Random solar/electrical/physics question…

New Topic
This topic has been archived, and won't accept reply postings.
 snoop6060 31 Aug 2022

I know there are a few clever clogs on here so figured I’d ask this. A bit random but hey ho.  I have a solar panel on my allotment and trying to work out if it could offer any sort of frost protection next spring. So produce some heat. Not a lot, just enough to stop frost inside a largish DIY propagator. Its 3m long and 80cm wide and is covered in bubble wrap at night. It sits inside a poly tunnel but that offers zero protection from frost pretty much. I see you can buy 12v heat cables that could possibly be used to heat the bed of the propagator area. They run at 15 watts / meter. Something tells there is zero chance this is going to heat anything up? They are quite expensive so trying to work out what chance I have of getting this to work before I buy one. I know there are a lot of variables. 

They run at 1amp / meter so I would be limited to 3 or 4m probably. I have 200aH of batteries but only a 100watt panel and it’ll be the dead of winter so doubt it’d be able to keep the batteries anywhere near full if I drain them (I can take them home and charge them but that’s a massive pita). I only need it when it’s gonna freeze. It’d be on a relay that will trigger a few hours before it’s supposed to freeze. 

I know there are other ways to do this eg paraffin but it’s costly. This would be free once I have the cable. But something tells me this isn’t going to work, not even close. What do you reckon? Raising the temp of bed even 5c would be enough. 
 

 wintertree 31 Aug 2022
In reply to snoop6060:

Suck it and see.  You could spend weeks building a model to try and find out, and you’d probably be miles wide of the mark.

Use a “12V battery protector” to stop it over-draining batteries or better use something like a Victron MPPT charge controller which has a 10A (on small ones) “load” output that turns on at night, mainly intended for solar lighting, and which turns off at a low battery threshold you can set by Bluetooth.

Probably the main thing is to not over-heat as that hammers power through increased thermal losses.  Look at “PTC” heaters which self-regulate to a design temperature through quantum tunnelling (clever stuff but cheap) or at building your own closed loop controller for temperature with a low power Arduino and a “1-wire” sensor.

2
 elsewhere 31 Aug 2022
In reply to snoop6060:

I don't know the heating requirements but googling gives cold frame heaters rated about 100W* so assuming they are rated for what is needed...

*or 6m of 15W/m

200Ah (12V) will give 100W at 8A for 25 hours which might be enough to be useful for a few days but it will run out if temperature doesn't rise during the day.

Solar cell - short days, low sun - not 100W so boost the battery a bit but not enough to keep heating going. As you mentioned, you're relying on battery to be topped up and fully charged by solar cell before the cold snap. Do you know if the battery is fully charged in winter?

Given that it's under bubble wrap and poly tunnel keeps the wind off it might work as you might not need the 100W for long.

 CantClimbTom 31 Aug 2022
In reply to snoop6060:

It'll generate during the day when you are less likely to need it and not at night when you probably want it. So you'll need some kind of battery and light sensor thrown in the mix - a bit like a garden led solar, probably also with a thermostat also so it kicks in when cold but not if above frost. Sounds like spiralling complexity?

1
OP snoop6060 31 Aug 2022
In reply to CantClimbTom:

It won’t run all day. It’d be on a relay that can be triggered however I like. Heat, time, light or via an mqtt msg based on the weather forecast. That already works in there. (My allotment is across the rd from my house so I have Wi-Fi just about).

It’s just a case of deciding if it’ll actually heat anything at all. 

OP snoop6060 31 Aug 2022
In reply to elsewhere:

> I don't know the heating requirements but googling gives cold frame heaters rated about 100W* so assuming they are rated for what is needed...

> *or 6m of 15W/m

> 200Ah (12V) will give 100W at 8A for 25 hours which might be enough to be useful for a few days but it will run out if temperature doesn't rise during the day.

> Solar cell - short days, low sun - not 100W so boost the battery a bit but not enough to keep heating going. As you mentioned, you're relying on battery to be topped up and fully charged by solar cell before the cold snap. Do you know if the battery is fully charged in winter?

Last winter yes, but there is barely any load in winter. A couple of esp8266s, occasionally a led light and a 1amp pump. So it just does nothing really. 

 MeMeMe 31 Aug 2022
In reply to snoop6060:

How well insulated (including under it) and sealed is your propagator?

The better you can stop heat loss the more viable it will be to heat it this way.

I built a very small propagator that was well insulated and sealed as a bit of an experiment. I also filled the base (inside the insulation) with black painted bricks to add some thermal mass. It has no active heating but stayed about 5 degrees above the lowest temperatures outside, although I didn't have it over winter more from early spring. I ended up with the opposite problem during sunny days, it over heated massively and I ended up putting a small fan in it powered by a tiny solar panel which helped moderate the temperature somewhat. I think a larger version would be less prone to temperature extremes.

Since it's just outside in my yard I could have run a cable to power some heating but I was keen what I could do passively.

