sustainability

Article Thermodynamic Analysis of ORC and Its Application for Waste Heat Recovery

Alireza Javanshir *, Nenad Sarunac and Zahra Razzaghpanah Faculty of Mechanical Engineering and Engineering Science, UNC Charlotte, Charlotte, NC 28223, USA; [email protected] (N.S.); [email protected] (Z.R.) * Correspondence: [email protected]; Tel.: +1-704-954-4395

Received: 6 October 2017; Accepted: 25 October 2017; Published: 29 October 2017

Abstract: The analysis and optimization of an organic Rankine cycle (ORC) used as a bottoming cycle in the Brayton/ORC and steam Rankine/ORC combined cycle configurations is the main focus of this study. The results show that CO2 and air are the best working fluids for the topping (Brayton) cycle. Depending on the exhaust temperature of the topping cycle, Iso-butane, R11 and ethanol are the preferred working fluids for the bottoming (ORC) cycle, resulting in the highest efficiency of the combined cycle. Results of the techno-economic study show that combined Brayton/ORC cycle has significantly lower total capital investment and levelized cost of electricity (LCOE) compared to the regenerative Brayton cycle. An analysis of a combined steam Rankine/ORC cycle was performed to determine the increase in power output that would be achieved by adding a bottoming ORC to the utility-scale steam Rankine cycle, and determine the effect of ambient conditions (heat sink temperature) on power increase. For the selected power plant location, the large difference between the winter and summer temperatures has a considerable effect on the ORC power output, which varies by more than 60% from winter to summer.

Keywords: organic Rankine cycle; working fluid properties; thermal efficiency; subcritical ORC; transcritical ORC; combined Brayton ORC cycle; techno-economic analysis

1. Introduction The increasing global energy demand, energy cost, and sustainability issues bring the need for waste heat recovery and use. According to the U.S. Department of Energy (U.S. DOE), manufacturing industries and power plants produce approximately 60% of the low-temperature waste heat [1]. Releasing the exhaust with temperatures higher than 350 ◦C directly to the environment represents a large waste of the primary energy. The recovery of exhaust heat and its use through the organic Rankine cycle (ORC) is an efficient and flexible method with simpler structure, higher safety, and lower maintenance requirements compared to the conventional heat recovery methods, such as steam Rankine cycle. Employing the low-grade energy (waste heat) by integrating the ORC into an energy system, such as a power plant, industrial facility, or large diesel engine (or other prime mover) increases the power output and system energy efficiency [2]. An organic working fluid is used in a Clausius–Rankine cycle (ORC), instead of water-steam in a conventional steam Rankine cycle. Due to the low temperature of phase change, organic working fluids represent a good choice for utilization of heat recovered from the low temperature heat sources [3]. In an ORC cycle, the superheated turbine exhaust due to generally lower boiling temperature and evaporation pressure, helps to avoid erosion of the turbine blades caused by the wet steam in a steam Rankine cycle [4]. Performance improvement of a regenerative ORC (ORC with a recuperator) in conjunction with a simple ORC (ORC without a recuperator) is the focus of the recent research work [5]. Due to the remarkable properties of the ORC for waste heat recovery from flue gases, a number of experimental studies were carried out and published in the literature. Zhou et al. [6] used a liquefied

Sustainability 2017, 9, 1974; doi:10.3390/su9111974 www.mdpi.com/journal/sustainability Sustainability 2017, 9, 1974 2 of 26 petroleum gas stove as the heat source maintaining the temperature in the range of 90 ◦C to 220 ◦C. R123 was chosen as a working fluid and a scroll expander was used to achieve a maximum power output of 0.645 kW and a cycle efficiency of 8.5%. Lemort et al. [7] and Quoilin et al. [8] also used the same working fluid and expander type, reporting the maximum cycle efficiency of 7.4%. Vatani et al. [9] employed an ORC to recover waste heat from the integrated direct internal reforming-molten carbonate fuel cells (DIR-MCFC) with a pre-reformer. A number of working fluids were used to simulate the cycle at optimum operating conditions resulting in highest thermal efficiency. The results indicate that both the energy and exergy efficiencies were increased by cathode splitting. In addition, a decrease in exergy loss was observed while using cathode splitting for all substances. Algieri et al. [10] studied the effect of internal heat recovery for sub-, trans- and supercritical ORC with dry working fluids. Mago et al. [11] compared the regenerative ORC based on the combined first and second law of thermodynamics analysis to a simple ORC. The study shows that ORC with dry working fluid not only requires less amount of waste heat to generate specified power output but also lowers irreversibility by increasing cycle efficiency. Acar [12] performed an energy and exergy analysis for the reheat-regenerative Rankine cycle. The results show that, despite the same energy and exergy efficiencies of the closed cycle, exergy analysis yields to a better understanding of the losses in the system. Evaluation of five different ORC was done by Wang et al. [13]. Since a portion of the heat from the turbine exhaust stream is recovered in the recuperator and beneficially used, a regenerative ORC has the lower rate of exergy destruction, compared to the simple cycles. Hung et al. [14] compared the effect of different types of working fluids on thermal efficiency improvement of the ORCs. The results show that, while the wet and isentropic working fluids do not have a significant effect on thermal efficiency (up to 2% improvements), dry fluids can improve thermal efficiency by more than 9%. A simple ORC using waste heat sources and different working fluids has been the focus of many papers and studies [15–19]. Saleh et al. [20] compared 31 different working fluids and their effect on the thermal efficiency for an ORC with different work cycle configurations. The effect of different working fluids on thermal efficiency was also studied by Lakew and Bolland [21] for a simple subcritical ORC operating in the 80 ◦C to 160 ◦C temperature range. Numerous criteria are as well as international safety and environmental protocols and agreements are considered during the fluid selection procedure. The ozone depletion potential (ODP), flammability, toxicity and global warming potential (GWP) are the criteria that need to be considered during the working fluid selection process. Papadopoulos et al. [22] used 15 criteria for the fluid selection; with environmental, safety, physical, chemical and economical properties being the five main groups. The best working fluid is selected based on the cycle thermal efficiency. Details are provided in [23]. Since there is no working fluid that satisfies all selection criteria [24], the fluid selection method should balance all the main mentioned criteria. The selection processes is divided into two steps: elimination and ranking [25]. Elimination is used to remove the unsuitable working fluids from the list before the ranking process is applied, having in mind that not all the properties have the same weight of evaluation. Roedder et al. [26] used a combination of the elimination and ranking methods to choose the best working fluid based on the 22 criteria. Iso-butane was identified as the best working fluid for a two-stage ORC. A combined cycle consists of several thermodynamic cycles, grouped into the topping or high-temperature and the bottoming or low-temperature cycles. Rejected heat from the topping cycle is utilized by the bottoming cycle to produce more power. For a low exhaust temperature of the topping cycle, organic Rankine cycle (ORC) is a suitable choice for the bottoming cycle, which performs better than the steam Rankine cycle [27–29]. The ORC power plants are commercially available on a small scale due to low maximum operating temperature, and are used in renewable energy, geothermal and low-temperature heat recovery applications [27]. Sustainability 2017, 9, 1974 3 of 26

Despite the extensive studies on ORC, the combination of a regenerative Brayton cycle as the topping cycle and an ORC as the bottoming cycle, i.e., the combined Brayton/ORC, has not been evaluated to the sufficient level of detail, and warrants more investigation. This paper is concerned with the exhaust heat recovery from the Brayton and steam Rankine cycles and its use in a simple ORC. The maximum temperature operating range used in this study is between 50 ◦C and 350 ◦C, which is well within the ORC operating range. In this temperature range, other cycles such as steam Rankine, Brayton, or Kalina cycles have a very low thermal efficiency. Thus, there is on benefit of using any of these cycles as a bottoming cycle for recovery and utilization of the low-grade waste heat. The effect of twelve working fluids on cycle performance was studied over the range of cycle operating conditions with the aim to identify the best working fluid for the specified set of operating conditions. The cycle calculations and simulations were performed by employing the Ebsilon Professional V11 (EPV-11) power systems modeling software [30]. EPV-11 is professional software used for a detailed design, analysis, and optimization of the power generation systems. Selection of working fluids resulting in best performance (thermodynamic efficiency or net specific work output) of the ORC for the specified operating conditions (maximum temperature and pressure, heat rejection temperature, and others) is a time-consuming and arduous task, especially when a large number of working fluids is being considered. The commonly used approach involves performing a number of parametric calculations over a range of operating parameters for a number of the candidate working fluids. The results are usually presented in a graphical form, for example: for each of the analyzed working fluids efficiency is plotted as a function of the maximum temperature and pressure. The best working fluid is selected by manually inspecting efficiency diagrams for all analyzed fluids. In addition, since the selection process is manual, there is a certain level of subjectivity and the potential for error involved. The best (most suitable) working fluid, for use in ORC in waste heat recovery applications, was selected based on the selection procedure developed by the authors and published in the previous studies [31–33]. The working fluid selection procedure allows for a non-subjective and exact determination of the best working fluid for the selected cycle operating conditions, as well as construction of performance maps which provide visual and easy-to-interpret information on the cycle thermal performance and the best working fluids. This selection procedure was applied to determine the best working fluids for the combined regenerative Brayton/ORC and steam Rankine/ORC applications. A techno-economic analysis of the ORC and combined Brayton/ORC cycles was performed to determine the total capital investment and levelized cost of electricity (LCOE), and allow economic comparison of the investigated cycles.

2. Thermodynamic Modeling and Working Fluid Properties The operating principle of the ORC and steam Rankine cycles are the same: compression of the liquid, phase change (evaporation) in the evaporator, expansion in the turbine (expander), and phase change (condensation) in the condenser [34]. The main components involved in a simple ORC (feed pump, evaporator, turbine, and condenser) are presented in Figure1. The working fluid is delivered to the evaporator by the feed pump where it is evaporated at approximately constant pressure using the externally supplied heat. In some ORC designs, a superheater is used to superheat the working fluid [4]. The saturated or superheated working fluid is expanded in the turbine (expander), which is driving an electric generator. The low-pressure, low-temperature working fluid leaving the turbine is condensed in the condenser. The pressure of the working fluid leaving the condenser as a saturated (or slightly subcooled liquid) is increased by the feed pump, completing the power cycle. Sustainability 2017, 9, 1974 4 of 26 Sustainability 2017, 9, 1974 4 of 26

Sustainability 2017, 9, 1974 4 of 26 Sustainability 2017, 9, 1974 4 of 26

Figure 1. Schematic of a simple ORC. Figure 1. Schematic of a simple ORC. Figure 1. Schematic of a simple ORC. The temperature-specific entropyFigure 1.(T-s) Schematic diagrams of a simpleof simple ORC. subcritical and transcritical ORCs The temperature-specific entropy (T-s) diagrams of simple subcritical and transcritical ORCs are are presentedThe temperature-specific in Figure 2. There entropy is no phase (T-s) change diagrams in the of supercritical simple subcritical cycle where and transcritical the working ORCs fluid presentedremainsare presentedThe in astemperature-specific Figure a homogeneous in 2Figure. There 2. There is supercritical no entropy is phase no phase (T-s) changefluid change diagrams throughout in in the the of supercritical supercriticalsimplethe entire subcritical power cycle cycle cycle. whereand where transcritical the the working working ORCs fluid fluid remainsareremains presented as a as homogeneous a homogeneous in Figure 2. supercriticalThere supercritical is no phase fluid fluid changethroughout throughout in the supercritical the the entire entire power powercycle cycle.where cycle. the working fluid remains as a homogeneous supercritical fluid throughout the entire power cycle.

