Energy- and exergy efficiencies of stationary LT and HT – fuel cell systems Summer school on electrochemical engineering, Palic, Republic of Serbia Prof. a.D. Dr. Hartmut Wendt, TUD

VAILLANT 5 kW PEMFC Hexis 1,5 kW SOFC UTC PC 25 C 250 kW PAFC FCE DF kW MCFC MTU HOT MODULE 250 kW, MCFC Siemens Westinghouse 100 kW SOFC

Do we need stationary fuel cells ? Yes, if they are cheap enough. And why ? Advantageous for distributed electricity generation in absence of grids or in sparsely populated areas. If properly matched to demands (electricity and heat) they help to reduce carbon dioxide emission. They allow to tap biomass in the form of biogas

What is the fuel for stationary Cells ? Hydrogen is the natural fuel for fuel cells, because it is the most reactive of all fuels and can easily be activated by Pt and other electrocatalysts. – But hydrogen is expensive Natural gas is most convenient, in highly developed economies, where there is a NG – grid. In absence of NG –grids liquified gases (propane/butane), ethanol/methanol, and also diesel are viable options

Hydrocarbons ? They must be converted into hydrogen – containing gas mixtures Carbon dioxide and water vapour do not interfere (much) with hydrogen conversion at the fuel cell anode Steam reforming: CH H 2 O > CO H 2

What is the fuel for stationary Cells ? Hydrogen is the natural fuel for fuel cells, because it is the most reactive of all fuels and can easily be activated by Pt and other electrocatalysts. – But hydrogen is expensive Natural gas is most convenient, in highly developed economies, where there is a NG – grid. In absence of NG –grids liquified gases (propane/butane), ethanol/methanol, and also diesel are viable options

Steam reforming of different fuels All these hydrocarbon fuels need chemical conversion into gas mixtures containing hydrogen STEAM REFORMING – established for hydrogen production from natural gas can be adopted for any of these fuels For stationary electricity production fuel cell systems are needed which are comprising 1.Heterogeneously catalysed steamreforming 2.Shift Conversion and for LT cells 3.CO -removal

FuelCell NG anode exhaust 20% of initial H 2 to reformer ReformerProx Air,20% oxygen Steam Stack gas PEMFC process scheme Shift Cathode exhaust (8% oxygen) to reformer Reactors:

Again: Why fuel cells ? Fuel cells systems are able to achieve in small units (100 kW to 500 kW) electrical efficiencies approaching 50%. (Internal combustion:30 to 40%) Fuel cells are environmentally more benign than combustion engines with low toxic emissions, low noise level and clean waste-water. Fuel cells systems can operate on NG and biogas But Fuel cells are (still awfully) expensive

Now : Thermodynamics The stress will be on gas processing of the whole process In order to decouple from cell thermodynamics and characteristics this assumption: The efficiency of the cells is independent on cell type:ή el = 50% ( respective to initially 1 mol of methane)

First Approach : Adiabatic Not Heat losses to environment considered, which are relatively low for large units (dispersed cogeneration 100 to 500 kW) And relatively higher for domestic cogeneration (1 to 5 kW)

Additional Assumptions Total process is adiabatic (1 st approach) – no heat losses to the environment, lower heat of combustion amounts to: 802kJ/mol methane Only methane as a fuel In the reforming and shift processes CH 4 +H 2 O -> CO + 3H 2 and CO + H 2 O -> CO 2 + H 2 all heat is preserved, that means, that the residual fuel`s heat is stored in hydrogen

Heat balance of LT fuel cell power plant – first law balance for a fuel cell power plant with postulated 50 % electric cell efficiency

First law heat balance: ok but 2nd law? Temperature niveau of off – heat is too low We need the evaporation heat for 3 mols of water to produce process steam - They must be supplied by burning additional methane (+17,7% of the lower heating value of methane) : postulated 50% eff. lowered to 42,3% Another 1% point is lost in the PROX – process by CO- and H 2 O-cooxidation Postulated 50% efficiency lowered to 41%

First law heat balance: ok but 2nd law? Temperature niveau of off – heat high enough We need no combustion to produce process steam. Therefore: postulated 50% eff. Not lowered No Fuel is lost by the PROX – process because HT – cells are insensitive towards CO

Second Approach: Heat Losses HT – Fuel Cell systems are for dispersed power generation: 100 to 500 kW el Losses amount to only 5% relative of methane input: result: Total efficiency is 47% LT –Fuel systems are for domestic cogeneration ánd due to higher surface to volume ratio of the HT – parts have relatively higher losses: 10% with respect to input energy. Total energy efficiency is down to 35 to 37%

Exergy efficiencies in LT and HT fuel cell power plants total exergetic yields: 42,5% 42,5%

Summary HT fuel cells are at an advantage compared to LT fuel cells as they can make use of internally produced high-temperature heat for their own gas process. LT-cell systems with methane redorming do not yield higher then 35% efficiencies – HT – systems may yield more than 50% efficiency