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C. Letellier wrote:It does cause me to wonder how much a vacuum thermos could be scaled up? If the glass was say 1/4" thick. Eliminate most of conduction and convection and if the outer layer is mirrored on the inside most of radiation too. Put that in a heavily insulated conventional shell and 100 watts would probably be enough. Put a large slab of some heat storage material in the over over the element.(slab of granite counter top maybe?) If you needed excess heat simply taking that or another slab to trade out to an optical concentrator would let you take the heat inside.
Or if the goal was a simmer pot a bit of piping and a evacuated tube solar collector would let you simmer clear to boiling with a completely passive, active collector.
That said if you can afford enough solar panels and good enough insulation it will certainly work. Probably ideally you would like a lot of mass and one oven you could get really hot and another oven that had a damper to the first oven so you can run a cooler oven to better utilize and store heat. One thing to remember is an oven will need a vent or otherwise you will end up with things like steamed bread.
One other thing to think about is pollution created making the solar cells vs pollution created doing some other form of cooker.
I have wondered on 2 answers in my thinking. 1. A heavily insulated oven tapping heat from a stratified water storage tank so it was always 140 degrees. Add a second insulating door that is in place most of the warm months. That way slow cooking in the oven is always there. When it is needed as an oven the added heat would be much smaller by 70 degrees. The dream over should be wood fired, gas and electric. Still dreaming there. 2. Thinking of an built in crock pot or set of crock pots running off the same hot water tank. There again slow cook. Easier to insulate. And a built in heater would let it get hotter at need. If it was heavily insulated with a heavily insulated lid it would use very little extra power.
William Bronson wrote:There are vacuum insulated pots with 12 VDC heating coils built in.
They are designed to keep foot hot over many hours.
I looked into them as an alternative to fast food ,cold meals or hunting for a microwave while on the road.
I think you could actually cook in them over the course of 6 or more hours.
Constant heat added to an insulated environment over time really adds up.
When I was an electrician you would sometimes see a hot box in use on big jobs.
Just a box lined with foil with a light bulb in it, it allowed 20 guys to heat up their meals, something a single microwave couldn't do when you only had 1/2 hour lunch.
Sure, they had to put the food in in the morning, but it worked, and took very little energy.
I just acquired a scrap electric water heater use as a "rainbarrel".
After stripping the insulation and filling it, I'm pretty sure there wasn't anything wrong with it.
I wonder if it could be used as the shell of a photovoltaic solar oven.
Robert Yaklin wrote:The electric cooking I do with solar is NOT direct DC but standard AC skillet and/or slow cooker(s). I do find that on cloudy days I can cook this way whereas using the solar oven on cloudy days is pretty much an exercise in futility. Interesting concept though of going direct DC. The cloudy day thing would apply for DC cooking as well I would think although it requires sufficient (excess?) solar panel wattage. In other words if I had only a single 300 watt panel it would not likely be much use on a cloudy day, but having 9 of the same wattage panels would likely work fine. On a sunny day with only one 300 watt panel one could likely be fine with a cooker that uses 300 or fewer watts. On darker cloudy days with my setup, I may only be able to use the slow cooker.
William Bronson wrote:OK, I just realized I had shared my reaction but failed to offer any useful advice.
To avoid the problem of water in a coil running through a mass that's hotter than the 100c, you can set an tank of water into the mass and run a coil through that.
Leave that tank unpressurized and open to the atmosphere and add a fill valve to keep it topped up.
You will lose water and energy to the atmosphere through evaporation, but a tall enough column (pipe for steam exhaust) would give the steam time to condense and return to the tank.
Alternatively keep an empty tank in the heated mass and add water when you need it.
It will flash into steam, which is very dangerous.
Don't close in the tank, leave it open to the atmosphere.
Instead run a large (huge!) diameter pipe from the tank through cooler space to a second(header) tank, set as high as possible where the condensed water can gather.
This set up will reduce the amount of energy lost on an ongoing basis.
Kenneth Elwell wrote:Sean, I get the idea of the "no moving parts", and that wires don't freeze. I also understand that the cost of PV panels has dropped quite a lot over the years.
Here's what I'm not quite getting: Solar PV is something like 15% efficient, and Solar Thermal is something like 85% efficient, so why use PV to make heat?
Do you have another need for that much electric that is a factor? Like a workshop or car charging? And then, why not store the electricity in batteries? That way you have the option to use electricity for anything: lighting, electronics/appliances, even for heat/hot water.
Have you looked at what has been done at Tamera in Portugal? They are using hot vegetable oil as a working fluid in a solar thermal system, with an insulated storage tank. That allows for >100C temps. (200C I think) and can be used on demand in a jacketed kettle to boil water for cooking, and for steaming.
Kenneth Elwell wrote:Sean, the article is about solar thermal collection, not PV. It would work with PV, perhaps in an off-grid scenario, but there’s lots of reasons to maybe do it differently.
The top one for me is lesser efficiency of PV vs. thermal when heat is the goal.
Next would be net-metered grid-tie PV, where over production (summer in Northern hemisphere) could be “saved” as credit with utility company or stored as cash payments (and used for anything you wish , even heating).
Then there’s the time scale... these systems are generally for balancing production and loads for electric utility plants in the scale of hours and days, not months. And they’re working with steam.
There are underground Annualized Thermal Inertia systems that use a heat pump for storage and delivery. If you have surplus PV it could power the heat pump, and you’d get cooling in the summertime as well. (Not just a stack of hot bricks)
