Thermodynamic Optimization of heat recovery ORCs for heavy duty Internal Combustion Engine: pure fluids vs. zeotropic mixtures

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Lillian Williams
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1 4 th International Seminar on ORC Power Systems September 13 th 15 th, 2017 Politecnico di Milano Milan, Italy Michele Tavano Costante Mario Invernizzi Emanuele Martelli

2 Motivations 2 Great diffusion of internal combustion engine, mainly in the transport sector Main advantages of recovering the waste heat: Increasing power generation and efficiency Reducing carbon footprint and fossil fuel consumption The technical and economic feasibility increases with engine size and for engines running mainly at constant load, like for power production and large naval engines Lacking of a systematic study of the ORC considering: The optimal working fluid selection The optimization of the heat integration with all the possible heat sources The optimization of the cycle variables

3 Objectives 3 1. Performing a systematic thermodynamic optimization of heat recovery ORCs for internal combustion engines 2. Devising an ad hoc optimization approach 3. Screening a large number of promising fluids: Conventional and recently developed fluids (HFE, HCFO, HFO) Binary zeotropic mixtures 4. Selecting optimal operative conditions and cycle configuration Economic and operational aspects, which require also the optimization of the heat exchanger configuration, are not considered in this work

4 Challenges 4 Extremely large number of possible fluids and mixtures Need of fluid selection criteria Several heat sources available: Exhaust gas Scavenge air Jacket water Several possible arrangements of the heat exchangers Need of considering the heat integration between the three hot streams and the cycle

5 Fluid selection 5 Selection criteria: Optimal heat integration with the hot flue gas stream (critical temperature close to flue gas inlet temperature) Condensation pressure at ambient temperature higher than 0.03 bar (preferably above atmospheric) Thermochemical stability (preferably up to the flue gas inlet temperature) Low environmental impact (GWP, ODP, ALT) No CFC, HCFC, PFC Measured fluid property data and validated Equation of State (NIST, REFPROP V9.1) OR Available Equation of State parameter estimation method (Lemmon 2001)

6 Heat Integration 6 The heat integration algorithm is the energy targeting method proposed by Maréchal and Kalitventzeff based on the heat cascade : No restriction regarding the possible matching of the flows The heat transfer is performed in a multiflow heat exchanger Linear bihavior of the streams (disctretization) Streams are divided into temperature intervals Heat transfer can occur between any hot and cold stream with ΔT ΔT min The variables are the ORC mass flow rate and cooling water The constraints are the energy balance of each temperature interval The objective function is the maximum energy efficiency (recovered power)

7 Cycle optimization problem 7 Optimization problem: (performed for each fluid/mixture selected) Objective function: Maximum mechanical/electric power generated by the heat recovery ORC Independent optimization variables: Evaporating pressure Condensing pressure Turbine inlet temperature Mass flow rate of ORC Composition (only for mixtures) Nonlinear constraints: Minimum vapor fraction in the expansion greater than 0.88 Minimum temperature difference in each heat exchanger grater or equal to 5 C Key Assumptions: Set of hot streams made available by an internal combustion engine Temperature of the available heat sink Ideal behavior of the ORC cycle components

Optimization algorithm 8 At the upper level, PGSCOM (evolutionary) optimizes the independent cycle variables and the composition (mixtures) REFPROP evaluates the thermodynamic properties of

8 Optimization algorithm 8 At the upper level, PGSCOM (evolutionary) optimizes the independent cycle variables and the composition (mixtures) REFPROP evaluates the thermodynamic properties of the fluids The energy targeting optimizes the ORC and cooling water mass flow rates The ORC mechanical power and efficiency are returned to PGSCOM as output of the optimized blackbox function

9 Heat recovery applications 9 Two large Diesel engines of the same size (10 MW), but with opposite features: Man S60MC6: twostroke with P el =10.2 MW and T fg =245 C Wärtsilä 46DF: fourstroke with P el =10.3 MW and T fg =354 C Different exhaust temperatures allows assessing the effects of this parameter on the optimal fluid selection, cycle configuration and ORC efficiency Flow Exhaust gas Scavenge air Jacket water Feature Units of measurement Man S60MC6 Wärtsilä 46DF Cycle type Twostroke Fourstroke Power output kw Efficiency (full load) % Mass flow rate kg/s Thermal power kw Temperature range C Mass flow rate kg/s Thermal power kw Temperature range C Mass flow rate kg/s Thermal power kw Temperature range C

10 Pure fluids results 10 Man S60MC6 (T fg =245 C): HCFO1233zde Transcritical cycle configuration with isentropic expansion Nonflammable* GWP=1 ODP=0 HFE245fa2 Transcritical cycle configuration with dry expansion and large regenerator Nonflammable* GWP=812 ODP=0 HFO1336mzz Transcritical cycle configuration with dry expansion and large regenerator Nonflammable* GWP=2 ODP=0 Fluid W out ORC η exe Turbine inlet temperature Turbine inlet pressure N of expander stages kw % C bar HCFO1233zde HFE245fa HFO1336mzz Novec TM *directly coupled with the flue gasses

11 Pure fluids results 11 Man S60MC6 (T fg =245 C): HCFO1233zde Exhaust gas inout Scavenge air inout Jacket water inout ORC turbine out Regenerator

12 Pure fluids results 12 Wärtsilä 46DF (T fg =354 C): Cyclopentane Subcritical cycle configuration with isentropic expansion and subatmospheric condensing pressure Flammable toxic GWP=11 ODP=0 Ammonia Transcritical cycle configuration with wet expansion Flammable toxic GWP=0 ODP=0 HCFO1233zde Transcritical cycle configuration with isentropic expansion Nonflammable* GWP=1 ODP=0 Fluid W out ORC η exe Turbine inlet temperature Turbine inlet pressure N of expander stages kw % C bar Cyclopentane Ammonia HCFO1233zde Novec TM *directly coupled with the flue gasses

13 Pure fluids results 13 Wärtsilä 46DF (T fg =354 C): HCFO1233zde Exhaust gas inout Scavenge air inout Jacket water inout ORC turbine out Regenerator

14 Mixtures results 14 Man S60MC6 (T fg =245 C): HCFO1233zde/HFC134a (90/10 wt%) Transcritical cycle configuration Nonflammable* GWP HFC134a =1430 HFO1336mzz/HFC134a (97/3 wt%) Transcritical cycle configuration Nonflammable* GWP HFC134a =1430 Isobutane/Pentane (56/44 wt%) Transcritical cycle configuration Flammable GWP<20 Fluid W out ORC η exe Turbine inlet temperature Turbine inlet pressure N of expander stages (Weight fraction wt%) kw % C bar HCFO1233zde (90) HFC134a (10) HFO1336mzz (97) HFC134a (3) Isobutane (56) Pentane (44) *directly coupled with the flue gasses

15 Mixtures results 15 Man S60MC6 (T fg =245 C): HCFO1233zde/HFC134a (90/10 wt%) Exhaust gas inout Scavenge air inout Jacket water inout ORC turbine out Regenerator

16 Mixtures results 16 Wärtsilä 46DF (T fg =354 C): Cyclopentane/CisButene (82/18 wt%) Transcritical cycle configuration Flammable Cyclopentane/Heptane (78/22 wt%) Subcritical cycle configuration Flammable Ammonia/Water (98/2 wt%) Transcritical cycle configuration Flammable Fluid W out ORC η exe Turbine inlet temperature Turbine inlet pressure N of expander stages (Weight fraction wt%) kw % C bar Cyclopentane (82) CisButene (18) Cyclopentane (78) Heptane (22) Ammonia (98) Water (2)

17 Mixtures (wt%) Pure fluid Comparison: Pure fluid vs. Mixtures 17 Man S60MC6 Wärtsilä 46DF W out ORC η exe η II ICE+ORC W out ORC η exe η II ICE+ORC kw % % kw % % HCFO1233zde Cyclopentane HFE245fa Ammonia HFO1336mzz HCFO1233zde HCFO1233zde (90) + HFC134a (10) Cyclopentane (82) + CisButene (18) HFO1336mzz (97) + HFC134a (3) Cyclopentane (78) + Heptane (22) Isobutane (56) + Pentane (44) Ammonia (98) + Water (2)

18 Conclusion 18 Pure fluids: Supercritical cycles can reach a better thermodynamic matching with the linear temperature profile of the hot flue gases (Lorentz cycle) HCFO1233zde appears to be a promising fluid both the engines analyzed, in particular with the lowtemperature exhaust gasses Relatively low critical temperature (transcritical cycle configuration) High molar mass (small enthalpy drop and number of expander stages) Condensing pressure higher than the atmospheric one Another promising pure fluid is Novec TM 649 Transcritical cycle configuration with subatmospheric condensing pressure Close to maximum efficiency Small enthalpy drop and volumetric ratio Nonflammable Low environmental impact (GWP = 1 ODP=0)

19 Conclusion 19 Mixtures: The use of optimized mixtures leads to an increase of the recovered mechanical power of around 3 percentage points The advantage of mixtures compared to pure fluids is lower than the values reported in the literature because the optimal cycle is transcritical (the temperature glide can be exploited only in condensation) The little efficiency advantage is likely not sufficient to compensate the reduction of heat transfer coefficient which affects mixtures Future works: Complete the analysis with a technoeconomic analysis of ORCs using HCFO 1233zde and Novec TM 649 Apply the optimization methodology to other heat recovery applications (e.g., low grade heat)

20 20 Thanks for your attention

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