How a Simple, Inexpensive Device Makes a Three-Stone Hearth as Efficient as an Improved Cookstove

H. S. Udaykumar
Women from the village of Karech at the end of their daily wood collection activity. The piles of wood behind the women were collected on their outing; each pile weighed about 50 pounds (approximately 20 kg).

In Brief

Approximately 2.7 billion people worldwide continue to use firewood for their energy and cooking needs leading to negative climate change, health, and ecological impacts. Efforts to replace the highly inefficient three-stone hearth stoves used in the majority of these homes with high efficiency cookstoves that cut wood use by half have met with mixed success; in India such efforts have by-and-large failed. Our team has been working on various solutions in the Mewar region of Rajasthan in western India. A resulting design for a simple, inexpensive (USD $1) device that may be simply placed in existing three-stone hearths has proven to cut wood use and diminish smoke to levels comparable to those achieved by the more expensive high efficiency cookstoves. In field tests in several households as well as in controlled laboratory experiments, it was demonstrated that this simple, unobtrusive, and inexpensive insert has the potential to significantly reduce smoke emissions and related respiratory discomfort and disease in the developing world.

Key Concepts

  • Firewood burning by the world’s poor for cooking is a major source of black carbon emissions, respiratory illness, and forest degradation.

  • Efforts to replace three-stone hearths with high efficiency cookstoves have met with only mixed success; the problem of firewood use for cooking persists.

  • In fieldwork in Rajasthan, India, we encountered reluctance among villagers to use high efficiency cookstoves due to design shortcomings and cost.

  • To remedy this, an inexpensive insert that can be placed in the three-stone hearth itself was designed and tested. This insert, called Mewar Angithi (MA), cut down smoke production and improved the hearth efficiency to the levels comparable to high efficiency cookstoves.

  • The insert is now being distributed in villages in Rajasthan as well as in locations in Kenya and Ghana.

Firewood burning is the second most significant cause of deforestation worldwide, behind only livestock production.1 Loss of biodiversity, habitat niches, and carbon stocks caused by deforestation are all major global concerns. The emissions from incomplete combustion of firewood also present a major source of atmospheric pollution and global warming.2,3 In particular, when deposited on the ice in the Arctic through wind currents in the northern hemisphere, soot or black carbon from firewood burning absorbs solar energy, whereas the original ice would have reflected it. This affects the “albedo,” or the fraction of solar energy reflected from the Earth back into space, contributing to accelerated warming and further ice loss.4 Firewood smoke has also become a potent human health hazard, contributing to a loss of as many as eight years in lifespan for the women who cook with firewood due to the constant inhalation of particulate matter as well as carcinogens such as benzopyrene.5,6

Much effort has been expended to mitigate the effects of firewood use among the three billion people in the Global South who still rely on biomass for their energy needs (mostly for cooking), but almost all of these efforts have been largely unsuccessful. The Global Alliance for Clean Cookstoves (GACC) has an ambitious plan to deploy 100 million High Efficiency Cookstoves (HECs) by the year 2020,7 but the plan has not yet been put into action due to technological and implementation hurdles. The Government of India has been trying to deploy HECs in rural India for the past two decades, but this intervention has been largely unsuccessful as well.8


H. S. Udaykumar
A woman in the village of Karech preparing a meal on a traditional three stone hearth, or chula. The evidence of soot emission can be seen on the wall behind the chulha.

The present group of authors has been working on issues related to forest degradation and firewood use in collaboration with the non-governmental organization (NGO) Foundation for Ecological Security in the areas around the Kumbalgarh Wildlife Sanctuary along the Aravali hill range in the semi-arid Mewar region of Rajasthan in India.9 In this location, despite previous efforts, the use of firewood is widespread and persistent (the authors were unable to find HECs in the local markets in the region). In the Bhil communities that reside on the hillsides along the forest preserve, women engage in the quotidian activity of firewood harvesting from the forest edge, which may take several hours each day depending on proximity to the forest. This harvested wood is burned in a three-stone hearth to prepare two meals each day.

