Every Boy Scout or Girl Guide probably had the experience of building a simple solar oven: an insulated box, some aluminum foil, and plastic wrap, and voila! On warm, sunny, summer days, you can bake. On cloudy days, well, you need another plan. The redoubtable [Kris De Decker] and [Marie Verdeil] provide one, with this solar-electric oven over on LowTechMagazine.
Now, you might be wondering: what’s special here? Can’t I just plug a full electric range-oven into the inverter hooked to my Powerwall? Well, yes, Moneybags, you could — if you had a large enough solar setup to offset the storage and inverter losses, that is. But if you only have a few panels, you need to make every watt count. Indeed, this build was inspired by [Kris]’ earlier attempt to power his apartment with solar panels on his balcony. His electric oven is one of the things that stymied him at that time. (Not because cooking took too much energy, but because it took too much power for his tiny battery to supply at once.)
That’s why this oven’s element is DC, driven directly from the panel: there are no batteries, no inverter, and no unnecessary losses. The element is hand-made to match the solar setup, avoid an unnecessary electronic thermostat, and is sized to keep the oven from getting too hot. The oven itself is tiny, only large enough for a single casserole pan, but it does cover 90% of their cooking. A smaller oven is obviously going to need less power to heat up, but it also makes it practical to wrap it in oodles of insulation to reduce losses even further.
Indeed, between the 5 cm of insulation and thermal mass from the mortar-and-tile interior, when preheated by the specified 100 W panel, this oven can retain its cooking temperature well after sunset. Instead of needing a battery, the oven is the battery. It’s really quite elegant. That does require a certain mental adjustment as well: “cooking temperature” here is only 120°C (248°F), about a hundred F less than most recipes in our cookbooks. This hack is almost like a solar-powered cross between an oven and a slow cooker.
It’s undeniably efficient, but we can only imagine it would take some getting used to. Of course, so does running a solar-powered website, and LowTechMagazine has kept that going since 2018.
Dr. Stone reference? “It’s really quite elegant” 🤩
Kris does some really great articles, be nice to see some more of them highlighted here too.
I do like this design concept, though I’m not sure I can put up with more slow cooking in the house – when it smells like food for hours and you can’t eat it yet is just not something I’m good at dealing with…
I’d also suggest to be useful to me it would need to be double the size (I’m rather large so a single oven tray just isn’t enough fuel just for myself most of the time, though obviously depending on the food in it), so I think I’d be tempted if designing something like this for myself to double up the height, increase the thermal mass inside by making the internal side walls thicker with a shelf groove – probably take an extra day to actually get up to running temperature as I’d suggest its got to end up in the ballpark of double maybe triple the thermal mass of this design and of course a larger volume of air, but also should then last longer at cooking temperatures once the sun goes down.
I could have missed it while skim reading but I think the solar panel is attached directly to the heating element in this design, that wouldn’t generate maximum energy from the panel. If the heating element draws too much current the voltage from the panel will collapse and it will generate little power so the element needs to be undersized but that leaves energy on the table. Adding an MPPT to the panel’s output should help significantly, then your heating element can be as big as you like.
Otherwise though I really like this idea! The parts are relatively cheap and would work anywhere in the world.
Pmax is available when PV panel and Heating Element are impedance-matched (or is it resistance?)
The V-I curve of a solar panel is basically the V-I curve of a diode turned sideways. For any voltage below some threshold, you get basically constant current out. After the threshold, as you reach towards the photocells’ maximum voltage, the current drops exponentially.
When you’re matching a resistor to that curve, the V-I curve of the resistor is a straight line starting from zero volts and zero amps, going up at some slope. What you want to do is pick a resistance so the line hits the “knee” point of the solar panel’s curve, just where it starts to drop off, which is where volts x amps is maximized.
Now the difficulty is that the amount of constant current you can get out of a solar panel is dependent on sunlight, so the entire V-I curve of the solar panel shifts downwards, and your target knee point on the curve moves. You would have to dynamically increase the resistance depending on the level of light that hits the panel to keep at the maximum power point.
Small differences don’t matter, but when the panel is getting half the sunlight, the discrepancy between a fixed resistor and the panel might drop the efficiency of the setup to half, so you’re getting a quarter of the power instead.
This is effectively clipping off the energy you might collect early in the morning or late in the afternoon, though for the small amount of energy you can get from a 100 Watt panel overall, the loss may not be worth the MPPT tracker and the complications you need to add to the system to use it.
For example, 100 Wp solar over the average day in the average location in the northern hemisphere is about a 100 kWh per year. That’s worth about 10-15 dollars in average electricity prices. If you lose 20% of that, who cares? If you spend $50 to solve this issue, you’re probably operating at a loss because it takes 20 years to recover the difference.
For reference:
https://commons.wikimedia.org/wiki/File:Solar-Cell-IV-curve-with-MPP.png#/media/File:Solar-Cell-IV-curve-with-MPP.png
The amount of energy the oven retains depends on its temperature, but so does the amount of heat that leaks out. Suppose you did add MPPT and gain more energy throughout the day; to store that energy you would need to make the oven hotter, but then you’re losing energy faster through the insulation.
