Solar powered attic fan without battery

**Mike90250** · 01-29-2019, 10:23 PM

Here's an idea. Wire a automotive light bulb socket in series with the panel and fan. Obtain a selection of bulbs that fit the socket and use the bulb wattage to control the speed. Perhaps a headlamp @ 55w would be right.

Or maybe the first thing to try is another 10w panel in series with the existing panel, and you will know you won't exceed the fan specs.

Remember - the fan only has so many revolutions in it before the bearings wear out. Higher speed = sooner wear out.

Some folks have had luck with about a 50w panel and a small automotive radiator fan. Those are made to work in high heat under hood applications.

**bcroe** · 01-29-2019, 10:51 PM

If the open circuit voltage of your panels will not damage the motor, just put more panels in parallel
(or larger panels at the same voltage), and your fan will run at max speed for a larger part of the day.
Tilting 2 panels east and west would probably give longest peak power.

Another approach is your idea of using a much larger panel, with higher Voc, and a buck regulator. I
expect with first sun, the buck (with inadequate input) will become a short through to the fan,
remaining that way until enough power is generated to reach the fan speed/voltage you have set.
Then the buck will start switching down the panel V to the V which you set for the fan, making
available more current which the fan will not use. But as Mike says, excessive V will wear or
damage the fan. Bruce Roe

**jflorey2** · 01-29-2019, 11:05 PM

Originally posted by noequus

I can use a buck converter to bring the voltage down

These often don't work for solar systems due to their negative input impedance characteristic. They tend to "stall" the panel at the converter's lowest turn-on voltage. You need a lot of power to overcome this, which means a lot of panel. You can ameliorate this by setting a high UVLO (around 14 volts) so you don't get stuck at a low voltage. But not all DC/DC converters have adjustable UVLO's.

it looks like fan speed is controlled by current in the systems they sell.

What sets the ultimate limit of motor speed (for most motors) is voltage. If the panel isn't beefy enough to get to Vmp, then it limits current, so you will see lower currents.

However, per what bcroe said, if the fan can handle 18-20 volts (the Voc of 12V panels) then just put more in parallel until you have your desired speed.

**bcroe** · 01-30-2019, 11:44 AM

Originally posted by bcroe

If the open circuit voltage of your panels will not damage the motor, just put more panels in parallel
(or larger panels at the same voltage), and your fan will run at max speed for a larger part of the day.
Tilting 2 panels east and west would probably give longest peak power.

Another approach is your idea of using a much larger panel, with higher Voc, and a buck regulator. I
expect with first sun, the buck (with inadequate input) will become a short through to the fan,
remaining that way until enough power is generated to reach the fan speed/voltage you have set.
Then the buck will start switching down the panel V to the V which you set for the fan, making
available more current which the fan will not use. But as Mike says, excessive V will wear or
damage the fan. Bruce Roe

I should add, if the larger panel has the same Isc as the original smaller, the fan will start through
the buck converter at the same sun as before. But the speed will rise to max setting much more
quickly. Multiple panels, keep them in parallel to keep the
Isc up. To control top speed, use an adjustable voltage buck. Bruce Roe

**bcroe** · 01-30-2019, 12:27 PM

I am going to add something about panel/buck/motor operation. Before starting the motor looks pretty
much like a short circuit. When enough current starts to flow to start the motor, it begins to generate
back voltage. This voltage subtracted from the panel Voc will slide up the panel V-I curve and make
less current available, so motor speed will soon stabilize at partial voltage and speed. However with
a (larger) higher voltage panel, the back voltage will be a smaller fraction of the total, so the motor
will rise to a MUCH HIGHER voltage/speed. And set speed will be reached sooner,

As sun produces more current, motor voltage/speed will rise to the set voltage of the buck, and the
buck will act as a cool V regulator. This protects and regulates the motor, certainly does NOT make
use of all panel capability. Besides earlier full speed operation, the buck give speed control.

