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Which is why this was invented: The Bio-Lite Home Stove - supremely efficient, burns clean, burns it all, and produces electricity. http://www.biolitestove.com/homestove/



Good article, but one aspect is missing. You could archive higher efficiency in cooking if you would use warm water from a solar water heater. If you want to boil photatos and would start with water that is at 70°C you would save a tremendous amount of energy.

Malcolm Gribble (Soluciones Apropiadas)


Good article. % efficiency isn't always proportional to fuel savings however. In Guatemala where we work, food is prepared by boiling and also using a griddle and therefore a water boiling test is not appropriate. Another extremely important factor is adoption rates, usability, cultural acceptance, performance, size and durability all affect this. Designing stoves with local end users helps to produce more culturally acceptible products that a one size fits all stove from a factory in china won't achieve.



Another choice would be the solar oven. It cooks only when there is enough sunlight to raise the heat sufficiently, but as a low-cost addition to whatever burning stove is at hand, can reduce both pollution and increase efficiency.




Bio-Lite is pretty neat, but it seems like for home use, you'd be better off using hand-made bellows, so you don't need batteries, especially lithium-ion batteries, to start the draft. I can understand that people might think pumping bellows is tedious or undignified, but still, it's more resilient. Like many tedious, undignified things.



I didn't think biomass electricity was going to be significant compared to solar/wind. But if we are going to compare biomass electricity for home stoves to biomass fueled home stoves, for a full comparison you'd also have to consider the costs of distributing the biomass to the home, compared to delivering electricity. Especially when the energy for hot water tanks is considered, that could get significant, though I'd guess not enough to tip the energy efficiency scale; might well tip the *cost* scale, given costs of labor and storage.

People also care about convenience, of course: a gas, microwave, or electric stove is instant (or nearly so for resistance-based electric) and fairly brain dead to use; extrapolating from charcoal grill operation and your own text, a traditional oven would take more time and skill.



Interesting article just a few points.

Electricity as well as Gas and fluid fuel can be easily controlled and switched on and off at will. That is nit so easy with solid fuel

Sometimes we only want to cook a cup of water or milk, in this case the microwave was most efficient as all other means of cooking need a too large container which is also heated. The smaller the quantity we want to cook the larger is the proportion of the container.

Usually cook fast is more efficient as the losses are in a shorter time. If we use to less power we will never get boiling water even we use a higher amount of energy.

The energy and time used to clean the pots etc is not considered. It is much higher with solid fuel.

If we actually cook food there are many ways for prepartion. A potato can be boiled, steamed, baked, fried, deep fried. It also matters how the potato is cut prior to cooking.

Many cuisines optimized the use of energy as a necessity. For example chinese cuisine. Everything is chopped to small pieces that it can be fried in a very short time in a wok. So high energy output but fast burning fuel can be used.

Gidon Gerber


I was just about to do some efficiency measurements at home, when I remembered that Tom Murphy had already done them for his fabulous "Do the Math" blog. His results are roughly consistent with the Quooker study [8].




You treated several aspects of the issue in some detail. No real consideration of labor time -- opportunity cost of providing your own solid fuel. Not usually pleasant or uplifting work. treating all particulate "pollution" the same, when particulate from cooking food are a whole different physiological effect than particulates from toxic fuels. So many more issues, positive and negative. Better to focus strongly on biogas, compost heat, solar, heat retention, and "cooking" without combustion (mashing, slaking, drying, fermenting).

Minor Heretic


Something to consider: Thermos cooking. There are various pots marketed as "thermos cookers" or "thermal cookers." What they are is a regular stainless pot with a stainless vacuum thermos container - an extremely wide mouth version of the one you might keep your coffee in while on the road.

A lot of cooking is simply the process of heating something in water to the boiling point and keeping it there for a period of time. You can keep it at temperature by adding heat or by preventing heat from escaping. Thermal cookers do the latter at zero fuel cost. Bring the food to boiling in the stainless pot, turn off the hob (or smother the fire), drop it into the vacuum thermos container, and wait. Add maybe 10% to the cooking time.

I have a 4 liter thermal cooker that keeps food steaming hot for 45 minutes to an hour. I figure that I'm knocking off 3/4 of my fuel use when cooking something for 15 minutes. That blows away any stove improvements. The longer the cooking time the more fuel saved. If the cooking time is very long, say for dried beans, the pot can be taken out after an hour, brought back to a boil, and put back in the thermos.

