Page 388 - Sustainable On-Site CHP Systems Design, Construction, and Operations

University Campus CHP Analysis 361

Cogeneration Units, with Duct Burners
3
Duct burner gas input 1,254 m /h
Additional steam output with duct burner 40,000 lb/h
3
Specific consumption 31.35 m /klb
TABLE 22-6 HRSG Duct Burner Parameters

Table 22-6 provides the CUP cogeneration units duct burner performance infor-
mation. When a cogeneration unit is at full output, additional steam can be generated
by combusting additional natural gas into the hot turbine exhaust before it enters the
HRSG boiler. The efficiency of a duct burner is generally higher as compared to a
comparable conventional boiler. At full load, additional hourly energy expense is
3
1254 m × $0.35/m = $439 and the additional output is 40,000 lb of steam. At bare
3
utility cost, the steam is produced at $10.97/klb, and delivered to campus buildings
at $14.33/klb.
Absorption Chiller
The operating parameters for this equipment are presented in Table 22-3. Excess steam
from the cogeneration units is used by the single-effect absorption chiller to provide
chilled water for campus cooling. As noted, excess steam is produced whenever the
campus steam consumption is less than the cogeneration plant output; and the cogen-
eration units must run at 100 percent output to meet the campus electrical load.
The output of the single-effect absorption chiller was valued, for purposes of this
study, based on the equivalent amount of electricity an electric-powered centrifugal
chiller would have consumed to provide the same amount of cooling as the single-effect
absorption chiller.
Thus, the equivalent output of the absorption chiller for 1 hour is the steam input
multiplied by the ratio of the efficiencies of the two chillers:
Revenue = 0.70 kWh/ton-h × $0.10/kWh/18 lb/ton-h × 1000 lb/klb = $3.88/klb
Therefore, $3.88/klb is the steam purchase price that will allow the absorption
chiller to compete with comparable centrifugal chillers to provide equivalent campus
chilled water demands, when the centrifugal units purchase electricity at $0.10/kWh.
As the cost of generating steam by the cogeneration units presented above is $5.14/klb,
with the particular set of parameter used in this study case it is more economical to run
the electric chiller than the single-effect absorption chiller. However, it is less costly to
use the steam in the absorber than to dump it in a steam condenser.

Electric Centrifugal Chillers
The electric centrifugal chillers appear on the analysis in order to establish a benchmarking
relationship between the amount of steam consumed by the single-effect absorber and the
amount of electricity required by electric centrifugal chillers, for the same amount of deliv-
ered cooling effect. Should the need arise for not operating the absorber, an equivalent
amount of cooling would have to be provided by the comparable electric centrifugal chill-
ers. The average efficiency of the electric chillers employed in this analysis is 0.70 kW/ton.