The energy efficiency of boiling cold vs hot tap water.

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

User avatar
King Author
Posts: 736
Joined: Sun Apr 12, 2009 12:30 pm UTC
Location: Pennsylvania, USA

The energy efficiency of boiling cold vs hot tap water.

Postby King Author » Fri Nov 20, 2009 4:25 pm UTC

So I do most of the cooking around here, and I constantly think about something that maybe someone more science-minded than I can figure out. When bringing water to a rolling boil, is it more energy-efficient to start with cold tap water or hot?

If you start with cold tap water, you don't depleat the hot water heater at all, but your stovetop is going to have to spit out a lot more gas to get the water up to boil. If, on the other hand, you start with hot tap water, you're taking from the hot water heater's reserves, but the water comes to a boil on the stovetop a lot faster.

How might one go about finding out which one consumes less energy? Both the stovetop and the hot water heater use natural gas, by the way.

Theoretically, since they're both using natural gas, it should cost about the same, but you've got to take into acount that refilling the hot water tank with a couple of cups of water is going to be rather easy, because as I understand it, the way thermodynamics works is that once you've got something going, it doesn't take as much energy to keep it maintained at a given level as it would to heat it to that level in the first place. On the other hand, since it is only a couple of cups being heated, it may be more energy efficient to just use the cold tap water for the simple reason that it doesn't take that much longer to heat a cold few cups than a tap-hot few cups of water on the stovetop.

Thoughts? I want to be as kind to my gas bill and the planet as I can when I'm cooking, but I just don't have the mental prowess to figure out this sort of problem on my own.
I have signitures disabled. If you do, too...you can't read this, so nevermind >_>

edgey
Posts: 14
Joined: Sat Oct 10, 2009 6:34 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby edgey » Fri Nov 20, 2009 4:26 pm UTC

I would imagine the boiler has a far easier job heating the water due to insulation.

User avatar
Qaanol
The Cheshirest Catamount
Posts: 3069
Joined: Sat May 09, 2009 11:55 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Qaanol » Fri Nov 20, 2009 4:39 pm UTC

edgey wrote:I would imagine the boiler has a far easier job heating the water due to insulation.

Agreed. The boiler is almost certainly more efficient.

The flip side is that the boiler heats a large quantity of water at a time. If you know you're only going to need a few cups of hot water now, and none at all for the rest of the day, it's probably more efficient to turn the boiler off completely and just boil the tap water on the stove. But if you also will take a hot shower or otherwise require the boiler, your best bet is most likely to take advantage of the already-hot water in the boiler.

On still another hand, there now exist what are called "on demand" water heaters. They do not store very much (or any?) hot water, they heat it as needed. In that case you're probably better off boiling cold tap water, simply because the water heater would have to heat enough water to fill the pipes as well as what you draw off the faucet. But again, it'll come down to relative efficiencies, so we'd need more information. Although having to wait half a minute for the on-demand hot water is probably incentive enough to just use cold water.
wee free kings

User avatar
Xanthir
My HERO!!!
Posts: 5426
Joined: Tue Feb 20, 2007 12:49 am UTC
Location: The Googleplex
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Xanthir » Fri Nov 20, 2009 5:11 pm UTC

Nod, the water wastage from waiting for the tap to turn hot is enough to justify always just boiling cold water. Unless you actively *need* hot water for something, don't wait for it.

Those on-demand water heaters are awesome. I'm planning to get one next year, especially since they come with a partial tax credit.
(defun fibs (n &optional (a 1) (b 1)) (take n (unfold '+ a b)))

User avatar
meat.paste
Posts: 404
Joined: Fri Apr 25, 2008 3:08 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby meat.paste » Fri Nov 20, 2009 9:34 pm UTC

Health concerns aside, when you wait for the hot water from the tap, you are really waiting for the hot water coming out of the hot water heater to heat up the pipes that lead to your tap. If you want to get a ballpark estimate of the energy dissipated in waiting for the hot water to arrive, measure the number of liters of water that go down the drain while you wait (wasting water as well).