OP snoop6060 31 Aug 2022
In reply to MeMeMe:

Not very to be honest. The bubble wrap is just pulled over some hoops late afternoon. I could probably improve this quite a bit but it needs to be quick and easy to open and close. It won’t overheat as it’s not shut in the day. Even in winter it would be full of dead plants by midday as the tunnel still gets pretty warm if the sun is out. 
 

I was thinking a brick base with the cable in a layer of sand. 

Post edited at 11:42
 wintertree 31 Aug 2022
In reply to snoop6060:

I just thought of a great way to over-complicate this and have some fun on the way.

Build a mini ground source heat pump with a buried pipe loop filled with water/anti-freeze going in to a set of water block > Peltier > heat sink > fan stacks, .  Water cooling PC enthusiasts mean there's a massive supply of radiators, fans, heat blocks with Peltier mounting and so on.  For a temperature rise of 10°C you might get a COP in the range of 2 to 3 effectively doubling or tripling the size of your solar panel and batteries.

1
OP snoop6060 31 Aug 2022
In reply to wintertree:

I have no idea how you do that but I do like to over complicate stuff so I’m gonna find out 😂

 Jamie Wakeham 31 Aug 2022
In reply to snoop6060:

I'm with Wintertree (his first post, not his slightly overcomplicated second one...).  Just stick it together and see how it performs.

My instinct is that it's not going to be great, because the bright sunny winter days when your PV will generate the most are also the days you don't need the additional heat.  Conversely the days when you really need extra heating are also the ones in which your 100W panel is going to be trickling a few watts at best.  I've had days in the dead of winter when my 5700W system produced less than 100W, averaged across the day.

You'll be fine if it's always one sunny day followed by a poor one... but I reckon that a run of several grey days will have your battery completely exhausted. It might mean that, when you do  have a week of crap weather, you'll need to switch in a second battery that you've charged at home.

1
OP snoop6060 31 Aug 2022
In reply to Jamie Wakeham:

The bright sunny days have the coldest nights usually. No clouds means the temps drop lower at night. 

 jkarran 31 Aug 2022
In reply to snoop6060:

Wouldn't it make more sense to use the propagator as a solar storage heater? Clear plastic lid, hinged sun tracking mirror boosting the solar gain then flipping down at night to kill radiation losses and insulate the top of the bed.

Jk

 Jamie Wakeham 31 Aug 2022
In reply to snoop6060:

Yeah but you'll have had a ton of solar gain, so (as long as the insulation is sufficient) it'll hold onto that heat.

 wintertree 31 Aug 2022
In reply to CantClimbTom:

> It'll generate during the day when you are less likely to need it and not at night when you probably want it

I was pondering this - once the battery is full, a low voltage DC solar charge controller will just stop charging and not collect available solar power, wasting it.  What the OP really wants is to set their solar charge controller to some fixed high voltage (e.g. 15 V) and use a diversion controller to apply the right charging profile to the battery by diverting all excess solar power to a second heater (one without temperature control).  They're sometimes called "Dump Load Controllers".  

Also, worth considering putting the batteries inside the tunnel to protect them from lower temperatures and to recover the thermal losses during charging and discharging.

OP snoop6060 31 Aug 2022
In reply to jkarran:

It’s sorta of what it is at the moment and it doesn’t really work. Though it’s a bit ramshackle. I could make a mini tunnel from the poly from my tunnel I guess and then just drape bubble wrap over that. It does have a few 5 litre water bottles as thermal mass but I guess I could add a few more. 

OP snoop6060 31 Aug 2022
In reply to wintertree:

That’s a good point actually, I had not considered that. Would a split charge relay work for that? I have an old one somewhere. Could probably make something if it doesn’t. 
 

edit: actually that would be useful in summer as well. I could have a water feature to wind up the nosey allotment folk that get arsed about water use. Haha. Look at my fountain 😂

Post edited at 14:39
 deepsoup 31 Aug 2022
In reply to snoop6060:

> I have no idea how you do that but I do like to over complicate stuff so I’m gonna find out 😂

I got that impression from your OP.

Sorry to go all fossil fuels on this, but is paraffin heating really that expensive?  100W amounts to about a litre or so a week in one of those burners like a low-tech lamp doesn't it?

I was just thinking about oil-fired central heating (that people have in their houses).  They use a bit of electricity to run the control system, ignite and extinguish the boiler as required but the heat itself comes from burning the liquid fuel. 

It'd be pretty heath robinson, but could you do something like that - use your battery/solenoids/servos/sensors/whatever to light and extinguish one of those little paraffin heaters as required?

(Probably unnecessary if you're visiting morning and evening to cover/uncover the frame with insulation - you could just light the burner in the evening when you're anticipating an overnight frost and knock it off again in the morning - but it'd be fun.)

Oh, and regarding this bit from your earlier post:

>It won’t overheat as it’s not shut in the day. Even in winter it would be full of dead plants by midday as the tunnel still gets pretty warm if the sun is out.

