While modern cooking stoves are convenient, when it comes to energy use they leave a lot to be desired. As we have seen in the previous article, the thermal efficiency of an electric hob does not exceed that of a conventional open fire. In both cases almost 90% of the primary energy is lost during the cooking process.
Cooking food could be achieved in a far more energy efficient way, especially if the cooking pot itself is insulated. This is the principle behind the fireless cooker, a well-insulated box that keeps food simmering with only the heat of the cooking pot itself. A fireless cooker doubles the efficiency of any type of cooking device because it shortens the time on the fire and limits heat transfer losses.
In the early twentieth century, fireless cookers were common additions to western kitchens, similar to the refrigerator or cooking stove. Some models even integrated fireless cookers with gas or electric hobs. These functioned by lowering an insulated hood over the cooking pot once the heat had been switched off.
Image: a fireless/gas cookstove from the 1920s.
In the previous article, we found that cooking food is an incredibly inefficient process. The thermal efficiency varies from 13% for electric hobs to 23% for gas hobs, and from 5 to 25% for open fires and crude biomass stoves. Cooking stoves also produce considerable levels of indoor air pollution -- especially in developing countries but also in the modern kitchens of wealthy households. These results show that in both developed and developing nations, energy efficiency and pollution levels in the cooking process can stand to be improved.
The proposed strategies to tackle these issues differ for poor and rich countries. In poorer countries, most efforts concentrate on improving energy efficiency and lowering the indoor pollution caused by biomass stoves. Rocket stoves, for example, can achieve thermal efficiencies of 45% or more, with only about half of the emissions. However, any type of improved biomass stove still requires wood for fuel, and continues to produce air pollution.
In the western world, the proposed solution is a more widespread use of the best available technology, such as electric induction stoves. However, these devices obtain a thermal efficiency of only 15%, which means that 85% of the energy generated for cooking is wasted. In summary, the present-day approach to improving the sustainability of cooking stoves is not very ambitious.
Why is cooking so inefficient?
To further improve upon the efficiency of cooking, we have to take a closer look at where the greatest energy losses are incurred. For electric hobs and microwaves, the most significant waste of energy can be attributed to power conversion losses. Converting fossil fuels or biomass into electricity produces an energy efficiency level of 20-45% depending on the power plant, which explains why electric stoves are among the least efficient cooking devices.
Gas stoves have the largest heat transfer losses of all modern cooking stoves. Picture: Ashley Bischoff @ Flickr.
The second most significant energy loss for electric stoves, and the most important one for all other cooking stoves, occurs during the transferral of heat from the cooking hob to the food in the cooking vessel. Not all heat produced by the fire reaches the cooking pot, and heat is lost through the walls and lid of the pot, as well as through escaping steam.
The cooking process is similar to heating an uninsulated building with all the doors and windows open
In order to bring water to a boil and to keep a dish simmering, the cooking stove has to continuously compensate for these heat transfer losses. This is similar to heating an uninsulated building with all the doors and windows open. Even the most performable stoves now available -- rocket stoves and wood gas stoves -- only achieve a maximal thermal efficiency of 40-50%.
Obviously, we could do better. With regards to potential improvements in cooking sustainability, four technologies deserve further attention: pot skirts, fireless cookers, pressure cookers, and solar cookers. While each of these is a solution in themselves, they are especially advantageous when used together.
A simple way to start improving cooking efficiency is by using a pot skirt. This device increases heat transfer efficiency between cooking stove and cooking pot. They work with all but the electric stove. A pot skirt is a vertical sleeve, usually of metal, that forces the hot gases from the fire to flow closely around the sides of the pot. Skirts can be insulated on the outside, which brings the additional benefit of decreasing heat losses from the sides of the pot.
A pot skirt. Picture: Ecozoomstove.
A pot skirt also reduces the effects of fire assymetry, which can be a problem for both outdoor and indoor cooking. Experiments in rooms with virtually no crossflow of air can show highly assymetric flame patterns, which decrease heat transfer efficiency. Tests on three types of stoves -- an open fire, a biomass rocket stove and a gas rocket stove -- showed that pot skirts can improve heat transfer efficiency by about 10-20% for a rocket stove, and by about 30% for an open fire. Since heat transfer losses are the main inefficiency for these types of cooking stoves, this is not a bad start.
