Page 69 - Sustainable On-Site CHP Systems Design, Construction, and Operations

48 CHP B a s i c s

without any trouble (as measured by the amount of overshoot, undershoot, or offset
compared to setpoint), while virtually no governor can handle a large step load change
without a change in engine speed, which can cause the generator to trip off-line. Diesel
engine generators can typically handle larger step loads than natural gas–fired engine
generators, which have a maximum step load of around 25 percent of full generator
capacity. Every manufacturer and model has a different maximum allowable step load,
and the allowable increase may depend on the percent the generator is loaded at the
time of the step load. For example, a 20 percent step load may be permissible at 80 percent
load, but not at 5 percent load. Governors also allow for decreased operator input as
well as for extended engine life. Overspeed trips are control devices installed on engines
that stop fuel flow when rotational speeds increase above safe operating conditions,
typically the result of governor failure or some other control failure.
Equipment Life, Operation, and Maintenance
Reciprocating engine equipment life, like with any machine, is a function of its operation
and maintenance (O&M), engine operating hours, engine run speed (e.g., low speed
versus high speed), and frequency of oil changes and overhauls. All affect the expected
useful life of an internal combustion reciprocating engine. Perhaps the most important
factor outside of engine rpm is the design, materials, and quality of construction of the
prime mover engine.
Regular maintenance, including engine cleaning, scheduled oil changes, and proper
water treatment, is important for the successful and efficient operation of a CHP plant.
Predictive maintenance needs are possible if instrumentation and/or controls are
installed which observe every aspect of engine operation and show any irregularities in
day-to-day engine operation. Observed increases in fuel usage, increased heat exchanger
approach temperature, and cylinder operating conditions, when trended over time, can
be symptomatic of larger problems for operators. When well maintained, including
timely overhauls, reciprocating engine generator sets can be expected to operate between
20 and 30 years, or sometimes longer.

Combustion Turbines
Combustion turbines and combustion turbine generators are gaining popularity in
CHP installations today, where power requirements are consistent throughout the day
and there is consistent use for the relatively larger quantity of high-grade thermal
energy year-round. Although the thermodynamics of combustion for turbines and
reciprocating engines is similar, the mechanical process is vastly different. In a multi-
stage combustion turbine, a multistage air compressor is mounted on a common shaft
to a multistage turbine. Outside air is ducted to the compressor, where the pressure and
temperature are increased before being delivered to the combustor, where the hot, com-
pressed air is mixed with fuel and ignited, creating high-pressure, high temperature gas
that is subsequently expanded in the turbine to provide shaft power. The shaft power
is used to drive the generator and to drive the compressor (in some cases the CTG is
designed with two turbines, one for each job).
Turbine capacity and efficiency is strongly dependent on the temperature of the air
entering the compressor, and, therefore, many combustion turbine systems precool the air
entering the CTG compressor. Precooling the combustion turbine inlet air provides more
air flow and greater compressor efficiency. Greater turbine inlet air flow can produce
more CTG power output. Some turbine inlet air cooling systems use evaporative cooling,
since air density is related to dry bulb temperature. Other turbine inlet air cooling systems