Page 56 - Sustainable On-Site CHP Systems Design, Construction, and Operations

CHAPTER 3

Power Equipment and

Systems

Lucas B. Hyman

Adam Stadnik

s discussed in previous chapters, combined heat and power (CHP), sometimes
called cogeneration, trigeneration, integrated energy systems, or total energy
Asystems, derives both electrical (or mechanical) power and useful heat from the
same fuel source. With any CHP method, the power and the heat are derived from the
oxidation of a fuel. In general, the processes fall into two major groups. One group
burns (oxidizes) the fuel to produce heat in order to produce rotary power for electric-
ity, where heat is a by-product. The second group derives electricity directly from the
chemical process of oxidation of fuel, and again heat is a by-product.
In the most common group, fuel is directly burned in a prime mover process. This
group includes internal combustion engines and combustion turbines. In each case,
oxidation of the fuel provides heat and the expansion of gases. The expanding gases
drive a mechanical process which outputs rotary power to drive an electrical generator.
With internal combustion, the hot gases expand inside a cylinder with a piston and the
expansion drives the piston. These prime movers are called internal combustion engines
and are closely related to vehicle engines. This group includes ones with a spark igniter
and ones which use heat of compression to ignite the fuel (diesel cycle) as discussed
further in this chapter.
In a combustion turbine, a hot compressed gas and air mixture produced by com-
bustion is allowed to expand through turbine blades, which produces rotary power for
electrical generation.
A second group of fuel-fired systems burns the fuel in a boiler to produce high-
pressure and often superheated steam. The steam produced is delivered to a steam
turbine. Expansion of steam through the turbine blades produces rotary power which
drives an electrical generator. The steam turbine process is often used in large commer-
cial electrical generation plants.
When a steam turbine process is used in CHP, the energy lost in condensing is put
to use in heating or cooling buildings or processes. For this to work, the discharged

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