THE SMOKELESS STOVE FINAL STOVE

CHALLENGE

Lower respiratory infections from cooking smoke are a major cause of mortality in the developing world, resulting in around 600,000 deaths a year in India alone. While several solutions exist for the urban poor or those who are otherwise connected to the power grid, the one effective smoke-reducing stove that does exist has only been disseminated to 5% of rural India, despite being in use since the 1980s.

BACKGROUND OF CHALLENGE

This Yunus Challenge was to develop a method to alleviate indoor air pollution (IAP), a major source of acute lower respiratory infections, the leading causes of disease and mortality in the developing world. IAP causes 600,000 deaths in India alone. The World Health Organization (WHO) estimates that pollution levels in rural Indian kitchens are 30 times higher than acceptable levels. When three of our team members visited slums around Ahmedabad, Gujarat during January, many of the houses became so filled with smoke that it became difficult to open one’s eyes indoors.

The Disease Control Priorities Project argues that “...an improved biomass stove is the most cost-effective intervention for South Asia and sub-Saharan Africa, the two regions with the highest solid fuel-related disease burden. ” The WHO calculates that if 50% of the population that cooked with solid fuels in 2005 switches to cooking on an improved stove by 2015, total benefits would be approximately $104 billion. As a country with one of the world's highest incidences of IAP, India is a natural target for our work. Additionally, the communities with which we plan to work use more noxious fuels than biomass, often even burning the carcinogenic plastic that they find in dumpsters. We provide a cheap and effective solution that removes the fuel-generated smoke from the house and avoids the cultural, financial, and compliance barriers that are inherent in current solutions.

INNOVATION

We developed a novel fuel-efficient stove that requires no external power, costs $5, and vents nearly all of the smoke that is produced in the process of cooking. Current stoves have not been disseminated widely in India because they are costly, non-portable, and/or not suited to existing cooking methods.

We will first implement our design in Chitravad, Gujarat, India, where our community partner, Aga Khan Planning and Building Services, will place the stoves into new houses that they are constructing.

Our design covers the cooking fire with a conical (truncated cone) steel structure, which has openings near the top for the pot and for a standard steel smoke vent. Our innovation lies in designing a simple, mobile structure that (1) prevents smoke from escaping into the house, (2) enables oxygen to flow into the base of the fire, and (3) lends itself to rapid dissemination.

Our Solution

In designing a smokeless stove to reduce IAP, we sought to evacuate all smoke without decreasing thermal efficiency and without employing external power. The materials that are required to implement this solution would have to be low-cost and locally available. An effective stove would require minimal compliance and be minimally obtrusive to people’s lifestyles and ways of cooking.

We designed a profiled conical funnel with a truncated tip that evacuates smoke through a connected “ventilation pipe” and breathes oxygen from an angled “air inlet” in its lower base, while heating a vessel that sits atop the structure. Since the fire is completely contained within this structure, our device harnesses the convective energies of the smoke toward the productive goal of ventilation. Smoke is driven down its energy gradient (up the ventilation duct and out of the house) by natural convection. By elevating the upper duct, we also take advantage of small transient wind currents that create a small pressure gradient to help force smoke out of the duct naturally. The two mechanisms, individually or in combination, ventilate a room very effectively.

DESIGN

The Design

The fire is kept in the middle of the conical structure, and is fed by a covered opening in the front that is 3.5" high and 6" across. The pot or pan is placed on top of the 6.43"-diameter opening, and the smoke exits through the 4"-diameter pipe up several feet and out of the house. The top conical piece and the bottom conical piece are held apart by four to five 1"-thick rectangular slabs that are equally spaced around the diameter, with a removable 3.5" x 9" piece that fits over the opening in front. The 4"-pipe is made from steel, and the conical base and upper funnel is also made out of steel.

Our central innovation lies in the gap between the top, 9"-conical piece, and the bottom, 3"-conical piece. The convection currents from the rising hot air and smoke, along with the small pressure gradient that is created by transient wind currents at the top of the 4"-pipe, drive a steady wind through the gap at the bottom and out the 4"-pipe at the top. Besides removing the smoke, the added oxygen causes the fuel to burn more cleanly and efficiently, extending the use of the scarce fuel even more than current designs that are marketed as "fuel-efficient", such as the Smokeless Chulha. Smoke does not diffuse back down because it must initially stay within the inner conical piece, and will not travel down far enough to escape. The dearth of oxygen inside the chamber also aids oxygen diffusion.

