# Energy-Exergy, Environmental and Economic Criteria in Combined Heat and Power (CHP) Plants: Indexes for the Evaluation of the Cogeneration Potential

## Abstract

**:**

## 1. Introduction

_{2}emissions and energy footprint. These factors are normalised with the weight coefficients and sixteen systems, with different CHP technologies and plant sizes, are compared. The authors stated that the results obtained with their method could be used to establish the most important factors that can influence the performance of a system. Nesheim and Ertesvag [5] have analysed the energy indexes that are adopted in the legislation of different countries and indexes based on exergy. Two plants have been simulated and compared using the aforementioned indexes. The authors have also discussed the importance of reference plants for separate electric and heat generation, and two possible choices have been underlined: best available technology (BAT) and an average of existing installations. Ertesvag [6] has continued the work he started in [5] and introduced a second law index named relative avoided irreversibility; industrial CHP and district heating CHP have been analysed as case studies, and both natural gas and biomass have been considered. He observed that some indexes overstate or underestimate improvements and, in some cases, exergetically poor systems could be favoured. In [7], Mancarella and Chicco have analysed global and local emissions in distributed cogeneration, and suggested some specific indicators. In [8], the same authors have introduced a new environmental index, trigeneration CO

_{2}emission reduction, which can be used to compare a trigeneration plant with conventional separate production of heat and power, and they have considered CHP as a subcase of the trigeneration analysis. They presented case studies, based on current technologies (microturbines, internal combustion engines, gas turbines, combined cycles), and discussed CO

_{2}emission reductions: the results mainly depend on the technology that was used for the combined production and on the composition of the energy generation mix. Kanoglu and Dincer [9] have analysed four cogeneration plants in which the heat is supplied to buildings: steam-turbine, gas-turbine, diesel-engine and binary geothermal plants. The adopted criteria were energy and exergy efficiencies, and the latter was more suitable when there were geothermal systems in the comparison. Ruan et al. [10] have studied CHP for commercial buildings (hotels, hospitals, stores and offices) and proposed an overall evaluation index that takes into account primary energy savings, CO

_{2}reduction, and payback. Sanaye and Ardali [11] have focused attention on microturbine CHP with the aim of estimating the optimal power and the number of units that maximize the annual profit, that is, the objective function; the payback period was also estimated. Kavvadias et al. [12] have discussed the design of a trigeneration system for a hospital building: two different electricity tariffs and different strategies to cover the loads were analysed. Different seasonal energy profiles were also taken into account. Particular attention was paid to energy indexes (overall efficiency, primary energy savings, system load coverage) and two economic indexes (annual operating profit and return of investment); no environmental or exergy indexes were considered. Wheeley et al. [13] have compared CHP systems for different industrial manufacturing applications using the simple payback, the internal rate of return and the net present value. The authors checked the effects of some factors on these indexes, such as operating hours, electric utility rate, facility thermal load, fuel type and fuel costs.

_{2}, CO and NO

_{x}. The authors concluded that CHP can be a cost effective technology for greenhouse cultivation and can help to reach emission reduction targets. However, tailored policies and support measures are necessary to promote CHP in the agricultural sector because new technical and management skills are necessary for the farmer, and higher investment are required. Maes and Van Passel [15] have studied an interesting policy aspect: the interference between different public policies promoting energy efficiency and CO

_{2}reduction. In fact, when authorities favour a technology, other energy technologies may in find themselves a less favourable position on the market. The authors analysed this aspect in a case study in which a hybrid energy system combined two complementary heating techniques: CHP and thermal solar panels. Two regions, the Netherlands and Flanders, were compared, and critical results were obtained for the latter: CHP has been favoured so much that solar panels are no longer of interest to investors. However, as far as the authorities are concerned, a more balanced policy would result in a larger CO

_{2}reduction for a lower cost.

**Figure 1.**The main quantities concerning the energy, exergy, emission and economic balances used to analyse the CHP (see the nomenclature for the meaning of the symbols).

## 2. CHP Energy Criteria

#### 2.1. Classical Efficiency and Electric-to-Thermal Ratio of a CHP-Unit

#### 2.2. Modified Efficiencies

#### 2.2.1. Weighted efficiency

_{el,SHP}) to obtain a surplus of electricity. In general, the weighted efficiency is a simplified exergy efficiency without the real exergy content of the heat being taken into account. Therefore, if the weight factors are not chosen carefully, they can lead to distorted results.

#### 2.2.2. Effective Electrical Efficiency

_{CHP}, is reduced by the portion of fuel that theoretically should be used if the heat is obtained from separate heat production (e.g., boilers). The advantage of this efficiency is that it can easily be compared with power plant efficiency.

#### 2.3. Energy Saving Indexes

#### 2.3.1. Introduction on Separated Heat and Power (SHP) Production

_{th-SHP}and F

_{el-SHP}are the fuel energies supplied to the separate plants. It is useful to define the total fuel energy supply to SHP as:

#### 2.3.2. Absolute Primary Energy Saving

_{CHP}and F

_{SHP}are fuel energy with and without cogeneration, respectively.

#### 2.3.3. Relative Primary Energy Saving

_{R}is one of the most important indexes, and it is at present used in European Union legislation [19] to promote cogeneration. Equation (16) can be rewritten as a function of other parameters, such as the electric-to-thermal ratio and the total efficiency, and these formulations are given in Equation (17):

#### 2.4. Potential Index for the Energy Saving

## 3. CHP Exergy Criteria

#### 3.1. CHP Exergy Efficiency

_{env}) associated to the environment. The term $\widehat{E}{x}_{CHP}^{F}$ corresponds to the exergy of the stream of fuel, which is mainly the chemical exergy; the tables of standard chemical exergies [21] can be used to evaluate this term.

#### 3.2. Exergy Saving Indexes

Type of index | Energy | Exergy |
---|---|---|

SHP efficiencies | ${\eta}_{el,SHP}=\hspace{0.17em}\hspace{0.17em}\frac{W}{{F}_{el,SHP}}\phantom{\rule{0ex}{0ex}}{\eta}_{th,SHP}=\hspace{0.17em}\hspace{0.17em}\frac{Q}{{F}_{th,SHP}}\phantom{\rule{0ex}{0ex}}{\eta}_{SHP}=\hspace{0.17em}\frac{W+Q}{{F}_{SHP}}$ | ${\epsilon}_{el,SHP}=\hspace{0.17em}\hspace{0.17em}\frac{W}{E{x}_{el,SHP}^{F}}\phantom{\rule{0ex}{0ex}}{\epsilon}_{th\_SHP}=\frac{E{x}^{Q}}{E{x}_{th,SHP}^{F}}\phantom{\rule{0ex}{0ex}}{\epsilon}_{SHP}=\frac{W+E{x}^{Q}}{E{x}_{SHP}^{F}}$ |

Absolute saving | $\begin{array}{l}PE{S}_{A}={F}_{SHP}-{F}_{CHP}=\\ =\left({F}_{el,SHP}+{F}_{th,SHP}\right)-{F}_{CHP}\end{array}$ | $\begin{array}{l}Ex{S}_{A}=E{x}_{SHP}^{F}-E{x}_{CHP}^{F}=\\ =\left(E{x}_{el,SHP}^{F}+E{x}_{th,SHP}^{F}\right)-E{x}_{CHP}^{F}\end{array}$ |

Relative saving* | $\begin{array}{l}PE{S}_{R}=\frac{PE{S}_{A}}{{F}_{SHP}}=\\ =1-\frac{{F}_{CHP}}{\frac{W}{{\eta}_{el,SHP}}+\frac{Q}{{\eta}_{th,SHP}}}=\\ =1-\frac{1}{\frac{{\eta}_{el,CHP}}{{\eta}_{el,SHP}}+\frac{{\eta}_{th,CHP}}{{\eta}_{th,SHP}}}\end{array}$ | $\begin{array}{l}Ex{S}_{R}=\hspace{0.17em}\hspace{0.17em}\frac{Ex{S}_{A}}{E{x}_{SHP}^{F}}=\\ \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}=1-\frac{E{x}_{CHP}^{F}}{\frac{W}{{\epsilon}_{el,SHP}}+\frac{E{x}^{Q}}{{\epsilon}_{th,SHP}}}=\\ \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}=1-\frac{1}{\frac{{\epsilon}_{el,CHP}}{{\epsilon}_{el,SHP}}+\frac{{\epsilon}_{th,CHP}}{{\epsilon}_{th,SHP}}}\end{array}$ |

_{R}

#### 3.3. Potential Index for Exergy Saving

## 4. CHP Environmental Criteria

_{2}, CH

_{4}, N

_{2}O, fluorinated gases), which are expressed in CO

_{2}equivalents. However, several other pollutants are also important: nitrogen oxides (NO

_{x}), sulphur oxides (SO

_{x}), particulate matter (PM), carbon monoxide (CO), unburned hydrocarbons (HC), etc. CHP technologies can reduce emissions due to the increase in efficiency, but in some cases the net emissions can increase, and a dedicated balance should therefore be conducted for each pollutant and each CHP technology. Before conducting a balance, it is necessary to define the boundaries of the area, and two cases are significant [7]: global scale balance, in which both the on-site plants and central power plants are taken into account, and local scale balance, in which only the on-site plants are taken into account. This distinction is important and the pollutant indexes have therefore been presented distinguishing between these two boundaries. A schematic drawing of an SHP is given in Figure 2A, while Figure 2B shows the case of a CHP-unit which is matched exactly to the power and heat demands.

**Figure 2.**Schematic comparison of the emissions (global and local scale).

**A**: SHP;

**B**: the CHP-unit is matched exactly to the power and heat demands;

**C**: the CHP-unit is not matched exactly to the power and heat demands; a CHP-system (composed of CHP-units, auxiliary boilers, and central power plants) has therefore been considered.

#### 4.1. Pollutant Saving Indicators (Global Scale)

#### 4.1.1. Absolute Pollutant Saving (Global Scale)

#### 4.1.2. Relative Pollutant Saving (Global Scale)

_{2}emissions are analysed, a further simplification can be made by introducing two hypotheses: the same fuel is used for CHP and SHP, and a complete combustion of the fuel is assumed, in this way, the emission only depends on the fuel characteristics. These conditions entail that the same emission factor is present as both the numerator and as the denominator, and Equation (35) can be simplified to:

_{R}. These considerations have been extended to trigeneration systems in [8].

#### 4.1.3. Potential Indexes for Pollutant Saving

#### 4.2. Pollutant Saving Indicator (Local Scale)

_{2}in order to establish where it has been produced). However, the local environmental impact is more important than the global one for other pollutants, because these pollutants could have adverse health effects on the local population. Therefore, a local emission balance should be conducted for each j-th pollutant in order to obtain the local indexes (superscript L). The main local saving indexes are shown in Table 2 and compared with the global indexes.

_{x}, SO

_{x}and PM) are discussed. As an alternative to the graphical comparison, an index could be calculated from the mean spatial distribution of the pollutant concentration, that is, a spatial integral is calculated over the area of the dominion. These evaluations are interesting, but require more detailed information (the orography of the site, weather data, the height of the stack, etc.) and are more complex.

Type of index | Global scale | Local scale |
---|---|---|

Absolute saving | $PO{S}_{A}^{j}={m}_{SHP}^{j}-{m}_{CHP}^{j}$ | $PO{S}_{A}^{L,j}={m}_{th,SHP}^{j}-{m}_{CHP}^{j}$ |

Relative saving | $\begin{array}{l}PO{S}_{R}^{j}=\frac{PO{S}_{A}^{j}}{{m}_{SHP}^{j}}=\frac{{m}_{SHP}^{j}-{m}_{CHP}^{j}}{{m}_{SHP}^{j}}\\ \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}=1-\frac{{\mu}_{fu,CHP}^{j}}{{\mu}_{fu\_el,SHP}^{j}\cdot \frac{{\eta}_{el,CHP}}{{\eta}_{el,SHP}}+{\mu}_{fu\_th,SHP}^{j}\cdot \frac{{\eta}_{th,CHP}}{{\eta}_{th,SHP}}}\end{array}\phantom{\rule{0ex}{0ex}}{(PO{S}_{R}^{j})}_{SYS}=\hspace{0.17em}1-\frac{{m}_{SYS}^{j}}{{m}_{SHP}^{j}}$ | $\begin{array}{l}PO{S}_{R}^{L,j}=\frac{PO{S}_{A}^{L,j}}{{m}_{th,SHP}^{j}}=1-\frac{{m}_{CHP}^{j}}{{m}_{th,SHP}^{j}}\\ \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}=1-\frac{{\mu}_{fu,CHP}^{j}}{{\mu}_{fu\_th,SHP}^{j}}\cdot \frac{{\eta}_{th,SHP}}{{\eta}_{th,CHP}}\end{array}\phantom{\rule{0ex}{0ex}}{(PO{S}_{R}^{L,j})}_{SYS}=1-\frac{{m}_{SYS}^{L,j}}{{m}_{th,SHP}^{j}}$ |

Potential index | ${{\displaystyle \pi}}_{POS}^{j}=PO{S}_{R}^{j}-{(PO{S}_{R}^{j})}_{SYS}$ | ${{\displaystyle \pi}}_{POS}^{L,j}=PO{S}_{R}^{L,j}-{(PO{S}_{R}^{L,j})}_{SYS}$ |

