Leisure batteries for greenhouse fan

[vivienz]

I want to rig up a simple electric fan system in our greenhouses to push some extra air through on hot days; it's already been peaking at 45C in there over the last week, so I want some more air circulation in addition to the shading that I'm putting in.

 

The greenhouses are a cheap and cheerful construction and aren't up to having holes cut in them to permanently support a fan or blower, so I'd like to use a 12v leisure battery and portable fan set up, with the plan that I can charge the batteries from the surplus PV generation from the house array. The battery and fan set up also means that the whole lot can be easily removed once the days have cooled. I don't want to have a solar panel installation there as I'm not confident it would stay in place with the strong winds we get here, plus it's more complicated than necessary.

 

Does anyone have any particular recommendations on the leisure batteries? They don't need to be lightweight as they will sit on the soil floor of the greenhouse, so lead acid is fine. I estimate that they may need to power a fan for up to 7 hours on a hot day to keep the maximum temps down to a non plant scorching level, driving a simple oscillating fan.

 

 

[SteamyTea]

Why not use a bathroom fan and an extension lead.

A normal leisure battery will probably store about 1 kWh.

So if you need 10 hours of forced air, the fan needs to be no more that 100 W.

[Jeremy Harris]

Had you thought about just fitting one or more solar panels to directly power fans?  That would make for an automatic system, with no maintenance required, as the fan would run whenever the sun was shining, and the more sun the faster the fan would run.  Not hard to put a simple system together, using a 12 V DC fan and a suitable small solar panel.  As a bonus, the solar panel(s) might also provide a bit of additional shading.

 

 

 

 

[vivienz]

4 minutes ago, Jeremy Harris said:

Had you thought about just fitting one or more solar panels to directly power fans?  That would make for an automatic system, with no maintenance required, as the fan would run whenever the sun was shining, and the more sun the faster the fan would run.  Not hard to put a simple system together, using a 12 V DC fan and a suitable small solar panel.  As a bonus, the solar panel(s) might also provide a bit of additional shading.

 

 

 

 

 

We thought about a direct solar power connection, Jeremy, but the roof already diffuses the light and I'm really not confident that anything attached to it would stay there over winter due to the lightweight construction and the howling gales over the winter.  The shading I'm planning for the greenhouse is only necessary for the clear vertical sides.  We currently have excess PV generation on sunny days and I'd quite like to be able to use it rather than push back into the grid, so this seems a good use. 

 

11 minutes ago, SteamyTea said:

Why not use a bathroom fan and an extension lead.

A normal leisure battery will probably store about 1 kWh.

So if you need 10 hours of forced air, the fan needs to be no more that 100 W.

 

Good idea, ST. I will have a look at bathroom fans, too.

 

Any thoughts on the battery itself?

[Jeremy Harris]

You're not looking at anything massive to run a ventilation fan, though: http://www.cpssolar.co.uk/Solar-Ventilation

 

Could be stand-alone, with a built-in solar panel, if a 160mm fan will fit OK:

 

[vivienz]

Thanks, Jeremy, that looks good.  Off to investigate.

[Jeremy Harris]

1 minute ago, vivienz said:

Thanks, Jeremy, that looks good.  Off to investigate.

 

 

There are probably a lot more choices around, including some smart solar powered fans with controllers made specifically for greenhouses, like this one (probably expensive though):

 

[SteamyTea]

The thing about using an extension lead and a cheap fan is that it will use your PV generation as you will almost only ever need cooling when it is sunny, the very times that you over generate.

 

12 quid.

 

20 metres of extension

https://www.amazon.co.uk/Rubberised-Outdoor-Extension-H-Frame-Carrier/dp/B077NDMC9Y

 

22 quid

 

Sorted

[vivienz]

Your logic is flawless, @SteamyTea , but we would need much more than 20m of cable to get power out to the greenhouses, hence my initial ponderings on the battery. I will ponder further.

[SteamyTea]

1 minute ago, vivienz said:

hence my initial ponderings on the battery. I will ponder further.

Where is your PV? How were you going to get the power from that to the battery?

 

As the fan is low power, you may well get away with smaller cable.

[vivienz]

The PV is on the house. It wouldn't be too much effort to put the batteries onto a trickle charge in the garage overnight.

[jamieled]

In terms of leisure batteries, we've been living in our wee caravan for a year or so now using a bog standard halfords leisure battery. It is charged off a 100w pv panel mounted on a pallet. Struggled a bit in winter but you wouldn't need it then anyway.