Benefits

Areas for Future Improvement

EXISTING TECHNOLOGIES

Although there are a few devices that have been unsuccessfully implemented by the government and a couple of different NGOs, most of these are not accepted by any particular community or are not in use to a significant extent (<<1% of indoor stoves). There are three popular types of stoves that are used in rural Indian communities that operate below the poverty line:

CONVENTIONAL STOVES

Competitive Advantage to Improved Stoves

Smokeless Chulah: The Indian government began implementing large-scale “smokeless chulha” programs in the 1980s, and continues to advocate them today. While they are cheap enough (between $5 and $7) and uncomplicated to construct and repair, they still reach only 5% of the population. House-based chulhas are built into the house itself, and the semi-permanent nature of the installation combined with the general lack of faith in government initiatives dissuade initial adoption. Additionally, the interviews we conducted amongst rural households and NGOs indicate that portability is a key factor in people's decision to use mud stoves; economic and social instability often require people to move at a moment's notice.

Sarai Cooker: The most popular technology among Indian NGOs is the stainless steel Sarai Cooker, which contains three chambers, each one of which can cook a different food. It is very portable, and in fact, that it can cook while it is being transported. However, the Appropriate Rural Technology Institute estimates that the smallest Sarai Cooker (which has an eight-liter capacity) costs $20, which is well out of the price range of the rural poor. Furthermore, because the cooker is fueled by charcoal, it is even more inaccessible to villages that make fires from twigs and plastic.

Upesi Stove: The Kenyan Upesi stove system is cheap, fuel-efficient, widely disseminated, and highly functional. The Upesi stove consists of an enclosed, cylindrical clay space that retains heat and directs smoke away from the person who is cooking. Unfortunately, it would not be useful in India, as the smoke that is generated by burning plastic is considerably more dense, and the smaller room sizes in India would mean that the smoke would soon fill the house anyway. Additionally, using the Upesi stove requires a significant change from the way the Indian villagers cook, as its clay surface takes a long time to heat and is not conducive to transmitting heat from small amounts of fuel.

Prolena Ecostove: Implemented in Nicaragua, this stove consists of an enclosed firebox with insulated walls and a "chimney," and is effective in eliminating cooking smoke from the room while maintaining high fuel efficiency. Despite its ability to funnel out smoke, however, the Ecostove has encountered serious distribution problems. It requires almost daily cleaning to maintain effectiveness, and it is encountering resistance in rural Nicaragua because it requires a significant change in cooking practices, as it would in India as well. Furthermore, at an estimated cost of $35, it is far beyond the reach of the average family in rural India.

IMPLEMENTATION

Our initial and primary community contact was with the Aga Khan Planning and Building Services of Chitravad, India. Through their influence within the village, we were able to install two of our stoves in houses for regular use. We also had the opportunity to have one stove fabricated in India by a local craftsman and test its ability to serve as a replacement to the conventional cooking method. We were able to measure the particulate and carbon monoxide emissions levels to validate our claim that the stove was smokeless, and we were able to establish our claim that our stove does not substantially change the thermal efficiency of cooking.

Beyond testing our alpha prototype, we were able to gather valuable feedback and testimony. Speaking with 10-15 of the women in the village afforded us with an invaluable perspective on how cooking is typically done in the village. Even the seemingly trivial details that we noticed from video-recording women cook, like the position they assume while cooking or the amount they wet their fuel, were useful for refining our device. We wanted to design a solution that would be accepted and achieve community compliance, and so incorporating the feedback we were given was key.

Furthermore, our relationship with the AKPBS afforded us many networking and problem-exposure opportunities both within Chitravad and around many of the 30 other villages they work within across India and Pakistan. My travels allowed me contact with a greater network of community and manufacturing partners both within and outside of Chitravad. Beyond implementing and field testing our device, we also gained valuable exposure to the culture of Gujarat and the particular cooking problems facing rural inhabitants.

TESTING

TESTING STRUCTURE LAYOUT

Theory of Heat Transfer and the Combustion Cycle

Wood, like many typical stove fuels, does is not directly flammable. It first undergoes pyrolysis, a degradation process that liberates combustion gases that mix with oxygen to ignite and sustain a flame. This flame produces carbon dioxide and water vapor as ideal waste products.

In a passively ventilated combustion process, as observed within a conventional stove, the products are only separated from the reactants by diffusion into the surrounding internal environment. Hence, combustion products are released into the internal atmosphere (the passively ventilated room) in a process that maintains the equilibrium pressure between the room and the environment, neither driving fresh oxygen-rich air into the room nor expelling combustion products. The lack of a driving pressure force results in an inefficient ventilation mechanism that is largely governed by simple diffusion. Thus a conventional stove, which is by nature passively ventilated, should cause an increase in the concentration of combustion products, including carbon dioxide, and a decrease in the concentration of combustion reactants, particularly oxygen.

