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Introduction To Remote Power Systems (continued)
Wire Sizing and Choosing A 12, 24 or 48 Volt DC System
The more voltage drop in a wire due to the resistance of the wire, the more power lost. We recommend that the wires in a solar electric system be
sized large enough so that the voltage drop of the wires is 2% or less, and so that the over all voltage drop of the entire system is less than 5%.
Changing from a system voltage of 12 volts to 24 volts, or 24 volts to 48 volts, reduces voltage drop in wires by 50%., and reduces the power loss in
wires by a factor of 4.
We usually recommend using the following to choose a system voltage size:
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If your long term system total watts is less than 1500 watts, then use a 12 volt system.
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If your long term system total watts is 1500 4000 watts, then use a 24 volt system.
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If your long term system total watts is 4000 + watts, then use a 48 volt system.
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Solar vs Wind
Solar systems are much more economical than wind systems for homes generally in sunny places like California, unless you need a tremendously
large amount of electricity. Wind becomes more economical than solar generally only if you need the electrical equivalent of several homes (i.e.,
gridtie sellback or a lodge). In climates like California's it usually makes more sense to purchase a few more solar modules rather than to have a hybrid
system. In considering a wind system it is important to derate for elevation, properly analyze wind speed, adjust the wind speed for the height of the tower
, and then calculate the actual electrical output. In climates with little sun and lots of wind, wind systems are likely to be more economical than solar. Wind
and solar can complement each other nicely throughout the year.
Choosing An Inverter: Modified Sine Wave vs True Sine Wave
True sine wave inverters provide electricity that is basically the same or better
quality as utility-supplied electricity. True sine wave inverters therefore supply electricity that is compatible with virtually any appliance or electrical device.
However, true sine wave inverters cost more than modified sine wave inverters. Thus modified sine wave inverters are commonly used. The
following is a list of the electrical devices that can have difficulty running on electricity produced by modified sine wave inverters:
- Laser printers.
- Some fluorescent lights (Phillips compact fluorescent and electronically ballasted lights like the Osram EL15 seem to work best.)
- Some radios, especially AM.
- All Macintosh Computers are likely to overheat the power supply (except some older models). Note: Most computers work just fine on either a
good quality modified sine wave inverter, or a true sine wave inverter. But occasionally a computer will crash (need to be rebooted) when the
source of electricity is being switched from an AC source (such as utility or electric generator) to the inverter. A stand alone solar system sized
large enough to operate year round without the need for a back up power source would, of course, never encounter this problem.
- Any power tool which employs "solid state" power or speed control. The components of some of these tools can be destroyed if powered by
electricity supplied by a modified sine wave inverter.
- Some battery chargers for cordless tools will be destroyed (Makita is an exception)
- Some washing machines with electronic timers.
- Some stereo and audio equipment.
- Some TVs.
- Some new furnaces because they have microprocessors (computer chips) in their controls.
- X-10 home automation systems.
Even the very popular Trace SW Inverters, which are a refined modified sine wave inverter in which the sine wave is made up of very tiny steps, have
problems running some appliances. Appliances which can have a problem with the electricity produced by Trace SW Inverters are:
- Some new furnaces because of the microprocessor in the furnace control. Note: In the event that a furnace does not operate properly, on
either a modified sign wave inverter or a full sign wave inverter, the electricity to the furnace controller can be modified further by installing a
power conditioner (also called a constant voltage transformer) on the line to the furnace controller only.
- X-10 home automation systems.
Other Advantages Of Trace SW Inverters
Trace SW Inverters include a gen start, which is compatible with most makes of generators, as a standard feature. Without the gen start an additional
control would be needed to auto start. They also work with smaller generators than modified sine wave inverters.
When true sine wave is required, the Exeltech MX Series or XP Series inverters are the highest quality and have a mean time before failure of 21 years.
Charge Controllers And Low Voltage Disconnect
Charge controllers prevent the batteries from being overcharged. Low voltage disconnect (LVD) prevents the batteries from being discharged too far.
Generally even a high quality deep cycle battery, such as an L16, should only be discharged 20% to 50%. Batteries similar to the L16 will last a lot longer if
they are discharged regularly only 20% to 50%. All solar electric systems, except possibly some small one module systems and DC water pumping applications, should have both a charge controller and a low voltage
disconnect. Most inverters now have LVD built into them. Also, charge controllers which have pulse with modulation will increase the life of batteries.
Maximum Point Power Tracking C.C.'s
The new Maximum Point Power Tracking C.C.'s can boost energy production up to 30% during winter with high voltage modules. The Solar Boost 50 by RV
Power Products will handle up to 40 amps at 24 volts with 97% efficiency.
Batteries
- Car (very short life) — not recommended
- Marine (short life) — the bare minimum
- Golf cart ((5 to 8 years)
- Deep cycle L16s (7 to 10 years)
- Industrial (10 to 30 years)
* Note: years stated are achievable only with proper maintenance.
Batteries outgas during high rates of charging, and in rare instances can explode. Therefore, be sure to isolate the batteries from the inverter and
charge controller. The batteries must also be vented unless they are gell cell. Use four series strings maximum, or you may start to have problems
equalizing the battery bank. A large battery bank with a lot of cells is more likely to have problems with a bad cell than is a large battery bank with fewer
cells. So the larger the battery cells, the less the likely hood of having a bad cell. Wiring from the battery bank must be from opposite terminals or the end batteries will wear out sooner than the other batteries.
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