This article references a 2023 article about lithium battery costs with price at 750 EUR per kWh (approx 12V 100Ah) for 30 years life couting 3 replacements if I read correctly note 5:
Storage is booming in my very much coal-powered country with residential storage going from 115MWh to 670MWh over the course of 2024.
As it stands it's enough to power the country for about two minutes, but that's two minutes gained in a year.
In the commercial space there's a total of 4.3GW of peak output commissioned to be deployed before 2030. That will greatly help in shaving the afternoon increase in electricity usage.
Just as a note pressure cookers like the Instapot are rated at 1000wh but they don't use that all the time IIRC — only initially when building the pressure. In simple terms it's like using 1kw for 10 - 15 mins and not an hour. So on average very efficient.
Has someone measured the time for max usage and average usage? Could be a good alternative with a 1kwh lfp battery with a lot of juice left over after pressure cooking(no pun intended)
An alternative that I experimented with and found to be very usable is one solar panel, a small camping battery ("portable power station"), and an Instant Pot. The total cost is not super high. The Instant Pot is power efficient and can cook a lot of food at once. Since it's battery powered you can start any time the battery is charged.
I had fun doing this over the summer with a 100W panel, a 1kWh battery (1500W max output), and various cooking appliances: rice cooker, hot water kettle, instant pot, induction cooktop. On a sunny Pacific Northwest day I could charge the battery around 50%, more if I rotated the panel diligently. Rice cooker and hot water kettle (5L) would use about 40-50% of it per usage. So during the summer it was handling all of my power needs for those two appliances. It was always fun getting to full charge and frantically finding novel ways to "not waste" the sunlight. Charging my power tool batteries, etc. One time I even charged my EV 1% but that wasn't very practical.
Some other interesting things I learned: the battery passively eats about 5-10W, and on a cloudy day the solar panel would only get 10W during the day. So in the cloudy PNW winters it can't even maintain the battery let alone charge it. The inverter eats another 30-50W or so, so you have to turn it off when you're not using the AC outlets. My battery lets me separately power AC and DC (USB-A and USB-C) so I was charging devices via USB and not wasting energy powering the inverter.
I don’t understand why people go this direction when propane-powered appliances are so much more appropriate. Much cheaper, readily available fuel, a few minutes to “reload” vs multiple hours to recharge the battery.
Similarly, I think there’s a niche market for a propane-powered espresso machine.
I can charge any battery powered appliances like that at home. Propane is only sold per bottle, not every petrol station has them(in fact - majority don't), you have to rent the bottle each time, the thing is stupidly heavy and has various restrictions on storage(can't park a car with it in an underground garage for instance, my insurance does not allow me to keep it inside my garage etc). Battery powered is just so much easier unless you're using it commercially and go through a lot of gas I guess.
Very neat. I guess what you cant do is cook a lot of food. The food needs to be in there a while and adding cold food takes energy out of the system which is say 0.8kwh per day. So like 20 minutes of microwaving a day of power. You probably need 2 or 3 for a family.
Why do the images look like someone took pictures of dotmatrix printer output?
---
Update: for whatever it's worth, I just asked the Magic 8 Ball (Perplexity):
Low-tech Magazine uses the option to display images as dithered primarily to reduce the energy consumption and data load of their website. Dithering is an old image compression technique that reduces the number of colors in images to just a few shades of gray (black and white with four levels of gray), which dramatically decreases the file size. The black-and-white dithered images are then recolored via the browser’s CSS, which adds no extra data load.
This approach makes images roughly ten times less resource-intensive than full-color high-resolution images, which supports the magazine’s goal of having a low-energy, solar-powered website. However, some images, such as graphs or those with crucial color information, may become less clear under dithering, so the website offers the option to turn off dithering for individual images to reveal the original, heavier images. This balances energy efficiency with the need for clarity when visual information depends on color or detail.
