*2.8. Steam Accumulator*

It is fundamental to consider the implementation of a steam accumulator for BMB, biomethane—fuelled FC, SES and HP due to the difference between the steam demand of the process and the steam supply. When the production is higher than the demand, the steam accumulator is charged, increasing the pressure inside while at the same time part of the steam condenses. When the steam accumulator is discharged due to a deficit of steam production, the pressure inside decreases and part of the water evaporates. Hence, the pressure level of the steam accumulator defines the load-level of the storage. To estimate the behaviour of the steam accumulator, the equilibrium model of Stevanovic et al. [43] is chosen (Equation (6)) . Even, a more complex and detailed non equilibrium model is available, Biglia et al. [44] proposed the utilisation of the equilibrium model. Depending on the mass and energy balance of the steam accumulator, the pressure change and the pressure *p* inside of the vessel with a constant Volume *V* can be calculated. The mass balance *m*˙ *nB* = *m*˙ *<sup>n</sup>*,*in* − *m*˙ *<sup>n</sup>*,*out* and the energy balance (*m*˙ · *h*)*nB* = (*m*˙ · *h*)*n*,*in* − (*m*˙ · *h*)*n*,*out* for liquid and vapourous considers the system boundaries as evaporation enthalpy *r* = *h* − *h* is also considered to depicture the evaporation and condensation inside of the accumulator. With the specific volume the saturated liquid *v* and vapourous *v* state, the total mass *M* is calculated.

$$\frac{dp}{dt} = \frac{(\dot{m} \cdot h)\_{1B} + (\dot{m} \cdot h)\_{2B} + (\frac{\frac{\dot{v}'}{M}}{v'' - v'} - h)(\dot{m}\_{1B} + \dot{m}\_{1B})}{M(\frac{d\dot{u}'}{dp} + \frac{\frac{V}{M} - v'}{v'' - v'} \cdot \frac{dr}{dp} - \frac{r}{v'' - v'} \cdot \frac{dv'}{dp} - r \cdot \frac{\frac{V}{M} - v'}{(v'' - v')^2} \cdot \frac{d(v'' - v')}{dp}) - V} \tag{6}$$

The steam accumulator is operated within a maximum pressure of 13 bar and a minimum pressure of 9 bar. A pressure difference of 4 bar is utilized. This pressure difference has a significant influence on the storage capacity of the steam accumulator. For the simulation runs, different storage vessel volumes should therefore be considered.
