Powering a small vent fan directly from a panel

JYanke · June 2020

I am wanting to power a very small 12v brushless fan directly from a 12v solar panel (no battery). The fan will only need to run during the day when sun hits the panel, and will be wired to a thermostat to only turn on when temp is above set point. Im looking for recommendations on appropriate panel size and an affordable charge controller that has a regulated 12v load output option that could handle this fan and application.

The fan specs are as follows:

DC 12v
operating voltage: 7-13.2V
input current: 2.4A (3.0A max)
input power: 36 Watts

any recommendations are greatly appreciated

Joe

mcgivor · June 2020

The solar output must exceed the consumption of the fan itself so for a 36W load perhaps a 50W panel would be approximately what would be required. There has to be some experentation, the fan will drag the voltage down to a certain degree but what that would be is undetermined, so perhaps the best method would be to utilize a DC to DC buck converter to ensure the voltage dosent exceed the the fan's requirements.

A change controller would likely exc,eed the fans requirements being that by design it is intended to charge a battery, the load terminals of the typical charge controller would reflect the battery voltage in most/all cases so I think it wouldn't be the best choice. The cost of each would likely be the same so my opinion is look towards a DC to DC buck converter.

BB. · June 2020

To add to McGivor's comment...

Buck converters that higher voltage/lower current DC and efficiently down convert to lower voltage/higher current DC. For buck converters, the input voltage should be something like 1.5 volts, or higher, than the output voltage (i.e., 13 volts output, then >14.5 volts input).

There are also boost controllers... They take low voltage/higher current DC voltage in, and output higher voltage/lower current output.

And lastly, there are buck-boost converters, they take a wide range of input voltages that can be higher, the same, or lower than the output voltage.

Buck converters are getting so cheap these days, many are sold in lots of 4 or more to get near $20 USD to make money on the sale (some Amazon examples):

https://www.amazon.com/s?k=buck+converter+adjustable (buck)
https://www.amazon.com/s?k=buck+boost+converter+adjustable (buck boost)

Just make sure the input voltage is compatible with your solar panel/array (Voc (open circuit) and Vmp (max power)) are sensitive to temperature... Typically, on very warm days/full sun/hot panel, the Vmp~17.5 volts becomes closer to 15 or 14 VDC... And when the sun is not hitting the panel at right angles (morning/evening sun), the Imp (current max power) is less... By 50% or less than noon-time sun. And on sub freezing days, Voc/Vmp can rise upwards of 20% or so (over the name-plate rating).

Why does this matter? DC brush-less fans use electronics internally to operate... In "corner conditions" it is very easy to have over voltage from the solar panel ruin the fan... For example, you can use a 24 volt fan on "12 volt" panels (really Vmp~17.5 volt) panels and avoid the DC converter. But the fan will run slower/lower RPM than its name plate rating most of the time.

Lots of factors to juggle (how much air you want to move, fan voltage, array voltage, buck or buck-boost or boost converter, etc.). Electronic circuits can be very unforgiving at times (over voltage usually kills semi-conductors).

-Bill

JYanke · June 2020

Thanks for the quick reply.

So the panel would wire directly to the buck converter and then set the output of the converter to 12v and connect the fan to the output?

Would you recommend fusing the lines anywhere between the panel and fan and should there be any other concerns related to eliminating a charge controller?

The other idea i had is just wiring the fan to my 12v battery bank on the main solar system. But I was concerned seeming that the bank is only 225 aH and wasn’t sure if the fan running all day would be too much on my small system?

BB. · June 2020

panel -> converter -> fan

That is about it. No fuses required (fuses are there to protect the wiring from short circuits)... As long as your wire AWG rating is > Isc (short circuit) of the solar panel, and output current of the converter, fuses are not required.

Loads that run for many hours per day can be, surprisingly, energy hogs. For example, using your 3 amp maximum fan on a 225 AH 12 volt battery bank (remembering suggesting that you should plan on 25-50% discharge of battery capacity in "normal" operation--longer battery life):

3 amps * 12 hours per day = 36 AH
36 AH / 225 AH battery bank = 0.16 = 16% of battery capacity
225 AH * 25% (i.e., 2 days of storage) = 56 AH per day suggested "nominal" loads on battery bank
36 AH (12 hour worst case fan loading) / 56 AH per day = 0.64 = 64% of daily suggested battery usage

On sunny days, probably not a huge issue... But if fan runs over night, or there are a couple days of clouds/winter/etc., after 3-6 days, yes, the fan can easily take your bank dead. And is a significant load on a "small" solar power system.

