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Exploration and Analysis of CO2 + Hydrocarbons Mixtures as Working Fluids for Trans-critical ORC
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Exploration and Analysis of CO2 + Hydrocarbons Mixtures as Fluidsoffor ORCMixtures as ExplorationWorking and Analysis COTrans-critical + Hydrocarbons 2
b a aand Cooling TheFeng 15tha,International Symposium on District Heating Working Fluids for Trans-critical ORC Lejun Danxing Zheng , Jing Chen , Xiaoye Dai , Lin Shia,*
Assessing the feasibility using heat Dai demand-outdoor Lejun Feng , Danxing Zhengof , Jing Chenthe , Xiaoye , Lin Shi * temperature function for a long-term district heat demand forecast a a a, Key Laboratory for Thermal Science and Power Engineeringb of Ministry of Education, Department ofaThermal Engineering, Tsinghua University, Beijing 100084, P. R. China b College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China a Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, P. R. China b a a b c P. R. China c Collegea,b,c of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, a
Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
b Veolia Recherche 291 Avenue Dreyfous Daniel, 78520 Limay,and France ORCs (Organic Rankine Cycles) are&ofInnovation, great significance for energy conservation CO 2 emission reductions c Abstract Département Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler, France owing to their usage in converting low grade waste heat to electrical power. The working 44300 fluid Nantes, properties are the key and hydrocarbon working fluids for use in ORCs are to getting good cycle performance. The advantages of CO 2 ORCs (Organic Rankine Cycles) are of great significance for energy conservation and CO 2 emission reductions +propane/n-butane/iso-butane/n-pentane/iso-pentane/neo-pentane. A investigated here for six binary mixtures: CO 2 owing to their usage in converting low grade waste heat to electrical power. The working fluid properties are the key transcritical ORC simulation program was used to predict the cycle performance for various hydrocarbon mass toAbstract getting good cycle performance. The advantages of CO2 and hydrocarbon working fluids for use in ORCs are fraction working a turbine inlet temperature of 453.15 K and inlet pressure of 1.3pc. The predicted cycle A investigated here fluids for sixforbinary mixtures: CO2+propane/n-butane/iso-butane/n-pentane/iso-pentane/neo-pentane. performance is used to divide the six working fluids into three categories to thermal efficiency of anthe District heating networks are commonly literature one performance of therelative most effective solutions for decreasing transcritical ORC simulation program addressed was used intothe predict the as cycle forthe various hydrocarbon mass The working fluidThese type systems (CO has increasing increasing ORC using pureemissions CO2. for 2+propane) greenhouse gas from the building require high investments arecreturned through the heat . The with predicted cycle fraction working fluids a first turbine inlet sector. temperature of 453.15 K and inlet pressurewhich ofefficiencies 1.3p hydrocarbon mass fraction while the second type have maximum thermal efficiencies at hydrocarbon mass fractions sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, performance is used to divide the six working fluids into three categories relative to the thermal efficiency of an ofprolonging 0.3,using andthe the third has maximums at a type mass(CO fraction of 0.2. In addition, the thermodynamics relative investment return period. The first working fluid ORC pure CO2. type 2+propane) has increasing efficiencies with increasing , is highest for CO +propane and then decreases for CO + n-butane, CO2mass +neo-pentane, efficiency, The main scope of this paper is to assess the feasibility of using the heat demand – outdoorCO temperature function for heat demand 2 2+iso-butane, 2at hydrocarbon mass fraction while the second type have maximum thermal efficiencies hydrocarbon fractions forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 +iso-pentane, and CO +n-pentane with the lowest. Then, the cycle efficiencies of the three working fluid types CO 2 2 of 0.3, and the third type has maximums at a mass fraction of 0.2. In addition, the thermodynamics relative buildings that vary in the bothperspective constructionofperiod and typology. molecular Three weather scenarios (low, medium, high) and three main district were analyzed from the hydrocarbon structure. The results show that longer efficiency, , is highest for CO2+propane and then decreases for CO2+iso-butane, CO2+ n-butane, CO2+neo-pentane, renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were and T , with this critical property leading to higher carbon chains and branched carbon chains have larger p c the cycle c efficiencies of the three working fluid types CO2+iso-pentane, and CO2+n-pentane with the lowest. Then, compared with results from a dynamic heat demand model, previously developed and validated by the authors. and T lead to higher efficiencies at high hydrocarbon efficiencies at low hydrocarbon mass fractions. Lower p cmolecular c were analyzed fromthat thewhen perspective of the hydrocarbon structure. The results show for thatsome longer main The results showed weather change is considered, the margin and of error be acceptable applications concentrations. Thus, there isonly a trade-off between the cycle efficiency the could hydrocarbon massleading fraction.to Finally, and T , with this critical property higher carbon chains and branched carbon chains have larger p c scenarios c considered). However, after introducing renovation (thethree errorbest in annual demand wasmixtures, lower than 20% for all weather the working fluid 0.3CO 2/0.7propane, 0.7CO2/0.3neo-pentane and 0.8CO2/0.2pentane, were c and Tc leadand to renovation higher efficiencies at high hydrocarbon efficiencies at error low hydrocarbon mass fractions. Lower pon scenarios, the value increased up to 59.5% (depending the weather scenarios combination mixture working fluid hadconsidered). the best further analyzedThus, according to their cycle characteristics. The efficiency 0.3CO2/0.7propane concentrations. there is a trade-off between the cycle and the hydrocarbon mass Finally, The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, thatfraction. corresponds to the cycle efficiency for these conditions. the three best working fluid mixtures, 0.3CO /0.7propane, 0.7CO /0.3neo-pentane and 0.8CO /0.2pentane, were
2 during the heating season 2 2 decrease in the number of heating hours of 22-139h (depending on the combination of weather and
/0.7propane mixture working fluid had the on bestthe further according their characteristics. The 0.3COincreased scenarios considered). Oncycle the other hand, function intercept for 7.8-12.7% per decade (depending ©renovation 2017 analyzed The Authors. Published by Elsevier Ltd. Keywords: transcritical ORC; to hydrocarbons+ CO efficiency; fraction 2mass 2; thermal Peer-review underfor responsibility ofsuggested the scientific of the IV International on ORC Power Systems. cycle efficiency these conditions. coupled scenarios). The values couldcommittee be used to modify the functionSeminar parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
Keywords: transcritical ORC; hydrocarbons+ CO2; thermal efficiency; * Corresponding author. Tel.: +86-10-62787613; fax: +86-13910338306. E-mail © 2017address:[email protected] The Authors. Published by Elsevier Ltd.
