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Trends in Technology: Alternative Power
Solar arrays are specified by their peak power output for “standard test conditions,” which refers to 1kW per square meter of irradiance; 25 degree C ambient temperature; and 1.5 times normal atmospheric pressure. Naturally you can assume that in practice you won’t get this much power out of the solar panel — and you’d be right. There are three factors that bring down the total power available per panel: manufacturing tolerance (subtract 5 percent); dirt, because as it collects on the array, the efficiency drops (typically 5 percent); and, since we’ll be powering a load with ac, we need to account for the efficiency of the inverter (typically 93 percent). So, for a panel “rated” at 200W of output under standard test conditions, we have: 200*(.95)*(.95)*(.93) = 168W max.
The next thing to consider is just how much sunlight you can gather each day. That of course is going to vary over the course of the year, and it’s going to be higher in sunny climes like the U.S. southwest. You’ll need to determine what your peak sun hours (PSH) are for the location you intend to use. (One example: bigfrogmountain.com/SunHoursPerDay.html) I’m going to use southern California in this example, with its six PSH (averaged over the year) per day.
Let’s go back to our load calculation now: we needed 2.5kW for each of the 24 hours per day, which equals 60kW-hours per day. Each panel will provide (168W)*(6 PSH) = 1,008W-hours per day. Dividing the needed power by the amount that can be collected per panel gives you 59.5 panels; obviously we’d round up to 60.
Now we can begin to see how practical our theoretical hybrid system is. If you consider a panel such as a Kyocera KD205, you’ll note that the dimensions are basically 60” by 39” by 1.4”. It weighs 40 pounds; 60 such panels will weigh 2,400lbs. (plus the weight of the framework). There are any number of ways to mount the panels, but say you were to put up two rows, each 30 panels wide; you would then have a rectangle 97.5’ wide, and 10’ high. The framework will be designed in such a way as to optimize the angle of the panels with respect to the sun.
Battery selection is the next step, but before we do that, we need to know a few things. For starters, each battery will have a depth of discharge specification that we’ll need to know. (Typically this is 80 percent.) Secondly, we’ll need to decide how many days of autonomy we want — basically the number of days the system will operate without any input from the PV array. For our purposes, let’s use three. So, to figure out our amp-hour rating, we divide our watt-hour need by the battery system voltage; we divide that result by our D-o-D figure; and finally we multiply that result by the number of autonomous days: 60,000/24 = 2500; 2500(3)/.8 = 9375 AH total.
Battery selection is interesting because it would seem to make sense to put two 12V batteries in series, and then add parallel branches to build the AH rating. (Batteries added in series keep the same AH rating; but when adding parallel branches, the AH ratings add.)
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