(A) (B) (C)

Figure 2. T-s(A) diagram for a simple ORC: (A) subcritical;(B) (B) subcritical with superheat;(C) and (C) (A) (B) (C) transcritical.Figure 2. T-s diagram for a simple ORC: (A) subcritical; (B) subcritical with superheat; and (C) Figure 2. T-s diagram for a simple ORC: (A) subcritical; (B) subcritical with superheat; and Figuretranscritical. 2. T-s diagram for a simple ORC: (A) subcritical; (B) subcritical with superheat; and (C) (C) transcritical.transcritical.The main components of a regenerative Brayton/simple ORC are shown in Figure 3. The main components of a regenerative Brayton/simple ORC are shown in Figure 3. TheThe main main components components of of a a regenerative regenerative Brayton/simpleBrayton/simple ORC ORC are are shown shown in Figure in Figure 3. 3.

Figure 3. Schematic representation of a combined regenerative Brayton/ORC cycle . Figure 3. Schematic representation of a combined regenerative Brayton/ORC cycle. FigureFigure 3. Schematic 3. Schematic representation representation ofof a combinedcombined regenerative regenerative Brayton/ORC Brayton/ORC cycle cycle..

In a combined power cycle, two or more thermodynamic cycles are combined to achieve higher efficiency and power output. Based on the operating temperature range, the cycles are divided into the topping and bottoming cycles. The Brayton cycle was selected as a topping cycle because the Sustainability 2017, 9, 1974 5 of 26

Sustainability 2017, 9, 1974 5 of 26 high turbine inletIn a combined temperature power (TIT)cycle, two is neededor more thermodynamic to achieve high cycles thermal are combined efficiency to achieve [34 higher]. The exhaust temperatureefficiency of a simpleand power Brayton output. Based cycle on (above the operatin 400 g◦ C)temperature is higher range, than the the cycles maximum are divided allowed into ORC operating temperature;the topping and thus, bottoming a regenerative cycles. The BraytonBrayton cycl cycle,e was having selected lower as a topping exhaust cycle temperature, because the was used high turbine inlet temperature (TIT) is needed to achieve high thermal efficiency [34]. The exhaust as a toppingtemperature cycle in of a combineda simple Brayton Brayton/ORC cycle (above cycle400 °C) in is this higher study. than the maximum allowed ORC operating temperature; thus, a regenerative Brayton cycle, having lower exhaust temperature, was 2.1. Thermodynamicused as a topping and Environmental cycle in a combined Properties Brayton/ORC of the cycle Working in this Fluids study.

Selection2.1. Thermodynamic of the working and Environmental fluid for theProperties ORC of the has Working an important Fluids role on the cycle performance. Twelve working fluids were analyzed in this study. Based on previous studies, the workings fluids Selection of the working fluid for the ORC has an important role on the cycle performance. selected forTwelve the analysisworking fluids have were the analyzed potential in this to givestudy. a Based good on ORC previous performance studies, the workings [31–33]. fluids Based on the slope of theselected saturated for the vapor analysis curve have inthe the potential T-s diagram to give a (Figuregood ORC4), performa workingnce fluids [31–33]. are Based divided on the into three main groups:slope dry, of the wet, saturated and isentropic. vapor curveThe in the positive, T-s diagram negative, (Figure and4), working infinite fluids slope are refers divided to into the dry, wet, and isentropicthree fluids,main groups: respectively. dry, wet, Inand an isentropic. ORC, the The slope positive, ofthe negative, saturated and infinite vapor slope curve refers is oneto the of the most dry, wet, and isentropic fluids, respectively. In an ORC, the slope of the saturated vapor curve is one importantof thermophysical the most important propertiesthermophysical of properties the working of the fluid,working having fluid, having a significant a significant impact impact on on thermal efficiency andthermal equipment efficiency and arrangement equipment arrangement [14]. The wet[14]. The working wet working fluid fluid has tohas be to superheatedbe superheated before its expansionbefore in the its turbine expansion to in maintain the turbine the to maintain maximum the maximum allowed allowed wetness wetness at the at turbine the turbine outlet outlet and avoid and avoid erosion damage to the turbine blading [35]. Previous studies on the isentropic fluids [35] erosion damage to the turbine blading [35]. Previous studies on the isentropic fluids [35] have shown have shown that there is not a strong relationship between the cycle efficiency and turbine inlet that there istemperature not a strong (TIT). relationship between the cycle efficiency and turbine inlet temperature (TIT).

Figure 4. Wet, dry and isentropic fluids. Figure 4. Wet, dry and isentropic fluids. For dry fluids, the highest cycle efficiency is achieved by maintaining the saturated steam For dryconditions fluids, at thethe turbine highest inlet cycle [11,36,37]. efficiency Superhea isting achieved the dry fluid by maintainingwill increase the the turbine saturated exit steam temperature and condenser loading without having a significant impact on the turbine power conditionsoutput. at the turbine inlet [11,36,37]. Superheating the dry fluid will increase the turbine exit temperature andDuring condenser the plant loadingdesign, the without environmental having and a sa significantfety characteristics impact of on working the turbine fluids should power output. Duringalso the be plant considered design, because the environmental they play a significant and safety role characteristicson the plant operators’ of working health fluidsand the should also be consideredenvironment. because Some they of play the environmental a significant and role safety on thedataplant for the operators’ selected working health fluids and are the shown environment. in Table 1. Pcr and Tcr are the critical pressure and temperature of the working fluids. P Some of the environmentalThe properties of and working safety fluids data considered for the selectedin this study working are the fluidsglobal warming are shown potential in Table 1. cr and Tcr are(GWP), the critical ozone depletion pressure potential and temperature (ODP), toxicity, of theflammability, working and fluids. corrosiveness. The GWP of a The propertiesworking fluid of is working a measure fluids of its effect considered on Global in Warming. this study Carbon are thedioxide global (CO2 warming) has been assigned potential (GWP), ozone depletiona GWP of potential 1. Thus, a (ODP), fluid with toxicity, GWP of flammability, 2 has the effect and on global corrosiveness. warming two The times GWP stronger of a working compared to the CO2. Working fluids which have higher ODP than zero cannot be considered for fluid is a measure of its effect on Global Warming. Carbon dioxide (CO2) has been assigned a GWP of 1. Thus, a fluid with GWP of 2 has the effect on global warming two times stronger compared to the CO2. Working fluids which have higher ODP than zero cannot be considered for power generation due to restrictions on their use imposed by the Montreal protocols [38]. The properties in Table1 were obtained from the GESTIS database [39]. As can be seen in Table1, ammonia is not a good choice of the working fluid for power generation due to its toxicity, flammability, and corrosiveness. Selection of the working fluid for a regenerative Brayton cycle used as a topping cycle in a combined Brayton/ORC cycle has a significant effect on performance. Nine working fluids were evaluated in this study to determine the best working fluid for the topping cycle. Table2 shows the Sustainability 2017, 9, 1974 6 of 26 properties of the working fluids used in this study. Based on previous studies, these workings fluids have the potential for good performance in a regenerative Brayton cycle [32].

Table 1. Physical, safety and environmental data of working fluids for ORC.

Physical Data Environmental and Safety Data Molar Mass T Fluid cr P (MPa) Type GWP ODP Toxicity Flamm-Ability Corrosi-Veness (kg/kmol) (◦C) cr 1 Butane 58.122 151.9 3.79 Dry 3 0 NO YES NO 2 Iso-butane 58.122 134.6 3.62 Dry 3 0 NO YES NO 3 Ammonia 17.03 132.2 11.33 Wet 0 0 YES NO YES 4 R11 137.37 197.9 4.40 Isentropic 4000 1 NO NO NO 5 R141b 116.95 204.3 4.21 Isentropic 600 0.11 YES NO NO 6 R152a 66.051 113.2 4.51 Wet 140 0 NO YES NO 7 R142b 100.5 137.1 4.05 Isentropic 1800 0.065 YES YES NO 8 R134a 102.03 101.0 4.05 Wet 1300 0 NO NO NO 9 Ethanol 46.068 240.7 6.14 Wet n.a. n.a. NO YES NO 10 R113 187.38 214.0 3.39 Dry 6130 1 NO NO NO 11 Iso-pentane 72.149 187.2 3.37 Dry 5 0 YES YES NO 12 R114 170.92 145.6 3.25 Dry 10.04 1 NO NO NO n.a., non-available.

Table 2. Physical data of the working fluids for a topping regenerative Brayton cycle.

◦ Fluid Molar Mass (kg/kmol) Tcr ( C) Pcr (MPa) 1 Air 28.965 −140.62 3.78 2 CO2 44.01 30.978 7.37 3 N2 28.013 −146.96 3.39 4 He 4.0026 −267.95 0.22 5 O2 31.999 −118.57 5.04 6 Methane 16.043 −82.586 4.59 7 Ne 20.179 −228.66 2.67 8 Ar 39.948 −122.46 4.86 9 Kr 83.798 −63.67 5.52

2.2. Calculation of Thermal Efficiency The analysis of the cycle performance was performed by neglecting the friction and heat losses in the pipes and heat exchangers, and assuming adiabatic turbomachinery (turbine and feed pump). An ORC is a considerably simpler and smaller power cycle compared to the Steam Rankine cycle with significantly smaller heat exchangers and considerably simpler connecting piping. In addition, the molar mass of most organic fluids and their density are higher compared to steam, resulting in comparatively smaller volumetric flows and, thus smaller equipment. Thus, friction in the pipes and heat exchangers, and the resulting pressure drop, does not have a significant effect on cycle performance and can be neglected. Different working fluids have different densities, which results in a difference in equipment size (for example CO2 vs. He). To include the difference in equipment size in the analysis, thermodynamic evaluation was supplemented by the techno-economic analysis to compare the capital investment costs, and the levelized cost of electricity (LCOE). Regarding the heat losses, turbomachinery is typically assumed to be adiabatic since any heat exchange with the surroundings is several orders of magnitude smaller compared to the energy flux through the turbomachine. The heat exchangers and connecting piping in the ORC are relatively small and, thus can be well insulated, minimizing heat losses.