mass fraction
under responsibility the Scientific Committee of The 15th International Symposium on District Heating and ©*Peer-review 2017 The Authors. Published byof Elsevier Ltd. Corresponding author. Tel.: +86-10-62787613; fax: +86-13910338306. Cooling. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. E-mail address:[email protected] © 2017 The Authors. Published by Elsevier Ltd. Ltd. Keywords: demand; Forecast; Climate 1876-6102 ©Heat 2017 The Authors. Published bychange Elsevier Peer-review under responsibility ofscientific the scientific committee ofInternational the IV International on ORC Power Systems. Peer-review under responsibility of the committee of the IV Seminar onSeminar ORC Power Systems. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 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. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 10.1016/j.egypro.2017.09.191
146 2
Lejun Feng et al. / Energy Procedia 129 (2017) 145–151 Feng LJ, Zheng DX, Chen J, Shi L / Energy Procedia 00 (2017) 000–000
1. Introduction Nomenclature m mass flow rate (kgs-1) Acronyms h specific enthalphy (kJkg-1) ORC organic Rankine cycle s specific entropy (kJkg-1K-1) HFCs hydrofluorocarbons ws output work for TUR (kW) CFCs chlorofluoro carbons wp output work for PUPM (kW) HCFCs hydro-chlorofluorocarbons q(TH) the input heat duty (kW) HCs hydrocarbons pTUR turbine inlet pressure (bar) ODP ozone depletion potential pOTUR turbine outlet pressure (bar) GWP Global warming potential TH average heat source temperature (K) Subscripts TL average cooling source temperature (K) i, j, k the material and energy flow Tb boiling point temperature (K) c critical Tc critical temperature (K) b boiling pc critical pressure (bar) ref reference Greek symbols th thermal th thermal efficiency (-) TUR turbine thermodynamics consummating degree (-) acentric factor Introduction ORCs (Organic Rankine Cycle) driven by waste heat are key technology for improving energy efficiencies [1]. Two impact factors have been reported to influence the cycle performance: working fluids and system configurations [2]. There are hundreds of possible ORC working fluids with extensive research on halohydrocarbon working fluids, including HFCs (hydrofluorocarbons), CFCs (chlorofluorocarbons) and HCFCs (hydro-chlorofluorocarbons). Although the tradition working fluids give good thermodynamic efficiencies, their negative environmental impact has limited their applications. Thus, there have been many studies searching for environmentally friendly working fluids that also give high thermodynamic efficiencies [3-5]. In addition, ORCs have many possible cycle configurations that can be divided into subcritical, transcritical and supercritical systems [6-8]. The best configuration often depends on the heat and sink source temperatures, the thermophysical properties and the working fluid behaviour. The major difference for subcritical, transcritical and surpercritical ORC systems is the working fluid heating process, as shown in Fig. 1. In surpercritical and transcritical ORC, the working fluid is heated directly from the liquid state to the supercritical state, bypassing the two-phase region (34). Compared with the subcritical ORC, surpercritical and transcritical ORC have a better thermal matching with the heat source, which resulting in less exergy loss [6]. However, the system will need a higher operating pressure for surpercritical ORC. Hence, the transcritical operating model was adopted in this work. Moreover, the choice of a working fluid is of key importance for a transcritical ORC [9, 10]. For pure working fluids, as shown in Fig. 1a, the constant evaporation and condensation temperature during the isothermal evaporation and condensation processes does not accurately match the temperature changes in the heat source and heat sink in the heat exchangers, which increases the irreversibilities. Thus, the traditional ORC system design with pure working fluids is not suitable for sensible heat sources. Unlike with pure working fluids, working fluid mixtures in a transcritical ORC can be heated directly from liquid to a supercritical vapor and condensed back to a liquid along variable temperature paths as shown in Fig. 1b, which results in better thermal matches with the source and sink fluids in the gas heater/condenser and reduces the exergy destruction in the heating and condensation processes[6]. Carbon dioxide (CO2) is an environmentally friendly natural working fluid with zero ODP (ozone depletion potential) and a negligible GWP (global warming potential), and it is non-flammable and non-toxic. In addition, CO2 has favourable thermodynamic and transport properties, so it has been extensively studied as a supercritical working fluid for ORC systems [13-15]. However, the disadvantage of CO2 is its low critical temperature, which limits the use of CO2 with many higher cooling source temperatures. Another disadvantage is its high critical pressure. Many researchers have tried to reduce the operating pressure for CO2 ORC syste ms by adding high critical temperature working fluids to the CO 2[16,17].
Lejun Feng et al. / Energy Procedia 129 (2017) 145–151 Feng LJ, Zheng DX, Chen J, Shi L / Energy Procedia 00 (2017) 000–000
(a) Pure working fluids
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(b) Mixture working fluids
Fig 1. The T-s diagram for the working fluid of ORC at different cycle configurations
Besides CO2, hydrocarbons are also environmentally-friendly natural working fluids with relatively high critical temperatures and low critical pressures, so they have also been investigated for use in ORC systems [18,19]. However, some of these hydrocarbon working fluids are flammable which limits their use. Zabetakis[20, 21] conducted experiments with propane and n-pentane in CO2 because these mixtures are not flammable for mixtures with CO 2 mass fractions greater than 0.3. However, the optimal composition of CO2 and HCs mixtures need to further explore. The present study analyzes mixtures of CO2 with six hydrocarbons, propane, n-butane, iso-butane, n-pentane, isopentane, and neo-pentane with various mass fractions. The analyses were based on a transcritical ORC simulation model that predicted the cycle efficiencies of the CO2 + hydrocarbon working fluids. 2. Selection of working fluids The candidate hydrocarbons were selected based on the number of carbon atoms and their branched chain structure. The hydrocarbons should have high critical temperatures and low critical pressures, so methane and ethane with critical temperatures close to CO 2, were not considered. Hydrocarbons with more than 5 carbon atoms have low cycle efficiencies; hence, only short carbon chain molecules were considered. Six hydrocarbons were tested, n-butane, iso-butane, n-pentane, iso-pentane, and neo-pentane, as additives in CO2 mixtures. The cycle efficiencies of these six mixtures were analyzed for various hydrocarbon mass fractions. The basic thermophysical properties of the working fluids are listed in Table 1, with all the data obtained from NIST REFPROP 9.1[22]. Table 1. Thermophysical properties of the working fluids Working fluids
molecular formula
molecular mass (g·mol-1)
Tb (K)
Tc (K)
pc (bar)
CO2 propane n-butane iso-butane n-pentane iso-pentane neo-pentane
CO2 CH3CH2CH3 CH3CH2CH2CH3 CH(CH3)3 CH3(CH2)3CH3 CH3CH2CH(CH3)2 C(CH3)4
44.01 44.10 58.12 58.12 72.15 72.15 72.15
194.69 231.04 272.66 261.40 309.21 300.98 282.65
304.13 369.89 425.13 407.81 469.70 460.35 433.74
73.77 42.51 37.96 36.29 33.70 33.78 31.96
cp (Jmol-1K-1) 298.15 K 37.24 73.47 98.35 96.51 119.80 118.65 120.60
0.22 0.15 0.20 0.18 0.25 0.23 0.20
3. Simulation and calculation 3.1. Parameters and basic assumptions The basic cycle parameters are listed in Table 2 with the following assumptions to analyze the transcritical ORC: (1) The system operates at steady state. (2) The fluid flowing inside the heat exchangers and pipes is isobaric, and the heat and friction losses are neglected. (3) The working fluid is a saturated liquid at the condenser outlet. (4) The vapor quality of the fluid exhausting the turbine is set to not less than 0.9 to avoid droplet erosion [6]. (5) The heat transfer fluids for the heat source and heat sink are low-grade gas and water, respectively. (6) The CO2 mass fraction rangewas between 0.3-0.9 to eliminate the flammability of hydrocarbons [20, 21]. (7) The input variable is the turbine inlet pressure.
