Application of environmental performance assessment of CHP systems with local and global approaches

Applied Energy xxx (2014) xxx–xxx

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Application of environmental performance assessment of CHP systems with local and global approaches Michele Bianchi a, Lisa Branchini b, Andrea De Pascale a,⇑, Antonio Peretto a a b

Università di Bologna – DIN, Viale del Risorgimento 2, 40136 Bologna, Italy Università di Bologna – CIRI ENA, via Angherà, 22, Rimini 47900, Italy

h i g h l i g h t s  Methods to indicate environmental impact of CHP systems are discussed.  Local impact is assessed with the Avoided Heat Generator method.  Global impact is measured introducing a new Pollutant Saving Index.  The analysis is focused on air pollutants emitted by CHP prime movers.  An assessment of CHP commercial units during normal operation is carried out.

a r t i c l e

i n f o

Article history: Received 28 October 2013 Received in revised form 5 March 2014 Accepted 7 April 2014 Available online xxxx Keywords: CHP Gas emission Environmental pressure Avoided Heat Generator Energy saving Prime mover

a b s t r a c t This paper is focused on methods to indicate the environmental impact in terms of air pollutants of CHP (Combined Heat and Power) systems. The aim is to combine the energy saving achievement with information concerning the environmental benefit, in comparison with the non-CHP scenario. Environmental impact of CHP production both on global and local scale should be taken into account. In particular, the method of the ‘‘Avoided Heat Generator’’ is highlighted in this study as a proper approach for CHP and is used for a local-scale environmental impact evaluation. This approach calculates the reduction of emission due to CHP operation, taking into account the amount of pollutant emitted by an equivalent heat generator, which provides the same thermal power of the CHP prime mover. Moreover, the recommended approach is compared in the paper with another proposed method, based on the PSI (Pollutant Saving Index) value, which is suitable to estimate the global-scale environmental impact. Numerical evaluations of the CHP environmental benefits, in terms of NOx, CO and CO2 emissions, are shown for several CHP systems with different technologies and electric power sizes, in order to provide a comprehensive overview of current CHP prime mover environmental performance and to serve as an application reference example of the two methods potential use, in the framework of the CHP units authorization procedure. The significant effect of the reference comparative scenario is also highlighted. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Combined Heat and Power (CHP) generation is recognized by regional and international policy makers [1] as one of the technically and economically viable strategies to face the rising trend of primary energy demand. Moreover, CHP production represents a sustainable path towards the reduction of GHG (Green House Gas) emissions, leading to a lower dependency on fossil fuels, with implications in the 3-E (Energy, Economic and Environment), as ⇑ Corresponding author. Tel.: +39 051 2093318. E-mail address: [email protected] (A. De Pascale).

reviewed by Angrisani et al. [2]. The CHP potential in different countries, depicted in Fig. 1, had been forecasted by IEA in [3]; Fig. 2 provides the actual scenario in terms of installed power [3], while Fig. 3 shows the Italian situation in terms of number of installed units and average nameplate size per installed unit [4]. The Prime Mover (PM) technologies used for CHP systems are mainly Internal Combustion Engines (ICE) and secondarily Gas Turbines (GT), typically with electric power size in a range of medium values (1–10 MW). For large industrial applications also Steam Turbine (ST) plants are currently installed with: (i) steam Extraction (Ex); (ii) Back Pressure (BP); or (iii) Combined Cycle (CC) arrangements. A large market potential for small size available

http://dx.doi.org/10.1016/j.apenergy.2014.04.017 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Bianchi M et al. Application of environmental performance assessment of CHP systems with local and global approaches. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.04.017

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Nomenclature

Fig. 1. CHP potential by IEA [2].

20%

Finland

Denmark

Latvia

Russia

Netherlands

Poland

Hungary

Czech Rep.

China

Austria

Italy

Germany

USA

UK

Spain

0%

Canada

10%

Sweden

G8+5

UK

USA

S.Africa

Russia

Mexico

Japan

Italy

India

Germany

China

France

0%

30%

Japan

5%

40%

France

10%

50%

Brazil

15%

60%

Mexico

2015 2030

Canada

constrained by the same environmental regulations and air pollution emission standards applied to non-CHP energy systems. Environmental issues related with CHP production are analyzed in various scientific documents and papers, e.g. Gullì [13] introduced the need to apply cost-benefit analysis; Canova et al. [14] discussed hazardous species characterization and emission factors of CHP micro-generators; and some values of CHP and SP emission factors are provided in [15–17]. Different methodologies for quantifying the emission of CHP and SP are discussed in studies by Mancarella et al. [18,19], by Fumo el al. [20] and by Rosen [21,22], mainly linking the environmental benefit of cogeneration to the reduction of primary energy [18], by allocating emissions to the input fuels [20] or measuring CHP profit with energy analysis effects [22]. Gullì highlights in [13] that small CHP systems can bring about lower electric efficiency, higher pollutant emission and reduced applicability of abatement technologies, in comparison with large power stations. Thus, issues caused by CHP systems size and affecting the environmental performance could act as a barrier to CHP diffusion. A correct and recognized quantitative analysis of distributed CHP production effects in terms of pollutant emissions, in comparison with SP, is becoming a concern for local authorities in many industrialized economies. The seek towards environmen-

Local CHP share of electricity production

25%

Brazil

of the Avoided Heat Generator of the Avoided Electric energy Generation Avoided of the prime mover in CHP operation electrical thermal

d D X k

ICE and Micro Gas Turbine (MGT) [5] or under development technologies such as Stirling, Organic Rankine Cycles (ORC) and fuel cell systems [6–8], can be also identified in the field of limited power output (1–100 kW). This potential is shown, in domestic applications, for example in the studies by Dentice d’Accadia et al. [8], by Peacock and Newborough [9] and by Bianchi et al. [10], highlighting the micro-CHP systems convenience, taking into account the actual heat and power demand of residential buildings. The large potential and techno-economic feasibility of CHP production in many sectors, replacing the Separated Production (SP) of heat and electricity, leads to the need for a proper environmental assessment of CHP penetration. The pursued aim of a CHP system should be to contextually minimize consumption of primary energy and environmental impact. On one side, the energy saving potential of CHP systems is computable with various performance criteria [1,11,12]. CHP production is often supported by incentive legislations privileging this CHP energy benefit. In EU countries the Primary Energy Saving (PES) index [1] is used to quantify the amount of primary energy resource saved with a CHP system, in comparison with SP. On the other side, dedicated regulations, prescriptions and guidelines for emission specification are typically not available for CHP production. Indeed, CHP systems are often

Additional CHP share of El. production (ref. 2005)

Subscript AHG AEG Avoid CHP el th

c

Symbols E energy (kW h) F fuel energy (kW h)

20%

g

dry flue gas per unit of fuel (N m3/kg) Lower Heating Value (kJ/kg) Primary Energy Saving (–) Pollutant Saving Index (–) mass concentration (mg/N m3) output-based emission (mg/kW h) total mass of emitted air pollutant (mg) oxygen vol. fraction of flue gas (%) input-based emission (mg/kW h) efficiency (–)

K LHV PES PSI

Abbreviation AHG Avoided Heat Generator BP Back Pressure CC Combined Cycle CHP Combined Heat and Power Ex Extraction GHG Green House Gas GT Gas Turbine ICE Internal Combustion Engine LCA Life Cycle Analysis MGT Micro Gas Turbine ORC Organic Rankine Cycle PM Prime Mover SP Separate Production ST Steam Turbine

Fig. 2. CHP 2009 global scenario by IEA [3].

