Solar Fan for Greenhouse

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jm3

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#279775 7-Nov-2020 21:29

Hi,

We recently erected a 6m tunnel house. We've quickly found that the air vents are inadequate. It has an auto roof vent, back louver window and main door has mesh or plastic.

I'd like to put a fan on the back wall to get some air flow on hot days.

I saw a project where someone used a 12v radiator fan powered directly from a solar panel to cool a roof space and thought that might work well for my situation.

My plan would be:
12v 80w radiator fan
Solar Panel 100w,200w,320w?
DC-DC Regulator
Bi-metal thermostat

This is the first solar project I've done. It seems cost prohibitive to get batteries involved.

Has anyone tried anything similar with or without success?
Can someone with solar knowledge offer advice on parts?

Cheers
JM

timmmay

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#2599662 7-Nov-2020 22:00

I looked into this for my greenhouse a while back. I eventually decided it was too hard and just didn't bother, but I made sure the doors at each end are open with mesh over the openings on hot days.

I've read that in summer it's best to change air every one to two minutes. How much air would a radiator fan move per minute compared with your greenhouse size. I guess something is anything is probably better than nothing.

jm3

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#2599683 8-Nov-2020 05:32

The volume of the greenhouse is around 1300 cubic feet. New radiator fans advertise moving around this amount or more per minute.

I've grabbed an old fan from the wreckers for testing, it came in a shroud with mount holes. I'll put a wooden frame in the back wall for mounting.

We are working on shading areas of the greenhouse too but I think the extra airflow is critical for fungal issues.

timmmay

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#2599688 8-Nov-2020 06:07

Sounds worthwhile then :) I'll be interested how it turns out for you. My greenhouse is about 2.5w x 2.5h x 4l which is about 25 square meters / 900 square feet, so half your size. I think better airflow would help a lot with the occasional fungal issues and keeping it cool in the middle of summer.

I can't help with solar panel sizing sorry, someone here hopefully can. I have an 80w panel but I just use it to keep batteries topped up. I guess if it's direct driving the motor go a bit oversized so it's putting some air through when it's cloudy and max air through when it's sunny, so if it's not producing the rated wattage the whole time that's probably ok? I guess the watt rating is in perfect circumstances, perfectly aligned with the full sun. Since it won't always be perfectly aligned you might double to 160W and round up to 200W? Some reading / experience required.

raytaylor

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#2599808 8-Nov-2020 10:17

I have a shed which I use some computer fans to keep cool - I undersized the solar panel so it doesnt start up on the darkest of days. The 4x fans use 5 watts each so I used a 10 watt solar panel.

However what I should have done is just a temperature switch.

So for an 80 watt fan, you would want an 80 watt solar panel. Try to find one with the lowest "Voc" so its as close to 13.8v as possible. 12v Solar panels are designed to recharge batteries so have a much higher voltage output, while your radiator fan is designed to run on 13.8v with the car alternator or 12v battery supply.

Jaycar sell solar panels for a reasonable price - you could go with a 120w or a 160w panel with no problem and use a thermostat to control the fan.

As long as the voltage isnt too high, the fan will just take whatever power it needs to run. The solar panel wont overload it with watts.

However upon looking at the jaycar panels and a few other suppliers, i am now a bit concerned the voltages might be too high for a 12v fan. It seems the current ones on the market sit around 18v-21v output. So my suggestion would be to look for a 24v truck fan and run it on a 12v (18-21v) panel. Or it may work with the 12v fan. I really dont know, fans might be more accomodating for excess voltage.

Jaycar doesnt sell a suitable thermostat switch so use aliexpress part ZR011

The good thing about the ZR011 is it has an NC and NO output so you could use a 20-40w 12v growing LED as a dump load on cloudy days when the temperature isnt so high and the fan isnt running if you wanted to specifically boost growth on some specific area of the greenhouse, or use a small reticulating water pump. When the temperature is below the set point power is sent to the NC output, and when above the set point it is sent to the NO output.

Ray Taylor

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Lias

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#2599809 8-Nov-2020 10:21

Watching with interest.. I was looking into something like this while ago to keep the garage cool in summer, couldn't find anything decent local, only in the US and it was getting a bit costly.

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jm3

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#2600097 8-Nov-2020 20:16

raytaylor:

So for an 80 watt fan, you would want an 80 watt solar panel. Try to find one with the lowest "Voc" so its as close to 13.8v as possible. 12v Solar panels are designed to recharge batteries so have a much higher voltage output, while your radiator fan is designed to run on 13.8v with the car alternator or 12v battery supply.

Jaycar sell solar panels for a reasonable price - you could go with a 120w or a 160w panel with no problem and use a thermostat to control the fan.

As long as the voltage isnt too high, the fan will just take whatever power it needs to run. The solar panel wont overload it with watts.

However upon looking at the jaycar panels and a few other suppliers, i am now a bit concerned the voltages might be too high for a 12v fan. It seems the current ones on the market sit around 18v-21v output. So my suggestion would be to look for a 24v truck fan and run it on a 12v (18-21v) panel. Or it may work with the 12v fan. I really dont know, fans might be more accomodating for excess voltage.

Thanks for the advice. If I over spec the solar panel I think I will use a DC/DC regulator to reduce the load back down. Ali have a few that take 36v or 48v and output 12v.

The solar light is a good idea and I will look into the ZR011 too.

Cheers

raytaylor

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#2600112 8-Nov-2020 20:43

Some motors have a high inrush/startup surge current so if its an 80 watt fan, and uses 12 volts then its probably an average of 6-7 amps - so you might want a regulator capable of 10 amps.

Ray Taylor

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MikeAqua

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#2600925 10-Nov-2020 11:42

If you are looking for a very simple solution, you can buy a solar roof vent with a integrated motor and battery. Places like Burnsco sell them. It would be easy to hack in and add a thermostat.

May or may not work in your situation, depending on how much sunlight the back wall gets.

I have one built into my new shed.

BTW usually a vent in a hothouse will be in the roof - I assume to to take advantage of convection.

Mike

Jase2985

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#2601085 10-Nov-2020 15:39

put some vents in down low and let the stack effect (convection) do its thing. thats what my old greenhouse had

jm3

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#2660473 21-Feb-2021 13:25

Finally an update on this project.

Equipment list:
Old Suzuki swift radiator fan
150w mono solar panel
Victron IP67 dc-dc 24/12-10A converter
Plus 18m solar cable, breakers and fuses...

The oversized panel works really well and runs the fan with full cloud, late evening I could hardly see the sun and it was still going.

Next step is temp/humidity control so the fan doesn't go all day. I've seen one on Ali that should work well.

Definitely scope to run some other devices with the extra power.

Hopefully the fan lasts but it was only $25 from the wrecker's so easily replaceable.