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Renewable Energy 89 (2016) 411e421

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Renewable Energy

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A tri-generation plant fuelled with olive tree pruning residues inApulia: An energetic and economic analysis

Riccardo Amirante a, Maria Lisa Clodoveo b, Elia Distaso a, Francesco Ruggiero c,Paolo Tamburrano a, *

a Department of Mechanics, Mathematics and Management (DMMM), Polytechnic of Bari, Via Re David 200, 70126, Bari, Italyb Department of Agro-Environmental and Territorial Sciences (DISAAT), University of Bari, Via Amendola 165/A, 70126, Bari, Italyc Department of Civil Engineering and Architecture (DICAR), Polytechnic of Bari, Via Orabona, 70126, Bari, Italy

a r t i c l e i n f o

Article history:Received 12 March 2015Received in revised form30 September 2015Accepted 30 November 2015Available online 22 December 2015

Keywords:Agricultural residuesTri-generationOrganic Rankine CycleEconomic analysis

* Corresponding author.E-mail address: paolo.tamburrano@poliba.it (P. Ta

http://dx.doi.org/10.1016/j.renene.2015.11.0850960-1481/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

This paper presents the energetic and economic analysis of a virtuous example consisting of a tri-generation system fuelled only with olive tree pruning residues and planned to be located next toBari Airport (Apulia, Italy). The main goal is to demonstrate the feasibility and convenience of producingcooling, heating and electrical power from olive tree pruning residues in those regions characterized by ahigh availability of this kind of biomass, such as Apulia. A strategic location was selected, namely BariAirport (Apulia), and this paper demonstrates the economic convenience of installing a commerciallyavailable Organic Rankine Cycle (ORC) unit of 280 kWe that is capable of satisfying the thermal demandsof the airport, with the addition of an absorption chiller for air conditioning in the airport buildings. Firstit is verified that the quantity of oil tree pruning residues available in the area surrounding the airportfully can satisfy the plant demand of feedstock. Then a detailed description of the components of theplant is provided. The performance of the plant is therefore evaluated in order to assess the thermo-dynamic competitiveness of a tri-generative system fuelled with this type of biomass. Finally, a detailedeconomic analysis is carried out with the aim of demonstrating the advantages that the plant can assurein terms of payback period (PBP), net present value (NPV) and internal rate of return (IRR). Two differenttypologies of government incentives are considered. In both of which, the PBP is 6 years with an IRR ofabout 21% and this points out the great economic attractiveness of the project. From an ecological pointof view, the plant can ensure a remarkable reduction in CO2 emissions.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

For over twenty years, governments have taken a number ofactions to solve the environmental problem concerning greenhousegas emissions and global warming caused by the excessive con-sumption of fossil fuels. Energy policies implemented to date havebeen promoting renewable energy exploitation, providing fullsupport by means of a number of incentives [1]. Such policies havealready led to a significant change in the energy mix, which iscontinuously replacing conventional fuels with renewable energysources. European countries planned to meet the 2020 targets onrenewable energies thanks to such a relevant paradigm shift inrenewable energy exploitation [2]. Biomass is a form of renewable

mburrano).

energy that can effectively be utilized to reduce the impact of theenergy production from fossil fuels on the global environment andcan be converted into useful forms of energy by using differentprocesses. Several techniques, plants and devices for extractingenergy fromwaste biomass are available. The techniques for energyextraction from waste biomass can be grouped in the following“families”: Combustion, Pyrolysis/Gasification, Bio-processing.Biomass energy systems can generally provide several advantagessuch as a low carbon footprint and a lower delivered energy costcompared to fossil fuels. In biomass-fuelled power plants theelectricity generation is usually coupled with the production ofheating and/or cooling with the aim of increasing the overall effi-ciency, since the electrical efficiency is low in the plants fuelledwith biomass.

Despite the energetic use of agricultural wastes can play animportant role in reducing the consumption of fossil fuels, such apractice is not so widespread as expected in those regions having

Nomenclature

a potential availability of residuesCc chipping cost [V/t]Cf fuel cost required for the transportation [V/t]Cfeedstock overall cost for feedstock procurement [V/year]Ch harvesting cost [V/t]Cl cost for loading and unloading the feedstock [V/t]Ct transportation cost [V/t]f pruning frequency per yearHdry lower heating value of dry biomass [kcal/kg]Hi lower heating value of wet biomass [kcal/kg]h enthalpy [kJ/kg]P pressure [bar]Pel total electrical power [kW]Pm weight of the feedstock transported [t]_Qc total useful cooling power [kW]Qol mass of olives per year [t/year/hectare]

Qpr mass of wet by-product (pruning residues) [t/year/hectare]

Q·

th total useful thermal power [kW]T temperature [�C]U percentage of humidityY ratio of the by-product to the olive yield_mb mass flow rate of fuel [t/year]hel overall electrical efficiencyhg total efficiency

AcronymsCCP combined cooling and powerCHP combined heating and powerCCHP combined cooling, heating and powerIRR internal rate of returnNPV net present valuePBP payback period

R. Amirante et al. / Renewable Energy 89 (2016) 411e421412

large availability of agricultural residues [3]. This paper is focusedon a particular type of agricultural residues largely diffused in farmsof the Mediterranean region, namely olive tree pruning residues,and aims to demonstrate that their energetic use can be veryprofitable in a tri-generation system, also thanks to the recent ad-vances both in the tri-generation technology and inmechanical andmanagement systems for harvesting, packaging and transportation[4]. It is planned to realize the tri-generative power plant in a regionhaving a large quantity of oil olive crops, namely Apulia (south ofItaly). The plant will be capable of satisfying the entire thermal andcooling demands of the buildings of Bari Airport as well as part ofthe electrical energy required by the Airport.

