An Assessment of Subcritical and Trans-critical Organic Rankine Cycles for Waste-heat Recovery

An Assessment of Subcritical and Trans-critical Organic Rankine Cycles for Waste-heat Recovery

Available online at www.sciencedirect.com ScienceDirect Energy Procedia 105 (2017) 1870 – 1876 The 8th International Conference on Applied Energy – ...

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Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 105 (2017) 1870 – 1876

The 8th International Conference on Applied Energy – ICAE2016

An assessment of subcritical and trans-critical organic Rankine cycles for waste-heat recovery Oyeniyi A. Oyewunmia*, Simó Ferré-Serresa, Steven Lecompteb, Martijn van den Broekb, Michel De Paepeb, Christos N. Markidesa a

Clean Energy Processes (CEP) Laboratory, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK b Department of Flow, Heat and Combustion Mechanics, Ghent University, 9000 Gent, Belgium

Abstract Organic Rankine cycle (ORC) systems are increasingly being deployed for waste-heat recovery and conversion in industrial settings. Using a case study of an exhaust flue-gas stream at a temperature of 380 °C as the heat source, an ORC system power output in excess of 10 MW is predicted at exergy efficiencies ranging between 20% and 35%. By comparison with available experimental data, the thermodynamic properties (including those in the supercritical region) of working fluids are shown to be reliably predicted by the SAFT-VR Mie equation of state; this verification is quite important as this is the first time that the SAFT-VR Mie equation of state is used for thermodynamic property prediction of working fluids in their supercritical state in trans-critical ORC systems. Various cycle configurations and the use of working-fluid mixtures are also investigated. ORC systems operating on trans-critical cycles and those incorporating an internal heat exchanger (IHE) are seen to be beneficial from a thermodynamic perspective, they are, however, more expensive than the simple ORC system considered (subcritical cycle with no IHE). Furthermore, ORC systems using pure working fluids are associated with slightly lower costs than those with fluid mixtures. It is concluded that a basic ORC system utilizing pure working fluids shows the lowest specific investment cost (SIC) in the case study considered. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-reviewof under responsibility of ICAE Peer-review under responsibility the scientific committee of the 8th International Conference on Applied Energy.

Keywords: ORC, SAFT-VR Mie, working-fluid mixtures, thermo-economic assessment, trans-critical

1. Introduction ORC systems can be used in a range of applications, including waste heat recovery, renewable heat (geothermal, biogas/mass) conversion, solar-thermal power, and present an opportunity to take advantage of low-/medium-temperature heat sources that would otherwise be either vented to the atmosphere (in the case of chemical/power plants) or simply not used (low-temperature solar, biomass or geothermal water)

* Corresponding author. Tel.: +44 (0)20 7594 1442 E-mail address: [email protected]

1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.548

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[1,2]. One of the main features of ORCs is the unique potential of using a broad range of working fluids, which allows one to design cycles based on their characteristic heat source. In particular, an internal heat exchanger (IHE, also called a recuperator or regenerator) can be utilized in ORC systems, especially those with dry or isentropic fluids, to supply an additional amount of heat in the heating up stage, since the working fluid is still superheated at the outlet of the expander. This can reduce the amount of thermal energy extracted from the heat source (thus relaxing the evaporator pinch limitations), thereby allowing the ORC to operate at higher working-fluid flowrates till the pinch conditions are reestablished, thus enabling higher thermal efficiencies and power outputs respectively, for the same heat source conditions. Furthermore, the performance of an ORC system can be enhanced by operating at trans-critical conditions, achieved by pumping up the fluid above its critical pressure, followed by (supercritical) heat addition till the critical temperature is exceeded, expansion, and subcritical heat rejection to the cooling stream. This may lead to an increase in thermal efficiency and power output [2], although it may also carry an increase in the system cost. Also, some working-fluid mixtures can lead to power and efficiency improvements especially in fully subcritical cycles [3-6]. These performance improvements are brought about by the temperature glides of the evaporating fluid leading to a better thermal match with the heat source and reducing the associated exergy losses. Thus in this work, we investigate the benefits and drawbacks of employing trans-critical cycle designs and working-fluid mixtures in ORC systems. 2. Methodology and case study An ORC setup along with the associated T-s diagram for a subcritical cycle without an IHE are shown in Fig. 1, with the fluid states at labelled points through the cycle. The cycle is modelled via energy balances across each component, accounting for the isentropic efficiencies of the pump (85%) and turbo-expander (75%) [1,7]. For the heat exchangers (which are of the shell and tube type), the energy balance is carried out on both the hot and cold streams, with the assumption of no heat losses in the system, and with a minimum pinch temperature difference of 10 °C. The heat addition and heat rejection processes are assumed to be isobaric, thus the working fluid is evaporated at a constant pressure (P23). The value of P23 in relation to the working-fluid’s critical pressure (Pcrit) determines whether the ORC is subcritical (P23 < Pcrit) or trans-critical (P23 > Pcrit). Details of the modelling equations can be found in general ORC literature [3,4,5,7].

