Available online at www.sciencedirect.com Available online at www.sciencedirect.com Available online at www.sciencedirect.com Available onlineScienceDirect at www.sciencedirect.com AvailableAvailable online onlineScienceDirect at at www.sciencedirect.com www.sciencedirect.com EnergyScienceDirect Procedia 00 (2017) 000– 000 EnergyScienceDirect Procedia 00 (2017) 000– 000 www.elsevier.com/locate/procedia Energy Procedia 00 (2017) 000–000 Energy Procedia 00 (2017) 000–000 ScienceDirectEnergy Procedia 00 (2017) 000–000 www.elsevier.com/locate/procedia ScienceDirectEnergy Procedia 00 (2017) 000–000 www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia EnergyEnergy Procedia Procedia 12900 (20(2017)17) 000891–898–000 www.elsevier.com/locate/procedia IV International Seminar on ORC Power Systems, ORC2017 www.elsevier.com/locate/procedia IV International13-15 Semina Septemberr on ORC 2017, Power Milano, Systems, Italy ORC2017 IV International Seminar on ORC Power Systems, ORC2017 IV International13-15 Semina Septemberr on ORC 2017, Power Milano, Systems, Italy ORC2017 IV International13-15 Semina Septemberr on ORC 2017, Power Milano, Systems, Italy ORC2017 Thermodynamic analysis13-15 of September a small 2017, scale Milano, combined Italy cycle for energy Thermodynamic analysis of a small scale combined cycle for energy Thermodynamicgeneration analysis fromof a s carbonmall scale neutral combined biomass cycle for energy ThermodynamicThe 15thgeneration Internationalanalysis fromof Symposium a s carbonmall scale on neutral District combined Heatingbiomass cycleand Cooling for energy R. Amirantegenerationa, P. De Palma froma, E. Distaso carbona, A neutral. M. Pantaleo biomassb, P. Tamburrano a* generationa froma carbona neutral biomassb a* aDepartmentR. Amirante of Mechanics,a, P. Mathematics De Palma anda Management, E. Distaso, Polytechnica, A. U Mniversity. Pantaleo of Bari, Viab, OrabonaP. Tamburrano 4, 70100 Bari, Italya* AssessingR. Amiranteb Departme thenta, ofP Agro. Defeasibility-anvironmental Palmaa, sciences, E. Distasoof University usinga, of A Bari,. Mthe Via. Pantaleo Amendola heat 165/A, bdemand, P 70125. Tamburrano Bari, italy- outdoora* aDepartmentR. Amirante of Mechanics,, P. Mathematics De Palma and Management, E. Distaso, Polytechnic, A. U Mniversity. Pantaleo of Bari, Via, OrabonaP. Tamburrano 4, 70100 Bari, Italy a temperatureDepartmentb Departme of Mechanics,functionnt of Agro Mathematics-anvironmental for and aManagement sciences, long University, Polytechnic-term of Bari, U niversitydistrict Via Amendola of Bari , 165/A, Viaheat Orabona 70125 demand Bari,4, 70100 italy Bari, Italy forecast aDepartmentb Departme of Mechanics,nt of Agro Mathematics-anvironmental and Management sciences, University, Polytechnic of Bari, University Via Amendola of Bari ,165/A, Via Orabona 70125 Bari,4, 70100 italy Bari, Italy Abstract b Department of Agro-anvironmental sciences, University of Bari, Via Amendola 165/A, 70125 Bari, italy a,b,c a a b c c AbstractI. Andrić *, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre AbstractThe aim of this paper is to investigate the thermodynamic performance of a novel small-scale power plant that employs a combined Abstractcyclea for the energy generation from carbon-neutral biomass, such as pruning residues. The combined cycle is composed of an TheIN+ aim Center of this for paperInnovation, is to investigate Technology the and thermodynamic Policy Research performance - Instituto Superiorof a novel Técnico small,-scaleAv. Rovisco power Paisplant 1, that 1049 employs-001 Lisbon a combined, Portugal externally fired Joule Braytonb cycle followed by a bottoming steam cycle. The topping cycle has the unique particularity of being cycleThe aim for of the this energy paper generationis to Veoliainvestigate from Recherche thecarbon thermodynamic &-neutral Innovati biomass,on ,performance291 Avenuesuch as Dreyfousof pruning a novel Daniel,residues. small -78520scale The power Limaycombined ,plant France cyclethat employs is composed a combined of an Thecomposed aim ofc of this a cost paper-effective is to investigate turbocharger the takenthermodynamic from the automotive performance industry, of a novel in place small of -ascale more power expensive plant commercial that employs micro a combined-turbine. extcycleernally for theDépartementfired energy Joule generationBrayton Systèmes cycle fromÉnergétiques followed carbon - neutralbyet Environnementa bottoming biomass, steam such- IMT cycle.as Atlantique pruning The toppingresidues., 4 rue Alfredcycle The hasKastler,combined the unique44300 cycle Nantes,particularity is composed France of beinof ang cycleThe turbocharger for the energy can generation be either fromdirectly carbon connected-neutral biomass,to the electric such asgenerator pruning (afterresidues. a fewThe modifications)combined cycle or is coupled composed (without of an composedexternally offired a cost Joule-effective Brayton turbocharger cycle followed taken by from a bottoming the automotive steam industry, cycle. The in place topping of a cyclemore hasexpensive the unique commercial particularity micro of-turbi beinne.g extmodifications)ernally fired with Joule a powerBrayton turbine cycle movingfollowed the by generator. a bottoming The steamuse of cycle.solid biomassThe topping in the cycle proposed has the plant unique is allowed particularity by the profese beinnceg Thecomposed turbocharger of a cost -caneffective be either turbocharger directly taken connected from the to automotivethe electric industry, generator in place (after of a morefew expensivemodifications) commercial or coupled micro (without-turbine. composedof an external of a combustor cost-effective and turbochargera gas-to-gas heattaken exchanger. from the automotive The warm flueindustry, gases in exhausted place of a by more the toppingexpensive cycle commercial are used inmicro a bottoming-turbine. modifications)The turbocharger with can a power be either turbine directly moving connected the generator. to the The electric use of solidgenerator biomass (after in thea fewproposed modifications) plant is allowed or coupled by the pr(withoutesence Thecycle turbocharger to produce steam, can bewhich either can directlypower a steamconnected expander. to the electric generator (after a few modifications) or coupled (without Abstractmodifications)of an external combustor with a power and turbinea gas-to moving-gas heat the exchanger. generator. The The warm use of flue solid gases biomass exhausted in the by proposed the topping plant cycle is allowed are used by in the a bottoming presence modifications)This paper thermodynamically with a power turbine assesses moving the novel the generator.combined cycThele use in theof solidconfiguration biomass infor the the proposed topping cycle plant that is allowed employs by a turbochargerthe presence ofcycle an externalto produce combustor steam, which and a cangas -powerto-gas aheat steam exchanger. expander. The warm flue gases exhausted by the topping cycle are used in a bottoming ofcoupled an external with a combustor power turbine and acapable gas-to- ofgas generating heat exchanger. 