In general, if you just need fan during daylight hours--A panel+fan (+converter) is a better solution. Battery powered devices--Just more things that can go wrong and/or using "expensive" battery+charger power to run a simple fan during the day.

-Bill

JYanke · June 2020

Thanks guys. Great info as always. So Im thinking the following:

12v 50 Watt solar panel-
Vmp: 17v
Voc: 20.23v

12ga MC4 cables 12ft each (panel to buck)

Adjustable voltage/current Buck Converter-
Input range: 5.3 to 32 vdc
Output range: 1.2 to 32 vdc and
0 to 12 amps

Adjustable thermostat controller-

compatible load: 20a 14vdc
power supply: DC 12v 200mA
Power consumption: <65mA

12v fan specs as before

Stranded wire from thermostat/buck to fan (at 10 feet maybe 16 ga??)

BB. · June 2020

Voltage Drop Calculator for 12 volts 10 feet 3 amps:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=13.17&voltage=12&phase=dc&noofconductor=1&distance=10&distanceunit=feet&amperes=3&x=42&y=35

Voltage drop: 0.24
Voltage drop percentage: 2.01%
Voltage at the end: 11.76

Generally aim for ~1-3% voltage drop for wire run in solar. Looks OK.

DC rated contacts on timer...

Timer would usually only be used on DC Battery supplied fan--Unless you are specifically after temperature control... In any case, typically need to supply stable DC 12 volt battery power to timer circuit, if not the fan itself.

Place the converter near the fan if you want more precise voltage control... But no big deal either way. Some down converters are "more stable" with short wiring on the output. Long wiring is inductive and can cause "ringing" in the converter output leads.

-Bill

JYanke · June 2020

Thanks a lot Bill. I will do some testing and see what works best. I have plenty of wiggle room on placement and shortening the wire runs. Not looking for any timer in this application, just a temperature thermostat. I am using this fan in a small attic space to promote ventilation. I want to set the thermostat to have the fan kick on when the attic is above 90 deg F or so. Here is the unit I was thinking of going with:

https://www.amazon.com/dp/B01J7VHS4I/ref=cm_sw_r_sms_c_api_i_p9L6EbXVSR2VV

BB. · June 2020

I am sorry JY, you did say thermostat, not timer...

There was a person here years ago that installed some solar attic fans, and his attic temperature did drop.

A few years later, he noticed that the fans had failed, but the attic temperature was still down... More or less, just adding additional venting (holes) was enough to do the trick. DC and Solar powered fans generally do not move that much air (10-35 Watts DC fan vs 200 Watt or so for a 120 VAC attic fan--2-3x or more airflow-CFM for AC versions?).

-Bill

mike95490 · June 2020

i like the 24V fan & 50W 12V solar panel. Only thing is, will the fan self-start or lock up at dawn ? Some fans are too smart !

Alternative is to use same panel, a 15V 50W zener diode as a crude, wasteful voltage regulator and your 13V fan. Getting fancy, you could stack a couple zeners to spread the heat dissapation (use three 5v 30W zeners)
https://www.digikey.com/products/en?keywords=zener

jonr · June 2020

On ebay you will find suitable buck converters that include a manual "MPPT" adjustment. While really just an input voltage drop limiter, it prevents the voltage collapse that you can get without it.

JYanke · June 2020

Thanks for all the input. I went with a 60 watt 12v panel:

vmp: 18.2v
imp: 3.3 a
voc: 22.2v
isc: 3.6a

The buck converter has adjustable output voltage and constant current settings as well as a nice little lcd display.

Set the fan up on test bench and functions well with the buck converter supplied with 18v power supply in and 12v 3 amp output. I varied the voltage on the power supply and didn’t notice any change in fan output until I dropped below 8 v or so.

The thermostat works well and has some good program functions. I extended the thermistor wires so it can be mounted further away.

Will see how it goes with the panel supplying power.

Thanks again for the help.

Joe

BB. · June 2020

Sounds good.... Just make sure the supply and fan starts up again with various input power drops (slow, clouds, birds flying overhead, etc.).

