A unit thermionic power cell (TPC) concept has been developed that converts natural heat found in high-temperature environments (460 to 700 C) into electrical power for in situ instruments and electronics. Thermionic emission of electrons occurs when an emitter filament is heated to gwhite hot h temperatures (>1,000 C) allowing electrons to overcome the potential barrier and emit into the vacuum. These electrons are then collected by an anode, and transported to the external circuit for energy storage

Greer, Harold F.

Manohara, Harish

Mojarradi, Mohammad

NASA Technical Reports Server

An  optical alignment mirror mecha-
nism (AMM) has been developed with
angular positioning accuracy of ±0.2 arc-
sec. This requires the mirror’s linear po-
sitioning actuators to have positioning
resolutions of ±112 nm to enable the
mirror to meet the angular tip/tilt accu-
racy requirement. Demonstrated capa-
bilities are ±0.1 arc-sec angular mirror
positioning accuracy, which translates
into linear positioning resolutions at the
actuator of ±50 nm.
The mechanism consists of a structure
with sets of cross-directional flexures that
enable the mirror’s tip and tilt motion, a
mirror with its kinematic mount, and two
linear actuators. An actuator comprises a
brushless DC motor, a linear ball screw,
and a piezoelectric brake that holds the
mirror’s position while the unit is unpow-
ered. An interferometric linear position
sensor senses the actuator’s position. The
AMMs were developed for an Astromet-
ric Beam Combiner (ABC) optical
bench, which is part of an interferome-
ter development. Custom electronics
were also developed to accommodate
the presence of multiple AMMs within
the ABC and provide a compact, all-in-
one solution to power and control the
AMMs.
This work was done by Bruno M. Jau,
Colin McKinney, Robert F. Smythe, and Dean
L. Palmer of Caltech for NASA’s Jet Propul-
sion Laboratory. For more information, con-
tact iaoffice@jpl.nasa.gov. NPO-47928
Space Mirror Alignment System
NASA’s Jet Propulsion Laboratory, Pasadena, California
18 NASA Tech Briefs, December 2011
A unit thermionic power cell (TPC)
concept has been developed that converts
natural heat found in high-temperature
environments (460 to 700 ºC) into electri-
cal power for in situ instruments and elec-
tronics. Thermionic emission of electrons
occurs when an emitter filament is heated
to “white hot” temperatures (>1,000 ºC)
allowing electrons to overcome the poten-
tial barrier and emit into the vacuum.
These electrons are then collected by an
anode, and transported to the external
circuit for energy storage.
The thermionic emission current den-
sity (A/m2) = AT 2e(–φ/kT); where A= a
constant, T = temperature (K), φ = work
function (eV), and k = Boltzmann con-
stant. The efficiency of emission in-
creases with decreasing work function of
the emitter material and increasing tem-
perature. For example, the emission effi-
ciency is much higher for cesium (φ =
2.4 eV) compared to pure carbon nano -
tube (CNT) (4.9 eV) and tungsten (4.5
eV). Additionally, the total current pro-
duced can be increased by enhancing
the emitter surface area.
In this proposed approach, the higher
emission efficiency of low-work function
metal is combined with the enormous
surface area achievable using CNT bun-
dles to produce mA to A range current at
lower temperatures of 460 to 700 ºC
range. This is achievable by conformally
coating CNT (see figure)bundle arrays
(or simply arrays of CNTs) with alkali
metals such as potassium (φ = 2.3 eV) or
cesium using an atomic layer deposition
process. Projected emission area of such
an alkali metal-coated CNT bundle array
(2-µm diameter, spaced 2 µm apart) over
a 4-in. (≈10-cm) diameter wafer is ≈3.0 ×
104 cm2. This leads to an estimated cur-
rent production of ≈500 µA (> 200
Wh/kg) at 460 ºC to ≈1.3 A at 700 ºC,
which is comparable to standard high-
temperature batteries (for example, for
Na-NiCl2, high-temperature batteries pro-
duce ≈90–130 Wh/kg), and sufficient to
power communication, computational
and control electronics, as well as sensors
Schematic representation of the Thermionic Power Cell (TPC) with alkali-metal coated CNT bundles to
enhance thermionic emission.
D
t
coated CNT bundles
Substrate
Anode
Thermionic 
electrons
ALD-
Thermionic Power Cell To Harness Heat Energies for
Geothermal Applications 
Possible uses include geothermal exploration, automotive, and renewable energy applications. 
NASA’s Jet Propulsion Laboratory, Pasadena, California 
https://ntrs.nasa.gov/search.jsp?R=20120001236 2019-08-30T18:46:50+00:00Z
NASA Tech Briefs, December 2011 19
The passive sizing system consists of a
series of low-profile pulleys attached to
the front and back of the shoulder bear-
ings on a spacesuit soft upper torso
(SUT), textile cord or stainless steel
cable, and a modified commercial
ratchet mechanism. The cord/cable is
routed through the pulleys and attached
to the ratchet mechanism mounted on
the front of the spacesuit within reach of
the suited subject. Upon actuating the
ratchet mechanism, the shoulder bear-
ing breadth is changed, providing vari-