Said in another way - if I have a tube with a volume of 1L connected to (but not in) a hot water supply, the tube will eventually be at equilibrium with its surroundings. The surroundings are assumed to be cooler than the hot water supply (or you wouldn't need the hot water supply). When I open a valve to collect the hot water, I let the 1L of water in the tube go away because it is cold. Assuming it requires no energy to heat the tube, I then collect the next 5L of hot water to boil my cabbage and turn off the valve. I now have 1L of hot water in the tube that slowly dissipates its energy into the surroundings. The net result is I use up 6L of hot water and throw one of those liters down the drain. The bigger the volume in your pipes and the smaller amount of hot water you need, the worse the efficiency becomes.

So, use cold water. If time is of the essence, then use hot water and accept the loss. If time and efficiency are important, consider heating cold water in a microwave before putting it on the stove. (back of the envelope calculation - 1.5 kW microwave boils 1L of water in about 4 minutes. deltaT=75C, Cv=4.184J/gC, so the water absorbs ~314 kJ and the microwave uses 360 kJ. Efficiency is about 87%, which I'm guessing is better than heating water on a stove top.)
Huh? What?

Random832
Posts: 2525
Joined: Wed Oct 10, 2007 4:38 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Random832 » Fri Nov 20, 2009 9:43 pm UTC

Heating water in a microwave can be dangerous due to superheating.

(basically, it won't start forming bubbles when it reaches the boiling point, so it'll heat to higher temperatures and then eventually when a bubble finally manages to get formed it explodes.)

User avatar
Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Tass » Fri Nov 20, 2009 9:51 pm UTC

meat.paste wrote:so the water absorbs ~314 kJ and the microwave uses 360 kJ. Efficiency is about 87%, which I'm guessing is better than heating water on a stove top.)


Yeah, but thats from electricity. The electricity has to be made from the gas (or something else) first. Using electricity directly for heat is never economical as a rule of thumb (damn convenient though).

User avatar
Charlie!
Posts: 2035
Joined: Sat Jan 12, 2008 8:20 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Charlie! » Fri Nov 20, 2009 10:32 pm UTC

Random832 wrote:Heating water in a microwave can be dangerous due to superheating.

(basically, it won't start forming bubbles when it reaches the boiling point, so it'll heat to higher temperatures and then eventually when a bubble finally manages to get formed it explodes.)

I was going to say that it's dangerous because most people use metal pans :P

Since I've boiled a fair amount of water in microwaves and never superheated any (never even heard any knocking [really fast bubble formation makes popping noises in slightly superheated stuff] that I can recall), I bet superheating is really unlikely, especially with a turntable-type microwave and typical kitchen equipment. I mean we're not talking fresh out of the box blown glassware, here. There's nucleation sites all over the place.
Some people tell me I laugh too much. To them I say, "ha ha ha!"

achan1058
Posts: 1783
Joined: Sun Nov 30, 2008 9:50 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby achan1058 » Fri Nov 20, 2009 10:48 pm UTC

Tass wrote:Yeah, but thats from electricity. The electricity has to be made from the gas (or something else) first. Using electricity directly for heat is never economical as a rule of thumb (damn convenient though).
You have nuclear energy for that purpose. While it is not necessary efficient, it's less polluting than gas is. Speaking of which, why don't people just send nuclear waste to the sun or something?

Random832
Posts: 2525
Joined: Wed Oct 10, 2007 4:38 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Random832 » Fri Nov 20, 2009 11:37 pm UTC

achan1058 wrote: Speaking of which, why don't people just send nuclear waste to the sun or something?


It takes more energy than you think to reliably drop something into the sun, as opposed to putting it in an unstable orbit where it might loop around and hit earth at some point in the future.

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26824
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby gmalivuk » Sat Nov 21, 2009 2:32 am UTC

Random832 wrote:
achan1058 wrote: Speaking of which, why don't people just send nuclear waste to the sun or something?

It takes more energy than you think to reliably drop something into the sun

Indeed. If we pretend Earth's orbit is perfectly circular at 150 million km from the sun, and ignore the initial energy expenditure needed to get something out of Earth's gravity well in the first place (which is itself definitely not negligible), then we need to change the total orbital energy from [imath]\frac{GM}{300000000000m}J/kg[/imath] to something like [imath]\frac{GM}{151000000000m}J/kg[/imath], to make it pass within 1 million km from the center of the sun (which is still outside its surface but probably close enough to take care of it). Which is 436.5 MJ/kg, or, in technical terms, rather a lot.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Carnildo
Posts: 2023
Joined: Fri Jul 18, 2008 8:43 am UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Carnildo » Sat Nov 21, 2009 3:09 am UTC

Tass wrote:
meat.paste wrote:so the water absorbs ~314 kJ and the microwave uses 360 kJ. Efficiency is about 87%, which I'm guessing is better than heating water on a stove top.)


