StirliQ+ Component development of the expansion Stirling generator with supercritical fluid as working & lubrication medium

Technical research and further development of details or components of the novel StirliQ engine, which has the potential to overcome the technical hurdles of conventional Stirling engines. On the basis of simulations as well as a laboratory plant, a narrowing down of the process parameters with regard to a resilient pre-dimension of apparatus components is carried out.

Short Description

Starting point / motivation

The novel StirliQ technology, which has been under development and research since 2016, represents an alternative to currently available CHP technologies. The technical innovation of this technology is the StirliQ engine, which, in contrast to the conventional Stirling engine, works neither with a gaseous nor with a liquid, but with a supercritical fluid as working and lubrication medium (combines the advantages of gaseous as well as liquid working media at an optimized operating point).

The conversion of thermal energy into mechanical energy also takes place by volume expansion as well as contraction, but with very low wear due to particularly slow piston movements (1 piston cycle lasts 1 minute) in an oil bath and high pressure between 100 and 170 bar.

As a result, this technology can be used very well with transient process parameters, which are prevalent in industrial waste heat recovery. The working medium is chemically inert and supersaturated with lubricant. The expansion chamber, in which heat transfer also takes place, represents the heart of the StirliQ engine.

The process conditions in the expansion chamber, in particular pressure and temperature, determine the compressibility, the isobaric coefficient of thermal expansion, the viscosity, the coefficient of thermal conductivity and the specific heat capacity. This enables the working medium to be continuously optimized in terms of gas or liquid properties.

Supercritical fluids are used as working fluids according to certain material-specific criteria, which have the highest possible spatial expansion coefficient with minimum compressibility. Thus, there are special requirements for the working medium: The compressibility of the working medium should reach a minimum at the operating point. This results in a minimum piston stroke speed, which overcomes the service life problems of a classic Stirling engine.

Contents and goals

In order for the StirliQ technology to be realized as a CHP plant in the intended industrial performance range and simple design, further comprehensive research work is required, which is the content of the present project:

  • Definition of the basic geometry of the expansion chamber and the heat input.
  • Exact definition of a reasonable balance between the pressure drop of the two heat transfer fluids and the apparatus size in order to maximize the StirliQ efficiency.
  • Definition of the expansion fluid and the associated heat exchanger material.
  • Elaboration / development of a control system.
  • Optimization of the tightness of the system.
  • Elaboration of integration possibilities into a higher-level industrial energy system or into the power grid.

The aim of the StirliQ+ project is, on the basis of the development steps to date, to design and research an almost maintenance-free, slow-rotating expansion generator which, by using a supercritical fluid as a working fluid and lubricant, makes it possible to lower the useful thermal limit temperature and has the potential to overcome the technical hurdles of conventional Stirling engines.


  • Investigation of different working media with regard to substance-specific criteria
  • Simulation of typical applications
  • Modeling and simulation of a StirliQ engine
  • Performance of sensitivity analyses
  • Iterative revision and adaptation loops to optimize the system
  • Establishment of a laboratory system
  • Execution, evaluation and analysis of laboratory test series
  • Elaboration of the system integration requirements of the StirliQ motor
  • System balancing for different use cases
  • Result workshops with experts
  • Development of recommendations for the (further) development of the StirliQ motor

Expected results

The results of the project are further detailed findings as well as improvements to previous developments, which serve as boundary conditions for further technology research. The process parameters of the StiliQ engine have been narrowed down to such an extent that a reliable predimensioning of apparatus components is possible.

Project Partners

Project management

4ward Energy Research GmbH

Project or cooperation partners

  • Prozess Optimal CAP GmbH
  • PK Haustechnik GmbH

Contact Address

DI Robert Pratter
ReininghausstraƟe 13a
A-8020 Graz
Tel.: +43 (664) 88 500 337