Thus, the dithered image feature is both an energy-saving measure and a distinct stylistic choice that aligns with the philosophy of reducing the environmental impact of web usage while maintaining visual storytelling appeal.
I looked into this and the same photos could be compressed as nice color JPGs of the same size, with a lot more detail. But it would require more computing resources on the viewing end. I think this is their main target, hardware required to decode.
Not sure, JPEGs were fine in Netscape on my 60 MIPS 5x86-133 29 years ago. Mortification of the flesh to do penance for the sin of humanity tasting the forbidden fruit of knowledge may be their main target.
This feels (needlessly) performative. I get the idea, but the low quality images make it harder to understand what they are showing in the photos, which makes it harder to then reproduce their work.
I suppose the vast majority of users will not need the higher resolution, so perhaps have it be a toggle to get the higher-resolution when needed.
Even if insulated, isn’t this setup always heating the home? The insulation slows down the heat transfer but if I’m following correctly it’s basically running the heat element anytime the sun is up. I think this would be counterproductive in my climate as I’m cooling the home most of the year which is an even more energy intensive process
Great project and build. I wonder if using a small fan would not help energy transmission, which is eventually the goal of cooking. It may also make the oven work like it’s at higher température while not consuming much energy. It would discharge energy from the walls faster using convection in addition to radiation.
Plus, you can experiment with tray materials depending on what you want to cook.
Suggestions inspired by my experience with convection/classic oven and copper pizza plates
I wonder, how hard would it be to make a vacuum insulated oven like a big thermos flask?
I know this is “low-tech” but still think it might be possible to fabricate something even if it means spending some of your power budget maintaining the vacuum with a pump.
It's from Low Tech Magazine. A low tech solution is not surprising. Chasing 20 or 30% solar generation efficiency gains isn't really something all that relevant when you're building an oven that you're going to leave switched on all the time whether you're cooking or not.
You've gotta count the cost of your time as negative for that to be true. The author built multiple ovens from scratch here, and every cooked meal could be done sooner if the temperature were higher.
With the panels I have on my balcony I get about 200 watts even on overcast days. I would love to have appliances that were optimized for lower peak consumption. But many appliances like the fridge have low average consumption but high peak consumption so that while the panels on average produce about as much as I consume per day in reality I still have to buy power from the grid.
> But many appliances like the fridge have low average consumption but high peak consumption
You need an “inverter drive” fridge. It’s effectively soft-start and stays on continuously instead of justt a binary on/off.
Many options available at the consumer level.
Dunno if it gets angry if it senses that it’s browning out its own circuit or just reduces its draw (vs increasing current draw). But does mean it can dial up and down its draw on a continuous basis.
Jerryrigging a setup of used batteries and an inverter for my fridge sounds like a great way to save two euros a months in exchange for burning my apartment down :D.
At what point is it easier to just use a microwave? For all the effort to biuld an insulated box, a small microwave consumes maybe 600w, easily doable with batteries and an inverter. Microwaves also get hotter (useful if you want to do more than poach food at 120c).
Actually, forget the inverters. It turns out loads of 12v/24/48v microwave ovens exist for the RV market.
>useful if you want to do more than poach food at 120c
Slow cooking almost always gives you excellent results with vegetables, meat or fish. There is already a market of crokpots, sous vide and steamers, that I wouldn't dismiss as "poach food".
And for things like a pizza or crispiness in general, that need higher temperatures, I doubt you can get decent results with a microwave.
The usual 180°C / 350°F set has more to do with convenience (highest temperatures that doesn't burn the outside before the core is cooked) for cooking things as quick as possible, than tastiness of the food.
wonder if you could increase fire resistance by moving the grout layer so it's not touching the bottom cork except in key spaces with a more effective insulator?
• Use lime cement rather than portland cement for the mortar. Portland will be damaged over time by the 120° temperature and may spall explosively, causing injury, particularly if you embed nichrome heating elements in it.