Climate Healers, an NGO based in Phoenix, Arizona tried deploying solar cookstoves in the villages of Rajasthan in 2010, but that effort was unsuccessful for a variety of socio-cultural reasons. Since then, Climate Healers, along with the Foundation for Ecological Security (FES), has been working with researchers at the University of Iowa on a stored-energy solar cook stove that can address the primary reasons for the unsuccessful deployment in 2010. However, progress on this project has been slow due to the difficulties of storing heat under the low-cost constraint. Meanwhile, the carbon offset mechanisms that Climate Healers planned to use to fund the deployment of these stored energy solar cookstoves have become mired in controversy and are largely defunct. Therefore, as of late 2014, the NGOs involved were considering a new course to get over these considerable procedural and technological hurdles. A simpler solution to the firewood use problem in Rajasthan that appeared worthy of exploration was the introduction of so-called HECs.

The High Efficiency Cookstove Solution

When compared to the traditional three-stone hearths that villagers in Rajasthan use, HECs pack several desirable features, including cutting down on fuel use, decreasing emissions, and improving portability and durability.10 Indeed, the higher efficiency of the HECs arises due to specific engineering improvements, primarily by ensuring more complete combustion of the wood by improving air supply, removal of ash and embers that may clog the air flow path, better direction of the flame (in the “rocket” stove concept for example) by directing the rising hot air through a channeled space that contacts the bottom of the cooking vessel,11 insulation of the sides of the stove to prevent heat loss, and a smaller opening at the front of the stove that prevents over-feeding of fuel while simultaneously restricting the loss of heat from the flame. All of these performance improvements derive from application of fluid mechanics and combustion principles in many research laboratories around the world, support from governments and international organizations (e.g. the World Health Organization (WHO), United States Agency for International Development (USAID) etc.), and the participation of individuals and communities. In recent times computer simulations have been used to design optimal wood-burning cookstoves.12 The upshot is that the best HECs are quite robust in their delivery of the promised efficiency increase of up to 100 percent over the three-stone hearths (TSH). The HECs on the market offer efficiency in the 20 to 30 percent range.13 However, in India the uptake of HECs has been low. Currently, only four percent of wood-burning stoves are HECs, while the rest remain the highly inefficient TSHs.7,8 Similar situations prevail in other countries as well.14-17


H. S. Udaykumar
Solar cookers provided by the NGOs to the village allowed the women to cook rotis, but were not successfully adopted.

During winter 2014–2015, members of a multidisciplinary research team from the University of Iowa, including students and NGO workers from FES and Climate Healers, conducted studies in the tribal villages in the Aravali hill region of Rajasthan in an effort to better understand the poor uptake of HECs. The team conducted careful observations of the cooking process as the women in the villages of Karech and Gogunda used two of the top-selling HECs globally (here labelled as A and B to preserve the anonymity of brand names) to cook their normal meals. Through the gracious assistance of the women in the villages and the interpreters who helped us communicate with them, the main reasons for the poor uptake of these stoves in the villages of Rajasthan became quite clear. They are the following:

    • The commercial HECs do not accommodate well the wide variety of wood-fuel types that are available in Rajasthan. For instance, the HECs cannot accept large pieces of wood without having them split lengthwise, which is very difficult for the women to do. Women tend to abandon these HECs since their traditional chulhas (mud and brick stoves) have no such size limitation.
    • HEC Stove A heated the clay tawa (a vessel for cooking rotis) too much in the center and not enough at the edges, with the result that the women had to constantly rotate the rotis (flatbreads), particularly the corn rotis, in order to cook their meal. Stove A was not nearly as efficient in its use of firewood for cooking as advertised.
    • The mouth of HEC Stove B was too large to fit the clay tawas used in Rajasthan, and the team had to jerry-rig a grill to hold the clay tawa in place. Perhaps as a result, much of the advertised efficiency of HEC Stove B could not be obtained as well.


H. S. Udaykumar
Placing the Mewar Angithi in the chulha prior to cooking.
  • In addition to the above performance-related deficiencies of the HECs, the users also identified potential safety issues. The HECs are made of metal, and the sides of the stoves became very hot as cooking progressed. The women expressed concern that children could make contact with the sides of the HECs and burn themselves.
  • The durability of the HECs were another issue of concern. In the three-week period of testing in the village, due to the rough handling and high temperatures, the HECs appeared to already exhibit some deterioration. The women expressed concern that the stoves may not last as long as the traditional chulhas and will need to be replaced as opposed to rebuilt (as in the case of chulhas).
  • The women typically used a large, slow-burning log in the chulha in the cold season as a source of heat for their homes. The HECs cannot accommodate large logs and thus cannot provide this ancillary role of home heating.
  • Though there were some savings in firewood use with the HECs, the women estimated the stoves were worth as little as one-fifth their actual retail prices. Even then, it appeared doubtful that the women would commit even to a reduced amount to cost HEC stoves.