2.2.1. Thermal Efficiency of a Simple ORC Based on the previous studies performed by the authors [31,32], thermal efficiency of a subcritical ORC without a superheat can be calculated as: Sustainability 2017, 9, 1974 7 of 26

ηth = ηt(c1 × FOM + c2) (1)

2 C1 = 1.207TEC − 3.973TEC + 2.831 (2) 2 C2 = −1.862TEC + 5.824TEC − 4.009 (3) where ηt and FOM are turbine isentropic efficiency and figure of merit, respectively. FOM is expressed by: 1 0.8 FOM = Ja0.1( ) (4) TEC C (T − T ) Ja = P,13 4 1 (5) h f g

Evaporation temperature T4 TEC = = (6) Condensation temperature T1 where CP,13 represents the average specific heat capacity calculated from the State Point (SP) 1 to SP 3, T1 is the temperature at the feed pump inlet, T4 is the maximum temperature (turbine inlet temperature, TIT), and hfg is the latent heat (enthalpy) of evaporation (vaporization). The quantity Ja is the Jacob number defined as the ratio of the sensible and latent heats. As can be seen in Equation (1), there is a linear relationship between FOM and ηth. According to Equations (1) and (4), ηth decreases as the Jacob number increases. Details are provided in [31,32]. Based on the results, the following expression for the specific heat input for a subcritical ORC without a superheat was developed: . qin . = h (Ja + 1) (7) m f g . where m is mass flow rate of the power cycle. Compared to a subcritical cycle without a superheat, thermal efficiency of a superheated subcritical cycle is affected by an additional variable; the superheat temperature. The following expression for thermal efficiency of a superheated ORC was proposed by the authors [31,32]

ηth = ηt(c1 Jat + c2) (8)

1.45 Jat = (Ja + k × Jas) (9)  11  0.1 T4 Tev T4 Tev (c11 × T + c12 + T ) (c21 × T + c22 + T ) c = 1 1 , c = 1 1 (10) 1  1.46Tev−41.9199 2  −0.44 82.875 T4 82.875 T4 Tev Tev

c11 = −0.5195T1 + 121.44, c12 = 0.5953T1 − 146.88 (11)

c21 = −0.5157T1 + 225.92, c22 = 0.5179T1 − 243.95 (12) where k = 1 for the wet and isentropic fluids, while for the dry fluids k = 1.5 [31]. Ja is the Jacob number and Jas is the superheat Jacob number.

CP,1ev(Tev − T1) CP,34(T4 − Tev) Ja = , Jas = (13) h f g h f g The results show that thermal efficiency decreases as the Jacob number Ja and superheat Jacob number Jas increase. Based on the results, the following expression for the specific heat input for a subcritical ORC with a superheat was developed:

. qin . = h (Ja + Jas + 1) (14) m f g Sustainability 2017, 9, 1974 8 of 26

For the supercritical ORC shown in Figure2C, a similar procedure was used to determine expression for thermal efficiency: ηth = ηt(c1Tmd + c2) (15)

Tmd = ln(TEC) − 0.8 × k(Tr − 1) (16) 3 2 c1 = 5.421Tr − 18.332Tr + 20.901Tr − 7.483 (17) 3 2 c2 = 3.272Tr − 11.065Tr + 12.454Tr − 4.63 (18)

T4 Tr = (19) Tcr where Tcr and Tmd are the critical and modified temperatures of the working fluid, respectively. The constant k has the same value as for a subcritical ORC with the superheat. For a transcritical ORC, as shown by Equation (15), at the critical point of the analyzed working fluids, thermal efficiency is a linear function of the dimensionless temperature Tr. Based on the results, the following expression for the specific heat input for a transcritical ORC with the superheat was developed: . qin . = cp14(T4 − T1) (20) m Finally, the net work output of all types of a simple ORC can be calculated as:

. . Wnet = ηthqin (21)

2.2.2. Thermal Efficiency of a Combined Regenerative Brayton/ORC Thermal efficiency of a combined cycle may be calculated as:

. . Wnet, Brayton + Wnet, ORC ηcombined = . = ηth,Brayton + A (22) qin,Brayton

Equation (22) provides insight into why combined cycles are more efficient compared to the simple cycles: efficiency of the combined cycle is a sum of the Brayton (topping) cycle efficiency and a positive quantity A. As will be shown later, the value of quantity A depends on selection of the working fluid for the bottoming (ORC) cycle. Based on the developed correlations and previous results [31,32], twelve working fluids listed in Table1 have been selected for this study. The use of these working fluids in a simple ORC has a potential to increase thermal efficiency and net power output compared to the other working fluids.

3. Power Block Cost Estimation The capital cost of a power block consists of the purchased cost of equipment, the labor and materials needed for the installation, such as the piping, the foundations and structural supports, the electrical equipment, the instrumentation and controls, and the indirect expenses. The indirect expenses cover the transportation costs for shipping equipment to the plant site, and salaries for the project personnel. Table3 shows components of the total capital investment cost considered in this study. More details can be found in [40]. The bare module cost, CBM, is the sum of direct and indirect costs for each unit as described in Turton et al. [41], and can be calculated by Equation (23).

0 CBM = CPFBM (23) Sustainability 2017, 9, 1974 9 of 26

0 CP is purchase cost of equipment in base conditions. Base condition represents the case where the equipment is made of the most common material, usually carbon steel operating at ambient pressure. FBM is the bare module factor. The data for the purchase cost of the equipment described in Turton et al. [41] were obtained based on the average CEPCI (Chemical Engineering Plant Cost Index) value of 397. The updated value of CEPCI of 556.8 for the year of 2015 was utilized for the present economic estimate. 0 The purchased equipment cost for base conditions, CP may be determined by employing Equation (24). 0 2 log10CP = K1 + K2 × log10 AA + K3(log10 AA) (24) where AA is the size parameter for the equipment, and K1, K2, and K3 are constants given in Table4.

Table 3. Components of the total capital investment.

Components of Total Capital Investment Formula

CTCI Total capital investment CTCI = CTDC + Cland + Croyal + Cstartup Cstartup Cost of plant startup Cstartup = 0.1 × CTDC Croyal Cost of royalties 0 4 0.7 Cland Cost of land Cland = 45 × 10 × (P/100MWNET) CTDC Total depreciable capital CTDC = 1.18 × CDPI CDPI Total direct permanent investment CDPI = 1.1 × CTBM CTBM Total bare module cost CTBM = ∑ CBM + Cspare + Cwf Cwf working fluid Cwf pump CSPARE Cost for spares 2.1CBM

Table 4. Coefficients used to calculate the price of different components in a power block.

Equipment K1 K2 K3 C1 C2 C3 B1 B2 FM FBM AA ORC turbine 2.2476 1.4965 −0.1618 ** ** ** ** ** ** 6.1 kW ORC pump 3.3892 0.0536 0.1538 −0.393 0.3957 −0.0022 1.89 1.35 1.5 ** kW ORC HEX 4.3247 −0.0303 0.1634 0.0388 −0.1127 0.0818 1.63 1.66 1 ** Area (m2) Motor pump 2.4604 1.4191 −0.1798 ** ** ** ** ** ** 1.5 kW ORC ACC HEX 4.0336 0.2341 0.0497 0.0388 −0.1127 0.0818 0.96 1.21 1 ** Area (m2) ORC ACC fan 3.1761 −0.1373 0.3414 ** ** ** ** ** 2.5 5 Q(m3/s) motors fans 2.4604 1.4191 −0.1798 ** ** ** ** ** ** 1.5 kW Brayton HEX 0.9420 0.8778 0 ** ** ** ** ** ** ** UA (W/K) Brayton turbine 3.5195 0.5886 0 ** ** ** ** ** ** ** kW regenerator 0.716 0.8933 0 ** ** ** ** ** ** ** UA (W/K) Brayton cooler 1.4843 0.8919 0 ** ** ** ** ** ** ** UA (W/K) compressor 3.1093 0.9142 0 ** ** ** ** ** ** ** kW Generator ** ** ** ** ** ** ** ** ** 1.5 kW Working fluid ** ** ** ** ** ** ** ** ** 1.25 ** Note: **, Value is not needed.

For the Brayton cycle analyzed in this paper, the primary heat exchanger is assumed to be a printed-circuit heat exchanger (PCHE). The high-pressure gas on the cold side and high-viscosity fluid on the hot side are considered for the PCHE. To represent the power density of different working fluids, the compressor data were modified by the density ratio ρair/ρ f luid. The A-frame, finned-tube air coolers were considered for all cooling processes. 0 The purchased equipment cost of the electrical generator, CP,gen, is calculated according to Equation (25) as presented in Ref. [42]

. 0.95 0 CP,gen = 690(Wgen) (25)

The bare module factor, FBM was calculated from:

FBM = B1 + B2 × FM × FP (26) Sustainability 2017, 9, 1974 10 of 26

2 log10FP = C1 + C2 × log10P + C3(log10P) (27) where B1 and B2 are constants given in Table4; FM is equipment material factor listed in Table4; and FP is the operating pressure factor. The relative pressure having unit of bar gauge (1 bar = 0.0 barg) is used in Equation (27), where pressure factors are always greater than unity. The constants C1, C2 and C3 are given in Table4. Since the ORC is running at temperatures lower than 350 ◦C, material changes were not considered for the components operating at different temperatures. The Brayton cycle operating temperature range is between 300 ◦C and 1000 ◦C, thus materials such as carbon steel, stainless steel, nickel alloys 625, 718, etc. need to be used for different operating temperatures. The material selection method for the expanders and heat exchanges is discussed in [43]. Tables5–7 show FBM value for components of the Brayton cycle operating at different maximum temperatures and pressures.

Table 5. Material selection for the primary heat exchanger and compressor in a Brayton cycle.

◦ Pmax < 10 MPa Pmax < 20 MPa Pmax < 30 MPa Tmax ( C) < Material FBM Material FBM Material FBM 500 Carbon steel 1 Carbon steel 1 Carbon steel 1 650 Stainless steel (347) 2.5 Stainless steel (347) 2.5 Stainless steel (347) 2.5 700 Stainless steel (347) 2.5 Stainless steel (347) 2.5 Nickel alloy(625) 3 750 Stainless steel (347) 2.5 Nickel alloy (625) 3 Nickel alloy (625) 3 900 Nickel alloy (625) 3 Nickel alloy (625) 3 Nickel alloy (625) 3

Table 6. Material selection for the turbine in a Brayton cycle.

◦ Pmax < 10 MPa Pmax < 20 MPa Pmax < 30 MPa Tmax ( C) < Material FBM Material FBM Material FBM 500 Stainless steel (347) 2.5 Stainless steel (347) 2.5 Stainless steel (347) 2.5 650 Stainless steel (347) 2.5 Nickel alloy (625) 3 Nickel alloy (625) 3 900 Nickel alloy (625) 3 Nickel alloy (625) 3 Nickel alloy (625) 3

Table 7. Material selection for the air-cooled cooler in a Brayton cycle.

