In March 2014 the latest Water Boiling Test (WBT) protocol and associated calculation sheet was released.
The WBT is a lab test for cookstoves that allows the tester to measure the efficiency and other characteristics of a cookstove including emissions of particulates and gases both inside and outside the house.
While a great tool for the stove tester, the results of this test can easily be misunderstood by anyone using test results to select the best stove for them or when comparing stoves. One issue is that the test outputs a percentage efficiency which is an attractive figure to the reader but doesn’t always reflect improved performance in the field. The efficiency percentage is the ratio of the fuel consumed and the energy transmitted to the water within the pot or pots. If you intend to create a super-efficient stove in the Tier 4 category, you can follow the steps shown in this video recently created by Dean Still of the Aprovecho Research Center in Oregon. The video shows that to achieve 45% efficiency, you need to focus on heat transfer to the pot by using a skirt around the pot of certain length and with the correct gap. The firepower is also important; add too much fuel, more heat is wasted and efficiency drops. By following these steps you could end up with a high efficiency stove that looks great on paper but fails to allow a cook to cook in the way she requires. The video shows that limiting the fire to 2kW is optimal, therefore manufacturers may optimize their designs to function at 2kW in order to get the best results and sell more stoves to people who buy based on these figures (we’re not talking about the end users here!). However the end user may actually want to use a pot larger than 5 liters requiring a larger fire. They may also want to use 2 pots or wood that isn’t kiln dried and cut to size. The concern is that the WBT is spreading beyond the lab where it belongs and is influencing decision makers, purchasers and manufactures in their design. There needs to be more focus on usability (including flexibility of use), acceptability and stove adoption.
The link between WBT results and real in-field fuel savings is even weaker with plancha stoves (stoves such as those used in Guatemala using a flat griddle for toasting and preparing tortillas). The reason being that the WBT measures heat into the water within the pot(s), however a typical meal prepared in Guatemala will make use of various pots pans and also the plancha. This can result in a stove with poor WBT efficiency performing well in the field and visa-versa.
When testing stoves it can be easy to get distracted from the issue. What are the goals? Why are we testing?
The main goals of a clean cookstove are: improving the health conditions of the stove user (in terms of burns and indoor emissions), reducing fuel consumption and deforestation, reducing greenhouse gas emissions and particulate emissions.
To achieve these goals, stove acceptance is key. A super high efficiency stove that doesn’t allow a family to cook in the way that they are used to will likely be sold, put to scrap or used alongside the traditional fire.
The in-field Kitchen Performance Test (KPT) is a great test for the manufacturer to use and gives data on real-life fuel savings as well as user feedback which can be used to judge the acceptability of the stove. A number of users are monitored over a few days using a traditional 3 stone fore or traditional stove. Surveys are carried out over a few days and their fuel usage is monitored. They are then given an improved stove to try for a month before the survey and fuel usage is re-measured. This test gives data for fuel savings based on the comparison of fuel consumption before and after the use of the improved stove and also gives the opportunity to get feedback from the cook. A limitation is that stove usage can vary regionally based on types of food prepared, altitude and climate and a large sample size can be required in order to gain statistically significant results. Consider a family who live in the mountains who use their open fire to heat their house and to cook. If they switch to a high efficiency stove with a chimney, a large portion of heat will not make it into the house and the family may either use an open fire for space heating alongside the stove or keep the stove running full power until the house warms up.
To summarize; The Water Boiling Test is a great tool for product development and has now been documented in a more concise manner with detailed explanation of all the calculations used. The ability of the WBT to be used to compare stove performance from location to location globally is questionable and care should be taken. Anyone reading WBT results should research the test first, understand the data and understand its limitations. Manufacturers can help educate their clients on stove testing data. Stove acceptance, user demands and adoption rates are of key importance. End users should be involved in stove design and should be given more choice in the stove they receive / purchase to drive the industry to better tend to their needs.
The water boiling test protocol and calculation spread sheets as well as controlled cooking tests and kitchen performance tests can be downloaded here:
The Aprovecho Research Centre hosts a wealth of information regarding testing here: