Solid Oxide Fuel Cell Micro Combined Heat and Power system: Choosing the right reformer
Authors
Jensen, Julian Ralf ; Olesen, Anders Christian
Term
4. term
Education
Publication year
2009
Pages
119
Abstract
Dette speciale undersøger, om en mikro-kraftvarme (mCHP) løsning er egnet til danske hjem. En mCHP-enhed producerer både varme og elektricitet i lille skala i selve huset. Der blev set på flere teknologier, bl.a. brændselsceller, forbrændingsmotorer og Stirling-motorer. Valget faldt på en fastoxid-brændselscelle (SOFC), fordi den kan opnå høj elvirkningsgrad og, via reformering, kan køre på naturgas og dermed udnytte det eksisterende danske gasnet. For at dimensionere systemet blev fem danske enfamiliehuse analyseret. Den almindelige driftsstrategi for mCHP er at følge varmebehovet, mens underskud eller overskud af strøm købes eller sælges til elnettet. Databehandlingen viste, at en termisk effekt på ca. 2 kW sammen med en varmtvandsbeholder på 350 liter kan dække husenes varmebehov. Da et typisk SOFC-anlæg har et varme-til-el-forhold på ca. 2:1, blev det vurderet, at anlægget vil kunne levere elektricitet i omtrent 90 % af tiden, mens elnettet håndterer resten. Kernespørgsmålet var, hvilken systemopbygning der giver højest virkningsgrad, især valg af reformer-teknologi. To teknologier blev sammenlignet: dampreformering (SR) og katalytisk partiel oxidation (CPO). Hovedkomponenterne blev modelleret (reformer, brændselscelle, katalytisk brænder, turbomaskineri, ejektorer, varmevekslere samt varmetab) og samlet i et samlet system i Engineering Equation Solver (EES). Litteraturen peger på, at recirkulation af katode-udstødningsgas kan øge effektiviteten, så fire varianter blev undersøgt: SR/CPO med og uden recirkulation. Fordi mange strømme skal opvarmes, blev et varmevekslernetværk udformet og optimeret med Pinch-analyse for at maksimere varmegenvinding. En optimeringsrutine blev brugt til at finde de input, der gav højest virkningsgrad. Resultaterne viser samlede virkningsgrader på niveau med moderne motor- og brændselscellebaserede systemer, og elvirkningsgraden var højere end i de sammenlignede systemer. Mere intern reformering øgede effektiviteten, men bør begrænses for at undgå kulaflejringer på anoden; det er langt lettere og billigere at udskifte katalysatoren i reformeren end brændselscellestakken. Anode-recirkulation blev ikke implementeret pga. konvergensproblemer, men forventes at kunne forbedre effektiviteten yderligere. Selvom SOFC-mCHP endnu ikke er omkostningsmæssigt konkurrencedygtig med traditionelle motorløsninger på grund af stakkens pris, kan den give miljøfordele om sommeren, hvis den fortrænger kulbaseret el fra nettet.
This thesis examines whether a micro combined heat and power (mCHP) system is suitable for Danish homes. An mCHP unit produces both heat and electricity on a small scale within the home. Several technologies were considered, including fuel cells, internal combustion engines, and Stirling engines. A Solid Oxide Fuel Cell (SOFC) was chosen because it can achieve high electrical efficiency and, through reforming, can run on natural gas and use Denmark’s existing gas network. To size the system, five Danish single-family homes were analyzed. A common operating strategy for mCHP is to follow the heating demand while buying or selling surplus or deficit electricity to the grid. Data processing indicated that about 2 kW of thermal output together with a 350 L hot water tank can cover the homes’ heating needs. Given a typical SOFC heat-to-power ratio of about 2:1, the unit would be able to supply electricity for roughly 90% of the time, with the grid balancing the rest. The core question was which system layout yields the highest efficiency, with a focus on reformer technology. Two options were compared: steam reforming (SR) and catalytic partial oxidation (CPO). The main components (reformer, fuel cell, catalytic burner, turbomachinery, ejectors, heat exchangers, and key heat losses) were modeled and integrated into a full system in Engineering Equation Solver (EES). Prior studies suggest that recirculating cathode off-gas can improve efficiency, so four variants were studied: SR/CPO with and without recirculation. Because several streams require heating, a heat-exchanger network was designed and optimized using Pinch Analysis to maximize heat recovery. An optimization routine was implemented to identify inputs that deliver the highest efficiency. Results show total efficiencies comparable to state-of-the-art engine- and fuel-cell-based systems, with electrical efficiency exceeding those compared. Increasing internal reforming significantly improved efficiency but should be limited to avoid carbon deposition on the anode; replacing the reformer catalyst is far easier and cheaper than replacing the fuel cell stack. Anode recycle could not be implemented due to convergence issues but is expected to further improve efficiency. While SOFC mCHP is not yet cost-competitive with conventional engine systems due to the high stack price, it can deliver environmental benefits in summer if it displaces coal-based grid electricity.
[This summary has been rewritten with the help of AI based on the project's original abstract]