Naked Science Forum
Life Sciences => The Environment => Topic started by: Letty on 03/12/2009 09:50:50
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Just to clarify (I didn't want to make the title too long):
I was wondering what uses up more energy when I want to make a nice cup o' tea:
- Filling my kettle up with hot water from the tap (it has to run for a bit before it gets warm), so it takes less time (and energy?) for the kettle to boil it;
- Or using cold water (thus not waiting for the tapwater to heat up), resulting in the kettle needing more time to get the water to boil.
Any suggestions?
Cheers!
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It's always said that tea should be made from freshly drawn water, not water which has been standing for however long, but that's beside the point here.
What you are asking, I suppose, is which is the cheaper and more environmentally friendly way to heat the water.
Since you must run the tap a while before the hot water from your tank filters down to the tap, you will be wasting maybe 3, 5 or even 8 litres of water in the first place. Yes, your kettle will boil much quicker if the water you put in is already hot, but the water you have run away, will then be replaced in your hot water tank with the same volume of cold water, cooling the temperature of the water in your tank. Now your tank heating system will need to reheat the water in your tank to the temperature set on the thermostat. Also, this will leave hot water in the pipes from your tank to the tap, which will cool very quickly, so next time you want a cuppa, you will again waste the 3, 5 or 8 litres of water standing in the pipes, which had been hot and has now gone cold again.
You will be far better off just putting as much cold water as needed to fill the kettle to the minimum marker, have your cuppa in the comforting knowledge that you have not wasted water or energy.
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You will be far better off just putting as much cold water as needed to fill the kettle to the minimum marker, have your cuppa in the comforting knowledge that you have not wasted water or energy.
Letty, I agree with Don in general.
However, if you have a perfect set-up then there might be a net gain.
We could (for simplicity) compare gas-to-electric-to-heat with gas-to-heat.
A CCGT generator is about 55% efficient.
Power grid efficiency is ~ 92%
Kettle is near as damn it 100%
So, about 50% overall.
Modern 'on-demand' gas boiler can be over 85% efficient, but from cold has to heat up a far bit of metal (heat exchanger, etc). But we'll assume that you've just finished having a shower & the kitchen tap is next to the boiler.
I guess we have to assume the kettle has to get the water from about 50degC to boiling in both approaches. But the efficiency of the electric route (10-50degC) is delivering only 50% of the gas' energy as opposed to 80%+ for the boiler.
Very cold weather (& high humidity?) will also hamper your home boiler, but have less effect at the power station.
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I did base my answer on Letty's statement:
it has to run for a bit before it gets warm
But yes you are right, a modern condensing boiler would make a difference.
But your point, peppercorn, about heating the fabric of the kettle itself, put in mind a point I had missed from my original post. That is, that the hot water from the tank will be cooled by the cold water pipes en route to the tap, so the water may leave the tank at say 50o but by the time it gets to the tap may be only 30o until sufficient water has passed through the pipes to heat them to 50o or there about.
The net effect then might be that 10 or more litres of water would be wasted in order to get water at the 50o temperature of the tank. Perhaps 5 litres of cold water followed by a further 5 litres of water progressing from 30 - 50o.
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I did look into a related question a couple of years ago concerning the relative energy-merits of boiling water for a hot drink in a kettle versus in a saucepan on a gas (and electric) hob.
As to the question on this thread, the answer is probably "it all depends".
If your tap hot-water comes from an electric immersion heater then it can't possibly be more efficient than the kettle, so use the kettle.
If the tap hot-water comes from a gas boiler, then it may be more energy efficient than just using the kettle - because heating hot water directly by gas is more efficient than burning gas/coal in a power station and the losses associated with heat-to-mechanical-to-electrical energy transfers.
However, as others have pointed out, if you have to run the tap then you may be wasting water (which the water-company used energy to process, purify and pump to your home) and as the pipe cools afterwards, it'll "lose" heat to the house. This 'lost' heat may be beneficial in winter (when it just supplements the heating in a small way), but is almost certainly unwanted (and may lead to further air-con energy-costs) in the summer.
In the grand scheme of things, if you're just making a few cups of tea/coffee per day, then any (potential) savings are really going to be quite small and you may as well just use the kettle and enjoy the tea made from fresh water!
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Thanks a mill for all your elaborate answers.
