A Method for Performance Evaluation of Cogeneration · PDF fileA Method for. Performance...

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A Method forPerformance Evaluation of

Cogeneration Systems(According to the Ministerial Decree)

Christos A. Frangopoulos

National Technical University of AthensSchool of Naval Architecture and Marine Engineering

1st S.E. Europe Region Workshop

Athens, Greece, 1 October 2009

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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

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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?

1. Introduction

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2. Fundamental Definitions

Fig. 1. A simplified picture of a cogeneration unit.

“Electrical” efficiency: ce

c

EF

η =

Thermal efficiency: CHPh

c

HF

η =

e hη = η + ηTotal efficiency:

(2.1)

(2.2)

(2.3)

Cogeneration UnitEc

HCHP

Fc

"Fuel" Energy(Primary Energy)

"Electricity"

Useful Heat

Hul: unavoidable losses

Hw: waste (avoidable)

"Electricity" shall mean electrical and/or mechanical energy.

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2. Fundamental Definitions

Threshold value of total efficiency (Directive Annex II):

(2.4)

For systems of type (b), (d), (e), (f), (g) and (h):

For systems of type (a) and (c):

thr 0,75η =

thr 0,80η =

Power loss coefficient:

Applicable in any system where the production of useful heat results in loss of electrical or mechanical power (e.g. in condensing-extractions systems).

c

CHP

EH−Δ

β =Δ

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non-CHP PartEnon-CHP

HCHP

Fnon-CHP

CHP Part ECHP

FCHP

Cogeneration Unit

Fc Ec

Hul-CHP: unavoidable losses

Hw: waste (avoidable)

Hul-non CHP: unavoidable losses

3. Calculation of“Electricity” from Cogeneration

If thrη≥ η then CHP cE E=

If thrη< η then CHP CHPE H C= ⋅

Fig. 2. CHP and non-CHP Parts of a cogeneration unit.

(3.1)

non CHP c CHPE E E− = −

(3.2)

(3.3)

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Additional Definitions

Procedure to calculate C

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.

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Total efficiency of the cogeneration unit in full cogeneration mode:


Additional Definitions


cogη

Total efficiency of the CHP Part: CHPη

If cog thrη ≥ η then (3.4)CHP cogη = η

If cog thrη < η then (3.5)CHP thrη = η

If cogη is not known, then CHP thrη = η

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Additional definitions in case of a cogeneration unit comprising a condensing-extraction steam turbine:


“Electricity” produced in fully condensing mode: max c CHPE E H= +β⋅ (3.6)

“Electrical” efficiency in fully condensing mode: maxe,max

c

EF

η = (3.7)

(3.8)Power loss coefficient: max c

CHP

E EH

−β =

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Power to heat ratio for a cogeneration unit comprising a condensing-extraction steam turbine:

e,max CHP

CHP e,maxC

η −βη=

η −η(3.9)

For units with no condensing-extractionsteam turbine:

and consequently:

0β =

e

CHP eC η=η −η

(3.10)

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Additional Calculations

“Fuel” energy for the CHP Part: CHP CHPCHP

CHP

E HF +=

η

“Fuel” energy for the non-CHP Part: non CHP CHPF F F− = −

(3.11)

(3.12)

“Electrical” efficiency of the CHP Part: CHPe,CHP

CHP

EF

η =

Thermal efficiency of the CHP Part: CHPh,CHP

CHP

HF

η =

Total efficiency of the CHP Part: CHP e,CHP h,CHPη = η + η

(3.13)

(3.14)

(3.15)

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For a cogeneration unit comprising a condensing-extraction steam turbine:

e,non CHP e,max−η = η

For a cogeneration unit with no condensing-extractionsteam turbine:

e,non CHP e−η = η

“Electrical” efficiency of the non-CHP Part

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4. Calculation of thePrimary Energy Savings

“Fuel” energy for separate production of “electricity”:c

Eer

EF =η

“Fuel” energy for separate production of heat:

CHPH

hr

HF =η

(4.1)

(4.2)

The Cogeneration Unit as a Whole

where

erη efficiency reference value for separate production of “electricity”

hrη efficiency reference value for separate production of heat

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The Cogeneration Unit as a Whole

Primary energy savings due to the cogeneration unit:

E H cPES F F F= + −

Primary energy savings ratiodue to the cogeneration unit: E H c

E H E H

F F F PESPESRF F F F+ −

= =+ +

(4.3)

(4.4)

e h

er hr

1PESR 1= −η η

+η η

or: (4.5)

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The CHP Part

“Fuel” energy for separate production of “CHP Electricity”:

