Microscale combustion (“microcombustion”), micro
power generation and micropropulsion
Quick Links
On-line
presentations:
Microcombustion and power generation
Micro solid oxide fuel
cells
Micropropulsion and
gas pumping
Microbial fuel cells
Papers
(in .pdf format):
Shao, Z,
Haile, S., Ahn, J., Ronney, P. D., Zhan, Z., Barnett, S. A., "A thermally self-sustained
micro Solid-Oxide Fuel Cell with high power density," Nature, Vol. 435, pp. 795-798 (9 June 2005). Download .pdf version from Nature.com website
Ronney, P. D., "Analysis of non-adiabatic
heat-recirculating combustors," Combustion and Flame, Vol 135, pp. 421-439 (2003). (As of 10/20/2006, this is the 2nd most cited
research paper (out of 1615) in any Combustion journal published on or after
this paper’s publication date.)
Pictures
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3D meso-scale burner (4x larger than final
design size of microscale burner) |
Experimental apparatus for testing 3D
meso-scale burner |
3-turn, 3D microscale burner built using EFAB
(3 mm tall) (partially folded) |
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2D meso-scale burner (3x larger than final
design size of microscale burner) |
Experimental apparatus for testing 2D
meso-scale burner |
3-turn, 3D microscale burner built using EFAB
(3 mm tall) (before folding) |
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Narrative
description
It is well
known that the use of combustion processes for electrical power generation
provides enormous advantages over batteries in terms of energy storage per unit
mass and in terms of power generation per unit volume, even when the conversion
efficiency in the combustion process from thermal energy to electrical energy
is taken into account. For example, hydrocarbon fuels provide an energy storage
density between 40 and 50 MJ/kg, whereas even modern lithium ion batteries
commonly used in laptop computers provide only 0.4 MJ/kg. Thus, even at only 5%
conversion efficiency from thermal to electrical energy, hydrocarbon fuels
provide about 5 times higher energy storage density than batteries. For this
reason automotive and aviation vehicles employ internal combustion engines for
prime moving and electrical power generation almost entirely to the exclusion
of batteries, even in vehicles whose mass may be less than 1 kg or more than 105
kg. In the past few years, many research groups from around the world have begun
to develop devices called Micro Electro-Mechanical Systems, or
"MEMS," typically borrowing technologies originally developed for
microelectronic devices. Recently much attention has been focused on the
application of MEMS devices to the production of electrical power, so-called
"Power MEMS" devices, typically in applications where
batteries are currently used.
Many groups
involved in Power MEMS are investigating scaled-down versions of
well-established macro-scale combustion devices (internal combustion engines,
gas turbines, pulsed combustors, etc.) There are numerous difficulties with
this approach, for example the fact that flames extinguish due to heat losses
if the dimension of the combustion chamber is too small, i.e. “microcombustion”
is more difficult than “macrocombustion.” Furthermore, even if flame quenching
does not occur, heat and friction losses become increasingly important at
smaller scales since the heat release due to combustion and thus power output
scales with the volume of the engine whereas the heat and friction losses scale
with the surface area.
For these
reasons we have developed two Power MEMS system concepts based on the
integration of the following four technologies:
• Microcombustion, heat transfer and thermal
management using a two-dimensional or three-dimensional toroidal "Swiss
Roll" counterflow heat exchanger and combustor. A U.S. Patent has recently
been issued to use for this design.
•
Power generation using thermoelectric elements or a single-chamber solid oxide
fuel cell. Professor Sossina Haile of Caltech is the
Principal Investigator for the fuel cell project. This “SOFC in a Swiss roll” concept was listed as one of six
technologies that will “change the world” according to Business 2.0 magazine (view article). (PDR note: I’m being a little more
cautious than Business 2.0 about forecasting the future.)
•
For the thermoelectric project, monolithic construction of entire device
including structural, thermoelectric and catalytic materals using electrochemical fabrication (EFAB) technique
pioneered by Adam Cohen, now President of Microfabrica,
Inc., Burbank, CA. Other
fabrication techniques including micro-Electro-Discharge Machining for metal
parts and directed assembly using colloidal inks for ceramic parts, the latter
of these in conjunction with Prof. Jennifer Lewis of
UIUC.
• To pump the gaseous reactants through
the combustor and (optionally) generate thrust with no moving parts, a catalytic combustion
driven thermal transpiration pump (described below).
These
approaches have the following advantages over "traditional"
approaches to micropower generation:
• No moving parts, thus no friction
losses
• Reduction of heat loss effects, thus
minimizing flame quenching problems and minimizing efficiency losses.
• Monolithic construction, requiring at
most one simple mechanical assembly step (for the electrochemical fabrication
technique.)
• Ability to use hydrocarbon fuels,
unlike some Power MEMS concepts which require hazardous,
low-energy-per-unit-storage-volume fuels such as hydrogen, or fuels derived
from solid rocket propellants.
The goal of
these projects is to produce practical working devices using a combination of
experimental examination of scaled-up model microcombustion, power generation
and propulsion devices, numerical modeling of macro- and micro-scale devices,
and micro-scale fabrication using the aforementioned techniques.
Support for
both power generation projects comes from the MicroElectricalMechanical Systems
Programs Office of the Defense Advanced
Research Projects Agency (DARPA), with Dr. Clark Nguyen as
the program officer. The
microscale pumping and propulsion project is supported by the Combustion
Science program of the NASA Office of Biological and Physical Research.
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