Organic Rankine Cycle
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Confessions of a
The night was dark and stormy. The rain pelted the skylights and flooded down the gutters while the wind howled menacingly through the trees and around the house, shaking the windows and chimney. The neighborhood was dark and no lights could be seen for miles around. The few radio stations that were still on the air reported that the utility power was out in the entire southern half of the state.
Fortunately, this home was powered with a photovoltaic renewable energy system with a back-up generator. It had been cloudy for several days preceding the storm and the generator had automatically come on to recharge the batteries the day before. The battery bank was sized to provide four days of cloudy operation.
It was near midnight when, surprisingly, the generator started up again. A quick check of the voltmeter revealed that the batteries were near the bottom of their charge. What! So soon? A question was raised. Was the generator filled with gasoline the day before? No one remembered, and no one wanted to go out and check the fuel gauge in the raging storm.
While everyone was trying to remember whether the generator had been topped up with gas, a quick check was made of the house loads. Was something on that shouldn’t be or was there some kind of fault that caused the batteries to be drained so quickly? No, everything appeared normal. There were no new hidden loads and the system ammeter showed that the existing connected loads were drawing only their low, energy- conserving amounts of current.
At this point, the lights dimmed a few times and went out as the generator slowly ran out of fuel and quit. A search with flashlights for the gasoline can found that it was empty. A quick trip to the gas station was aborted when everyone realized that, with the utility electrical outage, there was no power for the gas pumps.
Candles were lit and everyone settled down to try to figure out why the batteries had not kept their charge.
They were, after all, top-of-the-line golf-cart batteries that were only five years old.
Had they run dry and someone forgot to check that water was always over the plates? Well, it seems that that might have happened a couple of times every summer. And during the last year or so, it was getting necessary to check them every three or four weeks to add water. More water was always needed in the upper set of eight batteries on a 24-inch high shelf than was needed in the lower set of eight on the floor.
Did the temperature compensated charge controller work properly? Well, the home-brew charge controller didn’t have a temperature compensation device; the voltage set points were adjusted by the seat-of-the- pants method based on the season. The battery charger in the inverter had a temperature sensor, but somehow it had come away from close contact with the batteries. Anyway, it couldn’t be too important since only one sensor monitored the temperature of the batteries and they were at two different locations.
Was the temperature too high or too low in the garage where the batteries were mounted? Well, let’s see - it must get about 110°F on hot summer days in there and down to about 20°F on cold winter days. Maybe it was a little extreme for batteries that like to stay at 80°F.
Was the three-stage charger working properly? Well, no! This charge controller had only a single set point that held the batteries at that voltage. Did someone remember to lower the set point during the four-week long vacation every summer? Was that really needed? The set point was only a volt or so above the gassing voltage to get those batteries good and stirred up every cycle. But, during the summer vacation, the house loads were reduced to zip, and gee, the batteries sure did need a lot of water when the family returned home.
How about the battery installation? Did every cell get an equal share of current, voltage, temperature? The eight batteries on the floor had a one-foot longer cable than the set on the top shelf. No Z-style plumbing-type wiring was used; after all 2/0 cable was used between cells and those automotive-style posts and clamp-on terminals were plenty hefty. It was also a little cooler all the time near the floor than it was on the shelf.
Were the cable clamps always tight on the terminals and the clamps always tight on the cables? Now, that’s an interesting question. At times, when the voltmeter was used to measure the voltage drops in the cable, some cables measured 12 millivolts and some measured 150 millivolts - boy, some of those terminals were warm! Isn’t that normal?
When the batteries were equalized every month or so,
64 Home Power #54 • August / September 1996
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