30 kW The of electrical warm flue power gases. Furthermore,exhausted by the comparisontopping cycle between are used the in performance a bottoming Thiscycle paper to produce thermodynamically steam, which assessescan power the a novelsteam combined expander. cyc le in the configuration for the topping cycle that employs a turbocharger Districtcycleobtained toheating produce using networksthe steam, bottoming which are water cancommonly power Rankine a steamaddressed cycle expander. and ain bottoming the literature Organic as Rankine one of Cycle the mostis provided. effective solutions for decreasing the Thiscoupled paper with thermodynamically a power turbine capable assesses of the generating novel combined 30 kW of cyc electricalle in the configurationpower. Furthermore, for the toppingthe comparison cycle that between employs the a turbochargerperformance greenhouse©This 201 paper7 The gas thermodynamically Authors. emissions Published from assesses theby Elsevier building the novel Ltd sector.. combined These cyc systemsle in the configurationrequire high for investments the topping cyclewhich that are employs returned a turbocharger through the heat obtainedcoupled with using a powerthe bottoming turbine capablewater Rankine of generating cycle and 30 kWa bottoming of electrical Organic power Rankine. Furthermore, Cycle is the provided. comparison between the performance salesPeercoupled. Due-review withto under thea power changed responsibility turbine climate capable of the ofconditions scientific generating committee and 30 kW building of of electrical the renovationIV International power. Furthermore,policies, Seminar heat onthe ORC comparisondemand Power in Systemsbetween the future. the performance could decrease, ©obtained 2017 The using Authors. the bottoming Published water by ElsevierRankine Ltdcycle. and a bottoming Organic Rankine Cycle is provided. prolongingobtained usingthe investment the bottoming return water period. Rankine cycle and a bottoming Organic Rankine Cycle is provided. Peer©© 2012017-review7 The The underAuthors. Authors. responsibility Published Published by of Elseviertheby scientificElsevier Ltd. committeeLtd. of the IV International Seminar on ORC Power Systems. TheKeywords:©PeerPeer-review main 201-review7 scopeThe combined underAuthors. underof this responsibilitycycle; responsibility Publishedpaper biomass; is toby of ORC; assess Elsevierthe of Rankinescientificthe the scientificLtd feasibility cycle. committee committee of of using the IV theof International the heat IV demand International Seminar – outdoor on Seminar ORC tempe Power onrature SystemsORC function Power. Systems.for heat demand Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. forecast.Keywords: The combined district cycle; of Alvalade,biomass; ORC; located Rankine in cycle Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildingsKeywords: that combined vary incycle; both biomass; construction ORC; Rankine period cycle and typology. Three weather scenarios (low, medium, high) and three district renovation1.Keywords: Introduction scenarios combined cycle; were biomass; developed ORC; Rankine(shallow, cycle intermediate, deep). To estimate the error, obtained heat demand values were compared1. Introduction with results from a dynamic heat demand model, previously developed and validated by the authors. 1. IntroductionThe 2015 United Nations Climate Change Conference has stated that all nations have to promote the exploitation of The1. results Introduction showed that when only weather change is considered, the margin of error could be acceptable for some applications renewable1. Introduction sources for energy generation in order to reach the target of limiting global warming to less than 2 °C compared (the errorThe in2015 annual United demand Nations was Climate lower thanChange 20% Conference for all weather has stated scenarios that allconsidered). nations haveHowever, to promote after the introducing exploitation renovation of to preThe- industrial2015 United levels Nations [1]. ThisClimate goal Change can be Conferenceachieved by has boosting stated thatthe allemployment nations have of totechnologies promote the able exploitation to produce of scenarrenewableTheios, the2015 sourceserror United value for Nations energy increased Climategeneration up to Change 59.5% in order Conference(depending to reach the hason target thestated weather of thatlimiting all and nations global renovation warminghave toscenarios promote to less thancombination the 2exploitation °C compared considered). of electricalrenewableThe 2015 and sources United useful for thermalNations energy energyClimategeneration from Change in wind order Conference and to reachsolar theenergy has target stated (which of thatlimiting could all nations globalbecome warminghave more to effectivepromote to less than ifthe it 2exploitationwas °C csupportedompared of Therenewableto value pre-industrial of sourcesslope levelscoefficient for energy [1]. This increasedgeneration goal canon in orderaveragebe achieved to reachwithin bythe the boostingtarget range of limitingofthe 3.8% employment global up to warming8% of pertechnologies decade,to less than that able 2 corresponds°C to c omparedproduce to the withrenewableto pre energy-industrial sources storage levels for systems energy [1]. This[2]), generation hydropower,goal can in orderbe geothermalachieved to reach bythe energy boostingtarget asof welllimitingthe employmentas biomass. global warming This of technologies paper to less is focused than able 2 °C on to c theomparedproduce latter decreaseelectricalto pre -inindustrial the andnumber useful levels thermalof [1].heating Thisenergy hours goal from ofcan wind22 be-139h achievedand duringsolar byenergy the boosting heating (which theseason could employment (dependingbecome moreof ontechnologies effective the combination if itable was to supportedof