Mewar Angithi: An In Situ Modification of the TSH as a Solution to the Problem

Traditional TSHs or chulhas vary in size and shape to accommodate the different types of cooking vessels and foods cooked in them across the world. Our experience in Rajasthan showed that a single HEC stove cannot possibly replace all of these traditional stoves. Rather, significant fuelwood reductions can only be achieved with locally customizable solutions in different parts of the world. Tests with the HECs in the field did confirm however that they can cut wood use significantly (see Table 1) when compared to traditional chulhas. The low smoke effluence and main reductions in firewood use is due to the engineered airflow from below the fuel source in the HECs. Because it lacks such engineered airflow, the traditional chulha tends to accumulate embers that pile up and emit soot as they burn efficiently due to a lack of oxygen.

To address the air flow shortcomings of the three-stone hearth, the Mewar Angithi (MA), a simple metal device, was engineered to be placed in a traditional chulha in order to provide the same airflow mechanism in the traditional chulha as occurs in the HEC stoves. A traditional stove burns with good thermal efficiency at the start of the cooking session, but the efficiency deteriorates over time as the embers break off the fuelwood and pile up on the surface of the stove, impeding the airflow. Typically, this efficiency ranges from five to 15 percent. With the MA, airflow is directed from below the fuelwood through holes, as shown in Figure 1, thus improving efficiency even at the start of the cooking session. Since the MA provides the means to separate and drain the ash from the fuel, it maintains this engineered airflow throughout the cooking session, burning up the falling embers cleanly as well. Consequently, the MA facilitates the traditional chulha to maintain this improved thermal efficiency throughout the cooking session.

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Figure 1. (a) Schematic of the geometry of the top plate of the two-plate MA model used in the field test in Mewar. The top plate is shown with bends indicated by dashed lines. The bottom plate was of corresponding dimensions but solid. (b) Picture of the MA model used in the field test for which results are tabulated in Table 1. (c) Schematic of the single plate MA used in the lab studies. The bends are shown as dashed lines. (d) Picture of the single plate MA model showing top and side views.

For the chulhas found in the Mewar region of Rajasthan, this metal device can be constructed using a trapezoidal bottom metal plate, which can then be welded onto a bent-top metal plate with air holes in it. The top metal plate can be engineered by bending a square piece of porous metal into a trapezoidal shape with trapezoidal sides (see Figure 1). The resulting insert has a sloped bottom plate so that any ash collected on it can slide down to the exterior. The fuelwood is placed on the top porous plate, and as a result of the airflow from below the fuel source, any embers that break off from the wood would also burn up completely. In the engineering prototype, we used a square bottom metal plate that we also bent into a trapezoidal shape and punched holes in the side of the bent bottom plate so that the top and bottom plates can be held together with metal wires instead of being welded together. We found a porous mild steel metal sheet in the local market in Udaipur, Rajasthan as scrap metal from the metal washer industry. Such scrap metal sheets are ideal for constructing the MA and can be purchased in local markets at one-fourth the cost of solid metal sheets. We estimate the MA can be fabricated locally at a cost of less than US$1 each. Our initial MA pictured in Figure 1 cost about US$1. In addition, the MA can be easily reshaped and customized in different regions of the world to fit the sizes and shapes of the traditional stoves used in these regions. In recent field work in Rajasthan, we have found the need to customize the MA to the dimensions of the chulhas. Because of the simple, bent plate design of the MA, it is easily customized to individual chulha units.

Cook Stove Testing Results

Field Test

In preliminary tests involving the cooking of corn rotis and vegetable sabzi (curry), the MA achieved low smoke effluence (as estimated qualitatively by the users) comparable to both the commercial HECs, while providing around 60 percent reduction in firewood use (as measured quantitatively by the research team), which matches the advertised maximum savings of typical HECs. We investigated the users’ perception of HECs (two models: A and B) and compared it to the traditional chulha for cooking their typical meal of corn (makki), flatbreads (rotis), and a side dish of vegetables for families of different sizes.

