Outside the box - solar/batt electric heating. Brains trust

[TassieTiger]

It appears that there is a universal problem in relation to 12v heating. This problem has driven a caravanning/cabin/tiny home uptake into buying diesel blower heaters, mini gas heaters and small pot belly stoves for cabins.

So being a bit of a stubborn arse and wanting to undertake a few experiments to try and clarify the issue (hey Jarhead), I thought I might try and build something that might take the issue a step forward.

For those that dont know - heating from battery power has 2 main issues, modern electrical heating as it stands today, draws a lot of amps and the result is - it kill batteries;
for instance, a typical 2400watt bar / fan heater inverted to 12v battery power, would draw a massive 2400/12v = 200amps per hour!!! Now cables aren’t generally made for 200amps and that amount of draw is for 1 hour only, so even if you could get cables large enough, 8 x 200amps = 1600 amps for 1 night heating - you’d need a massive solar system to replenish batteries.

So what about the micro heaters they run off 600w ? Yeah...they are pathetically underwhelming and still draw massive amps. A dedicated 12v heater? Nope - you could get more heat from a candle.

Kicking some ideas around and wondering what people think of the following...effectively trying to emulate a column Heater from solar / batts., yes, it’s an experiment. Lol.
Equipment = 
2 x 200w 12v solar panels. 1 x 30amp MPPT controller. 2 x 260ah lead acid batteries. 1 x day / night switch ptc. 
1 alloy radiator. 2 x 12v cpu fans. 1 temp display switch. 3 x ceramic elements (these warm up to 220C each). 2.5 litres oil or glycol. 



So - I was going to wire up 3 x 12v 30w thermistor ceramic heating elements and submerge them into the bottom tank of a new commodore alloy radiator, filled with a light oil. The elements will be spread evenly in bottom tank and radiator will be temporarily sealed with plugs - in case oil doesn’t work - might consider glycol or water. 


2 small computer 12v fans will be mounted at a 45 degree angle, on the outside of the radiator, but towards the top - pushing air downwards from top tank to bottom tank at a 45 degree angle - so some air will be pushing through radiator and some air movement will be lost.

 So during day - once batts are at float charge, excess power will switch on ceramic elements and begin the heating process for night time use. Once day light falls, the batteries will take over. 3 x elements and 2 mini fans = Approx 100w in total and thst amount will be drawn for 8 hours. 800w/12v = approx 70amps for a night.

The 3 x elements will heat the oil and hopefully start some thermal movement through the radiator, with the lighter viscous oil floating above the thicker and heavier colder oil - might get a reticulation started...might not. 

The 12v fans will then help heat transfer from radiator core to the air itself, by blowing air passed the cooling fins - which should be quite hot as a result of the 3 ceramic elements being on for several hours prior to dusk. 

If I can get the oil to a constant temp well above 100C, then I’m hoping this experiment will produce enough heat to warm a 20ft x 10ft room to maybe 20 - 25 degs C. I don’t know enough about BTU’s nor the transfer rate of alloy radiators, nor oil liquid conductivity or even how the heating elements will actually perform in oil. A radiator cap will have spring tension removed as to ensure it doesn’t become a pressure vessel. 



What are peoples thoughts ?Will this work ? Better options for oil ? Radiator? Would a heat sink work better ? What air temps might be expected from this set up ?
Interesting to note that when researching this - I found a commercial 12v elec heater for sale that uses a steel barrel filled with water, heated by 12v bands wrapped around the barrel...2 x 100w bands rated to 180 degrees was apparently seeing the water give off 28 degrees...but I imagine condensation would be an issue with this set up.

[trekin]

Well, for out of the box thinking, how about this https://www.youtube.com/watch?v=bHQyT4IstrQ A lot less mucking around, dead simple to set up, larger thermal mass than the oil/water in your radiator setup. You could even preheat the bricks, in your firepit or left in the sun during the day, thereby reducing the draw on your batteries to bring them up to temp. links for the thermistors used are in the video discription.
And if you wanted to move the heated air around a bit more, one of these, https://www.bunnings.com.au/scandia-eco-fan_p3171516 Although I wouldn't pay that price, they can be got a lot cheaper, or you could build one yourself, plenty of Utubes showing how.
Edit to add; Insulation is the biggest factor in whatever setup you use. How is the insulation in your conex? roof?, walls?, floor?, double/tripple glazed windows? Well enough insulated, and the heat output of a candle will warm that space.

[eddievic]

Why not just an old conara.

Cooking oil is thin, but i don't know if it will pass through the fins of a radiator.

