Thermodynamic Analysis of an Open-Loop Thermosyphon Heat Engine for Combined Power Generation and Desalination from Low-Grade Waste Heat

A novel open-loop thermosyphon heat engine driven by low-temperature waste heat is proposed for simultaneous power generation and freshwater production. Large quantities of low-grade thermal energy from sources such as data centres remain underutilised due to the limited efficiency and mechanical complexity of conventional heat engines at low temperatures. The proposed system employs thermosyphon-driven circulation and gravity-assisted condensate return, eliminating mechanical pumping and reducing parasitic losses. A mathematical model was developed to evaluate system performance under low-grade heat input conditions. For a baseline case with 50% turbine isentropic efficiency and 5000 W thermal input, the model predicts an overall efficiency of 3.8% and freshwater production of 143 kg/day. A parametric study was conducted to identify the dominant performance parameters and assess sensitivity to operating conditions. While the predicted power output does not exceed that of optimised Organic Rankine Cycle systems, the proposed configuration offers reduced mechanical complexity and inherent freshwater production through phase change. Unlike membrane-based desalination systems, the open-loop design can accommodate high-salinity feeds, including concentrated brine streams, enabling high recovery operation. These characteristics suggest potential application in low-temperature waste heat recovery scenarios where simplified operation, high-salinity tolerance, and combined energy–water generation are desirable.