able upper torso sizing.
The active system consists of a series of
pressurizable nastic cells embedded into
the fabric layers of a spacesuit SUT. These
cells are integrated to the front and back
of the SUT and are connected to an air
source with a variable regulator. When in-
flated, the nastic cells provide a change in
the overall shoulder bearing breadth of
the spacesuit and thus, torso sizing.
The research focused on the develop-
ment of a high-performance sizing and
actuation system. This technology has ap-
plication as a suit-sizing mechanism to
Spacesuit Soft Upper Torso Sizing Systems
This system has application in medical devices for immobilizing injured limbs or applying
controlled pressure to areas of the body of a burn victim.
Lyndon B. Johnson Space Center, Houston, Texas
Spatial operators have been used to
analyze the dynamics of robotic multi-
body systems and to develop novel com-
putational dynamics algorithms. Mass
matrix factorization, inversion, diago-
nalization, and linearization are among
several new insights obtained using such
operators.  While initially developed for
serial rigid body manipulators, the spa-
tial operators — and the related mathe-
matical analysis — have been shown to
extend very broadly including to tree
and closed topology systems, to systems
with flexible joints, links, etc. This work
uses concepts from graph theory to ex-
plore the mathematical foundations of
spatial operators. The goal is to study
and characterize the properties of the
spatial operators at an abstract level so
that they can be applied to a broader
range of dynamics problems. 
The rich mathematical properties of
the kinematics and dynamics of robotic
multibody systems has been an area of
strong research interest for several
decades. These properties are important
to understand the inherent physical be-
havior of systems, for stability and con-
trol analysis, for the development of
computational algorithms, and for
model development of faithful models. 
Recurring patterns in spatial opera-
tors leads one to ask the more abstract
question about the properties and char-
acteristics of spatial operators that make
them so broadly applicable. The idea is
to step back from the specific applica-
tion systems, and understand more
deeply the generic requirements and
properties of spatial operators, so that
the insights and techniques are readily
available across different kinematics and
dynamics problems. 
In this work, techniques from graph
theory were used to explore the abstract
basis for the spatial operators. The close
relationship between the mathematical
properties of adjacency matrices for
graphs and those of spatial operators
and their kernels were established. The
connections hold across very basic re-
quirements on the system topology, the
nature of the component bodies, the in-
dexing schemes, etc. The relationship of
the underlying structure is intimately
connected with efficient, recursive com-
putational algorithms. The results pro-
vide the foundational groundwork for a
much broader look at the key problems
in kinematics and dynamics. 
The properties of general graphs and
trees of nodes and edge were examined,
as well as the properties of adjacency ma-
trices that are used to describe graph
connectivity. The nilpotency property of
such matrices for directed trees was re-
viewed, and the adjacency matrices were
generalized to the notion of block
weighted adjacency matrices that sup-
port block matrix elements. This leads us
to the development of the notion of Spa-
tial Kernel Operator SKO kernels. These
kernels provide the basis for the develop-
ment of SKO resolvent operators. 
This work was done by Abhinandan Jain
of Caltech for NASA’s Jet Propulsion Labora-
tory. For more information, contact
iaoffice@jpl.nasa.gov. NPO -47313 
Graph Theory Roots of Spatial Operators for 
Kinematics and Dynamics 
These concepts can be applied to modeling, simulation, and control of robots and other mechanisms. 
NASA’s Jet Propulsion Laboratory, Pasadena, California
and miniature motors. Large areas of
TPC or multiple TPC plates can be em-
ployed to produce much higher electrical
energy to power heavier systems.
This highly miniaturized, high-tem-
perature, long-life power source can be
supplementary to primary high-tempera-
ture battery. The concept applies famil-
iar thermionic emission principle for
power generation by harnessing the local
heat in the application environments.
The approach of power production and
design flexibility naturally provides an at-
tractive option to harness in situ heat to
produce power enough to operate elec-
tronics and miniature instrumentation.
TPC can be designed to support geo -
thermal explorations by harnessing heat
energy of the local environment.
This work was done by Harish Manohara,
Mohammad Mojarradi, and Harold F. Greer
of Caltech for NASA’s Jet Propulsion Labora-
tory. Further information is contained in a
TSP (see page 1). NPO-46967


Thermionic Power Cell To Harness Heat Energies for Geothermal Applications

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