Design, modeling, and performance of miniature reciprocating thermocompressor

Abstract

Research in the miniaturization of thermomechanical systems in recent years has proven to be both rewarding and challenging. Micro-scale devices have tremendous technological potential but come with a new set of design problems. One such problem is energy storage. Heat actuated systems, driven primarily by thermal energy, take advantage of the high stored energy density of hydrocarbon fuels, and are therefore desirable for small-scale applications. One concept for a heat-actuated compressor is called a thermocompressor, which is a variation of a traditional Stirling engine. Thermocompressors use constant volume heat addition to pressurize their working fluid without applying any mechanical work to the gas. Such devices would be well suited for numerous applications such as power cycles, heat pumps, refrigeration, and liquid pumps. This thesis develops a detailed thermodynamic analysis of the thermocompressor cycle providing insight into the operation of the cycle and the sensitivity of the cycle's performance to various design parameters including regenerator effectiveness, regenerator dead volume, hot side dead volume, and cold side dead volume. The developed model shows that the thermal efficiency is highly sensitive to regenerator effectiveness, which motivates a regenerator optimization study. As the regenerator surface area increases, its effectiveness increases, however, so does its dead volume. The regenerator model shows that there is an optimum balance between regenerator effectiveness and regenerator dead volume, which gives a predicted maximum thermal efficiency. The optimum regenerator effectiveness is approximately 96% with regenerator dead volume of approximately 26%, which corresponds to a maximum predicted thermal efficiency of 45-50% of Carnot efficiency. This raises questions about the practicality of the device particularly in achieving regeneration high enough to make the concept practical. More detailed modeling work would be beneficial to further understanding of this cycle. The experimental work of this thesis includes the development and testing of a first-order, miniature thermocompressor. Its intention is to explore the pertinent issues relating to the design, operation, and performance of thermocompressors. The thermocompressor's cylinder diameter is 0.945 inches, and its cylinder length is 1 inch with a displacement volume of 0.175 cubic inches. The reciprocating motion of the regenerator/displacer is driven by an electric motor to which it is magnetically coupled. Several tests were performed with the experimental apparatus. The primary interest was to measure pressure verses time using both nitrogen and helium as working fluids, over a range of temperature ratios from 1.2 to 2.0, operating at both 10 and 13 Hz. The pressure variation inside the cylinder is plotted as a function of time and compared to the regenerator/displacer position. In addition, some dynamic temperature measurements were made inside the cylinder during operation to indicate how much temperature variation exists in the hot space. The experimental results show that the thermocompressor produces pressure ratios of approximately 75% of the temperature ratios. Nitrogen and helium both appear to perform almost equally and operating speed does not appear to significantly affect the pressure ratio. As expected there is a linear relationship between temperature ratio and pressure ratio. Also, the temperature variation near the top of the hot side does not appear to fluctuate more than approximately 3 °C with N2 or 13.7 °C with He during operation.