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Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Mon Jun 06, 2011 8:49 pm
by John46
The energy content of wood is around 16.2 megajoules per kilogram (4.5 kWh/kg).
(according to the Sustainable Energy Development Office (SEDO), part of the Government of Western Australia)
Your fire would probably be around 70% efficient.
Wood heating is certainly a great idea IMO, if you have free wood.
Its sort of solar heating - just very indirect :) .

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Tue Jun 07, 2011 7:56 am
by Tracker
Gordon-Loomberah wrote:depending on the coefficient of performance, they would be ahead by a reasonable factor. However, unless they are using a different gas than what's in a heat pump hot water system, they wont bring the water to a boil... or are there heat pumps for boiling water in the kitchen available now :?:


The problem with the heat-pump, is that it would be possibly wasteful, in that it would create COLD within the environment (the kitchen), that would then need to be heated, during winter.
It's the reverse of a refrigerator, where in SUMMER, the created heat becomes a nuisance.
From my findings with the HW-Heat Pump, it is clear that in winter, I use significantly more power (60%) to heat the same volume of water.

I admit that my first response to the first question of "Which is the most efficient"? would have been √ŹNDUCTION"..
Not having played with them, I suppose the issue is the heat generated by the electronics.
I am aware that an installation requirement is that they have plenty of ventilation to move the waste heat.

The only other consideration in the "COST" of heating that water, is the actualy COST of the energy.
ie. the cost of the GAS used , rather than the WATTS equivalent of the energy. I'll try and calculate that from your maths

A most interesting experiment.!

I still ponder why any form of "Resistive Heating" differs in watts/cost, to another.
ie. I would have guessed that the Immersion-Heater and the "Resistive-Element-Jug" would have used the same WATTS.
I could have imagined that one of those "Conduction" jugs ( The two separated plates ), might be inefficient because some energy could be lost to other energies being created.
Could the immersion-heater be more efficient, because it's a gentle heater, with minimal energy lost to violent water aggitation.
Was the old-electric-jug, a wire-wound bobbin or an enclosed element. ?
I would still think that a quality jug with an enclosed element (replicating an immersion heater) would then be the same as the immersion heater.

PS -- There are such devices as Refrigerated (Heat-Pump) water heaters.
They tend to have two outlets.. One with boiling water and one with refirgerated water, so they do make the maximum use of the available energy.
I too, wonder how they could use a normal refrigeration gas to BOIL water. I think that the maximum temp you can get from a normal heat-pump is about 70DegC
Perhaps, they pre-heat, a tank of water, which then feeds (upon usage) another tank containing a resistive heater to boost the final temp to boiling.

PPS -- OT -- Last night in Sydney , the temp got down to 5DegC.
Last night I smelt burning coal. We have noticed burning wood smells for some time.
God help us if we end up like it was back in the 60's with the heavy smell of burning coal and smogs that blanketed the sun until mid mornings..
..
.

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Tue Jun 07, 2011 10:22 am
by zzsstt
Tracker wrote:Last night I smelt burning coal. We have noticed burning wood smells for some time.
God help us if we end up like it was back in the 60's with the heavy smell of burning coal and smogs that blanketed the sun until mid mornings..


All those nasty things that have been removed from our environment, so now people no longer suffer from allergies and asthma. Oh, hang on. People never used to suffer from those things, but now they do....Hmmm.

The experiment is interesting indeed, but not all encompassing. A wood fire that is heating the house surely boils the water for free - the fire would be burning anyway?

Taking it further, the most efficient method would differ from summer to winter. In summer any energy added to the room may require active cooling later, so the most effective method (extreme) would be to take the water outside and heat it with a direct solar collector that was mounted on the roof and thus remove heat that would otherwise enter the house and require removal. In winter the most efficient approach would be to use energy gathered outside and brought inside.

Then, of course, we have waste heat. Can we concoct a method for using waste heat from the fridge (or the water cooled computer) to preheat the water.....? ;)

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Thu Jun 09, 2011 10:30 pm
by Smurf1976
Tracker wrote:
Gordon-Loomberah wrote:PPS -- OT -- Last night in Sydney , the temp got down to 5DegC.
Last night I smelt burning coal. We have noticed burning wood smells for some time.
God help us if we end up like it was back in the 60's with the heavy smell of burning coal and smogs that blanketed the sun until mid mornings..

I don't want to sidetrack the thread too much, but is coal generally available to the public in Sydney? I'd have thought it wouldn't be but maybe it is? Just wondering...

