Page 401 - Sustainable On-Site CHP Systems Design, Construction, and Operations

374 Ca s e S t u d y 5

second utility power source does not count as redundant supply. On-site fuel must be
available for 3 days.
If the average electric demand is
• Less than 250 kW then the most likely technology will be a small reciprocating
engine gen-set or a microturbine gen-set, or possibly a fuel-cell
• Greater than 250 kW to about 800 kW the primary option is a reciprocating engine
• Between 800 kW to about 5 MW, either a reciprocating, combustion or steam
turbine is an option
There is a wide range in efficiency of these units. Diesel gen-sets are highly scalable
and less expensive than natural gas on a kilowatt basis. These packaged units are fac-
tory built and delivered to site as complete units that make installation a relatively
simple matter. Diesel systems, however, especially those above 1000 kW, are harder to
permit, are limited in run hours in most areas, and have on-site fuel storage issues.
Steam turbine generators for CHP systems that use natural gas as primary fuel in a
steam boiler can operate upward of 8000 hours per year. Some microturbines can run
up to 20,000 hours without substantial maintenance. Microturbines have found their
place among distributed resource technologies; a fact likely attributable to its status as
the only CHP technology currently eligible for U.S. federal tax credits. 8
Gen-sets may be operated at the standby rating for the duration of a power outage
but should not be used at the standby rating for continuous CHP operation. Generators
for standby use are frequently operated at a higher output and temperature rise than
are those for continuous use. Accordingly, the gen-sets can be classified according to the
fuel type, the load they carry and how long they can carry it. Some gen-sets can run
longer with de-rating factors (see Chap. 12 for more details).
A standard design approach features two or more smaller units as part of a building
block concept, in which additional units are added as capital is made available; thus
simplifying maintenance. When utility power fails, and the CHP system is balanced,
one or more generators will automatically start and be ready to pick up swing load.
While the lead generator continues to run, another generator is brought into synchro-
nism, paralleled automatically with the first.
In installations where there is a high ratio between the largest single generator and
total generation, frequency disturbances can be caused by a forced outage of a genera-
tor. For such a disturbance, the frequency variation can be controlled with the help of
other synchronous reserves.

Black Start
A CHP scheme that hosts a COPS will require some contingency arrangements to restart
in the event that rotating equipment comes to a standstill. The process of restoring a
stopped power system is commonly referred to as “black start.” On the macrogrid, a
black start involves isolated power stations starting individually and gradually being
reconnected to each other in order to form an interconnected system again.
Large diesel gen-sets are provided with much smaller gasoline engines for starting.
Smaller gas turbines can be started by electric motors supplied from station power bat-
teries backed up with black start generators. One gas turbine started by an internal
combustion engine will be able to start other gas turbines at the same location. One or
two diesels or gas turbines will be sufficient to start a much larger steam turbine unit.