How much energy is used in heating water?

In summary: It was a 50W panel and the label said:"Maximum Power: 50WMaximum Voltage: 17.5VMaximum Current: 2.86AOpen Circuit Voltage: 22VShort Circuit Current: 3.14A"In summary, the conversation is about trying to determine the energy needed to heat 2 cubic meters of water per day from 23C to 65C in order to power a water purifier in Malawi using solar panels. The initial calculation of 97.7kWh was deemed incorrect due to a mistake in units, and it was clarified that the machine is powered at 1.
  • #1
paulfitz1592
5
0
1. I'm trying to work out how much energy is used in heating 2 cubic metres of water per day from 23C to 65C. Basically trying to put a water purifier in Malawi and run it off solar panels. It currently uses 1.3kWh to run the machine but the initial stage involves pre heating water. In Malawi this won't be needed so I'm trying to work out how much less energy the machine will need to run.

2. I understand that you need 1 calorie to heat 1 ml of water 1C.3. Heating the water a total of 42C.
2 cubic meters of water is 2,000,000 ml.
Therefore the machine uses 84,000,000 calories to heat the water, or so I think.
If I use a calorie to kilowatt/hour as 1.163x10-6.
84,000,000 x 1.163x10-6 = 97.7kWh

This is obviously wrong and I'm unsure where, initially thinking maybe because the machine doesn't heat the water all at once as does it gradually over the course of 24 hours?

Any help would be appreciated. I understand this is probably silly on my part but its been a long time since I done physics.

Thanks
 
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  • #2
Water has a density 1000 kg/m^3
2m^3 = 2000kg
Q=ms(t2-t1)
=2000*4.156*42 J/Kg
=349188j/kg
=0.09699 kW hours
 
  • #3
You I messed up with the units I suppose
 
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  • #4
AbhinavJ said:
You messed up with the units I suppose

Doesn't surprise me! Thanks.
 
  • #5
Somebody messed up with units --- care to guess who?
 
  • #6
Bystander said:
Somebody messed up with units --- care to guess who?
Did I? Lol
 
  • #7
You both need to go through it again. One for confidence in calculation rather than intuition, and the other for attention to detail.
 
  • #8
Bystander said:
You both need to go through it again. One for confidence in calculation rather than intuition, and the other for attention to detail.
So, where am I wrong?don't get it
 
  • #9
AbhinavJ said:
So, where am I wrong?
AbhinavJ said:
=2000*4.156*42 J/Kg
 
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  • #10
Dear Paulfitz, welcome to PF :smile:

You correctly calculated the energy needed to heat up 2000 kg of water 42 degrees at 4.1868 kJ/kg/##^\circ##C.

What makes you think that the result is "obviously wrong" ?

It currently uses 1.3kWh to run the machine but the initial stage involves pre heating water
Does the machine use 1.3 kW of power while running, or does it use 1.3 kWh per day ? Where does the energy for the preheating come from ? Could it be that there is a gas bill to pay also ?
 
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  • #11
BvU said:
Dear Paulfitz, welcome to PF :smile:

You correctly calculated the energy needed to heat up 2000 kg of water 42 degrees at 4.1868 kJ/kg/##^\circ##C.

What makes you think that the result is "obviously wrong" ?

Does the machine use 1.3 kW of power while running, or dioes it use 1.3 kWh per day ? Where does the energy for the preheating come from ? Could it be that there is a gas bill to pay also ?

As far as I'm aware it uses 1.3kW whilst running so 1.3kWh continuous. My initial calcs involved saving 97kWh which isn't possible surely as it doesn't require this much energy to fully function? But I think my units are wrong and i should have divided by 1000 for kj/kg instead of kj?

We are planning on using black plastic tubing which in the African sun will heat the water to between 60-70C, I'm just trying to find ways to cut down the amount of solar panels needed to power the machine.
 
  • #12
AbhinavJ said:
You messed up with the units I suppose
Actually, you did. The heat capacity of water is 4.156 kJ/kg-C, not 4.156 J/kg-C.