I have read that dried beans, although cheap themselves, are too expensive to eat in some places because of the fuel cost for long simmering.

The other benefit is that the cook doesn't have to sit around tending a fire - multitasking!

I have looked into bulk pricing and they can be had for $20-$30. In a lot of places with expensive fuel that would pay back in a few months. Think of what a 3/4 reduction in charcoal use would do for deforestation in Haiti.

I have also experimented with a 40 ounce wide mouth thermos and a digital thermometer and found that the contents only lose 7-10 degrees C over 45 minutes.

Demand side management tends to win out over supply side.

Mark McClure


I've wondered whether a pot skirt would be a good idea - kind of a flue around the pot that directs otherwise wasted heat onto the sides of the cooking pot. It seems like a lot of heat is wasted from my gas stove due to heat blowing out to the sides. There is very little information on this on the web.

kris de decker


@ A.J., Minor Heretic, Mark McClure

Sorry to withold your comments for a while as they would have spoilt the follow-up article, which is now online: http://www.lowtechmagazine.com/2014/07/cooking-pot-insulation-key-to-sustainable-cooking.html

Like you say, insulated cooking pots are the key to sustainable cooking. The choice of the stove is of minor importance.

@ Mark: Thanks for mentioning the pot skirt, I had missed that one and included it in part two.

kris de decker


@ Gidon

Interesting to see that Murphy gets very similar results for modern cookstoves as he obviously was not aware of the study I talk about.



You may be interested in this clamp on heat exchanger for cooking pots. I purchased one over twenty years ago and it made a tremendous difference in water boiling speed.

I have used the Nissan thermal cooker for years also. I place it in box lined and covered with 2" polyiso insulation

Crispin Pemberton-Pigott


It would be interesting to know if the biomass water boiling tests consider the forestry losses, lighting the fire and the burnout phase in their systems analysis, as has been done for the power plant. If it is just the energy released from the fire during the heating phase converted into a fuel mass, then the comparison is completely inappropriate.

The combination of an old Russian thermal power station efficiency (35% delivered) connected to a low end Chinese induction cooker (88%) is higher than the efficiency of just the energy needed for a really well run, labour-intensive stove operated by a highly skilled cook continuously feeding an open fire like a baby bird.

If you want to heat water efficiently, do so in a device designed for heating: an electric kettle. It is much more efficient than a pot. If you want to heat water with biomass, do it in a water heater like a Kelly Kettle.

Most people use biomass fires to cook because they are poor and can't afford anything else. Once they have some money they abandon it as quickly as they can. This is not a surprise. Very short cooking is done with LPG when it is available even by very poor people. In Indonesia people cooking primarily with LPG often (70%) use biomass to heat water to save money. That is why 70% use LPG and 70% use biomass: fuel stacking.

One of the most frequent reasons for rejection of improved biomass stoves is the frequent feeding required, or preparing wood into small pieces. It is less effort to collect a lot more wood and stick with the traditional stove even if the emissions are much higher. It is a very complicated matter.

kris de decker


@ Crispin

An electric kettle is not suited for the preparation of food.

I'm not sure what you mean with your first two paragraphs. If they allude to the energy required for wood harvesting, then the answer is that people in developing countries usually gather brushwood by hand.

But even if wood would be harvested and transported with machines, the energy required for that would be much lower than the power conversion losses.

Crispin Pemberton-Pigott



I understand you do follow the argument that system analysis requires defining the bounds of the system, then making an analysis based on what is within them.

There are four major elements of cooking that define the total fuel use, the emissions per useful MegaJoule (MJ net)of the cooking cycle:

The stove product, the fuel, the user and the burn cycle from ignition to extinction.

Most 'efficiency ratings' of cooking stoves exclude a portion of the burn cycle.

Generally my question is whether on not the systems analysed had 'equal boundaries'. There are many inappropriate comparisons made for dramatic effect these days. Consider the energy required to deliver liquid petroleum gas, or liquid biofuels from palm trees (I am in Indonesia where both are promoted). Where are the boundaries for the traditional, fossil and biofuel systems drawn? Are they balanced?

