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Converting a 12 Volt System into a 24 Volt System
Bob-O Schultze KG6MM
©1994 Bob-O Schultze In the beginning it was a 12 Volt
battery and a radio. And the radio
begot the tape deck and rock ‘n roll and it was good. And the tape deck begot taillight bulb lighting and the CB radio, which begot ham radios and electronics projects, which begot the 12 Volt soldering iron, flashlight battery rechargers, and 12 Volt water pumps for killer showers and it was getting really good. But not great.
Then came the small inverter which begot computers, TVs, bigger stereos, better lighting, small electric tools, motors, and blenders for making Margaritas. Then the need arose for more powerful inverters to run businesses, microwaves, toaster ovens, well pumps, and larger power tools to build bigger houses to shelter all this good stuff and the children begotten as a result of the Margaritas. And wisdom dictated that the universe be reconfigured to 24 Volts to run more powerful inverters while still providing 12 Volt for the many wonderful (and spendy) 12 Volt goodies. And it was great, but now we had a few problems.
Kathleen and I finally decided that we needed a sine wave inverter to run a laser printer and other goodies we’d been drooling over. I wanted a big inverter to run my air compressor and other power tools. The new Trace SW4024 seemed perfect. Sine wave and lots of “snort”. But it required a 24 VDC input. At the same time, the Whisper 1000 was about to go in the air and the long wire run to the house called for either a higher voltage on the line or a spendy high power LCB. The handwriting was on the wall for a 24 Volt system.
Over the years, you tend to accumulate quite a few 12 Volt goodies. Not only do these represent a fairly large investment, but most of the gear is high quality stuff and is more efficient to operate using DC than any available ac replacements. However, it’s a good idea to re-evaluate each DC appliance in terms of value, life expectancy, overall system impact, and replacement
16 Home Power #41 • June / July 1994
cost of a comparable ac unit. In our case, the cost of buying a high power voltage regulator far outweighed the cost of replacing our 12 Volt RF-16 SunFrost and buying 117 vac power supplies for the ham gear. In different circumstances, where the major DC usage is lighting, for example, it may pay to replace older DC incandescent and fluorescent lamps and fixtures with some of the newer compact and circleline fluorescent lamps. If you make the switch, make sure that your wiring is up to snuff. Two conductor circuits without a separate ground work fine for low voltage DC loads, but won’t be safe in a 117 vac circuit.
Additionally, all our RE sources had to be reconfigured to 24 Volt. The DC fusing and circuit breakers had to be sized down to reflect the drop in amperage.
We bought a 20 Amp Vanner Voltmaster to power our 12 Volt loads from the 24 Volt battery bank. It has three inputs: –, +24, and +12. You tap half of your 24 Volt battery at +12 in addition to the major positive and negative 24 Volt connections. The Vanner monitors the voltage in both halves of the battery pack and electronically switches the load from one side to the other when a voltage imbalance occurs.
Rewiring the PV modules was easy and it actually eliminated a number of conductors, but it took some thought and different wire lengths to get the best configuration. Since the Wattsun tracker mounts the modules in two rows, it was possible to wire modules as pairs and parallel them as 24 Volt units. Running all the parallel connections at 24 Volts halves the currents on the wire and reduces line loss. The tracker to battery conductors were sized to carry twice the current at half the voltage than we had now, so the wire resistance and voltage drop went down significantly and we experienced a net gain in wattage delivered to the batteries. The conductors are 1/0 Cu wires with a one-way distance of 75 feet. Figuring an output of 36 Amps at 16 Volts at the modules, I calculated a 3.8% voltage loss from the tracker to the batts. Using 18 Amps at 32 Volts, the voltage loss drops to 0.9%. Under full sun conditions with the PV temperatures hovering at about 50°C, it roughly measures out to an extra 12 Watts. Free! Since my PV charge controller is a Heliotrope CC-60, all that was required was a flick of the DIP switch instead of control replacement.
With all the PV junction boxes opened anyway, it’s a good time to inspect, clean, and tighten all the wire terminal ends in the array. How do those spiders and tiny buggers get into a sealed J box anyway?
The hydroplant alternator needed to be upgraded with a rewound stator to maximize output at the higher
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