This paper aims to assess the feasibility and profitability of thissystem for tri-generation from the direct combustion of olive treepruning residues. As starting point the availability of biomass inApulia, particularly in the zone nearby the plant location, is quan-tified and compared with the needed amount of feedstock for theplant operation. Then the components of the proposed tri-generative plant are described in details. A thorough economicanalysis is finally exposed in order to evaluate the economic con-venience of the project. In conclusion an estimation of the annualquantity of CO2 that can be saved by the proposed plant is alsoprovided.

2. The agro-energetic Apulian model

2.1. Potential of olive tree pruning residues for energy generation inthe Mediterranean region

The simultaneous generation of electricity, heating and coolingfrom olive tree pruning residues in tri-generative plants can beinstrumental in increasing energy production from renewablesources. The Apulian context best fits this objective, by virtue of thehigh percentage of cultivated fields of olive trees and the conse-quential great quantity of pruning residues that are usually un-employed and burned on fields.

In the European Community, olive groves are mostly present inthe Mediterranean region. In fact, Mediterranean countries, led bySpain, produce 10million tons of olives per year, which is 75% of theworld production [5]. The Italian cultivation of olive trees isdiffused above all in southern and insular regions where about 80%of the Italian production of olives and oil olive is obtained. As a

matter of fact, the Italian region having the greatest extension ofolive cultivated land is Apulia with 377550 ha, followed by Calabria(194887 ha) and Sicily (161967 ha). Ever-increasing advances andresearch studies on the olive oil production technology demon-strate the importance of olive oil production upon Italian economy[6].

Every year, in these zones, farmers have the problem ofdisposing, at their own expenses, of tons of pruning residues. In Ref.[7], the management of pruning residues is considered: the authorsargue that pruning residues, despite having generally represented adisposal problem, can become a real opportunity for additionalrevenue if an energy recovery of such wastes is performed, usingthem as fuels for energy production; thus, in addition to elimi-nating the problem of disposal, the future commercialization ofsuch agricultural wastes can be a source of income rather than acost for farmers.

In the past, the inefficiency and low availability as well as thehigh costs of harvesting machines were all limiting factors inexploiting tree pruning residues. A large part of such residues wasusually burned on fields, while only the thickest branches wererecovered by farmers and used as fuel-wood [8,9]. A change infarmers' mentality is now possible by virtue of recent advances indesigning harvesting machines, which are more reliable and effi-cient than in the past, thus allowing farmers to be able to performcollection and harvesting processes effectively in terms of cost andtime. In this regard, new industrial pruning harvesters capable ofovercoming the limits of common small units were tested in Ref.[7], showing that the introduction of the industrial technology canbe instrumental in increasing the energetic use of pruning residues.More effective strategies in managing such residues can helpfarmers better exploit older and smaller sizemachines, especially insmall farms and in those groves where steep terrain and/or irreg-ular spacing do not allow the profitable use of large industrialharvesting units [7].

2.2. Availability of feedstock and plant demand

This section aims to assess whether the availability of raw ma-terial meets the requirements of the plant presented here, which isgoing to be realized at Bari Airport.

It should be noted that the by-product present on a field cannotentirely be used for energetic valorisation because of the

R. Amirante et al. / Renewable Energy 89 (2016) 411e421 413

permanence time on the field and weather conditions during theharvesting as well as the plot of the land (dimension and form). Inorder to evaluate the net potential availability of olive tree pruningresidues in Apulia accounting for all of these factors, a validcalculation method was recently proposed in Refs. [10,11]. Thismethod quantifies the quantity of olive tree pruning residues perhectare per year by means of the following equation:

Qpr ¼ QolYf$

a100

(1)

where Qpr is the mass of wet by-product (pruning residues) perhectare that is yearly obtainable, Qol is the mass of olives per yearobtained in the field, Y is the ratio of the overall by-product to theoverall olive yield, f is equal to the pruning frequency per year and ais the potential availability that takes into account the character-istics of harvesting techniques and field as well as climaticconditions.

Once coefficients a and f are known, equation (1) allows calcu-lating the precise value of Qpr achievable from a specific zone,provided that factor Y is properly estimated. To accomplish thistask, two equations have been retrieved in Ref. [10], specifically onefor the provinces of Bari and Foggia and the other one for theprovinces of Lecce, Brindisi and Taranto. These formulationscalculate the by product/product ratio, Y, as a function of the yieldof olives, Qol:

Y ¼ 0:566 þ 1:496Qol

for Bari and Foggia (2)

Y ¼ 0:305þ 1:401Qol

for Taranto; Lecce and Brindisi (3)

Using Equations (1)e(3), it is possible to depict the regional mapof the net pruning residues available per year, as shown in Fig. 1. Inaddition, Fig. 1 shows their energetic potential, which was obtainedby multiplying Qpr and the lower heating value (Hi) calculatedthrough equation (4):

Hi ¼ Hdry

�1� U

100

�(4)

where Hdry is the lower heating value of dry biomass and averages4200 kcal/kg for pruning residues, U is the average humidity per-centage present in the residuals collected on the field. This

Fig. 1. Plant location along with the distribution of olive pruning res

approach has general validity and can be applied to other zones ofItaly as well as other European countries, provided that coefficientsa, U and f, along with the relation between Y and Qol, are tuned inrelation to the specific zone.

The plant has been designed to ensure an electrical power (Pel)of about 280 kW. Using the expression of the electrical efficiency(hel) and knowing the lower heating value of the fuel, the quantityof biomass needed for the plant operation covering a period of ayear can be retrieved from the analytical expression of hel:

hel ¼Pel

m·bHi

(5)

where _mb denotes the fuel mass flow rate. Considering that theplant is expected to operate 8000 h per year and that its electricalefficiency is equal to 11.2% (as reported in the following section),and assuming the lower heating value of pruning residues presenton fields equal to 2540 kcal/kg (according to equation (4) andassuming 40% humidity remained in residuals on fields), it resultsthat _mb is equal to 6800 t/year.

Such a feedstock demand must fully be satisfied by the farmssurrounding the airport. In this regard, Table 1 reports the pro-duction of tree pruning residues in the municipalities nearest theplant location (average distance < 20 km), along with their averagedistances from the plant. The quantities of pruning residues in eachmunicipality were calculated by multiplying the hectares of landcovered by olive trees and the value of Qpr resulting from equation(1). The total production of pruning residues resulting from Table 1is 12656.02 t/year, which is well above the plant demand, thusdemonstrating that a very small area is sufficient to fully satisfy theplant demand. In particular, the complete production of treepruning residues in Modugno (569.47 t/year), Bari (771.18 t/year)and Bitonto (4986.49 t/year) together with a small part of theproduction in Giovinazzo (472.86 t/year) are capable of satisfyingthe annual demand of the plant. Only taking into account these fourmunicipalities, the average distance from the plant can be calcu-lated as a weighted mean (in which the weights are the quantity oftree pruning residues) and results to be equal to 10.1 km.

3. Description of the plant

3.1. Electrical and thermal demands of the airport

The designed plant is a tri-generative system employing acommercially available ORC for simultaneous production of

idues (t/year) and their energetic potential (tep/year) in Apulia.

Table 1Tree pruning residues production in the nearest municipalities and their distances from the plant.

Town Tree pruning residues production (t/year) Average distance (km)

Modugno 569.47 9Bari 771.18 10Bitonto 4986.49 10Giovinazzo 1937.61 13Bitetto 1292.99 15Bitritto 423.23 15Binetto 337.58 18Palo 2337.47 15Tot 12656.02 e

Fig. 3. Airport electrical demand.

R. Amirante et al. / Renewable Energy 89 (2016) 411e421414

electricity and useful heat combined with an absorption chillerused to generate chilled water for air conditioning. Fig. 2 depicts thethermal power required by the airport buildings during the year: itis concentrated in months comprised between November and Aprilwith a maximum demand of 410000 kW h in January. The electricaldemand of the airport, as depicted in Fig. 3, is always presentduring the year and is subjected to an increase in the summerseason, due to the air conditioning, with a peak of 1000000 kW h inJuly and August.

In the hot seasons (May, June, July, August, September, October),chilled water is needed to cool the buildings of the airport, whilstthe thermal power required by the thermal users of the buildings isnull. Contrarily, in the cold seasons (November, December, January,February, March, and April) the demand for cooling power is zero,while the buildings need useful thermal power. Because of such athermal and cooling demand of the buildings during the year, it isnot possible to perform a complete tri-generation, with the powerplant assuming a combined cooling and power (CCP) configurationin the hot season and a combined heating and power (CHP)configuration in the cold season.

The power plant has been designed in order to satisfy themaximum thermal demand, which occurs in January. Consideringthe overall number of hours over which this thermal demand isdistributed, the maximum thermal power provided by the plantmust be equal to about 1500 kW. In a similar way, the maximumcooling power achievable by the plant has been set equal to themaximum cooling power demanded by the buildings in July,namely 500 kW. As a result, the absorption chiller of the plant hasbeen chosen so as to provide a maximum cooling power of 500 kW.

3.2. Plant layout and components

Fig. 4 shows the layout of the plant along with the state pointinformation. The main sub-systems of the plant, namely thebiomass combustor, the ORC unit, the thermal users and the ab-sorption chiller are analysed in the following sub-sections.

Fig. 2. Airport thermal demand.