You will probably admit the title is at least slightly provocative. It is very difficult and time consuming to operate an open fire above an efficiency of 20% if ignition and extinction fuel is included (which it is in real life).

With reference to your implied questions: My first paragraph above asks a question where the line is being drawn around the biomass fuel supply. The reason is that unless the collection and preparation effort to get the biomass into the stove is considered, a major factor is not being included: people frequently reject stoves if it requires more effort to 'operate' including fuel gathering (a different type) or additional preparation.

Now, one can put an energy equivalence onto that, which is odd and difficult, but if the analysis is going to take a systems approach, and fuel lost in transport, discarded at the point of use, and if a portion is burned after cooking in an extinguishing fire, then comparisons between systems must include it.

When considering emissions, Yixiang Zhang at the China Agricultural University has a recent paper at Elsevier with a chart of total emissions of various sorts from a biomass stove showing that when the 'test' ended (providing the performance rating) in some cases there were more emissions of a couple of pollutants from the extinguishing fire than during the test itself. The question of 'when to stop' is therefore a relevant one when looking at a comparison of systems.

My second paragraph draws a line around the system excluding the energy needed to bring fuel to the power station. While not entirely comfortable with this exclusion, I am aware that some power stations (viewed as a simple box) can be placed near a mine or far from it. That variable means including it in the system provides different answers that are not caused by the technology being analysed.

Some people need space heating and cooking so it is appropriate to draw a larger box around the system in cold countries. A power station such as in commonly used on cold Asia is a combined heat and power plant and the system efficiency is very high even though the conversion to electricity is 35-40%. CHP plants are also used in Sweden as I recall.

This is relevant to your last line:

"But even if wood would be harvested and transported with machines, the energy required for that would be much lower than the power conversion losses."

This is conditioned, surely, on not considering the energy people expend on gathering and preparing the fuel and on a particular interpretation of 'power conversion' to the exclusion of others. Is the comparison generally true or in certain cases? The question I ask is not as provocative: Where are the system boundaries drawn when sustaining the claim that open fires can be more efficient than modern cooking stoves?

kris de decker


@ Crispin

For the system boundaries, see #16. Many of your questions are also answered in the studies that I am referring to.

Secondly, if you include the ignition and extinction phases for biomass stoves, you have to do the same for power plants. And if it concerns a power plant that runs for an extended time, should it be called more efficient because it runs for an extended time? You could also keep a fire or a biomass stove burning for an extended time, and then we're back to where we started.

The kind of analysis you propose (and this also concerns your other points) would be complex and time consuming, and it would lean upon a whole range of assumptions that are prone to discussion. I don't think such an analysis is necessary to show that modern cooking stoves are very inefficient.

Crispin Pemberton-Pigott



Thanks for taking the time to converse.

"Secondly, if you include the ignition and extinction phases for biomass stoves, you have to do the same for power plants."

That is reasonable and I concur. In fact when Bechtel wanted to start the new natural gas generating station on the waterfront in San Francisco that is exactly what happened.

A major problem (speaking as a stove tester) is that some test protocols include the ignition when it is obvious that smoke is emitted in substantial quantity and others do not. If the assessment is based on 'emissions' then leaving off some of the emissions when rating the product would be misleading. Not so? Where are the test boundaries?

The most misleading test would be one that ran everything up to power, so to speak, and then getting an emissions concentration level. That is how car testing started in 1970: nearly pointless because it makes dilution the solution to pollution. We are past that now.

On the 'burnout phase' this has not yet been well addressed. Zhang's paper demonstrates with several different products tested using China's current biomass stove protocol that the emissions after the 'official test' has ended can be substantial and variable.

"And if it concerns a power plant that runs for an extended time, should it be called more efficient because it runs for an extended time?"

I am not convinced power plants are all that efficient compared with a well made, certainly for a space heating stove. Having measured some amazing stoves the total emissions from small scale combustors are now lower than even a new power station. I think that is a great achievement. In other words bigger is not always cleaner on a per MJ basis.

"You could also keep a fire or a biomass stove burning for an extended time, and then we're back to where we started."

Rather obviously the point is people don't do that. We should study the cycle people use and see what products are really 'improved' Some stoves can be extinguished pretty quickly on demand. Others not at all.

