Civil Engineering Reference
In-Depth Information
The net carbon bene
t of the electricity generated by the domestic micro-CHP
systems is signi
cantly higher than for the condensing boilers at 88 %. Conclusion
on market potential [
20
] is micro-CHP can be competitive with condensing boiler if
their investment costs and maintenance are comparable.
An important characteristic of mCHP cogeneration systems is the ratio between
the electrical energy and the thermal energy of the primary engine. Between these
powers depends on the primary thermal engine and is called cogeneration index:
E
Q
c ¼
ð
3
Þ
where
E
—
is the amount of electricity from cogeneration,
is the power to heat ratio,
γ
and
Q
is the amount of useful heat from cogeneration (calculated for this purpose
as total heat production minus any heat produced in separate boilers or, in case
of turbines, by live steam extraction from the steam generator before the
turbine).
—
The power to heat ratio is of special importance, since the calculation of elec-
tricity from cogeneration is based on the actual power to heat ratio.
An over-unitary value of the cogeneration index shows the predominance of the
electricity produced by the mCHP system as compared to the thermal one. The
majority of cogeneration technologies have a subunitary cogeneration index.
The dependence of the mCHP system electric and thermal power on this index
may be determined by imposing that the cogeneration index should vary between
0.1 and 4, and assuming that the sum of electric power and thermal power is the
power unit.
E
þ
Q
¼
1
ð
4
Þ
Measured with this unit, electric power may be calculated in keeping with this
ratio, with the relation:
c
c þ
E
¼
ð
5
Þ
1
and the thermal power with the relation:
1
c þ
Q
¼
ð
6
Þ
1
The cogeneration index has a great in
uence on the mCHP performance indices
(PES and overall ef
ciency). If we assume that the cogeneration system has a fuel
saving of 5 % then, to produce the same output power and admitting an electric
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