There will be a point where fading sun lets the buck multiply available panel current longer, once
available power drops below motor set point requirement, the buck will revert to its short circuit
start mode. There are more complex circuits to try and capitalize on this short interval, perhaps not
worth it here. Bruce Roe

**SunEagle** · 01-30-2019, 03:21 PM

I don't know if increasing the amount of panel wattage will work or not.

I have a couple of those fans that were designed to be powered by a 10 watt panel. They will move their designated CFM when the sun is directly over head so increasing the panel wattage may over speed the fan blade and make it fail.

Something I was told was to make sure that I had enough of an opening on the other side of the attic to supply then same amount of air that the fan would pull out. With 2 fans the opening needed to be pretty big or I could create a negative pressure and pull coll air from the house.

So long story short the OP may need to add more fans or make sure the opening is big enough for the air being pulled by the fan and keep everything balanced..

**noequus** · 01-30-2019, 11:58 PM

Hi Everyone:

Thanks so much for this--I am starting to see how much more I need to learn about solar panels (and buck converters). Just to be clear: the basic problem is that the panel I want to use (it was free) is massively oversized for this attic fan--320 W, and I probably only want about 20 W going to the fan to get the appropriate level of airflow (more, but not too much--else causes problems like SunEagle mentioned).

Also (obviously) the V and I going to the fan can't exceed the fan's limits. And, it would be helpful to be able to fairly easily control the fan's RPM so I could experiment and find the right speed/airflow level.

The fan motor is designed to work with a range of panel wattages, from 10 W up to as much as 60 W. I could just buy a 24W panel from the same vendor as the fan came from, and probably get about the right amount of airflow, but those panels are expensive....and I got this other panel for free. Plus, I want to learn more about all this and have some fun. We already have a panel on the garage thats powering a raspberry pi computer, camera and some weather stuff, but that has a battery and controller; was a lot of fun and educational. But without a controller its clearly a different situation.

Sorry for the digression. Here's a specific question regarding the buck converter--I don't understand what "stalling" the panel means. A google search did point me to a thread on this forum that talked about "crashing" the panel (some or all of you were on it; link below) but it is still unclear to me. Seems related to... sunrise, sunset, or both and how the buck responds to those low voltages. If the panel stalls...does it recover once there is enough sun? Or does the system basically get stuck? I don't think the buck converters I have have adjustable UVLOs--need to check.

Here is the earlier discussion where crashing the panel was mentioned and buck converters discussed: https://www.solarpaneltalk.com/forum...buck-converter

Thanks again for your help!

**Mike90250** · 01-31-2019, 02:43 AM

Crashing or Stalling
The electronics needs 4V @ 400mA to start up PV panel puts out 8V @ 30mA, nothing happens till a little later, with more sun - Woohoo 400mA and it runs for 1.2 seconds, and flops over dead, still not enough sun. 10 sec later, tries again..... another hour of this nonsense, till the sun climbs high enough to meet the demand of the DC-DC converter and the panel can supply 400mA continuously. To protect things, good circuits have a count down, after 3 or 5 failures to start, the lock down till power is removed for 10 seconds. Most good brushless anti-stall muffin fans do this too.

So you need to find components that fail gracefully and attempt restarts.

**bcroe** · 01-31-2019, 10:38 AM

The original design relied on the panel supplying enough current to start the motor, and I would
stick to that idea to avoid complex and expensive controls. My guess is the fan is a brush motor.
Bruce Roe

**kswison** · 01-31-2019, 01:14 PM

Originally posted by noequus

My first post on this forum...hope its in the right place, and clear. All thoughts appreciated.

I have an attic fan that is powered by a 10W panel; it pulls out a lot of the hot attic air. However, its not quite powerful enough for this house and I'd like to increase its RPM to get better air exchange (but still not suck cool air from the house into the attic).

