Page 305 - Sustainable On-Site CHP Systems Design, Construction, and Operations

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Equation (17-10) shows, together with Eqs. (17-6) through (17-9), that more detailed
measurements could be used to isolate degradation of electrical generation efficiency to
either the engine (prime mover), the gearbox, or the electric generator. If no gearbox is
used in the system (e.g., in the case of microturbine used as the prime mover), η is
gearbox
set to 1.0 in Eq. (17-10).

Heat Recovery Unit
As discussed in Chap. 4, heat recovery units (HRUs) are an essential part of a CHP sys-
tem because they provide a means to recover heat from the exhaust gas of the prime
mover (turbine or reciprocating engine). Although there are several types of HRUs used
with CHP systems, only those that use indirect heating methods are considered in this
chapter: (1) indirect heating to provide hot water, (2) indirect heating to provide hot dry
air, and (3) indirect heating to provide process steam (described in the next section).
Some CHP applications use auxiliary firing (also called co-firing or supplemental firing)
to augment heat from the exhaust gases. Therefore, the HRU effectiveness equations are
developed assuming that there is auxiliary firing.

Effectiveness of Heat Recovery System
The effectiveness of the HRU is defined as the ratio of the actual heat transfer rate to the
maximum possible heat transfer rate, that is,
Q
ε = HRU, actual (17-11)
HRU Q
HRU, max
where Q is the rate of thermal energy gain across the HRU by the heat recovery
HRU, actual
fluid (e.g., heated water, heated air or water converted to steam) and Q is the
HRU,max
maximum possible rate of heat loss by the waste heat stream from the prime mover as
it passes through the HRU. If the cold-side material does not change phase in the HRU,
Q can be written as
HRU, actual
Q = (ρ vc ) T ( − T ) (17-12)
,
HRU, actual p HRU, w HRU, w o HRU, w i ,
where T is temperature of water exiting the HRU and T is the temperature
HRU,w,o HRU,w,i
of water entering the HRU.
The maximum possible heat transfer through the HRU, Q can be written (for
HRU,max
the non-phase-change case) as
Q = (ρ vc ) T ( − T ) (17-13)
HRU,max p HRU,min HRU, ex, i HRU, w i ,
where, (ρ vc ) is the smaller of the two quantities, (ρ vc ) (for the exhaust gas
p HRU,min p HRU,ex
flow) and (ρ vc ) (for the heat recovery stream). Although the temperature of the
p HRU,w
exhaust gas may change significantly across the HRU, Eq. (17-13) remains valid even when
(ρ vc ) = (ρ vc ) because the mass flow rate of exhaust gas (ρ v ) at the
p HRU,min p HRU,ex HRU,ex
HRU inlet equals its value at the outlet under steady-state conditions. Furthermore,
the heat capacity of the exhaust gas varies by less than 10 percent between representa-
tive HRU inlet and outlet conditions (see, for example, Kovacik 1982), further supporting
the assumptions implicit in using Eq. (17-13). To reduce errors associated with using a