MicroStirling


Acronym : MicroStirling

Project leader (in FEMTO-ST) : M.Barthès

Partnership : SYMME Laboratory, University of Sherbrooke and FEMTO-ST

Figure 1: Microfabricated micro-Stirling engine (3 engines in series)

Abstract :

This project concerns the development of a micro-engine for energy harvesting. It is a collaborative project between departments of the FEMTO-ST institute (MN2S & Energy) and is the result of a collaboration with academic partners (SYMME laboratory at University of Savoie and UMI-LN2, University of Sherbrooke, Canada) on the development of a miniaturized Stirling engine for energy recovery. The Stirling engine is based on a reversible cycle of isothermal compression and expansion of a gas which allows to convert a thermal energy into mechanical energy, or the opposite (to convert mechanical energy into thermal energy). This makes the Stirling engine a relevant element in many fields, and especially for the valorization of lost thermal energy at low temperature level.

Although there are functional demonstrators at the macro or mesoscopic scales, nothing exists at the micro-scale: international works on a miniature Stirling machine are only theoretical or numerical. The miniaturization of such an energy system is of great interest since it can be deployed in many areas and for multiple applications (for example, on-board cooling systems for which congestion constraints are very strong). During the previous project, a miniaturized Stirling engine demonstrator (based on a vertical stack of wafers) was realized using microfabrication technologies. Thermal, mechanical and fluid aspects were considered and led to an optimized solution in which the pistons/displacers are replaced by hybrid membranes (silicone polymer membrane embedding a silicon spiral flat spring). At these scales, the theoretical study of this demonstrator has highlighted insufficient knowledge on heat exchanges on the one hand and on alternate type flows on the other.

Concerning heat transfer aspects, the optimization of heat exchangers and of the thermal insulation between the hot and cold parts of the machine are two particularly important parameters to consider because of their large impact on the machine efficiency. Concerning micro-scale flows, there are several studies on permanent gas flows in the literature, even if the bibliographical results are often contradictory because surface condition or compressibility are not always (or cannot always) be taken into account and/or quantified. On the other hand, the specificity of Stirling type engines is the presence of alternating type flows (average speeds oscillating periodically between negative and positive values). Although there are a few studies on a macroscopic scale alternating gas flows, there is almost no data available on alternating flows and their associated pressure losses on micro-scales. From a technical point of view, the machine is difficult to characterize because commercial sensors are often more cumbersome than the machine itself, which requires the realization of dedicated sensors integrated into the machine during its manufacture. Dedicated temperature (RTD) and pressure sensors (membranes) will therefore be designed and manufactured using microfabrication techniques. 

From a scientific point of view, the objectives of this project are therefore to study both theoretically and experimentally: (i) permanent and alternating flows in order to deduce their phenomenology, behavior and scale laws (ii) geometries of {fluid - solid} heat exchangers optimized in the micro domain to ensure fast thermal exchange and to recover heat from the largest possible volume of gas, and (iii) geometries ensuring the best fluid transfer between the cold and hot zones of the machine, while minimizing hydraulic resistance.

 

Funding agency : EUR-EIPHI/I-site

Grant or funding obtained :   100k€

Start and end dates : 2018– 2022