The rather well-known pressure cooker takes a different approach. A pressure cooker is a sealed vessel which reaches higher water temperatures because of added steam pressure, making it more energy efficient and able to cook food faster. It is either operated electrically (as a standalone device or on an electric stove), or used in combination with a gas, biomass, coal or solar stove. The pressure cooker lowers both power conversion losses (because of shorter cooking times) and heat transfer losses (because it completely eliminates heat loss through evaporation).
Scientific studies on the energy efficiency of a pressure cooker could not be found. Manufacturers usually advertise energy and time savings of up to 70% when compared to cooking in a normal pot. If we assume these figures to be correct averages (which is probably overly optimistic), then the thermal efficiencies of cooking stoves start to look more promising.
If a pressure cooker is used on an electric stove, the cooking process would reach a thermal efficiency of 22%, which brings it on par with a well-tended three-stone fire. The combination of a gas stove with a pressure cooker would achieve a thermal efficiency of 39%, while the combination of a well-tended three-stone fire with a pressure cooker would obtain 40% thermal efficiency. The best result is achieved via the combination of pressure cooker and rocket stove, which is 62% effective. 
While we can see marked improvements with the pressure cooker, these vessels still lose heat through the walls and lid, and these losses are considerable. There are also heat transfer losses between the stove and the pot if the device is placed on a hob. However, if we bring food to a boil and then quickly put the pot in a well-insulated box, the heat transfer energy losses can be minimized to such an extent that the cooking process continues, without any further energy input.
Fireless cooker in a basket. Picture: Solar Cookers International.
This is the principle of the "fireless cooker" or "heat retention cooker", which is best described as the passive house concept applied to cooking. A passive house is a well-insulated building that requires little energy for space heating of cooling.
The fireless cooker is the passive house concept applied to cooking
The fireless cooker is the key to efficient cooking in poor and rich countries alike. It almost completely eliminates heat transfer loss and reduces cooking time on the fire or hob substantially, thus addressing the two largest energy losses in the cooking process. Fireless cookers can lower energy use by more than 80%, but the precise savings potential depends on many factors. Such factors include the insulation material, the design of the fireless cooker, the required cooking time of the dish, the food itself, and the swiftness with which the cooking pot is moved from the stove to the fireless cooker.
A classical fireless cooker. Picture: Natuurlijk Bewaren.
The Partnership for Clean Indoor Air (PCIA) has measured the energy savings of fireless cookers. In their test of 18 types of solid fuel cooking stoves, the energy savings of the fireless cooker amount to an average fuel reduction of 50%, which is the number we will use in this article.
If we combine an electric stove with a fireless cooker, we can double its thermal efficiency. Combined they reach 26%, which is still not very impressive, but at least achieves a higher energy efficiency than a gas stove alone. A gas stove used in conjunction with a fireless cooker obtains 46% thermal efficiency, while a well-tended fire with a fireless cooker attains 50%. A combination of a rocket stove with a fireless cooker is more than 80% efficient. 
These numbers could be further improved if we combine the fireless cooker with the pressure cooker. If we use a pressure cooker to bring food to a boil and then put the pressure cooker into a fireless cooker, we can cook at 40%-90% efficiency, depending on the cooking stove used. This compares to a maximum of 23% for our western cooking stoves, and 40% -- or at most 50% -- for improved biomass stoves.
In its simplest form, the fireless cooker is a wooden, metal or plastic container filled with straw, old clothes, styrofoam, paper or any other insulation material. It can even be a cooking pot wrapped into a sleeping bag. Usually 5 to 10 cm of insulation is applied on all sides, the upper layer often in the form of an easy-to-handle, scaled-down mattress or pillow. A more cost-effective technique to lower energy use is hardly imaginable.
Fireless Cookers in History
In some parts of the world, the concept of the fireless cooker has been known about for centuries. During the middle ages, Europeans used "hayboxes" and holes in the ground filled with straw. American Indians took a slightly different approach to limiting heat transfer losses by enclosing the heat source (fire-heated stones or clay balls) within the cookware. Some American Indian groups used "cooking baskets" for this purpose; tightly woven watertight baskets, which could be coated with clay for insulation. Others stone-boiled soups and stews in a hole that they dug in the ground, lined with animal hide.
The fireless cooker became popular in the western world in the years between the 1890s and the 1930s. A Norwegian "self-cooking apparatus" received an award at the 1867 World Exhibition in Paris. It was a simple yet elegant container with four layers of felt for insulation.
During the first decades of the twentieth century, the fireless cooker became a permanent fixture of many American and European households
Initially, the heat retention cooker was mainly used to make food more portable for use by people on the move such as fishermen, hunters and soldiers. Amsterdam trams (streetcars) had them onboard for the driver. However, during the first decades of the twentieth century, the fireless cooker also became a permanent fixture of many American and European households, an appliance often found next to the cooking stove.
The best models were made entirely out of metal lined with mineral wool insulation, and kept the cooking pot and insulating material separated for easy cleaning and durable construction. These devices were also used for cooling.
Another innovation from the early twentieth century was the fireless cooking gas range; a combination of gas stove, gas oven and fireless cooker. The device obviated the need to move cooking utensils from the hob to the fireless cooker by making use of insulated hoods -- "thermodomes" -- that could be lowered over the burners. The food was brought to a boil, the gas was shut off, and then the pot would be covered up by the thermodome. The inverted receptacle was raised and lowered with the assistance of a counterbalance.
Interestingly, the hood was partially lowered while the gas was burning. The interior thus became hot from the heat which would otherwise escape, ensuring that plenty of retained heat would be available for cooking after the gas was turned off. Later versions worked completely automatically, shutting off the gas and lowering the hood at a preset time.
Another attempt to merge fireless cookers with cookstoves was the deep well cooker (also known as the "thrift cooker"). Old ranges, both gas and electric, sometimes had one of their burners sunk into a hole in the cooktop. This "well" had heavily insulated sides and enclosed a specifically designed pot with an insulated lid and no handles on the sides.
A deep well cooker from the 1950s. Source.
With some models, the burners could double as a surface unit. Although they were not really fireless cookers -- the pot was on a low fire -- deep well cookers reduced heat transfer losses considerably.
Improved Fireless Cookers
The use of heat retention cookers declined in the 1930s, and then resurfaced during World War Two and the oil crises of the 1970s. Today the fireless cooker is mainly promoted for use in developing countries. NGO's that have introduced the technology are -- among others -- Practical Action, HELPS International and Solar Cookers International. The designs made for developing countries differ, from the insulated baskets of Solar Cookers International to the styrofoam insulated Wonderbag or ONIL.
Although heat retention cookers can be made cheaply with natural and locally available resources, they could just as well be mass-produced using more sophisticated materials. While it makes the devices less sustainable in production, plastic has made fireless cookers more practical, and superior insulation materials have improved their performance.
The thermal cooker is a compact, high-tech version of a fireless cooker
An important innovation in the western market is the so-called thermal cooker which appeared in the 1990s. The device is based on vacuum technology: the principle behind the thermos flask. The thermal cooker is comprised of a removable cooking pot, with handle and lid, that fits inside a vacuum flask which has a diameter ranging from 20 to 50 cm. The cooking pot is heated on the cooking stove (regardless of type) and then moved to and sealed in the flask. Find an exampe here.
A thermal cooker. The high-tech version of the fireless cooker. The cooking pot (left) is put on the fire and transferred to the vacuum flask (right) once to food has been brought to a boil. Picture: Thermal Cookware
In a thermos flask or thermal vacuum cooker the space between the dual walls of a cylinder is completely evacuated. With virtually no molecules of gas available, heat transfer by conduction and convection are almost eliminated and therefore thermal conductivities are extremely low. Insulation thickness is about one-seventh of that of rockwool and one-third that of petrochemical insulation foams for similar thermal resistance.