That sounds like an argument for increasing the thermal mass inside, concrete blocks perhaps or a tank of water?  Something that will even things out a bit by taking time to warm up and cool down, helping to prevent the environment from getting too hot as well as too cold during relatively short term fluctuations in the temperature outside.

OP snoop6060 31 Aug 2022
In reply to deepsoup:

I borrowed a GH heater from someone and it burnt £3 of parrafin in a single night. Just gave it them back. This is a much smaller space mind you. 

OP snoop6060 31 Aug 2022
In reply to deepsoup:

I borrowed a GH heater from someone and it burnt £3 of parrafin in a single night. Just gave it them back. This is a much smaller space mind you. 
 

There is quite a bit of thermal mass near it but not directly under it. 3 wheelie bins ~200 litres each full of water. 

 wintertree 31 Aug 2022
In reply to snoop6060:

>  Would a split charge relay work for that? I have an old one somewhere. Could probably make something if it doesn’t. 

In a very clunky sort of way that cycles the batteries a bit degrading them.  

Ideally you want the diversion controller to monitor charge current and voltage to apply both bulk and float charging stages as appropriate.  

> edit: actually that would be useful in summer as well. I could have a water feature to wind up the nosey allotment folk that get arsed about water use. Haha. Look at my fountain

Allotment politics, I know better than to get involved in that.  How do they feel about twee little windmills with their sails turned by a little electric motor?  You could make it in to a little Crazy Golf feature...

OP snoop6060 31 Aug 2022
In reply to wintertree:

To be fair everyone is mostly sound, it’s just the water thing. I have a massive poly tunnel with over 100 plants in it so every time we get the bill and it’s too big they all look at me. When I say big, it was £85 for our whole avenue last year, and I just offered to pay it on top of my rent to stop everyone getting arsey. They declined. 

Nobody minded when I built a pub complete with proper Angram beer engine. It’s the best beer garden in the local area as you can enjoy a proper pint sat under my hops. 

 CantClimbTom 31 Aug 2022
In reply to snoop6060:

If we're going to overcomplicate things...  although Wintertree's "Quantum tunnelling" would normally win hands down because "Quantum" anything sounds so very technical - have we considered an ultraviolet laser? I mean this is the other side of a road from OP's house, why not "just" mount a laser on the crest of his roof aimed at the propagator. It might even have spin off advantages such as undesirables loitering on the pavement across from his house may be dissuaded from staying, old ladies cycling past encouraged to cycle harder, many social benefits! But as a necessity anything involving a high powered laser MUST be controlled by a suitable switch such as https://www.instructables.com/Frankenstein-Light-Switch/

 deepsoup 31 Aug 2022
In reply to snoop6060:

> I borrowed a GH heater from someone and it burnt £3 of parrafin in a single night. Just gave it them back. This is a much smaller space mind you. 

Can't fault you on the facts there, that is pretty expensive!

I've never actually used one, but I was thinking of this kind of thing.  Which claims to run on about a litre per week of continuous use (about £3 worth?) - that should do at least a couple of weeks just burning overnight if they're true to their word.
https://www.futuregarden.co.uk/apollo-paraffin-greenhouse-heater-cold-frame

 CantClimbTom 31 Aug 2022
In reply to snoop6060:

If it's just a single exceptional night in a small propagator a 12 hour tealight might do, 2 if it's big one

OP snoop6060 01 Sep 2022
In reply to CantClimbTom:

I’ve made some of the 24hr candle things with that absolutely rank vegan baking fat (trex, seriously who would eat that). They work but I’m wanting a better solution late winter / early spring. Basically as I usually use 2 grow lights in the house in a tent. One of them usually used for much higher value crops than what I’m growing but 1) I now have a baby so am fairly sure I’m not have space to erect a grow tent as I’m getting thrown out of my study. And 2) they were pricy to run last jan so god knows what they’ll cost to run this January. 

 Moacs 01 Sep 2022
In reply to snoop6060:

https://hartley-botanic.co.uk/guides/greenhouse-power-calculator/ will give you some idea.  A 100W bulb with uninsulated ground would only manage a small bubble wrap tunnel

 CantClimbTom 01 Sep 2022
In reply to snoop6060:

Many years ago I had the job (at Pura foods, Canning Town) to unload big pallets coming off the end of the production line with a forklift, spin around drive round the corner and "throw" them into a roller racking slot really high up so they'd roll into the cold store. They made tons and tons of Trex each run and loads and loads of other household brands of solid vegetable oils all in the same production run (same stuff just different packaging). So Trex is nothing better or worse than any other solid veg oil. Same story with spreads, so many brands are all exactly the same muck.

I don't recommend Trex for tea lights though, it'll make a particular smell when it burns and definitely not last as long as a "real" candle.

Post edited at 12:36

New Topic
This topic has been archived, and won't accept reply postings.
Loading Notifications...