Proceedings of the Multi-Disciplinary Senior Design Conference Page 3 s3

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Project Number: P10461

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

FIRST GENERATION THERMOELECTRIC COOKSTOVE

Christopher Brol / ME Student (Team Lead) / Kevin Molocznik / ME Student
Aaron Dibble / ME Student / Ian Donahue / ME Student
Neal McKimpson / ISE Student

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Abstract

Haiti is the poorest country in the Western Hemisphere, with over 80% of the population living below the poverty line. In Haiti, the people are highly dependent upon biomass as a primary fuel source for cooking and heating, which has lead to two significant problems. First, the creation and use of charcoal fuel has caused a catastrophic rate of deforestation. Second, indoor cooking practices have led to smoke inhalation and a consequently increasing number of lower respiratory infections.

Figure 1: Boundary between Haiti and Dominican Republic, showing deforestation of Haiti.

The objective of this project is to design the mechanical and structural aspects to a biomass stove that will utilize a fan and thermoelectric unit. This design should address the concerns of deforestation and lower respiratory infections by significantly reducing fuel consumption and harmful emissions. This project has been supported by Dr. Robert Stevens and Dr. Richard Lux, Haiti Outreach – Pwoje Espwa (H.O.P.E.), a Rochester-based NGO, and the Environmental Protection Agency (EPA) through the receipt of a P3 Phase I grant. The final product for this project will be both, a prototype cook stove for experimentation and a cook stove constructed from a 55 gallon steel drum that has been optimized for the gasification and burning of a wood charcoal fuel source.

background

According to the World Health Organization more than three billion people depend on biomass as a primary fuel source for cooking. The practice of cooking with biomass has decimated ecosystems, requires an enormous amount of human effort to gather, and creates considerable health problems that continue to plague the world’s poorest populations.

In Haiti, the dependence on biomass fuels has created two significant problems upon which this project intends to address. First, the creation of charcoal in Haiti has caused a catastrophic rate of deforestation. Haiti has less than 3% of its forests remaining, and has consequently begun to import charcoal, and other biomass from the Dominican Republic. Deforestation is also directly correlated to the hurricane related death rate as these complications cause flooding and mudslides. Second, indoor cooking and smoke inhalation have led to an increase in lower respiratory infections throughout Haiti. Poor indoor quality leads to respiratory illness, the second largest killer, by environmental factors, of children in developing regions.

This project aspires to both, slow the rate of deforestation and to reduce the risk of respiratory infections. Manufacturing processes and capabilities that are available in Haiti, were considered throughout the design and build phases; the implementation of these stoves could create jobs to aid local prosperity. The goals for creating a better biomass cook stove for Haiti and other developing nations are as follows.

1. Reduce fuel use in order to slow the rate of deforestation and diminish the time and limited financial resources spent on fuel.

2. Affordability for traditional Haitian consumers, based on a $2/day income.

3. Create microenterprises for assembling the advanced stoves to generate wealth and develop local expertise for maintaining the stoves in order to improve chances of sustained stove adoption.

4. Implement a design that is intuitive, transportable and enhances conventional cooking techniques for traditional foods.

5. Improve the air quality for women and children.

6. Minimize the negative impact on the local and global environment by incorporating a life cycle analysis in the design process.

Wood charcoal is the primary fuel source for cooking. In an effort to increase the chance for the advanced stove to be adopted by the Haitian population, conventional cooking practices, including the use of charcoal should be maintained. In the first phase of the project, the gasification process was chosen as the best way to maximize cooking with wood charcoal. The gasification process is ideal for an improved biomass cook stove design, because all forms of biomass, including non-wood and waste-wood fuels, have an ideal burn rate; a gasifier can be optimized to meet the needs of various fuels. The second phase of the project is defined by creating a fully-adjustable prototype for testing and refinement, leading to the construction of a final, “charcoal-optimized” cook stove.