Tbh look into an inverter reverse cycle heater, or waste oil heater

[rc42]

If you want efficiency for heating then have a look at heat pumps, the latest range of small inverter split aircon units can provide 1kw of effective heat output from around 200w power input.
Certainly achievable if you have enough solar panels and batteries.

Alternatively, direct heat collection from the sun is far more efficient than solar PV electricity converted to heat, solar water heaters need far less roof space than a PV system with equivalent water heating ability. One or more of those being used to store hot water or oil could be used. Maybe your hot water storage could be circulated under floor during the night.

[eddievic]

Yes RC, i was taking to someone and they got heat pump on their pool and it uses $3 of electricity a day to heat his 50,000l pool.

Further to that a radiator heater like they use in europe

[trekin]

If you want efficiency for heating then have a look at heat pumps, the latest range of small inverter split aircon units can provide 1kw of effective heat output from around 200w power input.
Certainly achievable if you have enough solar panels and batteries.

Alternatively, direct heat collection from the sun is far more efficient than solar PV electricity converted to heat, solar water heaters need far less roof space than a PV system with equivalent water heating ability. One or more of those being used to store hot water or oil could be used. Maybe your hot water storage could be circulated under floor during the night.

Yes RC, i was taking to someone and they got heat pump on their pool and it uses $3 of electricity a day to heat his 50,000l pool.

Further to that a radiator heater like they use in europe

NOW, if I read the OP correctly, he'a asking for out of the box thinking, not an off the shelf solution. I'm assuming this is for his off grid hunting cabin out in the wilds of Tas., and is wanting a way to heat his cabin that is cheap and, more inportantly portable, and can run off a battery and small 200w solar panel.

[Blr243]

Today I was telling my customer about how I run an air conditioner into the side of my tent on summer hunting trips .......us Queenslanders are trying to control temprtature in the other direction

[eddievic]

If you want efficiency for heating then have a look at heat pumps, the latest range of small inverter split aircon units can provide 1kw of effective heat output from around 200w power input.
Certainly achievable if you have enough solar panels and batteries.

Alternatively, direct heat collection from the sun is far more efficient than solar PV electricity converted to heat, solar water heaters need far less roof space than a PV system with equivalent water heating ability. One or more of those being used to store hot water or oil could be used. Maybe your hot water storage could be circulated under floor during the night.

NOW, if I read the OP correctly, he'a asking for out of the box thinking, not an off the shelf solution. I'm assuming this is for his off grid hunting cabin out in the wilds of Tas., and is wanting a way to heat his cabin that is cheap and, more inportantly portable, and can run off a battery and small 200w solar panel.

RC has a very valid suggestion infact that is easy and off the shelf. And it might actually work.

Your suggestion was bricks, and an expensive fan. Good luck getting them hot in winter

[TassieTiger]

If you want efficiency for heating then have a look at heat pumps, the latest range of small inverter split aircon units can provide 1kw of effective heat output from around 200w power input.
Certainly achievable if you have enough solar panels and batteries.

Alternatively, direct heat collection from the sun is far more efficient than solar PV electricity converted to heat, solar water heaters need far less roof space than a PV system with equivalent water heating ability. One or more of those being used to store hot water or oil could be used. Maybe your hot water storage could be circulated under floor during the night.

Yes RC, i was taking to someone and they got heat pump on their pool and it uses $3 of electricity a day to heat his 50,000l pool.

Further to that a radiator heater like they use in europe

NOW, if I read the OP correctly, he'a asking for out of the box thinking, not an off the shelf solution. I'm assuming this is for his off grid hunting cabin out in the wilds of Tas., and is wanting a way to heat his cabin that is cheap and, more inportantly portable, and can run off a battery and small 200w solar panel.

You are of course correct Trek - I know I can plumb in a pot belly or invert a 1000w bar heater - but looking...outside the box.
That brick idea is gold - much easier. And yes, my cabin is steel outside, double wool insulated, then VJ pine lining - including roof. Bricks are a bit agricultural - what else might perform ? I need to test these thermistors which arrived today so...

I don’t know how a 1kw air system can run off 200w ?= it’s rated at 1kw for a reason. It’s also 1 thing to run a aircon fan, but add a heating element and things get messy quickly. Even a 12v x 1.0kw air con used in caravans will draw 85 amps per hour....do you have links to these 200w reverse cycle heaters?

Eddie - how do you propose to collect heat from the sun ? I agree it’s more efficient but you don’t really need heat during the day...

[cracker]

generator and a fan heater?

[TassieTiger]

Noisy...and gensets need fuel...
Looking for solar powered, electric heating...I honestly thought something would have already been invented.

[eddievic]

Mate i don't understand myself, lol but last inverter thing i got i looked at the specifications on the bix blew my mind.