## 5. CHP Economic Criteria

#### 5.1. General Criteria

- C
_{k}= costs at year k (C_{0}is the initial investment); - R
_{k}= revenues at year k; - d
_{k}= interest rate at year k; - N = lifetime of the system

#### 5.1.1. Absolute Present Value Saving

_{A}is also called net present value and can be obtained summing all the present values of the net cash flow. A greater PVS

_{A}value than zero does not guarantee a real economic advantage, and, two other indicators are often calculated considering Equation (42): the first is the rate of return of the investment (ROI), which is the root of Equation (42), when the PVS is set equal to zero and d is the unknown value; the second is the discounted payback period (DPB), which is the length of time necessary for the sum of the discounted net cash flows to be equal to the initial investment. The unknown value is therefore the time. This indicator is used as a screening value to evaluate whether to accept or reject a CHP investment.

#### 5.1.2. Relative Present Value Saving

#### 5.2. Potential Index for Economic Saving

#### 5.3. Other Indexes and Methods

## 6. Case Study

_{2}and another for NO

_{x}. The CO

_{2}savings is 22% for the CHP-System and 76% for CHP-unit. These values are much higher than the primary energy saving because the emission factors of the SHP system are high and the potential index, 54%, is therefore high. As far as the NO

_{x}emissions are concerned, the emissions at a local scale (dotted bars) are distinct from the emissions from the central power plant (hatched bars), as can be seen in the Figure 3. In the global scale balance, the NO

_{x}saving is 19% for the CHP-System and 63% for the CHP-unit. Therefore, a positive potential index of 44% can be observed. The local scale balance instead gives more critical results: the CHP-System shows a negative NO

_{x}saving (−33%), which means the local emissions are increased. This deterioration could further increase to −150% if cogeneration increases. The potential index is therefore negative, −117%, which means that an increase in cogeneration units leads to a deterioration of this environmental aspect.

^{CO2}are usually privileged but for a local government the goal is generally on improving the quality of the conditions of the people who are governed; for example in the case study, Figure 3, there is a deterioration of the NO

_{x}emissions due to the local presence of the CHP. Therefore, the local policy-makers could also constrain the incentives to achieve environmental improvements at a local scale, and the economic indexes could help them to calibrate the incentives. Moreover, these evaluations can be extended to other significant pollutants such as PM, CO, etc. However, the investors point of view is primarily focused on economic evaluation, but in a context where new constrains or incentives can be introduced by legislator at a later stage of investment, they should also take into account the energy and environment impact, in order to not penalize these aspects in the design phase. Therefore, they should simultaneously consider all the indexes as a function of different parameters: CHP technologies, fuels used, possible use of pollutant abatement systems, storage systems, etc. These analyses could be further extended by introducing a multi-objective optimization technique.

**Figure 3.**Case study results. Blue bars: primary energies; yellow bars: exergies; red bars: emissions (dotted bars refer to local scale); green bars: present values.

## 7. Conclusions

- it is possible to define saving indexes (both absolute and relative) for each aspect;
- in most cases, the indexes are also functions of the SHP, therefore, in order to establish the CHP improvements, knowledge of SHP data, that is, power plant efficiency, fuel-mix, heating boiler efficiencies, emission factors, etc., is fundamental;
- high efficiency of the CHP-unit is a necessary, but not sufficient, condition to achieve a real improvement. Therefore, five potential indexes have been introduced to compare the theoretical savings of a CHP-unit with the actual savings of a CHP-system, and are summarised in Table 3;
- further analysis could be conducted extending these indexes to trigeneration systems and/or including other aspects (e.g., social costs).

Aspect | CHP-system saving | Potential index |
---|---|---|

Energy | ${\left(PE{S}_{R}\right)}_{SYS}=1-\frac{{F}_{SYS}}{{F}_{SHP}}$ | ${{\displaystyle \pi}}_{PES}=PE{S}_{R}-{(PE{S}_{R})}_{SYS}$ |

Exergy | ${\left(Ex{S}_{R}\right)}_{SYS}=1-\frac{E{x}_{SYS}^{F}}{E{x}_{SHP}^{F}}$ | ${{\displaystyle \pi}}_{ExS}=\hspace{0.17em}Ex{S}_{R}-{(Ex{S}_{R})}_{SYS}$ |

