*2.4. Fuel Cell*

The number of newly installed Fuel Cells (FCs) has been rising steadily for several years. Although the number of FC in use is still relatively low compared to conventional energy generators, the constant research and development of the cells, for example in the automotive industry, contributes to the constant growth. Since the use of FC does not lead to combustion or gasification, the emissions are limited to a minimum. In addition, FC are usually operated as a combined heat and power (CHP) plant [32]. They provide both waste heat and electricity. Disadvantages are the very high investment costs and the short service life [35]. Depending on the electrolyte used, five different FC types can be distinguished: Alkaline Fuel Cell (AFC), Polymer Electrolyte Fuel Cell (PEFC), Phosphoric Acic Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC). The electrolyte

has an influence on the operating temperatures, efficiencies and the costs of the fuel cell. For example, the operating temperature of a PEFC is approx. 65–85 °C, while an SOFC is operated at approx. 700–1000 °C and MCFC at approx. 600–700 °C. Accordingly, only the MCFC and SOFC are suitable for the steam supply. In this paper a SOFC is used. It has good prerequisites for steam production due to its high temperatures and efficiencies. The amount of heat generated depends on the area of use and density of the cell, which varies depending on the operating point of the cell. Simplified, this represents the so-called heat-to-power ratio (HTPR), which describes the ratio of the electrical energy produced to the thermal energy [36]. In the SOFC in particular, biomethane/- gas is usable. Biomethane/-gas contains small amounts of hydrogen, hydrogen sulphide and other trace gases. These pollutants contained in the gas can damage both the components of the FC and normal engines and turbines. Therefore, it is necessary to pretreat the gas. For high-temperature FC, the removal of the sulphur compounds is sufficient. Typical processes for desulphurisation include fermenters, bioscrubbers and activated carbon filters. In addition to desulphurisation and processing, it is necessary for most FC types to reform the gas before use. This means that the methane is converted into hydrogen. For a SOFC, this process is not necessary because the reforming can take place within the cell. The fuel demand of the FC *EFC* depends on the steam demand *Esteam*,*demand* for FC and the part load efficiency *η* (Equation (2)). The electricity provided by the FC *PeL*,*FC* is defined by HTPR and the thermal power of the FC *Psteam*,*FC* (Equation (3)).

$$E\_{\rm FC} = E\_{\rm starm,dcmd} \cdot \eta (\frac{P\_{\rm FC,partload}}{P\_{\rm FC,max}})\_{\prime} \tag{2}$$

$$P\_{el,FC} = \dot{Q}\_{steam,FC} \cdot HTPR. \tag{3}$$

The simulation model is based on measurement data retrieved from an energy monitoring system. In addition, the simulation scenarios are designed for a constant load and the FC system is not exposed to fluctuations and behavioural changes. The created FC model is limited to the output data of a FC, which are defined and influenced by the heat-to-power ratio, the electrical and thermal efficiency as well as the reaction, heat and start up times.