(4.6)CHPE,CHP

er

EF =η

Primary energy savings due to the CHP Part:

Primary energy savings ratiodue to the CHP Part:

(4.7)CHP E,CHP H CHPPES F F F= + −

CHPCHP

E,CHP H

PESPESRF F

=+

(4.8)

or: (4.9)CHPe,CHP h,CHP

er hr

1PESR 1= −η η

+η η

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“Fuel” energy for separate production of “non-CHP Electricity”:

(4.10)

Primary energy savings due to the non-CHP Part:

Primary energy savings ratiodue to the non-CHP Part:

(4.11)

(4.12)

It is verified that:


The non-CHP Part

non CHPE,non CHP

er

EF −− =

η

non CHP E,non CHP non CHPPES F F− − −= −

non CHPnon CHP

E,non CHP

PESPESRF

−−

−

=

CHP non CHPPES PES PES−+ = (4.13)

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5. Systems with Auxiliary orSupplementary Firing

• Auxiliary firing: combustion with additional air.

• Supplementary firing:combustion without additional air.

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Fig. 3. System with auxiliary or supplementaryfiring used for cogeneration.

HRB

G

G

GT

ST

FGT FAS

HCHP

Auxiliary/Supplementary

firing

Cogeneration downstream the auxiliary or supplementary firing

“Fuel” energy for the cogeneration unit:

c GT ASF F F= + (5.1)

where

ASF “fuel” energy used for the auxiliary or supplementary firing

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Fig. 4. System with auxiliary or supplementaryfiring used for heat production only.

Only heat production downstreamthe auxiliary or supplementary firing

(5.2)

HRB

GGT

FGT FAS

H = HCHP + HAS

Auxiliary/Supplementary

firing

Heat produced with the aux. or suppl. firing:

AS AS ASH F= ⋅η

where

efficiency of the auxiliary or supplementary firing

ASη

Heat produced with the aux. or suppl. firing:

CHP ASH H H= − (5.3)

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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

cE 4 GWh=cF 10 GWh=

CHPH 2 GWh=

er 0,524η =

hr 0,90η =

thr 0,75η =

0β =

Results:e 0,4η = h 0,2η = 0,60η =⇒

thrη< η ⇒ CHP thr 0,75η = η =

C 1,1429= ⇒ CHPE 2,2857 GWh= non CHPE 1,7143 GWh− =

CHPF 5,7143 GWh= non CHPF 4,2857 GWh− =

e,CHP 0,4η = h,CHP 0,35η = CHP 0,75η = e,non CHP 0,40−η =

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Example 1

Primary energy savings of the cogeneration unit

EF 7,6336 GWh=

HF 2,2222 GWh=

PES 0,1442 GWh= − PESR 1,463 %= −

Primary energy savings of the CHP Part

E,CHPF 4,3621GWh= CHPPES 0,87 GWh= CHPPESR 13,21%=

Primary energy savings of the non-CHP Part

E,non CHPF 3,2716 GWh− = non CHPPES 1,0142 GWh− = − non CHPPESR 31%− = −

Note that: CHP non CHPPES PES PES−+ =

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Example 1

Effect of the efficiency reference value on theprimary energy savings of the cogeneration unit

PESR 1,463 %= −

Harmonized efficiency reference value(Commission Decision):

er 0,524η = ⇒

Efficiency reference value equal to the efficiency of the local electricity network, e.g.:

PESR 21,56 %=er 0,38η = ⇒

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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

cE 1574 GWh=cF 4295 GWh=

CHPH 1488 GWh=

er 0,514η =

hr 0,90η =

thr 0,80η =

0,24β =

Results:e 0,3665η = h 0,3464η = 0,7129η =⇒

thrη< η ⇒ CHP cog 0,82η = η =

C 0,6843= ⇒ CHPE 1018,3 GWh= non CHPE 555,7 GWh− =

CHPF 3056,5 GWh= non CHPF 1238,5 GWh− =

e,CHP 0,3332η = h,CHP 0,4868η = CHP 0,82η = e,non CHP 0,45−η =

cog 0,82η =

cog thrη > ηand

e,max 0,45η =

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Example 2

Primary energy savings of the cogeneration unit

EF 3062,3 GWh=

HF 1653,3 GWh=

PES 420,6 GWh= PESR 8,92 %=

Primary energy savings of the CHP Part

E,CHPF 1981,1GWh= CHPPES 577,9 GWh= CHPPESR 15,9 %=

Primary energy savings of the non-CHP Part

E,non CHPF 1081,1GWh− = non CHPPES 157,4 GWh− = − non CHPPESR 14,6 %− = −

Note that: CHP non CHPPES PES 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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