Monday, March 24, 2008

http://www.mhi-inc.com/Converter/watt_calculator.htm


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JOULE is the quantity of energy

WATT is the quantity of energy used or supplied per unit time (also called power)

http://www.mhi-inc.com/Converter/watt_calculator.htm

We often encounter power and energy separately: cars and motors are often priced by horsepower (power); light bulbs are often priced by their output watts (power); natural gas is sold by energy content namely therms (energy) and of course the supply-demand situation; electricity by kilowatt-hours (energy); furnaces are priced by kilowatts (power) and temperature capability Kelvin (K); and air conditioners by tons or BTU's per hour (power). 

Power is oomph! Power is the amount of energy that is converted in a unit of time.

Expect to pay more when demanding higher power.

When you purchase a energy conversion device (say electrical energy to heat), the device will often be rated by its "power".  This enables you to calculate how much energy you will use (and pay for) by simply multiplying the rated power multiplied by the hours used. Each electric unit you pay-for, is normally one kilowatt-hour (one unit).  When you pay for units, your US electric utility bill contains price for units used during normal usage and price for units used in a "demand penalty" condition.  You should always carefully watch this penalty number, and contact your electric utility provider to find ways for reducing this "penalty" charge.

The calculator(s) below will help you to convert energy from one unit to another and if you know the time over which the energy is delivered, it allows you to calculate the power.  And if you wanted to get a feel for the amount of energy used in terms of equivalent amount of gasoline burnt or in terms of how much ice the energy would melt-you can do that also.  Have fun!

Remember Energy is always conserved (such conservation is known as the first law of thermodynamics). Entropy is not. When you posses energy with the lowest entropy you posses the most useful form of energy for conversion to work. When you convert energy from one form to another, you create entropy (almost always)- and when you do, the process is called irreversible (because you generate entropy).  In ideal cases you may have a process that does not cause any irreversibility, and consequently no entropy is generated.  Electrochemical processes and processes where work is converted to work are such processes, but often not quite a 100% reversible.

Anytime heat is produced and transferred from one temperature to another (i.e. across a temperature gradient)- the process is irreversible because entropy is generated.

Energy

(used or delivered)

Time

(This is the time over which the energy was used or delivered)

Power
Joules

Watts
Watt Hours Kilowatts
Kilowatt Hours Horsepower
BTU BTU/hr
Therms tons
This is the average power in full sunlight falling on a square foot surface, directed toward the sun, and collecting hours each day.


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The following table illustrates how various energy storage systems would store this amount of energy:

Chemical gallons of gasoline ( pounds)
pounds of natural gas ( cubic feet at psi),
pounds of hydrogen Gas ( cubic feet at psi),
a lead-acid battery weighing pounds
Mechanical (Kinetic) a high-tech flywheel weighing about pounds
a pound car traveling at miles per hour
Thermal melting a block of ice weighing pounds
heating a gallon tank of water by degrees F.
evaporating pounds (about gallons) of water
heating a pound hunk of concrete by degrees.



Note unless a phase change is indicated the tables do not consider latent  heat.

To convert from the unit in the first column to the unit in the second, multiply by the conversion factor in the third column.

Example
1 Btu/hr = (1 x 0.0003929) Horsepower = 0.0003929 Horsepower

To Convert

To

Multiply By

Btu/hr

Horsepower

0.0003929

Btu/hr

Gram-cal/sec

0.07

Btu/hr

Foot-pounds/sec

0.2162

Btu/hr

Watts

0.2931

Btu/min

Kilowatts

0.01757

Btu/min

Horsepower

0.02356

Btu/min

Foot-lbs/sec

12.96

Btu/min

Watts

17.57

Foot-pounds/min

Kilowatts

0.0000226

Foot-pounds/min

Horsepower

0.0000303

Foot-pounds/min

Kg-calories/min

0.000324

Foot-pounds/min

Btu/min

0.001286

Foot-pounds/min

Foot-pounds/sec

0.01667

Foot-pounds/sec

Horsepower

0.000818

Foot-pounds/sec

Kilowatts

0.001356

Foot-pounds/sec

Btu/min

0.07717

Foot-pounds/sec

Kg-calories/min

1.01945

Foot-pounds/sec

Btu/hr

4.6263

Horsepower

Kilowatts

0.7457

Horsepower

Kg-calories/min

10.68

Horsepower

Btu/min

42.44

Horsepower

Foot-lbs/sec

550

Horsepower

Watts

745.7

Horsepower

Foot-lbs/min

33000

Horsepower (boiler)

Kilowatts

9.803

Horsepower (boiler)

Btu/hr

33479

Horsepower (metric)

Horsepower

0.9863

Kilowatts

Horsepower

1.341

Kilowatts

Kg-calories/min

14.34

Kilowatts

Btu/min

56.92

Kilowatts

Foot-lbs/sec

737.6

Kilowatts

Watts

1000

Kilowatts

Foot-lbs/min

44260

Watt (international)

Watt (absolute)

1.0002

Watts

Kilowatts

0.001

Watts

Horsepower

0.001341

Watts

Horsepower (metric)

0.00136

Watts

Kg-calories/min

0.01433

Watts

Btu/min

0.05688

Watts

Foot-lbs/sec

0.7378

Watts

Btu/hr

3.4129

Watts

Foot-lbs/min

44.27

Watts

Erg/sec

107

Watts (absolute)

Btu (mean)/min

0.056884

Watts (absolute)

Joules/sec

1

 

Efficiency

(first and second law efficiencies)

Rule: There is no process that can violate the first or second law of thermodynamics.

Here are some typical efficiencies encountered in everyday processes:
  • burning fossil fuel to get useable heat - about 85% (running a gas-fired water heater, or making steam to power a turbine...some heat goes up the smokestack).
  • burning fossil fuel to get electricity - about 33% (active world-wide research area to improve several types of efficiencies).  The big drop in efficiency comes from the second law limitation which states that not all heat (a disordered form of energy) can be converted to work (an ordered form of energy).
  • sunlight to electricity in a PV cell - about 10% (active world-wide research area to improve conversion efficiency).  Can you tell if this process has a second law limitation? Hint- think if any heat is produced.
  • putting electrical energy into a battery (charging it) and pulling it back out: about 90%
  • converting electrical energy into mechanical energy with an electric motor about 85%
  • Fuel Cell about 60% or lower.  Theoretically this can be as high as about 92%. Electrochemical processes are limited in efficiency by how much enthalpy of the reaction can be converted to free energy (there is an entropy of mixing of constituents which limits the conversion efficiency).
  • IC engine about 30%.
  • Wind energy about 59%.

 

All calculations are approximate only and should not be used for detailed engineering calculation requiring a high degree of precision.  Please check all values against more established calculators.  None of the values are guaranteed to be accurate.

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