Here is something from daiken
https://www.daikin.com.au/our-product-r ... chnologies

Look at the technical specifications

[TassieTiger]

Specs are showing the heating / cooling capabilities of the unit - not the power draw. We have a 20kw outdoor ducted system - but it’s 9kw heating capability. The calculations also allow for “off” time during the hours use - like a fridge, the power turns on / off to keep temp stable - so calcs can be an average of those off / on time. Power draw is often not advertised or if they are, they are complete bull shot.

[Larry]

The difference with a reverse cycle air con is that it does not use a heating element like in a electrical bar radiator. In the bar radiator it works on resistive wire that give a PF of near one. In that case the power used equals the power out. In reverse cycle air cons they are using the change in state of gas to liquid driven by a compressor motor and a fan to distribute heat/cold. With this tech you can get a heating capacity that is greater that the power being consumed electrically. So much for the conservation of energy formula where energy in must equal energy out.

[TassieTiger]

The difference with a reverse cycle air con is that it does not use a heating element like in a electrical bar radiator. In the bar radiator it works on resistive wire that give a PF of near one. In that case the power used equals the power out. In reverse cycle air cons they are using the change in state of gas to liquid driven by a compressor motor and a fan to distribute heat/cold. With this tech you can get a heating capacity that is greater that the power being consumed electrically. So much for the conservation of energy formula where energy in must equal energy out.

Electrical kW ≠ heating & cooling kW.
A reverse cycle output measures ability “move” air, not heat the air...they are efficient but not anywhere near what you think. Even the fan in a 1kw unit, consumes 350w.

[cracker]

i looked into putting a split system in my caravan and running it off batteries and an invertor/ solar and 240v charging set up, like a ship/shore type set up.
set up cost was huge. current draw on small split systems is low. 2/3 amps its very reasonable, they do spike up and down abit nothing crazy
its not an economical proposition, batteries just dont have the energy density of petrol
only way you could consider it was a generator when you dont have site power.

[TassieTiger]

i looked into putting a split system in my caravan and running it off batteries and an invertor/ solar and 240v charging set up, like a ship/shore type set up.
set up cost was huge. current draw on small split systems is low. 2/3 amps its very reasonable, they do spike up and down abit nothing crazy
its not an economical proposition, batteries just dont have the energy density of petrol
only way you could consider it was a generator when you dont have site power.

Do you mean the current draw for a 240v system is only 2 or 3 amps??
A true 1000w heater draws 4 amps on 240v but a massive 83amps as a 12v system.
When you walk into a renewable energy business and ask about going “off grid” generally they’ll try and talk you out of 2 items for off grid - refrigeration and heating.
A few years back, a person in the industry told me you couldn’t run a family sized 240v fridge on solar due to the massive draw....several years later, hundreds spent via experiments and I’ve now managed to achieve a family sized (albeit a small one) fridge running off solar - but even with 1000ah batteries, 1kw solar panels, a 3000w inverter and mppt controller - even with that equipment, if I get 4 days of little to no sun - I have to switch back to mains power.
Those household items that draw constant power (even intermittently) through the night - kill batteries...
I’m going to try and emulate / improve on the brick sand which with thermoistors, I tested one tonight and they get very hot, very quickly drawing 2.3 amps per hour so hopefully, I can transfer that heat to a “sink” that can be used as needed....

[trekin]

i looked into putting a split system in my caravan and running it off batteries and an invertor/ solar and 240v charging set up, like a ship/shore type set up.
set up cost was huge. current draw on small split systems is low. 2/3 amps its very reasonable, they do spike up and down abit nothing crazy
its not an economical proposition, batteries just dont have the energy density of petrol
only way you could consider it was a generator when you dont have site power.

Do you mean the current draw for a 240v system is only 2 or 3 amps??
A true 1000w heater draws 4 amps on 240v but a massive 83amps as a 12v system.
When you walk into a renewable energy business and ask about going “off grid” generally they’ll try and talk you out of 2 items for off grid - refrigeration and heating.
A few years back, a person in the industry told me you couldn’t run a family sized 240v fridge on solar due to the massive draw....several years later, hundreds spent via experiments and I’ve now managed to achieve a family sized (albeit a small one) fridge running off solar - but even with 1000ah batteries, 1kw solar panels, a 3000w inverter and mppt controller - even with that equipment, if I get 4 days of little to no sun - I have to switch back to mains power.
Those household items that draw constant power (even intermittently) through the night - kill batteries...
I’m going to try and emulate / improve on the brick sand which with thermoistors, I tested one tonight and they get very hot, very quickly drawing 2.3 amps per hour so hopefully, I can transfer that heat to a “sink” that can be used as needed....