• Similarly, don't use plaster of Paris inside the oven. It will also degrade over time at that temperature. Waterglass would work to fix tiles to wood but will foam up when heated if not crosslinked with polyvalent cations from, for example, chalk or iron oxide. Premixed refractory adhesives of this recipe are available from hardware stores in most countries.
• Use insulating fired-clay ceramic rather than cork or wool for insulation. It will need to be thicker but will not outgas volatile organic compounds into your food or degrade over time. Fireclay is not needed for this application; any normal pottery clay body should work. I used a ball clay body. Mix wet organic waste into the clay body at ratios between 1 and 3 parts organics to 1 part clay body before forming bricks and firing in an oxidizing atmosphere. The organics burn out in the kiln, which smells like hell during the firing. Suitable organic wastes include used yerba mate, used coffee grounds, cow manure, or sawdust made from untreated wood.
• Vermiculite or perlite from the garden store is another safe insulation option. A minimal amount of waterglass can hold it in place.
• Similarly, don't use chipboard (OSB) for the oven structure, as suggested in the article. It outgasses when hot, which it will be if you drape a blanket over the oven, as also suggested in the article. A very common type outgasses formaldehyde, which will condense in your food. You don't want to be eating formaldehyde for years. Similarly for most other composite wood products such as particle board and MDF, which at least the article does not specifically recommend. Masonite (high density fiberboard) should theoretically be fine, but I wouldn't risk it.
• Similarly, don't use hot glue as suggested in the article to attach the insulation. It's normally EVA with additives, but isn't specified for continuous operation close to its melting point, and might also outgas things you don't want condensing in your food for hours every day. I'd even be wary of wood glue. Waterglass with a polyvalent cation source should be fine.
• Mortar is porous, so if you want to be able to clean food off of mortar joints between tiles inside the oven, maybe seal it with waterglass. "Grout" is underspecified. You want a material that is food-safe at cooking temperatures; you don't want to go years eating whatever antifungal additives and plasticizers are in hardware-store grout for your bathroom floor, volatilized by oven heat.
• Electric cooking appliances should always include a thermal cutout device for minimal safety. These are cheap and available worldwide including by salvaging them from broken microwave ovens or electric kettles. They're very simple to use. No excuses. (This is mentioned in the "manual" as a "thermal switch", but only as an option, and not in the article.)
• Normally, high-temperature electrical connections should not be soldered as the "manual" suggests doing; they should use crimped or spring connections. Regular 63/37 electronics solder melts at 183°, which the oven as a whole might reach and which the nichrome could easily reach. It also contains lead, which is probably okay in an oven but seems a bit questionable to me, especially embedded inside mortar.
• Alternative methods of thermal energy storage and release seem like they might be worthwhile for this application, in order to lose less energy when not cooking, and in order to sustain an ideal cooking temperature for longer when cooking. Sensible heat storage like this has the disadvantage of constantly losing heat, which is an enormous disadvantage if the appliance is in your kitchen in a hot climate. Phase-change energy storage might be applicable, but I'm not sure what material melts in the right temperature range. TCES is probably applicable, but would need some design work. For example, plaster of Paris heats up to a usable oven temperature when hydrated, but keeping it from coalescing into a solid mass that is difficult to re-dehydrate conveniently. Phase-change or TCES materials might be portable enough to leave out on your balcony or roof terrace until it's time to cook—carrying them inside is not quite as convenient as flipping a switch, but it's far from the same level of hassle as building a wood fire.
• Storing sensible heat in a much thicker brick through which you can later circulate air might be another alternative, like electric night storage heaters. This would probably require a metal fan, but the amount of battery storage required to power the fan is minuscule compared to the amount you would require to power an electric heating element.
