Using a solar panel to power greenhouse fans

Forums › Home Workshop DIY › Using a solar panel to power greenhouse fans - some questions.

braaaaaaainssss

19 posts

Geek

#315915 29-Aug-2024 21:12

Disclaimer - I can't even claim "I only have highschool knowledge of physics" as I'm pretty sure I paid my friends to do my assignments in high school. I'm sorry if these questions are basic enough to prompt anyone to make an audible sigh when they open the topic.

I have acquired a solar panel that I want to use to power 3 fans for my small greenhouse. Two fans to create a slight breeze, and one to act as an exhaust fan, aided by the natural convection of the greenhouse.

I do not plan on using a battery in this system. I am fine with it only running when the sun is out.

The stats (attributes? personality traits?) of the solar panel are:

90w (maximum power)
22.3v (open circuit)
17.8v (maximum power voltage)
5.05a (maximum power current)

The fans I want to run are:

2x fans that are both 12v, 0.27a
1x fan (although not yet confirmed) that is 12v, 1.6a

My questions:

Is the easiest way to down-convert the voltage of the solar panel to 12v by using an MPPT solar controller - and if so, fine to do this straight to the fans (i.e. without a battery)? If so, cheap recommendations would be wonderful.
Do most MPPT controllers have undervoltage lockout built in?
Should I be concerned that the inrush current of starting the fans will exceed what is available via the panel -- and if so, should I try to manage this by using some type of soft-start thing for the larger fan?
Are there any obvious errors in this idea? I've pretended that electricity is witchcraft/doesn't exist for the last 20 years of my life.

Thanks in advance.

elpenguino

3425 posts

Uber Geek

#3276887 29-Aug-2024 23:28

Fans (motors) are a bit more tolerant of voltage swings. I think you'll be able to just use a resistor in series with each fan.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

SomeoneSomewhere

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#3276891 30-Aug-2024 00:29

Solar panels just reduce output voltage if you draw 'too much' current. This is totally fine.

I'm not sure how effective MPPT controllers will be without a battery to stabilize things.

I would be tempted to use a more basic PWM-type controller or just a shunt regulator to burn the extra power.

Standard buck regulators will be ineffective. They'll draw too much current, suck the voltage down, and get stuck on the wrong side of the power curve.

tweake

2391 posts

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#3277237 30-Aug-2024 17:48

a good way is to use a small pwm controller (small mppt are often fake). add a small sla battery. then add a ldr switch module to turn the fans on. that gives you far more control and more consistent fan speed.

SomeoneSomewhere

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#3277258 30-Aug-2024 18:51

If the goal is cooling/ventilation then power proportional to sunlight is probably a benefit over consistency.

Lead acid has longevity issues especially if there's regular cycling.

braaaaaaainssss

19 posts

Geek

#3277950 1-Sep-2024 21:58

SomeoneSomewhere:

Solar panels just reduce output voltage if you draw 'too much' current. This is totally fine.

I'm not sure how effective MPPT controllers will be without a battery to stabilize things.

I would be tempted to use a more basic PWM-type controller or just a shunt regulator to burn the extra power.

Standard buck regulators will be ineffective. They'll draw too much current, suck the voltage down, and get stuck on the wrong side of the power curve.

Thank you for your comments, much appreciated. I note your comment on buck regulators, which is helpful as I didn't really understand where these fit in this context (and had considered whether this was the solution). Thanks.

A question about PWM controllers:

I did a bit of internet searching about using a PWM controller without a battery connected, and the answers range from "no, you cannot do that, controllers need a battery to detect voltage, etc etc, it will explode very quickly if you try this" vs "yeah whatever it's fine". Any idea about this?
See thread here for a basic summary of my experience Googling this...

Should I just buy a cheap PWM controller off TradeMe and see if 1) it start without a battery, 2) it explodes?

elpenguino

3425 posts

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#3277953 1-Sep-2024 22:12

You haven't said what kind of motors are in the fans. Do you know?

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

SomeoneSomewhere

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#3277955 1-Sep-2024 22:14

I'm not sure whether a PWM unit would work. You could try; it shouldn't be able to blow anything up. Consider putting some big 25V capacitors on the output.

Some well heatsinked large 12V linear regulators e.g. 7812 might also be an option.

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braaaaaaainssss

19 posts

Geek

#3277956 1-Sep-2024 22:23

elpenguino:

You haven't said what kind of motors are in the fans. Do you know?

