Page 314 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Sustaining Operational Ef ficiency of a CHP System 287

Combining Eqs. (17-43) and (17-44), yields the relation for fan efficiency
v P ( − P )
η = Fan Fan,o Fan,i (17-48)
Fan
W
Fan,elec
Equation (17-48) provides the efficiency of the fan-motor combination rather than
the fan alone.

Desiccant System
A solid desiccant system can also be used in a CHP system for dehumidification because
it is capable of using a low-grade thermal source to remove moisture from the air, which
eliminates the overcooling and reheating steps typically employed in a conventional
cooling system for dehumidification, and thus can save electrical energy and associated
costs by using captured waste heat for the same process. The dry air produced by the
desiccant system can be used for industrial processes or space conditioning. A solid
desiccant system consists of a desiccant wheel, a supply (process) fan, an exhaust fan,
and a heat source for regenerating the desiccant. In a CHP system, exhaust gases, either
directly from the prime mover or indirectly after passing through an HRU, are used for
reactivation of the desiccant. In some cases, additional heating is provided by a duct-
burner, which supplements the heat in the exhaust stream.

Efficiency of Desiccant System
The efficiency of the desiccant system (η ) is defined as the ratio of dehumidification load
D
(rate of moisture removal) to the total electric and thermal power input for regenerating
the desiccant:
Q
η = d (17-49)
D Q + W
d,in d,elec
where Q = rate of dehumidification
d
Q = rate at which heat is used to regenerate the desiccant
d,in
W = total fan power input (for both the process and the regeneration streams)
d,elec
Q can be calculated using the following equation:
d
Q = Q − Q (17-50)
d d,total d,sensible
where Q is the rate of total heat transfer between the inlet and outlet on the supply
d,total
(air) side, given by
Q = v ( ρ ) hT ( ,DP ) − hT ( ,DP ) (17-51)
,,
,
d,total d a d a i d a o
,,
h(T,DP) and h(T,DP) are the specific enthalpies of the entering and leaving air
d,a,i d,a,o
(process) streams at the corresponding dry-bulb and dew-point temperatures (T and
DP, respectively). The mass flow measured either at the inlet or outlet of the process
stream is represented by the term ( vρ ) .
d,a
Q is the rate of sensible heat transfer to the process air stream between the
d,sensible
inlet and outlet of the desiccant system and can be calculated from:
Q = ( v c ) (ρ T − T ) (17-52)
,,
d,sensible p da , da i d a o
,,