Civil Engineering Reference
In-Depth Information
NOT RECOVERABLE HEAT
COMBUSTION AIR
LOSSES
TRIGENERATION PLANT
ELECTRIC GENERATOR
NATURAL GAS
HEAT RECOVERY
AND COOLING
ELECTRICAL ENERGY
LOSSES
100%
F
40%
H
40%
E
20%
L
50%
Net heat recovery index(1)
30.8%
Primary Energy Saving Index(2)
2,500
kW
1,000
kW
(3)
1,000
kW
500
kW
262.2
Sm3/h
1,000
kW (4)=(3)
0.75
COP absorption
750
kWcoling
4
COP cooling compressor to replace
188
kWelectrical compressor to replace
input data
(1) H/(H+E)
(2) (1-F/(E/
p
+H/
b
))*100
(4)= (3)
where
p
=electrical production and transmission efficiency=40% and
b
=efficiency of the boiler to replace=90%
natural gas LHV=34,325 kJ/Sm
3
h
h
Fig. 9.2
Basic scheme of a trigeneration plant
9.3
The Backpressure or Non-condensing Steam Turbine
Figure
9.1
represents a cogeneration plant with backpressure steam
turbine exhausting steam headers to the plant process. The efficiency
coefficients are those from Table
9.1
. Figure
9.1
may also represent a
utility plant designed to generate electric power if appropriate
coefficients are used.
The amount of power that can be produced by expanding steam in a prime mover
is limited by the Available Energy (AE) between the inlet and outlet of the steam
turbine. This energy is the enthalpy difference between the inlet superheated steam,
at high pressure and temperature, and the outlet steam at lower pressure along an
ideal isentropic expansion. The Mollier diagram or equivalent steam tables (see
Sect.
6.4
) can conveniently be used for this purpose (see Fig.
9.3
). Alternatively,
Theoretical Steam Rate tables such as those published by ASME can be used; these
report the Theoretical Steam flow Rate (TSR) required to generate 1 kWh in a
100 % efficiency expansion process (see Table
9.2
). TSR is the ratio between the
energy content of 1 kWh (3,600 kJ/kWh) and the Available Energy AE (kJ/kg); it
represents the amount of steam theoretically needed to produce 1 kWh:
