Sustainable Energy Mod.1: Fuel Cells & Distributed Generation Systems Dr. Ing. Mario L. Ferrari Thermochemical Power Group (TPG) - DiMSET University of Genoa, Italy
: fuel cell systems (hybrid systems)
Fuel Cell Power System Scheme
Hybrid System Aspects Fuel cells generate high temperature exhausts. Especially high temperature fuel cells generate exhausts at high exergy content (exhaust temperature: ~650 C for MCFC, up to 1000 C for SOFC. Gas turbine highest temperature (expander inlet): 900 C-1450 C. Steam plant highest temperature (expander inlet): 500 C-600 C. System efficiency increase possible with power system coupling.
Regenerative Brayton Cycle Fuel Cell Hybrid System (1/3)
Regenerative Brayton Cycle Fuel Cell Hybrid System (2/3) Fuel Cell/Regenerative Brayton Cycle Advantages Fuel cells generate high temperature exhausts. Simple cycle arrangement, minimum number of components. Relatively low compressor and turbine pressure ratio, simple machines. Relatively low fuel cell operating pressure, avoiding the problems caused by anode/cathode Pressure differential and high pressure housing and piping. Relatively low turbine inlet temperatures, perhaps 1950 F for SOFC and 1450 F for MCFC systems. Turbine rotor blade cooling not required. Relatively simple heat removal arrangements in fuel cells, accomplished by excess air flow. No internal heat transfer surface required for heat removal. Fuel conversion in cells maximized, taking full advantage of fuel cell efficiency. Adaptability to small scale power generation systems.
Regenerative Brayton Cycle Fuel Cell Hybrid System (3/3) Fuel Cell/Regenerative Brayton Cycle Disadvantages Tailoring of compressor and turbine equipment to fuel cell temperature and cycle operating pressure required (it is not clear to what extent available engine supercharging and industrial compressor and turbine equipment can be adapted to this application.) Large gas to gas heat exchanger for high temperature heat recuperation required. Efficiency and work output of the cycle sensitive to cell, compressor, and turbine efficiencies; pressure losses; and temperature differentials.
Rankine Cycle Fuel Cell Hybrid System (1/2)
Rankine Cycle Fuel Cell Hybrid System (2/2) Fuel Cell/Rankine Cycle Advantages Ambient pressure operation within the fuel cell. Heat recovery in a boiler, avoiding the high temperature gas to gas exchanger of a regenerative Brayton cycle. No gas turbine required, only fans for air and exhaust product gas flow. Steam available for cogeneration applications requiring heat. Fuel Cell/Rankine Cycle Disadvantages Fuel cell big size required (>10-50 MW). Not available for distributed generation. Inherently lower efficiency than regenerative Brayton and combined Brayton-Rankine cycles. requirement for cooling and feed water. greater complexity than regenerative Brayton cycle arrangement.
Combined Brayton-Rankine Cycle Fuel Cell Hybrid System (1/2)
Combined Brayton-Rankine Cycle Fuel Cell Hybrid System (2/3)
Combined Brayton-Rankine Cycle Fuel Cell Hybrid System (3/3) Fuel Cell/Combined Brayton-Rankine Cycle Advantages integrated plant and equipment available for adaptation to fuel cell heat recovery. high efficiency system for heat recovery. Fuel Cell/Combined Brayton-Rankine Cycle Disadvantages Fuel cell big size required (>10-50 MW). Not available for distributed generation. Complex, multi component, large scale system for heat recovery. Adaptation of existing gas turbine required to provide for air take off and return of hot depleted air and partially burned fuel. High pressure operation of the bulky fuel cell system required. Precise balancing of anode and cathode pressures required to prevent rupture of fuel cell electrolyte. Indirect heat removal required from fuel cells with compressed air, initially at low temperature, to enable significant conversion of the fuel flow in the cells.
Hybrid System Comparison Efficiency values not realistic: machine and cell efficiency for MW-size power plants, no thermal losses, no electrical losses, direct use of CH4 (no reforming), no auxiliary losses. Realistic efficiency values: 60%-68%.
PAFC Hybrid Systems (1/3)
PAFC Hybrid Systems (2/3) This cycle is very similar to the 11 MW IFC PAFC cycle that went into operation in 1991 in the Tokyo Electric Power Company system at the Goi Thermal Station (V = 750 mv and U f =80%). This is a 12 MW plant with V = 760 mv and U f =86%.
PAFC Hybrid Systems (3/3) Ambient air (stream 200) is compressed in a two-stage compressor with intercooling to conditions of 193ºC (380ºF) and 8.33 atm. The majority of the compressed air (stream 203) is utilized in the fuel cell cathode; however, a small amount of air is split off (stream 210) for use in the reformer burner. The spent oxidant (stream 205) enters a recuperative heat exchange before entering a cathode exhaust contact cooler, which removes moisture. The cycle exhaust (stream 304) is at approximately 177ºC (350ºF).
MCFC Hybrid Systems (1/15) Athmospheric MCFC based Hybrid system (1/2) Plant theoretically analyzed by National Fuel Cell Research Center (NFCRC) and National Energy Technology Laboratory (NETL) M
MCFC Hybrid Systems (2/15) Athmospheric MCFC based Hybrid system (2/2) Fuel cell parameters Gas turbine parameters On design performance
MCFC Hybrid Systems (3/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (1/9) Fuel cell layout (1/2) SHR: sensible heat reformer ECB: exhaust catalytic burner CCB: cathode catalytic burner
MCFC Hybrid Systems (4/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (2/9) Fuel cell layout (2/2)
MCFC Hybrid Systems (5/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (3/9) Fuel cell data
MCFC Hybrid Systems (6/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (4/9) Pressurization by auxiliary compressors Taking into account the power for the two auxiliary compressors the stack system efficiency is reduced to 40.1%
MCFC Hybrid Systems (7/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (5/9) Pressurization by turbocharger
MCFC Hybrid Systems (8/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (6/9) MCFC-mGT hybrid system (using a simple cycle gas turbine) Cell exhaust gas enters the gas turbine expander at about 3.5 bar and 700 C. Possibility of postcombustion in the ECB.