◦ Pmax < 10 MPa Pmax < 20 MPa Pmax < 30 MPa Tmax ( C) < Material FBM Material FBM Material FBM 500 Carbon steel 1 Carbon steel 1 Carbon steel 1 650 Carbon steel 1 Stainless steel (347) 2.5 Stainless steel (347) 2.5 900 Stainless steel (347) 2.5 Stainless steel (347) 2.5 Stainless steel (347) 2.5

The pumps are inexpensive but require maintenance to prevent leaks, therefore it is often recommended to provide funds for spares, Cspare, for the pumps [40]. 2 The cost of land, Cland, is related to size of power block [44] and is assumed to be 2.47 $/m in southern California while the cost of royalties, Croyal, is neglected in the present work. The cost of the working fluid, Cwf should be considered as the capital cost for an ORC. The amount of the working fluid needed may be expressed as the volume of liquid required to fill the whole ORC process [45]. Toffolo et al. [46], showed that about 370 kg of Iso-butene are needed for each kg/s of the working fluid for an ORC. More details can be found in [46]. For the Brayton cycle, the cost of the working fluid does not play a major role on the cost of the power block. Table8 shows the price of working fluids for ORC and Brayton cycles. The total operation and maintenance cost, COM, is the sum of the direct manufacturing cost, CDMC, and fixed costs, CFix. COM = CDMC + CFix (28) Sustainability 2017, 9, 1974 11 of 26

CDMC = CMain + CUT (29) The sum of the maintenance cost and utilities cost, which includes wages and benefits, salaries and benefits, materials and services, and manufacturing overhead is called the direct manufacturing cost. As defined in Ref. [40], maintenance cost CMain can be calculated as:

CMain = 0.0805CTCI (30)

Utilities cost is related to the dry cooling tower [44] and it is calculated as:

. 0.7 CUT = 2.1795 × (Wnet(MW)) (31)

Fixed cost includes cost of property taxes and liability insurance is equal to:

CFix = 0.02CTDC (32)

Annual operation hours and electricity generation are:

OHAnnual = CF × 24 × 365 (33) . Enet = Wnet × OHAnnual (34) where CF is the capacity factor of the power block and it is assumed to be 0.8. The Levelized Cost of Electricity (LCOE) of the project is calculated according to Equation (35) as described in [44]:

N COM,n C + n TCI ∑n=1 (1+d ) LCOE = nominal (35) N Enet,n ∑ = n n 1 (1+dreal )

 d   r   d = 1 + real × 1 + − 1 × 100 (36) nominal 100 100 where d is the discount rate and r is the inflation rate. The real discount and interest rates were assumed to be 0.055 and 0.025, respectively. N is economical lifetime of plant set to be 20 years, while n is the year of operation.

Table 8. Price of different working fluids.

Fluid Price ($/kg) Air 0 Butane 1.195 CO2 0.1 Ethanol 0.53 Helium 42.553 Iso-butane 0.86 R11 3.60 R141b 1.75

4. Results and Discussion As explained earlier, the analysis of cycle performance was performed by neglecting the friction and heat losses in the pipes and heat exchangers, and assuming adiabatic turbomachinery (turbine and feed pump). Sustainability 2017, 9, 1974 12 of 26

4.1. Effect of Working Fluid Properties on Thermal Efficiency and Specific Net Work Output Correlations for thermal efficiency developed by the authors [31,32] were used to investigate the effects of the working fluid properties on performance of a simple ORC. Equations (1), (5) and (8) show that, in a subcritical region, thermal efficiency is a function of the specific heat capacity and the latent heat of evaporation. It can be shown that the first derivative of Equations (1) and (8) with respect to CP in the subcritical region is negative (Equation (37)), meaning that at constant maximum and minimum temperatures, working fluids with higher specific heat capacity give lower thermal efficiency.

∂η th < 0 (37) ∂CP Sustainability 2017, 9, 1974 12 of 26 However, the first derivative of Equations (1) and (8) with respect to h f g in the subcritical region Correlations for thermal efficiency developed by the authors [31,32] were used to investigate is positive (Equationthe effects (38)), of the meaningworking fluid that properties at constant on performance maximum of a simple and ORC. minimum Equations (1), temperatures, (5) and (8) working fluids with highershow latent that, in a heat subcritical of evaporation region, thermal efficiency give higher is a function thermal of the specific efficiency. heat capacity and the latent heat of evaporation.

It can be shown that the first derivative of Equations (1) and (8) with respect to in the ∂ηth subcritical region is negative (Equation (37)), meaning> 0 that at constant maximum and minimum (38) temperatures, working fluids with higher specific∂h f g heat capacity give lower thermal efficiency. <0 (37) . Since the first derivatives of expression for Wnet (Equation (21)) with respect to specific heat However, the first derivative of Equations (1) and (8) with respect to ℎ in the subcritical capacity CP andregion latent is positive heat (Equation of evaporation (38)), meaningh fthat g in at theconstant subcritical maximum an regiond minimum are temperatures, positive, Equation (39), working fluids with higher latent heat of evaporation give higher thermal efficiency. working fluids with higher CP or h f g produce higher net power output. >0 (38) . . ∂W ∂ W Since the first derivatives of expressionnet > for0, (Equationnet > 0 (21)) with respect to specific heat (39) capacity and latent heat of evaporation ℎ in the subcritical region are positive, Equation (39), ∂Cp ∂h f g working fluids with higher or ℎ produce higher net power output.

The effect of critical temperature on thermal>0, efficiency>0 (Equations (1) and (8))(39) and net power ◦ ℎ◦ output (Equation (21)) for Tmin = 2 C and Tmax = 100 C is presented in Figure5. As the results show, . The effect of critical temperature on thermal efficiency (Equations (1) and (8)) and net power both ηth and Woutputnet increase (Equation as(21)) the for latentTmin = 2 °C heat and T ofmax = evaporation 100 °C is presented is in increased. Figure 5. As the An results increase show, in the specific heat, Cp, resultsboth in a higher and net increase power as the output latent heat and of evaporation lower thermal is increased. efficiency. An increase in the specific heat, Cp, results in a higher net power output and lower thermal efficiency.

(A) (B) Figure 5. Effect of latent heat of evaporation and specific heat capacity on thermal efficiency and net power Figure 5. Effectoutput of latent of a simple heat ORC of in evaporation the subcritical region: and specific(A) hfg; and heat(B) Cp. capacity on thermal efficiency and net

power output ofDepending a simple on ORC the application in the subcritical of a simple region:subcritical (A ORC,) hfg ;there and is ( Ban) Coptimump. specific heat capacity representing a tradeoff between thermal efficiency and net power output. The optimal value of Cp may be determined by developing a relationship between the LCOE and Cp and finding Dependingits onminimum. the application of a simple subcritical ORC, there is an optimum specific heat capacity representingEquation a tradeoff(15) shows that between in the supercritical thermal region efficiency, thermal efficiency and net is affected power by the output. critical The optimal temperature Tcr. It can be shown that the first derivative of Equation (15) in the supercritical region is value of Cp maypositive be determined (Equation (40)), bymeaning developing that at cons atant relationship maximum and betweenminimum temperatures, the LCOE working and Cp and finding its minimum. fluids with higher critical temperature give higher thermal efficiency. Equation (15) shows that in the supercritical region, thermal efficiency is affected by the critical temperature Tcr. It can be shown that the first derivative of Equation (15) in the supercritical region is Sustainability 2017, 9, 1974 13 of 26 positive (Equation (40)), meaning that at constant maximum and minimum temperatures, working fluids with higher critical temperature give higher thermal efficiency.

∂η th > 0 (40) ∂Tcr

Since the first derivatives of the expression for the net power output (Equation (21)) with respect to the specific heat Cp and critical temperature Tcr (expressed in dimensionless form as Tr) are positive, Equation (41), working fluids with higher Cp or Tcr produce higher net power output.

Sustainability 2017, 9, 1974 . . 13 of 26 ∂Wnet ∂Wnet >0, > 0 (41) >0 (40) ∂Cp ∂Tcr Since the first derivatives of the expression for the net power output (Equation (21)) with In addition,respect the to first the specific derivative heat Cp ofand Equations critical temperature (8), (15) Tcr (expressed and (21) in dimensionless with respect form to ask Trin) are the superheated subcritical andpositive, supercritical Equation (41), region working is negativefluids with higher (Equation Cp or Tcr produce (42)), higher meaning net power that, output. at constant maximum

and minimum temperatures, wet and isentropic>0, working>0 fluids give higher thermal(41) efficiency and net power output, compared to the dry fluids. In addition, the first derivative of Equations (8), (15) and (21) with respect to in the superheated subcritical and supercritical region is negative. (Equation (42)), meaning that, at constant maximum and minimum temperatures∂η , wet and∂W isentropic working fluids give higher thermal th < 0, net < 0 (42) efficiency and net power output, compared∂k to the dry∂k fluids. <0, <0 (42) The effect of critical temperature on thermal efficiency (Equation (15)) and net power output ◦ ◦ (Equation (21)) for TheTmin effect= 2of Ccritical and temperatureTmax = 200 on thermalC is presented efficiency (Equation in Figure (15)) 6and. As net thepower results output show, both ηth . (Equation (21)) for Tmin = 2°C and Tmax = 200 °C is presented in Figure 6. As the results show, both and Wnet increaseand as critical increase temperatureas critical temperature is increased. is increased.

Figure 6. Effect of critical temperature on thermal efficiency and net power output of a simple ORC Figure 6. Effectin of the critical supercritical temperature region. on thermal efficiency and net power output of a simple ORC in the supercritical region. 4.2. The Effect of Operating Conditions on Performance of a Combined Brayton/ORC Cycle Thermal efficiency of a combined regenerative Brayton/ORC cycle, shown in Figure 3, was 4.2. The Effect ofdetermined Operating over Conditions a range of operating on Performance conditions for of twelve a Combined working fluids Brayton/ORC listed in Table Cycle 1 for the bottoming (ORC) cycle, and nine working fluid listed in Table 2 for the topping regenerative Brayton Thermal efficiencycycle. The cycle of parameters a combined used in the regenerative calculations are Brayton/ORCsummarized in Table cycle,9. shown in Figure3, was determined over a range of operating conditions for twelve working fluids listed in Table1 for the Table 9. Cycle parameters for a combined regenerative Brayton/ORC cycle. bottoming (ORC) cycle, and nine working fluid listed in Table2 for the topping regenerative Brayton Parameter Value Reference cycle. The cycle parametersBrayton used cycle in the turbine calculations isentropic efficiency are summarized 0.90 in [34] Table 9. The temperature differenceORC between turbine isentropic the topping efficiency cycle turbine0.87 inlet[34] temperature (TC-TIT) and the heat source temperature ofCompressor 10 ◦C was and assumedpump isentropic in theefficiency calculations. 0.80 In [34] addition, the optimal pressure P2 (MPa) 5–30 Assumed ratio of the topping cycle was determined,T1 (°C), asT8 (°C) described in [4732–62]. At theAssumed optimal pressure ratio, the cycle network output reaches its maximum value.T3 (°C) Since the flow300–1000 rate of theAssumed working fluid in the bottoming Effectiveness 0.85 [34] cycle is dependent on the mass flow rate of the working fluid in the topping cycle, a constant gross power output of 100 MW was assumed for the topping cycle. The topping (regenerative Brayton) cycle exhaust (heat rejection) temperature (T6) for nine working fluids is presented in Figure7 over the range of operating conditions. Sustainability 2017, 9, 1974 14 of 26

Sustainability 2017, 9, 1974 14 of 26 Table 9. Cycle parameters for a combined regenerative Brayton/ORC cycle. Upper temperature difference for HEX (°C) 10 Assumed P10 (MPa), T10 > Tcr Pcr Parameter ValueAssumed Reference P10 (MPa), T10 < Tcr Psat Brayton cycle turbine isentropic efficiency 0.90 [34] ORC turbine isentropic efficiency 0.87 [34] The temperatureCompressor difference and pump between isentropic the efficiencytopping cycle turbine 0.80 inlet temperature [34] (TC-TIT) and the heat source temperature Pof2 (MPa)10 °C was assumed in the calculations. 5–30 In Assumedaddition, the optimal ◦ ◦ pressure ratio of the toppingT1 ( cycleC), T 8was( C) determined, as described 32–62 in [47]. At Assumed the optimal pressure ◦ ratio, the cycle network outputT reaches3 ( C) its maximum value. Since 300–1000 the flow rate Assumed of the working fluid Effectiveness 0.85 [34] in the bottoming cycle is dependent on the mass flow rate of the working fluid in the topping cycle, a Upper temperature difference for HEX (◦C) 10 Assumed constant gross power outputP (MPa), of 100T MW> Twas assumed for the toppingP cycle. 10 10 cr cr Assumed The topping (regenerativeP10 (MPa), Brayton)T10 < T crcycle exhaust (heat Prejection)sat temperature (T6) for nine working fluids is presented in Figure 7 over the range of operating conditions.

(A) (B) (C)

(D) (E) (F)

(G) (H) (I)

Figure 7. Topping cycle exhaust temperature for nine working fluids: (A) Air; (B) Ar; (C) CO2; (D) Figure 7. Topping cycle exhaust temperature for nine working fluids: (A) Air; (B) Ar; (C) CO2;(D) He; He; (E) Kr; (F) Methane; (G) N2; (H) Ne; and (I) O2. (E) Kr; (F) Methane; (G)N2;(H) Ne; and (I)O2.

As the results show, depending on the operating conditions, the topping cycle using CO2 as a As the results show, depending on the operating conditions, the topping cycle using CO as working fluid has the highest and lowest cycle exhaust temperature, followed by air and O2. For2 aexample, working for fluid a TIT has of the1000 highest °C, CO and2 produces lowest the cycle cycle exhaust exhaust temperature, temperature around followed 350 by °C. air For and the O 2. ◦ ◦ Forrange example, of analyzed for a TIToperating of 1000 conditions,C, CO2 produces the exhaust the temperature cycle exhaust of temperaturethe regenerative around CO2 350 BraytonC. For the range of analyzed operating conditions, the exhaust temperature of the regenerative CO2 Brayton Sustainability 2017, 9, 1974 15 of 26 cycle is between 100 °C and 350 °C, which is within the ORC operating range. Thus, the ORC can be selected as the bottoming cycle in the combined regenerative Brayton/ORC cycle configuration. Figure 8 shows the effect of exhaust temperature of the topping cycle (TC-ET) on the A value Sustainabilitygiven in Equation2017, 9, 1974 (22), where quantity A represents improvement in thermal efficiency 15of ofthe 26 combinedSustainability cycle2017, 9 ,with 1974 respect to the topping cycle. Five working fluids having the highest A 15value of 26 cycle(efficiency is between improvement 100 ◦C and with 350 respect◦C, which to the is topping within the cycle) ORC are operating shown in range. Figure Thus, 8. As the can ORC be seen can bein selectedFigurecycle is 8, between as for the the bottoming values 100 °C of and cycletoppi 350ng in °C, thecycle which combined exhaust is within regenerativetemperature the ORC Brayton/ORCloweroperating than range. 227 cycle°C, Thus, Iso-butane configuration. the ORC performs can be betterselectedFigure than as 8theother shows bottoming working the effect cyclefluids. of in exhaustR11 the iscombined the temperature prefe regenerativerred working of the toppingBrayton/ORC fluid for cycle TC-ET (TC-ET)cycle in configuration.the on 227 the °C Aandvalue 327 given°C range.Figure in Equation For 8 shows TC-ET (22), the higher effect where thanof quantity exhaust 327 °C, Atemperature ethanolrepresents gives of improvement the the topping highest cycle inA thermalvalue (TC-ET) (highest efficiency on the efficiency A of value the combinedimprovement).given in Equation cycle with (22), respect where to thequantity topping A cycle.represents Five workingimprovement fluids in having thermal the efficiency highest A ofvalue the (efficiencycombined cycle improvement with respect with to respect the topping to the toppingcycle. Five cycle) working are shown fluids inhaving Figure the8. Ashighest can beA seenvalue in(efficiency Figure8 ,improvement for the values with of toppingrespect to cycle the topping exhaust cy temperaturecle) are shown lower in Figure than 2278. As◦ C,can Iso-butane be seen in performsFigure 8, for better the values than other of toppi workingng cycle fluids. exhaust R11 temperature is the preferred lower workingthan 227 °C, fluid Iso-butane for TC-ET performs in the 227better◦C than and other 327 working◦C range. fluids. For R11 TC-ET is the higher preferred than working 327 ◦C, fluid ethanol for TC-ET gives thein the highest 227 °CA andvalue 327 (highest°C range. efficiency For TC-ET improvement). higher than 327 °C, ethanol gives the highest A value (highest efficiency improvement).

Figure 8. Effect of the topping cycle exhaust temperature on thermal efficiency improvement of a combined regenerative Brayton/ORC cycle relative to the topping regenerative Brayton cycle.

One of the main objectives of this study is selection of the preferred (most suitable, best) working fluid(s) for the given cycle operating conditions. A performance map of thermal efficiency for the combined regenerative Brayton/ORC cycle was developed to enable selection of the best workingFigure fluids 8. Effect for the of thetopping topping an dcycle bottoming exhaust cycles,temperat Figureure on 9. thermal efficiency improvement of a Figure 8. Effect of the topping cycle exhaust temperature on thermal efficiency improvement of a Performancecombined regenerative maps for Brayton/ORC the topping cycle cycle relative are presen to the tedtopping in Figure regenerative 9A,B. AsBrayton shown cycle. in Figure 9A, for thecombined minimum regenerative temperature Brayton/ORC of the combined cycle relative rege tonerative the topping Brayton/ORC regenerative Braytoncycle lower cycle. than 42 °C, dependingOne of on the the main maximum objectives temperature of this andstudy pressure, is selection CO2 orof airthe are preferred the preferred (most working suitable, fluids. best) One of the main objectives of this study is selection of the preferred (most suitable, best) working Asworking shown fluid(s) in Figure for the 9B, given for the cycle minimum operating temperature conditions. higherA performance than 42 map°C, CO of thermal2 is the efficiencypreferred fluid(s)workingfor the forcombined fluid the over given regenerative the cycle entire operating rangeBrayton/ORC conditions.of analyzed cycle Aoperating performancewas developed conditions. map to ofenable Performance thermal selection efficiency map of the for best thethe combinedbottomingworking fluids regenerativeORC for is presentedthe topping Brayton/ORC in an Figured bottoming cycle9C. Dependin was cycles, developedg Figure on the to9. TC-ET, enable Iso-butane, selection of R11, the or best ethanol working are fluidsthe preferredPerformance for the toppingworking maps and fluids. for bottoming the topping cycles, cycle Figure are presen9. ted in Figure 9A,B. As shown in Figure 9A, for the minimum temperature of the combined regenerative Brayton/ORC cycle lower than 42 °C, depending on the maximum temperature and pressure, CO2 or air are the preferred working fluids. As shown in Figure 9B, for the minimum temperature higher than 42 °C, CO2 is the preferred working fluid over the entire range of analyzed operating conditions. Performance map for the bottoming ORC is presented in Figure 9C. Depending on the TC-ET, Iso-butane, R11, or ethanol are the preferred working fluids.

(A) (B)

Figure 9. Cont.

(A) (B) Sustainability 2017, 9, 1974 16 of 26 Sustainability 2017, 9, 1974 16 of 26

Sustainability 2017, 9, 1974 16 of 26

(C)

Figure 9. PerformancePerformance map map of of thermal thermal efficiency efficiency for for a combined a combined regenerative regenerative Brayton/ORC Brayton/ORC cycle: cycle: (A) topping cycle for Tmin < 42 °C; (B◦ ) topping cycle for Tmin > 42 °C; and◦ (C) bottoming cycle. (A) topping cycle for Tmin < 42 C; (B) topping cycle for Tmin > 42 C; and (C) bottoming cycle.