Fig. 1. LEFT: Schematic diagram of a basic ORC engine. RIGHT: T-s diagram of an ORC with pure working fluid.

Working fluid properties are provided by the SAFT-VR Mie equation of state (EoS) [8]. The normal alkanes, butane, pentane, hexane and their binary mixtures are considered as working fluids for the subcritical and trans-critical cycles. The high temperature of the heat source (flue gas from a cement clinker kiln, with a flowrate of 185 kg.s-1 and temperature of 380 °C [9]) makes such cycles possible, including the option of deploying an IHE. The cooling stream is water at 25 °C; its flowrate is suitably adjusted to ensure

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its temperature increase is less than 30 °C and that the condenser pinch conditions are satisfied. In addition, CO2 is considered as a benchmark fluid, due to its broad use in the literature of supercritical cycles. The SAFT-VR Mie molecular parameters that completely describe each of the working fluids (the alkanes and CO2) are presented in Ref. [8]. The performance indices used in evaluating the different cycle/system configurations with the working fluids include the net power output (Ẇnet), the cycle exergy efficiency and the specific investment cost (SIC, in £ per kW) of the ORC system. The costs of the ORC systems are calculated by summing together the costs (in £) of the individual components (pump, expander and the heat exchangers) that are calculated using the CAPCOST costing method [10]. For the simulations, the objective is to maximize Ẇnet, with the working-fluids’ evaporation and condensation pressures, flow-rate and degree of superheat as decision variables; it is expected that, for a particular system with a selected working fluid, this objective is somewhat related to that of minimizing the systems SICs [3,4]. This relationship is further explored for trans-critical ORC systems. 3. Results and discussion 3.1. Thermodynamic property calculations We begin this discussion by comparing the predictions of the properties of the working fluids (for pure fluids and mixtures) obtained from the SAFT-VR Mie EoS with those obtained experimentally from the NIST database, as a form of validation. One of the advantages of this EoS lies in its simplicity; only five parameters (compared to the numerous parameters necessary in cubic EoS and correlated/fitted equations in computer programs like REFPROP) are required for the complete description of the non-associating fluids considered in this work [3,8], from which the required thermodynamic properties can be obtained. These comparisons are unique, and necessary in that this is the first time that the thermodynamic property prediction capabilities of SAFT-VR Mie are extended to the supercritical region of working fluids in ORC systems. The comparisons are illustrated in Figs. 2 and 3. In Fig. 2, we present the VLE temperature-density and temperature-specific entropy (T-ρ and T-s) coexistence envelope for n-pentane calculated with SAFT-VR Mie. Also included are the density, specific heat capacity (cp) and specific entropy isobars (at P = 5, 10, 20, 40 and 80 bar). These isobars have been chosen to span both the subcritical and supercritical regions and both condensed-liquid and superheated-vapour regions. The experimental values from NIST are plotted along for comparison. The descriptions of the fluid density, heat capacity and specific entropy in all the regions of interest for a trans-critical ORC system – the subcritical condensed-liquid and superheated-vapour phases as well as the supercritical region – are excellent. This degree of agreement is extremely challenging to achieve using an analytical EoS, especially for a calorific quantity such as the specific entropy and we are not aware of any literature where a similar degree of agreement has been reported. Although, slight deviations are noticeable close to the critical region, the predictions of the isobaric specific heat capacities from the SAFTVR Mie EoS are also seen to be consistent with the available experimental data. Calorific quantities such as entropies and enthalpies are very important in the simulation and description of power systems such as the organic Rankine cycle. These are presented in Fig. 3 in T-s and P-h vapourliquid spaces for the pure working fluids of interest and their mixtures (at x = 20%, 40%, 60% and 80% mass fractions). From Fig. 3, it is once again evident that the descriptions obtained with the SAFT-VR Mie EoS are in very good agreement with the available experimental data, for both the pure fluids and the mixtures. This further highlights the predictive power of this EoS, especially as these data (s, h) were not

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included in the refinement of the model parameters, thereby providing confidence in the reliability of the property predictions from SAFT-VR Mie for the working fluids of interest.

Fig. 2. Selected thermodynamic properties (ρ, cP and s) of n-pentane, at subcritical and supercritical pressures, as predicted using SAFT-VR Mie EoS (curves). Validation data from the NIST database are indicated by the symbols.