I have an electrical boiler indeed - whether it is a 'modern condensing' one I do not know. However, taking my landlord's cheapskate tendencies into consideration; I doubt it. ;-)
This is silly though: I was focusing so much on the energy used when heating up the water through the tap - that I simply forgot about the litres of nice, fresh water that just go straight through the drain; unused. (shame on me!)
Either way: all your answers are very useful and I must say, they have enlightened me! I'll go with the cold water from the tap from now on.
Cheers!
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Take one cup of cold water and put it in the microwave.
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Take one cup of cold water and put it in the microwave.
This one may be a smart idea in energy terms but a bit risky on the health-and-safety front.. water can be super heated in a microwave (heated to above its normal boiling point, and then when you go to get the cup out of the microwave it is liable to boil explosively and cover you in water at or around 100 C. Which would be nasty.
Putting a damp, wooden kebab skewar in the cup might help, by ensuring the boiling has places to start, but I wouldn't do it myself.
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I always heat my water in the microwave, and i never had it explode.
Instead of a wooden skewer, I usually add the tiniest amount of sugar in it, to prevent the superheating
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peppercorn - You wrote: "A Modern 'on-demand' gas boiler can be over 85% efficient." I live in the mountains and have a 300 gallon propane tank for heating and hot water. Both the furnace and hot water heater are claimed to be 95% efficient. [Indeed, my hot water heater requires an electric fan to exhaust the fumes. The miniscule pilot light itself will warm the 40 gallon tank enough to take one or two showers every week.] The furnace vents from a plastic pipe and emits little more then warmed water vapor.
But you are correct. It takes some time for that efficiently warmed water to reach the tap. The pipes from the hot water heater, though well insulated themselves, displace far more water then one or two cups of hot water delivered to the tap for tea. [In the States we mostly use coffee. Remember The Boston Tea Party?]
However, an open flame used to warm a tea kettle is WAY wasteful. Much of the heat escapes to the local room. This is fine in cold weather, and seems a reasonable method of tea production in such circumstances, especially if it is a natural gas burner. However, in warmer climes, you would be better off placing ambient temperature water from whatever source into a microwave rather then run the Air Conditioner to remove the excess escaped heat from the room.
This discussion is really neat. How many angels can dance on the head of a pin? Where I live I can gather dead wood from my extra lot and boil water for free. Yeah. Like I am a real Paul Bunyan.
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Both the furnace and hot water heater are claimed to be 95% efficient.
Yes - I see you've noticed that 95% efficient is over 85% efficient. And your point was?
However, an open flame used to warm a tea kettle is WAY wasteful. Much of the heat escapes to the local room. ... However, in warmer climes, you would be better off placing ambient temperature water from whatever source into a microwave rather then run the Air Conditioner to remove the excess escaped heat from the room.
I'd be surprised if the losses from a gas ring heating a tea kettle (if well matched) are higher than the losses in the cyclotron? Also the A/C is going to have the same job to do with a μW as with a gas ring. That said, traditional kettles are usually all metal, which is not the best solution.
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I understand what you mean about assimilating information from expert reports,- it is something I periodically have to do professionally, and is the sort of report that I consider the IPCC’s AR4 to be – which is why I attach significant weight to its conclusions. However, I would make a sharp distinction between peer reviewed scientific papers and (for example) a geotechnical assessment of a quarry or Traffic Impact assessment for a proposed development prepared by an expert.
I remain unconvinced that any individuals outside of mainstream academia are sufficiently knowledgeable in statistical techniques, atmospheric science, meteorology, metrology, geology, ecology, oceanography, glaciology, physics, (and of course) climatology to be able to understand all of the science concerned to a sufficient depth to critisize professional researchers working in their very narrow way. I accept that it could be argued that this is dangerous and that it could be argued that group think occurs but, looking at it from the other side there have been literally thousands of academic, peer reviewed papers, pointing towards the same conclusion that current observed climate change is not simply a function of natural variability. If there is scientific fraud as some allege, it is such a pervasive yet incredibly well concealed fraud that it begs the question why? I find tinfoil hats too sweaty so go for the simple explanation that AGW is occurring…
Many sceptics point to differences of opinion between academics as to the extent or likely future effects of anthropogenic climate change, turning this around, I have not corresponded (on this and other internet fora) with so called sceptics who share the same overall view either…
I am not saying that in order to be sceptical about the claims in the media in relation to AGW that you need to be a post doc, nor am I saying that all sceptical bloggers are entirely wrong, simply that in forming my own opinion on the matter I am very careful as to my sources and try to get as close to the original scientific work as I can understand. In this respect I trust AR4 more than I trust the opinion of Rush Limbaugh or Nigel Lawson.