NOMENCLATURE

P3 – People, prosperity and the planet; Environmental Protection Agency student design competition

Biomass – plant and animal material, especially wood, dung and agricultural waste products, used as a source of fuel

Gasification – the thermal conversion of biomass fuel into synthesis gas (syngas)

Pyrolysis – the chemical decomposition of condensed or organic substances by heating to produce gas and liquid products

Top Lit Up-Draft (TLUD) – efficient cook stove design using small biomass fuel sources

Thermoelectric – a solid state device that converts thermal energy to electrical power

FCR – Fuel Consumption Rate; rate that charcoal fuel is consumed

BT – Time to boil water (Table 1, Table 2 refer to boil time for 2L of water)

TF – Thermal Efficiency

Eco-points – metric used in Life Cycle Assessment that measures the level of environmental impact of a product

DESIGN methodology

Needs. The final cook stove recognizes the needs of two separate customers. First, Haiti Outreach – Pwoje Espwa (H.O.P.E.) acted not only as a design consultant, but also as a representative of the end user and their needs. The second customer was the Environmental Protection Agency through the P3 Student Design Competition.

Figure 2: 3D CAD model of final cook stove design.

The Haitian end user is primarily concerned with the initial and operating costs of the stove. For this reason, we tailored our design to meet the needs of a Haitian food vendor. A vendor will require a larger stove; however, they will have a greater investment interest in the stove for its potential to increase profits. If the technology is adopted by a vendor market, a trickle-down effect could open a market for the cook stove on an individual household scale. In addition to cost, the design will address the need to accept cookware typical of a Haitian vendor, to reduce fuel consumption, and the cook stove should be durable and portable. The project also recognizes the need for lesser emissions, reduced fuel consumption, sustainability and ease of manufacture and use.

Alternative Concepts. There are several schools of thought for designing a biomass cook stove. Two different stove configurations were initially considered. The first is similar to a rocket stove, with a long, vertical combustion chamber. With a vertical combustion chamber, there is direct single stage combustion, and it theoretically produces significantly less emissions because the gases and particulates are burned up as it travels up the combustion chamber. The top lit updraft stove was the second concept considered. This configuration is conducive to a forced-air gasification process.

Gasification. Based on feedback from technical reviews and consultations with H.O.P.E., the gasification stove was selected for further development because of the potential for higher fuel efficiency, controllable burn rates, and reduced emissions. The basic concept selected is shown in Figure 3, below.

Figure 3: Preliminary concept of first generation stove

For this concept, air is forced into an air channel through a side-mounted fan inlet locate at (A) in Figure 3. Within the air channel, the airflow acts as an insulator between the hot combustion chamber and the cool outer wall. In addition to insulation, the air is heated before entering the combustion chamber. Air enters the combustion chamber at the primary holes (B) and secondary holes (C). The initial stage of gasification occurs where the air enters the primary holes and mixes with the burning charcoal. This mixture of charcoal (biomass and oxygen) creates a pyrolysis reaction where fuel gas is released and travels up the through the combustion chamber. The fuel gas fully combusts when it comes into contact with the air entering the secondary holes. This two stage combustion process is more efficient because it captures and uses fuel gas that would otherwise be released into the atmosphere as with single stage combustion processes, such as with an open fire. The two stage combustion provides a more complete combustion of the fuel and consequently reduces emissions.

Combustion Chamber Design. Significant improvements to biomass stoves come via improving combustion and improving thermal efficiency. There are several factors that can be integrated into the stove to improve combustion.

1. Create a good draft.

2. Insulate around the fire for a hotter burn.

3. Use lightweight, heat-resistant materials for insulation to reduce warm-up time.

4. Allow air to circulate and contact all surfaces of the fuel.

5. Meter/limit fuel capacity.

6. Limit cold air intake into the combustion chamber.

7. Preheat intake air to maintain complete combustion.

The power output of the stove is a ratio of fuel mass and fuel energy to the change in time.

Power output equation.

