Instructables

Energy Efficiency of Bringing Water to the Boil

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When boiling water on a gas-fired stove three heat transfer mechanisms are in place: radiation, convection and conduction. Radiation and convection mainly occur between the flame and the bottom of the kettle or pan. Also, convection occurs when the flue gases escape and travel along the sides of the kettle. Conduction takes place where the bottom of the kettle touches its support.

When the gas is at minimum power (small flame) the combustion gases have more time to transfer their heat (through convection) to the pan compared to a situation where the gas is at maximum power (large flame). Moreover, the conduction losses through the kettle support may be less. This Instructable addresses the question whether the efficiency of a small flame is higher than the efficiency of a large flame.

The hypothesis tested in this Instructable is: with a small flame it will take less energy (i.e. less gas) to bring water to boiling temperature than it takes with a large flame. (The answer to this hypothesis can be found in Step 2)

The bonus question is: what is the average energy efficiency of boiling water on a gas stove? (The answer to this bonus question can be found in Step 3)

Not part of the initial research question is whether the amount of water in the kettle influences the efficiency of the process. But this was one of the unexpected outcomes of the experiments. (The answer to this second bonus question can be found in Step 4, which also features a discussion on the test setup)

To test the above hypothesis ten experiments were performed, five at minimum power and five at maximum power. In both series amounts of 200 g, 400 g, 600 g, 800 g and 1000 g (7.1 oz, 14.1 oz, 21.2 oz, 28.2 oz and 35.3 oz) were brought to boiling temperature. The mass is accurate to +/- 1 g (+/- 0.035 oz). The five water batches together each add to 3000 g (+/- 5 g) or 105.8 oz (+/- 0.2 oz).

The average starting temperature of the water was 13°C (55.4°F) and the average ambient temperature was 19°C (66.2°F). The time needed until reaching the boiling temperature (100°C or 212°F) was determined acoustically from the singing teakettle in which the experiments were performed. Time was measured in seconds using a stopwatch.

Step 5 in this Instructable does a suggestion for future work: the design of a condensing kettle to capture the latent heat in the flue gases.

In Step 6 some experiences from creating the graphs for this Instructable using Plotly (a collaborative data analysis and graphing tool) have been documented.The graphs are all available at www.plot.ly/~openproducts.

Step 7 finally spends some words on the CC BY license of this Instructable.

Take-home message
When bringing water to boiling temperature it is important to heat exactly the amount that is needed: boiling twice the volume of water needed means that half of the energy is going down the drain right away. Moreover, the boiling takes more time. The boiling process seems to become more efficient with a higher water level: using a smaller kettle or pan and filling it up completely is more efficient (and thus quicker) than a larger one that is mostly empty. Finally, it is of key importance to close the lid of the pan during the process to avoid convective and evaporative energy losses. The graphs in this Instructable have been generated using Plotly, which is a professional and convenient browser-based graphing environment, ideal for sharing data.

Previously openproducts released other Instructables in which energy efficiency and reduction of natural gas consumption was addressed: One-Armed Bandit - Mixer Tap Redesign (CC BY, 14 June 2013) and Energy Saving by Omitting Stand-by Energy Use in Combi Boiler through Remote Switches (CC BY, 30 July 2012).

The release of this Instructable (and all others) has been announced on Twitter.

 
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