1st S.E. Europe Region Workshop Athens, Greece, 1 October 2009
A Method for Performance Evaluation of Cogeneration Systems (According to the Ministerial Decree) Christos A. Frangopoulos National Technical University of Athens School of Naval Architecture and Marine Engineering
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1. Introduction
Directive – Annex II:
The calculation of electricity from cogeneration must be based on the actual power to heat ratio. Directive – Article 3, Definition (k):
Power to heat ratio shall mean the ratio between the electricity from cogeneration and useful heat when operating in full cogeneration mode using operational data of the specific units.
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1. Introduction Directive – Annex II:
a. Electricity production from cogeneration (ECHP) should be considered equal to total annual electricity production if the annual overall efficiency of the cogeneration unit is higher than a threshold value of 75% or 80% (depending on the type of the unit). b. If the annual overall efficiency is lower than the threshold value, then: ECHP= HCHP C 3
1. Introduction
Important questions to be answered: 1. Which is the correct definition and calculation of Power to Heat Ratio (C)? 2. How is the full cogeneration mode defined? 3. How is the quantity of electrical and/or mechanical energy from cogeneration calculated? (“CHP Electricity”, ECHP, in the following). 4. How is the correct Primary Energy Savings (PES) calculated?
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2. Fundamental Definitions Hul: unavoidable losses "Electricity"
Fc "Fuel" Energy (Primary Energy)
Ec Cogeneration Unit Useful Heat
HCHP
Hw: waste (avoidable) "Electricity" shall mean electrical and/or mechanical energy.
Fig. 1. A simplified picture of a cogeneration unit. “Electrical” efficiency:
Ec ηe = Fc
(2.1)
Thermal efficiency:
H CHP ηh = Fc
(2.2)
Total efficiency:
η = ηe + ηh
(2.3) 5
2. Fundamental Definitions
Threshold value of total efficiency (Directive Annex II): For systems of type (b), (d), (e), (f), (g) and (h): ηthr = 0,75 For systems of type (a) and (c): Power loss coefficient:
ηthr = 0,80 β=
−ΔE c ΔH CHP
(2.4)
Applicable in any system where the production of useful heat results in loss of electrical or mechanical power (e.g. in condensing-extractions systems).
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3. Calculation of “Electricity” from Cogeneration If
η ≥ ηthr
then
E CHP = E c
(3.1)
If
η < ηthr
then
E CHP = H CHP ⋅ C
(3.2)
E non −CHP = E c − E CHP
(3.3)
Hul-CHP: unavoidable losses
Cogeneration Unit FCHP Fc
HCHP CHP Part
ECHP Ec
Fnon-CHP
non-CHP Part
Enon-CHP
Hul-non CHP: unavoidable losses Hw: waste (avoidable)
Fig. 2. CHP and non-CHP Parts of a cogeneration unit.
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3. Calculation of “Electricity” from Cogeneration Procedure to calculate C Additional Definitions Full cogeneration mode: A cogeneration unit operating with maximum technically possible heat recovery is said to be operating in full cogeneration mode. In order for the numerical results of the calculations to be compatible and consistent with efficiency values specified in the Directive, the following definition is applied:
A unit operates in full cogeneration mode, if its overall efficiency is at least 75% if it is of type (b), (d), (e), (f), (g) and (h), or 80% if it is of type (a) and (c). These values shall be adapted to technical progress, as stated in the Directive. 8
3. Calculation of “Electricity” from Cogeneration Procedure to calculate C Additional Definitions Total efficiency of the cogeneration unit in full cogeneration mode:
ηcog
Total efficiency of the CHP Part:
ηCHP
If
ηcog ≥ ηthr
then
ηCHP = ηcog
(3.4)
If
ηcog < ηthr
then
ηCHP = ηthr
(3.5)
If ηcog is not known, then
ηCHP = ηthr
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3. Calculation of “Electricity” from Cogeneration Procedure to calculate C Additional definitions in case of a cogeneration unit comprising a condensing-extraction steam turbine: “Electricity” produced in fully condensing mode: E max = E c + β ⋅ H CHP (3.6) “Electrical” efficiency in fully condensing mode: Power loss coefficient:
E max Fc
(3.7)
E max − E c H CHP
(3.8)
ηe,max =
β=
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3. Calculation of “Electricity” from Cogeneration Procedure to calculate C Power to heat ratio for a cogeneration unit comprising a condensing-extraction steam turbine: C=
ηe,max − βηCHP ηCHP − ηe,max
(3.9)
For units with no condensing-extraction steam turbine: β=0 and consequently:
C=
ηe ηCHP − ηe
(3.10)
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3. Calculation of “Electricity” from Cogeneration Additional Calculations “Fuel” energy for the CHP Part:
FCHP =
E CHP + H CHP (3.11) ηCHP
“Fuel” energy for the non-CHP Part:
Fnon −CHP = F − FCHP (3.12)
“Electrical” efficiency of the CHP Part:
ηe,CHP =
E CHP FCHP
(3.13)
Thermal efficiency of the CHP Part:
ηh,CHP =
H CHP FCHP
(3.14)
Total efficiency of the CHP Part:
ηCHP = ηe,CHP + ηh,CHP (3.15) 12
3. Calculation of “Electricity” from Cogeneration “Electrical” efficiency of the non-CHP Part For a cogeneration unit comprising a condensing-extraction steam turbine: ηe,non −CHP = ηe,max For a cogeneration unit with no condensing-extraction steam turbine: ηe,non −CHP = ηe
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4. Calculation of the Primary Energy Savings The Cogeneration Unit as a Whole “Fuel” energy for separate production of “electricity”: E FE = c ηer
(4.1)
“Fuel” energy for separate production of heat: H CHP FH = ηhr where
(4.2)
ηer
efficiency reference value for separate production of “electricity”
ηhr
efficiency reference value for separate production of heat
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4. Calculation of the Primary Energy Savings The Cogeneration Unit as a Whole Primary energy savings due to the cogeneration unit:
PES = FE + FH − Fc Primary energy savings ratio due to the cogeneration unit:
PESR =
FE + FH − Fc PES = FE + FH FE + FH
or: PESR = 1 −
1 ηe ηh + ηer ηhr
(4.3)
(4.4) (4.5)
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4. Calculation of the Primary Energy Savings The CHP Part “Fuel” energy for separate production of “CHP Electricity”: FE,CHP
E CHP = ηer
(4.6)
Primary energy savings due to the CHP Part: PESCHP = FE,CHP + FH − FCHP
Primary energy savings ratio due to the CHP Part: or:
PESR CHP =
(4.7)
PESCHP FE,CHP + FH
(4.8)
1
(4.9)
PESR CHP = 1 −
ηe,CHP ηer
+
ηh,CHP ηhr
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4. Calculation of the Primary Energy Savings The non-CHP Part “Fuel” energy for separate production of “non-CHP Electricity”: FE,non −CHP
E non −CHP = ηer
(4.10)
Primary energy savings due to the non-CHP Part: PESnon −CHP = FE,non −CHP − Fnon −CHP
Primary energy savings ratio due to the non-CHP Part:
It is verified that:
PESR non −CHP =
PESnon −CHP FE,non −CHP
PESCHP + PESnon −CHP = PES
(4.11)
(4.12) (4.13)
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5. Systems with Auxiliary or Supplementary Firing
• Auxiliary firing: combustion with additional air. • Supplementary firing: combustion without additional air.
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5. Systems with Auxiliary or Supplementary Firing Cogeneration downstream the auxiliary or supplementary firing FGT
“Fuel” energy for the cogeneration unit:
FAS
GT
G
Fc = FGT + FAS
Auxiliary/ Supplementary firing
(5.1)
where
FAS HRB
ST
G
“fuel” energy used for the auxiliary or supplementary firing
HCHP
Fig. 3. System with auxiliary or supplementary firing used for cogeneration. 19
5. Systems with Auxiliary or Supplementary Firing Only heat production downstream the auxiliary or supplementary firing FAS
FGT
Heat produced with the aux. or suppl. firing:
H AS = FAS ⋅ ηAS
(5.2)
where GT
G
Auxiliary/ Supplementary firing
ηAS efficiency of the auxiliary or supplementary firing
Heat produced with the aux. or suppl. firing: H = HCHP + HAS HRB
H CHP = H − H AS
(5.3)
Fig. 4. System with auxiliary or supplementary firing used for heat production only. 20
6. Additional Rules
•
The calculations must be based on actual data collected during the reporting period.