Yeah, but thats from electricity. The electricity has to be made from the gas (or something else) first. Using electricity directly for heat is never economical as a rule of thumb (damn convenient though).


That's only true if your electricity comes from heat. If it comes from something else (say, falling water), it is.

achan1058
Posts: 1783
Joined: Sun Nov 30, 2008 9:50 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby achan1058 » Sat Nov 21, 2009 5:09 am UTC

gmalivuk wrote:
Random832 wrote:
achan1058 wrote: Speaking of which, why don't people just send nuclear waste to the sun or something?

It takes more energy than you think to reliably drop something into the sun

Indeed. If we pretend Earth's orbit is perfectly circular at 150 million km from the sun, and ignore the initial energy expenditure needed to get something out of Earth's gravity well in the first place (which is itself definitely not negligible), then we need to change the total orbital energy from [imath]\frac{GM}{300000000000m}J/kg[/imath] to something like [imath]\frac{GM}{151000000000m}J/kg[/imath], to make it pass within 1 million km from the center of the sun (which is still outside its surface but probably close enough to take care of it). Which is 436.5 MJ/kg, or, in technical terms, rather a lot.
Well, you don't really need to send it to the sun, but simply out of the earth's orbit.

User avatar
Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Tass » Sat Nov 21, 2009 9:12 am UTC

Carnildo wrote:
Tass wrote:
meat.paste wrote:so the water absorbs ~314 kJ and the microwave uses 360 kJ. Efficiency is about 87%, which I'm guessing is better than heating water on a stove top.)


Yeah, but thats from electricity. The electricity has to be made from the gas (or something else) first. Using electricity directly for heat is never economical as a rule of thumb (damn convenient though).


That's only true if your electricity comes from heat. If it comes from something else (say, falling water), it is.


Not as long as some of our electricity is generated from fossil fuels, and doing so is economically viable. Of course if the choice was between fossil fuels, and some cheap fusion powered electricity, then you are right.

The point was mostly that one can not just compare efficiencies between electric and other sources. The electric always wins.

It also depends on the temperature needed. If you need a million degrees, then electricity is basically the only way. If you only need twenty, then direct from electricity is very wasteful, but heat pumps will beat direct burning. If you need a few hundred (which is what burning is good at) then electricity will be a wasteful detour.

Technical Ben
Posts: 2986
Joined: Tue May 27, 2008 10:42 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Technical Ben » Sat Nov 21, 2009 4:14 pm UTC

I always avoided drinking water from the "hot water" tap as the boiler systems I have seen put me off it. If it's a storage heater, I have seen dead pigeons and other animals in the water tank (not the heating unit, but the large reserve in the attic) and the instant heaters in some places get a big collection of silt.

I don't know if boiling makes it safe, but I won't chance it.

Oh, googled this from the UK .gov information site.
Q-"Should I drink water from the taps in my bathroom?"

A- You should drink water from the cold bathroom tap only if the water comes directly from the supply main. Otherwise, you should always use water from the cold water tap in the kitchen. The cold water taps in the bathroom may be supplied from a storage tank in the loft so the quality may not be as good as that from the kitchen tap, which comes directly from the mains. Do not drink water from hot water taps as it may contain high levels of copper.
(emphases mine. Copper from the pipes?)
It's all physics and stamp collecting.
It's not a particle or a wave. It's just an exchange.

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26824
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby gmalivuk » Sat Nov 21, 2009 4:53 pm UTC

Wow, the only other place I knew of where water is stored in tanks above the tap like that was Mexico. (And I suspect it's because of this that you shouldn't drink the water there, rather than particularly bad water quality when it comes out of the main in the first place...)

Edit:
achan1058 wrote:Well, you don't really need to send it to the sun, but simply out of the earth's orbit.