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Table 2. Major cycle parameters for the simulations [16, 23, 24] Parameters Efficiencies Turbine isentropic efficiency/Pump isentropic efficiency/Electromechanical efficiency/Pump motor efficiency/Heat exchangers Heater pinch temperature difference/K Condenser pinch temperature difference/K Cooling water outlet temperature/K Others Turbine inlet pressure/bar Turbine inlet temperature/ K Working fluid mass flow rate/kgs-1
values 0.75 0.8 0.95 0.85 35 10 293.15 1.3 pc 453.15 1
3.2. Thermodynamic model Peng-Robinson (PR) equation is the most used cubic equation of state. PR equation has a more accurate description, high reliability and less parameters to the vapor-liquid equilibrium under high pressure. Hence, the Peng-Robinson equation was used here to describe the vapor-liquid equilibrium of the working fluids. 3.3. Calculational method and cycle performance evaluation The mass balance, energy balance, and vapor-liquid equilibrium of each unit in a transcritical ORC is written as: inout mi 0 (1) i
inout mi hi inout Q j inout Wk 0 i k j
(2)
f p, T , x 0
(3)
where i, j, k represent the two mixture components and the energy flow, m represents the mass flow rate and h represents the enthalpy. The cycle efficiency can be written as: w -w (4) th s p q TH where ws is the turbine work output, wp work absorbed by the pump, and q(TH) is the input heat. The thermodynamic efficiency relative to the second law of thermodynamics is given by the ratio of the thermal efficiency of the actual ORC cycle and that of an ideal Carnot cycle :
th
1 TL
(5)
TH
where TL and TH are the average temperature for the heat and cooling source. 3.4. Model validation Table 3. Simulation results comparison for different working fluids a a Working fluids TTUR /K pTUR th (%) /bar [6,16 25] Reference This work CO2 453.00 78.80 0.040 0.037 CO2 453.00 343.80 0.150 0.148 butane 411.15 30.15 0.155 0.154 iso-butane 394.15 28.86 0.138 0.137 pentane 459.15 28.65 0.184 0.185 0.5CO2/0.5R161 423.15 94.60 0.099 0.095 a TTUR and pTUR represent the TUR inlet temperature and pressure, respectively b Relative deviation of the thermal efficiency (δth): δth% = (th,this work -th,ref /th,ref) 100
th,this work -th,ref δth% 7.50 1.33 0.65 0.72 0.54 4.04
b
0.003 0.002 0.001 0.001 0.001 0.004
The model was verified be comparing the model predictions for five working fluids, CO 2, butane, iso-butane, pentane and 0.5CO2/0.5R161 with previous simulation results [6, 16, 25]. The differences were within 1% for most cases with the largest relative deviation of 7.50% for pure CO 2 at a turbine inlet pressure of 78.80 bar. The large
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relative deviations “7.50%” and “4.04%” is due to their small thermal efficiency, which is 0.04 and 0.099 respectively. However, their absolute deviation is only 0.003-0.004, this reflect the validation of this work from another aspect. The results are shown in Table 3. 4. Results and discussion
th and are given for various hydrocarbon mass fractions for each working fluid mixture and pure CO 2 in the transcritical ORC in Fig. 3 for a turbine inlet temperature of 453.15 K and a turbine inlet pressure of 1.3 pc. The mass fractions of the hydrocarbons added to the CO2 ranged from 0.1 to 0.7 since these mixtures are not flammable. The cycle efficiency increases with increasing hydrocarbon mass fraction for only the CO 2+propane system, the efficiencies of the other five working fluids all first increasing and then decreasing with increasing hydrocarbon mass fraction, so these five curves all had a maximum th, as shown in Fig. 2a. Hence, the six working fluid mixtures can be divided into three categories based on the th. th of the first kind of working fluids increases with increasing hydrocarbon mass fraction, with this kind of working fluid represented by CO 2+propane. The maximum th is 7.50% when the mass composition of the mixtures is 0.3CO2/0.7 propane. The second type of working fluid mixture are those whose th are higher than those of pure CO2 over the whole hydrocarbon mass fraction range which include CO2+iso-butane and CO2/neo-pentane as shown in Fig. 2a. The efficiencies of the neopentane/CO2 mixture are higher than those of the iso-butane/CO2 mixture with both having maximum efficiencies at a hydrocarbon mass fraction of 0.3. The third kind of working fluids have efficiencies that are higher than those of pure CO2 at lower hydrocarbon mass fractions, but then become lower than those of CO2 in high hydrocarbon mass fractions. These working fluids include CO2+n-butane, CO2+iso-pentane and CO2+n-pentane. The cycle efficiencies for these three working fluid mixtures are highest at a hydrocarbon mass fraction of 0.2. In addition, for these three mixtures, th is highest for CO2+n-pentane followed by CO2+iso-pentane and CO2+n-butane in low hydrocarbon mass fractions, and with the reversed order for higher mass fractions.