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M. Bianchi et al. / Applied Energy xxx (2014) xxx–xxx

of dry flue gas per unit of fuel mass in stoichiometric combustion) are factors depending on the input fuel used by the energy system; and gel is the electric efficiency of the PM. Nevertheless, the assessment of CHP emissions should take into account that cogeneration provides a double useful output, namely electric and thermal energy. Several approaches can be in principle applied in order to consider the benefit of cogeneration on emissions, as reviewed in [27]. Two of these approaches are here described and applied in the following paragraphs. 2.1. Avoided Heat Generator method

Fig. 3. 2010 Italian CHP plants data [4].

tal pressure indicators for energy systems is driven by the growing concern of citizens for the quality of their environment. The use of simple indexes useful to quantify Environment/ Energy/Economic performance of energy systems and compare alternative solutions is a clear target of researchers, industrial developers and market players, as highlighted for example in [23–25]. Methods to produce synthetic indices for energy systems pollution and to measure sustainability are currently under investigation by the EU [26]. Focusing on CHP systems and exhaust gas emissions, a comprehensive overview on different technical methods (both new and already introduced by other authors) for environmental assessment was provided in [27]; nevertheless, the adoption of an universally recognized approach to correctly indicate CHP pros in terms of pollutants reduction is still an open issue. The most promising methods are here firstly recalled and then further investigated, providing an application to several existing CHP commercial units representative of the current CHP market, taking into account the size effect and local/global potential impact. Therefore the carried out application, consists in a comprehensive overview of current CHP prime movers environmental performance, useful as reference also for future CHP units authorization procedures. In particular, two meaningful calculation methods are here shown and compared, namely the Avoided Heat Generator method and the Pollutant Saving Index method, respectively highlighted as most promising to quantify the environmental local and global impact of CHP systems.

The first significant approach identified and applied here, to show its benefits, is the Avoided Heat Generation (AHG) method. The approach, followed in order to take into account the double useful output of CHP in terms of emission, is described in Fig. 4; this methodology considers the CHP scenario in comparison with a non-CHP scenario, where the same PM operates producing only the electric output energy Eel, while a heat generator is also present to fulfill the thermal energy demand Eth. Fig. 4a shows the non-CHP scenario, where the PM produces a total emission D, corresponding to specific emission d = D/Eel; moreover, the additional heat generator is responsible for the total emission Dth. Fig. 4b shows instead the actual CHP scenario where the heat generator emission are avoided. Thus, in case of CHP operation of the system, a correction term should be introduced in Eq. (1), according to:

dCHP ¼ d  dAv oid

ð2Þ

where dCHP is the corrected specific emission of the system in CHP operation and dAvoid is the avoided emission term, in comparison with the non-CHP operation of the same energy system. According to this approach, the resulting reduced emission can be more correctly compared with the existing emission standard for non-CHP systems. In the framework of an electric energy output-based approach, the correction term is expressed as:

dAv oid ¼

Dth Dth gth kAHG gth ¼  ¼  Eel Eth gel gAHG gel

ð3Þ

where the electric efficiency gel and the thermal efficiency gth of the PM in CHP mode are highlighted, while kAHG represents the emission of the AHG per unit of its input fuel ðkAHG ¼ Dth =F AHG Þ and gAHG is the thermal efficiency of the same reference AHG.

2. Methods to assess CHP emissions 2.2. Pollutant Savings Index method The importance of expressing the amount of pollutant emissions of energy systems per unit of output energy is relevant, as remarked also in [27]. Indeed, the resulting specific emission is a clear indicator of the environmental pressure of the system, in the framework of a cost/benefit assessment. The output-based emission of a generic PM energy system (operable both in CHP and non-CHP mode) can be effectively referred to the produced electric energy, as main useful output.1 The resulting output-based emissions (indicated as d) can be calculated as:

d¼

k

gel

¼

21 3600 1 K c 21  X LHV gel



mg

N m3

 ð1Þ

where k is the input-based emission, i.e. per unit of fuel energy; c is the pollutant concentration in the flue gas; X is the oxygen vol. concentration in the flue gas; LHV (Lower Heating Value) and K (volume 1

This reference to the electric output seems more appropriate especially in case of typical ‘‘bottoming’’ CHP systems, where heat is recovered as secondary output of a ‘‘topper’’ PM (e.g., ICE, GT).

An alternative approach to measure the effect of CHP production on the environment, consists in simply calculating the difference between the total emission of the CHP system and the total emission of the separated production systems (a reference power plant and a heat generator, see Fig. 5). According to this concept, a normalized index named here Pollutant Saving Index (PSI), can be calculated as follows:

PSI ¼

ðDel þ Dth Þ  D Del þ Dth

ð—Þ

ð4Þ

representing the saving in total emission of the CHP system, in comparison with the reference SP; in particular, the avoided electric generation (AEG) is fully characterized in terms of electric efficiency gAEG and specific emission factor kAEG (emission of the AEG per unit of its input fuel: kAEG ¼ Del =F AEG ).

PSI ¼ 1  k

AEG

gAEG

d þ gkAHG

AHG

gth gel

ð5Þ

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M. Bianchi et al. / Applied Energy xxx (2014) xxx–xxx

(a)

(b) Fig. 4. Compared scenarios for CHP local assessment.

Fig. 5. CHP global emission assessment scenarios.

At least, a positive value of PSI should be achieved, in order to recognize environmental convenience of CHP production. It should be mentioned that other methods, which simply correlate the emission saving with fuel saving or with PES (see [27]), could be also introduced and are currently used in some local regulations, as recently described in [28] for the USA scenario. The PSI method instead, as proposed according to the above described formulation, is not applied yet in practical cases. The PSI method is more rigorous than the previous ones, if the aim is to identify the environmental global impact of the replacement of SP with a

CHP energy system. On the contrary, the local impact effect is not clearly quantified with PSI. Indeed, as shown in Fig. 5, the emission of large power plants are taken into account in PSI, even if this emission does not always cause a local environmental pressure, because of the potentially long distance. 3. Application to CHP and microCHP units The two above described methods can be used to compare different alternative CHP PMs, or to compare a given PM with the

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M. Bianchi et al. / Applied Energy xxx (2014) xxx–xxx

Electric efficiency

50%

Table 1 Example of reference values.

ICE MGT GT ORC Stirling

40%

Parameter

Value

Ref.

Thermal efficiency of the AHG gAHG NOx emission of the AHG (mg/kW hLHV) CO emission of the AHG (mg/kW hLHV) Natural gas CO2 emission (kg/kW hLHV)

0.90 200 70 100 0.20

[30] [30] class 2 [30] class 5 [31] –

Reference electric efficiency gAEG NOx emission of the AEG (mg/kW hLHV) CO emission of the AEG (mg/kW hLHV) CO2 emission of the AEG (kg/kW hLHV)

0.52 230 150 0.47

[30,32] [33,15] GT large size, [15] [33,34]

30%

20%

10%

0%

1

10

100

1000

10000

Electric power size [kW] Fig. 6. Electric efficiency data of CHP commercial units.

reference existing SP scenario. This becomes useful in the decision process undertaken by the plant proposers and by the local/regional authorities in charge of the authorization procedure for small/ medium size emitters, prior to installation, taking into account the manufacturer guaranteed emission values of the PM, as shown in this application study. The assessment of pollutant emissions due to CHP systems is worth of interest, especially taking into account the limited power size, in comparison with large power stations. Rated power values of CHP can be in general lower than 10 MWel in many industrial and tertiary applications, but lower ‘‘microCHP’’ units, with nominal size often lower than few kWel, are also currently under attention by the residential energy market. These power systems are typically installed nearby the users and connected to the electric grid at medium/low voltage level and they are potentially usable in CHP applications as alternative to the SP. In this size range the electric performance are rather limited, as shown in Fig. 6, reporting the nameplate electric efficiency of many existing PMs. Values of gel , required for Eq. (1), are mostly in the range 15–30% in case of power size up to 50 kWel, efficiency rises to 30–35% for size around 100 kWel and reaches values higher than 35–45% in case of larger power sizes up to few MWel. ICEs provide higher values in comparison with GTs and other micro-CHP systems. For these power systems the emission evaluation should be carried out considering the following points: – Most of the CHP applications are based on a ‘‘topping’’ configuration, with a topper PM (ICE, GT, MGT) for electricity production and a subsequent heat recovery. Therefore the output based approach with reference to the electric production is more appropriate; on the contrary, it does not seem correct to consider an ICE or a GT similar to a heat generator, assuming the same emission limits. – The AHG method allows to estimate the local environmental benefit of the CHP system; indeed, the method measures the environmental pressure affecting the receptors located close to the CHP/microCHP system receiving the pollutant emitted by the installed PM, but avoiding to receive the emission of the AHG. This method is especially useful for pollutants with low residence times and local effect, such as CO, NOx, and VOC. – The PSI method represents the most correct approach to express the global impact. Nevertheless, this emission saving is evaluated independently on the site in which the CHP plant and the substituted power plants are located. Therefore the PSI method should be used only for the pollutant species with a global impact effect, on national or regional scale or in critical areas

due to local climate and industrial concentration (e.g., the Pianura Padana Italian region, the Ruhr German district, etc.). On the contrary, the PSI method is not able to evaluate the local impact. Considering the above discussed points, for a local impact assessment of CHP systems, the most appropriate AHG method is primarily applied here. A parametric investigation has been performed on the influence of different factors affecting the avoided emission term dAvoid. The reference AHG is a key actor to be established; the AHG performance can be selected conventionally, using standard heat generator data. Table 1 provides realistic data, from different sources, of NOx, CO and CO2 concerning a representative AHG. The emission reduction is proportional to the emission of the AHG (Eq. (3)). The dAvoid term is plotted in Fig. 7, versus specific emission ðkAHG =gAHG Þ of the AHG, for a collection of typical values of the CHP gel/gth ratio. This general map can be used to calculate dAvoid for different: (i) pollutant species, (ii) PM size and technologies, (iii) reference AHGs and (iv) fuels. For example, given a PM, its avoided emission, when operated in CHP mode, can be calculated with the reference AHG of Table 1 and reported in Fig. 7. The NOx avoided emissions of a MGT, of an ICE and of a GT fuelled with natural gas and calculated considering kAHG = 200 mg/N m3 are shown in the same figure (points A, B and C; values are also shown in Table 2). The avoided emission can be compared with the actual emission d of the PMs in non-CHP operation (reported in Fig. 7, on the right side axis; values are in Table 2). The dAvoid term can be lower, or equal to the actual emission of the PM, or can become even higher, depending on the PM and on the reference AHG. In case of MGT (point A) and GT (B), the CHP mode provides emission similar to the AHG ; in case of GT, the calculated dAvoid is slightly higher than d. In case of ICE (C), typically characterized by higher electric efficiency and d values, the avoided emissions for a given AHG (kAHG =gAHG P 275 (mg/kW h)) tends to be lower. The importance of the AHG reference data is evident in Fig. 7. Emission saving results can obviously improve, if the selected reference AHG is less efficient or more pollutant, and vice versa; nevertheless, the general trend is captured by this investigation. In this study, the used reference thermal efficiency is the value suggested by the EU Directive [29] on CHP energy saving performance assessment and the NOx reference emission is equal to the European Standard for boilers [30]. In particular, the case of ‘‘mean’’ boilers (class 2 [30]) has been considered primarily, but also a comparison with ‘‘best-available’’ boilers (class 5 [30]) has been included in Table 2. It can be seen that by increasing the environmental performance of the reference scenario, the CHP benefit reduces drastically (points A0 , B0 and C0 in Fig. 7), becoming a fraction of the PM emission. The CHP environmental performance of the three above mentioned PM units in terms of PSI are summarized in Table 2, using the reference values in Table 1 for both thermal and electricity gen-

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M. Bianchi et al. / Applied Energy xxx (2014) xxx–xxx 1000

2500

δ η /η =0.25

Avoid

el

mg kWh

γ=500 mg/Nm3 (5%O ) 2 450

NOx

th

mg kWh

0.50

δ ICE

800

0.75

600

δ MGT

1.0

A

δ

400

Avoid

1.25

C

δ

B

MGT

δ AvoidGT Avoid

δGT

Output based NOx emission, δ [mg/kWh]

δ

2000

fuel: NG 1500

250 1000

500

γ =25 ppmvd(15%O2) 15 5

ICE

200

0 20%

A'

25%

30%

C' B'

35%

40%

45%

50%

Electric efficiency Fig. 8. Output based NOx versus electric efficiency for commercial ICEs, MGTs and small GTs.

0 0

ICE (Pel<100kW) ICE (100 kW < Pel < 1MW) ICE (1MW < Pel <10 MW) MGT GT (Pel < 10 MW)

100

200

λ

AHG

300

η

AHG

400

500

600

[mg/kWh] 2500

Table 2 Energy and NOx emission performance of CHP systems with selected ICE, GT and MGT as PMs.

Electric power size (kW) Electric efficiency Thermal efficiency Electric index = gel/gth PES NOx concentration (mg/N m3) NOx d (mg/kW h) NOx dAvoid (mg/kW h) (AHG class 2) NOx dAvoid (mg/kW h) (AHG class 5) PSI (NOx) (AHG class 2) PSI (NOx) (AHG class 5)

GT

MGT

ICE

7500 0.34 0.48 070 0.216 15 (15%O2) 271 313 110 0.643 0.508

100 0.30 0.55 0.55 0.158 50 (15%O2) 505 408 140 0.407 0.134

600 0.37 0.49 0.75 0.204 250 (5%O2) 759 295 100 0.029 0.391

600

Output based CO emission, δ [mg/kWh]

Fig. 7. Avoided emissions due to CHP operation: influence of AHG and PM characteristics; comparison with non-CHP emissions in case of NOx, for three PMs fuelled with NG.

ICE (Pel<100kW) ICE (100 kW < Pel < 1MW) ICE (1MW < Pel <10 MW) MGT GT (Pel < 10 MW)

500 2000

fuel: NG

3

1500

γ=300 mg/Nm (5%O ) 2

1000

γ=50 ppmvd (15%O2)

500

25 10 0 20%

25%

30%

35%

40%

45%

50%

Electric efficiency Fig. 9. Output based CO versus electric efficiency for commercial ICEs, MGTs and small GTs.

eration. It can be seen that the AHG method highlights an environmental convenience of CHP. On the contrary the calculated values of PSI are positive only for the GT and MGT but not for ICE, demonstrating that on a global basis the environmental impact of that ICE is questionable in terms of NOx. 3.1. Commercial CHP units emission results Figs. 8–10 present comprehensive data collections of the nameplate NOx, CO and CO2 specific emissions versus the electric efficiency, for several commercially available PMs (ICE of different size ranges, small GTs and MGTs) covering the entire spectrum of actual small CHP and l-cHP range of power size and operated with the most common fuel, i.e. natural gas. Concentration data c have been collected from commercial brochures and technical documents and have been converted here into output-based specific emissions. The rated NOx concentration values vary from 250 mg/N m3 to 500 mg/N m3(5%O2) in most of ICE cases, and in the rage of 5–30 ppm(15%O2) in case of GTs and MGTs. The positive effect of gel increase on d is evident for NOx, especially in case of high rated concentration values. The CO emission data in Fig. 9 also shows similar trends in the output-based