3.2.1. Biomass combustorOn the left of Fig. 4, the schematic representation of the biomass

combustor (dashed red box) is reported, where the top black boxindicates the combustion chamber. This is fed with pre-driedpruning residues by means of a pneumatic system, which allowsburning bigger size pieces of wood as well as leaves and tree barks.

The biomass combustor is equipped with radiative andconvective heat exchangers at its top for transferring heat from theflue gases generated from the combustion to the diathermic oil(denoted by the green line), thus increasing its temperature. Afterexiting the heat exchanger, the flue gases are subjected to partic-ulate and ash elimination. To accomplish this task, a cyclone and anelectrostatic filter, which is at present the best method of separa-tion for the smallest particles, are placed downstream of the heatexchanger.

Before being discharged into the atmosphere, the flue gases flowthrough a final heat exchanger (referred to as the exhaust regen-erator) which allows increasing the overall efficiency of the plant byrecovering most of the residual thermal energy of the exhaustgases.

In order to reduce the NOx emission (the limit value establishedby the normative is 200 mg/Nm3), the strategy consists in theintroduction of a certain quantity of urea in the combustionchamber, so that NOx can react with the injected urea to formmolecular nitrogen. To optimize the chemical reaction betweenurea and NOx, the temperature and reaction time must properly becontrolled. The best range of temperature is between 800 and1100 �C, with the optimum being equal to 1000 �C, while the bestresidence time ranges from 0.2 to 0.5 s.

3.2.2. ORCThe choice of an ORC system results from its peculiar charac-

teristics, specifically: the turbine isentropic efficiency can be as

87 °C

70 °C

INPUT POWER=2500 kW

Diathermic oilSat. Steam 260 °C

ELECTRIC POWER = 281 kWe

ORC

BIOMASS COMBUSTOR

wat

er 1

9.2

kg/s

ec87

°C

70 °C

ABSORPTION CHILLER

70 °C

87 °C

87 °C

70 °C

12 °C

7 °C

REGENERATOR

MDM=5.063 kg/sec

Water 2.11 kg/sec

Tank

Tank

CO

ND

ENSE

R

MAXIMUM THERMALPOWER =1516 kW

ELECTROSTATIC FILTER

EXHAUSTREGENERATOR

ABSORBER

GENERATOR CONDENSER

EVAPORATOR

COOLING TOWER

THERMAL USERS

HEATDISSIPATOR SYSTEM

87 °C

70 °C

87°C

70°C

310 °C

225 °C

9 bar

0.20

bar

260 °C

149 °C

Dry Steam215 °C

Flue gas 5544 kg/h

100°C

Fig. 4. Plant layout with state point information.

Fig. 5. CAD representation of the ORC unit [18].

R. Amirante et al. / Renewable Energy 89 (2016) 411e421 415

high as 90%, the turbine can rotate at very low rotational speed and,as a result, can directly be connected to the electric generatorwithout the need for a gear reducer. Furthermore, the turbineblades are subjected to low usury thanks to humidity absenceduring the steam expansion, and the system ensures short time formaintenance and long life of the components because this tech-nology is nowadays mature and reliable. All these advantages havecontributed to make ORC systems the most widespread technologyfor small-scale combined heating and power generation frombiomass [12,13]. However, thanks to recent technological advancesin designing gas to gas heat exchanger [14e16] and micro water-steam expanders, it was demonstrated in Ref. [17] that theemployment of small combined cycles as a valid alternative to ORCsystems for CHP from biomass will be feasible in the near futureand will be able to guarantee competitive thermodynamicperformances.

A CAD representation of the selected ORC unit is shown in Fig. 5.This unit is commercially available [18] and has been selectedamong commercially available units in order to satisfy themaximum thermal demand of the airport while ensuring themaximumpossible electrical efficiency. In fact, the unit is capable ofgenerating an electrical power of about 281 kW with an efficiencyof 16.4%, which is very high level of performance despite the small-scale application and despite the very high condensation temper-ature required by the thermal users. The thermodynamic cycle ofthe ORC is reported in Fig. 6 along with all the values of pressure,temperature and enthalpy; Fig. 7 shows the heat exchange both inthe boiler (which is made up of an economizer and an evaporator)and in the condenser (which also comprises a de-superheater).Both graphs have been retrieved by the authors using the dataprovided by the manufacturer. More details regarding the perfor-mance parameters of the ORC unit are provided in Table 2.

The proposed application is a high temperature configuration, as

the heat source comes from the biomass combustion. As occurs inmost of the commercially available ORC units to be used in hightemperature applications (i.e. biomass combustion), the workingfluid is not directly coupled to the flue gas, but a thermal oil is usedas a thermal vector between the combustor and the ORC, in order toprivilege safety and economic aspects. Indeed, the use of thethermal oil allows avoiding local overheating and allows the heatexchanger to operate at atmospheric pressure, as also discussed inRef. [19]. Moreover, the adopted temperature (about 310 �C) for thehot side of the thermal oil (therminol 66) ensures a very long oillife. The utilization of the thermal oil also allows operation withoutrequiring the presence of licensed operators.

The working fluid is a siloxane, namely MDM

Fig. 6. Thermodynamic cycle of the ORC.

