Proposed Project: Decoupled Stirling Engine

This project was submitted unsuccessfully for consideration for funding under the DIT Fiosraigh Research Excellence Award scheme. Perhaps I did not provide sufficient detail or maybe it was not clear what I was trying to achieve and, of course, I was up against very strong competition.


The objective of the proposed PhD project is to develop a novel decoupled Stirling Engine for sustainable energy applications. The concept has been described at an international conference and in a journal publication by Barry Cullen and Jim McGovern. This project will refine the concept and develop a prototype. Many sustainable energy applications could benefit from a heat engine of the type that is proposed. The expected benefits, compared to other Stirling engines, are significantly enhanced efficiency over a wide range of operating conditions and greatly enhanced flexibility of operation. These will be achieved through a novel heat-transfer-enhancement process and the decoupling of the thermodynamic cycle from dependence on any fixed-geometry mechanical linkage. The novel Stirling engine will be applicable to power production from low-to-moderate-temperature heat sources and will also be applicable to heat-pump or refrigeration installations. Potential application areas include solar thermal power production, waste-heat to power applications, micro-CHP (domestic scale), and thermally driven heat pumps or refrigerators.


The project has the objective of applying expertise relating to the Stirling cycle, simulation, modelling and finite time thermodynamics—which has been built-up by Jim McGovern and his research students and collaborators—to realize a practical implementation of a totally new type of Stirling engine machine for sustainable energy applications.


As currently envisaged, the decoupled Stirling engine cycle involves four independently acting motor/generators, four pistons, high- and low-temperature heat exchangers and a regenerator. Real-time control of the motion of the four pistons is to be realized via the motor/generators in accordance with the temperature levels of external heat source/sinks and a software algorithm that will respond to power demand (or, in some circumstances, power supply, heating demand or cooling demand), while optimizing plant performance.

The project will begin by reviewing the state of knowledge in relation to all engineering aspects of the proposed prototype machine. The review will include, for instance, a review of the literature relating to the design of regenerators for Stirling cycle engines and a review of reciprocating flow through heat exchangers (which is a key aspect of the prototype to be built). The PhD researcher will study the PhD thesis of Barry Cullen and will review its references for their relevance to the project. They will also review relevant publications of Jim McGovern and his collaborators and research students. Key researchers and research centres will be identified, including the identification of expertise available locally within the DIT and in Ireland. Contact will be made with potential collaborators with a view to developing mutually beneficial working relationships.

A close watch will be kept for similar or related work being conducted elsewhere, especially by any researchers that cite the papers that have already been published. A watch will be kept for peer reviewed international conferences in the general area of heat engines that operate with low grade heat sources, with a view to participating in these. Likewise, a watch will be kept for conferences and developments relating to the Stirling engine generally, e.g. Stirling engine technology that is used in association with parabolic concentrating solar collectors or as part of small-scale CHP installations. A ‘watch list’ of periodicals will be prepared.

The initial design analysis will be carried out and models developed using Wolfram Mathematica and Microsoft Excel. The approaches to be used have evolved from experience with other research projects. A modular and very flexible development process will be achieved and a high standard of presentation and self-documentation will be maintained for all parts of the work.

A small-scale machine will be designed from first principles containing all the necessary elements. The eventual working fluid would probably be helium, but neon or nitrogen could be considered for the initial prototype. A gas charge control system would have to be designed. Suitable motor/generator actuator/absorbers would be designed in conjunction with collaborators having specialized knowledge of these devices. Special mechanisms, such as rack and pinion, may have to be designed to link the pistons to the motor/generators.

A preliminary and then a comprehensive engineering model of the proposed prototype will be developed, evaluated and reviewed by colleagues and collaborators before any hardware is assembled. If found necessary, further iterations of this process will be undertaken until the prototype design has acceptable predicted characteristics and has a high likelihood of realizing the predicted performance. Hence, before the prototype is constructed a full predicted time evolution of temperatures, pressures and piston positions will be available as a reference with which actual test data can be compared. Costs will be minimized by analysing and modelling the evolving design thoroughly prior to its implementation.

Towards the end of the first year the researcher will present a DIT seminar paper and this will be followed by a presentation at a national or international conference. These presentations will relate mainly to the design phase of the project. The work will move on to the implementation of the design and the construction of the prototype and the testing rig. There will be interaction and collaboration with other researchers throughout this process.

In prototype development of this nature, problems are to be expected. These will be tackled in a professional way and, where necessary, supplementary investigations will be carried out, perhaps followed by re-design or modifications to the equipment or the approach being pursued.

A computerized control system will be designed in conjunction with collaborators. Programming will be implemented at high level. The achievement of high operating speeds will not be essential to the success of the project.

A number of academic colleagues in different universities will be consulted in relation to the heat transfer aspects. It is envisaged that all heat transfer components can be designed with reasonably high confidence using available data and analytical models.

The working prototype to be built and tested will accept heat transfer from a moderate-temperature heat source and reject heat to an ambient temperature heat sink. It will provide mains-quality single-phase electrical power. The experimental set-up will be instrumented for the measurement of the heat input and rejection rates, the inlet and outlet temperatures of the heating and cooling fluids, the net electrical power output, the instantaneous pressures within the system and the instantaneous positions of the four pistons.

The development of the experiment as outlined above will require a lot of investigation and analysis on the part of the PhD researcher and will also require a very significant amount of collaboration with individuals and groups who have specialized knowledge or expertise relating to the different aspects. Individuals and groups are already known in relation to most aspects. The researcher will also select and implement a suitable interface, such as Labview, between the hardware and the computer that will be used for control and will develop the software control algorithms.

A comprehensive testing programme will be devised and conducted that will allow simulation models of the decoupled Stirling engine machine to be validated. Performance results will be collated and prepared for journal publication.

Once a working prototype has been achieved it is envisaged that the testing phase for data gathering will proceed relatively quickly. It will be sufficient to demonstrate the viability of the concept conclusively and to present actual measured data.