What's the greenest way to boil my tea water?

June 6, 2012 8:11 AM

What is the most environmentally friendly way to boil water for, e.g., tea?

I was surprised to discover a difference of opinion among my family members on this. One side says that boiling a kettle over gas is best; they have on their side the relative cheapness and cleanness of gas. The other side advocates boiling a cup in the microwave; it's also quicker, and you don't boil the extra water like you would in the kettle.

Thoughts? Ways to calculate or guesstimate this? (Let's stipulate that nobody else is having tea since you have your kettle brewing, and whatever other simplifications are helpful. Maybe there are always 2 cups of water in the kettle, given that some will boil off and you want to have enough but not waste energy.)

posted by acm to science & nature (23 answers total)

I would guess that an electric kettle would be up there in terms of efficiency, but it may depend on what your source of electric power is: i.e., coal electric will almost necessarily be dirtier than hydro or nuclear.

I would guess that you lose a ton of joules boiling over gas heat, given that the air around the kettle heats up so much; not all of the energy goes straight into the water. (Obviously not all of the energy goes straight into the water with a microwave or electric kettle -- no system is perfect -- but to my first estimate, those are probably better.)
posted by supercres at 8:18 AM on June 6, 2012 [2 favorites]

Arguably, an electric kettle with just the right amount of water in it is right up there, since the element is extremely efficient, and transferring essentially all the heat it generates to the water. They also turn off automatically, or at least decent ones do.

Heating on a stove heats a LOT of air.

You want to consider the source of the electricity (natural gas, hydro, coal, wind) when doing any analysis of this type if you actually want to do it right.
posted by rockindata at 8:18 AM on June 6, 2012 [2 favorites]

Wikipedia says an electric kettle is 90% efficient as converting electricity into heat. Given that it operates for only a minute or two at a time, I'm not sure the source of the electricity is really going to be a significant issue.

A microwave boils water in 2-4 minutes, and draws a lot of power doing so. I can't see that being more efficient than the electric kettle.
posted by COD at 8:22 AM on June 6, 2012 [1 favorite]

Carbon emissions from electricity generating techniques will tell you relative differences between electricity sources. I still think those differences will pale compared to energy loss with an open flame (which is essentially what a gas stove is).
posted by supercres at 8:23 AM on June 6, 2012

If you're really concerned with energy use just make sun tea instead.
posted by mareli at 8:29 AM on June 6, 2012 [2 favorites]

I appreciate the input on electric kettles, but I hate having a single-purpose appliance stealing my precious storage space. Can we stick with what I have? Gas stove, electric microwave (vintage, but that should fall out of the simplifications). I think all the disagreeing households have the same two technologies at hand.

I guess we have different types of electricity, since ours is XX% renewables, while one family is mostly hydrothermal in HI, another grim PA coal. I guess I was just thinking about absolute joules per cup of tea, and feeling like electricity/microwaving is always wasty, but maybe I was undervaluing the heat leak with the stove flame...
posted by acm at 8:32 AM on June 6, 2012

Or if you can't wait for sun tea, just use a fresnel lens which you can get here (and elsewhere).

FWIW, when I taught 7th grade technology, I would do a demonstration with a smaller fresnel lens taken from a broken overhead projector. On a moderately sunny day, I could melt lead split shot in about 2 seconds and turn pennies into goo in about a minute.

Wear welding goggles if you want to look at it.
posted by plinth at 8:36 AM on June 6, 2012

I was undervaluing the heat leak with the stove flame

You are.
posted by the man of twists and turns at 8:36 AM on June 6, 2012 [9 favorites]

The mechanical engineer in me, without making any calculations, tends to agree that the microwave would be more efficient per your restrictions.

Nitpicky things to consider however would include the electronic display on the microwave and vampire power losses when it is not in use (but this might not be an issue if you have an older twist timer model), the rotating table in the microwave burning juice as well, and any potential efficiency differences in newer vs. dated microwave technology (which I can't think would be that geared towards efficiency, but rather towards controls and shielding improvements).

My gut feel for worst to best timeline view would be something like this: electric element stove --> induction stove --> gas stove --> purpose built electric kettle or microwave --> sun tea
posted by RolandOfEld at 8:42 AM on June 6, 2012 [1 favorite]

To be honest, if you want to stay sane you have to simplify some things when considering this, so I tried to stick to a more chemical/heat/electrical viewpoint in this instead of an economic analysis. Those can, in theory, go on forever if you want to go deep enough down the rabbit hole.

Say the electric energy is coal generated, ok that's clearly bad for the environment in umpity gazillion ways. Ok, now say it's wind power sourced. Great, but where did the materials for the wind turbines come from and where were they shipped from? It takes energy to make them initially and move them, shall we attempt to calculate that as well? Not saying people shouldn't think about these things, just that I didn't with my little off the cuff timeline above.

Oh, and I'm not downing renewable energy generation, please don't take that away from my little addendum here.
posted by RolandOfEld at 8:48 AM on June 6, 2012

What's your electricity source, an ancient coal burning power plant, or hydroelectric, or something else? That is a factor as well. If your kitchen is cold and you like to stand by the stove as you boil water, then the stove's relative inefficiency is beneficial to you.
posted by oneirodynia at 8:50 AM on June 6, 2012

Okay, this can totally be calculated. Heat capacity of water is 4J / gram-deg C. Assume 1L of water = 1kg, and heating from 20 to 100 deg C = 80 deg C. Therefore we have 80,000 gram-degC energy delivery required, which requires 320kJ of energy. Let's say this can be supplied with 350kJ of electricity assuming a 90% efficient kettle going on an earlier link. This is about 0.1kWh electricity by direct conversion (3.6MJ = 1kWh). Typical US grid is 500g CO2-eq per kWh which means we have 50g CO2 for the electric kettle.

