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Unregistered
05-03-2006, 06:52 PM
The calculator converts BTUs to megawatthours - but what energy source is that based on? Wouldn't the amount of heat energy in a fuel depend on which type of fuel it is (coal, oil, gas)?

Robert Fogt
05-04-2006, 02:50 AM
Btu and megawatt·hour are both units of energy, so the conversion between them is not dependent on the type of fuel or object.

A gallon of automotive gas and a gallon of kerosene both would generate a different amount of energy, but a Btu of energy from one is the same as a Btu of energy from another.

Unregistered
05-04-2006, 09:37 AM
Now it's clear - thanks!

Unregistered
08-02-2006, 12:45 PM
Reply: The potential available heat generating content of 1 kiloWatt-hour of electricity is 3,413 Btus. The heat required heat energy to generate 1 kiolwatt-hour depends upon the method of generation, the combustion of a fuel, the efficiency of the boilers, turbines etc. A general rule of thumb is to use the higher-heat value with average efficiencies assumed. That number is generally 10,000 btus/kilowatt-hr. If the electricity is generated from renewable sources and further not from combustion, such as by the us of hydro, wind or solar power the additional heat used to generate the electricity is 0.

Unregistered
01-15-2008, 06:22 AM
This is entirely not the case. You are confused. Both units are units of energy and as such are interchangeable. 1MWh will equal 3.413MMBTU, ie 1MWh of anything contains the same energy as 3.413MMBTU of anything else.
The confusion comes in when you assume that MWh refers to electricity and BTU to NatGas (although this is not necessarily the case).

01-25-2009, 07:36 PM
This is entirely not the case. You are confused. Both units are units of energy and as such are interchangeable. 1MWh will equal 3.413MMBTU, ie 1MWh of anything contains the same energy as 3.413MMBTU of anything else.
The confusion comes in when you assume that MWh refers to electricity and BTU to NatGas (although this is not necessarily the case).

No, you are also confused.

For example, if you are burning green wood (heat rating ~4500 Btu/lb) in a boiler that is 25% efficient, you can not simply multiply the 4500 times the number of pounds of wood times the conversion factor. You would have to multiply THAT by 0.25 because 75 percent of the energy is not converted into kWh, physically.

On the other hand, if you have produced 100 kWh of energy and you want to find the equivalent in Btu, simply multiply by the conversion factor.

JohnS
01-26-2009, 03:54 AM
No, you are also confused.

For example, if you are burning green wood (heat rating ~4500 Btu/lb) in a boiler that is 25% efficient, you can not simply multiply the 4500 times the number of pounds of wood times the conversion factor. You would have to multiply THAT by 0.25 because 75 percent of the energy is not converted into kWh, physically.

On the other hand, if you have produced 100 kWh of energy and you want to find the equivalent in Btu, simply multiply by the conversion factor.

You are making the errorneous assumption that kWh can ONLY refer to electrical power. While it is most commonly used for electrical power, that is not the case. In your example, 75% of the power is converted into (probably wasted) kWh of thermal energy. If some of the thermal energy can be captured for other useful purpose (eg hot water), that is called co-generation.

But any conversion of energy from one form to another involves inefficiency and waste heat. For example if you feed 1 kWh of electricity to an electric motor, you do not get 1 kWh of mechanical (shaft) power out, perhaps you get 80-90% of that.

As the use of hour in kWh messes up the coherence of the metric system, in any case, it would be much better to work in joules (1 kWh = 3.6 MJ), and recognize that ALL forms of energy may be expressed in joules (with a suitable prefix for the magnitude).

01-26-2009, 08:23 AM
Assumptions aren't erroneous if they are part of an example. I was simply showing by counterexample that this conversion depends on the context.

JohnS
01-26-2009, 12:38 PM
Assumptions aren't erroneous if they are part of an example. I was simply showing by counterexample that this conversion depends on the context.

I still disagree. A "conversion" invovles exactly the same energy, expressed in two different measurement systems.

Your "counterexample" involves the input/output analysis of a machine. Due to efficiency limits (and 2nd law) you never transform all the input energy to useful output energy. That has nothing to do with units, and involves lots of engineering analysis.

