Energy comparison

[controlnovice]

I'm trying to demonstrate the difference in energy cost of the different drivers we have in our plant: Gas, Steam, Electric and Air.

Gas and Electric are easy to convert to MMBtu, but I'm not sure about conversion for Steam and Air.

150# steam (we produce)
80# air (we produce)

I have read that it takes 7-8HP of electricity to make 1HP of air. But doesn't help with the comparison. Also read that 1000 lbs steam is about 1 MMBtu, but again, no nice cross ref chart or anything.


Is MMBtu not a good unit for comparison? Can someone help?

I'm electrical, so please forgive my ignorance.

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This is normally the space where people post something insightful.

 

[TheTick]

Going metric will probably help clarify things. Joules & watts always makes more sense.

As far as electricity to make compressed air, just put an ammeter on your compressor.

 

[controlnovice]

Yes, I have the kWh/scf for the cost of our air (based on kWh on compressor and totalized air flow).

I'm looking for a single unit for comparison for the energy the scf provides. (and steam)

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This is normally the space where people post something insightful.

 

[IRstuff]

BTU = .29307 Wh

TTFN

FAQ731-376

 

[BigInch]

"I'm trying to demonstrate the difference in energy cost of the different drivers".

You'll never compare apples to apples without considering the different efficiencies of each driver. As you've already found out. "I have read that it takes 7-8HP of electricity to make 1HP of air. But doesn't help with the comparison." That's because the total energy used in the compression process and during the reuse of air in the air motor to produce HP again is apparently only 12.5% efficient.

Different drivers will have different energy consumption per unit output as well as the processes that they are used to run. It is pointless to compare BTUs of gas burners used for direct heating applications with BTUs of gas used to run an air conditioning process.

The only way it makes any sense to compare energy use between two drivers is if the energy produced is used in the same manner. For example, you could directly compare an electric driver with a diesel driver, only if they were both driving similar pumps at identical flow rates. If the pumps were running at different flowrates and therefore different pump efficiencies, your answer would not be a consistant comparison.

Likewise it would not even make any sense at all to compare BTU of gas used to make BTUs of direct heat to BTUs of gas used to make BTUs of air conditioning.



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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mauricestoker]

If you are looking for a cost basis for comparison, why not use the dollar?

For steam, how much does it cost to generate 1 lbm 150 psi steam, and what is the energy source? If you have an electric steam gennie, then you would be looking at kWhr/lbm delivered, how much the kWhr costs, and enthalpy content of saturated steam at 150#. If burning dollar bills directly, you can skip a conversion.

 

[dcasto]

I English, metric, makes no difference, so everyone, get over that argument.

Lets just look at the system of air. The efficency of a compressor is about 78% for recips and 70% for centifugals. Then you can lose 5% to 15% in transport and storage of the air. A expander is about 40% to 60% efficent. so .7 * .6 *.9 = 35% efficency.

A boiler is 80% efficient and a steam turbine is 50%, so that is a 40% system.

A gas engine is 35% efficient.

and so one. You can get the exact efficencies from simulations or vendors.

see, not a SI FPS used in the killing of this question...

 

[IRstuff]

I think the OP is asking for a different type of efficiency, not just the heat engine efficiency, per se, but some more akin to cost of energy per product out for different forms of energy. Conversely, which form of energy results in the lowest net production cost?

However, the question is not something so trivially answered. You have to do a full analysis of how each form of energy might be used in the production process, and what the recurring and fixed costs are. Without knowing the specifics of the production process, no one can even begin to answer the question. Some production processes are amenable to pure electrical supply. Others might be more amenable to natural gas, or diesel, or coal.

Each specific energy form may have synergies within the production process, independent of its cost as a pure energy source alone. Other forms may require infrastructure that requires maintenance, etc.

TTFN

FAQ731-376

 

[dcasto]

Even with my simple analysis, you can do costs comparisons. A steam turbine has a net fuel to pump efficency of 80% boiler, 95% steam transport, 40% turbine, 90% mechanical or .8*.95*.4*.9 = 27.4%. the fuel cost $3.00/MMBTU so its 3/.274 = $11/MMBTU


air is electric based, so 75% efficient air comp, 90% delivery, 65% expander 90% mechanical = 43.8% with $.09/Kw-hr electricity or $.205/kw-hr

1 kw-hr = 3412 BTU

1kwhr =$.20 with air 1 kwhr is $.04 with steam.

 

[TheTick]

The reason I favor metric is that it is easier to understand the relationship between force, energy, and power in metric units. It will also be more readily understandable to an electro-type engineer who is already familiar with watts and joules.

Otherwise, a BTU-per-dollar is a BTU-per-dollar.