One correction. The article says,
"In contrast, an ISEC can be insulated on all sides, making it more energy efficient than a non-electric solar box cooker." This is arrant nonsense. Non-electric solar box cookers are typically over 50% efficient; retail solar photovoltaic panels are at best 24% efficient, so the efficiency of a solar cooking system that uses a retail PV module to capture sunlight is at best 24% minus the heat leakage through the insulation. (But to me it seems like nonsense to worry about efficiency in a context like this, since sunlight is free.)
Kris de Decker has an established practice of ignoring or denying indisputable factual errors like this when they are pointed out to him, so I fully expect this article to also remain permanently in error. I'd be delighted if he proved me wrong on this.
The "Hot Diodes!" paper from 02019 that de Decker cited (https://www.sciencedirect.com/science/article/pii/S235272851...) is promising. It points out that PV module voltage is nearly constant over a wide range of illuminance, so you can get much closer to the maximum power point with a nearly-fixed-voltage load like a string of diodes than with a linear resistance.
It occurred to me that this was a potential use case for obsolete CPUs, which are often conveniently removable from ZIF sockets in obsolete computers. Almost all chips have "clamping diodes" on every pin except their power rail pins, in order to keep those pins within 600mV of the power rails to prevent damage. If you connect the power rails backwards, the clamping diodes are forward biased and start to conduct; at least with old SSI chips and EPROMs, this can get the chip quite hot. Normally the clamping diodes are only rated for 0.5mA or so each but I'm guessing that often they won't actually melt until quite a bit more than that. In normal use, current microprocessors commonly draw over 100 amps at 1.2 volts or so, for which purpose they are packaged in ceramic with metal heatsink surfaces. I doubt you can get the whole 100+ amps through their protection diodes, but maybe you could get enough heat to cook with. Certainly the chips will withstand the 120° temperatures of this oven with no difficulty.
I should also mention that lime cement needs access to carbon dioxide from air to cure. If you sandwich it between ceramic tiles, it won't cure. Similarly for sealing it with waterglass: let it cure for a month first.
There are other candidate cements that are also safe for oven use, including some which can be made from common materials. Many that are too expensive for buildings because they cost over a thousand dollars per tonne would be fine for this purpose. Wood ash makes a perfectly adequate substitute for lime cement, for example. Some alternatives can cure without air; I found a very interesting recipe for "fire mud" in an Ayurveda textbook which is largely iron oxide and buffalo milk held together until heating with a little gum arabic, the reaction product presumably being a calcium iron phosphate similar to anapaite and some rodent teeth. Some industrial castable refractories use a similar formula, though aluminum phosphate is a more common backbone.
(Why would an Ayurveda textbook have a recipe for castable refractories, and why should we believe that it works? I haven't tried it, but its claimed use is for connecting ceramic vessels together to make distillation and calcination apparatus, and you'd hope that practitioners would notice if their distillation apparatus were unusable—even if they can't tell if the drugs they distill with it kill patients.)
This article references a 2023 article about lithium battery costs with price at 750 EUR per kWh (approx 12V 100Ah) for 30 years life couting 3 replacements if I read correctly note 5:
https://solar.lowtechmagazine.com/2023/08/direct-solar-power...
These days 1 kWh of LFP battery is 70-150 EUR depending on the brand, so far lower than referenced. And LFP cell lifetime is probably 20 to 30 years.
In a system it means that 1 kW of solar PV panels is about the same price as 1 kWh of battery, and similar lifetime.
Storage is booming in my very much coal-powered country with residential storage going from 115MWh to 670MWh over the course of 2024.
As it stands it's enough to power the country for about two minutes, but that's two minutes gained in a year.
In the commercial space there's a total of 4.3GW of peak output commissioned to be deployed before 2030. That will greatly help in shaving the afternoon increase in electricity usage.
There are also other types of batteries emerging from which options will emerge.
https://www.theguardian.com/australia-news/2024/feb/22/green...
Just as a note pressure cookers like the Instapot are rated at 1000wh but they don't use that all the time IIRC — only initially when building the pressure. In simple terms it's like using 1kw for 10 - 15 mins and not an hour. So on average very efficient.
