Page 84 - Sustainable On-Site CHP Systems Design, Construction, and Operations

62 CHP B a s i c s

thermal-to-power prime mover, the STG, has nearly 100 percent online availability.
Availability is a key issue in considering a CHP plant. First, there must be backup
system for use when the plant is down if the loads served are essential. If the backup
is utility power, there often will be large backup charges, and any energy charges for
power used during an outage will likely depend on when the demand is made on the
utility. Obviously, CHP plants with multiple prime movers are less impacted by down
time because other prime mover(s) should be available when one is being serviced.
The right combination of equipment and when service occurs can largely eliminate
down time, but no system will ever achieve 100 percent availability. Some maintain that
the probability of failure is directly proportional to the negative impact of such failure.
Overhauls involve the deconstruction of major components of the prime mover
equipment for the purposes of rehabilitation and/or reconstruction. Often times, equip-
ment manufacturers have backup/replacement equipment available at their factories or
service centers that can be installed at the CHP plant in place of the off-line equipment
to limit to CHP plant’s down time. It is recommended that prime mover equipment be
regularly overhauled at consistent intervals to ensure that the CHP plant operates con-
sistently and efficiently. CTGs are typically operated between 30,000 to 50,000 hours
between overhauls, which is notably different than their microturbine counterparts,
which are typically operated between 5000 and 40,000 hours between overhauls. IC
engines are recommended to operate between 24,000 and 60,000 hours between over-
hauls. Fuel cells are recommended to operate between 10,000 and 40,000 hours. STGs
can operate over 50,000 hours between overhauls; however, operating hours are depen-
dent on the cleanliness and quality of the steam used to operate the machines.
Start-Up Time
CHP plant prime mover equipment start-up times should be considered when evaluating
electric and thermal load profiles and selecting prime mover equipment to serve the
electric and thermal loads. Start-up times vary greatly between all different types of
CHP prime movers; from 10 seconds for diesel engines, 60 seconds for microturbines,
and 10 minutes to several hours for CTGs. Some prime mover equipment may take
even longer to start-up, from 3 hours to up to 2 days for fuel cells and from 1 hour to
1 day for some STGs. While steam turbine generators start quickly, significant time may
be required to bring boiler and steam distribution pipes to a proper operation point. It
should be noted that although IC engines have very fast start-up times, time required
to properly warm up the plant will be significantly longer, especially if crankcase heaters
are not in use.

Noise
The noise produced by the various technologies ranges from low enough so that no
enclosure is required to high enough so that both an engine enclosure and a building
enclosure are required. IC reciprocating engines tend to exhibit more low-frequency
higher amplitude linear vibrations than CTGs and STGs, which tend to exhibit higher-
frequency noise and vibration. Microturbines share the same type of noise characteris-
tics as CTGs, only to a lesser degree due to their relatively smaller sizes; however, noise
is increased when multiple units are arrayed and are operated at the same time. Fuel
cells produce the least noise of any of the CHP technologies. The high-pitched noise of
a steam turbine is far easier to attenuate than the rumble of an IC engine.