Glück Auf is loosely translated as "good luck to us all".

Tuesday, August 14, 2012

Design and Sizing Decisions for Solar Power Systems

I have reached a critical point in my Airstream renovation where it is necessary to make some very important design decisions for my electrical systems. I am trying to consider as many factors as reasonably possible but I have to prioritize safety, efficiency and cost.

Design Decision #1: What voltages will my system support?
Options: 12/24/48 Volts DC and 110/220 Volts AC

    Here are some estimates I have found for typical AC inverter cutoff voltage:
  • 12 volt battery, 11.5 volt minimum operation voltage, 10.5 volt cutoff;
  • 24 volt battery, 23 volt minimum operation voltage, 21 volt cutoff;
  • 12 volt battery, 46 volt minimum operation voltage, 42 volt cutoff

Lets assume that we want 100 amps at nominal voltage.

    Calculate the most optimal watts for each voltage at 100 amps:
  • 12v * 100 amp = 1,200 watts;
  • 24v * 100 amp = 2,400 watts;
  • 12v * 100 amp = 4,800 watts

AC inverters are "constant power" devices. Ohm's Law says power = voltage * current. If you run the inverter at minimum voltage it will draw more current. So the wiring fuses and all electrical components must be designed 125% of the minimums for NEC safety rating. You also have to remember that inverters are ~85% efficient.

    Calculate 125% of the Minimum Amps based upon Volt Cutoffs and Watts, at 85% efficiency:
  • 1,200 watts * 1/0.85 * 1/10.5v cutoff * 1.25 = 168 Amps minimum
  • 2,400 watts * 1/0.85 * 1/21v cutoff * 1.25 = 168 Amps minimum
  • 4,800 watts * 1/0.85 * 1/42v cutoff * 1.25 = 168 Amps minimum

Now notice the difference between Vbatt-minimum and Vcut-off of the Inverter.

    Calculate Voltage Drop:
  • 11.5 volts - 10.5 volts = 1 volt drop
  • 23 volts - 21 volts = 2 volt drop
  • 46 volts - 42 volts = 4 volt drop

    Larger DC voltage supports higher voltage drops and smaller gauge wire.
  • 48 volts can carry 4 times the power 4 times the distance as a 12 volt system using the same gauge wire.
  • Inversely, a 12 volt system requires 4 times the gauge of wire than a 48 volt system.
  • A 12 volt system carrying 240 watts would require 20 amps and a 1 volt drop.
  • A 48 volt system carrying 240 watts would require 5 amps and a 4 volt drop.
  • A 120 volt system carrying 240 watts would require 2 amps at a 3.6 volt drop.

All of these factors translate to a significant increase in cost of a 12 volt system verses a 48 volt system.

Design Decision #2: Which appliances will I use?
Options: 120VAC vs 12/24/48VDC

Most 12 volt devices are designed for cars and usually support 12 volts when the engine is off and ~14.2 volts when the engine is running. So, quite a few of these 12 volt adapters may not work well on a solar power battery bank and may get toasted during charging. A well designed AC inverter will be designed for 10.5-16 volts and output a consistent 120 VAC to appliances.

Another major factor to consider is energy conservation verses price. Some of the Energy Star rated 120 VAC appliances actually approach the efficiency of the off-grid DC appliances at a lower price and with longer life and more features. This leads me to believe that it might often make more sense to use 120 Volt AC appliances even though the inverters may cost you ~15% loss in efficiency.

Design Decision #3: What type and how many batteries will I use?
Options 12/24/48VDC

    Amp Hours(AH) vs Watt Hours (WH) vs Kilowatt Hours(kWH)
  • A 100 watt bulb running for 10 hours is 1,000 WH or 1 kWH
  • 1,000 WH / 240 VAC = 4.2 AH
  • 1,000 WH / 120 VAC = 8.3 AH
  • 1,000 WH / 48 VDC = 21 AH
  • 1,000 WH / 12 VDC = 83 AH

So, I think I have concluded that I should try to design my system for 48 VDC batteries and 120 VAC appliances.

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