DC Ceiling Fans Worth It?

Re: DC Ceiling Fans Worth It?

FanGuy,

More or less, there are three ways to bring electric power to an off grid home:

1) Simply connect a solar panel to a DC capable motor
2) Solar panels charging a battery and using the battery bus power to a DC motor
3) Use battery+AC inverter to an AC capable motor

The all have their pluses and minuses.

I like to look at the loads/needs first. So the motors in this case.

DC motors fall into to a couple of camps. The universal/brush motor. Cheap, work, take variable voltage pretty well, and you can get things like a Linear Current Booster to better match the solar panels (sort of) higher voltage/lower current to the DC motor's lower voltage/higher current needed to get started, run at slower speeds in low light (LCBs are done mostly for water pumping). LCB's are not cheap, but they do work and no battery needed if you only need the fan/pump/motor to run when there is sunlight available. And brushed motors require brush replacement every X,000 of hours (and commutator cleaning/turning once in a while too).

The second DC motor type are those with electronic commutation. Basically, the "brush/commutator" function is replaced with electronics. DC computer fan motors are a good example. Cheap, reliable, and simple to use.

From what I have seen/designed, if you can get away with daylight only operation (water pumping, fan/swamp cooler, even now some A/C systems), your solar power costs will be about 1/4 of that of a similar off grid system with battery bank (losses, more panels, battery costs, etc.).

If you need the power 24x7, then you are looking at a battery bank. And you have even more options.

If we drop universal/brushed motors as being unreliable for long term use--Then we are looking at brushless DC motors and AC motors.

The simpelist is an appropriate AC inverter driving an induction motor. The AC inverter may be around 85% efficient (if sized correctly) and all you need to do is make the solar array+battery bank ~1.18x larger to support the extra losses. Not a huge hit for the better reliability, the ease of sending 120 VAC vs 12 VDC longer distances, etc.

AC induction motors however have about 20% extra losses because of the way they work--"Induction" of the magnetic field in the rotor of the motor. If you replace the rotor with permanent magnets, use a poly phase windings (typically three phase), and a poly phase inverter of some sort (H-Bridge as in DC computer fans, or VFD--Variable Frequency Drive in larger motor applications), you can save ~20% of losses that and induction motor would have wasted.

Is it worth the time/money to go with the battery based "hi-tech" solution--It all depends. What is available (are PM fans available at reasonable prices), do you need the variable speed/soft start of a VFD, is the cost of a VFD worth it, etc.

You pretty much have to do the research and go through the paper designs to see what will work best.

In general, for specific applications, there are some neat tricks that can be done to make low cost Solar->VFD->3 phase motor applications very cost competitive (being done for water pumping).

For the average off grid home, there are some DC fans out there that are very nice for small cabins with DC battery systems. As I recall, they generally are not cheap.

But, once your power needs get over ~1,000 Watt*Hours per day, many times, it is better to just go with a 120 (or 230) VAC AC inverter. The reduction of current at the higher voltages just makes AC wiring so much easier. Plus deep cycle battery bank voltage can vary a lot over the day (from ~10.5 volts to 15-16 volts for a "12 volt system")--Which many DC appliances really have problems with.

Putting an AC inverter (even a smaller 300 Watt TSW inverter like this one) will power most of the loads a small cabin/RV will need pretty efficiently.

Just run everything on AC power. Most modern AC appliances with the Energy Star regulations are getting just about as energy efficient as native DC appliances. You only need larger AC inverters and battery bank when you start adding heavier loads (typically, installing a refrigerator is the break point between "small" and medium/larger off grid power systems.

And, if you are interested in the whole AC/DC motor application--Take a look at these water pumping threads:

WELL PUMP and Inverter QUESTION
Wind/solar for large scale pumping etc (out of my depth!)
could use knowledge - using Gould jet pump - transfering from 230vAC to ? DC (new link/thread 10/27/2012)
Help required to design off grid system (information on possibilities to connect "standard VFDs direct to solar panels) (new link 1/13/2013

And, general FAQ about Solar RE/Conservation, this thread:

Working Thread for Solar Beginner Post/FAQ

-Bill