Chet
 
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  • #13
Bystander said:
You both need to go through it again. One for confidence in calculation rather than intuition, and the other for attention to detail.
Thanks, that couldve been a major issue had such a question come in my exams
That's 4.18 j/g isn't it?
 
  • #14
1.3 kW is 1.3 kWh/h . If the thing runs 24 hours/day that means it uses 31.2 kWh/day. Still not enough to heat up 2 m3 of water from 23 ##^\circ##C to 65 ##^\circ##C in one day.
 
  • #15
AbhinavJ said:
Thanks, that couldve been a major issue had such a question come in my exams
That's 4.18 j/g isn't it?
Yes. j/g-C.
 
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  • #16
JA has edited his post #2 which makes the rest of the thread unintellegible. Great.

Let's get back to Paul and help him (her?) out. The 1.3 kW can't be right if the other numers are correct. What's missing ?
 
  • #17
BvU said:
JA has edited his post #2 which makes the rest of the thread unintellegible. Great.
I'm going to restore the original.

Chet
 
  • #18
BvU said:
1.3 kW is 1.3 kWh/h . If the thing runs 24 hours/day that means it uses 31.2 kWh/day. Still not enough to heat up 2 m3 of water from 23 ##^\circ##C to 65 ##^\circ##C in one day.
'It is electrically driven (rated at 1.3kWh) ' that's the information I have from the company.
 
  • #19
Any links you might want to share ?
 
  • #20
paulfitz1592 said:
'It is electrically driven (rated at 1.3kWh) ' that's the information I have from the company.
Sadly, it's not unusual to see confusion between kW and kWh in such a place, where you'd think they understood this stuff.
 
  • #21
haruspex said:
Sadly, it's not unusual to see confusion between kW and kWh in such a place, where you'd think they understood this stuff.
Sorry I'm unsure what you mean by that. Are you saying the machine is powered at 1300 kW and not 1300 kWh?
 
  • #22
'It is electrically driven (rated at 1.3kWh) ' that's the information I have from the company.
That is very weird. Normally the rating of a motor has the dimension of power, not energy. I don't think I would buy from such a company !

Power = energy / time. Units: Watt (= Joules/s), kW (=kJ/s), etc.

Energy: Units : Joule, kJ, kWh (= 1000 W an hour long, so 3600 kJ or 3.6 M J )
 
  • #23
paulfitz1592 said:
Sorry I'm unsure what you mean by that. Are you saying the machine is powered at 1300 kW and not 1300 kWh?
Yes, except that I think you mean "1.3kW (1300W) and not 1.3kWh". "Powered at 1.3kWh" doesn't mean anything. It's like saying a car's top speed is 100km.
To show how widespread confusion is on this, I was recently sent some usage data from an electricity supplier. The half-hourly data had a column titled "kW", but it turned out that the numbers were actually kWh. That doubled the usage.
 

1. How does the amount of water affect the energy required to heat it?

The amount of water directly affects the amount of energy required to heat it. The more water you need to heat, the more energy will be needed. This is because it takes a certain amount of energy to raise the temperature of a certain volume of water by a certain degree.

2. What factors besides volume of water impact the energy used in heating?

Aside from the volume of water, the temperature of the water and the desired final temperature also play a role in the energy required to heat it. Additionally, the type and efficiency of the heating source, such as a gas or electric water heater, can affect the amount of energy used.

3. Is it more energy efficient to heat water on the stovetop or in a microwave?

In general, it is more energy efficient to heat water on the stovetop. This is because microwaves use a lot of energy to heat water due to the way they work, while stovetops use direct heat from a burner to heat the water.

4. Does the initial temperature of the water impact the energy required to heat it?

Yes, the initial temperature of the water does impact the energy required to heat it. This is because the greater the difference between the initial temperature and the desired final temperature, the more energy will be needed to heat the water to the desired temperature.

5. Can using hot water instead of cold water save energy when heating water?

It depends on the situation. In some cases, using hot water instead of cold water can save energy because the initial temperature of the water is closer to the desired final temperature, requiring less energy to heat it. However, if the hot water must travel a long distance through pipes, it may lose heat and require more energy to heat it again. It is best to use cold water when heating water for most energy efficiency.

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