produce weather and renewableelectricalto pre-industrial and energy useful levels source thermal [1]., by This energyproposing goal from can a windnovel, be achievedand cost solar-effective byenergy boosting power (which theplant could employment capable become of moregeneratingof technologies effective electrical if itable was energy to supported produce from renovationwith energy scenarios storage considered). systems [2]), On hydropower, the other hand, geothermal function energy intercept as well increased as biomass. for 7.8 This-12.7% paper per is focuseddecade (dependingon the latter on the somewithelectrical energy forms and ofst usefulorage biomass systemsthermal that can [2]),energy be hydropower, considered from wind carbon geothermaland solar-neutral. energy energy (which as well could as biomass. become Thismore paper effective is focused if it was on supported the latter coupledrenewable scenarios). energy The source values, by proposingsuggested acould novel, be cost used-effective to modify power the plantfunction capable parameters of generating for the electrical scenarios energy considered, from and renewablewithThe energy term energy stcarbonorage source- neutralsystems, by means [2]),proposing hydropower, that the a novel,transformation geothermal cost-effective of energy biomass power as wellinto plant usefulas capable biomass. energy of Thisgenerating does paper not alter iselectrical focused the overall energyon the amount latterfrom improvesome theforms accuracy of biomass of heat that demand can be estimations.considered carbon-neutral. ofrenewable carbon presentenergy insource the environment., by proposing However, a novel, costnot every-effective form power of biomass plant capableis really of carbon generating neutral electrical, as discu energyssed in from [3]; someThe forms term ofcarbon biomass-neutral that meanscan be thatconsidered the transformation carbon-neutral. of biomass into useful energy does not alter the overall amount amongsomeThe forms termthose ofcarbon typologies biomass-neutral that of meansbiomasscan be thatconsidered that the can transformation be carbon considered-neutral. of biomasscarbon neutral into useful for energyenergy generationdoes not alter are the pruning overall residues, amount © 201of carbon7 The Authors. present inPublished the environment. by Elsevier However, Ltd. not every form of biomass is really carbon neutral, as discussed in [3]; providedof carbonThe term that present carbon they inare- neutralthe exploited environment. means on that site However, theto avoidtransformation additional not every of COform biomass2 generation of biomass into useful from is really feedstockenergy carbon does transportation neutralnot alter, as the discu [4overall-9]ssed. amount in [3]; Peeramong-review those under typologies responsibility of biomass of the Scientificthat can beCommittee consideredof carbonThe 15th neutral International for energy Symposium generation on areDistrict pruning Heating residues, and amongof carbonThe thoseresearch present typologies carried in the outenvironment.of biomassup to now that However,by can the beauthors nconsideredot every of this form carbon paper of biomassintendsneutral toforis developreally energy carbon ageneration novel, neutral small are, as- scalepruning discu powerssed residues, in plant [3]; provided that they are exploited on site to avoid additional CO2 generation from feedstock transportation [4-9]. Coolingamongcapable. those of generating typologies energy of biomass from solid that biomass can be directlyconsidered on site,carbon without neutral the for need energy of transforming generation itare into pruning syngas residues, or other providedThe research that they carried are exploited out up to on now site byto avoidthe authors additional of this CO paper2 generation intends from to develop feedstock a novel, transportation small-scale [4- 9power]. plant providedbiofuelsThe research [ that10-11 they] ,carried thus are exploitedproposing out up to on a now validsite byto alternative avoidthe authors additional to of“stand this CO paperalone2 generation ”intends Organic from to Rankinedevelop feedstock cyclesa novel, transportation (ORCs), small-scale which [4- 9power] .are, atplant the Keywords:capableHeat of generating demand; Forecast; energy C fromlimate solid change biomass directly on site, without the need of transforming it into syngas or other capableThe researchof generating carried energy out up from to now solid by biomass the authors directly of onthis site, paper without intends the to need develop of transforming a novel, small it into-scale syngas power or otherplant biofuels [10-11], thus proposing a valid alternative to “stand alone” Organic Rankine cycles (ORCs), which are, at the capablebiofuels of[10 generating-11], thus energyproposing from a solidvalid biomassalternative directly to “stand on site, alone without” Organic the need Rankine of transforming cycles (ORCs), it into which syngas are, or atother the ______biofuels [10-11], thus proposing a valid alternative to “stand alone” Organic Rankine cycles (ORCs), which are, at the * Corresponding author. tel. 080 5963470 - fax. 080 5963411. ______E-mail address: [email protected] * ______Corresponding author. tel. 080 5963470 - fax. 080 5963411. 1876 -*6102 ______Corresponding© 201 7 The author. Authors. tel. 080 Published 5963470 -by fax. Elsevier 080 5963411 Ltd. . * ECorresponding-mail address: author. [email protected] tel. 080 5963470 - fax. 080 5963411. Peer -review* Corresponding under responsibility author. tel. 080 of 5963470 the Scientific - fax. 080 Committee 5963411. of The 15th International Symposium on District Heating and Cooling. E-mail address: [email protected] 1876-6102 E-mail 2017address: The [email protected] Authors. Published by Elsevier Ltd. © Peer-review1876 -6102 under © 2017responsibility The Authors. of the Publishedscientific bycommittee Elsevier ofLtd. the IV International Seminar on ORC Power Systems.
10.1016/j.egypro.2017.09.213 Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 1876 -6102 © 2017 The Authors. Published by Elsevier Ltd.
1876Peer--review6102 © under 2017 responsibility The Authors. ofPublished the scientific by Elsevier committee Ltd. of the IV International Seminar on ORC Power Systems. 1876Peer--review6102 © under 2017 responsibility The Authors. ofPublished the scientific by Elsevier committee Ltd. of the IV International Seminar on ORC Power Systems. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems.
