Projects

Micro heat engines are essentially the miniaturized versions of the heat engines in the macro world. Development of micro heat engines themselves is an interdisciplinary effort spanning heat transfer and mechanical engineering. The heat engine developed by Till et al at IMTEK via Silicon micromaching is shown in Figure 1. The device is 16 ×14 ×1.5 mm3 in size. Integrating an electric power generator into the heat engine, to generate electric power, adds a further dimension, namely electrical engineering. However, we have successfully demonstrated electric power generation using a micro heat engine with integrated pyroelectric as well as electromagnetic generators. A schematic of the micro heat engine with a pyroelectric generator (PEG) is shown in Figure 2. When the engine chamber is in contact with the heat source, it gets heated up. Simultaneously the temperature of the air inside the sealed chamber rises, leading to a subsequent increase in the pressure inside the chamber. Once the pressure exceeds a threshold, the bistable membrane flips to its second state. Now the engine chamber is in contact with the heat sink through the PEG. The engine chamber loses heat and the air inside is cooling down. Subsequently the pressure inside the chamber drops. Once this pressure drops below a threshold, the membrane flips back to its original position. The whole process repeats, ensuring a periodic oscillation of the engine chamber between the heat source and sink. Thus, these devices transport heat periodically from a heat source to a heat sink, with a part of the heat transported being converted to mechanical work. Simultaneously, the PEG is also getting heated up and cooled down periodically. The PEGs by virtue of their inherent pyroelectric property converts these thermal oscillations into oscillating electric fields or charges, thereby generating usable electrical power. The electrical power generated as well as the operational frequency of the device is shown in Figure3. The device generates 3 µW of electrical power from a spatial temperature difference of nearly 80 K. We are currently in the process of developing new heat engines with integrated electric power generators. We are also focusing on the development of new techniques to enhance heat transfer at the dynamic thermal interfaces which are often found in micro heat engines.