Monday, November 23, 2009

The most efficient way to boil water

This is a follow up of a great article by Pablo at the Treehugger http://www.treehugger.com/files/2009/11/electric-kettle-stove-or-microwave.php where he tested the efficiency of different ways of boiling water. By bringing 350ml of 17°C water to a boil and measuring the energi used, he found the efficiency of using a pot on the stove to be 30.5%, a microwave 47%, and an electric kettle 81%.

I have been thinking of how to make coffee in the morning. I can use the drip brewer or the Frensh press pot. So I first repetead the Pablo test with our 2000W electric kettle. It took 77 seconds to bring the water the 83 C° up to a boil. Actually slightly shorter, but I timed it to the point where the kettle turned it self off. The specific heat of water is 4.180 kJ/kgK, which means that it takes 4,180 kWs to heat one liter one C°. Needed energy transfered to the water is then 0.350 l * 83C°*4.180 kWs which is 121.4kWs. The kettle averaged 2000 W and used 77 seconds, which is 154 kWs. From this I get the efficiency of my electric kettle to be 78%, which is close to what Pable got. Then I tried the drip brewer. It averaged 950 W and after 153 seconds it was done, but the next 15 minutes it turned it self on again four times for 6 seonds each time. So at the time of automatic shut off it had been running for 177 seconds. 168.2 kWs turns out to be an efficiency of 72%. I did this test with 350 ml so it could compare to Pablos test.
For us, a more realistic water use is typical 700ml, so after waiting for an hour, I redid the tests with twice as much water. The result from this was that the drip brewer was 95% efficient and the electric kettle 88% efficient. So for our family, the drip brewer is the most efficient way boiling water for coffee.

Summary in Norwegian: Det kreves store mengder energi for å varme opp vann. Derfor er det lønnsomt å være bevisst på hvordan man varmer opp vann. Konklusjonen fra denne testen er at en kaffe trakter er den minst energikrevende måten å lage kaffe på.

5 comments:

Luke said...

Svein,
Good drip pot test. Thanks for contributing to the conversation at blog.plotwatt.com! I'd be interested to know how it changes with both volume (presumably linearly during the brew cycle) and time.

One other thought: most drip pots do have the nice feature of keeping the coffee warm with pretty low additional energy (unlike the press), but with either a press or drip an insulated thermos / carafe can obviate the need for that additional energy altogether.

Thanks again!

Anonymous said...

What about embodied energy in appliances. A stove will last years and can be mended economically whereas small appliances need replacing every couple of years and need energy for materials, manufacture, distribution and reycling.

tave said...

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Lucas said...

A electric kettle you should own. :)

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