Page 393 - Sustainable On-Site CHP Systems Design, Construction, and Operations

366 Ca s e S t u d y 4

Campus load = 25,000 lb/h Campus load = 50,000 lb/h

600 600
400 400
200
200
Net result ($/h) –200 0 Net result ($/h) –200 0 0.15

0.13
0.11
–400 0.11 0.15 –400 0.09 0.13
0.09 –600 0.07 Elec. rate ($/kWh)
–600 Elec. rate ($/kWh) 0.20 0.30
0.20 0.07 0.05
0.30 0.40 0.50
0.40 0.05 Gas rate ($/m )
3
3
Gas rate ($/m ) 0.50
FIGURE 22-4 Net result versus natural gas and electricity rates.

Conclusions

• The break-even point between operating two cogeneration units or one
cogeneration unit and a duct-fired HRSG is approximately 29,415 lb/h.
• When the campus steam load exceeds 50,000 lb/h, both cogeneration units
should be run at full capacity with the HRSG duct burners fired as required.
• When the campus steam load falls below 50,000 lb/h (or the combined cogene-
ration steam capacity with unfired HRSGs), but above the break-even point of
29,415 lb/h, all the excess cogeneration steam should be used to run the single-
effect absorption chiller in the chiller plant.
• When campus steam demand falls below the break-even point of 29,415 lb/h,
one cogeneration unit should be shut down. However, the remaining cogenera-
tion unit should be operated by firing the HRSG duct burner as required to
meet campus demand but with no steam sent to the absorber.
• When campus steam demand is less than 25,000 lb/h (the output of one
cogeneration unit with unfired duct burner), all the excess cogeneration steam
should be used by the absorption chiller.
• The break-even point between one cogeneration unit operating with the
absorber, and a conventional boiler, is below the boilers minimum firing rate,
therefore, in all cases, a cogeneration unit should be operated.
• If the second cogeneration unit is operated when campus steam demand is
between 25,000 lb/h and the break-even point, this study identified a method
for calculating the additional cost burden to CUP.