Solar Energy 101: Part III

a guest post by Ken Whiteside, Director of Business Development at ONTILITY, LLC.

Solar 101
 

So far in parts 1 and 2, we’ve looked at how PV cells work and how those cells are arranged into modules, and how modules are configured into a solar array. In the little three-circuit array described in Part 2, we ended up with about 402VDC at just under 8 Amps. How do we make use of that DC electricity?

One of the key factors that held up the widespread adoption of PV as an energy source was the development of an efficient, reliable inverter to convert DC to AC. For years, PV was for the most part restricted to off-grid applications where DC loads were used because there wasn’t a sufficiently cost-effective way to convert the DC current from the PV array to 120/240 volt AC current. Furthermore, that voltage had to be constant, the frequency a steady 60 Hz, and delivered as a clean sine wave. Otherwise, digital equipment wouldn’t work and feeding electricity to the utility grid would be impossible.

 In the past 20 years, technology has solved the problem. We now have super-efficient (96%+ conversion rates), highly reliable inverters that produce clean, steady power acceptable to all utilities. These inverters are equipped with self-regulation circuitry that shuts them down if the power source makes it impossible to deliver current within the specifications set by the grid operators and the NEC. They also contain anti-island shut-off isolating the PV array from the grid in the event of a power failure.
 
Each inverter has both minimum and maximum voltage limits. It takes a certain voltage to start the inverter and if the voltage exceeds its maximum, it shuts down. If the array isn’t producing the minimum voltage, the inverter is off and no power is available to loads. Likewise, if array voltage exceeds the inverter maximum, it shuts down to protect itself and other system components. So the solar array and the inverter must be matched, a critical part of system design and equipment specification.
 
So how much AC electricity would we get from our example system? Our 402VDC at 7.8 amps results in an array output of 3,135.6 Watts. Using a very general DC/AC conversion factor of 0.7 (Losses due to wiring, temperature, inverter efficiency and other factors. Did I mention that this gets complicated?), the AC output from the inverter would be 2,194.9 Watts or about 2.2kW.
 
OK, enough calculations. In the next installment, we’ll look at the other parts that make up a PV system.
 
Ken Whiteside photo Ken Whiteside has been a fan of solar energy for decades. His first hands-on experience was installing solar on off-grid houses around Telluride, Colorado in the 1990’s (summer in the San Juan Mtns. - somebody had to do it). From his home in Austin, Ken writes and works for widespread adoption of solar electricity, smart energy production and use, and sustainability.
 

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