Fig 2.Transcritical ORC thermal efficiencies (a) and relative efficiencies (b) for various mixtures for a turbine inlet temperature of 453.15 K and a turbine inlet pressure of 1.3 pc.
The relative thermal efficiencies, , relative to the Carnot cycle shown in Fig. 2b change with the hydrocarbon mass fraction. The relative cycle efficiencies mostly decrease with increasing hydrocarbon mass fraction with the smaller hydrocarbon mass fractions having less irreversibilities, which suggests that the hydrocarbons cause more irreversibilities than pure CO2. The relative efficiencies are higher for the CO2+propane mixture followed by CO2+iso-butane, CO2+ n-butane, CO2+neo-pentane, CO2+iso-pentane and CO2+n-pentane. The molecular structures of the hydrocarbons shown in Fig. 3 show that the main chains of the first and second kinds of working fluids have 3 carbon atoms, while the third kind of fluids have 4 or 5 carbon atoms. The first category working fluids (CO2+propane) have the fewest main chain carbon atoms, and this mixture have the smallest molecular volume. Hence, the intermolecular force is weak and lead to smaller pc and Tc, pc and Tc are the critical pressure and temperature of the CO2/hydrocarbons mixtures, respectively. The higher HCs mass composition will lead larger pc and Tc, and the working fluid mixtures more easily condense and have better match with the heat source, which will directly obtain larger th and . Therefore, the th is smaller than pure CO2 when the HCs mass composition is smaller, and then larger than pure CO 2 along with the increasing HCs mass composition. For the second type working fluids, the main chain carbon atoms is 3 and have the branched chain. The branched
Lejun Feng et al. / Energy Procedia 129 (2017) 145–151 Feng LJ, Zheng DX, Chen J, Shi L / Energy Procedia 00 (2017) 000–000
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chain enhanced the molecular volume and intermolecular force, the more branched chain, the larger pc and Tc. Hence, theth of CO2+neo-pentane system larger than CO2+iso-butane, but is opposite. For the third category working fluids, there is a trade-off relation between the intermolecular force and the HCs mass composition. When the HCs mass composition is smaller, the more main chain carbon atoms and branched chain will lead to a higher th. However, the vary tendency is opposite when the HCs mass composition is larger. The influence of HCs mass composition prevail over the molecular structure.
Fig 3. The molecular structure of HCs
The potential of using these working fluid mixtures was further evaluated for the three working fluids with the highest thermal efficiency for each kind working fluid, 0.3CO2/0.7propane, 0.7CO2/0.3neo-pentane, and 0.8CO2/0.2pentane. The power generated and the cycle efficiencies of these three working fluid mixtures in a transcritical ORC are compared with those of pure CO2 in Table 4. Th e turbine inlet temperature was 453.15 K and the turbine inlet pressure was 1.3 pc. Table 4 Cycle efficiencies of CO2 and CO2+hydrocarbons as the working fluids in a transcritical ORC Tevaporator (K) working fluids pTUR/ pOTURa CO2 461.75 1.67 0.3CO2/0.7propane 377.58 3.04 0.7CO2/0.3neo-pentane 397.13 2.04 0.8CO2/0.2pentane 394.60 1.67 a pOTUR represents the turbine outlet pressure
q(H)/ (kW) 332.65 487.69 380.35 350.70
wp (kW) 7.27 13.54 13.34 13.56
ws (kW) 26.52 50.14 40.90 40.25
th/% 5.79 7.50 7.25 7.61
/% 56.51 62.94 60.49 61.10
Table 4 lists the operating pressure ratio, the evaporator temperature and heat duty, pump work, the turbine output work, th and. The turbine output power is higher for all three mixtures than for pure CO2, and th is increased by 29.5% for 0.3CO2/0.7propane, 25.22% for 0.7CO2/0.3neo-pentane and 31.43% for 0.8CO2/0.2pentane. The hydrocarbons increase the cycle pressure ratio and reduce the operating pressure of the transcritical ORC which makes the cycle which allows more flexible operating conditions. th and are highest for the 0.3CO2/0.7propane mixture. Thus, the CO2+hydrocarbon working fluid mixture mentioned will give significantly better operating efficiencies in transcritical ORCs. 5. Conclusion CO2 is the potential working fluid for ORC. However, its low critical temperature and high critical pressure limited its develop. HCs were selected as the additive for CO2 to improve its thermophysical property. The potential CO2+HCs mixture working fluids were explored in this work, and some conclusions are obtained as follows: (1) The transcritical ORC was modelled using a cycle simulation program. (2) The binary CO2+hydrocarbon system uses the advantages of CO2 and the hydrocarbon as the working fluid in a transcritical ORC. The cycle efficiencies were calculated for six working fluid mixtures: CO2+propane, n-butane, iso-butane, n-pentane, iso-pentane and neo-pentane.