Has someone measured the time for max usage and average usage? Could be a good alternative with a 1kwh lfp battery with a lot of juice left over after pressure cooking(no pun intended)
Rated at 1000 watt hours?
I used my Instapot ONE time as a hotpot thing, keeping a broth hot, almost cooking, for 2 hours with no lid. It died that day (thermistor).
This is fun, I'm curious to try it.
An alternative that I experimented with and found to be very usable is one solar panel, a small camping battery ("portable power station"), and an Instant Pot. The total cost is not super high. The Instant Pot is power efficient and can cook a lot of food at once. Since it's battery powered you can start any time the battery is charged.
I had fun doing this over the summer with a 100W panel, a 1kWh battery (1500W max output), and various cooking appliances: rice cooker, hot water kettle, instant pot, induction cooktop. On a sunny Pacific Northwest day I could charge the battery around 50%, more if I rotated the panel diligently. Rice cooker and hot water kettle (5L) would use about 40-50% of it per usage. So during the summer it was handling all of my power needs for those two appliances. It was always fun getting to full charge and frantically finding novel ways to "not waste" the sunlight. Charging my power tool batteries, etc. One time I even charged my EV 1% but that wasn't very practical.
Some other interesting things I learned: the battery passively eats about 5-10W, and on a cloudy day the solar panel would only get 10W during the day. So in the cloudy PNW winters it can't even maintain the battery let alone charge it. The inverter eats another 30-50W or so, so you have to turn it off when you're not using the AC outlets. My battery lets me separately power AC and DC (USB-A and USB-C) so I was charging devices via USB and not wasting energy powering the inverter.
There are now commercial ovens with LiFeO4 batteries in then that do basically just that.
Except they're stupid expensive.
I don’t understand why people go this direction when propane-powered appliances are so much more appropriate. Much cheaper, readily available fuel, a few minutes to “reload” vs multiple hours to recharge the battery.
Similarly, I think there’s a niche market for a propane-powered espresso machine.
Sunlight is free, nichrome nearly so. For these low temperatures any metal would work if you make it thin enough.
I can charge any battery powered appliances like that at home. Propane is only sold per bottle, not every petrol station has them(in fact - majority don't), you have to rent the bottle each time, the thing is stupidly heavy and has various restrictions on storage(can't park a car with it in an underground garage for instance, my insurance does not allow me to keep it inside my garage etc). Battery powered is just so much easier unless you're using it commercially and go through a lot of gas I guess.
[dead]
Very neat. I guess what you cant do is cook a lot of food. The food needs to be in there a while and adding cold food takes energy out of the system which is say 0.8kwh per day. So like 20 minutes of microwaving a day of power. You probably need 2 or 3 for a family.
Why do the images look like someone took pictures of dotmatrix printer output?
---
Update: for whatever it's worth, I just asked the Magic 8 Ball (Perplexity):
Low-tech Magazine uses the option to display images as dithered primarily to reduce the energy consumption and data load of their website. Dithering is an old image compression technique that reduces the number of colors in images to just a few shades of gray (black and white with four levels of gray), which dramatically decreases the file size. The black-and-white dithered images are then recolored via the browser’s CSS, which adds no extra data load.
This approach makes images roughly ten times less resource-intensive than full-color high-resolution images, which supports the magazine’s goal of having a low-energy, solar-powered website. However, some images, such as graphs or those with crucial color information, may become less clear under dithering, so the website offers the option to turn off dithering for individual images to reveal the original, heavier images. This balances energy efficiency with the need for clarity when visual information depends on color or detail.
Thus, the dithered image feature is both an energy-saving measure and a distinct stylistic choice that aligns with the philosophy of reducing the environmental impact of web usage while maintaining visual storytelling appeal.