emission. The severe excursion of emission values is due to the different implemented emission abatement technologies. It should be mentioned that data used to calculate both CO and NOx output-based emissions are rated concentrations, i.e. max values guaranteed by the PM manufacturers in normal conditions; indeed, in actual operation at base load, d values could be lower. Eventually, calculated full load CO2 output-based emission are shown in Fig. 10 for the same PMs. For this pollutant, data presented clearly depend only on the gel value, for a given fuel (input-based emission is constant, equal to 198 g/kW h if natural gas is used). Indeed, the input-based CO2 concentration is not affected by the technologic level of the combustor; moreover, CO2 capture or removing techniques have not been considered here. Fig. 10 shows that, starting from the considered NG fuelled conditions, the obtained points of CO2 emission, for the investigated PMs, would shift vertically upward in case of adoption of C-rich fuels and downward in case of CO2 capture. It is interesting to observe that output-based NOx and CO emissions of GTs and MGTs show, in most of the cases, lower values than ICEs, despite the lower characteristic electric efficiency. On

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M. Bianchi et al. / Applied Energy xxx (2014) xxx–xxx

300g/kWh

700 C-rich fuels CO2 capture tech.

600

500

100g/kWh 400 20%

25%

150g/kWh 30%

35%

ICE (100 kW < Pel < 1MW) ICE (1MW < Pel <10 MW) MGT GT (Pel < 10 MW)

AHG

250g/kWh

NG

ICE (Pel<100kW)

fuel: NG λ =100 mg/kWh 800

[mg/kWh]

800

λ=198g/kWh

Avoid

900

1000 ICE (Pel <100kW) ICE (100 kW < Pel < 1MW) ICE (1MW < Pel <10 MW) MTG GT (Pel < 10 MW)

ηAHG=90%

600

CO avoided emission, δ

Output based CO2 emission, δ [mg/kWh]

1000

400

η th =0.75

200 40%

45%

50%

η

Electric efficiency 0 20%

Fig. 10. Output based CO2 versus electric efficiency for commercial ICEs, MGTs and small GTs.

th

=0.4

25%

30%

35%

40%

45%

50%

Electric efficiency Fig. 12. Avoided CO versus electric efficiency for commercial ICEs, MGTs and small GTs.

NOx avoided emission, δAvoid [mg/kWh]

1000 ICE (Pel <100kW) ICE (100 kW < Pel < 1MW) ICE (1MW < Pel <10 MW) MGT

800

Local assessment of CO2 via the dAvoid indicator has not been considered, due to the global effect of this chemical species.

GT (Pel < 10 MW)

3.1.2. Pollutant Saving Index results Figs. 13 and 14 provide, in comprehensive maps for the same large number of commercial PMs, the calculated values of PSI respectively for NOx and CO2 (both species have a global environmental impact). The reference SP input data (to characterize the AHG and AEG) used to obtain PSI are the ones provided in Table 2. The NOx PSI values are positive or negative, depending on the selected PM technology, size range and model and they tend to be higher for systems with lower efficiency, but a direct correlation between PSI and gel is not evident. The GT and MGT PSI values are positive (ranging between 0.3 and 1.0) for all the considered models, confirming, for such systems, the environmental advantage of

600

η

th

=0.75

400

200

fuel: NG λAHG=200 mg/kWh ηAHG=90%

0 20%

25%

η

th

30%

35%

40%

=0.4

45%

50%

1.5

Electric efficiency Fig. 11. Avoided NOx versus electric efficiency for commercial ICEs, MGTs and small GTs.

the contrary, the output-based CO2 emissions, strictly linked with efficiency, show higher values for GTs and MGTs, because of the lower gel values.