Fig. 3. LEFT: T-s phase boundaries (saturation curves) for pure working fluids. MIDDLE: T-s phase boundaries for Hexane + Pentane mixtures. RIGHT: P-h phase boundaries for Butane + Hexane mixtures. Predictions using SAFTVR Mie EoS are indicated with curves, while validation data from the NIST database are indicated by the symbols.

3.2. Waste-heat recovery ORC simulation results As mentioned earlier, one of the objectives was to investigate the application of zeotropic working-fluid mixtures in trans-critical ORCs. Thus, simulations (to maximize Ẇnet) were carried out to determine mixture combination(s) that deliver higher net power outputs than the constituent pure fluids (Fig. 4). Of the 3 combinations tested, only in two cases did a mixture provide an improvement over the constituent pure fluids – in Hexane + Pentane (xC5H12 = 0.6) and Butane + Hexane (xC6H14 = 0.9). Therefore, we proceeded to carry out simulations (to maximize Ẇnet) employing these fluid mixtures and the selected pure working fluids in subcritical and trans-critical ORCs, including cycles with supercritical CO2 for comparison. The results of these simulations are presented Fig. 5, where we have the power outputs from subcritical and trans-critical cycles, and the SICs of the subcritical ORC systems. As expected, the trans-critical cycles deliver higher power outputs than the subcritical cycles. Furthermore, the ORC systems with internal regeneration deliver a higher power output than those without one, since the internal heat exchanger (IHE) helps to increase the thermal efficiency of the cycle thus making more heat available for subsequent conversion to power. The improvement provided by the IHE depends on the state (temperature) of the working fluid exiting the turbine – cycles with the working fluid exiting at higher temperatures are likely to experience a higher additional power provided. One would ordinarily expect indices like the thermal and exergy efficiencies to follow the same trends as the power output. Amongst the pure working fluids, butane generally leads to ORC systems (both subcritical and transcritical) with the lowest power output while pentane leads to those with the highest power outputs. This is

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especially true for basic ORC systems (i.e., those without IHE). This is also true for subcritical systems with IHE while for trans-critical systems with IHE, hexane delivers the lowest power output; cycles with pentane result in the highest power output in all cases. Thus one can infer that the IHE is most effective (in terms of additional power generated) in trans-critical ORC systems with butane as the working fluid. The systems with the two working-fluid mixtures (C5+C6 – 60% pentane + 40% hexane and C6+C4 – 90% hexane + 10% butane) are seen to deliver higher power outputs than those with the constituent pure fluids in both the trans-critical and subcritical cases, for systems with and without IHE. Systems with pure pentane provide more power than those with C6+C4, at least for cases with no IHE. However, when an IHE is considered, systems with C6+C4 are seen to generate more power than those with pentane, further buttressing the earlier inference that the IHE is most effective in systems containing butane as (part of) the working fluid. Estimates of the costs of the ORC systems are also presented in Fig. 5. These SIC figures are presented in units of pounds per kW (£.kW-1) such that each absolute system cost in £ ($/£=0.75) is normalized by the net power ratings of that system. As expected, the systems based on trans-critical cycles are more expensive than those based on subcritical cycles, even though they deliver higher power outputs. An examination of the individual component costs reveals that this is due to the costs associated with the higher pressure equipment (evaporator and turbine) required for the trans-critical systems, and the lower overall heat-transfer coefficients of the supercritical working fluids in such systems. With regards to the SICs of systems using mixtures, these are not always lower than those of systems using pure fluids. Systems with pure pentane are seen to have lower SICs than those with C5+C6, while those with pure butane are seen to have lower SICs than those with C6+C4. Thus, the improved power output provided by the cycles with working-fluid mixtures comes at a disadvantage of additional system costs.

Fig. 4. Power output from trans-critical ORC plants with working-fluid mixtures at different evaporation pressures; x represents the mass fractions. LEFT: Butane + Pentane. MIDDLE: Hexane + Pentane. RIGHT: Butane + Hexane.

Fig. 5. Performance indices of optimised subcritical (P23/Pcrit < 1) and trans-critical (P23/Pcrit > 1) ORC plants with pure working fluids and working-fluid mixtures. LEFT: Net power output for cycles without IHE. MIDDLE: Specific investment costs (SICs) for cycles without IHE. RIGHT: Net power output for cycles with IHE.

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Fig. 6. Performance indices of optimised supercritical CO2 and ORC plants without regeneration (– IHE) and with internal regeneration (+ IHE), with pure working fluids (butane, pentane, hexane and CO2 respectively) and workingfluid mixtures. LEFT: Exergy efficiency. MIDDLE: Net power output. RIGHT: Specific investment costs (SICs).