Three households (labeled Houses 1, 2, and 3) were selected, and in each household, the feedback of the users (women) was sought on HEC-A and HEC-B compared to the chulha. In each household, the amount of wood procured for each meal was measured before cooking the meal, and the wood left over after cooking was measured, thereby obtaining the weight of wood used per meal. Unfortunately, results from House 3 were often unreliable due to operational difficulties that caused large variations in meal size, type, and preparation. Since the meal sizes varied over household, and even day-by-day for any given household, the performances of these stoves were normalized by the meal size and are shown in Table 1 in terms of wood use per meal unit (taken to be one roti).

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Table 1. Results in the field for tests performed with two different HECs, the traditional chulha and chulha with the MA.

The MA (Figure 1a) was placed in the chulha in the homes tested and the amount of wood used was measured. The results indicated that significant gains in efficiency were made even in the TSH using this simple low cost device (see rows labeled TSH+MA in Table 1; for TSH+MA two tests—on different days—were performed in House 1 due to operational difficulties in House 3 as noted above), resulting in approximately 60 percent reduction in wood use relative to TSH alone. During the course of cooking, it was also observed by the research team as well as the users that there was a significant reduction of smoke with the HECs as well as with the TSH+MA case relative to that produced with the TSH alone, though no measurements of emissions were taken in the field. This limited initial field test was promising for the TSH+MA, but the sample size was small and there remained concern over the day-to-day variability of cooking and the lack of quantification of emissions.

Laboratory Tests

To ascertain whether the results for the MA would hold under more controlled and rigorous tests, the Government of India approved the creation of the Cook Stove Testing Center at the Maharana Pratap University of Agriculture and Technology, which conducted standardized tests by boiling 9.4 liters of water in a traditional stove, with and without the MA. The tests showed that the MA reduced wood use by 63 percent and eliminated 89 percent of soot. One kilogram of fuelwood was used without the MA as per standard testing protocols, with wood moisture content of 0.4. The results of the lab tests are summarized in Figure 2.

The MA device in the test was constructed using a refined version of the design that we reported earlier (Figure 1b). It was built using just one square foot of a scrap metal sheet and can be assembled with simple 90 degree bends, with no need for welding. The resulting device has a simple rectangular cuboidal shape, with dimensions of 6″ × 8″ × 1.5″ and therefore, maintains the surface slope of the traditional stove. The bends occur in a symmetric fashion so that the assembled device is structurally stable even when the heavy fuelwood is placed on it. Figure 1d shows the top and front views of the device, and Figure 1c shows the angles that need to be made to create the device. In laboratory tests, we have seen that the design of the MA, particularly with regard to the size and placement of the holes, influences its performance. Insufficient holes would impede the separation of the embers and ash, while too many holes would not sufficiently obstruct airflow, and therefore the rising plume that provides oxygen to the flame would not be strong enough. The precise shape of the holes may not play a role. For example, rectangular holes (akin to those in “grates”) with equivalent blockage (i.e. overall orifice area) may prove equally effective. We are currently performing tests to determine the dependency of the performance of the MA by varying the size and distribution of holes and will report on this work in future publications.

The dimensions of the device were chosen to be compatible with the traditional chulhas used in the Mewar region of Rajasthan, India. However, the dimensions of the MA can be easily adjusted in different regions of the world to fit the sizes and shapes of the traditional stoves used in those regions. Thus, the MA could prove to be an easily malleable solution for the cook stove deployment problem that has been vexing the world’s policymakers to date.

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Figure 2. Lab test results comparing the performance of a chulha with and without the insert.

In recent work in the field, the MA has been supplied to four villages in the Udaipur district of Rajasthan, in the foothills of the Aravalli hills. The data for this implementation study is being accumulated, including performance and emissions of the chulhas in several homes with and without the MA. Preliminary face-to-face queries of the village residents who used the MA indicate that they confirm that the device significantly reduced smoke in their cooking spaces. Further quantitative data on the adoption rates and performance will be reported in the near future. Outreach activities in Ghana and Kenya are also under way and the results from those areas will be reported as well.