Estimation of transient thermal efficiency of a biomass cookstove

Biomass cookstoves are used by many people throughout the developing world for daily cooking, which unfortunately brings inherent risks to individuals and the environment. During the past several decades, extensive efforts have been made to mitigate the harm by developing clean burning, high efficiency biomass cookstoves. However, comparison and selection of cookstoves has proven to be difficult. Generally, cookstoves are characterized using the Water Boiling Test (WBT) or a variant thereof. The WBT protocol identifies several metrics used to assess cookstoves, such as average thermal efficiency (ηavg), turn-down ratio, time to boil, and several pollutant-based metrics. Unfortunately, the results of lab tests often are not replicated by actual users in the field. Additionally, results may vary from lab to lab and sometimes within the same lab. To provide an additional tool for assessment, this study demonstrates how to predict a cookstove's average thermal efficiency with time (ηt) which enables analyses and comparisons not possible using ηavg obtained at the end of the WBT. Model predictions were validated with several experiments. Insights and benefits of time-dependent ηt analysis include reduced testing times, identification of the reasons ηavg varies in repeated experiments, comparison of ηt at specified times even when the length of tests differ, identification of ηt characteristics between cold and hot starts, and determination as to if and when ηavg has reached steady state. The only required modification of the current WBT protocol to obtain ηt is to record water temperature with time.