Maybe a solar oven? Unlike Li-ion batteries and solar cells, you can make a solar oven without mass production and using materials fabrication technologies available 2000 years ago. (You can make it cheaper if you use modern mirrors instead of polished bronze.) Typical solar cells are 16% efficient; your pre-medieval-materials solar oven will be about 50% efficient.
Just make sure you prepare your evening meal at noon in the Winter... Solar ovens are not a practical means of cooking unless you've dedicated your life to being off-grid and have few other obligations.
I admit that I haven’t! However, I think you’re talking about baking in a crappy solar oven, not baking in a solar oven in general. For example, http://www.solare-bruecke.org/infoartikel/Papers_%20from_SCI... describes a 3.4m² kilowatt prototype solar oven successfully used to cremate chunks of goat meat at temperatures of 800° to 900° in under an hour.
Larger solar ovens (usually known as “solar furnaces”) can reach 4000°, adequate not just for cooking or even cremation but also for firing terra-cotta, blowing glass, calcining lime, firing earthenware, making glass, firing porcelain, calcining portland cement, making borosilicate glass, casting aluminum, melting porcelain, purifying metals by zone melting, casting iron, smelting iron, casting steel, melting lime refractory, making quartz glass, casting basalt, boiling aluminum, vaporizing steel, vaporizing quartz, vaporizing lime refractory, melting tungsten, and sublimating graphite. Although people have been doing most of these with solar ovens for a while, especially the famous solar furnace at Odeillo, you can’t do all of these with Iron Age materials; apparently if you want to do anything useful with melted tungsten, you have to use Ta₄HfC₅ or similar.
Of course none of this works when it’s cloudy or dark, although molten-salt heat storage might be a usable solution to that even at a household scale.
Are you really saying that a metal bucket full of saltpeter (tree stump remover) is “exotic” compared to a giant electrical battery based on lithium, a metal which doesn’t exist in nature, violently explodes on contact with water, and needs a solid-state electronic charge controller to keep the battery from exploding when you charge it? Why would you say something like that?
By itself it's not exotic. Using it in molten state to cook things is exotic, yes.
Lithium clearly exists in nature. Elemental lithium might not, but they don't make the batteries from that either.
Molten salt also explodes on contact with water.
A charge controller is unnecessary. It's just a cheap way to improve efficiency. The bucket of salt would have a similar controller.
Also since price isn't a part of the equation for whether it's exotic, feel free to mentally replace the lithium ion battery with a lead acid one, made out of utterly boring materials and full of water.
You have some good points. I didn't realize that lithium-ion batteries didn't actually contain metallic lithium! I thought they did, just like nonrechargeable lithium batteries, but you're right. Thank you!
However, I don't think it's true that the charge controller is just a cheap way to improve efficiency; it's actually necessary to keep the battery from exploding like a firebomb when you try to charge it.
The bucket of molten salt has a much higher energy density for cooking purposes, although you're right that this only becomes an overwhelming factor if you consider joules per dollar instead of joules per kilogram. Lead-acid batteries are 0.17 MJ/kg; the particular molten saltpeter mix commonly used for energy storage is 0.16 MJ/kg just from the melting, plus another 0.3 or 0.4 MJ/kg from melting, depending on the temperature range. The practicality difference is that saltpeter costs US$1/kg, while lead-acid batteries cost about US$6/kg.
>However, I don't think it's true that the charge controller is just a cheap way to improve efficiency; it's actually necessary to keep the battery from exploding like a firebomb when you try to charge it.
You only need to worry about it when you're riding near the edge of what the batteries can handle. If your solar array outputs 4 volts and needs a minimum of 6 hours to mostly-charge the batteries, and your batteries are in a nice cool basement, I'm pretty sure you can just hook them all together. It wouldn't be ideal but I don't think you'd have any real fire hazard.