Well like I said, I ignored getting off of Earth's surface in that calculation. So in addition to the still-not-negligible energy required to move it to a solar orbit that's farther in or farther out than Earth's, you've also got to spend at least 62.5 MJ/kg just to escape from Earth's gravity well. (If there were some way to use nuclear energy to do this, it wouldn't be so bad, since according to this site, it seems we can get almost 309MWh of electricity for every kg of spent fuel. Which is several orders of magnitude more than is needed to eject that fuel from Earth's gravity well.)
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

Technical Ben
Posts: 2986
Joined: Tue May 27, 2008 10:42 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Technical Ben » Sat Nov 21, 2009 6:53 pm UTC

It's only for the older hot water tanks and for use in the bath. I think it might be as an overflow when the boiler overheats/pressure gets to much and to make sure you get a constant water pressure. Not used for drinking water or anything else. Combie boilers do not use them.

I'd just say I prefer not to. Warm water just makes me go :yuk:.
Would all the pipes waste energy? Your loosing a lot just filling up the space between the tap and the boiler.
It's all physics and stamp collecting.
It's not a particle or a wave. It's just an exchange.

User avatar
meat.paste
Posts: 404
Joined: Fri Apr 25, 2008 3:08 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby meat.paste » Mon Nov 23, 2009 10:05 pm UTC

gmalivuk wrote:
Random832 wrote:
achan1058 wrote: Speaking of which, why don't people just send nuclear waste to the sun or something?

It takes more energy than you think to reliably drop something into the sun

Indeed. If we pretend Earth's orbit is perfectly circular at 150 million km from the sun, and ignore the initial energy expenditure needed to get something out of Earth's gravity well in the first place (which is itself definitely not negligible), then we need to change the total orbital energy from [imath]\frac{GM}{300000000000m}J/kg[/imath] to something like [imath]\frac{GM}{151000000000m}J/kg[/imath], to make it pass within 1 million km from the center of the sun (which is still outside its surface but probably close enough to take care of it). Which is 436.5 MJ/kg, or, in technical terms, rather a lot.

I'm confused. Wouldn't the ejection of a mass from the earth into the sun result in the net release of energy? The payload should be picking up speed as it falls towards the sun. I see how that energy would come from the Earth's orbital energy, but if we ejected the mass so that it would be stationary relative to the sun (ejected at -orbital velocity of the Earth), that should be a velocity or 0.94Gm/yr, or 29.8 km/s. This is 444MJ/kg, which is in fine agreement with your answer. So, never mind.

Still, it seems there should be a lower energy way to do this other than cancelling out ALL the orbital motion. If the mechanics worked out to have the wast never pass within 20 Earth radii, could we lower the energy bill? Maybe transfer some of the orbital energy to another body?
Huh? What?

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26824
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby gmalivuk » Mon Nov 23, 2009 10:32 pm UTC

meat.paste wrote:Still, it seems there should be a lower energy way to do this other than cancelling out ALL the orbital motion. If the mechanics worked out to have the waste never pass within 20 Earth radii, could we lower the energy bill?

Yes, but at the very least we still have to get it out of Earth's gravity well in the first place, which I made a subsequent post about. (For another body to give some momentum, we'd have to first get it to that other body, which still requires getting it away from Earth.)
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
meat.paste
Posts: 404
Joined: Fri Apr 25, 2008 3:08 pm UTC

Re: The energy efficiency of boiling cold vs hot tap water.

Postby meat.paste » Mon Nov 23, 2009 11:35 pm UTC

I did see the post about the escape velocity from the Earth, I was just ignoring it (I calculate it at 62.7 MJ/kg). If I understand the orbital mechanics properly, then the total energy required to launch a payload out of Earth's gravity well and remove the Earth's orbital velocity should be 506.7 MJ/kg (simple sum of the velocities involved) . By the way, I'm calculating the energy required per kilogram to move something by finding 0.5*(delta-V)^2. I'm getting close to the values you get, gmalivuk, but they are not the same. I suspect my method is overly crude for celestial mechanics. How do you find "the answers"?

I was looking at wikipedia and noticed that the total velocity change required to simply eject the mass from the solar system (starting at the Earth's surface and launching in the direction of our orbital motion) is only 16.7 km/s or 139 MJ/kg.