Fig. 7. Temperature vs thermal power in the boiler (top) and condenser (bottom).

R. Amirante et al. / Renewable Energy 89 (2016) 411e421416

(octamethyltrisiloxane, molecular formula ¼ C8H24O2Si3, criticaltemperature ¼ 290 �C, critical pressure ¼ 14.20 bar, molecularweight ¼ 236.5 kg/kmol). The siloxanes are the most used moleculesin high temperature CHP applications, because they have the desired

Table 2Specifications of the biomass combustor, exhaust regenerator and ORC valid both foand power configuration.

Plan component Description

Biomass combustor Input powerThermal power transferred to tHeat exchanger efficiencyMass flow rate of the flue gases

Exhaust regenerator Flue gas temperature at the exhFlue gas temperature at the exhMaximum mass flow rate of waWater temperature at the inletWater temperature at the outleMaximum thermal power transEfficiency of the exhaust regene

ORC Thermal power transferred to tGross electrical power of the stNet electrical powerTurbine isentropic efficiencyORC overall electrical efficiencyMass flow rate of cooling waterTemperature of the water at theTemperature of the water at theThermal power achievable in thORC thermal efficiencyORC efficiency (thermal þ elect

characteristics that best fulfil the high working temperatures [20].The use ofMDMas theworkingfluid results from the fact thatMDM isthemost suitable in cycles having high condensation temperatures. Infact, in the proposed plant the condensation temperature must be ofthe order of 100 �C to satisfy the needs of the heating network (hotwater at 87 �C is needed in the plant). Despite the very high conden-sation temperature, the use of MDM allows the back-pressure of theturbine to be kept as low as possible, by virtue of its very lowcondensationpressure at100 �C (20kPa). Asa result, theworkingfluidcan be expanded in the turbine to 20 kPa, and, at this exit pressure, thetemperature of the steam is still very high (215 �C). This temperaturelevelmakes the employmentof the regeneratormandatory in order tomaximize the efficiency of the cycle.

3.2.3. Thermal users and absorption chillerIn the condenser of the ORC system, the working fluid disposes

of the remaining heat to the cooling water (azure line) which isused either in an additional heat exchanger for thermal uses or inthe absorption chiller (purple dashed box) using a solution oflithium bromide salt and water for cold generation. When thethermal power produced by the plant exceeds the demand for

r the combined cooling and power configuration and for the combined heating

Value

2500 kWhe Diathermic-oil 2141 kW

85.6%5544 kg/h

aust regenerator inlet 220 �Caust regenerator outlet 100 �Cter entering the exhaust regenerator 2.11 kg/sof the exhaust regenerator 70 �Ct of the exhaust regenerator 87 �Cferred to the water 150 kWrator 81%he working fluid 1713 kWeam turbine 300 kW

281 kW85%16.4%

(condenser) 19.2 kg/scondenser inlet 70 �Ccondenser outlet 87 �Ce condenser 1366 kW

80%rical) 96%

R. Amirante et al. / Renewable Energy 89 (2016) 411e421 417

chilled water or useful thermal power, the excess thermal power isdissipated through a heat dissipator.

In the configuration proposed, two insulated water tanks areused to regulate the flowof the hotwater used as thermal vector forthe absorption chiller and for the thermal user: a large part of thehot water (at 70 �C) is sent from the bottom tank to the condenserof the ORC cycle, while the remaining part is sent to the exhaustregenerator. The hot water exits these two devices at higher tem-perature (87 �C) and is conveyed into the top tank, which is used todistribute the hot water either to the thermal user or to thegenerator of the absorption chiller. Finally, the hot water at lowertemperature (70 �C) exiting either the generator or the thermaluser is delivered back to the bottom tank in order to have acontinuous operation mode. In Fig. 4, the heat dissipator system isdepicted below the heat exchanger indicating the thermal users;the heat dissipator is activated by acting on a three-way valve,which is also capable of regulating the thermal power to be dissi-pated according to the demand of either the thermal users or theabsorption chiller.

On the right hand side of Fig. 4, it is possible to observe thecomponents of the absorption chiller, specifically the absorbercoupled with the generator, the condenser, the lamination valveand the evaporator. In the configuration proposed for the absorp-tion chiller, a cooling tower is used to dispose of the heat trans-ferred to the condenser coolant. The choice of an absorptionrefrigerator instead of a compressor refrigerator results from thefact that the former allows the recovery of the surplus heat;furthermore, an absorption refrigerator does not need acompressor to realize the refrigeration cycle, which results in asubstantial reduction in the electric power compared to a standardcompressor refrigerator. It should be noted that novel and effectivestudies have been conducted in order to recognise the optimum hotwater temperature for absorption chillers [21,22].

The selected unit is a commercially available lithium bromideabsorber that is capable of providing the maximum efficiency (72%)with a hot water temperature of 87 �C. This choice was made inorder to maintain the same condensation temperature of the ORCboth in Summer and in Winter. This choice allows the ORC unit tooperate constantly at its design conditions and allows thecomplexity of the regulation system to be reduced.