As for the smoke in the past, was that from household fires or was it from the coal-fired power plants that used to be near the city? I know they existed and I've seen photos generally taken when they weren't running. I've often wondered how much of an impact they had on air quality since there wouldn't have been much in terms of emissions controls back then.

As for heating water, don't forget that for those using grid power any consideration of electric versus gas should include the upstream energy use at the power station etc if the basis for comparisson is impact on the enviornment or use of resources (as distinct from purely cost or some academic criteria). Gas maybe less efficient in the kitchen, but it's a lot more efficient than fuel generated electricity to produce which offsets its lower efficiency in use to some extent.

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Thu Jun 16, 2011 11:57 am
by lad
I believe that it takes 4.18 joules of energy to raise the temperature of 1 ml of water by 1 degree celcius.

375 Ml X Raise from 12 to 100 (88degrees) X 4.18 = 137940j or 137.94KJ

using an online converter 33kj = 38.316666667 W h

This is the best possible theoretical requirement of energy to bring this water to the boil. Every real world method of raising water temperature will have losses so by comparison we are measuring these losses.

Looks like an opportunity for a considerable efficiency gain

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Thu Jun 23, 2011 8:42 pm
by Cherokee Solar
Hi Gordon,

On my test 12v solar system here I have a 12v kettle. It draws 10Ah and takes around 25 minutes to boil about two cups of water. So at 10Ah for about 0.42 of an hour = about 4.2amps. So to convert to watts equals about 50w.

The morningstar 15a controller has probably never seen such big loads before!

Clearly, this is by far the slowest, but most efficient method of heating a cup of tea. I challenge anyone to find a more efficient method energy wise. It's just really slow... ;)

By the way, a mate of mine left the 12v kettle here many years ago which certainly must add something to the efficiency! :)

Regards

Chris

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Thu Jun 23, 2011 10:32 pm
by Gordon-Loomberah
Hey Chris, I'm glad you arrived at 50, which is the right number, in the end :) Some dodgy units along the way though! :o

It draws 10A (for 25 min = ~4.2Ah)
@ 12V = 120W
120W * 25mins = 50Wh


I'd expect it to be slightly more efficient than the 240V electric jug I used, as it avoids the inverter inefficiency, but because it takes so long, more heat is lost to the surroundings through radiation, conduction, and convection, which decrease the overall efficiency somewhat. If it was a higher wattage, it should be a bit more efficient. Insulating the kettle would help too!

My parents used to have a 12V immersion heater for heating cups of tea whilst travelling, I guess they are still available...I reckon that would be more efficient again, and hard to beat, especially in a cup of low specific heat and poor conductivity like polystyrene.

Still, if the sun is shining, not that it has been the past couple of 8C max days, I'm not worried by a little inverter inefficiency :)

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Thu Jun 30, 2011 3:51 pm
by australsolarier
gorden,

i was just wondering, maybe the experiment should be repeated with 2 liters of water. i mean the pan for the induction stove would be fairly large in relation to a cup of water. or if you have such a tiny pan it also might not be very effective.
the large pan would radiate out a fair bit of heat.

i read somewhere induction stoves are 20% more effective than hotplate stoves.

slower warming of water should be less effective, as the container has heaps more time to radiate heat. so the more time it needs the more energy lost.

then the microwave and the induction, there could be different efficiencies, like different engine technologies have better or lesser mileage.

thanks for the experiment
urs

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Thu Jun 30, 2011 4:18 pm
by australsolarier
come to think of it, a cup of water for the microwave is probably not the ideal shape either, a plate would be better, as it probably catches more rays to heat the water

Re: Boiling water for a cuppa - which is the most efficient way?

PostPosted: Thu Jun 30, 2011 6:40 pm
by Gordon-Loomberah
australsolarier wrote:i was just wondering, maybe the experiment should be repeated with 2 liters of water. i mean the pan for the induction stove would be fairly large in relation to a cup of water...


It was just a regular kettle, the same as I used on the gas, so it was a fair comparison between those 2 heat sources. Yes the size of the container relative to the volume of water is important, and so is the speed of heating, as I mentioned in reply to Chris's slow boil test.
I was just doing the test for 1 large cup of water, the standard size used by my wife for her hot drinks... we dont often have a need for 2l of boiling water ;)
However, yes I may do a test with a larger volume of water at some time when I have power and some time to spare :)