More efficiency in one part, less efficiency in the other – negating a great deal of your gains. Of course you could make the thermal mass bigger to better store that energy, but then it might not reach temperature on days when there’s less sun available. To make that work, you’d have to make it much bigger and store energy over multiple days, which gets you into other trouble and cost.
For the point of the oven, as long as it maintains high enough temperature to cook your food, anything extra is extra. You get no real return on investment by making it more complicated as it were.
For this size of an oven and use case, this is a local optimum of multiple combined factors. Other such points could be found, but you’d have to balance all the variables together, not just adding an MPPT controller and calling it a day.
This guy is all about simple, low tech and low embedded carbon solutions. As long as the heating element is well-matched to the solar panel, an MPPT really isn’t needed. The solar oven is only useful when the panel is at close to peak output, so it doesn’t really matter if its efficiency is low when the panel only has enough light to produce 20% power.
I guess you could argue that an MPPT would let you use the oven earlier in the day, as it would increase the efficiency while the sun is coming up.
‘well matched to the solar panel’ if the panel behaves consistently sure. In my area weather varies a lot and so does panel output. It could be 10%-100% of rated output on any given day so if I wanted my oven to work reliably I’d have to size it for the lower end and accept a reduced cooking temperature. Alternatively I could target maximum energy, accept a variable temperature and adjust my cooking times.
I think the writer has more consistent sunshine.
Not necessarily. In your case you’d have to add more heat mass and insulation, making the oven physically bigger, but then it would maintain a steady cooking temperature all through the day (useful at any time).
This is the problem with storing solar energy in batteries as well. The minimum level is to shift your energy from noon to night, but on cloudy days you don’t have any energy to shift, so you’re not getting enough – noon or night.
The next level is to average out the output, and the time scale jumps straight up from a few hours to a few days: a 24-fold difference in the number of batteries you need to buy.
Fortunately for us, the volume of bricks grows in the cube of linear dimensions, so getting 24 times more bricks in the box only requires you to make it 2.89 times bigger on the side. I mean, it’s still going to be a box the size of a small fridge, but at least it’s practically and economically feasible for most folks. Bricks don’t cost as much as batteries.
Their idea was to use heat bricks, so the oven actually reaches maximum temperature after the day, around late afternoon. Then it’s useful for another 4-5 hours, and then it cools down till the next morning.
If you increase the energy input, or add more insulation etc. the oven would become hotter sooner, but this is not necessary since the actual cooking happens later when the people return home from work and school.
If the oven became significantly hotter, above 200 C, there would be a risk of fire and the insulation materials (cork) would have to be replaced with something else. As the device is intended to operate unsupervised through the day, you don’t actually want it to become hotter than necessary to cook your food.
You could add a thermostat, but then you’d have to mind that the thermostat keeps working. Eventually it will not, and the oven either shuts down or overheats, probably while you’re not at home. If on the other hand the oven cannot reach unsafe temperatures even during summer heat waves, because it’s only reaching 120 C on an average day, you don’t have to do anything. It’s safe as it is.
These things are like the tyranny of the rocket equation. If you add more of something, you then have to add more of another thing, which demands you to add more of a third thing, and the whole thing blows out of proportion.
Or put a immersion heater into a Thermos bottle; then start your cooking with the preheated water. For a most simple solution the solar cell output shouldn’t be more than you can shed safely by ejecting steam, though.
But then you have to mind that the water doesn’t boil off, and cooking times become longer.
Much cooler – figuratively – than using solar panels to power a children’s “easy bake oven.”
I’m not sure how the modern version worked, but the pre-LED-lighting-era ones were just a light bulb in an insulating “oven” with a way to get the food in and out. It worked fine for things like brownies. The ones I remember ran directly on household mains electricity without any kind of transformer, but it would’ve probably worked fine if hooked directly to a battery of suitable voltage.
Hard to find a high-wattage incandescent bulb these days though (luckily!) The Easy-Bake oven was a nice demonstration of their staggering inefficiency (95% heat, 5% light).
Look for ‘heat bulbs’.
They are 100% efficient…at making heat.
The 5% light fraction turns into heat when it lands.
End run on rules didn’t work in the fascist EU.
Move.
They had their last laugh though, or did they?
You can still buy a 100 Watt lightbulb online, but you cannot find a fixture that can tolerate the heat. That’s because they changed the materials and designs to fit LED bulbs, so if you screw in an old-fashioned incandescent bulb, it starts to melt the plastic and smoke the paper.
I found that out the hard way when moving houses, because I have a stash of 75 Watt halogen bulbs that’ll last me forever. I wanted a new ceiling light fixture, but oh, the power rating on the new ones is only 40 Watts at best.
But that is easily remedied. Just need to pick a fixture with multiple sockets, each rated at 40 Watts. Even if they do manage to ban imports of incandescent bulbs bought online, they can’t ban 40 Watt oven bulbs because there’s no LED device that can operate at 300 C.