In the StirliQ+ project, the StirliQ technology, researched since 2016, was further developed to harness thermal energy in the low-temperature range (< 100 °C). Coupled with a generator, the system converts thermal energy into electricity. The StirliQ engine is based on the Stirling engine principle, but instead of using a gaseous or liquid working medium, it operates with a supercritical fluid. Supercritical CO₂, in particular, exhibits exceptional properties in this range, such as low specific heat capacity and very high reciprocal isothermal compressibility. The preceding project already demonstrated that the StirliQ engine offers significant efficiency advantages over conventional Stirling technology.

The project objectives were cost optimization, maximizing efficiency within physical limits, ensuring long-term operational stability, and enabling power control. To achieve these goals, a novel process design based on an axial piston engine was investigated.

Component development and simulation

As part of component development, key parts for operation with supercritical CO₂ were designed, adapted, and manufactured. These included the shaft seal, piston rings, and valve plate, aimed at minimizing clearance losses caused by leakage. To support this, a simple calculation model based on Hagen-Poiseuille principles was developed, and its results were used to optimize the above components.

In parallel, simulation models were created in Aspen HYSYS and DWSIM to represent various thermodynamic operating points of the process. An additional Python model provided deeper insights into process design and enabled realistic simulations of the StirliQ process under variable operating conditions. Using this Python model, different operating windows (pressure, temperature, etc.) were investigated, leading to key findings for improving efficiency. Under certain conditions, R32 was also considered alongside CO₂ as a working medium to evaluate the technology in broader system contexts. For example, it was shown that different temperature levels may require different working media to maximize efficiency.

Integration into energy systems

To assess integration of the StirliQ system into larger energy systems, two application scenarios were analyzed. The aim was to utilize previously untapped waste heat sources to generate electricity. Various thermodynamic boundary conditions and load profiles were considered to estimate, among other factors, the annual electrical energy output of the StirliQ+ system. For future real-world implementation, framework conditions were defined that should be taken into account when integrating the StirliQ engine.

Experiments and optimization

A process control system with data acquisition was developed for the laboratory plant. This included visualization of process and measurement data, a PID-based control and regulation concept, and the extension of measurement capabilities through soft-sensor strategies. Initial experiments focused on verifying component tightness and leakage resistance. Parameter studies conducted on the test bench confirmed the basic operability of the StirliQ process and allowed optimization of the initial working fluid mass.

Outlook

Based on the results so far, the realization of a demonstration project appears highly promising. Key opportunities lie in the system's high flexibility, scalability, and the possibility of integrating existing technologies (e.g., standard inverters). The main challenges and risks concern ensuring long-term stability and validating the technology under changing and real-world operating conditions.

Project management

4ward Energy Research GmbH

Project or cooperation partners

• Prozess Optimal CAP GmbH
• PK Haustechnik GmbH

DI Robert Pratter
Reininghausstraße 13a
A-8020 Graz
Tel.: +43 (664) 88 500 337
E-Mail: robert.pratter@4wardenergy.at
Web: www.4wardenergy.at