Bill

JYanke · June 2020

So I just finished testing the set up with the solar panel connected. It was clear skies and sunny when I began. The converter showed 21 volts input from panel and had no issues outputting 12v to the fan. I manipulated the thermostat to go above set point so the fan would turn on and off as expected.

However...clouds rolled in and the problems began.

The incoming voltage began jumping all over the place on the buck converter dropping in and out with the varying sun intensity. Fan speed dropped dramatically and I watched the output voltage do the same.

The output would essentially pulsate in a way through various voltages below 12 v. This also caused the digital thermostat to kick off then back on throughout the voltage fluctuations. I removed the thermostat and wired the fan directly to the buck output thinking maybe the thermostat kicking out was causing an issue. However, same response. I tried lowering the buck output voltage to accommodate the lower incoming from the panel but no avail. Any thoughts?

BB. · June 2020

This is the problem with "current sources" like solar panels...

A solar panel outputs current (mostly) in proportion to the amount of sunlight... So, say 4 amps in full sun from zero volts to Vmp (~18 volts).

When connected to a non-linear power device (like your DC to DC converter), once the solar input drops the available current below that required by the converter, then the output voltage collapses. Say Vpanel fall from 18 volts to 12 volts, all at 3 amps max input... Then a bit less sun, current falls below 3 amps. DC converter tries to draw more than 3 amps (because of falling panel voltage) and then collapses panel voltage to minimum input supported by supply:

Say 3 amps @ 12 volts (made up numbers for example)
P=V*I= 12 volts * 3 amps = 36 volts to fan
Say >4 amps at panel, and panel will produce 0-18+ volts
18 volts * 4 amps = 72 Watts from panel (more than 36 watts for fan)
3 volts * 4 amps = 12 watts input to DC converter (less than 36 watts for fan)

So, you see "two stable" regions for panel--18 volts and 3 volts (as example).

That is the "Magic" MPPT does. It looks for the maximum output power from the array (18 volts * 4 amps)... And would only draw enough current from the panel to keep Vmp @ ~18 volts (~2 amps * 18 volts = 36 watts to fan).

But with "simple" DC to DC converters, they run into the issue where a cloud passes over, and the converter "stalls" trying to supply power to fan, but this power draw keeps the Vpanel voltage low--And it gets "stuck" until something changes.

Changes can include panel simply outputting more current and drawing Vpanel up. Another way is to interrupt the Fan Power (open switch and keep fan off), to let Vpanel get back to ~18 volts, then start fan again. And it is very possible that your DC converter can get into a "restart" cycle until there is "more sun" (fan starts, Vpanel is ~18 volts), or there is not enough sun, and Vpanel does not get to ~18 volts when fan starts, and voltage collapses again--Until sun sets).

-Bill

mountainman · June 2020

Fwiw As an experiment I connected a honda accord fan to a 100 watt panel 19 vmp 5.3 imp.
Slows down when a cloud goes by
But keeps turning.
It really hums in full sun.
I manually connect it from 9-5 5 days a week for 3-4 months a year in summer
3 years and still going.

RCinFLA · June 2020

The more electronics, the tougher not getting humidity intrusion problems. A high power shunt 12v zener diode mounted on a sideways aluminum bar across inside opening in outgoing air flow would be simple. They are about $10 for a 50 watt zener. Needs good heatsinking so needs to be in outgoing airflow..

NLVMike · June 2020

I went through a bit of similar testing. I have a 40' shipping container that needed ventilation, and I have a bunch of solar components laying around. After a relatively short period of frustration, I just bought one ready made in a kit from Remington Solar. Took less than an hour to cut the hole in the roof and install it. Fan started running as soon as I took it out of the box. 25 watt panel and DC motor. Runs great.

All that said, it was not cheap. It seems like I could have built it fairly easy, but I would need to find the motor that could handle the power fluctuations. This one runs whenever it has direct sun, and thermostat is about 82 degrees. If I so much as wave my hand over it, the motor just stops running.

JYanke · June 2020

Thanks again for all the info. I ordered a buck converter with MPPT adjustment like jonr suggested and will see how that goes.

This is not a typical attic vent application and is fairly unique so Im just doing some experiments. I do have some complete package Honeywell solar gable exhaust fans in the gable ends of the main Cabin that work very well and have given me no problems for the past 10 yrs. i don't have the room for these type to be installed in this application and they weren’t very cheap.

Ill update after the mppt buck arrives.

Joe

Powering a small vent fan directly from a panel

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