Page 34 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Overview 13

a low overall plant system efficiency due to insufficient thermal energy use, as well as
it is possible to have a low electric power generation efficiency but have a high overall
plant system efficiency due to maximum heat recovery and thermal energy use.

Generators and Electrical Distribution Systems
The generator and electrical distribution system are key components of a CHP system
and there are numerous electrical issues and challenges that must be understood in
order to properly plan, design, construct, and operate a successful, sustainable CHP
system. The type of CHP system has an effect on the generator type, its design charac-
ters, and protections required. The generator, which must be grounded, supplies power
to the switchgear, which feeds the CHP plant and facilities. As discussed in Chap. 11 of
this book, there are a variety of utility interconnection rules, standards, and require-
ments to help ensure that the generator and electrical system are protected in case of
system power outages, shorts, and other malfunctions such as electrical system voltage
spikes and sags. There are also a number of generator types and configurations and
these are also discussed further in Chap. 11.

Heat Recovery Boilers
A heat recovery boiler is similar to a typical fuel-fired boiler, except that instead of having
a combustion chamber or firebox, the unfired pressure vessel extracts heat from the
prime mover exhaust to produce either hot water or steam. Maximum steam pressure
is a function of the flue exhaust gas temperature. As previously noted, a heat recovery
boiler that produces steam is known as a heat recovery steam generator (HRSG).

Alternative Use of Heat Transfer Fluids
A non-volatile fluid-based heat recovery system incorporating a hybrid heater has been
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proposed as an improvement on HRSG. This approach utilizes oil designed for use as
a heat transfer fluid which has very good resistance to overheating. In particular, the oil
can be heated up to 600°F, and, if overheated, it creates small particles of burned oil
which stay in solution rather than coating the walls of a heat exchanger. The oil is used
to transfer heat from the combustion turbine exhaust stream to a hybrid heater used for
steam generation. Claimed advantages of the oil-based system are as follows:

• Much smaller thermal mass of oil and water in the system as compared with a
HRSG, thus allowing much quicker response to varying thermal input.
• Low-pressure operation of the oil loop, which reduces the mechanical requirements
of the exhaust heat exchanger, making it more robust to thermal cycling.
• Relaxed mechanical requirements for the exhaust heat exchanger and removing
the steam generated from exhaust stream allows for more compact heat exchanger
design.
• Reduced exhaust heat exchanger pressure drop, which results in slight improvement
in power generation.
• Lower overall installation cost.
When used for steam generation, the hot oil approach may result in reduced total
heat recovery due to pinch point issues. However, the hot oil could also be used directly