A proven concept to increase efficiency of energy production is cogeneration. By co-producing electricity and heat, efficiency can rise up to 100 percent because all heat produced as by-product of electricity generation is used for heating. In several countries, like the, the cogeneration concept is downscaled and implemented at district level (meso-cogeneration varying from 5-500 kW of electrical power) or even at household level (micro-cogeneration up to 5 kW of electrical power). Downscaled cogeneration fits within the category of decentralized energy production.
To avoid the waste of heat, in general heat demand determines the amount of operating hours - as electricity will either be used or fed into the net whereas feasible transportation possibilities of heat are rather limited. This implies that the electrical efficiency of the cogeneration system should be as high as possible to minimize co- production of heat.
Four technologies for cogeneration are common being the engine which has an electrical efficiency up to 25% of which only 15% is proven in practice; the gas turbine which has an electrical efficiency up to 43%; the piston engine with an electrical efficiency up to 35%; and fuel cells which can have an electrical efficiency up to 60%. In general the larger the system is, the higher the electrical efficiency and the lower the maintenance costs. Moreover when the electricity-efficiency of a cogeneration system increases the amount of fossil fuels needed decreases. Additionally the emissions decrease, next to that because electricity is more expensive than heat, the economic feasibility of the system increases.
Currently most market developments in the are directed towards micro-cogeneration systems based on technology, which have the lowest efficiency for electricity generation – but do have a total efficiency of nearly 100%, thus produces a lot of heat. As the main limitation of this system is that the quantity of electricity produced is directly dependent on the heat demand, this contrasts the argumentation above. Because on the one hand, in periods of high heat demand, the electricity grid must be able to accommodate for the large amounts of electricity generated. On the other hand in periods of low heat demand the installations are used at partial capacity or not used at all, which decreases the feasibility of the system. Moreover our dependence on central power plants for electricity generation remains as in summer decentralized cogeneration will hardly be used.
Current cogeneration systems are lacking the flexibility to adapt to power demand and price. In this paper we will explore the innovative concept of tri-generation using the thermodynamic possibilities of an internal reforming fuel cell for flexible heat, power and hydrogen production. In an internal reforming fuel cell natural gas is converted into hydrogen which is converted by the fuel cell into power and heat. However, when increasing the input of natural gas, more hydrogen can be produced than is needed by the fuel cell for its own consumption. Hydrogen can thus be extracted and becomes a third product of the fuel cell, next to electricity and heat. Moreover, heat is converted into hydrogen because the reforming reaction is endothermic. In other words: because chemical energy is produced, less waste heat results. Thus, system efficiency, in terms of hydrogen and electric power production can be increased up to 80 - 90%. This means only 10-20% of waste heat results.
The flexibility of the tri-generation system is even more important. Within certain limits large variations in production ratios of the various products of the system can be obtained, being electric power, hydrogen and heat. This flexibility can be used in the built environment in a combined heat and power application. The flexibility of the fuel cell can thus be used to adapt heat production to local heat demand. Moreover, in times of low heat demand, the installation does not stand idle but can be used for hydrogen production instead, increasing potential profit and thus overall economic feasibility. The hydrogen produced can for example be used for fueling fuel cell vehicles of the local residents; moreover the amount of hydrogen produced can be adapted to the (growing) demand for hydrogen for transport. Finally, the flexibility of the fuel cell on the input side can be used to mix locally produced biogas in almost any mixture with natural gas to fuel the fuel cell. Moreover, in a context of decreasing energy demand for space heating, this concept can be part of the transition to low temperature district heating. The potential of the concept of tri-generation for a further transition to a more sustainable energy supply will be explored and compared to other options in terms of feasibility; sustainability now and in the future; implementation aspects; and the prevention of undesirable lock-in effects. The paper will describe the advantages and disadvantages in terms of efficiency, feasibility and compatibility with the desired transition.