6

Just like the title said.

In my house, we are very heavy tea drinkers. So my mother tends to boil a lot of water every day. She usually does this with a small amount of water (1-2L) every time she boils the water.

This make me wonder: I know that it takes the same amount of energy to heat 1 L water ten times and 10 L water once, since both of them are essentially same. But in practice, does heating 1 L water ten times consume more energy (in gas or electricity) than just heating 10 L water in bulk once, due to the design of kettle, stove, heat transfer or something?

I tried searching the net for this but found no conclusive answer.

  • 2
    Kinetic energy is 'moving' energy. You mean chemical energy (from gas), or electrical energy (from electricity). – Eric Deloak Nov 27 '15 at 17:54
  • @EricDeloak: edited the question, including your point – PJTraill Dec 3 '15 at 15:14
  • Some points: if you boil 10 l in advance, you may sometimes bol too much. As answers indicate: heat losses are a problem; don’t use a kettle on a hot plate; make sure you boil the right amount. I have marked the scale on our kettle with the amounts needed for various pots and numbers of cups. If you keep the water standing all day, you may find it acquires an unpleasant taste. I am disappointed that none of the answers so far have given typical figures, so one can make a quantitative judgement. – PJTraill Dec 3 '15 at 15:18
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Unless she's actually using the 1-2 litres of tea each time, there's more savings from simply boiling less water. We bought a small electric kettle with a flat bottom and glass walls to encourage people to boil the one cup of water they actually want, and that has worked reasonably well.

small kettle

If you can make 10 litres of boiling water and insulate the container then you might save energy, especially if you can fill it with tea at a slightly lower temperature rather than needing boiling water every time. The savings are as much from the lower temperature as the extra insulation.

You can also buy an insulated version of the big tea urns used at conference centres and some offices, and that will be a good start on your project. It'll be easier to super-insulate one of those than to start from scratch, and it's also easier to turn it off at night when you don't need it to stay hot.

insulated tea urn

If you have a decent electricity supply you might be better off with an "instant hot water" heater in the kitchen, so that the whole tea-making process is faster (this is what you tell her) as well as using less electricity (which is what you tell whoever pays the bills). The reason offices use them is the time saving, so they don't pay people to stand in the kitchen waiting for the kettle to boil, but the good ones also save energy (instead of having a really big tank of hot water to insulate they have a tiny little one, so they lose less heat)

The inside of those looks like this

instant hot water heater

  • Also, you might find that using hot water from a solar hot water system works even better, assuming you can afford one and have a roof you can install it on. – Móż Dec 6 '15 at 10:58
  • And heating with gas is generally more energy efficient than doing it electrically. – Jan Doggen Jan 28 '16 at 12:30
  • @JanDoggen can you explain that a bit more? Resistive electric heaters are normally close to 100% efficient while gas heaters lose heat up the flue. – Móż Jan 28 '16 at 20:05
  • You don't lose it at the heating point, but in the generation of the electricty. – Jan Doggen Jan 29 '16 at 8:25
  • @JanDoggen so somewhere between the solar PV array and the water heater? I think there are so many unknowns in your statement that the best you can say is that "in different places one or the other is more efficient". Some parts of India have piped gas and coal power, others have bottled gas trucked great distances to compete with local wind farms. Those questions are very local. – Móż Jan 29 '16 at 8:35
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The heat loss will be proportional to the surface area. If you boil ten litres, one litre at a time (rather than all ten litres at once), there's typically a lot more surface area, so the heat loss will be higher during boiling, so it will take more energy.

However, once boiled, it will then lose energy. So if you're boiling water for consumption many hours later, you probably will end up using more energy, because when you want that water hours later, you're going to have to reheat it to a suitable temperature.

  • So in other words... It make no much difference... so much for my energy saving scheme... sigh – Satori Wita Nov 26 '15 at 18:23
3

A. It's energy efficient to boil exactly that amount of water which is actually needed for tea preparation. To heat more water means some water is not used and heat will be wasted into the air.

B. But if you need to prepare for example 5 liters of the tea at once, it is more effective to heat it at once, and not to heat it 5 times 1 liter, due to following reasons:

  1. detection of 100°C is not exact and almost all heaters will switch off the power some moments after the water already boiled. It's better if this happens once instead of 5 times.

  2. water is usually heated in cylinder shapes so if you take the formulas for cylinder Volume V=πr^2h and Area A=2πr(r + h), you will get nonlinear dependency between Volume and Area(surface). The heat waste is directly dependent on the surface area.

    If you make some reasonable modeling, you will get that cylinders for 5 liters have 1.5-2.5 times more of Area (surface) and if you multiply surface 5 times, you can get numbers of 10-30 times more in volume, depend on the shape of your cylindric heater.

    I recommend to calculate it for your case/heater. This is the theory, there are many ways to reduce heat waste of the heater and different heaters will have a different index of heat waste on the top, bottom, and sides.

0

This is one of those messy questions. The answer, "It depends"

Consider a 1 candle power heater. It heats 1 cup of water to boiling in 10 minutes. Total energy used: 1/6 candle power hour. Now try to heat 10 liters of water. Due to the much larger pot, it never gets it to boiling. 1 candle power gets it to something above luke warm, and never get any warmer. Total energy used Infinite. And you get people impatient for their tea.

The most efficient water heater is one that works so fast that there are no loses. Few of us have flash boilers in the kitchen. Failing that, one that is insulated and has the element surrounded by water will do better than anything with a separate water vessel and heater.

Practical advice: Heat the water in the pot in the microwave. The microwave is very efficient. It heats the pot at the same time. If you don't need a pot full, use a thermal mug and a teabag in the microwave. Put the tea in first or after.

My suspicion is that this is not as efficient as the insulated immersed elment kettle. The microwave conversion is not perfect, nor is all of the energy absorbed by the items in the microwave. However pouring boiling water into a pot can result in the water dropping 10-15C. The water used to heat the pot is discarded. Overall I suspect that using the microwave is a win, but I cannot back that up with actual tests or data.

The inside of a microwave reflects microwaves. So the waves bounce until they are absorbed. This is one reason you don't run them while empty. I couldn't find actual figures for MW efficiency, but one indication: Energy that isn't absorbed has to show up somewhere. The exhaust temperature from my microwave doesn't get warm until there is steam coming off the food, so emperically it is quite efficient.

  • You say microwave is very efficient: how does the energy to boil a 1 L pot in the microwave compare with that in a type 2 to 3 kW kettle? (Perhaps you could edit it into your answer.) – PJTraill Dec 3 '15 at 15:07

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