20, 25, 30, 40, 50 feet.  There are also 30 & 30 foot extender cables.  Each panel will need two cables + & - .

Combiner Boxes serve as a junction point for the power cables exiting the solar panels.  They come in various configurations and may allow series and parallel connections to establish the DC voltage output.  Most include positions to add fuses and/or circuit breakers. Lightning protection is a mandatory component for all solar PV installations.  Both the DC side and AC side need protection.  Protection begins with an effective ground such as a copper rod at least 8 feet deep with 1/0 or larger cable to the DC and AC grounding points of the system.  The Lightning arrestor is a special silica device that below a certain voltage threshold is an insulator, and above that voltage becomes a conductor.

DC Disconnect box requirements are usually dictated by local codes.  Generally a conventional four position AC service panel with DC rated breakers will do.  Most kits do not include these boxes so you will need to check your local code requirements and purchase them separately, generally less then $100.  Locate this panel next to the inverter for easy service.

Inverters take the DC power from the solar panels and invert that to AC power.  A clean AC power signal is a must;  for a refresher on inverters review Section 309.  Grid Tied systems must use an inverter approved by the local utility.  Grid Tied requirements are more stringent to assure a clean output is generated, that proper surge and protection is implemented, and that the output power wave is synchronized with the commercial AC side. Inverter placement should be located indoors, near the AC Power Panel, and preferably in an environment free of excess dust and heat.  Some inverters have fans which may require annual dust removal etc.

AC Power Panel is the same thing as the main power panel for your home.  The output from the inverter should pass through an AC disconnect box (strongly recommended) and local codes need to be checked for requirements.  Generally something similar to the DC disconnect will apply.  A dedicated AC Disconnect will provide the best location for AC surge and lightning protection devices for the solar electric system.

Kilowatt-Hour Meters provided by the utility company may be sufficient, however most utilities will require a separate meter just to measure the volume of electricity produced by the solar electric system.  These separate kWh monitoring units (Cumulative kWh Meters) are often not included in the kit and are generally under $100.  They make a great feature for homeowners to visually see the value of the system they purchased.

95% of the solar PV installations are Grid Tied with no battery or generator backups.  Off grid systems will almost always require a backup system or there is no power when the sun is not shining.  The main component differences between a Grid Tied System and Off Grid system are Battery Banks and Charge Controllers.

Every Off-Grid system will have a dynamic set of complexities so we'll only touch on a few key points.

Battery Banks use lead-acid batteries that differ from your car in sizes and voltage.  Each cell of the battery provides 2 volts so a 6 volt has 3 cells, and 12 volt has 6 cells.  Storage batteries are designed for "Deep Cycle" service meaning they will typically cycled between 100% charge to 15% charge.  Car batteries aren't designed for deep cycle service and won't last very long under home service. 

Batteries are sized as Voltage x Amp hours.  Section 305 has brief summary about DC electricity, but to simply, multiply the volts x amps to get the watt hours.  So a 600 Amp hour 6 volt battery would store 3600 watt hours.  Typically battery storage is sized at about 6 times the DC Watts of the system operating for 1 hour.  For example, a 3000 Watt DC solar panel array at 1 hour is 3000 Watt-Hours;  Battery Capacity may be sized about 18,000 Watt-Hours.  Again, the complexities of the specific application will determine the sizing of battery storage.

DC Charge Controller is required to maintain an adequate charge level for the batteries.  Some Charge Controllers are connected to the DC Disconnect to feed the batteries, and some Inverters may have an integrated Charge Controller.