Page 386 - Sustainable On-Site CHP Systems Design, Construction, and Operations

University Campus CHP Analysis 359

Operating Modes for Cogeneration Plant
The cogeneration plant contributes to the campus electrical and steam loads, and offers
a versatile, reliable, and independent source of power that produces electricity at higher
energy efficiency than fossil fuel utility power plants, provided that there is a use for the
steam being produced.
However, energy efficiency and economic efficiency do not always correlate. The
cost-efficiency of a cogeneration plant is inextricably linked to its ability to use 100 per-
cent of both of the cogenerated outputs, electricity, and steam. Furthermore, the cost of
natural gas must be such that in comparison to grid purchased kilowatthour and pro-
ducing steam using conventional means, it allows for a positive cash flow in sufficient
quantity to pay for capital cost repayment or return on investment (ROI) and mainte-
nance and other operating costs.
Currently, during the late spring, summer, and early fall, the demand for steam
by the campus buildings is less than the steam output of the two cogeneration units,
50,000 lb/h. At these times, plant operators use the excess steam into the single-effect
absorption chiller. This chiller uses up to a maximum of 25,000 lb/h to generate up to
1400 tons of cooling in the form of chilled water. Using the absorber eliminates the need
to generate these 1400 tons of cooling using an electrical chiller and reduces the CUP
electric demand and load accordingly.
This interdependency between the equipment in the CUP can be summarized as
follows:

• Electricity. The cogeneration units operate to match the electrical load of the
campus, which translates into continuous full-load operation.
• Steam. As a result of the cogeneration, approximately 50,000 lb/h of steam is
generated. When the campus steam load is higher than the output from the
unfired HRSGs, duct burners, or supplementary boilers are used to generate
the balance of required steam. When the campus steam load is below 50,000
lb/h uses for the steam must be found to minimize dumping.
• Chilled water. When the campus steam load is lower than 50,000 lb/h the steam
plant pressure control is achieved by modulating the absorption chiller to
maintain steam pressure, thus utilizing the absorption chiller as a steam dump.
If the absorber cannot use the excessive steam, then the steam is dumped in a
steam condenser that uses cooling tower water to condense the excess steam for
thermal balance. It should be noted that using the dump condenser, the most
onerous operating option, occurs only in case of equipment malfunctioning and
is not a regular procedure.

When the campus steam load is over 50,000 lb/h, the plant can take full advantage
of cogeneration and it is understood that the supplementary boilers would operate only
when the cogeneration unit HRSGs and turbine exhaust duct burners, both operational,
cannot meet the load.
This case study investigates the operation of the cogeneration plant at various cam-
pus loads lower than 50,000 lb/h, in order to develop an optimized operational strategy
for the cogeneration units. To summarize, the following scenarios are presented:
• Two cogeneration units plus an absorption chiller
• One cogeneration unit plus duct burner