Page 36 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Overview 15

• Store excess thermal production as heat when the power demand exceeds the
heat demand; either cool or heat storage must be able to productively discharge
most of its energy before it is dissipated to the environment.
• Sell excess power or heat through approved paralleling protocols on a mutually
acceptable contract basis to a user outside of the host facility (off-site). Often the
buyer is the local utility, but sometimes it is nearby or “over the fence.”

Quality of Heat
The quality (temperature and pressure) of recovered energy needed by the facility is
another major determinant in selecting the prime mover. If high-pressure steam is
required using a topping-cycle, the only option is to use a CTG with a HRSG.

General System Sizing
As discussed in this book, proper CHP system sizing is critical to the sustainability of a
CHP system. For example, if a CHP system is oversized, it is likely that the facility will
not fully be able to utilize the waste heat, heat dumping will occur, overall system
efficiencies will be low, and economic expectations may not be realized. If a CHP system
is undersized electric and thermal loads may not be served and economic opportunities
will be forgone, for example.
Note that CHP systems fall into two process categories:
1. Topping-cycle. A CHP process in which the energy input to the system is first
used to produce useful power output, and at least some of the rejected heat
from the power production process is then used to provide useful thermal
energy to the facilities.
2. Bottoming-cycle. A CHP process in which the energy input to the system is first
applied to produce useful thermal energy, and at least some of the rejected heat
emerging from the thermal application is then used for power production.

Bottoming-cycles are typically used for facilities or industries that are heat load
driven. That is, facilities that typically require large amounts of heat for their process.
The topping-cycle has several variations and can be sized to meet the following:
• A portion of the facilities electric load (peaking plant)
• The facilities base electric load
• The facilities total peak electric load
• A portion of the facilities thermal load
• The facilities base thermal load
• The facilities peak thermal load
Unless power or thermal energy is to be exported from the site, the sizing variations
listed above sets the “edges of the envelope” with respect to CHP plant size, and as
discussed in Chap. 8 the various options need to be carefully studied. For example, a
CHP plant sized to meet the peak electric demand provides maximum energy cost
savings and maximum reliability, but may be large and relatively expensive to con-
struct. Further, for many of the hours in a year, the peak demand cogeneration system