Unregistered
06-10-2009, 01:23 PM
actually there is much confusion on the conversion of Btu and joules

see
en.wikipedia.org/wiki/BTU
and
en.wikipedia.org/wiki/Calorie

there are several definitions of a btu
mean, 39F, 60F, and thermochemical

the Gass Processors Suppliers Association has a very good overview of it (an a better source than wikipedia)

gpaglobal.org/gpsa/pdf/SIerrata04_05.pdf
(see page 2 - it will not let me post direct links)

I am actually making a conversion table for our company today and will be using the exact definitions they have listed from the Internation Conference on the Properties of Steam. May not seem like much but we are processing millions of btu's per hour and after a few conversions your decimal error will propagate to the third decimal place.

Unregistered
12-03-2009, 02:31 PM
Neither poster is confused... But they are writing from different perspectives. Of course you can convert kwh directly to mmbtu without taking into account fuel efficiencies and such, but the answer you get is of absolutely no value unless you are doing homework in a high school physics class. In the real world, you would need to know what quantity of fuel would yield said output, and so all of the assumptions of fuel type and efficiency come into play. But then, this sort of confusion is why energy analysts like me have a job.

Unregistered
12-22-2009, 02:21 AM
to summarize, and for analytical purposes, what is mechanics of converting gas price into KWr price? as you see different reports (and graphs) expressing it in different units (such as an offcial gas consumption/price not in bcf but in TWr) and you always wonder what assumptions (efficiency, generation type, fuel type, etc) those analysts had made. Please provide an example. Of course another question why at all use gas price expressed in watt hours?

12-22-2009, 03:13 AM
Reply: The potential available heat generating content of 1 kiloWatt-hour of electricity is 3,413 Btus. The heat required heat energy to generate 1 kiolwatt-hour depends upon the method of generation, the combustion of a fuel, the efficiency of the boilers, turbines etc. A general rule of thumb is to use the higher-heat value with average efficiencies assumed. That number is generally 10,000 btus/kilowatt-hr. If the electricity is generated from renewable sources and further not from combustion, such as by the us of hydro, wind or solar power the additional heat used to generate the electricity is 0.

It is all very useful but I'm a little confused with this conversation. If I see the gas price/consumption expressed in c/KWr (instead of per bcm/bcf, MMBTU), how should I understand that or put another way what is an international standard (e.g IEA) of converting gas price from BTU/MJ into watthours (e.g. KW, MW) and how should I interpret given multiple? What assumptions an international community makes?

trereamurry
12-22-2009, 09:08 AM
Many thanks for the replies So Seacam do you think I should take the 80,000 bit off and use the part for the 100,000 BTU?

Better to go higher than lower?

Sounds like Im a contestant on Play Your Cards Right

Thanks again.

JohnS
12-22-2009, 09:20 AM
It is all very useful but I'm a little confused with this conversation. If I see the gas price/consumption expressed in c/KWr (instead of per bcm/bcf, MMBTU), how should I understand that or put another way what is an international standard (e.g IEA) of converting gas price from BTU/MJ into watthours (e.g. KW, MW) and how should I interpret given multiple? What assumptions an international community makes?

I don't know what the r is in your notation, A kilowatt-hour, kWh, is a kilowatt for an hour. A kilojoule is a kilowatt for a second. As there are 3600 s in an hour, 1 kWh = 3600 kJ = 3.6 MJ, exactly, and under all conditions.

Unfortunately, there are about five "flavors" of BTUs depending on starting and ending temperatures of the water, but they are not GREATLY different. 1 kWh is ABOUT 3412 BTU, give or take, depending on flavor of BTU used (calorie has the same problem).

If the gas is measured by volume (cubic feet, cubic meters, etc) it is necessary to know the heat content of the gas to know how much energy you have bought. In english units, natural gas is very roughly 1000 BTU/SCF but may vary 5% or more with source of gas. (Biogas from landfills is about half CO2, and only about 500 BTU/SCF).

All of the above is talking about exactly the same energy, only measured in different unit systems. Transformation of energy involves losses. It might take 10 MJ of thermal energy to generate 1 kWh (3.6 MJ) of electricity, the missing 6.4 MJ representing the inefficiency of conversion. Much of the discussion in this thread revolves around not understanding these two fundamentally different concepts.

Unregistered
03-22-2010, 11:04 AM
The calculator converts BTUs to megawatthours - but what energy source is that based on? Wouldn't the amount of heat energy in a fuel depend on which type of fuel it is (coal, oil, gas)?

I am trying to do the conversion for biomass fuel pellets that substitutes for coal. They will be burned in a stoker coal boiler.