Thermal efficiency of a combined regenerative Brayton/ORC cycle is shown in Figure 10A. At Performance maps for the topping cycle are presented in Figure9A,B. As shown in Figure9A, the maximum temperature of 1000 °C, minimum topping cycle temperature of 32 °C, and Pmax of 30 for the minimum temperature of the combined regenerative Brayton/ORC cycle lower than 42 ◦C, MPa, the combined Brayton/ORC cycle has a thermal efficiency of 55%. Figure 10B shows the A depending on the maximum temperature and pressure, CO or air are the preferred working fluids. value given in Equation (22), i.e., the improvement in2 thermal efficiency of the combined As shown in Figure9B, for the minimum temperature(C) higher than 42 ◦C, CO is the preferred working regenerative Brayton/ORC cycle with respect to the regenerative Brayton2 cycle. Since the cycle fluid over the entire range of analyzed operating conditions. Performance map for the bottoming exhaustFigure temperature 9. Performance of amap regenera of thermaltive efficiency Brayton for cycle a combined is quite regenerative high, the Brayton/ORCrejected heat cycle: used (A )by a ORC is presented in Figure9C. Depending on the TC-ET, Iso-butane, R11, or ethanol are the preferred bottomingtopping ORC cycle increases for Tmin < 42thermal °C; (B) efficiencytopping cycle of thefor Tcombinedmin > 42 °C; cycleand (C by) bottoming up to 15%-points cycle. (A = 0.15). In working fluids. addition, as shown in Figure 10B, efficiency improvement increases as maximum temperature of the ThermalThermal efficiencyefficiency of of a a combined combined regenerative regenerative Brayton/ORC Brayton/ORC cycle cycle is shownis shown in Figurein Figure 10A. 10A. At the At topping cycle is increased. This◦ is because higher maximum temperature results◦ in a higher exhaust maximumthe maximum temperature temperature of 1000 of 1000C, minimum °C, minimum topping topping cycle cycle temperature temperature of 32 ofC, 32 and °C,P maxandof P 30max MPa,of 30 temperature (T6) (see Figure 7), thus the topping cycle is providing higher temperature heat to the the combined Brayton/ORC cycle has a thermal efficiency of 55%. Figure 10B shows the A value bottomingMPa, the combined ORC cycle, Brayton/ORC which increases cycle efficiency has a ther of themal bottoming efficiency cycle.of 55%. Figure 10B shows the A givenvalue ingiven Equation in Equation (22), i.e., (22), the improvementi.e., the improv in thermalement in efficiency thermal of efficiency the combined of the regenerative combined Brayton/ORCregenerative Brayton/ORC cycle with respect cycle to with the regenerative respect to Braytonthe regenerative cycle. Since Brayton the cycle cycle. exhaust Since temperature the cycle ofexhaust a regenerative temperature Brayton of a cycle regenera is quitetive high,Brayton the rejectedcycle is heatquite used high by, the a bottoming rejected heat ORC used increases by a thermalbottoming efficiency ORC increases of the combinedthermal efficiency cycle by of up the to combined 15%-points cycle (A by= up 0.15). to 15%-points In addition, (A as= 0.15). shown In inaddition, Figure as10 B,shown efficiency in Figure improvement 10B, efficiency increases improvement as maximum increases temperature as maximum of the temperature topping cycle of the is increased.topping cycle This is is increased. because higher This is maximum because higher temperature maximum results temperature in a higher results exhaust in temperaturea higher exhaust (T6) (seetemperature Figure7), ( thusT6) (see the toppingFigure 7), cycle thus is providingthe topping higher cycle temperature is providing heat higher to the temperature bottoming ORCheat cycle,to the whichbottoming increases ORC efficiencycycle, which of the increases bottoming efficiency cycle. of the bottoming cycle.

(A) (B)

Figure 10. Thermal efficiency and A value for a combined regenerative Brayton/ORC cycle: (A) thermal efficiency; and (B) A value.

Figures 11A,B shows the net power output in MW generated by the combined regenerative Brayton/ORC cycle and the bottoming ORC, respectively. By the using waste heat from the topping cycle at Tmax of 1000 °C, the power output of the bottoming ORC exceeds 13 MW (25% of the total power output). This is in contradiction with lines 428–429, which state that the constant power (A) (B) output of 100 MW was assumed for the topping cycle. FigureInFigure addition, 10.10.Thermal Thermal as shown efficiency efficiency in Figure and andA value 11B,A value forfor a the combinedfor amaximum combined regenerative toppingregene Brayton/ORCrative cycle Brayton/ORC temperatures cycle: (A cycle:) thermal lower (A) than 700 °C,efficiency;thermal the efficiency;minimum and (B) A and value.temperature (B) A value. of the topping cycle has a significant effect on the power generated by the bottoming ORC. However, for maximum temperatures higher than 700 °C, the minimumFigures topping 11A,B cycleshows temp the eraturenet power does output not have in MW a significan generatedt effectby the on combined power output regenerative of the Brayton/ORC cycle and the bottoming ORC, respectively. By the using waste heat from the topping cycle at Tmax of 1000 °C, the power output of the bottoming ORC exceeds 13 MW (25% of the total power output). This is in contradiction with lines 428–429, which state that the constant power output of 100 MW was assumed for the topping cycle. In addition, as shown in Figure 11B, for the maximum topping cycle temperatures lower than 700 °C, the minimum temperature of the topping cycle has a significant effect on the power generated by the bottoming ORC. However, for maximum temperatures higher than 700 °C, the minimum topping cycle temperature does not have a significant effect on power output of the Sustainability 2017, 9, 1974 17 of 26

Figure 11A,B shows the net power output in MW generated by the combined regenerative Brayton/ORC cycle and the bottoming ORC, respectively. By the using waste heat from the topping ◦ cycle at Tmax of 1000 C, the power output of the bottoming ORC exceeds 13 MW (25% of the total power output). This is in contradiction with lines 428–429, which state that the constant power output of 100 MW was assumed for the topping cycle. In addition, as shown in Figure 11B, for the maximum topping cycle temperatures lower than 700 ◦C, the minimum temperature of the topping cycle has a significant effect on the power generated by the bottoming ORC. However, for maximum temperatures higher than 700 ◦C, the minimum topping cycle temperature does not have a significant effect on power output of the bottoming ORC. Sustainability 2017, 9, 1974 ◦ 17 of 26 ◦ For example, at Tmax = 700 C, increasing the minimum temperature of the topping cycle from 32 C ◦ to 62 SustainabilityC results 2017 in, a 9,500% 1974 increase of the power output of the ORC (i.e., from 2 to 10 MW).17 However, of 26 ◦ at Tmax = 1000 C, increasing the minimum temperature of the topping cycle results in less than 1% increase of the net power output of the bottoming ORC.

(A) (B)

Figure 11. Net power output of the combined regenerative Brayton/ORC cycle and bottoming ORC: Figure 11. Net power output(A) of the combined regenerative Brayton/ORC cycle(B) and bottoming ORC: (A) combined Brayton/ORC cycle; and (B) bottoming ORC. (A) combinedFigure 11. Brayton/ORC Net power output cycle; of the and co (mbinedB) bottoming regenerative ORC. Brayton/ORC cycle and bottoming ORC: (A) combined Brayton/ORC cycle; and (B) bottoming ORC. The results of the techno-economic analysis, i.e., the total capital investment, CTCI, and LCOE for Thea combined results of Brayton/ORC the techno-economic cycle obtained analysis, ov i.e.,er thethe totalrange capital maximum investment, and minimumCTCI, and LCOEcycle for The results of the techno-economic analysis, i.e., the total capital investment, CTCI, and LCOE for temperatures, and two maximum pressures (10 and 30 MPa), are presented in Figure 12. a combineda combined Brayton/ORC Brayton/ORC cycle obtainedcycle obtained over theov rangeer the maximum range maximum and minimum and minimum cycle temperatures, cycle and twotemperatures, maximum and pressures two maximum (10 and pressures 30 MPa), (10 are and presented 30 MPa), are in Figurepresented 12. in Figure 12.

(A) (B)

(A) (B)

Figure 12. Cont. Sustainability 2017, 9, 1974 18 of 26 Sustainability 2017, 9, 1974 18 of 26

Sustainability 2017, 9, 1974 18 of 26

(C) (D)

Figure 12. Total capital investment ($/kWnet) and LCOE ($/MWh) of combined Brayton/ORC cycle: Figure 12. Total capital investment ($/kWnet) and LCOE ($/MWh) of combined Brayton/ORC cycle: (A) CTCI at Pmax = 10 MPa; (B) CTCI at Pmax = 30 MPa; (C) LCOE at Pmax = 10 MPa; and (D) LCOE at Pmax = (A) C at P = 10 MPa; (B) C at P = 30 MPa; (C) LCOE at P = 10 MPa; and (D) LCOE at 30TCI MPa. max TCI max max Pmax = 30 MPa. As Figure 12 shows, the total capital investment and LCOE decrease as the maximum cycle (C) (D) Aspressure Figure is increased.12 shows, For the example, total capital at Tmin investment= 32 °C, Pmax = and 10 MPa LCOE and decreaseTmax = 1000 as °C, the the maximum total capital cycle investmentFigure and 12. LCOE Total capital of ainvestment combined ($/kW Brayton/ORCnet) and ◦LCOE ($/MWh) cycle ofare comb 1000ined Brayton/ORC$/kWnet and cycle: 25 ◦$/MWh, pressure is increased. For example, at Tmin = 32 C, Pmax = 10 MPa and Tmax = 1000 C, the total respectively.(A) CForTCI at thePmax =same 10 MPa; maximum (B) CTCI at P andmax = 30 minimu MPa; (C)m LCOE temperatures at Pmax = 10 MPa; and and maximum (D) LCOE at pressurePmax = of 30 capital investment30 MPa. and LCOE of a combined Brayton/ORC cycle are 1000 $/kWnet and 25 $/MWh, MPa, the values of CTCI and LCOE are 940 $/kWnet and 24 $/MWh, i.e., 6 and 4 percent lower. respectively. For the same maximum and minimum temperatures and maximum pressure of 30 MPa, The effectAs Figure of the 12 maximumshows, the cycletotal capitaltemperature investme isnt more and complex,LCOE decrease since asmo there maximumexpensive cycle materials the values of CTCI and LCOE are 940 $/kWnet and 24 $/MWh, i.e., 6 and 4 percent lower. need pressureto be used is increased. as temperature For example, is atincreased, Tmin = 32 °C, re Psultingmax = 10 MPain step and changeTmax = 1000 in °C, cost. the totalFor capitalexample, a Thesignificant effectinvestment ofincrease the and maximum in LCOE total capitalof cyclea combined investment temperature Brayton/ORC can be is moreobserved cycle complex, are at T1000max = since 500$/kW °C,net more sinceand expensive 25for $/MWh, Tmax > 500 materials °C needa to more berespectively. used expensive as temperature For stainless the same steel is maximum increased, has to andbe resulting usedminimu insteadm in temperatures step of changecarbon and steel in maximum cost. for Forall componentspressure example, of a30 significantof the ◦ ◦ increaseregenerative inMPa, total the capitalBrayton values of investment cycle CTCI and (RBC). LCOE can are be 940 observed $/kWnet and at 24T $/MWh,max = 500 i.e., 6 C,and since 4 percent for lower.Tmax > 500 C a more The effect of the maximum cycle temperature is more complex, since more expensive materials expensiveThe stainless total capital steel has investment to be used and instead LCOE ofof carbon the CO steel2 regenerative for all components Brayton cycle of the (RBC) regenerative and need to be used as temperature is increased, resulting in step change in cost. For example, a combined Brayton/ORC cycle are compared in Figure 13 over the range of Tmax from 300 °C to 900 °C, Brayton cyclesignificant (RBC). increase in total capital investment can be observed at Tmax = 500 °C, since for Tmax > 500 °C Pmax of 10 and 30 MPa, and Tmin = 32 °C. As the results show, the combined Brayton/ORC cycle has a The totala more capital expensive investment stainless andsteel LCOEhas to be of theused CO instead2 regenerative of carbon steel Brayton for all cyclecomponents (RBC) of and the combined significantly lower total capital investment and LCOE, compared to the regenerative◦ Brayton cycle.◦ Brayton/ORCregenerative cycle Brayton are compared cycle (RBC). in Figure 13 over the range of Tmax from 300 C to 900 C, Pmax For example,The totalat T maxcapital = 550 investment °C and◦ P maxand = 30LCOE MPa, of LCOEthe CO 2of regenerative the combined Brayton Brayton/ORC cycle (RBC) cycle and is 17 of 10 and 30 MPa, and Tmin = 32 C. As the results show, the combined Brayton/ORC cycle has a $/MWhcombined (43%) lowerBrayton/ORC compared cycle toare the compared RBC. As in Figurementioned 13 over before, the range because of Tmax fromof the 300 material °C to 900 change, °C, a significantly lower total capital investment and LCOE, compared to the regenerative Brayton cycle. significantPmax of increase 10 and 30 in MPa, total and capital Tmin = investment32 °C. As the and results LCOE show, occurs the combined at Tmax = Brayton/ORC500 °C. cycle has a ◦ For example,significantly at Tmax lower= 550 total Ccapital and investmentPmax = 30 and MPa, LCOE, LCOE compared of theto the combined regenerative Brayton/ORC Brayton cycle. cycle is 17 $/MWhFor (43%) example, lower at T comparedmax = 550 °C and to the Pmax RBC. = 30 MPa, As mentionedLCOE of the before,combined because Brayton/ORC of the cycle material is 17 change, $/MWh (43%) lower compared to the RBC. As mentioned before, because of the material◦ change, a a significant increase in total capital investment and LCOE occurs at Tmax = 500 C. significant increase in total capital investment and LCOE occurs at Tmax = 500 °C.