The result is a much more compact fireless cooker, which could easily become part of any western kitchen as a standard, built-in device next to the cookstove. Smaller thermal cookers could be used to make hot food portable. Vacuum insulation is also available in the form of insulation panels, which you could use to build a compact yet superinsulated fireless cooker yourself. (But although home production is possible, one would have to adapt to available sizes -- it's not possible to cut the panels as this would destroy the vacuum).
All too often, fireless cookers are pictured as an emergency device aimed at campers, refugees or survivalists. However, a relatively simple device that can double the efficiency of whatever cooking technology you have at your diposal deserves more credence than that. The fireless cooker should be a commonplace item in every kitchen. Aside from its energy saving potential, its use in the western world would also encourage its acceptance in the developing world.
In the beginning of the twentieth century, time savings were the main sales argument for fireless cookers. This seems odd, because the average cooking time doubles compared to the traditional cooking process. Fireless cookers do afford the cook more time, however, by reducing the amount he or she spends in front of the stove or fire.
Once the cooking pot has been transferred to the fireless cooker, it requires no further attention and the cook is free to do something else, even if it's outside the house. It's impossible for the food to boil over, and there is no fire hazard to keep an eye on. Furthermore, a dish can stay hot for up to 6 hours or more, so the timing of the cooking process becomes more flexible.
A fireless cooker also increases the capacity of a cooking stove, whether it runs on electricity, gas, coal, wood or solar energy. You can put a new dish on the fire while the other one is simmering in the fireless cooker. With every fireless cooker you add, the capacity of the cooking stove increases further.
Solar Cookers + Fireless Cookers
This feature is especially interesting in combination with a solar cooker. A fireless cooker increases the capacity of a solar cooker, but it also allows you to cook if there is not much sun available. When a fireless cooker is used to complete the cooking process, a solar cooker requires as little as half an hour of sunshine to cook dinner.
Fireless cookers essentially act as batteries, storing energy in hot food. They greatly increase the usefulness of solar cookers, making them appropriate even on cloudy days and in countries where there is less sunshine. Furthermore, the combination of solar cooker and fireless cooker allows you to prepare a meal that can be served hours after sunset.
When viewed alongside all other cooking appliances, the solar cooker is the ultimately sustainable stove. It requires zero fuel and produces zero air pollution. Even if gas or solid fuel stoves could reach a thermal efficiency of 100%, they would still require resources like wood or coal, and they would continue to produce air pollution. The solar cooker is the only cooking device that doesn't face these issues.
The fireless cooker greatly increases the usefulness of a solar cooker
There exist many designs for solar cookers. The simplest type is the solar box cooker, which is not much more than an insulated box with a glass plate on top. The glass allows solar radiation to enter, heating up the interior, while the insulated walls decrease heat loss. There is not much difference between a solar box cooker and a fireless cooker, and both appliances could be merged into one design. Solar box cookers can also work under cloudy conditions because they are able to exploit diffuse radiation.
Parabolic solar cookers incorporate a more complex design, and use curved mirrors to focus solar radiation on a focal point. They work faster, produce higher temperatures, and have the ability to fry, roast and barbeque food. They are, however, more challenging to build, they require frequent orientation to the sun, they can be dangerous, and they only work in clear weather conditions. Panel cookers -- such as the CooKit -- incorporate elements of both box and parabolic cookers.
A parabolic solar cooker. Picture: Solar Cookers International.
The solar cooker is not the only way to take advantage of solar energy for cooking. Electric cookstoves or microwaves run by electricity from PV solar panels can also be considered solar powered cookstoves. However, converting solar energy into electricity to convert it to heat in order to boil water is needlessly complex, energy inefficient, and very expensive compared to taking advantage of solar heat in a direct way by using a solar cooker.
Indoor Solar Cooking
Like improved biomass stoves and fireless cookers, solar cookers are mainly promoted in developing countries as an alternative to the use of open fires. The technology is distributed by some 500 organisations, companies, and individuals, united in the Solar Cookers World Network. The promotion of panel and box cookers is mostly aimed at households and refugees, while the more sophisticated parabolic cookers are generally reserved for large-scale cooking in institutions.
The promotion of solar cookers in developing countries has produced improved technology that can be useful all over the world. For example, it is now possible to cook indoors using solar energy. This can happen in two ways: either by focusing a parabolic cooker through a wall aperture and then reflecting the sunlight onto a cooking pot, or by using concentrated sunlight to generate steam which is then transported through pipes to a nearby indoor kitchen.
Indoor solar cooking. Image: Solare Brücke.
Both approaches were demonstrated in the Scheffler Community Kitchens in India. These cooking installations are applied on a very large scale, for example at the Shirdi Temple where a solar cooked lunch is served to over 50,000 people per day. However, using solar energy indoors can also happen on a much smaller scale, as is demonstrated by the system pictured above.
Solar cookers used in developing countries are usually not the most efficient devices. Improving them with more sophisticated materials greatly increases their usability
Although a remarkable and highly sustainable piece of equipment, most solar cookers used in developing countries are not the most efficient. As Appropedia notes, solar cookers are "solar concentrators where precision and efficiency have been sacrificed for ease of construction and use of readily available materials".
The performance of these cookers could be enhanced if we built them in a more sophisticated way. For example, low-E window glass makes a solar box cooker much more efficient, as most heat that escapes from the box is through the glass. Solar Cookers International notes that the ongoing development of more efficient models continues to push the practicality of solar cookers into higher latitudes.
Making Cooking Sustainable: "Integrated Cooking"
Combining cooking stoves with solar cookers, fireless cookers, and pressure cookers turns an inefficient process into a year-round sustainable system that dramatically cuts greenhouse gas emissions, fuel use, and air pollution. This holds true for poor and rich countries alike, regardless of which type of cooking stove is used.
Increasingly, NGO's are betting on a combination of solar cookers, fireless cookers and improved biomass stoves, an approach that is known as "integrated cooking".  In "integrated cooking", solar cookers are used whenever possible, while the improved biomass stove offers a solution when solar energy is not available. The fireless cooker is used in combination with both, increasing the capacity of the cooking system and maximizing energy efficiency. For an example, see this video.
A similar system in the western world could even utilise electric or gas stoves instead of improved biomass stoves. Because the use of fireless cookers, pressure cookers and solar cookers shortens the use of electric or gas stoves considerably, their low efficiency becomes less of a concern. If the energy use of gas and electric stoves is substantially reduced, it also becomes more realistic to supply this smaller amount of energy by renewable sources, such as wind power.
Increasingly, NGO's are betting on a combination of solar cookers, fireless cookers and improved biomass stoves
Solar cookers and fireless cookers are good examples of the kind of technology that we aim for here at Low-tech Magazine. They can be cheap and easy to make, they are truly sustainable, and yet they are superior to any cooking technology available in pre-industrial times. The extensive use of water power and wind power in history seems to suggest that solar cooking goes back many centuries, but that is not the case. The first experimental solar box cookers only appeared in the 18th century, and parabolic cookers only showed up in the late 19th century.
Solar cookers and fireless cookers might have a very low-tech image, but they integrate well with high-tech materials. Before the Industrial Revolution, we had no tin or aluminium foil, no vacuum technology, no plastic containers, and no thermally insulated glass. Cooking with fireless cookers, pressure cookers and solar cookers is not a return to the now impractical or defunct gadgets of the past. Rather, it is an innovative approach that optimizes existing knowledge and technology with the aim of radical energy efficiency.
Kris De Decker (edited by Jenna Collett)
Thanks to all readers who have urged me to write about fireless cookers.
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 This is a rough calculation as I have assumed that the cooking time is equally divided between the cooking stove and fireless cooker, and that the energy use of a cookstove is the same whether it brings water to a boil or merely simmers it. This leads to either an overestimation or an underestimation of the combined thermal efficiency, depending on the technology used. For example, a rocket stove is especially efficient at high power output and much less so while simmering water, so that the combined efficiency of rocket stove and fireless cooker is higher than mentioned.