3.3. Efficiency analysis

All the components of the power plant shown in Fig. 4 arecommercially available, and their performance parameters areclearly indicated by the manufacturer. Table 2 reports the specifi-cations along with the setting chosen for the biomass combustor,the exhaust regenerator and the ORC system. As indicated in thistable, a great part (2141 kW) of the input power (2500 kW) istransferred to the diathermic oil according to the efficiency of theheat exchanger (85.6%), while the residual thermal energy of theexhaust gases is partly recovered through the exhaust regeneratorand is transferred to the hot water (maximum thermal powerrecovered¼ 150 kW). Not all the thermal energy transported by thediathermic oil (2141 kW) can be transferred to the ORC, because ofthe efficiency of the heat exchanger of the ORC system (80%); as aresult, the thermal power in input to the ORC results to be equal to1710 kW. For this value of input power, the chosen ORC system iscapable of producing 281 kWe, while a thermal power of 1366 kWis still available in form of hot water exiting the condenser. Theoverall efficiency (including both the electrical and the thermalpower produced) of the ORC system, excluding the heat exchangersand the other components of the plant, is equal to 0.96.

The setting shown in Table 2 is valid both for the summer sea-son, when only air conditioning is needed (combined cooling and

power configuration), and for the rest of the year, when the ab-sorption chiller is unnecessary and the thermal energy transferredto thewater in the condenser can be recovered only for thermal use(combined heating and power configuration).

Table 3 reports the setting regarding only the former case(combined cooling and power configuration). In this case, it isnoteworthy that cold water at 7 �C is available for the coolingsystem with a maximum cooling power of 500 kW. The electricalefficiency (hel) is defined by equation (5) and is equal to 11.2%, whilethe overall efficiency (hG) is 31.2% according to equation (6):

hG ¼ Pel þ Q·

c

m·bHi

(6)

where Q·

c denotes the cooling power. In contrast, Table 4 reportsthe setting regarding the latter case (combined heating and powerconfiguration), when chilled water is not needed. In this case, amaximum useful thermal power of 1516 kW can be produced(35.5 kg/s of hot water at 87 �C) in January. The maximum overallefficiency, hG, results to be equal to 71.8%, according to equation (7):

hG ¼ Pel þ Q·

th

m·bHi

(7)

where Q·

th denotes the useful thermal power.The maximum potential of the proposed plant in the two

different configurations is also illustrated by the bar charts of Fig. 8for completeness.

Fig. 9 shows how the overall efficiency of the plant changes withthe months. The partial load strategy consists in producing thesame electrical power (281 kWe), while dissipating the excessthermal and cooling power through proper dissipation systems. Inthis manner the ORC operates at its design conditions so that theefficiency of the ORC is not reduced at partial loads; as a result, theelectrical energy obtained from biomass is always the maximumpossible. This strategy is also justified by the large availability offeedstock in the zone surrounding the airport (see Section 2).

4. Economic analysis

In this section the economic advantages that the plant can ensureare analysed. In particular, the payback period (PBP), the net presentvalue (NPV) and the internal rate of return (IRR) are evaluated. Toaccomplish this task, it is necessary to quantify the periodic cashflow of the investment, in terms of annual costs and incomings.

4.1. Cost estimation

The main costs to be sustained are the overall cost for therealization of the plant and the annual cost required for the pro-curement of the necessary feedstock.

The cost of the plant, given by the sum of the costs of thecomponents and the installation costs, amounts to 2900000 V.

The yearly cost due to the procurement of the necessary feed-stock (Cfeedstock), expressed in V/year, can be calculated as follows:

Cfeedstock ¼ m·b*ðCh þ Ct þ CcÞ (8)

where _mb is the fuel mass flow rate, Ch is the harvesting cost, Ct isthe transportation cost and Cc is the chipping cost.

The cost of harvesting depends on the machines used for thispurpose. In this analysis, a machine (referred to as the baler)capable of both collecting tree pruning residues and grouping theminto cylindrical bales, as with the forage waste, is considered. This

Table 3Specifications of the absorption chiller and efficiency parameters valid for the summer configuration (combined cooling and power configuration).

Summer configuration (electricity þ cooling)

Absorption chiller Maximum mass flow rate of hot water through the generator 9.77 kg/sTemperature of hot water entering the generator 87 �CTemperature of hot water exiting the generator 70 �CMaximum thermal power transferred to the generator 695 kWMaximum mass flow rate of cold water through the evaporator 23.89 kg/sTemperature of cold water at the evaporator inlet 12 �CTemperature of cold water at the evaporator outlet 7 �CCOP 72%

Plant efficiency Net electrical power 281 kWUseful thermal power e

Maximum cooling power 500 kWElectrical efficiency 11.2%Maximum overall efficiency 31.2%

Table 4Specifications of the thermal users and efficiency parameters valid for the combined heating and power configuration.

