Fabrication and characterization of a micro capillary evaporator for MEMS based power generation
Daniel A. Carpenter, M.S.
Washington State University
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.
School:Washington State University
School Location:USA - Washington
Source Type:Master's Thesis
Keywords:microelectromechanical systems evaporators electric power production
Date of Publication: