Fabrication and characterization of a micro capillary evaporator for MEMS based power generation

Daniel A. Carpenter

A Microelectromechanical System (MEMS) based micro capillary evaporator for power generation was fabricated and characterized. The main goal of this work is to determine the efficiency of an evaporator, where efficiency is defined to be the amount of energy used to evaporate fluid over the amount of energy put into the evaporator. An energy balance on the evaporator is used to determine this efficiency and track where input energy goes. The evaporator is fabricated on 10mm by 18mm silicon die with a5mm square, 2µm thick membranes at the center. Two concentric, annular platinum Resistance Temperature Detectors (RTDs) surround a platinum resistance heater located in the center of the membrane. A 10mm diameter capillary wicking structure is fabricated over the RTDs and heater on the membrane using an epoxy-based resist, SU8. The SU8 wick pattern, which went through several iterations, consists of structures with 10µm wide channels, with 10µm wide lines and 10µm thick sidewalls. A series of evaporation experiments are reported. The efficiency of the wicks and the energy balance for the experiments is documented. Both steady state and transient evaporation tests are detailed. During a five-minute period of steady state evaporation the energy into the evaporator is 14.47J. Energy carried away by evaporation is 3.36J. Energy conducted across the membrane is 15.7J. The corresponding electrical power into the evaporator is 48mW with, 11mW carried away by evaporation, and 52mW conducted across the membrane. Energy dissipated in the evaporator heater minus the energy conducted across the membrane and energy carried by evaporation balance within 27%. RTD1 measured a temperature of 40°C and RTD2 30°C, giving a ∆T of 10°C. The efficiency is 23%. A transient evaporation test run at 1Hz and 50% duty cycle results in energy carried away by evaporation of 2.51J with energy into the evaporator of 16J over 5 minutes and 13.9J conducted out across the membrane. Energy in minus energy out thus balances within 2%. The electrical power in is 53mW with 8mW of power carried away by evaporation, and 46mW of power conducted out across the membrane. The efficiency of energy of the evaporation in transient operation is 15.66%. RTD1 measured a temperature of 42°C and RTD2 of 22°C, giving a ∆T of 20°C.

Fabrication and characterization of a micro capillary evaporator for MEMS based power generation

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