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(3) The six working fluids were divided into three categories based on their cycle efficiencies relative to pure CO2. The efficiency for the CO2+propane system increased with increasing propane concentration, while the thermal efficiencies of the other five working fluid mixtures all increased to a maximum and then decreased with increasing hydrocarbon concentration. The relative thermodynamic efficiency relative to the Carnot efficiency was highest for CO2+propane, followed by CO2/iso-butane, CO2/ n-butane, CO2/neo-pentane, CO2/so-pentane, and CO2/n-pentane. (4) Comparisons the molecular structure of the hydrocarbons showed that the hydrocarbons with longer main chains or with branched chain with the same number of main chain carbon atoms have larger pc and Tc, which lead to higher thermal efficiencies at low hydrocarbon mass fractions. However, with high concentrations of the hydrocarbons in the mixture, the thermal efficiency and relative efficiency are higher with lower pc and Tc, so there is a trade-off between the thermal efficiency, the relative thermal efficiency and the hydrocarbon mass fraction. (5) Compared the cycle performance of 0.3CO2/0.7propane, 0.7CO2/0.3neo-pentane and 0.8CO2/0.2pentane with the pure CO2, Overall consideration, 0.3CO2/0.7propane have a better cycle performance. Acknowledgements This work was supported by the Science Fund for the National Key Research and Development Program of China (No. 2016YFB0901405) and for Creative Research Groups (No. 51621062). References [1] Quoilin S, Broek M. Techno-economic survey of organic Rankine cycle (ORC) systems. Renew Sust Energ Rev 2013;22:168-186. [2] Bao JJ, Zhao L. A review of working fluid and expander selections for organic Rankine cycle. Renew Sust Energ Rev 2013;24:325-342. [3] Wang EH, Zhang HG. 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Exploration and Analysis of CO2 + Hydrocarbons Mixtures as Fluidsoffor ORCMixtures as ExplorationWorking and Analysis COTrans-critical + Hydrocarbons 2
b a aand Cooling TheFeng 15tha,International Symposium on District Heating Working Fluids for Trans-critical ORC Lejun Danxing Zheng , Jing Chen , Xiaoye Dai , Lin Shia,*
Assessing the feasibility using heat Dai demand-outdoor Lejun Feng , Danxing Zhengof , Jing Chenthe , Xiaoye , Lin Shi * temperature function for a long-term district heat demand forecast a a a, Key Laboratory for Thermal Science and Power Engineeringb of Ministry of Education, Department ofaThermal Engineering, Tsinghua University, Beijing 100084, P. R. China b College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China a Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, P. R. China b a a b c P. R. China c Collegea,b,c of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, a
Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
b Veolia Recherche 291 Avenue Dreyfous Daniel, 78520 Limay,and France ORCs (Organic Rankine Cycles) are&ofInnovation, great significance for energy conservation CO 2 emission reductions c Abstract Département Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler, France owing to their usage in converting low grade waste heat to electrical power. The working 44300 fluid Nantes, properties are the key and hydrocarbon working fluids for use in ORCs are to getting good cycle performance. The advantages of CO 2 ORCs (Organic Rankine Cycles) are of great significance for energy conservation and CO 2 emission reductions +propane/n-butane/iso-butane/n-pentane/iso-pentane/neo-pentane. A investigated here for six binary mixtures: CO 2 owing to their usage in converting low grade waste heat to electrical power. The working fluid properties are the key transcritical ORC simulation program was used to predict the cycle performance for various hydrocarbon mass toAbstract getting good cycle performance. The advantages of CO2 and hydrocarbon working fluids for use in ORCs are fraction working a turbine inlet temperature of 453.15 K and inlet pressure of 1.3pc. The predicted cycle A investigated here fluids for sixforbinary mixtures: CO2+propane/n-butane/iso-butane/n-pentane/iso-pentane/neo-pentane. performance is used to divide the six working fluids into three categories to thermal efficiency of anthe District heating networks are commonly literature one performance of therelative most effective solutions for decreasing transcritical ORC simulation program addressed was used intothe predict the as cycle forthe various hydrocarbon mass The working fluidThese type systems (CO has increasing increasing ORC using pureemissions CO2. for 2+propane) greenhouse gas from the building require high investments arecreturned through the heat . The with predicted cycle fraction working fluids a first turbine inlet sector. temperature of 453.15 K and inlet pressurewhich ofefficiencies 1.3p hydrocarbon mass fraction while the second type have maximum thermal efficiencies at hydrocarbon mass fractions sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, performance is used to divide the six working fluids into three categories relative to the thermal efficiency of an ofprolonging 0.3,using andthe the third has maximums at a type mass(CO fraction of 0.2. In addition, the thermodynamics relative investment return period. The first working fluid ORC pure CO2. type 2+propane) has increasing efficiencies with increasing , is highest for CO +propane and then decreases for CO + n-butane, CO2mass +neo-pentane, efficiency, The main scope of this paper is to assess the feasibility of using the heat demand – outdoorCO temperature function for heat demand 2 2+iso-butane, 2at hydrocarbon mass fraction while the second type have maximum thermal efficiencies hydrocarbon fractions forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 +iso-pentane, and CO +n-pentane with the lowest. Then, the cycle efficiencies of the three working fluid types CO 2 2 of 0.3, and the third type has maximums at a mass fraction of 0.2. In addition, the thermodynamics relative buildings that vary in the bothperspective constructionofperiod and typology. molecular Three weather scenarios (low, medium, high) and three main district were analyzed from the hydrocarbon structure. The results show that longer efficiency, , is highest for CO2+propane and then decreases for CO2+iso-butane, CO2+ n-butane, CO2+neo-pentane, renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were and T , with this critical property leading to higher carbon chains and branched carbon chains have larger p c the cycle c efficiencies of the three working fluid types CO2+iso-pentane, and CO2+n-pentane with the lowest. Then, compared with results from a dynamic heat demand model, previously developed and validated by the authors. and T lead to higher efficiencies at high hydrocarbon efficiencies at low hydrocarbon mass fractions. Lower p cmolecular c were analyzed fromthat thewhen perspective of the hydrocarbon structure. The results show for thatsome longer main The results showed weather change is considered, the margin and of error be acceptable applications concentrations. Thus, there isonly a trade-off between the cycle efficiency the could hydrocarbon massleading fraction.to Finally, and T , with this critical property higher carbon chains and branched carbon chains have larger p c scenarios c considered). However, after introducing renovation (thethree errorbest in annual demand wasmixtures, lower than 20% for all weather the working fluid 0.3CO 2/0.7propane, 0.7CO2/0.3neo-pentane and 0.8CO2/0.2pentane, were c and Tc leadand to renovation higher efficiencies at high hydrocarbon efficiencies at error low hydrocarbon mass fractions. Lower pon scenarios, the value increased up to 59.5% (depending the weather scenarios combination mixture working fluid hadconsidered). the best further analyzedThus, according to their cycle characteristics. The efficiency 0.3CO2/0.7propane concentrations. there is a trade-off between the cycle and the hydrocarbon mass Finally, The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, thatfraction. corresponds to the cycle efficiency for these conditions. the three best working fluid mixtures, 0.3CO /0.7propane, 0.7CO /0.3neo-pentane and 0.8CO /0.2pentane, were
2 during the heating season 2 2 decrease in the number of heating hours of 22-139h (depending on the combination of weather and
/0.7propane mixture working fluid had the on bestthe further according their characteristics. The 0.3COincreased scenarios considered). Oncycle the other hand, function intercept for 7.8-12.7% per decade (depending ©renovation 2017 analyzed The Authors. Published by Elsevier Ltd. Keywords: transcritical ORC; to hydrocarbons+ CO efficiency; fraction 2mass 2; thermal Peer-review underfor responsibility ofsuggested the scientific of the IV International on ORC Power Systems. cycle efficiency these conditions. coupled scenarios). The values couldcommittee be used to modify the functionSeminar parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
Keywords: transcritical ORC; hydrocarbons+ CO2; thermal efficiency; * Corresponding author. Tel.: +86-10-62787613; fax: +86-13910338306. E-mail © 2017address:[email protected] The Authors. Published by Elsevier Ltd.