Winter configuration (electricity þ heating)

Thermal user Maximum mass flow rate of hot water available for thermal users 21.31 kg/sTemperature of hot water available for thermal users 87 �CTemperature of hot water returning to the bottom tank 70 �C

Plant efficiency Net electrical power 281 kWMaximum thermal power available for thermal use 1516 kWCooling power e

Electrical efficiency 11.2%Maximum overall efficiency 71.8%

Fig. 8. Maximum potentiality of the plant in the summer season (cooling and power)and in the winter season (heating and power).

Fig. 9. Overall effciency vs months.

R. Amirante et al. / Renewable Energy 89 (2016) 411e421418

machine was constructed for forage and in a second time modifiedfor olive tree pruning residues. A bale has a diameter of 1.50m and awidth of 1.20m, themediumweight varies between 400 and 450 kg,and the time necessary to produce a bale is about 15min. Accordingto the specification provided by the manufacturer, the operationcosts for the harvesting operation can be estimated to be Ch¼ 26V/t.

The overall transportation cost per tons of feedstock (Ct) is givenby the sum of the cost of the fuel required for the round trip (Cf) andthe cost for loading and unloading the feedstock (Cl), divided by theweight of the feedstock transported (Pm):

Ct ¼Cf þ ClPm

(9)

According to typical vehicles for transportation of feedstock, themaximum weight of feedstock that can be loaded on the vehicleamounts to Pm ¼ 18 t.

To calculate Cf, the average speed of the vehicle during the roundtrip can be assumed equal to 30 km/h with a fuel consumptionequal to about 40 V/h in the case of transporting 18 t of feedstock,which results in a fuel cost per km equal to 1.33V/km. The distanceof a round trip can be taken equal to the average distance of thefarmers from the plant, namely 10.1 km (see Section 2.2). Withthese assumptions, Cf amounts to about 26.9V (multiplying 1.33V/km by 20.2 km, considering the round trip). With regard to Cl, thetime to load and unload 18 t of feedstock can be assumed equal to1 h (45 min necessary to load and 15 min to unload); if these op-erations are performedmanually, Cl averages 25V, according to theaverage salary of a worker [23].

Hence, substituting Cf ¼ 26.9 V, Cl ¼ 25 V and Pm ¼ 18 t inequation (9) results in a total transportation cost (Ct) equal to 2.9V/t.

The chipping stage allows obtaining a solid biofuel in the form ofchips with dimensions suitable for the biomass combustor. The costof the chipping phase can be estimated to be equal to Cc ¼ 5 V/t.

In conclusion, it has been demonstrated that Ch ¼ 26 V/t,

R. Amirante et al. / Renewable Energy 89 (2016) 411e421 419

Ct ¼ 2.9V/t and Cc ¼ 5V/t. Substituting these values in Equation (8)and considering that _mb ¼ 6800 t/year, the cost required for theprocurement of the necessary feedstock amounts toCfeedstock ¼ 230500 V/year.

4.2. Incomings estimation

The main incomings are represented by the avoided costs of hotwater and electricity generation.

The first contribution can be calculated from the examination ofthe annual thermal demand of the airport shown in Fig. 2, whichamounts to 1680000 kWhth. Considering that 1 Nm3 of methaneaveragely costs 0.30V and that the quantity of methane required toproduce 1680000 kWhth is about 159200 Nm3 according to thecurrent technology, it results that the cost avoided for thermalpower production is equal to 47760.00 V/year.

The avoided cost of electricity results from the fact that theoverall electrical energy required by the airport is partly providedby the power plant. The plant produces an electrical power of281 kWover 8000 h, so the overall electrical energy provided to theairport buildings results to be equal to 2248000 kW h/year. Inaddition, the plant allows saving an electrical energy of610000 kW h necessary for the cooling of the buildings (see Fig. 3).Summing up these two contributions and considering that thecurrent price of electricity in that area is 0.12 V/kWh, the overallavoided cost of electricity is 342960.00 V/year.

Further incomings are given by government incentives, whichcan be grouped into two categories independent from each other.The first one is equal to 75% of the overall capital cost of the plantand regards CCHP biomass efuelled plants built in the south re-gions of Italy with a capital cost comprised between 2 and 25 mil-lions of Euros. With this incentive, the initial plant cost of2900000.00 V is lowered to 725000.00 V.

The other one regards all CCHP biomass efuelled plants andensures a bonus per every KWhe produced during the initial 15years. The bonus is equal to 0.227V/kWhe in the first year and thenis reduced by 2% per year.

4.3. Analysis of the investments

The estimation of costs and incomings achieved in Sections 4.2

Fig. 10. Annual cash flows of the investment vs number of years, con

and 4.3 allows evaluating the annual cash flow of the investments.Fig. 10 shows the annual cash flow during a period of 15 years: thesituation regarding the first kind of incentive is depicted in red,while the other case is plotted in blue.

The first type of incentives leads to having an initial outlay lowerthan the second type, but also a lower annual cash flow in all thesubsequent years. This results from the fact that the first type of in-centives onlyensures a reduction in the initial cost of the plant,whilstthe second type gives a bonus for each electrical kWh produced, thusensuring additional annual incomings for the subsequent 15 years.

