OT: Solar battery power

[geoffbeaumont]

Grossly off topic (the battery didn't even come out of a land rover), but there are a few members on here who've got some experience with solar setups, so I'm hoping I can pick your brains...

We have a partly installed heat sink system in our greenhouse. Fairly simple idea, a 3" pipe runs from up near the apex of the roof into a well under the paving slabs down the centre of the floor, where it splits into smaller pipes packed along rocks and stones. At the other end of the week another 3" pipe vents low down. There's a fan in the intake pipe to circulate air. During the day the rocks are warmed and it helps avoid excessively high temperatures, then during the night the heat is released back to keep the greenhouse warmer. As a bonus it circulates air reducing humidity.

Or at least it would if I finished it off...

The fan is a 12V inline boat bilge vent one, and I have just had to change the battery on my car so I have a 75AH lead acid battery which was getting dubious for starting but should still be happy enough running this fan.

The missing bit is a means of charging the battery. We're planning on a small solar panel, which even for this simple rig is I guess going to need some kind of controller/battery protection. If it incorporates the ability to switch the load on and off (above and below fixed temperature) that would be useful, but could easily be added later separately.

So... There's lots of small solar panels and controllers on the market. Most of them making grand and probably overblown claims as to what they're capable of... How do we figure out what we'll actually need to run this?

Don't want to spend a fortune - but also want to do this once and finish up with something reliable.

As an aside, this is what self-directed home education looks like in our house. One of my son's projects!

[FridgeFreezer]

How much power does the blower take?

Any of the cheapy solar controllers that will handle the load would do it - the ones like I've got in the Ambulance have a programmable "load" output that allows you to only run the load when the sun's out / battery is healthy, they cost about £10-£20, then you just need a panel that's rated about enough to run the load.

[geoffbeaumont]

The fan is 3 Amps, so if I remember my physics correctly, 36 Watts.

As far as I can see pretty much any controller will handle that. How much headroom do I need to allow on the solar panel rating? I'm guessing a 40W panel likely won't deliver 36W (never mind charge the battery at the same time) in our weather?

Is MPPT overkill for this? Looks like at the budget end of the market I could get IP65 (probably a good idea) or MPPT but not both. Then again, sounds like it's a bit of a gamble whether cheaper controllers really are MPPT... 

[miketomcat]

My 100w panel will deliver 3 amps fairly well but it's flat on the roof so sub optimal mounting. Angled and tracking would see better results but I just added a second 100w panel. It's difficult to tell what it will deliver because once the battery's full it drops back to under an amp.

Mike

[FridgeFreezer]

3A * 24H = 72Ah per day you need to run the fan 24/7

A 100W panel will deliver an average 10W (ish) over any given 24H period (which is including night time) = 10W / 12V = 0.8333A which = ~20Ah per day ish, more on sunny days, less over winter.

Our 100W panels can top 5A on a hot & sunny summers day but only when the sun's overhead beating down.

As you can see you'll need a fair chunk of panel to get to a solid reliable 72Ah. It may be more cost-effective to pull a mains cable through to the greenhouse and just run it like that, 864Wh/day is less than one unit of electric so maybe 15-30p/day.

I'd say jump straight to an over-spec panel and a cheap eBay controller, the amount that the big names want for MPPT controllers you can just buy a bigger panel (or a 2nd panel & controller) and get more electric that way, and the cheapy £15 controllers have better information display and configurability!

These cost me maybe £20 each because I wanted the nice display, they claim to be MPPT but I've not checked.

As you can see there's configuration for it to switch the load on/off automatically based on battery voltage or at "night" when the solar voltage drops off, so this would let you easily run the fan until the battery charge drops off and then shut off to avoid damaging the battery.

This setup would mostly run our fridge & lights in the camper (~28Ah/day) when we were also driving around every few days to top the battery off, when we were parked for longer periods it would drop so we just added a 2nd panel & duplicate controller and now it's fine.

[miketomcat]

I have a similar system to fridge on the caravan build but I've only got one mppt30, two panels and two 110ah battery's. Last trip we were parked for 5 days in autumn and it struggle to keep up with the fridge so I turned it off overnight when it was cooler anyway. Hense I've added a second panel but haven't been out yet, though it's keeping the battery's fully charged at the moment.

Mike

[geoffbeaumont]

Sounds like we really need to implement temperate based switching, if only to reduce the time the fan is running...

Running mains to the greenhouse would involve trenching an armoured cable in. At present the nearest power is about 50m away, so not a trivial undertaking. I'm going to run power to the shed when I rebuild it this year, which is closer. But would probably still involve a 20m+ trench. Not hugely appealing! There's another lean-to greenhouse attached to the shed, which once replaced will likely get a similar setup, but as there'll be mains in the building already I'm planning to use that there.