The vendor sells a variety of panel sizes for this fan and motor, up to 60W; with the more powerful panels for use in larger homes. All panel sizes are 12V (nominally, ~18V Voc), and the 60W panel has an Ioc of 3.6A. The vendor said it was best not to exceed that for the fan motor; I don't have full specs on the motor but the vendor told me its okay for 1-38V. Its labeled "1.7A, 38.2V".

A friend recently gave me a large panel (320W; Voc=44V, Ioc=9A), and it would be nice to use it to power the attic fan. So...I would like to adjust the power sent to the fan so it is:
1) Within spec of the fan
2) Provides the needed ventilation, but not too much
3) Ideally, easy to adjust the fan RPM (this will help me find the right ventilation level).

The fan motor has no problem starting with the 10W panel, and it only needs to run during the day; no battery needed or wanted.

I can use a buck converter to bring the voltage down, but its not clear to me how to also control the current...or, actually, its not clear what combination of V and I to use. Since the panels the vendor sells only vary in the current they can provide (all same V, but different wattages) it looks like fan speed is controlled by current in the systems they sell. So on my system (conceptually) the V could be set to match their panels (ie, nominally 12V), and then current could be varied and (conceptually) I could match the current to one of the panels they sell that is sized better for my house.

But its not clear to me if that is possible, or practical--or even if that's what I would really want to do. In the end, I just want to control the fan speed. So: perhaps you-all have some thoughts on the right way (or, rather, a good but safe way) to do that. Thanks in advance!

*******************
I am going to caution you on installing exhaust fans into an attic of a residential building, or really any heated building if you are in a cold climate. The building codes generally require a minimum amount of venting, generally 1:150 for roofs less than 1 in 6 slope, or 1:300 for roofs over 1:300. That's 1 sq.in. of clear venting per 300 sq.in. of insulated ceiling area. It may vary in some areas, but this is pretty general. That total require venting is best split 50/50 top to bottom of the attic for a balanced air flow through the attic. So, if you require 300 sq.in. of total venting, I would recommend splitting it so you have 150 sq.in. of inlet venting at the soffits and 150 sq.in. of outlet venting at the peak.

In attics that are having frosting issues or excessive ice damming the total clear venting area could be doubled to provide increased natural air flow with convention venting. I analyze and fix building like this all the time.

I do not recommend installing exhaust fans in attics as the exhaust fans negatively pressurize the attic space. This negative pressure places suction forces on the existing ceiling vapour barrier, in turn sucking even more warm air and humidity (vapour) from the interior of the house up into the attic.

Humidity (vapour) in the attic causes frosting issues. Warm moist air escapes from the interior of the house and as the vapour touches cold/frozen surface within the attic, it condenses back to moisture, and if there is not enough air flow through the attic to dry it, it will freeze as frost. Only to melt later and leak back down into the house.

The other thing is does is suck warm air out of you house, causing your furnace to have to provide make up air and then heat up cold out side make up air before it blows it around your house.

Also, in many cases, most home owners would never go up into an attic to check if the fan is still working, so it become a maintenance issue.

So, in reality, the money that you think you are saving my installing a solar powered fan, is just going out the exhaust fan as you are now paying to heat up replacement/make up air within the house.

... and don't fool yourself... residential vapour barriers are NEVER 100% and leak all kinds of moisture up into attics. That's why we need to vent attics in the first place... to remove the moisture/vapour.

I strongly recommend simply fixing the attic venting by opening up more natural inlet venting at the soffit and adding new or larger roof vents to the top of the roofs for increased natural convection through the attic space. I never use mechanical fans to resolve attic venting issues.

They use to call this a poor man's air conditioning, cause as you increase natural air flow through the attic you also reduce the heat build up. This keeps the house cooler in the summer and the house cools off faster in the hot summer evenings, making it easier to sleep at night.

Hope this helps someone.