And just to stay vaguely on topic...
Even if natural gas is used to generate the electricity to power the microwave, I still think the efficiency will be better than directly heating the water with burning gas.

According to this, turbine efficiencies can be near 50%. For a stove top, the natural gas is only about 40% efficient (http://michaelbluejay.com/electricity/cooking.html). For the calculated natural gas to electricity to microwave conversion, the total efficiency is about the same. So, my gut feel is wrong. Ah well.
Huh? What?

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26824
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby gmalivuk » Tue Nov 24, 2009 1:23 am UTC

meat.paste wrote:I did see the post about the escape velocity from the Earth, I was just ignoring it (I calculate it at 62.7 MJ/kg).

That's just the difference of using 11.18km/s versus 11.2km/s.

By the way, I'm calculating the energy required per kilogram to move something by finding 0.5*(delta-V)^2. I'm getting close to the values you get, gmalivuk, but they are not the same. I suspect my method is overly crude for celestial mechanics. How do you find "the answers"?

Well the equation I used first, that you quoted above, didn't involve completely killing its orbital motion, for one thing. Rather, as I said, it put it in an orbit that brought it within a few hundred thousand km of the sun's surface, which I figure should be close enough for our purposes. The equation I used for that was just the specific orbital energy for an elliptical orbit. (Other assumptions are as stated in that post: circular orbit, exactly 150 million km from the center of the sun, not worrying about escaping from Earth.)
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
King Author
Posts: 736
Joined: Sun Apr 12, 2009 12:30 pm UTC
Location: Pennsylvania, USA

Re: The energy efficiency of boiling cold vs hot tap water.

Postby King Author » Tue Nov 24, 2009 8:52 pm UTC

@Everyone: I'll trust that your math mojo is righteous, rather than try to understand it, and use cold tap water from now on.

Random832 wrote:
achan1058 wrote: Speaking of which, why don't people just send nuclear waste to the sun or something?


It takes more energy than you think to reliably drop something into the sun, as opposed to putting it in an unstable orbit where it might loop around and hit earth at some point in the future.


Fry: Who cares? That won't be for hundreds of years.
Farnsworth: Exactly! It's none of our concern.
Fry: That's the 20th century spirit!

gmalivuk wrote:Indeed. If we pretend Earth's orbit is perfectly circular at 150 million km from the sun, and ignore the initial energy expenditure needed to get something out of Earth's gravity well in the first place (which is itself definitely not negligible), then we need to change the total orbital energy from \frac{GM}{300000000000m}J/kg to something like \frac{GM}{151000000000m}J/kg, to make it pass within 1 million km from the center of the sun (which is still outside its surface but probably close enough to take care of it). Which is 436.5 MJ/kg, or, in technical terms, rather a lot.


I always thought that was just a sci-fi trope -- isn't there no drag in space? So, you'd only need enough fuel to point yourself in the right direction, then get to as fast as you can. From there, you don't need fuel to sustain your flight because there's no resistance. That is, if you threw a football in space, it'd go on forever. Barring micrometeroids and the gravity of planets and stuff, of course.

Technical Ben wrote:I always avoided drinking water from the "hot water" tap as the boiler systems I have seen put me off it. If it's a storage heater, I have seen dead pigeons and other animals in the water tank (not the heating unit, but the large reserve in the attic) and the instant heaters in some places get a big collection of silt.

I don't know if boiling makes it safe, but I won't chance it.

Oh, googled this from the UK .gov information site.
Q-"Should I drink water from the taps in my bathroom?"

A- You should drink water from the cold bathroom tap only if the water comes directly from the supply main. Otherwise, you should always use water from the cold water tap in the kitchen. The cold water taps in the bathroom may be supplied from a storage tank in the loft so the quality may not be as good as that from the kitchen tap, which comes directly from the mains. Do not drink water from hot water taps as it may contain high levels of copper.
(emphases mine. Copper from the pipes?)


I don't think that's how it works around here. I live in a house in the U.S. and I've never even heard of an attic storage tank.
I have signitures disabled. If you do, too...you can't read this, so nevermind >_>

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26824
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby gmalivuk » Tue Nov 24, 2009 9:00 pm UTC

King Author wrote:I always thought that was just a sci-fi trope -- isn't there no drag in space? So, you'd only need enough fuel to point yourself in the right direction, then get to as fast as you can. From there, you don't need fuel to sustain your flight because there's no resistance. That is, if you threw a football in space, it'd go on forever. Barring micrometeroids and the gravity of planets and stuff, of course.

