When your batteries are fully charged, perhaps you could dump excess electricity from your solar pv panels into heaters that are embedded into a cob bench (assuming you have a rocket mass heater).
These ceramic heaters go as low as 25 Watts. A controller could programmatically switch additional heaters into the circuit depending on how much power is coming from the solar panels.
I imagine ideally you'd want a heater that works on DC so it would "appear" (in the electrical sense) just like a 12v battery to your charge controller, negating additional power conversions. Perhaps you could build your own heater using nichrome wire set into the thermal mass?
Ah just had a thought - if you did make your own heater with nichrome wire, you'll potentially have issues where the wire expands and contracts and will eventually lose a good physical contact with the cob. Not sure how you could mitigate that effect.
You could go in a different direction and dump the electricity into a water heater.
I love the idea. Get some silicon insulated copper wires (rated for 500F/250C).
I would get 3 zone (left, middle, right), and each zone would have 4 heating element (800w, 400w, 200w, 100w).
Then i would have the controller alternate between each zone so that for even heating and also which heating element to maximize the power.
I am not sure why you selected 12v, most new solar panels are 24V and 250w solar panel.
I really like this idea alot.
What about a water being your mass? A DC heating element in a dead hot water heater. You can circulate the water through a nice heavy salvaged radiator. If you want to incorporate it into a rmh, embed some metal coils into the mass.
When heated in air, most metals then oxidize quickly, become brittle, and break. However, when heated to red-hot temperatures, nichrome wire develops an outer layer of chromium oxide, which is thermodynamically stable in air, is mostly impervious to oxygen, and protects the heating element from further oxidation.
...which means it shouldn't rust in water.
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Insulated house and insulated hot water tank still lose heat, so the insulated wire will still eventually give its heat to its environment.
However the insulation that I was referring to was electrical insulation, so as to start and terminate the circuit at the controller.
So that the entire wire heat up evenly (aka the entire house heats up evenly, vs it closing the circuit 5inches in when it touches the thermal mass uninsulated.
Also by insulating the wire with this inert silicon, you are not giving high energy electron to catalyst unwanted chemical reactions in your thermal mass.
The insulation also buffer the transfer of heat to the thermal mass preventing thermal shock/rapid heating and cooling (expansion & cooling)
Another point that I like about this particular wire insulation is that it will not melt or catch on fire at 150F or 100F but instead at a crazy 500F.
I like that extra fire insurance.
I was literally just looking at wind turbines and specifically about dumping the excess energy into a mass, and needed a break then saw this topic. LOL
You can dump the excess energy still being produced from your PV setup or turbines into a heating element. This is new to me so I'm still learning about it, yet it seems fairly simple. Once the batteries are fully charged, and the PV panels or turbine is still producing power it needs to be cut off or dumped somewhere. I like the idea of using the power to heat a mass of some sort instead of just cutting off the excess power being generated.
I am going to set it up to help supplement the energy needed to keep a water heater tank full of hot water that I can use in the house for washing and such.
Good ideas. Me also. I've been doing loads of reading on this subject and energy recently as I'm not using my excess energy from my panels. I have a stainless steel beer keg that had ports welded into it for brewing batches of beer. The top was cut out and it can heat water or any liquid. A giant soup pot. I figured it could be a good "dump load" because it has a 1" port welded in for a water heater element. There are lots of uses for kegs. I would consider embedding one in the cob stove mass as a preheater for my domestic water.
If you invest in nice solar charge controllers and inverters they come with Auxillary switches that will activate and deactivate a relay at programmamble voltages. Then a conventional water heater or other AC load can be controlled.
There are lots of other ways to use the surplus energy using an inverter. Sure there is a little loss of efficiency. The Instapot programmable pressure cookers are great. Toss in the ingredients, set the settings, and you can time it to come on when your batteries are full. I've been using my bread maker in the afternoon. Also the eWave countertop convection oven is working good. It takes more power than the other two and if I don't time it just right I've needed to run a small generator a little to boost the inverter. But the convection oven cooks very quickly.
Also someone showed me a cool old school water heater element that never touches the water. It's in a steel tube and it might be ni chrome wire? It could be inserted into cob and easily pulled out of the tube and checked.
What are you finding out?
You can get these 12v heated car seats for quite cheap. Rather than permanently embed a heater into the cob, perhaps you could just lay it on a prepared, flat surface. These heated seats are cushioned, so require some sort of weight on them to really get the heat to propagate - so just make a separate, heavy block of cob that you can manually place over the heat pad to create a sort of sandwich. The performance might be improved by actually disassembling the heat pad and removing as much of the foam padding as possible.
Strangely, you actually can buy heat mats without the cushioning but they're inexplicably expensive.
EDIT: I'm an idiot. They're 'expensive' because you get 4 of them. Per unit they are very affordable.
Another idea - use a germination heat mat. However these all seem to run directly on AC, and I personally would prefer a purely DC system just to keep it simpler.
On a larger scale, many electrical grid systems use off-peak excess electrical energy to pump water up into dams located on the tops of hills etc. They then either use the water pressure directly, or it's converted back to electrical via a water turbine later.
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