10.1016/j.egypro.2017.09.213 1876-6102 2 Amirante et al./ Energy Procedia 00 (2017) 000–000 892 R. Amirante et al. / Energy Procedia 129 (2017) 891–898
state of the art, the best technology for the direct transformation of solid biomass into electrical and thermal energy [12]. With regard to commercially available small scale ORC plants, the electrical efficiency is usually below 15% when the generated electrical power is lower than 100 kW [13]. The electrical efficiency usually increases with the increasing size of the plant; e.g., an ORC plant produced by an important manufacturer [14] is capable of generating 200 kWe with an electrical efficiency of about 16.5%. The proposed power plant has the particularity of being composed of an externally fired Joule Brayton cycle serving as the topping part, plus a Rankine cycle serving as the bottoming part. The novel idea is to apply a combined cycle (gas turbine + Rankine cycle) to dirty fuels (such as solid biomass) and small-scale generation. Instead, as it is widely recognised, combined cycles are only suited for both clean fuels (to prevent the gas turbine blades from being damaged) and large-scale generation. The novel configuration of the combined cycle was assessed thermodynamically in [10], with the assumption of using water as the working fluid of the bottoming cycle and for a generated electrical power of about 100 kW. The thermodynamic optimization performed in [10] showed that a maximum electrical efficiency of about 0.25 can be achieved, which is a very good level of performance for such a low power plant size. The configuration proposed in [10] was also capable of producing useful thermal power, thus achieving a combined heat and power (CHP) arrangement with a maximum overall efficiency (electrical plus thermal efficiency) of about 0.7 [10]. This paper further investigates this novel typology of power plant by exploring its thermodynamic feasibility for lower useful electrical power (less than 50 kW) and in a full electrical power generation arrangement. Furthermore, the main aim of this paper is to compare the plant configuration employing the bottoming water Rankine cycle with a similar plant configuration employing an ORC as the bottoming cycle. The comparison between the two plant configurations is performed in terms of overall electrical efficiency (useful electrical power over input thermal power).
2. Methodology
2.1 Proposal of plant configurations
The architecture of the proposed small scale combined cycle is shown in Figure 1. Clean air is compressed by a centrifugal compressor and delivered into a gas to gas heat exchanger aimed at increasing the air enthalpy. At first the high temperature air expands through a turbine (T1) connected to the centrifugal compressor and then is conveyed into a second turbine (T2) connected to an electric generator. The first turbine has the only task of driving the compressor, whereas the second turbine serves as the power turbine producing the shaft work output. Both turbines are moved by clean air, therefore their mechanical integrity is not at risk although dirty fuels are burned in the combustor. The air exhausted from the power turbine is delivered into the external combustor, where the combustion between the air and biomass generates high temperature flue gases. These exit the combustor and enter a cyclone separator (needed to clean the flue gases); afterwards, the flue gases enter the high temperature heat exchanger (HTHE) and transfer their thermal power to the clean air. After leaving the heat exchanger, the flue gases are conveyed into the bottoming cycle of the plant. In the configuration presented here, only the compressor is responsible for moving the air flow in the circuit, without the need for an additional fan; as a result, the discharge pressure of the power turbine can be greater than the atmospheric pressure in case of non-negligible pressure losses.
Chimney HRSG G + G a b G SuperHeat. External b Depuration Eco Vapor. 7 HTHE 9 8 6 combustor pp 2 C D 3 G V E T C 1 T B 2 ~ ~ SE
F 5 A Ga 1 4
Fig.1. Scheme of the combined plant: topping part employing two turbines (right) and bottoming part employing a water Rankine cycle (left)
Summarizing, the topping cycle presents the following innovative aspects: I. The compressor-turbine group does not need to be manufactured ex-novo, but a turbocharger and a power turbine for trucks can be used for this purpose. II. The combustion chamber is external instead of being internal (as occurs in typical gas turbine plants). AmiranteR. Amirante et al./ Energyet al. / ProcediaEnergy Procedia 00 (2017) 129000– (2017)000 891 – 898 3 893
III. The input thermal power is provided to the working fluid (clean air) by means of a gas to gas heat exchanger, which has to withstand the high temperatures required by a gas turbine while ensuring negligible pressure drops. Item I), namely, the use of a turbocharger taken from the automotive industry, has the potential to reduce the capital costs of the plant, because this technology is mature and cheap. The plant analysed thermodynamically in [10] was based on a single-shaft configuration (i.e., one turbine and one compressor); instead, this paper analyses the two-shaft configuration (two turbines and one compressor), which can provide advantages at partial load. Items II) and III) are necessary for the use of dirty fuels instead of clean ones. In order to burn solid biomass effectively, either a fluidized bed combustor or a moving grid burner can be used to accomplish this task. Several models of combustors are available on the market that can be used to perform the combustion of solid biomass with the high temperature air discharged from the turbine [10]. Several models are also characterized by excellent efficiency, as high as 90% [10]. With regard to the gas to gas heat exchanger, it is the most critical component, as several companies tried to design high temperature gas to gas heat exchangers, mostly with negative results due to high temperature cracking. However, the design of this component can take advantage of the fact that the pressures and the temperatures as well the gas flow rate of the proposed cycle are not as high as those of typical industrial gas turbines. The problematics concerned with the application of gas to gas heat exchangers were discussed thoroughly in previous papers [15-18], with two configurations being proposed by the same authors so far. The first one is called “the immersed particle heat exchanger (IPHE)” and is based on the concept of using an intermediate ceramic medium (namely small ceramic particles) to transfer heat from two fluids without mixing them. The second configuration employs a shell and tube heat exchanger with the particularity of being made of metallic materials capable of withstanding very high temperatures, such as a Nichel alloy or a stainless steel. The IPHE has a higher potential than the Nichel alloy heat exchanger, because the former does not employ metallic surfaces for the heat exchange and therefore can withstand higher temperatures [19]. Concerning the dimensions of the heat exchanger, it would not be an issue, because the heat exchanger would be very compact, with small size and reduced material content, considering that the flow rates of air and flue gas are very low in this type of power plant. Figure 1 also shows a schematic representation of the bottoming cycle. The flue gases exiting the topping cycle are delivered into a heat recovery steam generator (HRSG) allowing the remaining enthalpy content to be transformed into useful energy. The architecture shown in Fig. 1 makes use of water as working fluid for the bottoming cycle. The use of water for such a low temperature level and gas flow rate was unfeasible in the past, because of the poor efficiency of steam expanders. However, recently there have been several advances in the design of steam turbines, which are more efficient than in the past even for very low levels of gross power. For example, the ‘‘Green steam turbine’’ is a commercially available model whose cost amounts to a few thousand Euros (by virtue of the single stage configuration) and which is capable of generating a maximum electrical power of 15 kW, with a maximum pressure of 10 bar and a maximum temperature of 225-250 °C [20]. Using the data provided by the manufacturer, and assuming an efficiency for the electric generator of about 95÷97%, the authors estimated an isentropic efficiency of approximately 50%, which represents a high level of performance despite the very low electrical power.