•
For cogeneration units at the construction phase or during the first year of operation, when there are no sufficient data, specifications from the manufacturer or results obtained with a simulation model of the particular unit can be used.
•
If neither specifications nor results of a simulation model are available, then the default value for the power to heat ratio can be used, but for the first year of operation only.
•
For micro-cogeneration units, specifications from the manufacturer can be used.
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7. Numerical Examples Example 1 Data: Gas engine cogeneration unit
Fc = 10 GWh
E c = 4 GWh
ηer = 0,524
ηthr = 0,75
H CHP = 2 GWh
ηhr = 0,90
β=0
Results: ηe = 0, 4
ηh = 0, 2
η < ηthr C = 1,1429
⇒
⇒ ⇒
η = 0,60
ηCHP = ηthr = 0,75
E CHP = 2, 2857 GWh
E non −CHP = 1,7143 GWh
FCHP = 5,7143 GWh
Fnon −CHP = 4, 2857 GWh
ηe,CHP = 0, 4 ηh,CHP = 0,35
ηCHP = 0,75
ηe,non −CHP = 0, 40
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Example 1
Primary energy savings of the cogeneration unit PESR = −1, 463 % PES = −0,1442 GWh FE = 7,6336 GWh
FH = 2, 2222 GWh Primary energy savings of the CHP Part
FE,CHP = 4,3621 GWh
PESCHP = 0,87 GWh
PESR CHP = 13, 21 %
Primary energy savings of the non-CHP Part
FE,non −CHP = 3, 2716 GWh Note that:
PESnon −CHP = −1,0142 GWh
PESR non −CHP = −31 %
PESCHP + PESnon −CHP = PES
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Example 1 Effect of the efficiency reference value on the primary energy savings of the cogeneration unit Harmonized efficiency reference value (Commission Decision): ηer = 0,524 ⇒
PESR = −1, 463 %
Efficiency reference value equal to the efficiency of the local electricity network, e.g.: ⇒ ηer = 0,38
PESR = 21,56 %
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Example 1 Comments on the Results • The CHP Part of a cogeneration unit may be of “high efficiency” (higher than 10%), but the unit as a whole may have negative Primary Energy Savings. This is not a proper application of cogeneration. • The efficiency reference value has a strong effect on the Primary Energy Savings of a cogeneration unit: a unit may have negative PES compared with the best available technology (harmonized efficiency reference value), but a very positive one compared with the local electricity system.
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Example 2 Data: A combined cycle cogeneration unit
Fc = 4295 GWh
E c = 1574 GWh
ηcog = 0,82
ηer = 0,514
H CHP = 1488 GWh
β = 0, 24
ηhr = 0,90
Results:
ηe,max = 0, 45
ηthr = 0,80
ηe = 0,3665
ηh = 0,3464
⇒
η = 0,7129
η < ηthr
ηcog > ηthr
⇒
ηCHP = ηcog = 0,82
and
C = 0,6843 ⇒ E CHP = 1018,3 GWh
E non −CHP = 555,7 GWh
FCHP = 3056,5 GWh
Fnon −CHP = 1238,5 GWh
ηe,CHP = 0,3332
ηh,CHP = 0, 4868
ηCHP = 0,82
ηe,non −CHP = 0, 45 26
Example 2
Primary energy savings of the cogeneration unit PES = 420,6 GWh FE = 3062,3 GWh
PESR = 8,92 %
FH = 1653,3 GWh Primary energy savings of the CHP Part FE,CHP = 1981,1 GWh
PESCHP = 577,9 GWh
PESR CHP = 15,9 %
Primary energy savings of the non-CHP Part FE,non −CHP = 1081,1 GWh Note that:
PESnon −CHP = −157, 4 GWh
PESR non −CHP = −14,6 %
PESCHP + PESnon −CHP = PES
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Example 2 Comments on the Results • Even a cogeneration system of high nominal efficiency, such as a combined cycle plant, may have periods of operation with low efficiency. As a consequence, the annual total efficiency may be lower than 10%, and the plant looses the benefits of high efficiency cogeneration. • Therefore, optimal design and operation of cogeneration systems is of crucial importance.
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Thank you for your attention
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