Page 203 - Sustainable On-Site CHP Systems Design, Construction, and Operations

176 De s i g n

Operational Flexibility
As discussed, having a variety of uses for the thermal output improves operational flexi-
bility. Likewise, having a number of pieces of the same equipment to meet the demand
improves operational flexibility and can allow for equipment backup so that operations
will not be impacted by the loss of a single piece of equipment. The number and size of
any selected CHP equipment item depends on the size and nature of the loads served
(e.g., what is the peak load and how does it vary by day and season), on the unit sizes
available in the marketplace, and depends on the cost to provide multiple units, as
multiple units always cost more than a comparable single unit but can offer greater
CHP plant operational flexibility. In contrast, however, the greater the number of equip-
ment units, the less expensive it is to add a backup unit. Note, it is often not economi-
cally feasible to provide a backup prime mover generator, and the local utility typically
provides electric power backup. Exceptions to not providing a backup prime mover
include, for example, manufacturing processes, where loss of power could damage
valuable product, and revenue generating operations, where loss of power could cause
severe economic loss (e.g., lose all of the guests at a hotel and pay for their trouble).
Another advantage of using multiple smaller pieces of equipments is the ability for the
system to operate efficiently at part load or at less than design conditions. For example,
if a cooling water flow rate is required to be only 30 percent of the design flow rate at
given condition, than operating one of three pumps at 100 percent flow rate will likely
be more efficient than trying to operate one of one pumps at 30 percent flow rate. Turn-
down will also improve with multiple equipments.

Plant Equipment Location and Layout
Layout of proposed CHP equipment is an important part of the CHP design process. A
number of factors, including access for maintenance must be incorporated and recon-
ciled. First, all applicable code regulations must be satisfied with respect to distance
from property lines, for example, for fuel or ammonia storage, as well as for the pro-
posed exhaust stack. The prime mover and generator, as well as other plant equipment,
should be arranged to minimize the length of interconnection piping and conduits to
fuel supply, power connections, and thermal points of connection. Connection to the
electrical power distribution for the facility served is also a key issue.
Further, each piece of equipment has a minimum maintenance envelope that must
be kept clear for any required servicing, maintenance, and/or repair, and the mainte-
nance envelopes need to be incorporated into the design layout.
Electrical panels and switchgear also have minimum clear areas in front of (and
sometimes at the back of depending on maintenance access points) the electrical equip-
ment, with the required clear distance depending upon the electrical equipment voltage
(greater clear distance is required for higher voltages).
An operator’s room with clear view to all the major components is a clear asset of a
good layout.
Maintenance and Servicing
As noted, key maintenance and servicing issues need to be kept in mind during the
layout of the proposed CHP plant. For example, CTG engine replacement typically
occurs every 30,000 hours or so of operation. Usually, in order to minimize downtime,
a replacement engine is provided, and the old engine is rebuilt for a new customer.
Space must be provided to remove the engine from the CTG, and then easily to remove