Chimney G + G G a b External b Depuration Eco. Vapor. 7 HTHE 9 8 6 combustor pp 2 3 B C D ≡ E G V
~ SE C T1 T2 ~ Regen.
A’ F F’ 4 5 Ga A 1
Fig. 2. Second scheme for the combined plant: topping part employing two turbines (right) and bottoming part employing an ORC (left)
Figure 2 reports a second configuration proposed for the bottoming cycle. In this case, the working fluid is based on an organic molecule, thus the bottoming part is to be defined as an Organic Rankine Cycle (ORC). The main difference with the previous architecture is that the presence of the super-heater is not mandatory in the scheme of Fig. 2, while a regenerator is employed to increase the efficiency of the cycle. The two configurations are compared using the thermodynamic model described in the following subsection. It should be noted that the bottoming cycle can be used either as a power plant for the only generation of electrical energy or as a plant for the generation of electrical and thermal energy (cogeneration). In the latter case, the condensation pressure must be raised in order to increase the condensation temperature, thus allowing high temperature heat recovery from the vapour condensation. Such a plant configuration was
894 4 AmiranteR. Amirante et al./ Energy et al. Procedia/ Energy 00 Procedia (2017) 000 129–000 (2017) 891–898
discussed in a previous paper [10], whereas the present analysis is only concerned with the generation of electrical power, so the condensation temperature will be kept very low to maximize the electrical power generated.
2.2 Thermodynamic model
The cycles of Figures 1 and 2 were modelled by means of a calculation code using the libraries “Fluid Prop” for the evaluation of the fluid enthalpies [21]. The flue-gas properties were assumed equal to those of air; the latter was modelled with the “gas mix model”, which implements the ideal gas equation of state with a temperature dependent specific heat. The water was modelled by means of the IF97 model, which implements the thermodynamic and transport properties of water and steam according to the IAPWS -IF97 industrial standard [21]. The organic fluid was modelled with the “FreeStanMix” subroutines [21]. The following equations were employed for the simulation. The isentropic efficiency of the compressor and turbines ( , ) were calculated as follows (see Fig. 1 and Fig. 2 for explanation of symbols):
𝑖𝑖𝑖𝑖,𝑐𝑐 𝑖𝑖𝑖𝑖,𝑡𝑡1 𝑖𝑖𝑖𝑖,𝑡𝑡2 𝜂𝜂 𝜂𝜂 , 𝜂𝜂 , . ℎ2𝑖𝑖𝑖𝑖−ℎ1 ℎ3−ℎ4 ℎ4−ℎ5 (1) 𝜂𝜂𝑖𝑖𝑖𝑖The,𝑐𝑐 = equilibriumℎ2−ℎ1 , 𝜂𝜂of𝑖𝑖𝑖𝑖 ,𝑡𝑡the1 = compressorℎ3−ℎ4𝑖𝑖𝑖𝑖 -turbine 𝜂𝜂𝑖𝑖𝑖𝑖,𝑡𝑡2 =groupℎ4−ℎ states5𝑖𝑖𝑖𝑖 that the power produced by the first turbine equals the power(1) absorbed by the compressor: (2) , 𝐺𝐺𝑎𝑎(ℎ2−ℎ1) 𝑚𝑚,𝑡𝑡1 𝑎𝑎 3 4 𝜂𝜂𝑚𝑚,𝐶𝐶 where 𝜂𝜂 η𝐺𝐺m,t1(ℎ and− ηℎm,c) denote = the mechanical efficiency of the first turbine and compressor, respectively. Furthermore, the pressure increase provided by the compressor, namely p2-p1, must be equal to the sum of the pressure jump across the two turbines, p3-p5, and the overall pressure drop along line 5-9 of the circuit, denoted by ∆ploss (assuming p2=p3) : (3) p2-p1= p3-p5+∆ploss . The power generated by the second turbine (power turbine) was evaluated as follows: (4) , and denoting the useful power and the mechanical efficiency of the power turbine, respectively. The heat 𝑃𝑃𝑢𝑢,𝑇𝑇𝑇𝑇 = 𝜂𝜂𝑚𝑚,𝑡𝑡2𝐺𝐺𝑎𝑎(ℎ4 − ℎ5) exchange within the gas-to-gas heat exchanger implies that the following equation must be satisfied: 𝑃𝑃𝑢𝑢,𝑇𝑇𝑇𝑇 𝜂𝜂𝑚𝑚,𝑡𝑡2 , (5)
w( here𝐺𝐺𝑎𝑎 + 𝐺𝐺𝑏𝑏)(ℎ6 accounts− ℎ7)𝜂𝜂𝑡𝑡ℎ ,for𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 the =thermal𝐺𝐺𝑎𝑎(ℎ3 dispersions− ℎ2) from the HTHE towards the environment. Instead, the heat exchange efficiency, ԑHTHE, is the ratio of the exchanged thermal power to the ideally exchanged thermal power, namely: 𝜂𝜂𝑡𝑡ℎ,𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 . (6) ℎ3−ℎ2 𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 ԑ The thermal= ℎ6−ℎ2 balance of the external combustor was calculated as follows: , (7)
b , b , i and denoting the combustor efficiency, the mass flow rate of the fuel, the lower heating value of the fuel 𝜂𝜂𝑏𝑏𝐺𝐺𝑏𝑏𝐻𝐻𝑖𝑖 + 𝐺𝐺𝑏𝑏ℎ𝑏𝑏 + 𝐺𝐺𝑎𝑎ℎ5 = (𝐺𝐺𝑎𝑎+𝐺𝐺𝑏𝑏)ℎ6 and the inlet enthalpy of the fuel, respectively. The term b accounts for both the imperfect combustion and the thermal 𝑏𝑏 dispersion𝜂𝜂 towardsℎ the environment. The thermal dispersion from the cyclone separator was included in this term. The overall power generated by the bottoming cycle was𝜂𝜂 calculated as follows: , (8) 𝑝𝑝𝐵𝐵−𝑝𝑝𝐴𝐴 𝑢𝑢,𝐵𝐵𝐵𝐵 𝑚𝑚,𝐸𝐸 𝑖𝑖𝑖𝑖,𝐸𝐸 𝑉𝑉 𝐸𝐸 𝐹𝐹𝐹𝐹𝐹𝐹 𝜂𝜂𝑚𝑚,𝑃𝑃𝜂𝜂𝑦𝑦,𝑃𝑃𝜌𝜌𝑙𝑙 𝑃𝑃 and = 𝜂𝜂 denoting𝜂𝜂 𝐺𝐺 (ℎ the− mechanicalℎ ) − and isentropic efficiencies of the expander; and ηy,P are the mechanical and hydraulic efficiencies of the pump. The liquid density is denoted by ρl. The heat power exchanged between the flue gases 𝑚𝑚,𝐸𝐸 𝑖𝑖𝑖𝑖,𝐸𝐸 𝑚𝑚,𝑃𝑃 and𝜂𝜂 the fluid𝜂𝜂 of the bottoming cycle (either water or organic fluid) was calculated through𝜂𝜂 the following equation: = , (9)