Probably best to read the Low-Tech Magazine site About page explaining what they do as a solar-powered site:
https://solar.lowtechmagazine.com/about/the-solar-website
There is a button under the image that will show you the color version. As others mentioned, the site's about page explains the rationale.
You can also find this information in the web site itself: https://solar.lowtechmagazine.com/about/the-solar-website/#h...
It would seem strange if that was the purpose since the first photo on the website is ~40kb
I looked into this and the same photos could be compressed as nice color JPGs of the same size, with a lot more detail. But it would require more computing resources on the viewing end. I think this is their main target, hardware required to decode.
Not sure, JPEGs were fine in Netscape on my 60 MIPS 5x86-133 29 years ago. Mortification of the flesh to do penance for the sin of humanity tasting the forbidden fruit of knowledge may be their main target.
This feels (needlessly) performative. I get the idea, but the low quality images make it harder to understand what they are showing in the photos, which makes it harder to then reproduce their work.
I suppose the vast majority of users will not need the higher resolution, so perhaps have it be a toggle to get the higher-resolution when needed.
All the comments here underline the un-needlessness of this performative act.
Even if insulated, isn’t this setup always heating the home? The insulation slows down the heat transfer but if I’m following correctly it’s basically running the heat element anytime the sun is up. I think this would be counterproductive in my climate as I’m cooling the home most of the year which is an even more energy intensive process
All cooking appliances are heating the home if you view it in that lens. Fridge, normal cooktop, oven, airfryer, dishwasher.
Only when you cook, though, except for the fridge, which produces very little heat.
Yes this is exactly my point, I don’t cook ~12 hours a day every day. Usually less than 1 and even use the oven less often, most days not at all
Right. Although actually cooling is less energy-intensive—removing 3000 watts of heat only costs about 1000 watts of electricity.
Stick it in the balcony then?
That conversation with my wife would be comical
Great project and build. I wonder if using a small fan would not help energy transmission, which is eventually the goal of cooking. It may also make the oven work like it’s at higher température while not consuming much energy. It would discharge energy from the walls faster using convection in addition to radiation.
Plus, you can experiment with tray materials depending on what you want to cook.
Suggestions inspired by my experience with convection/classic oven and copper pizza plates
https://www.italiancookshop.com/products/hammered-copper-rou...
I wonder, how hard would it be to make a vacuum insulated oven like a big thermos flask?
I know this is “low-tech” but still think it might be possible to fabricate something even if it means spending some of your power budget maintaining the vacuum with a pump.
I think some simple MPPT circuitry would be a smart investment for this, rather than a fixed resistance connected directly to a solar panel.
It's from Low Tech Magazine. A low tech solution is not surprising. Chasing 20 or 30% solar generation efficiency gains isn't really something all that relevant when you're building an oven that you're going to leave switched on all the time whether you're cooking or not.
An MPPT would double the cost of this setup.
You've gotta count the cost of your time as negative for that to be true. The author built multiple ovens from scratch here, and every cooked meal could be done sooner if the temperature were higher.
With the panels I have on my balcony I get about 200 watts even on overcast days. I would love to have appliances that were optimized for lower peak consumption. But many appliances like the fridge have low average consumption but high peak consumption so that while the panels on average produce about as much as I consume per day in reality I still have to buy power from the grid.
> But many appliances like the fridge have low average consumption but high peak consumption
You need an “inverter drive” fridge. It’s effectively soft-start and stays on continuously instead of justt a binary on/off.
Many options available at the consumer level.
Dunno if it gets angry if it senses that it’s browning out its own circuit or just reduces its draw (vs increasing current draw). But does mean it can dial up and down its draw on a continuous basis.
batteryhookup is a great source for used batteries, it's pretty affordable to build your own setup to run stuff like a fridge
Jerryrigging a setup of used batteries and an inverter for my fridge sounds like a great way to save two euros a months in exchange for burning my apartment down :D.
