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Old 10-13-15, 05:31 AM   #1
bernard
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Default How much energy to heat 100 liters of water?

There is something that has been confusing me for a while, and it is related with "types" of energies when it comes to heating water.

Here is an example:
volume of water to heat: 100 liters (26.4 gallons)
required water temp = 45C (113F)
mains water temp = 10C (50F)

So to heat 100 liters to upper temperature of 35C, with an electric heater:

100 liters * (45-10) = 3500 kcal

1 kcal is equal to 1/860 kWh, so:

3500kcal / 860 ≈ 4 kWh (13.6 kBtu/h)

But this is an electric energy needed to heat upper 100 liters by 35C.


What If I wanted to heat the same amount of water with gas? How would this example look like then?
And what would be the result then? Thermal energy in kWh?

How would I compare the thermal energy and electrical energy, even though they have the same units (kWh)?

Thank you for the reply.

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Old 10-13-15, 06:06 AM   #2
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Here is a website that has a lot of energy conversions. Does this help?

http://www.onlineconversion.com/energy.htm

This site has the theoretical conversion. Burning natural gas has an efficiency factor (electricity is 100% "efficient" - no conversion loss) that must be used. For example, a high efficiency water heater might be 90% efficient.




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Old 10-13-15, 07:47 AM   #3
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Thank you for sharing the link Steve. That is a very interesting page.
But I still do not understand it.

If efficiency (EF) of electricity is 100% (or 1.0), then in order to calculate the amount of electricity needed to heat 100 liters of water by 35C, we have:

a) electricity:
volume * (deliveryT-mainsT) / EF
100 * (45-10) / 1.0 = 3500 kcal
3500kcal / 860 ≈ 4 kWh (13.6 kBtu/h)


Natural's gas efficiency is lower than 1.0. I found this table:
View image: fuel type efficiencies

Let's assume that we chose the natural gas instantaneous fuel type: EF = 80% (0.8). In that case the amount of energy needed to heat 100 liters of water by 35C would be:

b) natural gas
volume * (deliveryT-mainsT) / EF
100 * (45-10) / 0.8 = 4375 kcal
4375kcal / 860 ≈ 5 kWh (17 kBtu/h)


But 5 kWh of what? Not an electric energy, but what? Thermal energy?
How can I compare 4 kWh of electric energy with 5 kWh of Thermal energy?
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Old 10-13-15, 08:03 AM   #4
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With different fuels, the number of BTUs, calories or other energy units would be exactly the same to heat a specific quantity of water up by a number of specific degrees.

However, combusted fuels have different heat outputs per unit volume. Liquid fuels use a true volume (liters, gallons, etc) and gaseous fuels use a volume at a specified pressure and temperature.

What you want to calculate (I think) is the amount of a fuel, and its cost, to provide that amount of heat.

Per volume, when combusted, diesel has more heat than gasoline. Per volume, natural gas has more combusted heat than propane. Peat has less heat (per weight) than wood, but coal has a higher heat (by weight) than the other two. The costs of each is different.

It gets even more confusing when you use the power of falling water (as with dams) to describe energy as water in itself has no energy. The energy with dams is the result of stored potential energy by virtue of gravity (up hill water can release power when it flows down hill).

Energy to heat water is just that, but differing fuels have different inherent energy densities per volume. Some fuels are burned, other situations, such as in heat pumps, simply require you move energy from one state to another.

Hope this helps.

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Old 10-13-15, 08:30 AM   #5
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Thank you Steve.

I am not interested in costs.

I simply want to know how can 4 kWh of electric energy be compared with 5 kWh of thermal energy (is this even thermal energy)?

Is the only way of comparing these two types of energy by multiplying them with their costs per unit:

4 kWh of electric energy * 1$/kWh = 4 $
5 kWh of thermal energy (?) * 0.5$/kWh = 2.5 $

?

There is no other way of comparing these two types of values without costs involved?
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Old 10-13-15, 02:02 PM   #6
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A watt is a watt. They are not two different types of values to be compared. Simply put, it takes more power input to heat with gas than with electric BECAUSE some of the input is lost due to the inefficiency of how gas heating works. IF you could directly extract the available energy from gas without any losses due to non-perfect heat exchangers, the numbers would be exactly the same.
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Old 10-13-15, 02:07 PM   #7
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Thank you bmxeroh,

I am not interested in knowing why do these two values differ, but how they can be compared (without interfering with costs).

Indeed watt is watt.
But upper "4" is 4 kWh of electric energy.
While upper "5" is 5 kWh of thermal (?) energy.

These two can not be compared, as they are different types of energy.
Or did I get this wrong?
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Old 10-14-15, 08:11 AM   #8
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Quote:
Originally Posted by bernard View Post
Thank you bmxeroh,

I am not interested in knowing why do these two values differ, but how they can be compared (without interfering with costs).

Indeed watt is watt.
But upper "4" is 4 kWh of electric energy.
While upper "5" is 5 kWh of thermal (?) energy.

These two can not be compared, as they are different types of energy.
Or did I get this wrong?
Those two can be directly compared. They are essentially defined amounts of heat energy. Btu's are also a raw amount of heat. As with all physical systems, the heat you put in is greater than what you get out. No matter what, some will be lost.

For example, the transformer that feeds your house might be 99% efficient. For every 99 kWh of energy that flows through your meter, 1 kwh is lost at the pole transformer. This value can be directly converted into btu. In the same sense, a gas furnace may be 80% efficient. For every 80 btu of usable heat, 20 btu is lost as waste heat, mainly up the chimney.

I believe your questions lie within the bulk value of raw energy and the relationship with time and rate. A watt is a rate (joules per second), as is horsepower. This translates directly to units like lbf/sec, btu/sec, kcal/min, etc. The raw heat value is determined by multiplying by time to get watt-hours or kWh. A definite number of kWh equals a converted number of btu, kcal, joules, foot-pounds, etc. This conversion of energy between forms (electrical, thermal, mechanical, chemical, nuclear, etc.) is one of the gears that drives science. It is a well-travelled path. Power is a rate, energy is a bulk quantity.

Did I completely miss the point here?

Last edited by jeff5may; 10-14-15 at 12:49 PM..
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Old 10-14-15, 08:26 AM   #9
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Thank you for the reply Jeff.

I am actually not interested in units other than kWh. I live in a metric country.
The reason why I put gallons and kbTu was as a way of respect, because I can see that majority of users at ecorenovator.org/forum, are from USA.


My question was:
How can I compare 4 kWh of electric energy with 5 kWh of thermal energy, when these two are two different types of energies?
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Old 10-14-15, 11:15 AM   #10
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1kw of electricity equals 1kw of heat. Its a direct relationship.

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