The calculation of the payback period and net present value canbe instrumental both in evaluating the profitability of the invest-ment and in recognizing which of the two typologies of govern-ment incentives is more suitable. These indicators are plotted inFig. 11 for both categories of incentives, with the assumption thatthe discount rate is equal to 8%. It can immediately be noticed thatthe investment has a PBP value of 6 years for both cases; further-more, we can state that the second typology of government in-centives (a bonus for each electrical kWh produced) ensures moregains than the first one. In fact, at the end of the 15th year the NPV is646399.00 V in the first case and 2386248.00 V in the second one.

These results can be generalized considering what happens atthe changing of the discount rate. For both types of governmentincentives, Fig. 12 shows the NPV of the investment after the 15-thyear as a function of the discount rate. It is clear that this graphconfirms that the second type of incentives is more convenientthan the other one, regardless of the discount rate chosen.

From the examination of Fig. 12, we can also appreciate the in-ternal rate of return (IRR) of the investment. It is almost constantregardless of the typology of the incentive, being equal to about 21%in both cases. This value points out the profitability of the invest-ment once again.

5. Ecological considerations

This final section evaluates the quantity of CO2 that will not bereleased into the atmosphere after the proposed power plant is builtat Bari Airport. Toperformthis evaluation, itmust be considered that1 Nm3 of methane weighs 0.7 kg, and 2.75 kg of CO2 are normallyproduced per kg of methane in a stoichiometric reaction.

With regard to the heat generation, the quantity of methane

sidering the two typologies of government incentives separately.

Fig. 11. Net present value of the investment vs number of years for the two typologies of government incentives.

Fig. 12. Net present value of the investment after the 15-th year, as a function of the discount rate for the two typologies of government incentives.

R. Amirante et al. / Renewable Energy 89 (2016) 411e421420

required to produce 1680000 kW h is 159200 Nm3, thus the annualquantity not released into the atmosphere will be 306460 kg.

With regard to the electricity production, supposing a referenceelectrical efficiency of 0.60, it results that the methane necessaryfor the electrical generation is 316050 kg. Thereby, the avoidedrelease of CO2 for electricity generation will be equal to 869140 kg.Overall, the mass of CO2 avoided per year will be 1175600 kg.

In conclusion, it results that the proposed tri-generative plantcan be very important in the Apulia context from an ecologicalpoint of view. Furthermore, the plant can lead to a double ecologicalgain, because in addition to avoiding the combustion of fossil fuelsfor energy production, it can also avoid that a lot of olive pruningresidues are burned on fields with considerable quantities of CO2released into the atmosphere. Such a practise is prohibited by theItalian law and produces a great quantity of dioxin because of thelow temperature combustion, while the produced pollutants arealmost completely eliminated in the biomass combustor of theproposed tri-generative plant.

6. Conclusions

For over twenty years ever increasing attention has been givento the ecological problem in national politics. One of the mosteffective ways to reduce CO2 emissions is the use of biomass as fuelfor energy production. This research is focused on the energetic useof agricultural wastes in Apulia, in particular olive tree pruningresidues. At present, such agricultural residues are burned onApulian fields (despite being prohibited by the Italian law) with aconsequential loss of possible energy exploitation.

In the real case considered, pre-dried olive tree pruning residuesare directly used as solid fuel in a tri-generative power plant ofabout 280 kWe planned to be located near Bari Airport.

The Apulia context was analysed with respect to the availabilityof feedstock, finding that the resource of biomass present in a verysmall area surrounding the plant location is sufficient to satisfy theenergetic needs of the plant. Afterwards, the plant was described indetail. It is composed of a biomass combustor, a co-generative ORC

R. Amirante et al. / Renewable Energy 89 (2016) 411e421 421

system and an absorption chiller, which ensures chilled water inthe summer. Moreover, to reduce the quantity of pollutant com-ponents like NOx and CO, an electrostatic filter will be positioneddownstream of the exhaust regenerator.

In the winter season, when the production of chilled water isunnecessary, the plant will be able to produce a maximum thermalpower of 1516 kW in the form of hot water at 87 �C along with a netelectrical power of 281 kW. In contrast, part of the thermal powercan be transferred to the generator of the absorption chiller withthe aim of producing chilled water (maximum cooling power of500 kW) in the summer season. In the former case (combinedheating and power configuration), the overall efficiency of the plantis 71.8%, while, in the latter case (combined cooling and powerconfiguration) the plant is able to produce electricity and coolingpower with a maximum overall efficiency of 31.2%.

The main economic advantages come from the avoided costs ofelectricity and methane necessary for generating the thermal po-wer. Two typologies of government incentives have been consid-ered: according to the first one, the government finances apercentage of the plant cost, contrarily, according to the other one,the government provides a bonus for every electrical kWh pro-duced. In both cases, the payback period is 6 years and the internalrate of return is about 21%, highlighting that the proposed project ishighly convenient from an economic point of view. From anecological point of view, the plant is remarkably eco-efficient,ensuring a reduction of 1175600 kg/year in CO2 emissions.

Although this research activity is concernedwith a specific zone,namely the area surrounding Bari Airport, the results can beapplied to other zones of the Mediterranean region, which hascontinuous availability of residues from olive tree pruningpractices.

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