Good point - thanks. I don't know. I picked up two of these for very cheap off TradeMe with the intent of them being the ones that provide a gentle breeze -- then was planning on using a server fan or similar mounted into the back wall of the glasshouse as the exhaust fan.

I guess I'll go and do some reading on motors... is there any preference re: brushed/brushless for this type of applicatioon?

braaaaaaainssss

19 posts

Geek

#3277957 1-Sep-2024 22:24

SomeoneSomewhere: I'm not sure whether a PWM unit would work. You could try; it shouldn't be able to blow anything up. Consider putting some big 25V capacitors on the output.

Some well heatsinked large 12V linear regulators e.g. 7812 might also be an option.

Thank you.

SomeoneSomewhere

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#3277958 1-Sep-2024 22:34

7812 won't quite do the 1.6A fan; look at one of these options:

https://www.digikey.com/en/products/detail/texas-instruments/LM1085IT-12-NOPB/363562

https://www.digikey.com/en/products/detail/microchip-technology/MIC29300-12WT/771587

One regulator for the big fan, the other shared between the two smaller ones.

You'll want two separate large heatsinks ideally in the airflow as these will get hot under full sun. Note that the tab is electrically connected to the centre lead.

SomeoneSomewhere

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#3277959 1-Sep-2024 22:46

Brushed fans have a limited lifespan because they've got physical carbon brushes rubbing against a commutator. They won't last running 80 hours a week. You want brushless and with good bearings.

braaaaaaainssss

19 posts

Geek

#3278649 3-Sep-2024 20:30

SomeoneSomewhere:

Standard buck regulators will be ineffective. They'll draw too much current, suck the voltage down, and get stuck on the wrong side of the power curve.

One final question... could you elaborate on this, or point me in the right direction of understanding it?

Some cursory internet searches seem to say that people tend to use something like this product to achieve stable 12v output from a non-12v panel -- if it worked it would forgo the problems associated with solar controllers that need to be connected to a battery. But it sounds like you're suggesting that this is not very efficient, or would use too much current to make the system work as a whole?

Thanks - again, I appreciate your comments.

SomeoneSomewhere

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#3278676 4-Sep-2024 01:35

I wrote a nice long post then realised that the failure mode I was imagining (and have seen) generally requires a boost converter, not a buck converter. Doh! See below for what I wrote anyway:

So, solar panels have a voltage-current curve that looks like this:

Each line represents a different amount of sunlight.

Power is the product of voltage and current. With full voltage and no current (bottom right), you get no power. With full current and no voltage (top left), you get no power. You want to be somewhere in the top right.

The maximum power point is about where the knee in the graph is (in this case, about 30V; your panels are a bit over half that). Below this voltage, the panel is pretty much a constant-current source, with the current available being proportional to sunlight.

With both buck and boost converters, if the output needs more power, they draw more current. This works great if you're below and right of the maximum power point: drawing more current gets you more power. If you're to the left, it doesn't work so well: drawing more current reduces the voltage, giving you less power. So you draw more current again. The converter gets stuck somewhere on the left of the graph, where it's drawing as much current as the panel will supply but not getting any useful power. It's a bit like drowning in 2ft of water.

With a boost converter, the input voltage can go down to a volt or two and input current can be lots, even if the output current is something the panel could easily supply. E.g. the buck converter is drawing 8A 1V, poor efficiency in these conditions gives an output of 1A 5V, yet the load is designed for 1A 24V and could run off the panel directly even in poor lighting, or even if you disconnected the load for a second to fill the input and output caps and let the panel voltage recover.

With a buck converter, you've got less of an issue as input voltage can't go below output voltage. It's possible to imagine say a 3V 10A load: it could be supplied by 30V 1A, but the panel will get 'stuck' at 3V 1.8A: getting more power means drawing less current, and simple regulators assume an input voltage that doesn't change based on the current they draw.

I'm not too fussed about that situation because it's no worse than doing it with a linear or shunt regulator; we still only get the amount of current that the panel can provide. It does mean that in low light conditions (where MPP current is less than design load current), the fans might not be producing as much power as they could with a proper MPPT tracker.

Buck converter should be an OK option.

jm3

86 posts

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#3278677 4-Sep-2024 05:31

I'm doing this currently in my green house with a 150w solar panel and a Victron IP67 DC to DC converter. It will accept 15-40 vdv and step it down to 12v. I started with a car radiator fan then moved to a bunch of smaller ones.

jm3

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#3278680 4-Sep-2024 06:41

More airflow is always great but if you are trying to reduce temps in summer I would suggested shading part of the roof is another tool to help manage the environment.