As designed, the advanced stove should output 1,250 to 5,400 Kcal/hr of cooking power, at a charcoal consumption rate of 1.6 kg/hr when simmering as for rice and beans; this rate is considerably less than that of traditional cook stoves. Because design models are very general, making design improvements is an iterative process that requires experimentation. The prototype of the first generation stove was therefore designed for combustion chamber variations including distance between primary and secondary holes, hole-count and controllable airflow. Airflow control should also allow for some control over burn rates and creating cooking conditions suitable for boiling and simmering. To prevent over fueling the stove and minimize wasted fuel, the combustion chamber was sized to contain enough fuel for a one hour run time, enough to boil and simmer a rice meal.

Thermal Efficiency. The second means of stove design improvement is to increase thermal efficiency. Because of the lightweight material of the stove, heat loss can occur through the walls of the stove, as well as through poor heat transfer to the cook pot itself. Because there is very little information available for designing a thermally efficient biomass stove, the final cook stove design will rely on iterative, experimental data to balance an optimal thermal efficiency with other variables. Reducing side losses must be done without dense insulating materials to maintain fast start up times, and therefore less time and fuel requirements for stove warm-up. To further reduce side losses, the first generation stove will employ an aluminum radiant barrier.

The fuel consumption rate is calculated as a ratio of fuel consumption versus operating time.

Fuel Consumption Rate Equation.

Percent thermal efficiency is the ratio of mass of product in the cook pot, heat capacity and temperature change versus fuel mass and fuel energy.

Thermal Efficiency Equation.

There are several factors that can improve heat transfer to the cook pot itself.

1. Increase the pot surface area.

2. Direct heat through narrow channels around the pot.

3. Maintain turbulent, fast-moving airflow around the pot to avoid boundary layer effects.

4. Use materials that have a high heat thermal conductivity.

The stove is designed with a secondary skirt to fit a range of pot sizes and shapes while still maintaining narrow gaps to promote good heat transfer to the pot. Energy, therefore, is directed towards the stove skirt through the use of air channels.

Sustainability/Manufacturability. Sustainability and manufacturability are critical when designing a good stove for an impoverished target market. In researching possible stove materials, it was discovered that Haitians have access to 55 gallon steel drums used for transporting oil and food. The Haitian people currently have a process in place for reconditioning these barrels for creating metal art. These drums are made from 18GA cold-rolled sheet steel, which was determined to be structurally capable of supporting the weight of the largest pots identified during the needs assessment stage of the project. By using the recycled steel drums which are relatively inexpensive and utilizing a locally available skill set, the primary stove can be built for less than $10 with the exception of the power unit and fan.

Figure 4: Proof of concept. Initial stove start up (left), and first test within P10451 test stand (right).

Results and discussion

As stated previously, the prototype cook stove design has the capability to vary the distance between the primary and secondary combustion chamber holes, hole-count and variable airflow. Experimentation for the cook stoves were divided into two series. Test Series A served as a series of proof of concept tests to understand the stove response to design modifications. The purpose of Test Series B was to determine fuel consumption rate, boiling time, thermal efficiency and power output of the prototype design.

Proof of Concept. Test Series A was a progression of tests that produced a temperature versus time profile of stove operation as it responded to design modifications and problems, including: variation of fuel loads, skirt modifications, the addition of a secondary skirt and the addition of an aluminum heat barrier. Series A tests were measured from a cold start.

Test Series B. Test Series B began once all design problems that were encountered during the proof of concept had been addressed. Test Series B assumed measurements start under warm-start conditions. The results are more conclusive than Test Series A.

FCR (Kg/Hr): / BT (min): / TF(%): / Power (kW):
Test B-2 / 0.9 / 8 / 14.9 / 6.88
Test B-3 / 0.53 / 8 / 24.4 / 4.01
Test B-5 / 1.83 / 4.5 / 32 / 7.45
Test B-6 / 1.53 / 8 / 9.82 / 13.16
Test B-7 / 0.816 / 5 / 36.3 / 5.52

Table 1: Test Series B results.