mass fraction
under responsibility the Scientific Committee of The 15th International Symposium on District Heating and ©*Peer-review 2017 The Authors. Published byof Elsevier Ltd. Corresponding author. Tel.: +86-10-62787613; fax: +86-13910338306. Cooling. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. E-mail address:[email protected] © 2017 The Authors. Published by Elsevier Ltd. Ltd. Keywords: demand; Forecast; Climate 1876-6102 ©Heat 2017 The Authors. Published bychange Elsevier Peer-review under responsibility ofscientific the scientific committee ofInternational the IV International on ORC Power Systems. Peer-review under responsibility of the committee of the IV Seminar onSeminar ORC Power Systems. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 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. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 10.1016/j.egypro.2017.09.191
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Lejun Feng et al. / Energy Procedia 129 (2017) 145–151 Feng LJ, Zheng DX, Chen J, Shi L / Energy Procedia 00 (2017) 000–000
1. Introduction Nomenclature m mass flow rate (kgs-1) Acronyms h specific enthalphy (kJkg-1) ORC organic Rankine cycle s specific entropy (kJkg-1K-1) HFCs hydrofluorocarbons ws output work for TUR (kW) CFCs chlorofluoro carbons wp output work for PUPM (kW) HCFCs hydro-chlorofluorocarbons q(TH) the input heat duty (kW) HCs hydrocarbons pTUR turbine inlet pressure (bar) ODP ozone depletion potential pOTUR turbine outlet pressure (bar) GWP Global warming potential TH average heat source temperature (K) Subscripts TL average cooling source temperature (K) i, j, k the material and energy flow Tb boiling point temperature (K) c critical Tc critical temperature (K) b boiling pc critical pressure (bar) ref reference Greek symbols th thermal th thermal efficiency (-) TUR turbine thermodynamics consummating degree (-) acentric factor Introduction ORCs (Organic Rankine Cycle) driven by waste heat are key technology for improving energy efficiencies [1]. Two impact factors have been reported to influence the cycle performance: working fluids and system configurations [2]. There are hundreds of possible ORC working fluids with extensive research on halohydrocarbon working fluids, including HFCs (hydrofluorocarbons), CFCs (chlorofluorocarbons) and HCFCs (hydro-chlorofluorocarbons). Although the tradition working fluids give good thermodynamic efficiencies, their negative environmental impact has limited their applications. Thus, there have been many studies searching for environmentally friendly working fluids that also give high thermodynamic efficiencies [3-5]. In addition, ORCs have many possible cycle configurations that can be divided into subcritical, transcritical and supercritical systems [6-8]. The best configuration often depends on the heat and sink source temperatures, the thermophysical properties and the working fluid behaviour. The major difference for subcritical, transcritical and surpercritical ORC systems is the working fluid heating process, as shown in Fig. 1. In surpercritical and transcritical ORC, the working fluid is heated directly from the liquid state to the supercritical state, bypassing the two-phase region (34). Compared with the subcritical ORC, surpercritical and transcritical ORC have a better thermal matching with the heat source, which resulting in less exergy loss [6]. However, the system will need a higher operating pressure for surpercritical ORC. Hence, the transcritical operating model was adopted in this work. Moreover, the choice of a working fluid is of key importance for a transcritical ORC [9, 10]. For pure working fluids, as shown in Fig. 1a, the constant evaporation and condensation temperature during the isothermal evaporation and condensation processes does not accurately match the temperature changes in the heat source and heat sink in the heat exchangers, which increases the irreversibilities. Thus, the traditional ORC system design with pure working fluids is not suitable for sensible heat sources. Unlike with pure working fluids, working fluid mixtures in a transcritical ORC can be heated directly from liquid to a supercritical vapor and condensed back to a liquid along variable temperature paths as shown in Fig. 1b, which results in better thermal matches with the source and sink fluids in the gas heater/condenser and reduces the exergy destruction in the heating and condensation processes[6]. Carbon dioxide (CO2) is an environmentally friendly natural working fluid with zero ODP (ozone depletion potential) and a negligible GWP (global warming potential), and it is non-flammable and non-toxic. In addition, CO2 has favourable thermodynamic and transport properties, so it has been extensively studied as a supercritical working fluid for ORC systems [13-15]. However, the disadvantage of CO2 is its low critical temperature, which limits the use of CO2 with many higher cooling source temperatures. Another disadvantage is its high critical pressure. Many researchers have tried to reduce the operating pressure for CO2 ORC syste ms by adding high critical temperature working fluids to the CO 2[16,17].
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(a) Pure working fluids
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(b) Mixture working fluids
Fig 1. The T-s diagram for the working fluid of ORC at different cycle configurations
Besides CO2, hydrocarbons are also environmentally-friendly natural working fluids with relatively high critical temperatures and low critical pressures, so they have also been investigated for use in ORC systems [18,19]. However, some of these hydrocarbon working fluids are flammable which limits their use. Zabetakis[20, 21] conducted experiments with propane and n-pentane in CO2 because these mixtures are not flammable for mixtures with CO 2 mass fractions greater than 0.3. However, the optimal composition of CO2 and HCs mixtures need to further explore. The present study analyzes mixtures of CO2 with six hydrocarbons, propane, n-butane, iso-butane, n-pentane, isopentane, and neo-pentane with various mass fractions. The analyses were based on a transcritical ORC simulation model that predicted the cycle efficiencies of the CO2 + hydrocarbon working fluids. 2. Selection of working fluids The candidate hydrocarbons were selected based on the number of carbon atoms and their branched chain structure. The hydrocarbons should have high critical temperatures and low critical pressures, so methane and ethane with critical temperatures close to CO 2, were not considered. Hydrocarbons with more than 5 carbon atoms have low cycle efficiencies; hence, only short carbon chain molecules were considered. Six hydrocarbons were tested, n-butane, iso-butane, n-pentane, iso-pentane, and neo-pentane, as additives in CO2 mixtures. The cycle efficiencies of these six mixtures were analyzed for various hydrocarbon mass fractions. The basic thermophysical properties of the working fluids are listed in Table 1, with all the data obtained from NIST REFPROP 9.1[22]. Table 1. Thermophysical properties of the working fluids Working fluids
molecular formula
molecular mass (g·mol-1)
Tb (K)
Tc (K)
pc (bar)
CO2 propane n-butane iso-butane n-pentane iso-pentane neo-pentane
CO2 CH3CH2CH3 CH3CH2CH2CH3 CH(CH3)3 CH3(CH2)3CH3 CH3CH2CH(CH3)2 C(CH3)4
44.01 44.10 58.12 58.12 72.15 72.15 72.15
194.69 231.04 272.66 261.40 309.21 300.98 282.65
304.13 369.89 425.13 407.81 469.70 460.35 433.74
73.77 42.51 37.96 36.29 33.70 33.78 31.96
cp (Jmol-1K-1) 298.15 K 37.24 73.47 98.35 96.51 119.80 118.65 120.60
0.22 0.15 0.20 0.18 0.25 0.23 0.20
3. Simulation and calculation 3.1. Parameters and basic assumptions The basic cycle parameters are listed in Table 2 with the following assumptions to analyze the transcritical ORC: (1) The system operates at steady state. (2) The fluid flowing inside the heat exchangers and pipes is isobaric, and the heat and friction losses are neglected. (3) The working fluid is a saturated liquid at the condenser outlet. (4) The vapor quality of the fluid exhausting the turbine is set to not less than 0.9 to avoid droplet erosion [6]. (5) The heat transfer fluids for the heat source and heat sink are low-grade gas and water, respectively. (6) The CO2 mass fraction rangewas between 0.3-0.9 to eliminate the flammability of hydrocarbons [20, 21]. (7) The input variable is the turbine inlet pressure.