ICE (Pel <100kW) ICE (100 kW < Pel < 1MW) ICE (1MW < Pel <10 MW) MGT GT (Pel < 10 MW)

NOx PSI 1.0

0.5

0

3.1.1. Avoided emission results Figs. 11 and 12 report, for the same wide range of examined PMs, the calculated avoided NOx and CO emissions (evaluated assuming the reference values reported in Table 2). Once fixed the reference AHG, the dAvoid values depend only on the thermal to electric efficiency ratio, according to Eq. (3). Indeed, the calculated dAvoid points in Figs. 11 and 12 are confined between two lines corresponding to gel =gth values in the range of 0.4–0.75. Moreover, the figure shows that the avoided emissions are of the order of magnitude of the actual emissions of Figs. 8 and 9; especially in case of GTs, dAvoid can fully compensate d values. The replacement of the AHG is less convenient for the smallest ICEs, but significant avoided emissions still occur.

-0.5

-1.0

-1.5 20%

25%

30%

35%

40%

45%

50%

Electric efficiency Fig. 13. NOx PSI versus electric efficiency for commercial ICEs, MGTs and small GTs.

Please cite this article in press as: Bianchi M et al. Application of environmental performance assessment of CHP systems with local and global approaches. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.04.017

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could be useful to guide future implementation of CHP environmental regulation in countries where local and/or global impact in comparison with the actual scenario should be assessed, when large number of small CHP units are going to be installed. It should be remarked that, in this study, only actual flue gas emissions during operation are considered; the comprehensive environmental impact derived from a LCA approach is not included. A larger LCA investigation, which was out of the scope of this study, could take into account the sources and related emissions of pollutants (both in CHP and SP), which should be added to the amount of pollutants evaluated with the methods described here.

0.8

CO

2

PSI 0.7

ICE (Pel <100kW) ICE (100 kW < Pel < 1MW) ICE (1MW < Pel <10 MW) MGT GT (Pel < 10 MW)

ηth =0.75

0.6

η

th

=0.4

0.5

References 0.4

0.3 20%

25%

30%

35%

40%

45%

50%

Electric efficiency Fig. 14. CO2 PSI versus electric efficiency for commercial ICEs, MGTs and small GTs.

CHP also on a global scale vision. On the contrary, in the case of ICEs, the CHP emission can be higher than the reference plants emission. The PSI value in this case can be as low as 1.0, for various systems, therefore resulting not convenient, according to this pollutant global impact assessment. Instead, according to the results in Fig. 14, the CO2 PSI values are always positive (lower impact than in SP), for all the considered PMs (ranging between 0.4 and 0.6). This result is due to the fact that all the considered CHP systems are characterized by positive values of PES (assuming a full heat recovery). Thus, the fuel saving achieved with CHP in comparison with SP is also responsible for reduced CO2 emissions in the CHP operating mode. It can be also observed that, in case of CO2, PSI values are strongly correlated with the electric efficiency: systems with higher gel (ICEs) are privileged (in comparison with GTs and MGTs). 4. Conclusions Emission saving due to cogeneration of heat and power is here assessed for CHP systems fully developed and available on the market, by applying indicators for a local and global scale assessment and comparing environmental performance of a large number of different units, to provide a comprehensive overview of the current CHP state of the art in terms of NOx, CO and CO2 specific emission. The analysis was carried out to assess the potential of CHP operation for small/medium power PMs, by comparing in particular ICEs and GTs, in simplified (nameplate) operating conditions. Indeed, a limit of this investigation consists in the fact that only full load conditions have been taken into account, neglecting actual part-load operations, which are frequent in several CHP applications, where PMs undergo load-following operating modes. The study highlights that CHP production can dramatically reduce the local environmental impact in comparison with SP. As shown for several engines and gas turbines representative of the current market, the NOx and CO avoided emission values could be comparable with the nameplate emission levels of the PMs, if the considered reference heat generator technology level is limited. On the contrary, the global impact, evaluated with the indicator PSI, still remains an issue for NOx emissions, and could be even magnified by a distributed CHP generation system. These results

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Please cite this article in press as: Bianchi M et al. Application of environmental performance assessment of CHP systems with local and global approaches. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.04.017