w(here𝐺𝐺𝑎𝑎 + 𝐺𝐺𝑏𝑏)(ℎ7 −takesℎ8) 𝜂𝜂into𝑡𝑡ℎ,𝐻𝐻𝐻𝐻𝐻𝐻 account𝐻𝐻 𝐺𝐺𝑉𝑉 (theℎ𝐸𝐸 −thermalℎ𝐵𝐵) dispersion towards the environment. Equation 9 can be employed to determine the mass flow rate of the vapour, . It should be noted that the previous equations neglect the slight reduction 𝑡𝑡ℎ,𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 in the mass𝜂𝜂 flow rate of fuel stemming from the ash content; this assumption was made because the ash content is usually 𝑉𝑉 very low in flue gases generated by the combustion𝐺𝐺 of pruning residues [22]. The enthalpy of point B must be determined using different procedures in the two cases. When water is employed (Fig.1), hB can be assumed equal to the enthalpy of the saturated liquid exiting the condenser (hA). On the other hand, when the ORC is employed (Fig.2), the value of hB can be calculated by using the following simplified equations: AmiranteR. Amirante et al./ Energy et al. / Procedia Energy Procedia00 (2017) 129000– (2017)000 891 – 898 5 895
= , (10)
𝐵𝐵 𝐴𝐴′ 𝐹𝐹 𝐹𝐹′ ℎ −=ℎ ℎ − , ℎ (11) ℎ𝐵𝐵−ℎ𝐴𝐴′ ԑregԑ𝑟𝑟𝑟𝑟𝑟𝑟 denoting ℎ𝐵𝐵𝐵𝐵𝐵𝐵− theℎ𝐴𝐴′ heat exchange efficiency of the regenerator, and the ideal enthalpy of point B (namely, the liquid enthalpy calculated for p=pC and T=TF). The minimum temperature difference between gas and vapour, denoted by ∆Tpp, 𝐵𝐵𝐵𝐵𝐵𝐵 can be calculated in both configurations using the following relations:ℎ = (12)
𝑎𝑎 𝑏𝑏 𝑝𝑝𝑝𝑝 8 𝑆𝑆 𝐶𝐶 𝐵𝐵 (𝐺𝐺 + 𝐺𝐺 +∆T)(ℎ pp − ℎ ) 𝐺𝐺 (ℎ − ℎ ) (13) where𝑇𝑇𝑝𝑝𝑝𝑝 = suffix𝑇𝑇𝐶𝐶 pp stands for the flue gas conditions at the pinch point. Finally, the global (electrical) efficiency of the combined cycle was calculated as follows:
(14) 𝑃𝑃𝑢𝑢,𝑇𝑇𝑇𝑇+𝑃𝑃𝑢𝑢,𝐵𝐵𝐵𝐵 𝜂𝜂𝑔𝑔 = 𝐺𝐺𝑏𝑏𝐻𝐻𝑖𝑖 3. Results
The aim of this paper is to investigate the global (electrical) efficiency of the combined cycle for a small-scale application, namely for an overall useful electrical power of less than 50 kW. Considering that the useful electrical power of the bottoming cycle is expected to be a half of that of the topping cycle, the latter was fixed equal to 30 kW. The calculation of the topping cycle was therefore performed for PU,TC= 30 kW and by fixing some parameters according to the available technologies, as reported in Table 1. Specifically, the pressure at the outlet of the compressor was set to p2 = 3.5 bar (which is a value achievable by commercially available turbochargers); the temperature of the flue gas entering the gas-to-gas heat exchanger was set equal to T6=850 °C. This value represents the maximum temperature of the cycle and was fixed in order to preserve the integrity of the gas-to-heat exchanger, which, in this analysis, is supposed to be a tube-and-shell exchanger made of a high temperature material (Nichel alloy or stainless steel). The efficiency of the gas- to-heat exchanger was set to the value of εHTHE=0.8, in order to have an optimum compromise between the HTHE dimensions and the cycle efficiency (according to previous discussions [10]). The efficiency of the compressor and the two turbines were considered equal to = 0.75 and = 0.8, respectively (which are very common values for commercially available turbochargers). The mechanical efficiencies of the compressor and the turbines were set to the 𝑖𝑖𝑖𝑖,𝑐𝑐 𝑖𝑖𝑖𝑖,𝑡𝑡1 𝑖𝑖𝑖𝑖,𝑡𝑡2 value of 0.98. The thermal losses of the𝜂𝜂 topping cycle were𝜂𝜂 taking= 𝜂𝜂 into account as follows: th,HTHE = 0.95 and b = 0.9 (see equations 5 and 7 for the explanation of these terms). The lower heating value of the fuel was taken equal to i = 14000 kJ/kg, which corresponds to the lower heating value of pruning residues with 20% of humidity.𝜂𝜂 With regard𝜂𝜂 to the air temperature at the inlet of the first gas turbine (T3), it depends on T6 and εHTHE (efficiency of the HTHE). Given T6 and εHTHE, it results that T3=722.5° C, which should be a temperature level largely tolerable by turbochargers having their turbine blades made of Nichel alloys. As far as the bottoming cycles are concerned, two calculations were performed, one for the water cycle (Fig.1) and the other one for the ORC (Fig.2). With regard to the former (see Fig. 1), the commercially available “Green steam turbine”, having an estimated isentropic efficiency equal to 0.5, is proposed to be used as the water steam expander. The temperature at the inlet of the