[Editor's Note: This article came to us by way of the Editor of the Haleakela Times. That paper had published our "Makini Awili" article about creating a grid beating electrical appliance. This article appears to be a letter to the paper in response to our article. The fact that no UPS is currently available for the job at hand was why we proposed
the invention of one.]
by Franklin Russell on 17 January 2007
This is a response to the person who was considering modifying a UPS for off the grid power use.
The UPS (uninterruptible power supply) is an important part of my off the grid system. I would never modify a UPS though, as the writer was suggesting. This could cause a fire and failure of the UPS. A UPS is a miniature off the grid power system. So one could understand why the writer thought you could just attach PV modules to the battery. The problem is that the UPS has a cheap inverter and a sealed battery.
Inverters take DC power from PV panels, fuel cells etc. and turn it into AC (alternating current). Here in the USA we use 60 cycle per second 110 volt AC. This is a difficult job for an electronic device to emulate the perfect sine wave AC provided by the spinning dynamo that Tesla invented. A sine wave should be perfectly smooth with no bumps or over voltages. Here is where the problem arises, a UPS uses a cheap modified sine wave inverter. The wave form looks
like stair steps instead of a pure sine wave. This is called square wave distortion. Your appliances may not work right and heat up. Your power supply in your computer will burn out sooner. If you try to plug a UPS into a modified sine wave inverter it will alarm and not work because the voltage goes above 160, setting off the alarm.
If you get a surge capacitor and put it on the output of a cheap inverter it is supposed to smooth it out and regulate the voltage. I have never tried this though.
Modern expensive inverters have pure regulated sine wave output with 5% distortion max. The cost is much higher but your gear will work cooler and more reliably. Also you can plug a UPS into the output to protect your computer and entertainment system in case of a shut down from low battery voltage or a brown out from a washing machine motor kicking in. I use a UPS and highly recommend them for off the grid systems. Some folks don't understand how to shut a
UPS off. Many require pushing in and holding a momentary switch to truly shut it down. It will alarm and heat up as the battery in the UPS is providing current to the UPS inverter. This can be a problem. Read the manual.
Batteries for off the grid are typically flooded lead acid and have deep cycle plates in them. Charging a sealed lead acid battery could be dangerous without a correctly set charge controller. Only use deep cycle batteries for off the grid use. Even if you take care of them they will still wear out in 5-7 years. Batteries are a weak point still. Someday we may be able to get NiMH or L-ion batteries for deep cycle use. Until then, we are stuck with lead and sulfuric
acid clunkers.
For Juan's system, I would recommend an Exeltec sine wave inverter 600Watts for about $600. Then four golf cart batteries about $250, and two or four amorphous PV panels at $375 each. And a small MPPT charge controller about $250. This should keep your office or cash register going long enough to get through an eight hour power outage for under $2000.
I live off the grid and sell PV (www.SmartRoofSystems.com) systems that integrate into the roof. There is no way around the cost of these systems. You get what you pay for when it comes to renewable energy systems. Take advantage of state and federal tax credits and accelerated depreciation to help pay for your system. There are lenders like Wells Fargo that will loan on solar systems.
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