February 24, 2017

More Thermal Cookery

The old stove on left features a "thermodome" that lowered over the cooking pot after the heat was turned off.

OK, after our recent experiment, I admit I am obsessed with thermal cookery. Or to be more exact, I have always been obsessed with energy efficiency.

The efficiency of a modern electric cook stove has a thermal efficiency of about 15%, meaning that 85% of the energy is wasted. This is partly because converting fossil fuels to electricity produces an energy efficiency of 20-45%, depending on the power plant.

There is much room for improvement, since this is about the same efficiency of an open fire, a truly ancient technology.

One way we can improve on the efficiency of the cooking process is by using thermal retention which prevents heat loss to the point that food continues cooking without additional energy.

Using thermal retention to cook foods can save up to 80% of the energy required for normal cooking, depending on the thermal cooker and heat source used, and on how long it takes to get the hot pot into the cooker.

An electric stove, when used with a thermal cooker, doubles its efficiency to about 26%, which still is not that good, but much better than the electric stove alone.

This old stove shows a "thermowell", a built-in thermal cooker. With the pot shown, one could cook three different foods in one pot. 

Around the early 1900s, kitchen stoves were designed with heat retention features, and were called "fireless cookers". Some old stoves had insulated bells that lowered over cooking pots to retain heat (marketed as "thermodomes"). Others had thermal cookers or insulated "wells" built right into the cooktop that pots could be lowered into for thermal cooking.

One such stove boasts of a "New and Improved Thermowell" that does one hour of cooking with only 10 minutes of energy use. Another was sold as the stove "that cooks with the gas turned off".

A 1950s era electric stove that still featured a built-in thermal cooker.

As power became more abundant, such efficiencies were lost. The good news is that we can re-introduce this energy saving technology quite easily in our modern kitchens with materials we may already possess.

Sample Cooking Times In A Thermal Cooker

White rice: 5 min on heat, 1-2 hours in cooker
Brown rice: 10-15 min heat, 2 hours in cooker
Potatoes: 5-10 min heat, 1-2 hours in cooker
Creamed soups: 2 min heat, 1 hour in cooker
Dried beans (soaked): 10-15 min, 3-4 hours in cooker

It is hard to imagine a more cost-effective technique to lower energy use than a homemade, DIY thermal cooker. See our post about trying out our first DIY thermal cooker HERE.


  1. Anonymous2/25/2017

    That 1950s stove pictured is so cool - the built in thermal cooker, the extra space to put down a hot pan. I've always loved the look of the British Aga cooker, and the elements are seen in these 1950s and earlier stoves, with their many ovens and warming compartments. One thing I want to learn to do is use my rice cooker to cook more things than just rice. I've seen mentions in articles about that and I would figure using the rice cooker is less energy use than my stove top or oven. Many ways to reduce energy use. Thanks for the research, its great!-- Mary

    1. Mary,

      Rice cookers and slow cookers are more efficient than stove top, but not as efficient as thermal retention set ups. It does take some getting used to, though. I have not tried to make rice in mine yet. Or porridge oats. Or yogurt. Lots of possibilities, and we are still finding new ways to reduce our energy use.

  2. My (now ex) mother-in-law had a stove with a deep well in it back in the 70's. I would have loved to have taken it when she bought new, but we had gas hookup instead of electric. Just goes to show that the old ways are useful and sometimes much better than the new.

    1. Marla,

      We sure have been tricked into thinking that anything new has to also be better. Not so. That is neat that you have seen first hand a stove with a thermal well. I would like to have one now. But the camping cooler works just fine, and I already have that.

      Hope your week is great. It will be March by the end of it!


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