Page 389 - Sustainable On-Site CHP Systems Design, Construction, and Operations

362 Ca s e S t u d y 4

In the break-even analysis, the expense per ton-hour is 0.70 kWh/ton-h × $0.10/kWh
= $0.07/ton-h and the revenue is valued at $0.11/ton-h.
Break-Even Analysis
The object of this case study analysis is to compare the economic advantage of the fol-
lowing two scenarios:
1. Operating both cogeneration units and the single-effect absorption chiller
2. Operating only one cogeneration unit and firing its turbine exhaust duct burner
to meet campus steam demand

Economic Model
For the two scenarios listed above, the expenses and revenues are summarized in
Tables 22-7 and 22-8 for a 1-hour period at constant campus steam consumption. These
tables reflect the economic model for this case study.
Tables 22-9 and 22-10 depict the calculations based on equipment data for this case
study, with the following notations:

3
R is gas rate ($/m )
g
R is electricity rate ($/kWh)
e
R is steam rate ($/klb)
s
R is cooling rate ($/ton-h)
c
Note that for comparison purpose, the electric chiller revenue is equal with the rev-
enue for the absorption chiller, and the electric chiller expense is the corresponding cost
for electric energy used to generate the same cooling output as the absorber.
Results of Analysis
In Tables 22-7 and 22-8, the net revenue for scenario 1 is equal to (Revenue 1 − Expense 1)
and for scenario 2, is equal to (Revenue 2 − Expense 2). The break-even point (in lb/h cam-
pus steam) between scenario 1 and scenario 2 is where the net revenues are equal, namely:
(Revenue 1 − Expense 1) = (Revenue 2 − Expense 2)

Equipment Item Expense per Hour Revenue per Hour
Cogeneration units Natural gas Cogen gas cost
Electricity Electricity revenue
HRSG Steam Campus steam revenue
Cogeneration units Fixed cost
Absorption chiller Steam Absorber steam cost
Cooling energy Campus cooling revenue
Total “Expense 1” “Revenue 1”
TABLE 22-7 Evaluation Model for Two Cogeneration Units and Absorption Chiller