Page 87 - Handbook of Energy Engineering Calculations

4. Determine the electrical output of the cogeneration plant
Since the efficiency of the turbine is already factored into the exhaust
enthalpy of the cogeneration turbine, use the relation, P = W (h − h )/3413,
s
1
2
where the symbols are as defined earlier. Or, P = 60,000(1448 − 1315)/3413
= 2338 kW.

5. Compute the total energy output of the cogeneration plant
2
Assuming that the latent heat of the steam at 200 lb/in (abs) (1378 kPa) is
available for industrial process heating, the total energy output of the
cogeneration scheme = electrical output + (steam flow, lb/h)(latent heat of the
exhaust steam, Btu/lb). Since, from the steam tables, the latent heat of steam
2
at 200 lb/in (abs) (1378 kPa) = 834 Btu/lb (1939.9 kJ/kg), total energy
output of the cogeneration cycle = (2338 kW)(3413) + (60,000)(834) = 58
MM Btu/h (61.2 MM kJ/h).
Since the total energy output of the conventional cycle was 23 MM Btu/h

(24.3 MM kJ/h), the ratio of the cogeneration output vs. the conventional
output = 58/23 = 2.52. Thus, about 2.5 times as much energy is derived from
the cogeneration cycle as from the conventional cycle.

6. Find the comparative efficiencies of the two cycles
The boiler input = (weight of steam generated, lb/h)(enthalpy of superheated

steam at boiler outlet, Btu/lb − enthalpy of feedwater entering the boiler,
Btu/lb)/(boiler efficiency, expressed as a decimal). Or, boiler input = (60,000)
(1448 − 200)/0.82 = 91.3 MM Btu/h (96.3 MM kJ/h). The efficiency of the

conventional cycle is therefore (23/91.3)(100) = 25 percent. For the
cogeneration cycle, the efficiency (58/91.3)(l0) = 63.5 percent.

Related Calculations. This real-life example shows why cogeneration is such
a popular alternative in today’s world of power generation. In this study the
cogeneration scheme is more than twice as efficient as the conventional cycle

—63.5 percent vs. 25 percent. Higher efficiencies could be obtained if the
2
boiler outlet steam pressure were higher than 1000 lb/in (abs) (6890 kPa).
However, the pressure used here is typical of today’s industrial installations

using cogeneration to save energy and conserve the environment.
This procedure is the work of V. Ganapathy, Heat Transfer Specialist.
ABCO Industries, Inc.