Environmental global scale | ${\left(PO{S}_{R}^{j}\right)}_{SYS}=\hspace{0.17em}1-\frac{{m}_{SYS}^{j}}{{m}_{SHP}^{j}}$ | ${{\displaystyle \pi}}_{POS}^{j}=PO{S}_{R}^{j}-{(PO{S}_{R}^{j})}_{SYS}$ |

Environmental local scale | ${\left(PO{S}_{R}^{L,j}\right)}_{SYS}=\hspace{0.17em}1-\frac{{m}_{SYS}^{L,j}}{{m}_{th,SHP}^{j}}$ | ${{\displaystyle \pi}}_{POS}^{L,j}=PO{S}_{R}^{L,j}-{(PO{S}_{R}^{L,j})}_{SYS}$ |

Economic | ${\left(PV{S}_{R}\right)}_{SYS}=\hspace{0.17em}1-\frac{P{V}_{SYS}}{P{V}_{SHP}}$ | ${{\displaystyle \pi}}_{PVS}=PV{S}_{R}-{(PV{S}_{R})}_{SYS}$ |

_{2}show both a positive relative saving and a positive potential saving; the local scale NO

_{x}emission indicate a negative saving and a negative potential index; the present value saving is positive, but the potential index is negative. These different trends can be useful both to characterize a particular CHP-system, but also to compare different solutions: size of the CHP unit, the number of the units, the CHP technologies, etc. From the policy-makers’ point of view, all the information from the indexes can be used to better calibrate the CHP incentives, which should take into account energy, environmental and economic aspects.

## Nomenclature

## Latin Symbols

C | cost |

d | interest rate |

Ex | exergy |

Ex^{F} | exergy associated with fuel F |

Ex^{Q} | exergy associated with heat Q |

$\widehat{E}x$ | rate of exergy |

ExS | exergy saving |

F | fuel energy supply (lower heating value) |

$\widehat{F}$ | rate of fuel energy supply |

m | mass |

N | lifetime of the system |

NC | number of CHP-units |

NB | number of auxiliary boilers |

PES | primary energy saving |

POS | pollutant saving |

PV | present value |

PVS | present value saving |

Q | heat |

$\widehat{Q}$ | rate of heat |

R | revenues |

T | thermodynamic temperature |

W | electric or mechanical energy |

$\widehat{W}$ | rate of electric or mechanical energy |

## Greek Symbols

α | weight factor |

ε | efficiency expressed as exergy ratio |

$\widehat{\epsilon}$ | efficiency expressed as exergy rate ratio |

η | efficiency expressed as energy ratio |

$\widehat{\eta}$ | efficiency expressed as energy rate ratio |

λ | electric-to-thermal ratio of the CHP-unit (refer to energy) |

$\widehat{\lambda}$ | electric-to-thermal ratio of the CHP-unit (refer to rate of energy) |

μ | emission factor |

π | potential index |

## Subscripts

A | absolute |

B | auxiliary boilers |

CPP | central power plants |

env | value associated with the environment |

ef | effective |

el | electric |

fu | fuel |

k | k-th year |

O&M | operation and maintenance |

R | relative |

SYS | CHP-system |

th | thermal |

we | weighted |

0 | initial time in the economic evaluation |

## Superscripts

F | referring to fuel |

in | input |

loss | loss due to irreversibility |

j | j-th pollutant |

L | local scale |

out | output |

Q | referring to heat |

W | referring to electric or mechanical energy |

## Abbreviations

CHP | Combined Heat and Power (synonymous of cogeneration) |

CHP-unit | a single cogeneration plant |

CHP-system | a system composed of CHP-units, auxiliary boilers and central power plants |

SHP | Separated Heat and Power |

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**MDPI and ACS Style**

Torchio, M.F.
Energy-Exergy, Environmental and Economic Criteria in Combined Heat and Power (CHP) Plants: Indexes for the Evaluation of the Cogeneration Potential. *Energies* **2013**, *6*, 2686-2708.
https://doi.org/10.3390/en6052686

**AMA Style**

Torchio MF.
Energy-Exergy, Environmental and Economic Criteria in Combined Heat and Power (CHP) Plants: Indexes for the Evaluation of the Cogeneration Potential. *Energies*. 2013; 6(5):2686-2708.
https://doi.org/10.3390/en6052686

**Chicago/Turabian Style**

Torchio, Marco F.
2013. "Energy-Exergy, Environmental and Economic Criteria in Combined Heat and Power (CHP) Plants: Indexes for the Evaluation of the Cogeneration Potential" *Energies* 6, no. 5: 2686-2708.
https://doi.org/10.3390/en6052686