Working fluid filling effects on a small ORC unit for geothermal power generation

Quasi-static thermodynamic models of ORC units are generally based on the thermodynamic cycle, the mass and energy balance equations, and the characteristic curves of fluid machines. For instance, in Gao, et al. [1] a similar approach is used to study the effect of Scroll expanders of different displacements on the ORC performance, and by Altun, et al. [2] to obtain a thermodynamic evaluation of an operating geothermal ORC power plant, rated at 3 Mwe. A wider perspective on advances and challenges in modeling ORC and other kinds of systems for low-grade thermal energy recovery may be found in the review by Ziviani et al. [3]. A higher level of ORC modeling, including the dynamic behavior of the system, is instead presented in Carraro et al. [4]. The amount of fluid inside the unit is not considered but is generally regarded as adequate to realize the thermodynamic cycle considered. In reality, the amount of fluid affects the cycle and, ultimately, the unit performance. This is particularly true for small ORC units, where the filling of the shell-and-tube heat exchangers (in particular the evaporator) can significantly change the value of the heat transfer surfaces. The work starts from a conventional quasi-static model of a small ORC for geothermal applications. The case study refers to a small ORC unit for low-temperature heat input produced by Kaymacor [5]. The quasi-static model has been integrated with the effect of filling, taking into account the geometry of the evaporator which is the most critical component in this respect. The results obtained from the model are consistent with the first experimental values available for the considered unit and show how a correct filling is essential to ensure the actual achievement of the expected performances of the ORC unit.