solar powered motor

Oopsies, that should say "120V 60Hz 4A fan motor"

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I gathered from your later post that this is a 120VAC, 4A fan. That's 4*120 or about 500 watts, total. It's certain you will need more than one 85 watt solar panel to run it. Also, solar panels are usually DC and have a rated output voltage and current at some expected solar irradiance and incidence to the panel. If they are rated 85 watts, it may be that this is only in optimal conditions at the equator, with no clouds or dust, and assuming 1000W/m^2 of sun. In Texas, maybe 30N of the equator, you can expect somewhat less as the cosine(30) is about 87%. And that's at high noon. So you may need to "derate" the numbers to make certain you buy enough panel area.

So, taking a look at the panel you linked. It shows "Power at STC" of 85W. What is STC? Well, it's that 1000W/m^2 I just mentioned above. The sun puts out about 1364 W/m^2 at Earth's distance from the sun. The Earth reflects away about

30% of that, leaving about 960 W/m^2. At the equator when the sun is directly overhead. Well, that's not going to happen in Texas, hot as it may seem sometimes. You need to derate the panel. You can probably call up your local power company and get better figures for your area, but you can make some rough estimates. At noon, you might expect about 85% of 85 watts or about 72 watts. But figuring over some useful time during the day, you should expect even less. More, if the panel runs hot (and it will), the rating gets worse as panels deliver -80mV/C, above 25C. Call it 60 watts as a realistic figure for now. On good days. That tells you that you will need 9 or maybe even 10 of them to run the motor.

You will need a converter (inverter) of some kind. However, the one you linked, I think, may be described on this page in more detail:

It works best, according to the author, at an input supply no more than 17V. (Says, 10.5V to 17V.) It doesn't do any voltage boosting, nor 60Hz AC inverter stuff, so it really won't drive a 120VAC motor. Even if the power situation worked out. (The schematic there above looks a lot like what would be associated with the product you linked.)

You need to look for a device that can deliver 500 watts of

60Hz AC power, sourced from DC solar panels that can actually provide MORE than 500 watts at your location and time of operation. You may want a true sine wave inverter, not a modified sine wave inverter, as you are driving a motor.

None of this is going to be entirely cheap.

Solar panels delivering power synchronized to the AC system of a home are about $8.5/watt when installing under 10kW, in California:

A recent publication for my state, Oregon, shows a figure of $22,500 for 3kW as a "market average" before federal and state tax credits are accounted for. That $22,500/3kW works out to about $7.5/watt.

Assuming a similar figure, you would be looking at 500 times $8.5 (or $7.5) or about $4,250.00 (or $3,750.00) to run your motor. You don't need the synchronization if the motor is the only thing hooked to it. So it will be cheaper. But I would expect more than $2,000.00. But this may be made much better depending upon your tax situation, current fed and state tax incentives, etc.

Maybe someone better informed can add their thoughts or correct mine.

Jon

That's assuming that the panel is horizontal. If it's oriented directly toward the sun, cos(a) is 1 regardless of latitude and time of day. These still affect atmospheric absorption, but that's less significant than cos(a) (k^(-sec(a)) as a first approximation).

Slight difference ;)

12V*4A = 48W, which is feasible given enough sunlight. 120V*4A = 480W, which isn't feasible under any circumstances.

[snippety snip]

No corrections; your numbers are reasonable. Just impressed that you cranked this out at midnight on a Saturday. ;-)

--
Rich Webb     Norfolk, VA

Yes.

Or put another way, it's not relevant when oriented squarely and ignoring rayleigh, aerosols, and the rest.

Atmosphere does have a significant impact that can be quite close to cos(z). I tend to use cos(z) now as a reasonably good approximation for surfaces normal to the sun even though the reason isn't the obvious one.

I remember some fairly nasty looking models (more than a few, actually) that include CO2, O2, O3, H2O, rayleigh scattering, total molecular, and aerosols. Total transmittance at 84N in one such model is 10%. Although for different reasoning, it's not far from cos(z). At my latitude it is about 73% or so. I'm at 45N. Again, not far from cos(z). It surprised me when I saw the models' plots. But then I just figured that this only means I can use cos(z) as an okay approximation _for all the wrong reasons._

What's a person to do if reyleigh and molecular and aerosol scattering get you to about the same place? And when I go to my local power company to look at their polar charts for solar panels installed in my area, it turns out that the numbers experienced in practice here are close to this kind of rough-shod guessing of peak output. One home I went over to, about 6 months ago, to look at their recorded performance (they used a computer to log the data for the year prior) came out about as I'd expect applying nothing more complex, as well.

Jon

Thanks for all the interesting and educational (and quick) responses.

