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Decentralized Energy Conversion

Nowadays more than 40% of the final energy in Germany and many other European countries is consumed for space heating and hot water supply in residential and commercial premises. Although this energy is needed at temperatures just marginally above ambient level, most of it is still provided by direct combustion of high-value fossil fuels. On the other hand, such low-grade heat is discharged to the environment in large quantities (e. g. at power stations or industrial sites), since it is technically or economically infeasible to convey it to potential consumers.

So, extensive economies can be achieved by decentralizing energy conversion systems and relocating them to sites where low-grade heat can be usefully consumed. This can be realized either by small cogeneration systems, i. e. by so-called combined heat and power generation (CHP), or by heat pumps, which may be driven mechanically (i.e., by electric compressors in most cases) or thermally (by a burner or possibly by waste heat).

The so-called regenerative gas cycles offer several options to realize such decentralized conversion systems at potentially high efficiencies and without any major constraints to the operating temperature levels. Furthermore, the driving heat flows may come from virtually any source, including biomass, solar power, and waste heat of sufficient temperature. Therefore, these cycles are under particular investigation in this research group.

The most well-known of these, the Stirling cycle, has been used as a prime mover in a variety of application fields for almost two centuries now. Presently, Stirling engines form the core component of commercially available Mini- and Micro-CHP systems. Furthermore, the reversed Stirling cycle is used for mechanically driven heat pumps and refrigerators, particularly in cryogenic application fields.

Another regenerative gas cycle, the Vuilleumier cycle, may either be used for cryocoolers, too, or for thermally driven heat pumps. For about three decades, it has been investigated as a potential domestic heating system in several R&D-projects with very promising results. In addition, several other regenerative cycles can be conceived, which may serve as heat transformation systems in a most general sense. Due to the similarity of all these cycles, it is possible to devise convertible energy supply systems that can be adapted to various demand situations. Furthermore, such cycles are suited for waste heat utilization at industrial locations, e. g. generating compressed air or refrigeration capacity – maybe even at cryogenic temperatures. These ideas and their realization will be pursued by this group by current and future R&D-projects.


Current Research Projects in the group:


Development and optimization of regenerators with axially varying matrix structures


This project aims to increase the efficiency of regenerative (gas) cycles by developing and optimizing regenerators with axially varying matrix structures. Therefore, analytical and numerical models are applied to the design and the optimization of such regenerators, the performance of which is experimentally investigated afterwards.

read more: Sebastian Peveling

Direct Generation of Compressed Air from Waste Heat by Reciprocating Thermocompressors


This project aims at the development of a multi-stage reciprocating thermocompressor to generate compressed air from thermal sources, preferably waste heat. In this context, analytical and numerical modelling as well as constructive considerations are required to develop a practically and economically competitive solution.

read more: Fabian Fischer


Experimental Investigation of Appendix Gap Losses in Stirling Cycle Machines


The aim of this work is to investigate appendix gap losses experimentally by direct measurements of gas and wall temperatures for the first time and thus validate and improve the existing modeling approaches. So far these losses have only been investigated theoretically or by indirect experimental balancing procedures.

read more: Jan Sauer


Completed Research Projects in the group:


Analytical Modeling of Appendix Gap Losses in Stirling Cycle Machines


Appendix gap losses may amount to approximately 10 % of the heat input to a Stirling engine. They are caused by the annular gap between the insulating dome of a piston or displacer and the adjacent cylinder wall. The aim of this work is to develop a new analytical model for these losses, since the existing modeling approaches appear to be insufficient and are partly based on very gross and possibly questionable assumptions.

read more: Analytical Gap Model


Combined Cooling, Heat and Power by a Convertible Stirling-Vuilleumier Hybrid Engine


Regenerative gas cycles offer the opportunity to combine a Stirling engine and a thermally driven Vuilleumier heat pump in a single machine. Such a system can respond to varying demands of heat, power, and possibly cold, and may therefore be used as a decentralized micro-trigeneration unit, i.e. a combined cooling, heat and power (CCHP) system, operation of which can be toggled in a demand-dependent way. In the present research project, a first prototype was developed and manufactured at the Chair of Thermodynamics and is currently investigated experimentally with particular respect to the various thermal and mechanical losses.

read more: Convertible CCHP




Foto von PD Dr.-Ing. Hans-Detlev Kühl

PD Dr.-Ing. Hans-Detlev Kühl


TU Dortmund
Fakultät Bio- und Chemieingenieurwesen
Emil-Figge-Str. 70
44227 Dortmund

Raum G2-515