The other day John was ruminating on the energy efficiencies of slow cooking using 'crock pots.' My memory was jogged. I once set my students at a design college the assignment of designing and making a working prototype product that would reduce cooking energy by at least a measurable 25%. "You expect us to design a totally new type of stove in three weeks?" they cried. "Impossible!" Only it wasn't.With a little positive encouragement, discussions on heat transfer, insulation and a visit to a materials reuse centre, the students eventually twigged that they didn't need to become instant engineers to achieve the requested result. As I recall, nearly all the teams passed, and most exceeded the 25% efficiency requirement. Many of their designs were variation of the venerable hay box, yet conjured direct from their own imaginations, without having seen such a device.
A 'hay box' is simply a sturdy container larger than its companion cooking vessel. It is lined with an insulative material that traps a lot of air (like hay or straw). Once the cook pot is placed inside an insulated lid is closed.
The heat contained in the food, especially water dense meals, continues to cook the food because the insulation reduces the ability of the contained heat to escape.
Yes, there is a time penalty, but a very significant energy efficiency gain. Plus your stove/oven is freed up for other pots and pans, and you can't burn meals that are slow cooked!
In its simplest form the idea is the ages old teapot cosy. A modern form for backpackers is the Optimus Terra cookset, (pictured right) which includes a close fitting nylon/neoprene warp for the camp pots. (Jetboil and AntiGravityGear do similar things)
A commenter ('kd') over at Instructables, where they show you how to make your own hay box (pictured left and mid) reckons he/she uses one tenth of energy used to cook a meal by the related method of adding grains and boiling water to a large thermos flask.