A modern thermal cooker uses the concept of the Haybox whereby placing hay or straw around a cooking pot of heated food the meal continues to cook without fuel.
The earliest dates from medieval times and consisted of two earthenware pots one within the other. The technique involved bringing food up to temperature on the fire as usual, but rather than allowing it to cook over the flame, the hot pot was placed in a 'nest' of hay, moss, dry leaves or other insulating material, in a box or hole in the ground, and covered. The heat in the pot was conserved for a considerable length of time, and the food inside would cook slowly, without the need for supervision. Both time and fuel were saved in this way, and foods that needed long slow cooking, such as pulses or tough meat, benefited from this technique. An example of this type of cooker was found by The Monmouth Archaeological Society's year-long excavation inside two shops at 69-71 Monnow Street, Monmouth. In 69 Monnow Street what maybe the first archaeological evidence of "cooking without fire". The remains of the pots that were found can been seen in The Peoples Collection WalesMedieval Instructions for cooking without fire taken from an Anglo-Norman manuscript in the British Library 
Take a small earthenware pot, with an earthenware lid which must be as wide as the pot, then take another pot of the same earthenware, with a lid like that of the first this pot is to deeper than the first by five fingers, and wider in circumference by three; then take pork and hens and cut into fair-sized pieces, and take fine spices and add them, and salt; take the small pot with the meat in it and place it upright in the large pot, cover it with the lid and stop it with moist, clayey earth, so that nothing may escape, then take un slaked lime, and fill the large pot with water, ensuring that no water enters the smaller pot; let it stand for the time it takes to walk between five and seven leagues and then open your pots, and you will found your food indeed cooked.
In the mid 1990s the thermal cooker was developed in Asia. It consisted of two stainless steel pots, one within the other. The inner pot, was used to bring the food to the boil and the outer, twin walled with a vacuum between the walls, was used as the container to keep the cooking process continuing.
To use the thermal cooker the food is put into the inner pot and brought to the boil, simmered for about 10 minutes and then placed in the outer pot for continued cooking. There are a number of thermal cookers on the market. Some use insulation material between the outer pot walls, others, use a vacuum.
Thermal cookers with two inner pots allow two items to be cooked at the same time, such as curry and rice. All thermal cookers are capable of cooking many dishes from soups to puddings. Cakes and bread can also be cooked by partly submerging the cake/bread tin in boiling water.
Vacuum flask cooking was introduced to the Asian market in the mid-1990s. The vacuum cooker (燜燒鍋) is a stainless steel vacuum flask. The flasks come in various sizes ranging from Script error: No such module "convert". in diameter and Script error: No such module "convert". tall. A removable pot, with handle and lid, fits inside the vacuum flask. The pot and contents are heated to cooking temperature, and then sealed in the flask. The flask simply reduces heat loss to a minimum, so that the food remains at cooking temperature for a long time, and cooks without continued heating. Note that the food is not cooked in a vacuum. It is cooked inside a vacuum flask. The hollow evacuated wall of the cooker thermally insulates its contents from the environment, so they remain hot for several hours.
Vacuum flasks appeal to Cantonese cooks because many Cantonese dishes require prolonged braising or simmering. When these cookers were first introduced in the US, they sold very quickly in the larger Asian supermarkets. The slow cooker is used for a similar purpose; but instead of minimising heat loss, sufficient heat is applied to the non-insulated slow cooker to maintain a steady temperature somewhat below the boiling point of water. A slow cooker allows any desired cooking time; the more energy-efficient vacuum flask must cook within the time taken for the food to cool below cooking temperature.
The historical equivalent of the vacuum cooker is the haybox, nowadays perhaps using more modern insulating material than the original hay or straw. This works on the same principle but has much poorer heat retention.
The pot is filled with food and heated to cooking temperature, usually to boiling. It is then sealed inside the vacuum flask for several hours. The flask minimises heat loss, keeping the food hot enough to continue cooking.
Some flasks also come with 2 or 3 smaller pots so you can cook 2+ dishes at once.
- long cooking means more tender meat
- minimizes fuel, energy use, CO2 emissions
- Saves water - less evaporation
- Saves food - no burning, no cleanup
- keeps flavour, nutrients in
- convenient - cooks while you are at work or sleeping
- can take while travelling or to picnics
- reduces smoke, odor, humidity, grease buildup in kitchen
- easy cleanup
- Safer - no power cord, outside not hot, spill-proof, reduces injuries
- Reduces toxic fumes which means less respiratory problems and other diseases, particularly in children
Thermal Cooker guide
If a large part of the cooking time is spent at temperatures lower than 60 °C (as when the contents of the cooker are slowly cooling over a long period), a danger of food poisoning due to bacterial infection, or toxins produced by multiplying bacteria, arises. It is essential to heat food sufficiently at the outset of vacuum cooking. 60 °C throughout the dish for 10 minutes is sufficient to kill most pathogens of interest, effectively pasteurizing the dish. The best practice is to bring briefly to a rolling boil then put the pot in the flask. This keeps it hottest longest.
Variants of thermal cooking, though a better name would be "heat retention" cooking:
- Wonderbag is an insulated bag to put around pots you already own.
- Haybox cooking uses hay or sawdust to provide the insulation around the pot.
A different kind of vacuum cooker is used in the candy manufacturing industry to cook candies at low air pressures.
Sous-vide cooking is cooking at temperatures under boiling, usually in a plastic bag and a temperature-controlled water bath in order to have tender, succulent flesh.
- Moorhouse 1987, 26; Hieatt and Jones 1986, 874, No.6
- Low temperature cooking#Tenderisation
- Vacuum Flask Cooking - incredibly efficient cooking - Green Cooking Wiki
- Thermal cooker guide | Smart Living: Cooking World
- A Practical Guide to Sous Vide Cooking
- Thermal Insulated Cookware Thermos Stainless Steel Cook & Carry System
- Green Cooking Creative ways to cook greener.