Why the Stirling engine's 200-year-old design still struggles to scale

The Stirling engine, invented over 200 years ago, remains a unique piece of technology. It works by using temperature differences to generate power, with a design that allows for high theoretical efficiency. Yet despite its potential, recent years have seen little commercial use in large-scale projects.

Robert Stirling developed the engine in 1816. Its operation relies on a cycle of four phases: isothermal expansion, isochoric heat rejection, isothermal compression, and isochoric heating. A key component, the regenerator, boosts efficiency by storing and reusing heat during these stages.

The engine comes in multiple designs, including alpha, beta, gamma, and free-piston configurations. These variations suit different applications, from small educational models to industrial systems. Over time, Stirling engines have found roles in combined heat and power plants, space missions, and solar thermal projects. However, their adoption in solar thermal power plants has stalled. Between 2021 and 2025, no new commercial installations were recorded. Earlier efforts, such as Spain's PE 10 plant in Puerto Errado (2009) and US projects, failed to scale due to high costs and low efficiency. Swedish and Danish firms like Ripasso Energy and Cleanergy also struggled to make the technology commercially viable.

While the Stirling engine's design remains theoretically efficient, real-world challenges have limited its use. The absence of new solar thermal installations highlights ongoing barriers. For now, its applications stay niche, focused on smaller systems and specialised fields.