Assuming your thermistors are PTC (positive temperature coefficient) then they will draw most current when heating the thermal mass (bricks etc) to temp, then 'idle' back and draw very little current to maintain temp.
Thermal mass is the ability of a material to absorb, store and release heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Some interesting reading on thermal mass here; https://www.greenspec.co.uk/building-de ... rmal-mass/

[eddievic]

The reality is, its hard to reinvent the wheel.

I haven't followed your conversation earlier but why don't you want to use a solid wood heater, or a diesel heater. Both even though are not efficient but the fuel is packed with energy.

[TassieTiger]

The reality is, its hard to reinvent the wheel.

I haven't followed your conversation earlier but why don't you want to use a solid wood heater, or a diesel heater. Both even though are not efficient but the fuel is packed with energy.

Fair questions. For you. Lol.

Several reasons really - young kids want to stay over night by themselves (11-14) and the wood heater that is currently semi plumbed in there, needs monitoring - and place is pine lined and of course monoxide poisoning always a worry - even though I have alarm.

Diesel heater is another fuel that needs to be ferried in and monitored, but this is prob the best low worry option.

I have a huge fire pit only 3m from cabin - how easy to carry a few coals in I know...but there is something appealing about solar / elec heating that is completely worry free where kids are concerned. I’m also contemplating opening the cabin to Ronald McDonald house for kids to go bush - so need to sort something left - but that’s a way off when I finish flush toilet etc.

BUT - a heap of solar equipment has fallen into my lap very cheaply. It wasn’t until I started researching solar heat, that I realised that there is a huge gap. Caravaners and tiny house peeps have been searching for a long time for a electric heating solution - I thought opportune timing to undertake some experiments and see what I could come up with. And some ppl on here - you excluded of course lol - are quite smart and can think left of field.

Treks info so far has me doing some experiments today - those little thermistors are amazing! How amazing ? I hooked up one last night - left it for 20 mins, went back and picked it up and now I have a square fuken blister on my hand! Yep - never said that I was one of the smart ones.

[TassieTiger]

i looked into putting a split system in my caravan and running it off batteries and an invertor/ solar and 240v charging set up, like a ship/shore type set up.
set up cost was huge. current draw on small split systems is low. 2/3 amps its very reasonable, they do spike up and down abit nothing crazy
its not an economical proposition, batteries just dont have the energy density of petrol
only way you could consider it was a generator when you dont have site power.

Do you mean the current draw for a 240v system is only 2 or 3 amps??
A true 1000w heater draws 4 amps on 240v but a massive 83amps as a 12v system.
When you walk into a renewable energy business and ask about going “off grid” generally they’ll try and talk you out of 2 items for off grid - refrigeration and heating.
A few years back, a person in the industry told me you couldn’t run a family sized 240v fridge on solar due to the massive draw....several years later, hundreds spent via experiments and I’ve now managed to achieve a family sized (albeit a small one) fridge running off solar - but even with 1000ah batteries, 1kw solar panels, a 3000w inverter and mppt controller - even with that equipment, if I get 4 days of little to no sun - I have to switch back to mains power.
Those household items that draw constant power (even intermittently) through the night - kill batteries...
I’m going to try and emulate / improve on the brick sand which with thermoistors, I tested one tonight and they get very hot, very quickly drawing 2.3 amps per hour so hopefully, I can transfer that heat to a “sink” that can be used as needed....

Assuming your thermistors are PTC (positive temperature coefficient) then they will draw most current when heating the thermal mass (bricks etc) to temp, then 'idle' back and draw very little current to maintain temp.
Thermal mass is the ability of a material to absorb, store and release heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Some interesting reading on thermal mass here; https://www.greenspec.co.uk/building-de ... rmal-mass/

Very interesting trek - thank you. Mind is off and running...to where, I don’t know

[Larry]

Heating water on the roof or by the wood stove and then circulating it in the floor or even just some hydronic heater bars is probably the cheapest and most trouble free ways of heating. for cooling you could run pipes in the ground with a couple of valves to change from roof to ground when the temperature suits.

[TassieTiger]

Heating water on the roof or by the wood stove and then circulating it in the floor or even just some hydronic heater bars is probably the cheapest and most trouble free ways of heating. for cooling you could run pipes in the ground with a couple of valves to change from roof to ground when the temperature suits.

Have thought about this at length Larry - there is a version called a wet bsck that is similar again and in use in many places around the world - but what youve described has additional benefits in being able to self reticulate, because as the hot hot water boils, it rushes to the top of the system, forcing cold water back down to the source of heat. The only issue with undertaking this at mine - I built on something of a rock shelf lol.