Notes from problem 1: -exergy of natural gas is about 20,000 BTU/lbm -engine con

Question

Notes from problem 1:

-exergy of natural gas is about 20,000 BTU/lbm

-engine consumes gas at the rate of 250 lbm/hr

-output from the driveshaft is 750 hp (work output of shaftwork is 100%)

-exergy contained in the engine is constant, the process is steady.

The engine of Problem 1 is used to turn a generator, which yields 85% of the a) What is the electric output, in k W? b) Find a residential gas bill (or contact WE Energies) and locate the current cost of natural gas in this area. If the engine and generator run steadily for one week, how much is spent for natural gas? c) Find a residential electric bill (or contact WE Energies) and locate the current cost of electricity in this area. For one week's usage, how much would it have cost had the electricity been bought from the electric utility, rather than generating it on-site? Which of these two methods (generate on-site vs. purchase) is preferable? List advantages and disadvantages for each option, and indicate which are most important, in your opinion. d)

Explanation / Answer

given, energy per lbm of natural gas = 20,000 BTU

fuel used per hour by the engine = 250 lbm/hr

power of drive shaft P = 750 hp = 559275 W

2. for 85 pc effeciency

a. electric output = 0.85*559275 = 475383.750 W = 475.38375 kW

b. running the engine for one week, natural gas required = 7*24*250 = 42000 lbm

cost of natural gas = $ 200 per lbm

so total running cost = $ 8400000 for one week

c. energy in the form of electricity produced by the generator for one week = 24*7*475.38375 = 79864.47 kWh

cost of electricity is $0.12 per kWh

so net cost = $ 9583.7364

d. buying electricity from the electricity company is way more cheaper and that is because of the fact that tehy generate electricity from Dams and other renewable sources of energy.

Using a generator creates independence , but also is costly and generates more green house gases than the Dams

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