steam turbine, TE, was maintained constant and equal to 250 °C, which is the maximum temperature that the turbine can withstand according to the manufacturer specifications. The maximum water pressure was set equal to 10 bar, while the condensation pressure was considered equal to PF=PA=0.1 bar. These pressure levels are consistent with the maximum and minimum pressures required by the Green steam turbine. With regard to the ORC (Fig. 2), toluene was selected as the working fluid because its thermodynamic properties are compatible with the temperature of the flue gases exiting the HTHE. The condensation pressure and the maximum pressure level of the vapour were set equal to the values employed for the water cycle, namely 0.1 bar and 10 bar, respectively. The efficiency of the regenerator was set equal to 0.6. The isentropic efficiency of the ORC expander, which is supposed to be a single stage axial turbine, was calculated by using the method proposed by Astolfi and Macchi [23]; according to this method, and using the operating conditions of the proposed plant (size parameter=0.037m and volume ratio=75.7), a maximum isentropic efficiency of approx. 0.53 can be achieved by properly designing the turbine. In the present analysis, a conservative value of is,E=0.5 was considered for the ORC turbine. In both cases (water cycle and ORC), the thermal dispersions towards the environment were taken into account by fixing th,HRSG=0.95 (see equation 9); the𝜂𝜂 temperature difference at pinch point was taken equal to ∆Tpp=10K. It should be noted that, in both cases (water cycle and ORC), the vaporization pressure was calculated by the code so as to respect the assigned𝜂𝜂 value for ∆Tpp. Furthermore, in both cases, the temperature of the exhaust gas exiting the HRSG, T8, was set equal to 150 °C to avoid the formation of acid rain caused by the sulphur content of biomass, typically 0.1–1%. The efficiencies of both the electric generators and the electric motors moving the pumps were not considered in this analysis. With these assumptions, the code calculated the mass flow rates (Ga, Gb, GV) and the unknown enthalpies in the circuit,
6 Amirante et al./ Energy Procedia 00 (2017) 000–000 896 R. Amirante et al. / Energy Procedia 129 (2017) 891–898
the useful power of the bottoming cycle (Pu,BC), the input thermal power (Gb ) and the overall efficiency (ηg).
Table 1. Fixed parameters for the topping cycle TableHi 2. Fixed parameters for the bottoming cycles Fixed parameters Value Fixed parameters Rankine cycle ORC 30 kW Fluid Water Toluene
p2 3.5 bar TE 250 °C - 𝑃𝑃𝑢𝑢,𝑇𝑇𝑇𝑇 T6 850 °C pA 0.1 bar 0.1 bar
0.75 TA 45.83 °C 45.35 °C 0.8 0.95 0.95 𝜂𝜂𝑖𝑖𝑖𝑖,𝑐𝑐 0.98 - 0.6 𝜂𝜂𝑖𝑖𝑖𝑖,𝑡𝑡1 = 𝜂𝜂𝑖𝑖𝑖𝑖,𝑡𝑡2 𝜂𝜂𝑡𝑡ℎ,𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 HTHE 0.8 T8 150 °C 150 °C 𝜂𝜂𝑚𝑚,𝑐𝑐 = 𝜂𝜂𝑚𝑚,𝑡𝑡1 = 𝜂𝜂𝑚𝑚,𝑡𝑡2 ԑ𝑟𝑟𝑟𝑟𝑟𝑟 T3 722.5 °C 0.5 0.5 ԑ b 0.9 0.98 0.98 𝜂𝜂𝑖𝑖𝑖𝑖,𝐸𝐸 th,HTHE 0.95 0.8 0.8 𝜂𝜂 𝜂𝜂𝑚𝑚,𝐸𝐸 = 𝜂𝜂𝑚𝑚,𝑃𝑃 i 14000 kJ/kg ∆Tpp 10 °C 10 °C 𝜂𝜂 𝜂𝜂𝑦𝑦,𝑃𝑃 The calculation was performed by changing the pressure drop in line 5-9 (denoted by ∆ploss), so as to investigate the influence of ∆ploss on the plant performance. Figure 3 reports the overall efficiency of the combined cycle, as a function of the pressure drop. As expected, the increase in the pressure drop in line 5-9 causes a great decrease in the efficiency, evidencing that the components placed downstream of the power turbine need to be designed as effectively as possible as far as the associated pressure drops are concerned. However, in both cases, the overall efficiency is very high, showing that, even for large pressure drops in line 5-9, the combined cycle has a great potential, in spite of the small-scale application. Figure 3 shows that, for a given pressure drop, the overall global efficiency of the combined cycle employing toluene is higher than that of the combined cycle employing water. However, the difference is slight, highlighting that, for such a low level of electrical power and by virtue of the employment of the Green steam turbine, the bottoming water cycle is very competitive. Figure 4 reports the mass flow rates in the circuit, showing that the air mass flow was increased with the pressure drop in order to achieve the target power of 30 kWe for the topping cycle. Nevertheless, the range calculated for the air mass flow rate results to be consistent with the mass flow rate values of typical turbochargers for trucks. As expected, because of the presence of the regenerator and the absence of the superheater in the ORC, the mass flow rate of toluene is remarkably greater than that of the water.