Angling the panel isn't a problem, and the fan is needed less on overcast days so it's not a showstopper if it stops then.

[geoffbeaumont]

So, 200W of panels (overkill? We'll see!), brackets, wires and an IP65 controller* ordered. I'll report back once we've cocked upcompleted the installation.

* This one, used from the original seller. Bit more than I'd reckoned on spending but near enough half list price.

[sean f]

Not a subject I know much about but have been looking for some sort of system to keep the truck battery topped up when I am away (it could easily not get used for a couple of months depending on my schedule), tried a cheap panel in the windscreen plugged into the lighter socket and not convinced it did anything and certainly not enough. Plugging in a mains powered system is not an option where the truck is kept.

So now thinking a bigger panel on the rear and controller such as being discussed here, with the controller hard wired in with a disconnect switch or simple unplug when using the vehicle and the panel removable.

The controller in the link above and a medium sized (100w?) panel should do the job to act as a trickle charge as far as I can see, possible over kill but I tend to over engineer rather than under!.

Just to educate me on this, looking at the battery options for the above controller would a standard car battery class as "flooded"?.

[miketomcat]

Lead acid is flooded, AGM is often spiral wound (Optima), gel are generally completely sealed and deep cycle. Then you slip down the slippery slope of lithium.....

At least this is my understanding and I'm generalising.

Mike

[FridgeFreezer]

100W panel would be waaaay overkill, I'd say any of the small panels that claim to be ~25-50W (about A4-A2 paper size) would be more then enough for the average Defender unless you're got some parasitic drain or something. My Mini seemed pretty happy with a very carp ~5W panel (~30cm x 10cm ish) just keeping it topped off when last stored, the 127 has nothing at all (battery cutoff on the house battery, main one untouched) and stands for months and months and has always started first time.

Something hungry like a D3/4 or modern RR might want an amp or so and push you into 50-100W territory if you were being REALLY belt & braces, I guess the setup is relatively inexpensive Vs the faffing about it saves and the potential to damage a battery by having it go flat.

[FridgeFreezer]

Not that I've been bored at work  but I made an online calculator thing that lets you work out likely panel & battery specs / performance:

https://fuddymuckers.co.uk/tools/solarcalc.html

Something like this:

Have a play and let me know!

[geoffbeaumont]

2 hours ago, FridgeFreezer said:

Not that I've been bored at work  but I made an online calculator thing that lets you work out likely panel & battery specs / performance:

https://fuddymuckers.co.uk/tools/solarcalc.html

Something like this:

Have a play and let me know!

Handy tool - could use a little CSS tweaking do make it more mobile friendly (happy to have a crack at that if you don't want to).

[FridgeFreezer]

3 hours ago, geoffbeaumont said:

Handy tool - could use a little CSS tweaking do make it more mobile friendly (happy to have a crack at that if you don't want to).

Feel free, as you can tell I am not a jedi master at web / CSS I run out of patience very quickly with all that stuff.

[geoffbeaumont]

Promised an update, so here goes.

 

Solar panels mounted on a simple OSB/roofing batten frame. Not an ideal location as it'll be shaded part of the day by a kids den which is off to the right - I'll likely move them once the raised area to the right of the compost bins has been cleared and tidied up.

Mounted the controller on a bit of OSB, trip on the left doubles as a switch for isolating the solar panels. The take away tubclassy waterproof enclosure on the right contains the under-voltage protection circuit and load fuse (which doesn't need to be in the tub, but it was tidier that way):

The intake duct at this end takes in hot (very much ish at this time of year) hot air at the top of the greenhouse and passes it into the heat sink under the paving slabs. Ducted fan to the left of the louvre:

The air is vented low down at the other end so that it circulates around the greenhouse:

And before @FridgeFreezer calls me out for it, yes I did go round and crop the cable ties after I took the photos!

Took a little fiddling with the under-voltage protector to get it running nicely (it's got a couple of potentiometers to adjust the voltage it cuts out and back in at). To start with it was just sitting rattling it's relay as the fan kicked in and out. Don't know if I've got it set sensibly yet - it would have been a good idea to set it up on the lab power pack first. Seems like it might not cut out very cleanly, time will tell.

So, tweaking aside it's working nicely - only real problem is that the fan may be a tad OTT. It's shifting air nicely, but it's not quiet... Think I may need to step the voltage down and run it slower. If I do that right I should be able to reduce the current too - my electronics is rusty, but I'm pretty sure I remember at least one reasonably efficient way of dropping voltage...