453906 Improved Thermal Efficiency Analysis of Biomass Cookstoves

Tuesday, November 15, 2016: 5:20 PM
Union Square 17 & 18 (Hilton San Francisco Union Square)
Cameron M. Quist, Chemical Engineering Department, Matthew R. Jones, Mechanical Engineering Department and Randy S. Lewis, Chemical Engineering Department, Brigham Young University, Provo, UT

Biomass is an important sustainable energy source, especially in the developing world. One common use of biomass in the developing world is through biomass cookstoves. Biomass cookstoves are used by approximately 2 billion people worldwide on a daily basis. In order for this use to be as sustainable as possible, efforts to reduce wood usage are important to help minimize environmental and health effects associated with pollution and deforestation. Currently the effects of biomass cookstoves on deforestation have been shown to be relatively small, though minimizing wood usage can reduce health, safety and environmental risks.

Because of these risks, the safety and efficiency (both thermal and combustion) of biomass cookstoves has been considered and improved by many researchers through the design of cookstoves and associated auxiliary features such as pot skirts. Once a new cookstove or auxiliary feature has been designed, the design is assessed and optimized. The most common test for assessing cookstoves is the Water Boiling Test (WBT). The WBT consists of three tasks. The first task is to heat water from room temperature to boiling using a “cold” cookstove that hasn’t been in use. The second task involves repeating the first task but starting with a “hot” cookstove that was recently used and still has thermal energy in the cookstove structure. The final task directly follows the second task by lowering the feed rate of fuel and keeping the water a few degrees below boiling for 45 minutes.

Measurements of emissions (CO , CO2, particulate matter, etc.), fuel used, water boiled off, and the time to complete each task are performed during the WBT. Metrics, including thermal efficiency, time to boil, burning rate, firepower, turn-down ratio, and emissions per fuel burned are then calculated from the measurements. Unfortunately, some of these metrics are averages based on endpoint analysis (measurements taken at the beginning and end of the test). This work expands upon the use of the thermal efficiency metric by developing and validating a model that can be used to estimate the average thermal efficiency at any point during the boiling process. This transient average thermal efficiency can be used to gain insight into the attributes of tested cookstoves. Such insights can include the minimal testing time required to determine the endpoint thermal efficiency used for cookstove comparisons, the effects of testing perturbations on thermal efficiency, and the difference in the behavior of thermal efficiency between cold starts and hot starts.

The WBT protocol along with the developed method for estimating thermal efficiency with time was performed using a typical Peruvian cookstove made of two small parallel walls of bricks, modified with both a grate under the fire and pot skirts. The studies using this stove showed that the time required to reach steady-state thermal efficiency was significantly different for cold and hot starts. Additionally, the time and temperature difference required to reach steady state thermal efficiency was significantly lower than required to actually boil the water as required by the WBT. The experimental runs with time-based endpoints were also found to be in good agreement with the estimation shown in this work. This expansion of the WBT protocol should allow for more efficient analysis and optimization of biomass cookstoves.

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