Page 292 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Operation and Maintenance Services 265

CTG and STG Optimization
Optimizing the performance of a CHP plant can be a constant challenge for the plant
operator. The plant design should provide for the ability to use all of the steam or hot
water produced in the heat recovery unit as the electric load changes so that engine
turndown is minimized. Note that typically if there is an opportunity to provide steam
from the HRSG to processes or thermal loads, the most efficiency is gained serving
those processes or thermal loads first, although greater value may be obtained from a
power-production-first strategy.
Heat recovered in the form of steam or hot water can also be used for thermal-
powered cooling. In a combined cycle plant, additional steam may be used to drive the
STG if there is a demand for additional electricity. The CHP plant will typically operate
at the highest efficiency when the STG is allowed to follow the steam availability,
varying the electricity output. If additional electricity is needed and the STG is not at
full load, more steam can be directed to the STG via supplemental duct fire, also known
as firing the duct burner.

Duct Burner
The duct burner has the potential to enhance the performance of a CHP plant by greatly
increasing the output of steam from the HRSG by means of supplemental duct fire. The
plant operator must monitor and control the amount of steam produced with the duct
burner and be able to determine when plant conditions are appropriate for the use of
duct burner firing so as to optimize plant operation. The cost of the fuel used for
supplemental duct fire can be offset by a measurable increase in plant efficiency. The
plant operator should have the necessary efficiency data at the control console to enable
his decision in any event.

Inlet-Air Cooling
A more direct enhancement to the efficient operation of a CTG is through inlet-air
cooling particularly during seasonal warmer outdoor conditions. The plant operator
will be required to closely monitor the CTG performance and adjust the set point of the
inlet-air temperature control to boost the output of the generator to improve toward
International Organization for Standardization (ISO) conditions. This is usually neces-
sary during a hot and humid afternoon when the electric load is at its highest. The
inlet-air cooling process can also extract the excess moisture from the air (if cooling coils
are used), to enhance performance of the combustion turbine engine.

Balance of Plant
In a combined cycle plant, the steam produced in the HRSG is often directed, as a first
priority to plant thermal loads or processes. The plant operator must continually monitor
all of the loads in the plant and determine the most efficient way to address the loads.
He or she must always have an awareness of the overall balance of the plant and how
individual equipment performance and weather create imbalances.
In a district heating and cooling plant with CHP, the heating and cooling load vari-
ations will require operators to seek a balance of the CHP prime movers, STGs, thermal
chillers, and building heat production. The more diverse the plant design is the more
opportunities the plant operator has to maximize efficiency as loads change. For exam-
ple, the plant operator should be able to select either electric- or steam-driven chillers to
deliver building cooling. In many CHP designs, absorption chillers are used to produce