Solar Energy 101: Part II

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

Solar 101
 

Now that we know a little about the PV effect, let’s see how that small stream of electrical current from each PV cell becomes useful energy.

PV cells are organized into the sealed units you are accustomed to seeing; solar modules. PV modules produce DC current and typically range in output from 160 – 310 Watts, depending on the materials and technology used.
 
PV modules are connected together in series into what are called strings. Then these module strings are configured into arrays to produce the optimal combination of voltage and current to satisfy the system design requirements. Actual output calculations are complicated by lots of factors, so let’s start with a simple example: A specific 260 Watt module operating at its maximum power (under load) would produce 33.5 volts according to manufacturer specs. That voltage results in approximately 7.8 amps of DC current. A single string of 12 of these modules would produce an output of 402 volts at 7.8 amps (recall that when devices are connected in series, that is positive to negative to positive to negative, etc., voltage adds and current remains constant) . Then if we combined three identical strings in parallel (positive to positive, negative to negative, whereby voltage remains constant and current is cumulative), we’d have a three-string or -circuit array with a total DC output of 402 volts at 23.4 amps.
 
So we have cells aggregated into modules which are connected together into circuitswhich are configured into an array.
 
OK, remember the complications I mentioned? Let’s look at a couple of the most significant ones. First, sunlight isn’t constant and the current from a PV cell varies with the amount of light hitting it – a cloud passes by, the current drops. Secondly, there’s temperature and it too has an impact on PV cells – the hotter they get, the lower the voltage. These variables affect overall power output. On a hot clear day, current will be high (plenty of sunshine), but lower voltage because of the heat will cause output power to drop. Likewise, a cold (high voltage), day with high thin clouds will produce lower output than a cold clear day because the clouds (lower light intensity) lower the current from the cells.
 
All of this becomes important when we are selecting modules and matching them with inverters, which we’ll talk about next time.

 

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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