THERMO-ECONOMIC ANALYSIS OF ZEOTROPIC MIXTURES AND PURE WORKING FLUIDS IN ORGANIC RANINKE CYCLES FOR WASTE HEAT RECOVERY 3rd International Seminar on ORC Power Systems, Brussels (Belgium) Florian Heberle and Dieter Brüggemann
Introduction Zeotropic mixtures as working fluids in ORC power systems Zeotropic mixtures are potential working fluids for ORC power systems. The temperature-glide at phase change leads to temperature match with heat source and sink. Compared to pure components lower irreversibilities and higher efficiency is obtained. In the context of a thermo-economic evaluation, a reduction of heat transfer characteristics due to additional mass transfer resistance has to be taken into account for zeotropic mixtures. A comparison to pure working fluids is performed to clarify, if the efficiency increase overcompensates the additionally required heat transfer surface. Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 2
Introduction General approach Boundary conditions / Fluid selection Simulations / Second law analysis Design of key components Cost estimation / Economic evaluation Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 3 Page 3
Boundary conditions injection injection production production preheater preheater evaporator evaporator drill hole drill hole drill hole drill hole heat transfer Subcritical and saturated cycle geothermal geothermal medium H H H H water water 2 2 3 3 4 4 Heat input of 3 MW by pressurized water turbine turbine generator generator at 6 bar and 150 ° C 5 5 Additional boundary conditions: pump pump cooling condenser condenser cooling water cooling water Parameter Value medium 1 1 mass flow rate of heat source ṁ HS 10 kg/s ORC - working fluid ORC - working fluid outlet temperature of heat source T HS,in 80 ° C inlet temperature of cooling medium T CM,in 15 ° C temperature difference of cooling medium ΔT CM 15 ° C 0.8 ∙ p crit maximal ORC process pressure p 2 isentropic efficiency of feed pump η i,P 75 % isentropic efficiency of turbine η is , T 80 % efficiency of generator η G 98 % Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 4 Page 4
Fluid selection Investigated working fluids • Pure fluids: R245fa, isobutane, isopentane • Zeotropic mixture: isobutane/isopentane Composition is varied in discrete steps of 10 mole-% @ condensation isobutane/isopentane 14 @ evaporation 12 temperature glide (K) 10 8 6 4 2 0 0 20 40 60 80 100 mole fraction of isobutane (%) Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 5 Page 5
Simulations / Second law analysis • The minimal temperature difference in the evaporator and condenser are chosen as independent design variables in order to identify the most cost- efficient process parameters. • Pressure and heat losses are neglected in the pipes and components. Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 6 Page 6
Simulations / Second law analysis • The minimal temperature difference in the evaporator and condenser are chosen as independent design variables in order to identify the most cost- efficient process parameters. • Pressure and heat losses are neglected in the pipes and components. P P P P G Pump Fans η net • Second law efficiency: II E m e HS HS HS e h h T (s s ) where HS 0 0 0 and T 0 = 15 ° C; p 0 = 1 bar Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 7 Page 7
Design of key components Preheater and evaporator – Predefined design specification • Shell and tube heat exchanger for preheater and evaporator (TEMA-E-type) • Inner diameter of the tubes: d i = 0.02 m • Wall thickness of the tube: s = 0.002 m • Maximum flow velocities (VDI Heat Atlas): u l = 1.5 m/s and u g = 20 m/s • Squared layout: d o P d t 1 22 . o P t Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 8 Page 8
Design of key components Preheater and evaporator – Heat transfer correlations • Shell side (preheater, evaporator) Single phase; pressurized water: Kern (1950) • Tubes side (preheater) Single phase; pure fluid & mixture: Sieder and Tate (1936) • Tubes side (evaporator) Two phase; pure working fluid: Steiner (2006) Two phase; zeotropic mixture: Schlünder (1983) Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 9 Page 9
Design of key components Air cooled condenser – Predefined design specification • A tube bank staggered arrangement is considered. • Cross-flow heat exchanger with finned tubes. • Layout: d F p F t F d Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 10 Page 10
Design of key components Air-cooled condenser – Heat transfer correlations • Air side Single phase; air: Shah and Sekulic (2003) • Tubes side Single phase; pure fluid & mixture: Sieder and Tate (1936) Two phase; pure working fluid: Shah (1979) Two phase; zeotropic mixture: Bell and Ghaly (1973), Silver (1964) Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 11 Page 11
Cost estimation Purchased equipment costs (PEC) of the major components • PEC in US $ for ambient operating conditions and a carbon steel construction 2 log PEC K K log (Y) K log (Y) 10 1 2 10 3 10 • Equipment cost data according to Turton et al. (2003) component Y ; unit K 1 K 2 K 3 Pump (centrifugal) kW 3.3892 0.0536 0.1538 Heat exchanger (floating head) m 2 4.8306 -0.8509 0.3187 m 2 Heat exchanger (air cooler) 4.0336 0.2341 0.0497 Turbine (axial) kW 2.7051 1.4398 -0.1776 Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 12 Page 12
Cost estimation Purchased equipment costs (PEC) of the major components • PEC in US $ for ambient operating conditions and a carbon steel construction 2 log PEC K K log (Y) K log (Y) 10 1 2 10 3 10 • Equipment cost data according to Turton et al. (2003) component Y ; unit K 1 K 2 K 3 Pump (centrifugal) kW 3.3892 0.0536 0.1538 Heat exchanger (floating head) m 2 4.8306 -0.8509 0.3187 m 2 Heat exchanger (air cooler) 4.0336 0.2341 0.0497 Turbine (axial) kW 2.7051 1.4398 -0.1776 • Consideration of inflation and the development of raw material prices PEC PEC CEPCI / CEPCI k , 2014 k , 2001 2014 2001 • Total investment costs (TCI) of the ORC modul according to Bejan et al. (1996) TCI 6 32 . PEC k , 2014 Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 13 Page 13
Economic evaluation Economic boundary conditions and parameters • Economic boundary conditions parameter lifetime 20 years interest rate ir 4.0 % annual operation hours 7500 h/year 0.02 ∙Z̈ CI Cost rate for operation and maintenance 0.2 ∙PEC total Costs for process integration C PI Power requirements of the air-cooling system 5 kW e /MW th 0.08 € /kWh Electricity price Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 14 Page 14
Economic evaluation Economic boundary conditions and parameters • Economic boundary conditions parameter lifetime 20 years interest rate ir 4.0 % annual operation hours 7500 h/year 0.02 ∙Z̈ CI Cost rate for operation and maintenance 0.2 ∙PEC total Costs for process integration C PI Power requirements of the air-cooling system 5 kW e /MW th 0.08 € /kWh Electricity price • Calculated economic paramters (c E Z ) C F ,tot F ,tot k C tot,ORC P,tot k SIC c P,tot E E P P,tot P,tot net costs per unit exergy (Bejan et al.) specific investment costs Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 15 Page 15
Results Minimization of costs per unit exergy – R245fa • Minimal costs per 0 6 unit exergy are identified for costs per unit exergy (€/GJ) each working 9 5 fluid. • In case of R245fa 8 5 c p,total minimal for Δ T PP,E = 1 K and Δ T PP,C = 13 K. 8 57 9 T PP,C (K) 1 0 • Corresponding 1 1 LCOE = 1 2 6 56 5 106.6 € /MWh 4 1 3 3 2 T PP,E (K) 1 1 4 0 Thermo-economic analysis of zeotropic mixtures and 12.10.2015 pure working fluids in ORC for WHR - F. Heberle and D. Brüggemann Page 16 Page 16
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