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Solar Electricity: How it Works

poster1Most solar electric systems we install today are “grid-tied” and have no batteries. These types of systems send electricity to your loads (electricity-using devices) or to the grid (selling it back to the utility for use elsewhere). In most states, the utility is obligated to buy back the excess. Solar electricity will always flow to the loads first, since that is the path of least resistance.

All this means that you can have solar electricity without the need for batteries. However, if you want to have electricity during power outages, you will still want to get batteries, or alternatively, a back-up generator. If you have frequent power outages (say several times per month), then battery backup is a good idea. If power outages are not frequent (once a year or less), then it will be more cost-effective to get a generator instead, along with a pure grid tie solar electric system. One of the great things about a pure grid tie solar electric system is that it has very minimal maintenance requirements.

A “pure grid tie” solar electric system consists of

  • Photovoltaic (PV) solar modules
  • Roof rack or ground-mount racks
  • Inverters
  • Disconnects
  • Wiring, conduit, junction boxes

The majority of PV modules on the market today are made with silicon solar cells. This fact is one reason why the widespread use of solar power is possible and perhaps even inevitable! Silicon is the second most common element on Earth. Sand is made of silicon dioxide!

The solar cell is a semiconductor device, the simplest one in fact. It is a single P-N junction, like a simple diode. In contrast, your computer’s silicon brain has something like 200 million P-N junctions! The P-N junction creates an intrinsic electric field. This means that when photons (from the sun) free an electron in the material, it feels a force. Force leads to motion and moving electrons constitute a current. So the basic miracle of the semiconductor is the intrinsic electric field, which leads to current flow. This is the miracle upon which our entire electronic civilization is based! Thus, high school students should be learning about this in science class.

Solar cells are made into PV modules. These are made with tin-plated copper wiring, polyvinyl fluoride back sheets, ethylene vinyl acetate encapsulant (EVA), tempered glass covers and aluminum frames. The embodied energy in a solar module can be generated by that module in about two year’s time, so its “energy payback” is quite good. This means that someday, PV will power PV factories.

The PV modules are the most durable part of the system, with life expectancies of over 30 years.

The inverter converts direct current (DC) from the solar array to alternating current (AC). It also:

  • Synchronizes to the utility’s grid power
  • Shuts off when there is no grid or the utility voltage or frequency is bad or if the utility sine wave is not clean
  • Restarts automatically when grid power is restored
  • Will open the DC circuit if there is a ground fault to protect against DC arcing and fire

So the inverter is a pretty smart gadget. It is even able to distinguish between real grid power and the output of small generators. So you can run your critical loads with a generator and the inverter will still not try to back feed the grid when it is down.

The PV system is “interconnected” by running the inverter output to a dedicated breaker in your circuit box.

Typical wiring scheme: