- Development of a general simulation model for geothermal surface plants using TESPy: Performance assessment under variable operating conditions- Dipartimento di Energia (Politecnico di Milano)

Abstract

Geothermal binary power plants, and Organic Rankine Cycles (ORCs) in particular, provide dispatchable, near-baseload renewable electricity and are, thermodynamically, the renewable technology closest to a conventional fossil-fuel or nuclear cycle. This thesis develops and validates a thermodynamic simulation model of a high-enthalpy, two-phase geothermal ORC binary plant using TESPy, an open-source, graph-based Python framework, pursuing two objectives of equal weight: assessing plant performance for working-fluid selection, and rigorously documenting TESPy’s capabilities and limitations as a modelling tool for future students and researchers. The methodology proceeds from a review of TESPy’s architecture and internal Newton-Raphson solver, through three progressive validation benchmarks of increasing complexity, to a full-scale plant model integrating a phase separator, parallel evaporators, a preheater, and an air-cooled condenser. Four hydrocarbon working fluids, n-butane, isopentane, n-pentane, and cyclopentane, are compared through a multi-variable net-power optimization at a fixed resource temperature and a parametric sweep across a wide geofluid temperature range, complemented by a component-level exergy analysis. The validation stage reproduces reference thermodynamic states with close to exact numerical agreement against a developer reference cycle and an academic exercise, and close agreement with Aspen Plus for a real, documented binary plant (147.92kW against a 147.9kW reference). At plant scale, the results confirm that the proximity between a working fluid’s critical temperature and the geofluid temperature, rather than the subcritical pressure margin alone, governs net power output and thermal efficiency: n-pentane, isopentane, and cyclopentane reach thermal efficiencies between 14.85% and 15.79% at a 160◦C resource, while n-butane collapses toward its own critical point once a minimum reinjection temperature is enforced. Across a 140–230◦C parametric sweep, only cyclopentane remains solvable over the full range, and net power increases monotonically with resource temperature for every fluid. Beyond these physical findings, this thesis establishes TESPy as a reliable, well-documented open-source simulation framework for future geothermal ORC studies.