Page 327 - Sustainable On-Site CHP Systems Design, Construction, and Operations

300 Op erations

• Exhaust gas temperature, T
Turbine, ex
• Rate of useful heat output, Q
th
• Chilled water supply temperature, T
evap,w,o
• Chilled water return temperature, T
evap,w,i
• Temperature of water entering the HRU, T
HRU,w,i
• Temperature of water leaving the HRU, T
HRU,w,o
• Exhaust gas temperature leaving the HRU, T
HRU,ex,o
• Current electric power output, W (kW)
elec
24 hours
• Average daily electric energy output, ∫ W elec dt (kWh/day)
0 t
/
• Average electric power output over the last n hours, ∫ W elec dt n (kW)
−
tn
24 hours
• Daily average hourly electric power output, ∫ W elec dt/24 (kW)
0
• Cooling tower water inlet temperature, T
CT,w,i
• Cooling tower outlet temperature, T
CT,w,o
• Cooling tower approach, T − T
CT,w,o wb
• Cooling tower range, T − T
CT,w,i CT,w,o
The system monitors these performance parameters and conditions and provides alarms
to the operators when conditions deviate significantly from baseline values. A hypothetical
sequence of values is shown in Table 17-3 to illustrate a scenario, where monitoring of these
parameters assists operators in detecting and correcting a system performance problem
much quicker than would be possible without such a monitoring system. Thirty minutes
has been used for illustrative purposes, and monitoring of an actual system would likely be
done using a much shorter time interval than the 30-minute interval used in the table.
Conditions at 13:00 are consistent with those for several immediately preceding
time steps (values not shown in the table), and the system is running properly. At 13:30,
deviations for a few performance variables (COP , η , Q , and T ) from the
AbChiller CT th CT,w,o
values at 13:00 can be seen, but their magnitudes are so small that no problems are
apparent. In fact, these deviations are all within the range of normal variations likely to
be observed during normal, fault-free, operation.
At 14:00, some substantial changes in performance variables are evident. The value-
weighted energy utilization factor has decreased by about 4.5 percent (from 1.12 to
1.07), not enough to be alarming by itself, but if this persists over the long run, fuel cost
increases will be substantial. The fuel utilization efficiency has also decreased from
59 to 54 percent, and the effectiveness of the heat recovery unit has decreased from 63
to 54 percent (i.e., by 14 percent), tending to indicate that something is wrong with the
heat recovery. The electric generation efficiency has not decreased, but the COP of
the chiller has dipped from 68 to 60 percent, and most alarmingly, the overall cooling
tower efficiency and electric-utilization efficiency of the cooling tower have decreased by
26 percent (from 70 to 52 percent) and 50 percent (from 7.0 to 3.5), respectively. The out-
put of the chiller has also decreased from 1180 to 1000 kW . These observations direct
th
operator attention immediately to the cooling tower, which clearly has some sort of