(A) (B)

(A) (B)

Figure 13. Cont. Sustainability 2017, 9, 1974 19 of 26

Sustainability 2017, 9, 1974 19 of 26

Sustainability 2017, 9, 1974 19 of 26

(C) (D)

Figure 13. Total capital investment ($/kWnet) and LCOE ($/MWh) of the RBC and combined Figure 13. Total capital investment ($/kW ) and LCOE ($/MWh) of the RBC and combined Brayton/ORC cycle: (A) CTCI at Pmax = 10 MPa;net (B) CTCI at Pmax = 30 MPa; (C) LCOE at Pmax = 10 MPa; and Brayton/ORC(D) LCOE cycle: at (PAmax) =C 30TCI( CMPa.) at Pmax = 10 MPa; (B) CTCI at Pmax = 30 MPa;(D (C) ) LCOE at Pmax = 10 MPa;

and (D) LCOEFigure at 13.Pmax Total= 30capital MPa. investment ($/kWnet) and LCOE ($/MWh) of the RBC and combined 4.3. Validation of Results for the Combined Brayton/ORC Cycle Brayton/ORC cycle: (A) CTCI at Pmax = 10 MPa; (B) CTCI at Pmax = 30 MPa; (C) LCOE at Pmax = 10 MPa; and 4.3. ValidationThe of(D) Results LCOE results at P forobtainedmax = the 30 MPa. Combined in this study Brayton/ORC by using the Cycle EPV-11 modeling software were validated against the results published in the literature. The combined regenerative Brayton/ORC cycle 4.3. Validation of Results for the Combined Brayton/ORC Cycle The resultsutilizing obtained CO2 in the in topping this study cycle and by Iso-pentane using the EPV-11in the bottoming modeling ORC software was modeled were by validatedDunham against et al. [34] using the Engineering Equation Solver (EES). Table 10 presents the cycle parameters used the results publishedThe results in theobtained literature. in this Thestudy combined by using the regenerative EPV-11 modeling Brayton/ORC software were cycle validated utilizing CO2 in in the calculations. The heat input to the topping cycle is assumed to be 100 MW. the toppingagainst cycle the and results Iso-pentane published in in the the bottoming literature. ORCThe combined was modeled regenerative by Dunham Brayton/ORC et al. cycle [34] using the utilizing CO2 in the topping cycle and Iso-pentane in the bottoming ORC was modeled by Dunham Engineeringet al. Equation [34] using Solverthe Engineering (EES). Equation Table 10 Solver presents (EES). theTable cycle 10 presents parameters the cycle used parameters in the used calculations. The heat inputin the tocalculations. the topping The heat cycle input is assumedto the topping to becycle 100 is as MW.sumed to be 100 MW.

Figure 14. Thermal efficiency for different turbine inlet temperature for combined Brayton/ORC cycle.

Figure 14. ThermalTable efficiency 10. Cycle for parametersdifferent turbine for Combined inlet temperature Brayton/ORC for cycle.combined Brayton/ORC Figure 14. Thermal efficiency for different turbine inlet temperature for combined Brayton/ORC cycle. cycle. Parameter Value Compressor isentropic efficiency 0.80 Table 10.TableCycle 10. Cycle parametersPump parameters isentropic for for efficiency Combined Combined Brayton/ORC Brayton/ORC 0.80 cycle. cycle. Topping cycle turbine isentropic efficiency 0.9 RecuperatorParameter effectiveness Value 0.85 BottomingParameterCompressor cycle turbine isentropic isentropic efficiency efficiency 0.870.80 Value PumpHeater isentropic pressure dropefficiency (%) 0.80 5 CompressorTopping isentropicMinimum cycle turbine temperature efficiency isentropic (°C) efficiency 0.932 0.80 Pump isentropicRecuperator efficiency effectiveness 0.85 0.80 Bottoming cycle turbine isentropic efficiency 0.87 Topping cycle turbineHeater isentropic pressure drop efficiency (%) 5 0.9 RecuperatorMinimum effectiveness temperature (°C) 32 0.85 Bottoming cycle turbine isentropic efficiency 0.87 Heater pressure drop (%) 5 Minimum temperature (◦C) 32 Topping cycle maximum pressure (MPa) 20 Sustainability 2017, 9, 1974 20 of 26

Topping cycle maximum pressure (MPa) 20 Sustainability 2017, 9, 1974 20 of 26 4.4. Combined steam Rankine/ORC Cycle 4.4. CombinedSteam Rankine steam Rankine/ORCcycle is the most Cycle widely used thermodynamic cycle for power generation. As a heat Steamengine, Rankine it rejects cycle large is amou the mostnts of widely low-temperature used thermodynamic grade heat. cycle Its efficiency for power ranges generation. from about As a 32%heat engine,(efficiencies it rejects are largegiven amounts on the ofhigher low-temperature heating value grade (HHV) heat. basis) Its efficiency for the rangessubcritical from live about steam 32% (efficienciesconditions, to are 45% given for on the the advanced higher heating ultra-supercriti value (HHV)cal live basis) steam for the conditions subcritical [34]. live Most steam (about conditions, 95%) ofto the 45% exiting for the power advanced generation ultra-supercritical fleet is subcritical, live steam about conditions 5% is supercritical, [34]. Most (aboutand only 95%) a small of the number exiting powerof the units generation operate fleet at isultra-supe subcritical,rcritical about live 5% issteam supercritical, conditions and and only efficiency a small numberof about of 40% the units[34]. Commercialoperate at ultra-supercritical operation of the advanced live steam ultra-supercritical conditions and efficiency units is expected of about within 40% [ 34the]. next Commercial 10 years [48].operation of the advanced ultra-supercritical units is expected within the next 10 years [48]. The efficiency efficiency of the steam Rankine cycle could be improved by addition of a bottoming cycle. cycle. Due to the low temperature of the rejected heat, thethe ORCORC isis aa goodgood choice.choice. An analysis ofof aa combinedcombined steam steam Rankine/ORC Rankine/ORC cycle cycle was was performed performed to to determine determine the the increase increase in powerin power output output that that could could beachieved be achieved by adding by adding a bottoming a bottoming ORC ORC to the to utility-scale the utility-scale (600 MW) (600 steamMW) steamRankine Rankine cycle. Thecycle. analysis The analysis included included the effect the ofeffe ambientct of ambient conditions conditions (heat sink (heat temperature) sink temperature) on the poweron the power output output of the bottomingof the bottoming ORC. ORC. The analyzed utility-scale st steameam Rankine cycle employs a steam turbine with two double low-pressure (LP) exhausts. The The steam from the LP exhausts is condensed in two steamsteam condenserscondensers (A and B) placedplaced inin aa serialserial arrangement,arrangement, FigureFigure 1515.. The cold cooling water from the coolingcooling tower (CT) flowsflows throughthrough condenser condenser A first,A first, then then through through condenser condenser B. Since B. Since the temperature the temperature of the coolingof the watercooling at water the entrance at the entrance to condenser to condenser B is higher B is compared higher compared to condenser to condenser A, the pressure A, the (and pressure saturation (and saturationtemperature) temperature) in condenser in B iscondenser higher compared B is higher to condenser compared A (Tableto condenser 11). The A hot (Table cooling 11). water The from hot coolingcondenser water B is from circulated condenser back B to is the circulated CT. back to the CT.

Figure 15. Schematic of the Steam Rankine cycle wi withth two double exhaust LP turbines.

For the purpose of the analysis, it was assumed that the heat rejected in the steam condensers For the purpose of the analysis, it was assumed that the heat rejected in the steam condensers (latent heat of condensation) can be utilized as the heat input to the ORC. Two ORCs were (latent heat of condensation) can be utilized as the heat input to the ORC. Two ORCs were employed, employed, one for the each condenser. It was also assumed that addition of the bottoming ORC does one for the each condenser. It was also assumed that addition of the bottoming ORC does not not affect performance of the existing steam Rankine cycle because the temperature (and affect performance of the existing steam Rankine cycle because the temperature (and corresponding corresponding saturation pressure) at which heat is rejected in the condensers A and B remained the saturation pressure) at which heat is rejected in the condensers A and B remained the same. same. Although adding a bottoming ORC to the existing steam Rankine cycle would eliminate the Although adding a bottoming ORC to the existing steam Rankine cycle would eliminate the difference in saturation temperature and pressure between condensers A and B, the analysis was difference in saturation temperature and pressure between condensers A and B, the analysis was performed for conditions corresponding to the condenser A and B operating conditions to illustrate performed for conditions corresponding to the condenser A and B operating conditions to illustrate the effect of the heat source temperature. Properties of the ORC heat source used in the analysis are the effect of the heat source temperature. Properties of the ORC heat source used in the analysis are summarized in Table 11. In addition, the ORC analysis was performed over a range of minimum summarized in Table 11. In addition, the ORC analysis was performed over a range of minimum temperatures and for twelve working fluids listed in Table1 to determine the net power output of temperatures and for twelve working fluids listed in Table 1 to determine the net power output of each of the two ORCs associated with condensers (Cases) A and B. The cycle parameters used in the each of the two ORCs associated with condensers (Cases) A and B. The cycle parameters used in the analysis are summarized in Table 12. analysis are summarized in Table 12. Sustainability 2017, 9, 1974 21 of 26

Table 11. Properties of the heat source.