Who said anything about drag? I was only ever using gravity in my computations. (Earth's atmosphere, incidentally, is another thing that adds to the energy requirment.)

If you throw a football in space, it'll fall into a star or planet, or it will orbit a star or planet. Unless you throw it really really hard, and it's able to escape the solar system you're in.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

masher
Posts: 821
Joined: Tue Oct 23, 2007 11:07 pm UTC
Location: Melbourne, Australia

Re: The energy efficiency of boiling cold vs hot tap water.

Postby masher » Tue Nov 24, 2009 9:05 pm UTC

Are you also taking into account all of the cold water from the hot tap that you have to waste until the hot water comes out?

Xanthir wrote:Those on-demand water heaters are awesome. I'm planning to get one next year, especially since they come with a partial tax credit.


They aren't normal?

We've got a solar-boosted on-demand gas heater - 'tis fantabulous

User avatar
Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Tass » Tue Nov 24, 2009 9:10 pm UTC

gmalivuk wrote:
King Author wrote:I always thought that was just a sci-fi trope -- isn't there no drag in space? So, you'd only need enough fuel to point yourself in the right direction, then get to as fast as you can. From there, you don't need fuel to sustain your flight because there's no resistance. That is, if you threw a football in space, it'd go on forever. Barring micrometeroids and the gravity of planets and stuff, of course.

Who said anything about drag? I was only ever using gravity in my computations. (Earth's atmosphere, incidentally, is another thing that adds to the energy requirment.)

If you throw a football in space, it'll fall into a star or planet, or it will orbit a star or planet. Unless you throw it really really hard, and it's able to escape the solar system you're in.


Wasn't the point rather that because there is no drag you have to kick the football damned hard to actually make it hit the star, assuming you start in orbit around the star?

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26824
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby gmalivuk » Tue Nov 24, 2009 9:14 pm UTC

My understanding was that King Author thought I was assuming the thing would somehow be using energy the whole time in flight. But my computations were actually for instantaneous delta-V.* Yes, you "only" have to get the thing moving in the right direction, but considering that Earth is already moving around the sun at 30km/s, that's by no means a trivial endeavor.

* In fact, the amount of energy you need to expend to change your orbit ends up being higher than my calculations would indicate, because you can never apply all of that energy at precisely the one point where its effect would be maximized. So you have to use more energy than that over a period of time, which would be several minutes at least to drop something into the Sun. (It's actually more energy efficient to loop it around Jupiter , so it can shed orbital energy there and fall in, than it is to try and send something directly to the Sun.)
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
King Author
Posts: 736
Joined: Sun Apr 12, 2009 12:30 pm UTC
Location: Pennsylvania, USA

Re: The energy efficiency of boiling cold vs hot tap water.

Postby King Author » Wed Nov 25, 2009 3:01 pm UTC

gmalivuk wrote:My understanding was that King Author thought I was assuming the thing would somehow be using energy the whole time in flight. But my computations were actually for instantaneous delta-V.* Yes, you "only" have to get the thing moving in the right direction, but considering that Earth is already moving around the sun at 30km/s, that's by no means a trivial endeavor.


You're right -- knowing nothing about physics, I saw the magnitude of that formula and thought you were calculating for fuel being constantly consumed throughout the whole flight (erm, is "flight" the proper term for space travel?). That's the energy required just to get it started? Yikes! We desperately need that space elevator.
I have signitures disabled. If you do, too...you can't read this, so nevermind >_>

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26824
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: The energy efficiency of boiling cold vs hot tap water.

Postby gmalivuk » Wed Nov 25, 2009 3:19 pm UTC

Remember, though, that sending something to the sun is really hard compared to other trips. We could go to Mars and the Asteroids and Jupiter each for considerably less energy than it would take to actually get to the sun.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
Adacore
Posts: 2755
Joined: Fri Feb 20, 2009 12:35 pm UTC
Location: 한국 창원

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Adacore » Thu Nov 26, 2009 2:34 pm UTC

meat.paste wrote:According to this, turbine efficiencies can be near 50%. For a stove top, the natural gas is only about 40% efficient (http://michaelbluejay.com/electricity/cooking.html). For the calculated natural gas to electricity to microwave conversion, the total efficiency is about the same. So, my gut feel is wrong. Ah well.