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maz - You wrote: "I remain unconvinced that any individuals outside of mainstream academia are sufficiently knowledgeable in statistical techniques, atmospheric science, meteorology, metrology, geology, ecology, oceanography, glaciology, physics, (and of course) climatology to be able to understand all of the science concerned to a sufficient depth to critisize professional researchers working in their very narrow way."
I know more about Roman Era Warming then just about any of these people. Why? Because I spent a couple of days googling Roman Era Warming, and I really really doubt few if any of these other clowns have the list I developed. Here it is. And I have more if needed. I get actually exasperated by people who do not seem to have even a clue about this, or defer to simple minded hockey stick slickers.
http://www.co2science.org/subject/r/summaries/rwpeuropenorth.php
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PS - There has not been one reply to either of my posts asking
1)How Much Have Sea Levels Changed Over Time?
2)Does Deep Ocean Volcanic Activity Vary Over Time?"
I confess, I was ready to sandbag on number one. RE the Maldivian Island Scuba Guy in the Fish Tank over in Copenhaagen. "By the end of this century, sea level may have risen by between 30cm and 50cm according to the various IPCC scenarios. Our records suggest a maximum of 20cm. Neither of those levels would pose any real problem — simply a return to the situation in the 17th and the 19th to early 20th centuries, respectively." http://www.spectator.co.uk/coffeehouse/5595813/why-the-maldives-arent-sinking.thtml
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RE Deep Sea Volcanic Activity. If there is one consensus I accept, it is that surface volcanism can immediately and dramatically effect climate. However, MOST volcanic activity takes place in the deep oceans. Yet I have NEVER seen this variable even mentioned in the current climate debate. It is a small scandal in its own right, IMHO.
For instance, I have seen studies that suggest oceanic volcanism ended the last Big Ice Age. I have also seen studies that suggest such volcanism could account for various Glacier Melts, and other observed oceanic temperature hot spots.
Current climate models are, apparently, insufficient to account for current cooling. [According to the recent hacked East Anglia Emails]. I suggest the models are fatally insufficient whatever their findings if they do not include deep ocean volcanic activity.
Really.
Really. How much genious does it take to just PONDER the variability of oceanic volcanism. If nothing more then to dismiss them. Am I the only person on the face of the planet that has noticed this omission?
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Well, well! WE are definitely 'Off topic' now, aren't we!
I also see the single post rule is being ignored yet again.
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pepper
I admit to ignorance on 'the single' post rule. I offer my regrets and pledge not to make more then one post at a time. However, I can not help but observe no one has actually addressed any of my multiple posts except to point out I should not make multiple posts.
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What the heck have the last six post to do with hot water heating in a kettle or hot water heater? At first I thought a mistake occurred and accidentally posted in the wrong thread, but no, without missing a step, the discussion continued.
Tss tss, no discipline.
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What the heck have the last six post to do with hot water heating in a kettle or hot water heater? At first I thought a mistake occurred and accidentally posted in the wrong thread, but no, without missing a step, the discussion continued. Tss tss, no discipline.
LOL!
However, I can not help but observe no one has actually addressed any of my multiple posts except to point out I should not make multiple posts.
Put your posts in a relevant thread & I'll be happy to respond to them (as long as it's not repeating the ol' stuck record of 57 other posts!) - surely you can use one you've just started, I don't know, something like 'Is global warming a liberal conspiracy?'!
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BACK ON TOPIC: What uses more energy: hot water or kettle?
A kettle. For the simple reason the hot water heater is insulated in order to conserve energy, and a kettle is not. An open flame is just about by far the absolute least inefficient way heat anything but the large open space within which it is burning. And that presumes you do not need to vent the exhaust gases as dangerous.
Just use a frign microwave and be done with it. One other choice is a sink mounted hot water dispenser. I had one of these and it just simplified everything. It consisted of a very small insulated hot water heater mounted under the sink. It was fed from a two stage water filter and could dispense ambient temperature filtered cool water, or 190 degress F filtered hot water. It was so effective we began to use high quality instant coffee. Tea was a sinch.