Boil kettle or heat water in pan

[andrea...@btopenworld.com]

Which is greener, using a kettle to boil just the necessary water to
add to a pan to cook potatoes, or throwing cold water straight into
the pan with the potatoes in the first place, then waiting for longer?

Gas is running out, electricity can be renewable.

Cheers, Chalky

[bandon]

have you timed how long it takes both ways?
have you tasted both one after another?
try it.

[gor...@gmail.com]

This is what happens when people start to think without base knowledge
or exerience or thermodynamics (or physics) training. The heat
required to raise the temperature of water through a range of
temperature is calculable. The formula is:- H[heat required]=M[mass in
kilos of water]xR[range of temperature in celsius]xS[the specific heat
capacity of water]
The energy source will not affect the amount of energy in joules
required to increase the temperature of water. What you are asking is
which is greener; the heat source for the cooker vs the heat souce for
the kettle. In an all electric house, the answer is simple. In any
other house hold, the debate will be long and arduous dependant on
many factors, such as natural gas versus LPG, oil fired versus gas
versus, green energy supplier and so it goes on.

[melani...@googlemail.com]

Well, actually, your physics is oversimplified. It's a question of
insulation and conductivity - that is, how much energy is effectively
transferred from your heat source to the water and potatoes. The
decision on how to heat and electrify your home is separate from this.

The answer: let the potatoes come to room temperature during the day
(saving a tiny amount of energy). Good cooking techniques then
suggest you should put the potatoes into boiling water, however you
boil the water. Then, either use a nice, energy efficient, limescale
free kettle to boil the water - or, if you don't have one, use a
saucepan, with a lid. Finally, once your potatoes are ready, do
something useful with the hot water, don't just tip it down the
sink!

Hope this helps :)

Melanie

[cho...@googlemail.com]

The kettle is far more efficient in transferring energy to the water
than by boiling in a pan, however heat (or any other form of energy)
that comes from mains electricity has already been through an
inefficient stage of conversion from the energy source (usually heat)
so vast amounts of energy have already been lost through harnessing
and transmitting the power.

Overall, one kWh of heat from mains electricity typically results in
about 450-500g of CO2, wheras one kWh of heat released by burning
natural gas typically releases about 190g of CO2. So (if you assume
that the kettle is 95% efficient) even if you only capture 50% of the
energy released (by burning the gas) as heat in the water it is still
more 'carbon efficient' to boil the water.

I think it's a good point that mains electricity is partly from
renewable sources, but considering that renewable energy production as
a fraction of the total production is only around 4.7% it's not really
a bit effect.

Tim.

[cho...@googlemail.com]

er.. that is to say that it's more efficient to boil the water in a
pan (with the lid on!) :)

[michael....@googlemail.com]

Hi. I am a physicist at the UK's National Physical Laboratory and this
particular question was irritating me intensely last month. Tim has
all the correct ideas but the results of my tests surprised me. I
heated 1 litre of water using an electric kettle, a microwave, and a
gas kettle sitting on the hob. I recorded the time for 1 litre of
water to boil and then added up the energy used, taking account of the
inefficiency of power generation. This is easy for a microwave and an
electric kettle, but tricky for gas. I just used the rated power of
the burners on the gas hob.

As expected, using gas does use less energy - but only at low power.
The amount of heat leaking around the kettle at high powers was so
significant that on the most powerful gas burner, the energy used was
actually greater than the energy used by the electric kettle. I think
for small-bottomed pans teh heat leak would be even greater. You can
read about these tedious and energy wasting experiments here.

http://web.mac.com/michaeldep/Protons/Protons_for_Breakfast_Blog/Entries/2008/1/1_Which_kettle_to_choose:__Gas_or_Electric.html

All the best

Michael

On 6 Feb, 16:15, cho...@googlemail.com wrote:

[John Ackers]

That's a really good analysis. Well done for making the effort. How
accurate do you think the 1kW, 1.75kW, 3kW gas burner ratings are?

[michael....@googlemail.com]

I got those specifications out the cooker manual. They will depend on
gas pressure (which I guess is quite significant) , and the calorific
content of the particular sample of gas (probably not very
significant). It is (as you say) the weakness in teh whole analysis.
However I couldn't think of an easy way around it.

I think the answer woul dbe to use a more sophisticated gas kettle
which had a much better design of base. Or alternative to measure the
air flow and tempertaure above the kettle. However it was clear that a
significant fraction of the flames heat was not being passed to the
water. At the highest setting the handle was impossibly hot!