MCFC Hybrid Systems (9/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (7/9) CFC-mGT hybrid system (using a regenerated cycle gas turbine) Scheme 1 Elimination of cathodic recycle. CCB outlet temperature is obtained with additional fuel (18% of the total fuel flow).
MCFC Hybrid Systems (10/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (8/9) CFC-mGT hybrid system (using a regenerated cycle gas turbine) Scheme 2 Elimination of cathodic recycle. Elimination of fuel injection in the CCB. CCB outlet temperature is obtained with additional fuel in the ECB (21% of the total fuel flow). With this approach machine efficiency is increased.
MCFC Hybrid Systems (11/15) Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (9/9) Performance comparison
MCFC Hybrid Systems (12/15) Pressurised MCFC based Hybrid system by TOYOTA (1/4) Fuel cell data
MCFC Hybrid Systems (13/15) Pressurised MCFC based Hybrid system by TOYOTA (2/4) MCFC/mGT hybrid system layout
MCFC Hybrid Systems (14/15) Pressurised MCFC based Hybrid system by TOYOTA (3/4) MCFC/mGT hybrid system: performance estimation
MCFC Hybrid Systems (15/15) Pressurised MCFC based Hybrid system by TOYOTA (4/4) MCFC/mGT hybrid system: 2005 World EXPO test results
SOFC Hybrid Systems (1/10) Athmospheric SOFC based Hybrid system (1/2) FC Fuel compressor SOFC- Fuel Cell B- Burner C Compressor E Expander REC- Recuperator FP Fuel pre-heating Critical component (T 7 =1000 C) β= 4; P FC ac =1198 kw; P GT ac =422 kw; P HS =1550 kw; η FC =0.44; η HS =0.55
SOFC Hybrid Systems (2/10) Athmospheric SOFC based Hybrid system (2/2)
SOFC Hybrid Systems (3/10) Pressurised SOFC based Hybrid system (1/6) Critical component FC Fuel compressor SOFC- Fuel Cell B- Burner C Compressor E Expander REC- Recuperator FP Fuel pre-heating M_fuel = 0.018 M_air =1.05 P FC /P HS = 83% β = 4 Fuel Cell Power = 474.8 kw Hybrid System Power =565.8 kw Fuel Cell Efficiency = 0.53 Hybrid System Efficiency =0.62
SOFC Hybrid Systems (4/10) Pressurised SOFC based Hybrid system (2/6) Theory of Pressurisation 1 Ideal cell voltage is a function of pressure (Nernst Equation) Cathode O 2 partial pressure E ideal = G n e F + R T n e S F ln Ideal cell voltage increases with increasing pressure 0 ( ) p O 2 1 p H 2 2 O p H 2 Anode partial pressure ratio remains constant
SOFC Hybrid Systems (5/10) Pressurised SOFC based Hybrid system (3/6) Theory of Pressurisation 2 Cell voltage = Ideal voltage - Losses E = E cell ideal η activation η ohmic η diffusion Electrode process losses reduce with increasing pressure Mass transport losses reduce with increasing pressure At constant current, cell voltage and hence power increase with pressure
SOFC Hybrid Systems (6/10) Pressurised SOFC based Hybrid system (4/6) 1,05 1 0,95 Nernst 0,9 0,85 V [V] 0,8 0,75 Real 0,7 0,65 0,6 0,55 0 2 4 6 8 10 12 14 p [atm]
SOFC Hybrid Systems (7/10) Pressurised SOFC based Hybrid system (5/6) DIRECT BENEFITS OF PRESSURE Pressurisation reduces pressure drops caused by flows Pressurisation reduces pumping work required to overcome pressure drops: allows greater power density through reduction in passage size; reduction in stack volume big driver for overall system cost. Reduces area and cost of heat exchangers Increases cell performance: 50% stack efficiency; translates to /kw.
SOFC Hybrid Systems (8/10) Pressurised SOFC based Hybrid system (6/6) Pressurised and atmospheric HS compared Identical stack in pressurised and atmospheric configurations Near term SOFC stack Underlying stack efficiency 50% System efficiency exceeds stack efficiency for pressurised case Atmospheric recuperator must be done with exotic material Pressurised recuperator can be stainless steel
CERAMIC H HX T H T SOFC Hybrid Systems (9/10) ALSTOM Turbine (7.9 MWe) Mass Flow Pressurised SOFC based Hybrid system combined with a steam plant Pressure Ratio Isentropic Efficiency Inlet Temperature Compressor 29.47 kg/s 14.13 0.851 Is. Blading 288 K Turbine 26.81 kg/s in 29.74 kg/s out 12.42 0.876 Is. Blading 1422 K Water 1ST HRSG 11 4ST 2ST ST CO 3ST No Recuperator Fuel Cell Power = 19.2 MW GT+SOFC Power = 23.51 MW Steam Turbine Power = 2.4 MW Fuel Cell Efficiency = 0.635 GT+SOFC Efficiency = 0.78 Plant Efficiency =0.85 Plant Power= 25.82 MW P FC /P HS = 81.5% M FUEL /M AIR =0.0205 TIT calc =1174 K 1F FC 2F C 1 11S FP 10S 2 Cooling Air 10 9 3F 3 RRR R 5 4F 8 B 6 CERAMIC HX HT 7 E DC AC 4
SOFC Hybrid Systems (10/10) Pressurised SOFC based Hybrid system combined with a STIG plant