I suspect the electricity to microwave has a lower efficiency than direct gas heating. As well as turbine efficiency you then have transmission and transformer losses, plus the microwave efficiency. In addition, most power plants will need a fair amount of auxiliary power (electricity used to run things like fans, pumps, lights, gas cleaning equipment, &c. in the power plant) which will reduce the overall efficiency of the plant.

In terms of cost economy, however, which was the original question, I'm not so sure. It's really damn hard to get accurate price data for energy sources (which is stupid, you'd think with the current energy savings focus they'd make it a legal requirement for companies to publish their exact prices), but I think it's around 12p per kWh for electricity and 7p per kWh for gas in the UK, although those figures might be a few years out of date. With those numbers and the efficiencies mentioned earlier, the electricity works out at about 14p per kWh and the gas at about 17p per kWh to heat water.

In terms of actual energy cost, a single hob has a typical maximum wattage of around 1kW (varies quite substantially depending on the size and type of the hob in question and I have no idea if this figure is really right, which screws the rest of the maths). Using that for 15 minutes to boil water would be about 0.25 kWh [1000J/s * 60s/min * 15min / 3600000J/kWh], so, assuming a gas hob and the price figures above, would cost about 2 pence. If we assume it takes ten minutes on a hob of 1kW gas input to heat your litre of water to boiling temperature (1.2 pence in gas), or alternatively you could use a microwave to input the required 314kJ using your 87% efficient microwave (~360kJ input, so 0.1kWh, 1.2 pence in electricity). So it works out almost exactly the same price.

Hob costs work out reasonably small compared to some other uses of power such as using an oven (half an hour in an electric 4kW oven would cost 24 pence in electricity), since you tend not to use hobs for as long - if you were simmering a stew or something for hours, it might compare, but the actual initial heat input to get it boiling (the hot water vs cold water to start problem) is relatively small.

NB: I considered doing a full thermodynamic analysis of the boiling pan problem - I did it at uni, but it would take ages to look up all the right conductive and convective heat transfer coefficients, and you'd need to make a ton of assumptions, so I decided against. The calculation I did above assumes that your hob transfers heat to the water with about 52% efficiency. I should probably check that...

User avatar
Tass
Posts: 1909
Joined: Tue Nov 11, 2008 2:21 pm UTC
Location: Niels Bohr Institute, Copenhagen.

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Tass » Fri Nov 27, 2009 7:02 am UTC

Twelve pence for a kWh? Wow thats cheap. We pay something like 2.15 danish kroner, thats more than double. (According to google 0.263GBP)

By the way, this discussion is beginning to remind me of 309 :lol:

User avatar
Mot
Posts: 82
Joined: Wed Mar 10, 2010 10:03 am UTC
Location: Welgevonden, Stellenbosch, South-Friggin-Africa

Re: The energy efficiency of boiling cold vs hot tap water.

Postby Mot » Mon Apr 19, 2010 8:27 pm UTC

I have a similar query about hot water from a heater vs hot water from a kettle. The application in question is doing the dishes (My housemate assumes the role of chef, I do the dishes).

Given an electric heater (large water supply kept warm) and an electric kettle, which would be more efficient:*
a) Using the kettle to heat cold water; or
b) Using water from the heater?

I'm sure the answer will change depending on variables such as the temperature of the heater and the length of the pipes between the heater and the sink(or at least the volume of water therein).
The reason this concerns me is because doing the dishes takes a fairly small amount of hot water and the heater is quite far away from the sink(much more cold water comes out the tap before hot water than the amount of hot water which is actually used, so efficiency must be quite low).

*Ignore water wasting. I am purely interested in electricity use
Robo captain, do you not realize that by destroying the human race because of their destructive tendencies, we too have become like, well...it's ironic. (mmm) Cause we...
Silence! Destroy him!
DOO! (boo!) Dooboodoo!


Return to “Science”

Who is online

Users browsing this forum: No registered users and 13 guests