Characterization of a dynamic micro heat engine with integrated thermal switch

Abstract

Progress toward the realization of an external combustion dynamic micro heat engine is documented. First, the development of a thermal switch suitable to control heat transfer to and from the micro heat engine is described. Second, the integration of a thermal switch with an engine is detailed. The thermal switch is shown to be an effective means to control heat transfer into the engine from a continuous heat source and out of the engine to a continuous heat sink. The use of the thermal switch is shown to enable engine cycle speeds up to 100 Hz, engine efficiencies up to 0.095% and power output up to 1.0 mW. The internal irreversibility of the engine is measured to be 23%.

Power production from phase change in MEMS and micro devices, a review

“…Response time was high due to the poor conductivity of the working fluid, taking 15 s for full actuation. Actuators using refrigerants or other liquids as their working fluids exhibited response times on the order of several milli-seconds in transient operation [30]. To address this issue, carbon-doped paraffin has been utilized as an alternative working fluid [35].…”

“…Whalen et al noted maximum operating frequencies of 40 Hz and declining thermal efficiencies with increased operating speeds beyond 11 Hz [31,37]. Work performed by Weiss et al addressed the cooling issue through the introduction of a cold thermal switch [30]. The switch was mechanically actuated using a silicon-based contacting surface maintained at 15 C. This contacting surface was allowed to intermittently make and break contact with the actuator lower membrane.…”

Effect of finite heat input on the power performance of micro heat engines

“…the inverse of the thermal resistance (3.8 K/W) of the switch in its "on" state [20], i.e. K S ¼ 0.263 W/K.…”

“…(13) and (15), the effect of the leakage length and thermal conductivity on the limit of maximum power and power density of micro heat engines can be obtained, which can then provide practical design guidance. For instance, since the maximum power is dependent on various thermal conductances within the heat engine, it suggests that the development of micro scale heat exchangers such as the P3 thermal switch [20] could have a large effect on the power output. It is important to note that in extreme and ideal cases when K S ¼ N or when K L ¼ 0 , Eq.…”

Resonant operation and cycle work from a MEMS-based micro-heat engine

“…Similar to the thermal switch used for heat addition work, the thermal switch operated by closing a gap between itself and the microengine evaporator membrane. In these experiments, the switch was maintained at a cool temperature so that contact with the evaporator membrane controlled heat rejection from the micro-engine (Weiss et al 2006).…”