Parameter Condenser (Case) B Condenser (Case ) A Sustainability 2017, 9, 1974 21 of 26 Mass flow rate of exhaust steam (kg/s) 165.63 165.63 ◦ Exhaust steam temperatureTable ( C) 11. Properties of the49.73 heat source. 38.38 Exhaust steam pressure (MPa) 0.012 0.0067 Latent heat of evaporationParameter (kJ/kg) Condenser 2382.5 (Case) B Condenser (Case 2409.8 ) A Mass flowquality rate of exhaust steam (kg/s) 165.63 1 165.63 1 Exhaust steam temperature (°C) 49.73 38.38 Heat input (MWth) 394.89 399.13 Exhaust steam pressure (MPa) 0.012 0.0067 Latent heat of evaporation (kJ/kg) 2382.5 2409.8 Tablequality 12. Cycle parameters for the1 bottoming ORC. 1 Heat input (MWth) 394.89 399.13 Parameter Value Reference Table 12. Cycle parameters for the bottoming ORC. Turbine isentropic efficiency 0.87 [34] Pump IsentropicParameter efficiency Value 0.80 Reference [34] MinimumTurbine temperature isentropic (◦ efC)ficiency 0.87 2–30 [34] Assumed Pump Isentropic efficiency 0.80 [34] Maximum pressure (MPa) Psat Assumed Minimum temperature (°C) 2–30 Assumed Maximum pressure (MPa) Psat Assumed Since the condenser in the topping steam Rankine cycle would be used as the evaporator in the Since the condenser in the topping steam Rankine cycle would be used as the evaporator in the bottomingbottoming ORC, the ORC, steam the steam exhausted exhausted by by the the LP LP turbineturbine exhausts exhausts would would be condensed be condensed in the ORC in the ORC evaporatorevaporator providing providing heat input heat toinput the bottomingto the bottoming ORC. ORC. Figure Figure 16 shows16 shows the the schematic schematic ofof thethe combined steam Rankine/ORCcombined steam cycle. Rankine/ORC cycle.

FigureFigure 16. Schematic 16. Schematic of theof the combined combined steam steam Rankine/ORC Rankine/ORC cycle. cycle.

Depending on the geographical location of the steam Rankine power plant, the minimum ORC Dependingtemperature on the would geographical follow seasonal location variations. of A the 2 °C steam to 30 °C Rankine range, corresponding power plant, to the summer minimum ORC temperatureand would winter followconditions, seasonal respectively, variations. was assumed A 2 ◦C toin 30the◦ Canalysis. range, The corresponding effect of the tominimum the summer and temperature and working fluid on the net power output of the bottoming ORC is presented in Figure winter conditions, respectively, was assumed in the analysis. The effect of the minimum temperature 17. and working fluid on the net power output of the bottoming ORC is presented in Figure 17. As the results presented in Figure 17 show, ethanol produces the highest power output for both cases. For Case B (heat source temperature of 49.73 ◦C) and minimum temperature on 2 ◦C, one bottoming ORC would provide additional 45 MW power output (90 MW total, or 15% of the current plant output). At the same minimum temperature, the additional power output for Case A would be 36 MW (72 MW total). As the minimum ORC temperature increases, the power output of the ORC decreases linearly; at the minimum ORC temperature of 30 ◦C, the ORC power output would be 17.5 MW and 6.5 MW for Cases A and B, respectively. The lower power output is due to the smaller difference between the cycle maximum and minimum temperatures (19.73 ◦C and 8.38 ◦C) and, thus, diminishing efficiency of the ORC. The difference in power output between R11 and R141b is negligible. Since, using R11 and ethanol has flammability and ODP issues, R141b was selected as the best working fluid for the bottoming ORC. Sustainability 2017, 9, 1974 22 of 26

With R141b, the maximum additional power output generated by the ORC is 35 and 44 MW for Cases

A and B, respectively.Sustainability 2017, 9, 1974 22 of 26

(A) (B)

Figure 17. Net ORC power output of ORC for Cases A and B: (A) Case A; and (B) Case B. Figure 17. Net ORC power output of ORC for Cases A and B: (A) Case A; and (B) Case B. As the results presented in Figure 17 show, ethanol produces the highest power output for both As showncases. inFor Figure Case B 17(heat, during source thetemperature winter of when 49.73 the °C) minimumand minimum temperature temperature ison low2 °C, (around one 2 ◦C), bottoming ORC would provide additional 45 MW power output (90 MW total, or 15% of the current additionalplant power output). output At the of same 70 to minimu 88 MWm temperature, can be produced the additional by the power bottoming output for ORCCase A for would Cases be A and B, ◦ ◦ respectively.36 MW The (72 large MW differencetotal). between the winter (2 C) and summer (30 C) temperatures has a significant effectAs the on minimum the ORC ORC power temp output.erature increases, the power output of the ORC decreases linearly; Althoughat the minimum adding theORC bottomingtemperature of ORC 30 °C, to the the ORC steam power Rankineoutput would cycle be 17.5 can MW generate and 6.5 MW more power for Cases A and B, respectively. The lower power output is due to the smaller difference between the (i.e., the “fuelcycle free” maximum megawatts), and minimum it requires temperatures additional (19.73 °C theand 8.38 capital °C) and, investment. thus, diminishing The capital efficiency investment for the ORCof the and ORC. temperature ranges used in this study is around 3000 $/kWnet. A more detailed analysis is neededThe difference to determine in power LCOE output of between the Rankine/ORC R11 and R141b cycle. is negligible. Since, using R11 and ethanol has flammability and ODP issues, R141b was selected as the best working fluid for the 5. Conclusionsbottoming ORC. With R141b, the maximum additional power output generated by the ORC is 35 and 44 MW for Cases A and B, respectively. The analysisAs shown and in optimization Figure 17, during of the an winter organic when Rankine the minimum cycle temperature (ORC)used is low as(around a bottoming 2 °C), cycle in the Brayton/ORCadditional power and output steam of 70 Rankine/ORC to 88 MW can be produced combined by the cycle bottoming configurations ORC for Cases was A and performed B, in respectively. The large difference between the winter (2 °C) and summer (30 °C) temperatures has a this study. Parametric calculations were performed to evaluate the thermodynamic performance significant effect on the ORC power output. (thermal efficiencyAlthough and adding net the power bottoming output) ORC to of the the steam combined Rankine cycle Brayton/ORC can generate more cycle power over (i.e., a range of operatingthe conditions. “fuel free” megawatts), A thermodynamic it requires additional analysis the of capital the subcritical,investment. The superheated capital investment subcritical, for and transcriticalthe ORC ORC and was temperature performed ranges using used Ebsilon in this study Professional is around 3000 V11 $/kW (EPV-11)net. A more power detailed systems analysis modeling is needed to determine LCOE of the Rankine/ORC cycle. software. A techno-economic analysis was performed for the combined Brayton/ORC cycle to determine5. the Conclusions total capital investment and levelized cost of electricity (LCOE). The effectThe of analysis the working and optimization fluid properties of an organic on cycleRankine performance cycle (ORC) used was as investigated a bottoming cycle by performingin analysis forthetwelve Brayton/ORC working and steam fluids. Rankine/ORC The results combined show thatcycle workingconfigurations fluids was withperformed higher in this specific heat capacity orstudy. latent Parametric heat of evaporationcalculations were produce performe higherd to netevaluate power the output thermodynamic in a subcritical performance region, while (thermal efficiency and net power output) of the combined Brayton/ORC cycle over a range of working fluids with higher specific heat capacity or latent heat of evaporation produce lower or higher operating conditions. A thermodynamic analysis of the subcritical, superheated subcritical, and thermal efficiency,transcritical respectively. ORC was performed In a using supercritical Ebsilon Professional region, workingV11 (EPV-11) fluids power with systems higher modeling specific heat capacity orsoftware. critical A temperature techno-economic produce analysis higher was perf thermalormed efficiencyfor the combined and net Brayton/ORC power output. cycle to determine the total capital investment and levelized cost of electricity (LCOE). In the case of a regenerative Brayton/ORC, the results show that CO2 and air are the best working The effect of the working fluid properties on cycle performance was investigated by performing fluids for the topping cycle. Depending on the exhaust temperature of the topping cycle, Iso-butane, analysis for twelve working fluids. The results show that working fluids with higher specific heat R11 and ethanolcapacity areor latent the heat preferred of evapor workingation produce fluids higher for net the power bottoming output in ORC a subcritical cycle, region, resulting while in highest efficiency of the combined cycle. For the cycle operating conditions used in this study, the results show that, by the using the waste heat from the topping cycle, the maximum power output of the bottoming ORC exceeds 25% of the total power output, increasing thermal efficiency of the combined cycle by 15%. A performance map is constructed and presented as guidance for selection of the best working fluid(s) for the topping and bottoming cycles for the specified set of cycle operating conditions. Sustainability 2017, 9, 1974 23 of 26

The results of a techno-economic analysis show that a combined Brayton/ORC cycle has significantly lower total capital investment and LCOE, compared to the regenerative Brayton cycle. An analysis of a combined steam Rankine/ORC cycle was performed to determine the increase in power output that could be achieved by adding a bottoming ORC to the utility-scale (600 MW) steam Rankine cycle. Considering the flammability and ODP issues, R141b was selected as the best working fluid for the bottoming ORC. The total additional power output, generated by the bottoming ORC, is a strong function of the sink temperature and varies from 88 MW in the winter to 35 MW in the summer, i.e., by more than 60% for Case B. In conclusion, recovery and utilization of waste heat in the ORC is an efficient and cost effective method for increasing power output and thermal efficiency of power cycles. The magnitude of the improvement is, however, highly dependent on the ORC sink (minimum) temperature, and the difference between the maximum and minimum operating temperatures of the ORC.

Acknowledgments: This research was supported by Energy Production and Infrastructure Center (EPIC) at UNC Charlotte and American Public Power Association (APPA). Author Contributions: Alireza Javanshir, Nenad Sarunac and Zahra Razzaghpanah wrote the paper. Alireza Javanshir developed the models and analyzed the data under supervision of Nenad Sarunac and Zahra Razzaghpanah collaborated in developing the literature review and data analysis. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

C Cost CP Specific heat [kJ/kg·K] d Discount rate h Enthalpy [kJ/kg] h f g Latent heat [kJ/kg] . m Mass flow rate [kg/s] P Pressure [MPa] . Q Heat rate [kW] r Inflation rate s Specific entropy [kJ/kg.K] T Temperature [K] . W Power [kW] Greek symbols η Efficiency ε Effectiveness Subscript 1–10 State points in the cycle c Compressor cr Critical point ev Evaporation f Working fluid in Inlet max Maximum md modified min Minimum net Net p Pump r Dimensionless reg Regeneration t Turbine th Thermal Sustainability 2017, 9, 1974 24 of 26

Acronyms EPV-11 Ebsilon Professional V11 FOM Figure of merit GWP Global warming potential Ja Jacob number ODP Ozone depletion potential ORC Organic Rankine cycle TCI Total capital investment TIT Turbine inlet temperature LCOE Levelized cost of electricity

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