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Table 2. Major cycle parameters for the simulations [16, 23, 24] Parameters Efficiencies Turbine isentropic efficiency/Pump isentropic efficiency/Electromechanical efficiency/Pump motor efficiency/Heat exchangers Heater pinch temperature difference/K Condenser pinch temperature difference/K Cooling water outlet temperature/K Others Turbine inlet pressure/bar Turbine inlet temperature/ K Working fluid mass flow rate/kgs-1
values 0.75 0.8 0.95 0.85 35 10 293.15 1.3 pc 453.15 1
3.2. Thermodynamic model Peng-Robinson (PR) equation is the most used cubic equation of state. PR equation has a more accurate description, high reliability and less parameters to the vapor-liquid equilibrium under high pressure. Hence, the Peng-Robinson equation was used here to describe the vapor-liquid equilibrium of the working fluids. 3.3. Calculational method and cycle performance evaluation The mass balance, energy balance, and vapor-liquid equilibrium of each unit in a transcritical ORC is written as: inout mi 0 (1) i
inout mi hi inout Q j inout Wk 0 i k j
(2)
f p, T , x 0
(3)
where i, j, k represent the two mixture components and the energy flow, m represents the mass flow rate and h represents the enthalpy. The cycle efficiency can be written as: w -w (4) th s p q TH where ws is the turbine work output, wp work absorbed by the pump, and q(TH) is the input heat. The thermodynamic efficiency relative to the second law of thermodynamics is given by the ratio of the thermal efficiency of the actual ORC cycle and that of an ideal Carnot cycle :
th
1 TL
(5)
TH
where TL and TH are the average temperature for the heat and cooling source. 3.4. Model validation Table 3. Simulation results comparison for different working fluids a a Working fluids TTUR /K pTUR th (%) /bar [6,16 25] Reference This work CO2 453.00 78.80 0.040 0.037 CO2 453.00 343.80 0.150 0.148 butane 411.15 30.15 0.155 0.154 iso-butane 394.15 28.86 0.138 0.137 pentane 459.15 28.65 0.184 0.185 0.5CO2/0.5R161 423.15 94.60 0.099 0.095 a TTUR and pTUR represent the TUR inlet temperature and pressure, respectively b Relative deviation of the thermal efficiency (δth): δth% = (th,this work -th,ref /th,ref) 100
th,this work -th,ref δth% 7.50 1.33 0.65 0.72 0.54 4.04
b
0.003 0.002 0.001 0.001 0.001 0.004
The model was verified be comparing the model predictions for five working fluids, CO 2, butane, iso-butane, pentane and 0.5CO2/0.5R161 with previous simulation results [6, 16, 25]. The differences were within 1% for most cases with the largest relative deviation of 7.50% for pure CO 2 at a turbine inlet pressure of 78.80 bar. The large
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relative deviations “7.50%” and “4.04%” is due to their small thermal efficiency, which is 0.04 and 0.099 respectively. However, their absolute deviation is only 0.003-0.004, this reflect the validation of this work from another aspect. The results are shown in Table 3. 4. Results and discussion
th and are given for various hydrocarbon mass fractions for each working fluid mixture and pure CO 2 in the transcritical ORC in Fig. 3 for a turbine inlet temperature of 453.15 K and a turbine inlet pressure of 1.3 pc. The mass fractions of the hydrocarbons added to the CO2 ranged from 0.1 to 0.7 since these mixtures are not flammable. The cycle efficiency increases with increasing hydrocarbon mass fraction for only the CO 2+propane system, the efficiencies of the other five working fluids all first increasing and then decreasing with increasing hydrocarbon mass fraction, so these five curves all had a maximum th, as shown in Fig. 2a. Hence, the six working fluid mixtures can be divided into three categories based on the th. th of the first kind of working fluids increases with increasing hydrocarbon mass fraction, with this kind of working fluid represented by CO 2+propane. The maximum th is 7.50% when the mass composition of the mixtures is 0.3CO2/0.7 propane. The second type of working fluid mixture are those whose th are higher than those of pure CO2 over the whole hydrocarbon mass fraction range which include CO2+iso-butane and CO2/neo-pentane as shown in Fig. 2a. The efficiencies of the neopentane/CO2 mixture are higher than those of the iso-butane/CO2 mixture with both having maximum efficiencies at a hydrocarbon mass fraction of 0.3. The third kind of working fluids have efficiencies that are higher than those of pure CO2 at lower hydrocarbon mass fractions, but then become lower than those of CO2 in high hydrocarbon mass fractions. These working fluids include CO2+n-butane, CO2+iso-pentane and CO2+n-pentane. The cycle efficiencies for these three working fluid mixtures are highest at a hydrocarbon mass fraction of 0.2. In addition, for these three mixtures, th is highest for CO2+n-pentane followed by CO2+iso-pentane and CO2+n-butane in low hydrocarbon mass fractions, and with the reversed order for higher mass fractions.