Now, natural gas. This needs to be empirical because of the difficulty in measuring the energy transfer from gas output to water heating, but I'll use the result from the man of twists and turns, and say 0.2kWh. Twice as much energy. But because the combustion happens at the stove instead of some power plant, and because it is heating water directly instead of turning a turbine to generate electricity (with associated losses), it may come out better, depending on the emissions factor of combustion. Which is 0.25 kg CO2-eq / kWh according to something I have in front of me. Therefore total emissions are ... 50g CO2 for the gas stove. Huh.

This means, assuming all these assumptions are correct, that if you are in a place with a very dirty electricity grid with a lot of coal, you should use gas. If you have a lot of hydropower and renewables in your local grid, you should use electricity. Though if the microwave really does draw 0.03kWh, it will beat them both.
posted by PercussivePaul at 8:52 AM on June 6, 2012

PercussivePaul: Does your figure for 500g CO2-eq per kWh stand for generated or delivered? The grid has transmission losses that are non-negligible.

Gas also has to be delivered somehow as well and maintenance costs (if plumbed directly) or delivery costs would have to be factored in to be a fair comparison.

Plus we didn't even get into the respective environmental impact of obtaining the respective fuel sources from the earth...

See what I mean about that rabbit hole thing?
posted by RolandOfEld at 8:58 AM on June 6, 2012 [1 favorite]

Oh, and the excess heat generated from the stove could either be considered a positive or a negative thing depending on if you were running the air conditioning to cool the room or the furnace to heat the room at that given moment in your home. Make sense? *sigh* Why do I do this to myself...
posted by RolandOfEld at 9:08 AM on June 6, 2012 [1 favorite]

Ooops, sloppy. The empirical link was for a smaller amount of water. Divide my kettle numbers by 3 to make it 300mL each, and then the electric kettle ends up winning.

The emissions factors I gave should include all of the backend processes including extraction, generation, etc. I don't have enough time to really check them. The end result is that it's something of a wash, anyway.
posted by PercussivePaul at 9:12 AM on June 6, 2012

Wikipedia says an electric kettle is 90% efficient as converting electricity into heat.

Actually the link says greater than 90% with the only lost energy going to heat the kettle (light weight plastic nowadays), the loss to evaporation, and standby losses (the hot water will lose heat to the room once it goes over room temperature). If you aren't overfilling your kettle I wouldn't be surprised if it's over 95%, possibly well over.

At any rate a gas flame on pot (pretty well the worst shaped heat exchanger you could imagine) is going to have an effiecency of around 50%; half the heat is going to be wasted. A microwave approaches electric kettle efficiencies; it's pretty well only got the same losses + heating of the magnetron and transformer. Which is greener is going to come down to where the electricity is coming from.
posted by Mitheral at 9:12 AM on June 6, 2012

The lates Do the Math post was precisely about this.
posted by Bangaioh at 9:17 AM on June 6, 2012 [3 favorites]

I don't have enough time to really check them. The end result is that it's something of a wash, anyway.

Yep, totally understand and can relate.

I tend to fret over these things like the OP and nine times out of ten I discover the thing I'm fretting over is a wash and the best thing I can do for the environment boils down to things like make sun tea every now and then, drink less soda/packaged foods, bump the A/C offline, and work on driving less and biking more, etc, etc, etc.

The truly green option is nearly always 'reduce consumption' but I will say we have this electric kettle and we use it every day with our french press for coffee and I'm 100% certain it's better than having a percolator or warming the stovetop (ours is electric) up to heat water.
posted by RolandOfEld at 9:20 AM on June 6, 2012

for what it's worth, it's less fretting (in this case, heh), than general curiosity/mental exercise. although the consensus seems strong enough that I guess I should switch, short of the comfort value of kettle-huddling. I think I'm even most of the way to the electric kettle now!

thanks for all the great mullings!
posted by acm at 9:30 AM on June 6, 2012

As for it being a single-purpose appliance, that's true in the strict sense; it only boils water. But, I use mine to boil water to make pasta, beans, soup, etc. An electric kettle can get the water to a boil much more quickly than the stove. I've also used it to make hard-boiled eggs! I love my electric kettle.
posted by infodiva at 9:56 AM on June 6, 2012 [5 favorites]

Interesting to see the much lower numbers that the Do The Math post found -- 45% for the microwave, something in the 60s or 70s for the kettle. (in the 20s for the gas.) That's measured by his actual metered use of electricity or gas, not accounting for upstream inefficiencies. Still, more believable to me than the 90s being cited here initially. And, of course, there's still the likelihood that one overfills the electric kettle just like the old-fashioned one. We're getting back into the range of indeterminacy again! :)

Um, I think I'm glad I just use a hot-water-spigot at work, or this would be keeping me up nights!!
posted by acm at 10:06 AM on June 6, 2012

None of the methods are particularly efficient. The most important factor, which overwhelms all of the efficiency differences, is to only heat exactly the amount of water needed and no more. Given most people's habits, using the microwave to heat a mug of water is most exact, but if you use a gas or electric kettle, use your mug to measure the water you place in the kettle. The correct amount of water is more important than efficiency.
posted by JackFlash at 11:09 AM on June 6, 2012 [2 favorites]

Just for fun, how about a metal cup placed directly on top of an induction stove :-) That tops out at 84% efficiency - just less than the kettle - but no energy is wasted heating the kettle itself.
posted by primer_dimer at 2:15 AM on June 7, 2012