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Trends in Technology: Alternative Power
“The system uses (24) 4V 1,550AH flooded-cell batteries, wired in series-parallel to make a 48V system. These batteries provide a total of 2,500AH storage at 48V. The solar array uses (68) 75W panels to give us a peak input of 5kW, although this is only at prime conditions, as solar panels lose so much efficiency when not precisely aimed at the sun. The system on average is able to produce between 2kW and 3kW, most of which is pushed to the batteries. A sophisticated digital charge controller manages the flow of energy from the panels to the batteries. Due the potential weather at the site, the panels are stationary and tilted to favor winter sunlight. While possible, we do not change the tilt during summer months as it is simply too much work. 120Vac is created by a Trace 4kW pure-sine inverter and it has been going strong for 13 years now. We have a backup on-site, too. We also have a LPG-fired 8kW generator (which at 9,200’ MSL is de-rated to a mere 3kW), which is started automatically when the system voltages drop below a specific threshold.”
I asked Jeremy about the design of the battery bank — specifically why lower-voltage batteries were chosen for their system. “My understanding on why we use low-voltage batteries is mostly storage space and simplicity, but there are other considerations. Each 4V battery is about 1,500AH (6kW-hrs) so you get a fairly large bank with few batteries. By comparison, a 12V deep cycle marine battery can store about 200AH, or 2.4kw-hrs of energy. When you calculate the load banks with Ohm’s Law, you see that in order to obtain 48V at 2,500AH, you’d need about 100 12V batteries to do what you can with 24 4V batteries. That is an enormous number of inter-connections that must be cleaned and maintained, plus the regular ‘watering’ of each cell. In a 12V battery, that’s six cells per battery, so you have a lot of work to keep them all topped off. The plates inside each low-voltage flooded battery are designed for more stressful cyclical use. They are less prone to pitting from charge/discharge cycles and have more surface area to resist sulfate build-up, which is why they tend to last so much longer than small, higher voltage batteries — the warranty on the 4V batteries is 15 years, if they are properly maintained. By comparison, a good marine battery has a warranty of 2 or 3 years in deep-cycle service. So, you get a lot more bang for your buck with the big batteries.”
“Green” power is all the rage right now, and many vendors are available to sell you a system. As we’ve seen, it’s completely practical, as long as your power requirements aren’t too high. Just remember all of the design factors involved when you are determining the overall capital and operating costs of the system.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at firstname.lastname@example.org.
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