After the "DOH" moment of opening the box and correcting from "12V" to "120V", and particularly after reading everyone's input, I seem to have no other choice but to (A) wire it to the house supply or (B) buy a properly rated motor. It's for an attic fan, and the 12V motors don't seem to move enough air to make it worthwhile, which leaves me with wiring it to the house DC.

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Thanks for all the interesting and educational (and quick) responses.

After the "DOH" moment of opening the box and correcting from "12V" to "120V", and particularly after reading everyone's input, I seem to have no other choice but to (A) wire it to the house supply or (B) buy a properly rated motor. It's for an attic fan, and the 12V motors don't seem to move enough air to make it worthwhile, which leaves me with wiring it to the house DC.

I don't know what you pay per KW-Hour but for me it would take ten years to break even on equipment cost vs. using the utility power. Use thermostat control and the duty cycle will most likely be less than 100. Lesson: Solar panels are not very efficient, when you can use utility power, use it. They make power cheaper than you can.

Tom

Ditto that, it looks like I have alot more homework to do.

Thanks for your taking the time Jon to present these facts and analysis which certainly put some perspective on the Green Energy Hype.

First Solar Inc. claims to have reduced their manufacturing costs to less than $1 per watt, but I don't know how close we are to having comparable products available for average consumers. I expect some nano-engineered material will eventually be developed that makes it worthwhile to harvest the abundant sunlight.

John M Houston, TX

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All distributed (small, self-contained) power systems are expensive simply because they cannot take advantage of economies of scale, if for no other reason.

Solar PV is expensive beyond that basic fact, too, right now. Even done on a large scale or with concentrating systems that track the sun, it appears to still be so expensive that no one does it without some kind of incentive designed to change the economic calculations. Even in places with the best lighting, year around, it's still not otherwise done (unless there are other considerations that factor in.)

The tax incentives are substantial. So it can be worth some consideration.

Here in Oregon at 45N, on this side of the Cascades, we get about 1000 kW-hr/year per "1000 watt module." Using roof top installations. At $1/watt, that module would cost $1000 and generate 1000 kW-hr in a year. At a standard $0.10/kW-hr for electricity, this would produce an "income" (reduction of costs) of $100 per year for an up-front investment of $1000. Ignoring interest rates, maintenance, loss of use, and aging effects, this pays off in about 10 years. (Still longer, in reality.)

Eastern Oregon's figure is 1250 kW-hr/yr. I'm not sure what the figures are for Texas, but probably somewhat better than that.

And as you point out, when we even get to that point for average consumers is still another question entirely. Those tax incentives are what makes it happen at all for individuals, if at all.

By the way, many power companies (certainly the one here) try to buy those watts as "green energy watts" so they can sell them, themselves. In choosing to sell them that way, you are no longer generating carbon-free energy (assuming you ever imagined that carbon dioxide wasn't produced in making and installing the systems in the first place) because those green credits get sold to folks who intend on generating lots of atmospheric carbon. You have to refuse to accept the benefit, if you want to imagine any "green-ness" to your installation. Even then, you remain in doubt as getting a fully burdened accounting that everyone agrees with seems as hard as finding a chicken with teeth.

Green plants achieve very high efficiency, I have read. Obviously, it is worthwhile to them! We've a planet covered with their successful use. Of course, it's for their own use and doesn't directly make electricity.

(Hmm. Some physicist+microbiologist might come up with a modified chloroplast that moves electrons across some potential barrier, but then you'd still need to let it use _some_ of the energy in self-maintenance and perhaps replication.)

Solar cell technology is working towards better efficiency. I've no idea when all this gets into the mainstream and out of some lab, though. It's been a dream for so long, I am not sure when to expect to wake up from it or who to believe that is trying to wake me up. ;)

Jon

On Jun 19, 9:05=A0am, "John M" wrote: > Thanks for all the interesting and educational (and quick) responses. >

This is one example of many

We have two of these (or ones like it) in our attic. Roof is black composition and the attic isn't much warmer than outside. The fans are virtually silent and do a good job.

G=B2

Don't make it harder than it has to be.

Your fan wants 12V, your solar panel puts out 21 OPEN CIRCUIT. No solar panel does any useful work Open Circuit that's just how they specify it. At any load expect much less than 21 volts like maybe 15 if the fan alone is the load.

Can a 12 volt fan withstand more than 12 V? Hell yes. Even a brush type motor will survive 2-4 over voltage at load without dying.

try it and see.

And solar panels - cells in series are limited to the output of the cell(s) in the shade. They almost never deliver peak output (since only people living on the equator would ever know)

He could charge a battery (or super capacitor) and drive the fan with a

10% duty cycle. It's typical usage of PV.