• Julian Ralf Jensen
  • Anders Christian Olesen
4. term, Energy Engineering, Master (Master Programme)
In this thesis the eligibility of a micro combined heat and power (mCHP) system has been investigated. Focus was pointed at the potential of mCHPs in danish homes. Different technologies were discussed including fuel cells (FC), internal combustion engines and stirling engines. A Solid Oxide Fuel Cell (SOFC) was chosen as the technology of interrest, since it has the potential of a very high electrical efficiency and because it can, via reforming, be run on natural gas and thereby use the extensive distribution network that is present in Denmark. To determine the size of the system an analysis was made of five danish single family homes. The common way of running a mCHP system is to let it follow the heating demand and to buy or sell the deficiency or surplus of electricity from or to the electric grid. Via data processing, it was concluded that a 2 [kW] thermal output together with a 350 [L] hot water storage tank would cover the heating demand of the houses. Since the heat to power ratio of a common SOFC system is 2:1 it was investigated how much of the electricity demand would be covered. It was found that in 90 % of the time the mCHP system would be able to supply electricity to the houses. The core of this thesis project was to find out which topology of the mCHP system would yield the highest efficiency, especially which reformer technology would be most suitable. Two different technologies were chosen and compared: steam reforming (SR) and catalytic partial oxidation (CPO). All the different main components in the system were modeled: reformer, FC, catalytic burner, turbomachinery, ejectors, heat exchangers and the heat loss of the vital components. All these sub-models were connected in a total system which was programmed in Engineering Equation Solver (EES). In the literature it was found that recirculation of the cathode off-gas would increase the efficiency, so a total of four models were made: SR without recirculation (Case 1) SR with recirculation (Case 1 CR), CPO without recirculation (Case 2) and finally CPO with recirculation (Case 2 CR). As there are several streams that need to be heated and there is a lot of heat in the exhaust gas a heat exchanger network is a necessity; this was set up and optimised via Pinch Analysis to obtain the highest amount of heat recovery. Due to the complexity of the system an optimisation routine was implemented to find out which system inputs would yield the highest efficiency. Total efficiencies comparable to a state-of-the-art combustion engine and FC based system were found. The electrical efficiency exceeded those of the compared. Due to convergence issues it was not possible to implement anode recycle in the system although it is expected that it could increase the efficiency even more. An interesting finding was, that increasing the amount of internal reforming significantly increased the system efficiency. The amount of internal reforming should however be kept as low as possible due to the internal reforming can cause carbon depositions on the anode during long-time operation; it is much easier and cheaper to replace the catalytic material in the reformer than in the FC stack. With the emerging technology of SOFCs they are still not compatible with regular IC systems due to the high price of the FC stack. However, studies have shown that in the summer, when the heating demand is almost non-existing and the mCHP is operating almost only to supply electricity, the SOFC mCHP unit can produce an environmental benefit if grid electricity produced from coal is displaced.
Publication date2009
Number of pages119
Publishing institutionAalborg Universitet
ID: 17636251