>lower cooking temperature of about 120°C
At what point is it easier to just use a microwave? For all the effort to biuld an insulated box, a small microwave consumes maybe 600w, easily doable with batteries and an inverter. Microwaves also get hotter (useful if you want to do more than poach food at 120c).
Actually, forget the inverters. It turns out loads of 12v/24/48v microwave ovens exist for the RV market.
>useful if you want to do more than poach food at 120c
Slow cooking almost always gives you excellent results with vegetables, meat or fish. There is already a market of crokpots, sous vide and steamers, that I wouldn't dismiss as "poach food".
And for things like a pizza or crispiness in general, that need higher temperatures, I doubt you can get decent results with a microwave.
The usual 180°C / 350°F set has more to do with convenience (highest temperatures that doesn't burn the outside before the core is cooked) for cooking things as quick as possible, than tastiness of the food.
How does it compare to direct solar cooking?
wonder if you could increase fire resistance by moving the grout layer so it's not touching the bottom cork except in key spaces with a more effective insulator?
Some design alternatives:
• Use lime cement rather than portland cement for the mortar. Portland will be damaged over time by the 120° temperature and may spall explosively, causing injury, particularly if you embed nichrome heating elements in it.
• Similarly, don't use plaster of Paris inside the oven. It will also degrade over time at that temperature. Waterglass would work to fix tiles to wood but will foam up when heated if not crosslinked with polyvalent cations from, for example, chalk or iron oxide. Premixed refractory adhesives of this recipe are available from hardware stores in most countries.
• Use insulating fired-clay ceramic rather than cork or wool for insulation. It will need to be thicker but will not outgas volatile organic compounds into your food or degrade over time. Fireclay is not needed for this application; any normal pottery clay body should work. I used a ball clay body. Mix wet organic waste into the clay body at ratios between 1 and 3 parts organics to 1 part clay body before forming bricks and firing in an oxidizing atmosphere. The organics burn out in the kiln, which smells like hell during the firing. Suitable organic wastes include used yerba mate, used coffee grounds, cow manure, or sawdust made from untreated wood.
• Vermiculite or perlite from the garden store is another safe insulation option. A minimal amount of waterglass can hold it in place.
• Similarly, don't use chipboard (OSB) for the oven structure, as suggested in the article. It outgasses when hot, which it will be if you drape a blanket over the oven, as also suggested in the article. A very common type outgasses formaldehyde, which will condense in your food. You don't want to be eating formaldehyde for years. Similarly for most other composite wood products such as particle board and MDF, which at least the article does not specifically recommend. Masonite (high density fiberboard) should theoretically be fine, but I wouldn't risk it.
• Similarly, don't use hot glue as suggested in the article to attach the insulation. It's normally EVA with additives, but isn't specified for continuous operation close to its melting point, and might also outgas things you don't want condensing in your food for hours every day. I'd even be wary of wood glue. Waterglass with a polyvalent cation source should be fine.
• Mortar is porous, so if you want to be able to clean food off of mortar joints between tiles inside the oven, maybe seal it with waterglass. "Grout" is underspecified. You want a material that is food-safe at cooking temperatures; you don't want to go years eating whatever antifungal additives and plasticizers are in hardware-store grout for your bathroom floor, volatilized by oven heat.
• Electric cooking appliances should always include a thermal cutout device for minimal safety. These are cheap and available worldwide including by salvaging them from broken microwave ovens or electric kettles. They're very simple to use. No excuses. (This is mentioned in the "manual" as a "thermal switch", but only as an option, and not in the article.)
• Normally, high-temperature electrical connections should not be soldered as the "manual" suggests doing; they should use crimped or spring connections. Regular 63/37 electronics solder melts at 183°, which the oven as a whole might reach and which the nichrome could easily reach. It also contains lead, which is probably okay in an oven but seems a bit questionable to me, especially embedded inside mortar.
