Page 315 - Sustainable On-Site CHP Systems Design, Construction, and Operations

288 Op erations

where T and T are dry-bulb temperatures at the inlet and outlet of the air side of the
d,a,i d,a,o
desiccant system, respectively. The term Q represents the regeneration energy input:
d,in
Q = ( v c )ρ T ( − T ) (17-53)
d,in p d,ex d,ex, i d,ex, o
where T and T are dry-bulb temperatures at the inlet and outlet of the regenera-
d,ex,i d,ex,o
tion stream, respectively.
Desiccant System Performance Monitoring Calculations
The performance monitoring algorithms, with the exception of the auxiliary fuel input,
are all based on Eqs. (17-49) through (17-53). The density of supply air and exhaust
gases should be evaluated at the same conditions as the inlet flow rate is measured. The
specific heats of the supply air and the exhaust gases are assumed constant across
the desiccant system, which is a good assumption for both air and exhaust gas because
the variations across the inlet and outlet are small.
Equations for the efficiency of CHP components are summarized in Table 17-1.
System-Level Performance Monitoring
System-level monitoring is needed to ensure that the overall CHP system is performing
up to specifications and that significant degradation in performance has not occurred.
If degradation is detected and quantified, monitored component-level information can
be used to isolate the cause of degradation and correct it. This process is illustrated in
the “Application Scenarios” section later in this chapter.
The system shown in Chap. 1 represents the most complete building-scale CHP
system. Many of the other CHP configurations used in practice can be derived by speci-
fying the prime mover and eliminating components.
To monitor the system-level performance of CHP systems, two metrics for efficiency
and several other metrics calculated from sensed conditions or measured directly
are used. The two efficiency metrics are the overall fuel utilization efficiency (η ) [as
F
defined in Eq. (17-1)] and the value-weighted energy utilization factor (EUF ), ∗ which
VW
is defined as
+
Y
W elec elec ∑ Q Y th,j
th,j
EUF = j (17-54)
VW
∑ Q Fuel,j Price F Fuel,j
j
Here, W is the net electrical power output, Q represents the net rate of useful ther-
elec th,j
mal energy output from thermal recovery and/or conversion process j (e.g., the cooling
provided by an absorption chiller) with the sum being over all thermal recovery and
conversion processes in the system delivering energy for end use (e.g., an absorption
chiller or a desiccant unit), and Q is the total rate of input of fuel energy to the CHP
Fuel
system. For systems with fuel used for supplemental heating (e.g., for a heat recovery
unit, steam generator, or desiccant regenerator), Q is the sum over all fuel inputs to
Fuel
the system, that is,
Fuel ∑
=
Q Q Fuel, j (17-55)
j
∗ The EUF was introduced by Timmermans (1978) and later elaborated upon by Horlock (1997).
VW