0,23 flow rate of air Overall Efficiency (ORC) kg/s 0,22 0,8 flow rate of water Overall Efficiency (water) 0,21 flow rate of Toluene 0,2 0,6 0,19 0,4 0,18 0,17 0,2 0,16 0,15 0 0 0,05 0,1 0,15 0,2 0 0,05 0,1 0,15 0,2 Overall pressure drop [bar] Overall pressure drop [bar]
Fig. 3. Overall electr. efficiency (ηg) vs pressure drop (∆ploss) Fig. 4. Flow rates (Ga, GV) vs pressure drop (∆ploss)
It should be noted that, in both cases, the increase in the air flow rate determines a consequential increase in the flow rate of both the water and toluene. As a result, the useful electrical power increases with the pressure drop, as shown in Figure 5 which reports the electrical power generated in the bottoming cycles and the overall input thermal power. The latter increases more than proportionally with the degree of pressure losses, because the overall efficiency has a corresponding decreasing trend, as shown in Fig. 3. Figure 6 reports additional parameters, useful for the comparison, such as the vaporization pressure in the HRSG and the temperature of the flue gases entering the HRSG. It is noteworthy that, for a given pressure drop, the vaporization pressure of the water Rankine cycle is slightly lower than that of the ORC, both having a decreasing trend with the increasing pressure drop. This is because the vaporization pressure was changed to satisfy the imposed temperature difference at pinch point (10 °C) and the discharge temperature (150 °C). Finally, Fig. 7 provides the comparison between the thermodynamic diagrams of water (Fig. 7a) and toluene (Fig. 7b) for a selected case (∆ploss=0.1 bar). The difference in terms of efficiency between the bottoming ORC and the bottoming water cycle is not as high as expected, because the temperature of the flue gas exiting the topping cycle is sufficiently high to make the steam cycle competitive, in addition to the fact that the selected operating conditions do not allow a single stage ORC turbine to be Amirante et al./ Energy Procedia 00 (2017) 000–000 7 R. Amirante et al. / Energy Procedia 129 (2017) 891–898 897
designed effectively. However, the difference between the bottoming water cycle and bottoming ORC would increase if a more efficient ORC expander was employed, such as a multi stage axial turbine. In this regard, Table 3 shows that, for a selected case (∆ploss=0.1 bar), the global plant efficiency would be remarkably improved with the adoption of a two stage ORC turbine, which can achieve efficiencies as high as 70% according to the method proposed in [23]. 35 kW 30 °C bar 310,00 9 Electr. power Topping 25 8,8 Electr. power Bottoming ORC 20 308,00 8,6 Electr. power Bottoming Water 8,4 15 306,00 8,2 10 8
304,00 7,8 5 0 0,05 0,1 0,15 0,2 7,6 Gas temperature entering the HRSG kW 302,00 7,4 300 Input thermal power Vaporization pressure (ORC) 7,2 250 Vaprorization pressure (water) 300,00 7 200 0 0,05 0,1 0,15 0,2 150 Overall pressure drop [bar] 0 0,05 0,1 0,15 0,2 Overall pressure drop [bar]
Fig. 5. Electrical power and input thermal power vs pressure drop (∆ploss) Fig. 6. Temperature of the gas entering the HRSG (T7) and
vaporization pressure (pC), vs pressure drop (∆ploss)
450 450 T [°C] T[°C] 8.11 bar
7.73 bar 300 300 E 0.1 bar C D C D 0.1 bar 150 B 150 F
F' F A A 0 0 0 2 4 6 8 10 -1,5 -1 -0,5 0 0,5 1 1,5 S [kJ/kg] S [kJ/kg] Fig.7. Thermodynamic diagrams (temperature-entropy): water (left) and toluene (right)
Table 3. Expander isentropic efficiency, electrical power and plant efficiency as a function of the type of expander (∆ploss=0.1 bar)
Expander Isentropic efficiency Electrical power bottoming cycle Plant efficiency (electrical) Water steam turbine 50% 10.60 kW 18% Single stage ORC turbine 50% 12.65 kW 19% Two stage ORC turbine 70% 17.37 kW 21%
4. Conclusions
The aim of this paper is to investigate a novel, small scale, combined cycle for energy generation from biomass, by exploring its thermodynamic feasibility for very low electrical power generated (less than 50 kW) and in a full electrical power generation mode. The thermodynamic analysis regarded both the topping cycle and the bottoming one. Concerning the former, it was analysed with the hypothesis of being mainly composed of a cheap turbocharger from the automotive industry (instead of a more expensive commercial micro-turbine), an external combustor, a gas-to-gas heat exchanger and a power turbine moving an electric generator. Two configurations were analysed for the bottoming cycle, the first one being a water Rankine cycle (employing the highly efficient “green steam turbine”) and the second one being an Organic Rankine cycle (employing toluene because of the temperature levels). The results have shown that the combined cycle, in spite of the economic solutions and the very low electrical power, can achieve very competitive levels of global efficiency (up to 22%). The comparison between the water Rankine cycle and the ORC, performed by employing a single stage turbine, has shown that the global efficiency achieved by the latter
8 Amirante et al./ Energy Procedia 00 (2017) 000–000 898 R. Amirante et al. / Energy Procedia 129 (2017) 891–898
is slightly greater than that of the former. This results from the fact that toluene better adapts to the temperatures of the combined cycle. The difference was not as high as expected because of the high temperature of the flue gas exiting the topping cycle and because, for the selected operating conditions, the isentropic efficiency of a single stage axial ORC turbine is limited to about 50%. However, it was demonstrated that the use of a two-stage ORC turbine with an isentropic efficiency of 70% can allow a remarkable improvement in the plant performance. Forthcoming studies will investigate the comparison between the bottoming water cycle and the bottoming ORC in the case of a CHP arrangement, in which the use of an ORC is expected to be more profitable compared to the full electrical generation case study.
Acknowledgments
The research presented in this work is co-funded by “Fondo di Sviluppo e Coesione 2007-2013 – APQ Ricerca Regione Puglia, Programma regionale a sostegno della specializzazione intelligente e della sostenibilità sociale ed ambientale - FutureInResearch”
References
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