"The kind of analysis you propose (and this also concerns your other points) would be complex and time consuming,..."

The conceptual analysis should precede the drafting of the test method to ensure representative and fair comparisons are made that apply to the real world.

"...and it would lean upon a whole range of assumptions that are prone to discussion."

Exactly. Never assume anything - observe and reproduce.

"I don't think such an analysis is necessary to show that modern cooking stoves are very inefficient."

I understood the claim in the title was that open fires were (or could be) very efficient. Still, when showing that modern stoves are very efficient there are two distinctly different approaches taken to testing: one reports the energy value used, and the other reports the fuel mass needed (or its energy equivalent) needed to conduct a replication of the burn cycle typical in the community. You catch the difference? One reports the energy released by the fire (or an approximation of it) and the other measures the fuel needed each time the stove is re-lit. They provide different answers and different ratings.

Some stoves create a lot of unusable char left over which is thrown away or allowed to burn out. In a nutshell, is that fuel consumed (because you have to get more raw fuel tomorrow) or is it 'unused energy' because we were really trying to find out the heat transfer efficiency from fire to pot? The answers can be very different because the questions are quite different, like, 200%.

Power stations and old stove tests assume that energy consumption equals fuel consumption. The UNFCCC prefers that the measurement should show the raw fuel demand per burn cycle or per day or per year, meaning, biomass resource impact where 'better' means 'uses less of the resource'.

I am sure we agree on that. That is why testing should take place in a realistic context so the predicted performance is achieved in the field.

Pete Sherwood


@Crispin & @Kris

speaking of test boundaries, I didn't see where the cost of manufacturing the stove(s) and cooking appliances was taken into account (except the comment Kris made in regard to some societies making their own items). This changes ALL the metrics.

Some modern stoves can be used for decades, however, the energy consumption to manufacture them and provide them to consumers is astronomical even when factoring in, say, a decade, or more, of use.

Now, if the cost of building and maintaining a modern power supply station as well as the costs associated with energy supply lines (and maintaining them) is accounted for, the results should be adjusted again. It's ugly and excessively complicated and I am not asking anyone to perform this feat (nearly impossible anyway) but it changes quite a few assumptions regarding the true cost of meal preparation metrics.

I only bring these items up because as we look forward to off-grid living they are matters that I have pondered. We're trying to move away from the excesses of western-style living and to at least a slightly less global impact lifestyle. At the same time, we don't want to go back to the dark (or is it neolithic) ages, so to speak.

Frankly, I am struggling with the fact that a $10 (US) tank of propane (when used just for cooking), lasts us between two and three months, cooking most every day. Inefficient or not, I'd spend far more than ten dollars worth of time gathering and preparing fuel for an allegedly more efficient rocket stove.

Purple Library Guy


So OK, if the electric stove's inefficiency is largely derived from efficiency losses burning fuel in power plants to generate the electricity, and in transmission across long distance power grids, that raises a couple of questions.

1. What is the efficiency if the electricity is generated by a hydroelectric dam? By the efficiency definition being used, where the question seems to be purely "How much got burned?" presumably infinite, since an electric stove drawing on that power source causes no combustion at all. But that's ridiculous.
2. If off-site considerations are being included for electric stoves, why not for anything else? Just because it's hard to come up with a simple "How much got burned" measurement? The overall efficiency comes down to how much energy all told is required to boil that water. The infrastructure and extraction for gas, coal and electricity all require energy use; dams and coal mines and gas wells take energy to create, dig, or operate, as do pipelines and transmission wires. Even burning wood requires that it first be chopped and transported, and may involve waste wood that gets left behind on the clear-cut. If you're going to go off-site in your calculations, it's going to be meaningless if you don't go all the way.
3. So, what if you have photovoltaic panels on your roof and use their electricity to power your electric stove? No power plant, no grid. What is the efficiency of the heating of the electric burners themselves before you decide they will be the only technology to be looked at as part of a whole system rather than at the stove level? Must be pretty high for the remainder to have been in double digits after multiplying by 40%, twice--after all, if the stove's efficiency was 100%, multiply by .4 and then .4 again and you have 16%.

Crispin Pemberton-PIgott


@ Purple Library

The are a number of questions raised by your last post. Basically, who wants to spend their life gathering, chopping and preparing fuel to cook the next meal? No one I know.