An actively ventilated stove, like the Smokeless Stove, actively expels the combustion products from the combustion chamber before they can enter the surrounding environment. In doing so, it creates a negative pressure within the stove and surrounding ventilated environment that ideally serves to drive in fresh air from the external surroundings. Thus, an actively ventilated combustion process leaves the carbon dioxide and oxygen concentrations in a room unchanged because combustion products are expelled and reactants are continuously replenished by the surrounding external air.

In either combustion process, the energy released in burning wood at a particular rate gives rise to a heat transfer that can be gainfully employed by a stove. The net heat transfer of a stove can be measured by the stove’s ability to increase the temperature of a substance of mass m, with specific heat, by a temperature, over a time period. Assuming we neglect any heat transfer from the substance into the environment, the first law of thermodynamics and the energy constitutive relationship yield:

FIRST LAW OF THERMODYNAMICS AND THE PROPERTY CONSTITUTIVE RELATIONSHIP

Instrumented Stove to Emulate Cooking Environment

A four-walled shed was assembled, such that one wall is open for passive diffusion. Both stoves were individually positioned in a far corner from the open wall. The Smokeless Stove was connected to a duct that leaves the room, whereas the Three-Stone stove, when in use, had a seal in the place of that duct.

In order to establish the claim of reduced emissions from the Smokeless Stove, product and reactant concentrations were measured. The 02/2007 Vernier CO2-DIN and O2-DIN sensors were placed at the center of the room to measure combustion product and reactant concentrations, respectively. To compare the thermal efficiency of the two cooking methods, the heat transfer rate and power output potential for both approaches was measured. To accomplish this, an equivalent amount of fuel, 194 g of ground and compacted wood, was added to each stove and used to boil 418 g of distilled water. Within the water, a temperature probe, Vernier TMP-BTA, was used to determine the net heat transfer to the water, per the energy constitutive relationship.

During a total of five trials, gas concentrations and water temperatures are measured in an environment that emulates the typical cooking environment of a rural slum. Measurements are initially taken without cooking to establish a baseline control group. Following the control, subsequent 10-minute measurements are taken at a sampling frequency of 0.25 samples/second. For each stove, the same amount of fuel is tested twice, so that data is collected over a total of five trials, including the initial control.

Results of Measurement

The Smokeless Stove confirmed expectations, eliminating emission of exhaust gases into the internal environment and maintaining a consistent concentration of reactants. A stable CO2 concentration was observed in the Smokeless Stove, confirming no significant variation from baseline measurement of 645 ppm at the 95% confidence level. Beyond a subjectively noticeable decline in air quality, the Three-Stone stove, demonstrated significant increases in CO2 concentration at a rate of 10.8 ± 1.2 ppm/min, as expected. The CO2 concentration measurements in time can be seen for both stoves below.

CO2 CONCENTRATION COMPARISON FOR STOVES

Similarly, stable O2 concentrations were observed in the Smokeless Stove, confirming no statistically significant variation from the initial baseline measurement of 21.8% volume of O2 at the 95% confidence level. The Three-Stone stove, demonstrated a significant decline in the room’s O2 concentration, at a rate of 0.59 ± .05 %/min, as expected. The O2 concentration measurements in time can be seen for both stoves in Figure 4. Together, these findings corroborate the claim that the Smokeless Stove has reduced or eliminated emissions and is successful in evacuating exhaust gases.

O2 CONCENTRATION COMPARISON FOR STOVES

These improvements in emissions come at the small cost to thermal efficiency. The Smokeless Stove exhibits a power output that is approximately 9% less than the conventional Three-Stone stove. It was observed to attain a power output potential of 207 ± 5 W, in comparison to the conventional stove’s 226 ± 8 W, for the same amount of fuel. Plots of the water temperature in time for the first trials of both stoves have been included below within Figure 5. We surmise that these differences in power output are minimal and, although statistically significantly, will not represent noticeable differences to an end user.

TEMPERATURE COMPRARISON FOR STOVES

In summation, when compared to the conventional cooking stove, the Smokeless Stove demonstrated no significant CO2 emissions and no change to the O2 concentration of the internal environment at the 95% confidence level. The improvements in emissions come at the minor cost of thermal efficiency, however, because the Smokeless Stove attained a lower power output potential of 207 ± 5 W, in comparison to the conventional stove’s 226 ± 8 W, for the same amount of fuel.

CURRENT PROGRESS

Our device is currently in use in two different households, undergoing realistic duty cycle testing. We regularly interface with our community partners to integrate feedback into our final design, which is expected to be deployed in 2010 by the Aga Khan Planning and Building Services in an initial set of earthquake-struck rebuilt homes.