Fig 2.Transcritical ORC thermal efficiencies (a) and relative efficiencies (b) for various mixtures for a turbine inlet temperature of 453.15 K and a turbine inlet pressure of 1.3 pc.
The relative thermal efficiencies, , relative to the Carnot cycle shown in Fig. 2b change with the hydrocarbon mass fraction. The relative cycle efficiencies mostly decrease with increasing hydrocarbon mass fraction with the smaller hydrocarbon mass fractions having less irreversibilities, which suggests that the hydrocarbons cause more irreversibilities than pure CO2. The relative efficiencies are higher for the CO2+propane mixture followed by CO2+iso-butane, CO2+ n-butane, CO2+neo-pentane, CO2+iso-pentane and CO2+n-pentane. The molecular structures of the hydrocarbons shown in Fig. 3 show that the main chains of the first and second kinds of working fluids have 3 carbon atoms, while the third kind of fluids have 4 or 5 carbon atoms. The first category working fluids (CO2+propane) have the fewest main chain carbon atoms, and this mixture have the smallest molecular volume. Hence, the intermolecular force is weak and lead to smaller pc and Tc, pc and Tc are the critical pressure and temperature of the CO2/hydrocarbons mixtures, respectively. The higher HCs mass composition will lead larger pc and Tc, and the working fluid mixtures more easily condense and have better match with the heat source, which will directly obtain larger th and . Therefore, the th is smaller than pure CO2 when the HCs mass composition is smaller, and then larger than pure CO 2 along with the increasing HCs mass composition. For the second type working fluids, the main chain carbon atoms is 3 and have the branched chain. The branched
Lejun Feng et al. / Energy Procedia 129 (2017) 145–151 Feng LJ, Zheng DX, Chen J, Shi L / Energy Procedia 00 (2017) 000–000
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chain enhanced the molecular volume and intermolecular force, the more branched chain, the larger pc and Tc. Hence, theth of CO2+neo-pentane system larger than CO2+iso-butane, but is opposite. For the third category working fluids, there is a trade-off relation between the intermolecular force and the HCs mass composition. When the HCs mass composition is smaller, the more main chain carbon atoms and branched chain will lead to a higher th. However, the vary tendency is opposite when the HCs mass composition is larger. The influence of HCs mass composition prevail over the molecular structure.
Fig 3. The molecular structure of HCs
The potential of using these working fluid mixtures was further evaluated for the three working fluids with the highest thermal efficiency for each kind working fluid, 0.3CO2/0.7propane, 0.7CO2/0.3neo-pentane, and 0.8CO2/0.2pentane. The power generated and the cycle efficiencies of these three working fluid mixtures in a transcritical ORC are compared with those of pure CO2 in Table 4. Th e turbine inlet temperature was 453.15 K and the turbine inlet pressure was 1.3 pc. Table 4 Cycle efficiencies of CO2 and CO2+hydrocarbons as the working fluids in a transcritical ORC Tevaporator (K) working fluids pTUR/ pOTURa CO2 461.75 1.67 0.3CO2/0.7propane 377.58 3.04 0.7CO2/0.3neo-pentane 397.13 2.04 0.8CO2/0.2pentane 394.60 1.67 a pOTUR represents the turbine outlet pressure
q(H)/ (kW) 332.65 487.69 380.35 350.70
wp (kW) 7.27 13.54 13.34 13.56
ws (kW) 26.52 50.14 40.90 40.25
th/% 5.79 7.50 7.25 7.61
/% 56.51 62.94 60.49 61.10
Table 4 lists the operating pressure ratio, the evaporator temperature and heat duty, pump work, the turbine output work, th and. The turbine output power is higher for all three mixtures than for pure CO2, and th is increased by 29.5% for 0.3CO2/0.7propane, 25.22% for 0.7CO2/0.3neo-pentane and 31.43% for 0.8CO2/0.2pentane. The hydrocarbons increase the cycle pressure ratio and reduce the operating pressure of the transcritical ORC which makes the cycle which allows more flexible operating conditions. th and are highest for the 0.3CO2/0.7propane mixture. Thus, the CO2+hydrocarbon working fluid mixture mentioned will give significantly better operating efficiencies in transcritical ORCs. 5. Conclusion CO2 is the potential working fluid for ORC. However, its low critical temperature and high critical pressure limited its develop. HCs were selected as the additive for CO2 to improve its thermophysical property. The potential CO2+HCs mixture working fluids were explored in this work, and some conclusions are obtained as follows: (1) The transcritical ORC was modelled using a cycle simulation program. (2) The binary CO2+hydrocarbon system uses the advantages of CO2 and the hydrocarbon as the working fluid in a transcritical ORC. The cycle efficiencies were calculated for six working fluid mixtures: CO2+propane, n-butane, iso-butane, n-pentane, iso-pentane and neo-pentane.
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(3) The six working fluids were divided into three categories based on their cycle efficiencies relative to pure CO2. The efficiency for the CO2+propane system increased with increasing propane concentration, while the thermal efficiencies of the other five working fluid mixtures all increased to a maximum and then decreased with increasing hydrocarbon concentration. The relative thermodynamic efficiency relative to the Carnot efficiency was highest for CO2+propane, followed by CO2/iso-butane, CO2/ n-butane, CO2/neo-pentane, CO2/so-pentane, and CO2/n-pentane. (4) Comparisons the molecular structure of the hydrocarbons showed that the hydrocarbons with longer main chains or with branched chain with the same number of main chain carbon atoms have larger pc and Tc, which lead to higher thermal efficiencies at low hydrocarbon mass fractions. However, with high concentrations of the hydrocarbons in the mixture, the thermal efficiency and relative efficiency are higher with lower pc and Tc, so there is a trade-off between the thermal efficiency, the relative thermal efficiency and the hydrocarbon mass fraction. (5) Compared the cycle performance of 0.3CO2/0.7propane, 0.7CO2/0.3neo-pentane and 0.8CO2/0.2pentane with the pure CO2, Overall consideration, 0.3CO2/0.7propane have a better cycle performance. Acknowledgements This work was supported by the Science Fund for the National Key Research and Development Program of China (No. 2016YFB0901405) and for Creative Research Groups (No. 51621062). References [1] Quoilin S, Broek M. Techno-economic survey of organic Rankine cycle (ORC) systems. Renew Sust Energ Rev 2013;22:168-186. [2] Bao JJ, Zhao L. A review of working fluid and expander selections for organic Rankine cycle. Renew Sust Energ Rev 2013;24:325-342. [3] Wang EH, Zhang HG. 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