• Alternative methods of thermal energy storage and release seem like they might be worthwhile for this application, in order to lose less energy when not cooking, and in order to sustain an ideal cooking temperature for longer when cooking. Sensible heat storage like this has the disadvantage of constantly losing heat, which is an enormous disadvantage if the appliance is in your kitchen in a hot climate. Phase-change energy storage might be applicable, but I'm not sure what material melts in the right temperature range. TCES is probably applicable, but would need some design work. For example, plaster of Paris heats up to a usable oven temperature when hydrated, but keeping it from coalescing into a solid mass that is difficult to re-dehydrate conveniently. Phase-change or TCES materials might be portable enough to leave out on your balcony or roof terrace until it's time to cook—carrying them inside is not quite as convenient as flipping a switch, but it's far from the same level of hassle as building a wood fire.
• Storing sensible heat in a much thicker brick through which you can later circulate air might be another alternative, like electric night storage heaters. This would probably require a metal fan, but the amount of battery storage required to power the fan is minuscule compared to the amount you would require to power an electric heating element.
One correction. The article says, "In contrast, an ISEC can be insulated on all sides, making it more energy efficient than a non-electric solar box cooker." This is arrant nonsense. Non-electric solar box cookers are typically over 50% efficient; retail solar photovoltaic panels are at best 24% efficient, so the efficiency of a solar cooking system that uses a retail PV module to capture sunlight is at best 24% minus the heat leakage through the insulation. (But to me it seems like nonsense to worry about efficiency in a context like this, since sunlight is free.)
Kris de Decker has an established practice of ignoring or denying indisputable factual errors like this when they are pointed out to him, so I fully expect this article to also remain permanently in error. I'd be delighted if he proved me wrong on this.
The "Hot Diodes!" paper from 02019 that de Decker cited (https://www.sciencedirect.com/science/article/pii/S235272851...) is promising. It points out that PV module voltage is nearly constant over a wide range of illuminance, so you can get much closer to the maximum power point with a nearly-fixed-voltage load like a string of diodes than with a linear resistance.
It occurred to me that this was a potential use case for obsolete CPUs, which are often conveniently removable from ZIF sockets in obsolete computers. Almost all chips have "clamping diodes" on every pin except their power rail pins, in order to keep those pins within 600mV of the power rails to prevent damage. If you connect the power rails backwards, the clamping diodes are forward biased and start to conduct; at least with old SSI chips and EPROMs, this can get the chip quite hot. Normally the clamping diodes are only rated for 0.5mA or so each but I'm guessing that often they won't actually melt until quite a bit more than that. In normal use, current microprocessors commonly draw over 100 amps at 1.2 volts or so, for which purpose they are packaged in ceramic with metal heatsink surfaces. I doubt you can get the whole 100+ amps through their protection diodes, but maybe you could get enough heat to cook with. Certainly the chips will withstand the 120° temperatures of this oven with no difficulty.
I should also mention that lime cement needs access to carbon dioxide from air to cure. If you sandwich it between ceramic tiles, it won't cure. Similarly for sealing it with waterglass: let it cure for a month first.
There are other candidate cements that are also safe for oven use, including some which can be made from common materials. Many that are too expensive for buildings because they cost over a thousand dollars per tonne would be fine for this purpose. Wood ash makes a perfectly adequate substitute for lime cement, for example. Some alternatives can cure without air; I found a very interesting recipe for "fire mud" in an Ayurveda textbook which is largely iron oxide and buffalo milk held together until heating with a little gum arabic, the reaction product presumably being a calcium iron phosphate similar to anapaite and some rodent teeth. Some industrial castable refractories use a similar formula, though aluminum phosphate is a more common backbone.
(Why would an Ayurveda textbook have a recipe for castable refractories, and why should we believe that it works? I haven't tried it, but its claimed use is for connecting ceramic vessels together to make distillation and calcination apparatus, and you'd hope that practitioners would notice if their distillation apparatus were unusable—even if they can't tell if the drugs they distill with it kill patients.)