**noequus** · 02-01-2019, 01:20 AM

Thanks, Mike90250. Sounds similar to a problem that, say, a raspberry pi computer has when you try to power it without a battery. I actually bought a device to deal with that for USB connections, cuts off at 3.5V so basically waits until the panel can provide enough power for the pi to start up and be stable. Unfortunately, I can't find it now...In any case I have some research to do.

Bruce Roe: Yup, the motor is brushed. Its actually the second motor that's been in the fan--the other one was in there for 15 years or more and was still working fine, but after some discussion with the vendor about increasing the airflow a bit, for various reasons they just sent me a new one, newer model but still brushed. Its been working for 3 or 4 months...though doesn't run during the winter since the temperature threshold that activates the switch isn't crossed.

kwison; Thanks for that. Agree. Here in southern CA we've never had a vapor issue (before or after installing the fan). When I added the fan I actually did add some additional venting area, and I do hope that, once I get the additional power to the fan and more rpms that I can find the sweet spot that pulls more hot air out, but does not start to suck air in from the living space--that would be counter productive if the AC is on...The fan itself is installed is installed in a hood we had made so we could service it easily. Still have to go up on the (flat) roof but seems like I need to go up there all the time anyway to clean out the gutters or whatever. So in our case maintenance is fortunately not an issue. Not sure exactly how hot the attic gets on a hot summer day but it is freakin hot and getting hotter (last year the temp at Burbank airport reached 114F--unheard of, broke the old record by something like...4 or 5F.).

As an aside...in addition to powering the fan (assuming I get that going properly...) the panel will also power a weather station (assuming we get things together...), and I'l put sensors inside the attic as well as embedded on the (white-painted) roof; on the garage the roof sensor there gets up to 140F.

Anyway, thanks you-all for your help.

**kswison** · 02-01-2019, 01:52 PM

Yes, in Southern CA you are not in a COLD climate, so you would not have the vapour issues. I am in Canada and we have COLD to EXTREME COLD climates. Vapour in attics is always a concern. The northern US States would have similar vapour issues in those COLD climates.

**Paul Land** · 02-01-2019, 05:26 PM

Originally posted by jflorey2

These often don't work for solar systems due to their negative input impedance characteristic. They tend to "stall" the panel at the converter's lowest turn-on voltage. You need a lot of power to overcome this, which means a lot of panel. You can ameliorate this by setting a high UVLO (around 14 volts) so you don't get stuck at a low voltage. But not all DC/DC converters have adjustable UVLO's.

What sets the ultimate limit of motor speed (for most motors) is voltage. If the panel isn't beefy enough to get to Vmp, then it limits current, so you will see lower currents.

However, per what bcroe said, if the fan can handle 18-20 volts (the Voc of 12V panels) then just put more in parallel until you have your desired speed.

My attic fans run/don't run without circuit board controllers @ 12v .It had controllers and i switched them out to simple 12v fans and simple a Am/Pm rechargeable internal batt timer switch rewired for fans batts hold full charge for 6+ months

timer.jpg

**noequus** · 02-09-2019, 11:24 AM

Originally posted by Mike90250

Crashing or Stalling
The electronics needs 4V @ 400mA to start up PV panel puts out 8V @ 30mA, nothing happens till a little later, with more sun - Woohoo 400mA and it runs for 1.2 seconds, and flops over dead, still not enough sun. 10 sec later, tries again..... another hour of this nonsense, till the sun climbs high enough to meet the demand of the DC-DC converter and the panel can supply 400mA continuously. To protect things, good circuits have a count down, after 3 or 5 failures to start, the lock down till power is removed for 10 seconds. Most good brushless anti-stall muffin fans do this too.

So you need to find components that fail gracefully and attempt restarts.

Sounds like whether these start/fail/start/fail/start cycles eventually lead to a "real" startup, once there is sufficient sunlight, depends on the device. Since I don't know about these devices, is it reasonable to just try it and see what happens?

Are those cycles bad for the electronics (either in the panel or the device that is trying to start?)

Solar powered attic fan without battery

Solar powered attic fan without battery

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