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Trends in Technology: Alternative Power
Let’s take a specific example that’s easy to find: Surrette batteries. (rollsbattery.com). In order to get the AH rating we need, you could use an array of 54 12V 357 AH batteries (27 parallel branches each made up of two batteries in series); or, by using 4V 1350 AH batteries, you can achieve the necessary AH rating by building an array of 42 batteries (seven parallel branches, each made up of six batteries in series). It turns out that the cost of the 4V array is somewhat less (on the order of 5 percent) but there are some other important factors: the 4V battery array (when flooded) weighs 13,230lbs.; the 12V array weighs 14,688lbs. The 4V batteries need 48 square feet of floor space; the 12V batteries need 92 square feet. (I’m neglecting the height differences because no matter what, the room will need to be tall enough for you — and you’re much taller than either type of battery.)
So now we know how large the PV array is, and how large the battery array is. What I want to point out is that in calculating the overall cost of a hybrid system such as this, you’ll add up the hard costs of the solar cells, and the batteries, but you also must include room indoors for the batteries, as well as the cost of whatever structure you use to hold up the solar cells.
Since we’re describing a hybrid system, we must consider the alternate power source as well. Consider a small generator first, the purpose of which is really two-fold: It can operate the entire ac load should the need arise, although normally it is used to keep the battery array from discharging too far in the event that dc input from the PV array is lacking (take a look at Figure 2 again.) Obviously, in this configuration the generator must be large enough to handle the complete ac load.
Another source of power for charging the batteries is a wind-generator. (See Figure 3 at RadioMagOnline.com) I’ve written before about using wind for power: radiomagonline.com/infrastructure/power/radio_sun_wind_fuel//index2.html. Since the efficient use of wind-generators requires the installation of another tower, and depends upon clearance around the wind-generator itself, I doubt that there will be many instances favorable to this type of system.
Another important item in the system is the controller, which is often a combination unit that not only serves as the inverter, but as the battery charger as well. In some systems, the charger/voltage regulator that feeds dc to the batteries, either from the PV panels, or a wind turbine, is added into the system.
Let’s look at a real-world example of an off-grid, hybrid energy system. KLRD is one of Educational Media Foundation’s stations in Southern California (licensed to Yucaipa, which is about 70 miles east of Los Angeles). It has an ERP of 590W, but with a HAAT of about 3,400’, and height above sea level of over 9,000’, it can be heard over a very wide area. The Mt. San Gorgonio site is very isolated, and in fact, the site’s Engineer, Jeremy Preece, tells me that access can only be gained during winter months by flying into the site. I asked Jeremy to describe the hybrid power system that KLRD has been using successfully since 2001. “The (KLRD) system was designed to comfortably support a constant load of about 500W, or 12kW-hours per day. In reality we use closer to 15kW-hours per day since we have added more ancillary equipment and increased TPO for an upgrade a couple years ago.
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