Page 302 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Sustaining Operational Ef ficiency of a CHP System 275

The fuel utilization efficiency (η ), can be defined as
F
⎛ ⎞
⎜ W elec + ∑ Q th, j⎟
η = ⎝ j ⎠ (17-1)
F Q
Fuel
a metric for overall CHP system performance. Here, W is the net electrical power
elec
output, Q represents the net rate of useful thermal energy output from heat recovery
th,j
process j with the sum being over all thermal recovery processes in the system, and
Q is the total rate of input of fuel energy to the CHP system. This is the most com-
Fuel
monly used indicator of CHP system efficiency, although as noted in Katipamula and
Brambley (2006), it fails to account for the quality (exergy) of the different energy
streams. Equation (17-1) is specialized to a specific CHP system configuration later in
this chapter. To account for the quality of the various energy streams, we also use the
value-weighted energy utilization factor (EUF ), which is discussed in more detail
VW
later in this chapter and in Katipamula and Brambley (2006).
The generic components of CHP systems include combustion turbines, microtur-
bines, or reciprocating engines as prime movers; electric generators; heat recovery units
(which are heat exchangers); steam turbine–driven centrifugal chillers or absorption
chillers (which convert waste heat from the prime mover to useful chilled water for
cooling); supplemental electric-drive vapor compression chillers to help meet cooling
loads during times when the thermal-powered chiller cannot or does not meet the entire
load; ∗ cooling towers; desiccant systems for dehumidifying air; and pumps for moving
liquid and fans for moving air.

Commissioning Verification
Commissioning verification (CxV) is a process by which the actual performance of the
individual components in a CHP system and the performance of the CHP system as a
whole are verified to comply with the designers’ and manufacturers’ specified per-
formance. Furthermore, for new systems, commissioning should include a systematic
series of activities, starting in the planning phase and continuing through design, instal-
lation, and start-up, aimed at ensuring that the owner’s project requirements are met
and the CHP system operates correctly. Before start-up, the process should include
inspection and testing of all components in the CHP system to ensure proper compo-
nents are installed, they are installed correctly, and they perform properly.
Another goal of this chapter is to provide the reader with algorithms that can be used
to automate parts of the process for verifying that commissioning has been done correctly
and resulted in a CHP system that meets design and operational expectations. Although
CxV can include active testing of components and subsystems, this chapter focuses on veri-
fying performance to ensure that the system has been adequately commissioned and to
provide indicators of commissioning still needed when deficiencies are found.

∗ Vapor compression chillers used for this purpose often are not considered part of the CHP system, but
because use of thermally powered chilling must be optimized as part of a larger system that includes
vapor compression chilling, they must be included in decisions made by the supervisory controller
regarding how much thermally powered chilling and how much vapor compression chilling to use to
meet the total cooling load.