Environmental Engineering Reference
In-Depth Information
Naturally, 0.81 kW
el
is complimented through the electric utility to provide for
the 1.8 kW
el
needed, which means the electric utility requires 2.25 kW
el
of power.
●
In the second case, the primary energy that needs to be supplied into the energy
service networks and eventually into the residence is 9.85 kW. Therefore, when com-
pared to the first case a primary energy saving of 2.65 kW is obtained, which is
equal to a 21% difference. Now, if the load demands this comparison are required
constantly for an hour and taking the average domestic retail costs in the UK for
electricity and natural gas, 13 p/kWh and 4 p/kWh, respectively [188]; yields savings
of 35 pence if the CHP system is present - without taking capex nor incentives into
consideration.
From the above example, it is clear that the interpretation of performance indi-
cators is key to analyse the viability of CHP systems, permitting us to compare and
acknowledge the relevance of alternative solutions. A set of indicators are common in
the cogeneration literature; however, in this topic only the indicators relevant for mod-
elling the TCOPF tool are covered. The approach undertaken is a generic model
for energy conversion focusing on the instantaneous input and output of the power
flows, while considering the device as a black box characterised by its linear energy
conversion efficiencies.
Considering a CHP device that converts the natural gas input into electrical and
thermal power in node
k
, as indicated in Figure 4.10, yields the equations below.
The electrical power generation efficiency can be described as:
P
chp
G
G
chp
D
P
chp
G
η
el
=
=
(4.20)
F
chp
D
GHV
·
⎧
⎨
P
chp
G
is the net electrical generated power of the CHP system (J/s
=
W
el
)
G
chp
D
where
is the natural gas demand expressed in power (J/s
=
W
th
)
⎩
F
chp
D
is the natural gas demand flow rate (m
3
/
s)
Similarly, the thermal power generation efficiency can be detailed as:
T
chp
G
G
chp
D
T
chp
G
η
th
=
=
(4.21)
F
chp
D
GHV
·
where
T
chp
G
is the useful net thermal output power of the system (J
/
s
=
W
th
).
η
el
P
G
chp
G
D
chp
CHP
η
th
T
G
chp
Figure 4.10
Efficiency formula indicators in a CHP unit
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