I have a more fundamental objection to the comparison which is from a technical perspective. The comparison is made using 'energy per litre' and titled 'specific energy consumption. This is a metric used in a test method called the Water Boiling Test v4.x. It is not calculated correctly and in fact does not represent energy consumption. I have a problem with that. If one makes comparisons based on the use of suspect measuring sticks, then the results are hardly valid.

The WBT 4.2.3 is available from the GACC website. I invite anyone who is interested to examine how that number of calculated and see how it compares with the energy taken from the resource pile. In short, the 'energy' released from a fire to cook something is not the same as the total energy taken from the forest because some of the fuel is wasted - varies depending on the stove type. Stoves rating 'well' one the metric do not rate so 'well' when the energy needed from the forest is calculated properly. I suspect the same is true when the systems are analysed holistically, as noted in comments above.

As for improved stoves burning other fuels, I have seen a methanol stove with a system efficiency over 70%. The stick-burning ZamaZama stove from South Africa is 33-35% efficient.

Even if the whole-system performance of an electric stove is half of that, very few people will select something else if they can afford electricity because it gives them access to a kettle, a rice cooker, a toaster, a microwave, a George Forman Grill, and a refrigerator so they don't waste so much food. Plus they can talk to their friends about it on their cell phone. A very large number of people on this planet plan to do exactly that.

Anna Marie Byrne


Your figures are out. Water specific heat capacity is 0.0012kWh/litre.kelvin . (3600kj/kwh). By my working the rocket stove in your 733g of wood example , boiling 5 litres water, is about 14% efficient, not 25 - 50%. And this was the best biomass option.

Found this article when researching efficiency of electric pressure cookers which from their figures are about 85% efficient inelectric (electricity only about 40% efficient in Primary energy depending on the country ) so overall 35% efficient.

Please check through otherwise it's misleading people towards biomass.

Crispin Pemberton-Pigott


"...20 liter can rocket stove (image at the right) -- convincingly beats the efficiency of the well-tended open fire. It requires 733 grams of wood (12,579 kJ) to bring five litres of water to boil and simmer for 45 minutes, only 65% of the 1,112 grams of wood (19,496 kJ) required by the well-tended open fire. The thermal efficiency of the rocket stove varies between 23 and 54%. [7]"

Please everyone note that the 'energy' needed to boil and simmer the water is not the 'amount of fuel' used. The test method employed in the referenced source document subtracts the energy content of remnant char, treating is as unburned raw fuel because it didn't burn, even though it was wasted. This is a legacy of the WBT referred to above. It reports 'energy released' and converts that to an equivalent mass of dry fuel. It does not report the mass of fuel the stove needs each time it replicates the task which is always higher for a stove that produces a lot of char. The larger the amount of char, the more misrepresentative the claim for fuel efficiency.



Solar cooking plus a thermal pot can increase your availability of days dramatically in some locations.

Where I live is a Mediterranean climate and I would say you could solar cook 75% of all days of the year just with a simple portable cooker, I have a range.

Most people who haven't used one grossly underestimate the potential of a solar cooker, remember around 1kW of energy per sqm of the sun in a clear sky. Slow cooker is 100-200W depending on settings, so plenty of energy available from the sun when it can see you.

If I was to build a new house I would have an inbuilt solar cooker with a tracking mirror on the roof directing sun down a shaft with a thermal storage cavity and a thermal mass in it, with the oven drawing down from the stored heat with a fan with a temp/flow controller when in use.

This would mean a conventional fan forced oven electric heater could be used on any days the thermal storage was exhausted, but I would see no problems in getting at least 3 days worth out of a fairly small space of thermal storage.

Perlite is the stand out insulator if kept dry, maybe expanded clay hydroponic balls in a pinch, LNG tankers and storage facilities use Perlite as it is the best material available for the price by miles.

You could easily design a well insulated thermal storage cavity to lose less than 1 degree an hour when boxed in, depending on some conduction and the mass to surface area ratio.

This would mean a conventional fan forced oven electric heater could be used on any days the thermal storage was exhausted, but I would see no problems in getting at least 3 days worth out of a fairly small space of thermal storage.

I love a fire, but to rely on one to cook all year round is not compatible with modern life in many ways.

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