Friday, 22 April 2011

Zero carbon tea

A friend of mine asked for advice on how to run a zero-carbon cafe at a fair in Cambridge on June 10-11th. "Do you realise how much energy it takes to boil water?" I asked. "We'll use small cups," she said. We are looking at three different methods: pedal power, sunlight and biomass.

Consider boiling water for 1 cup of tea, say 125ml (about half the size of the mug in the picture - I like large mugs!). Heating this quantity of water from 20C to boiling point, ignoring the heat capacity of the kettle, takes 42 kJ (0.012 kWh).

Suppose I use a pedal powered generator with 80% efficiency and I maintain 150 W output. At that rate it will take me 6 minutes to boil the water for my tea - and I might want a shower afterwards as well.

June 10th is practically midsummer and if the weather is good the insolation at noon should be about 1000 W/m2 (1). At just 40% efficiency I could boil my mug of water in 5 minutes using a solar collector area of 0.4 m2. By 4pm the sun will be lower but the insolation could still be 900 W/m2. Even in midwinter, the noon insolation in good weather is 500 W/m2, half the midsummer power.

However, a solar collector of this size will have to be some kind of concentrator system which needs direct sunlight. If the sky is even a bit hazy it won't work too well. Using the same sort of evacuated tube technology we use in solar thermal panels you can make a solar kettle which will still work in light cloud. These devices typically only have one tube though, so the collection area is small. A Navitron C-zero kettle has been shown to boil 2 litres of water in 3 hours (2). Over the 2 days of the fair we could manage 8 litres or 64 cups.

Wood stoves are not very efficient, but even at 10% efficiency I could boil my cup of tea with just 28g of seasoned wood (15 GJ/tonne) (3), whatever the weather. 64 cups would need less than 2kg. However, wood grows on a yearly cycle - perhaps 10t/ha. 2kg of wood - would need about 2 m2 of woodland for a year.

(3)  Renewable Energy - power for a sustainable future, edited by Godfrey Boyle

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