The performance indices (including the exergy efficiency) of the best system (that with the highest net power) for each working fluid are presented in Fig. 6. Also included are the indices for systems operating on supercritical CO2, for comparison purposes. Without regeneration, the system with pentane is the most efficient (more than the mixtures), while that with CO2 is the least efficient. The CO2 system also produces the least net-power output due to the large compression duty involved; ordinarily, its expander power output is indeed higher than those of the ORCs. More so, it is the most expensive system – more than twice as expensive as the ORC systems. This is due to the very high costs associated its high pressure equipment, as all the processes in the CO2 system (compression, heat addition/rejection and expansion) are carried out at supercritical pressures (> 74 bar). As expected, ORC systems with IHEs are more efficient and deliver more power than those without it. They are, however, more expensive (in £.kW-1), such that while regeneration may improve the thermodynamic performance of ORC systems, the additional cost associated with this component makes it undesirable, at least in the present case study. 4. Conclusions We have presented an evaluation of pure fluid and working-fluid mixtures in ORC systems while considering both subcritical and trans-critical cycles for waste-heat recovery and power generation from a flue-gas stream at a temperature of 380 °C. Working fluid properties were obtained reliably from the SAFTVR Mie EoS. The advantages and drawbacks of using a regenerator (also called recuperator or internal heat exchanger, IHE) were also evaluated. In comparison with supercritical CO2 systems, the ORC systems deliver higher net-power outputs and are more cost effective (in terms of the specific investment costs, £/kW). Amongst the ORC working fluids, pentane was the most cost-efficient, leading to cheaper systems than those with the working-fluid mixtures. Also, it can be concluded that ORC systems with trans-critical cycles perform better (in terms of power and efficiency) than subcritical cycles, but are however more expensive. The same conclusion can be drawn for ORC systems with regenerators, whose added cost makes them undesirable. Therefore, a subcritical ORC system without internal regeneration appears as the most cost effective option for waste-heat recovery in our chosen case study.

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Acknowledgements Oyeniyi A. Oyewunmi gratefully acknowledges the funding awarded to him by the Nigerian government which allowed him to embark on this research. References [1] Le VL, Feidt M, Kheiri A, Pelloux-Prayer S. Performance optimization of low-temperature power generation by supercritical ORCs (organic Rankine cycles) using low GWP (global warming potential) working fluids. Energy 2014;67:513–526. [2] Schuster A, Karellas S, Kakaras E, Spliethoff H. Energetic and economic investigation of organic Rankine cycle applications. Appl Therm Eng 2009;29:1809–1817. [3] Oyewunmi OA, Taleb AI, Haslam AJ, Markides CN. On the use of SAFT-VR Mie for assessing large-glide fluorocarbon workingfluid mixtures in organic Rankine cycles. Appl Energ 2016;163:263–282. [4] Oyewunmi OA, Markides CN. Thermo-economic and heat transfer optimization of working-fluid mixtures in a low-temperature organic Rankine cycle system. Energies 2016;9:448. [5] Lecompte S, Ameel B, Ziviani D, van den Broek M, De Paepe M. Exergy analysis of zeotropic mixtures as working fluids in organic Rankine cycles. Energ Convers and Manage 2014;85:727–739. [6] Lecompte S, Huisseune H, van den Broek M, Vanslambrouck B, De Paepe M. Review of organic Rankine cycle (ORC) architectures for waste heat recovery. Renew Sust Energ Rev 2015;47:448–461. [7] Gao P, Jiang L, Wang LW, Wang RZ, Song FP. Simulation and experiments on an ORC system with different scroll expanders based on energy and exergy analysis. Appl Therm Eng 2015;75:880–888. [8] Lafitte T, Apostolakou A, Avendaño C, Galindo A, Adjiman CS, Müller EA, Jackson G. Accurate statistical associating fluid theory for chain molecules formed from Mie segments J Chem Phys 2013;139:154504. [9] Karellas S, Leontaritis A-D, Panousis G, Bellos E, Kakaras E. Energetic and exergetic analysis of waste heat recovery systems in the cement industry. Energy 2013;58:147–156. [10] Turton R, Bailie RC, Whiting WB, Shaeiwitz JA. Analysis, synthesis and design of chemical processes. Pearson; 2008.

Biography Oyeniyi A. Oyewunmi is a Doctoral student in the Clean Energy Processes (CEP) Group of Imperial College London, where he obtained his M.Sc. degree in Process Systems Engineering in 2013. His research interests include computer-aided molecular design and thermo-economic optimization of heat engines such as the organic Rankine cycle and the two-phase thermofluidic oscillator known as the ‘Up-THERM’ engine.