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Heating water for coffee - efficiently.
Heating water for coffee - efficiently.
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Old 25th April 2018, 02:47 PM   #1
Pano is offline Pano  United States
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Heating water for coffee - efficiently.
Default Heating water for coffee - efficiently.

A bit of an engineering question. How to heat water quickly and efficiently?

Every morning I make a cup of coffee in a small french press. I use 350cc of water starting at 21C. I place the water in the microwave oven to bring it to a boil and this takes about 2 minutes.
The microwave is rated at 1.58kW (how much of that is actually radiated I don't know)

According to what I can calculate, the power needed to raise the water temp by 79 degrees in 2 minutes is 970 watts. That makes my microwave heating 60% efficient. Is that good, or can I do better?
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Old 25th April 2018, 02:55 PM   #2
MuddJester is offline MuddJester  United States
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I don't know about microwave ovens, but wouldn't a simple resistive heater be 100% efficient (neglecting losses through air cooling and evaporation)? Like one of those purpose made water boilers?

If you're looking to get higher efficiency, try thermoelectric coolers. The heat coming out on the hot side is the electrical energy put in plus the heat moved away from the cold side. For heating, better than 100% (I know, I know, doesn't make sense at first).
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Old 25th April 2018, 02:56 PM   #3
billshurv is online now billshurv  United Kingdom
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Heating water for coffee - efficiently.
A kettle is close to 100% efficient less heat loss from the container provided it will boil that little. I have a 3kw kettle, let me time that for the same amount of water.
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Old 25th April 2018, 03:00 PM   #4
multisync is offline multisync  Canada
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Many many years ago I used an electric immersion type of heater element that I put in the coffee mug and it boiled the water in no time. It looked like a soldering iron heating element. Since the electric heater was in the mug and the only heat loss was through the mug sides and a little bit from the water to air contact. I would say it was fairly efficient. I remember seeing a parabolic solar water heater, Im guessing it might be impractical when its raining or cloudy.
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Old 25th April 2018, 03:05 PM   #5
Pano is offline Pano  United States
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Heating water for coffee - efficiently.
Thanks. I was thinking of the little immersion heaters like I used to carry traveling. But typically those are 300W, and not very fast. But I have seen 1000W and 1500W models. Might be too big, tho.

Bill, thanks for looking at the kettle timing for me. The formula that seems most common is this:
volumeX4Xrise/3412 is the power (kW) need to raise the volume (liters) by degrees C in one hour. So in my case 0.35x4x79/3412=0.0324 for 1 hour.
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Old 25th April 2018, 03:13 PM   #6
kevinkr is offline kevinkr  United States
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Heating water for coffee - efficiently.
Mike something like this might work for you and would probably be an efficiency improvement over the microwave, it's probably slightly slower..

Amazon.com: Bonavita 0.5L Mini Kettle, Stainless Steel: Kitchen & Dining

I use something similar at work to boost the luke warm hot water from water filter to something actually suitable for brewing tea.

This one has the added benefit of no plastic in contact with the water.
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Old 25th April 2018, 03:20 PM   #7
Juergen Knoop is offline Juergen Knoop  Germany
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Quote:
Originally Posted by Pano View Post
I use 350cc of water starting at 21C.
a slightly larger, say half a liter, electric kettle made of plastics will be most efficient and cheap too. Metal or glass ones will have more thermal losses.
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Old 25th April 2018, 03:31 PM   #8
DPH is online now DPH  United States
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The energy cost to manufacture an additional kettle is going to take years of use (if ever) to recoup. Your 60% isn't far off other efficiencies I'm seeing. Also, seeing as this is 2 minutes a day, not a lot of time per day to integrate these advantages.

Gotta think cradle to grave with these things.
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Old 25th April 2018, 03:31 PM   #9
Pano is offline Pano  United States
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Heating water for coffee - efficiently.
Thanks again. I've been looking at the 2 cup models like Kevin posted. Didn't know they came that small. None have a stated power on Amazon, but seem to boil 2 cups (473cc) in 4 minutes. So an estimated 650-700 watts.
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Old 25th April 2018, 03:32 PM   #10
mkane77g is offline mkane77g  United States
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Heating water for coffee - efficiently.
I also heat water in the microwave for pour overs. 8oz a shot. Takes 2min 5 sec. to attain a temp of 205.

Last edited by mkane77g; 25th April 2018 at 03:45 PM.
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