A thermionic photovoltaic energy conversion device comprises a thermionic diode mounted within a hollow tubular photovoltaic converter. The thermionic diode maintains a cesium discharge for producing excited atoms that emit line radiation in the wavelength region of 850 nm to 890 nm. The photovoltaic converter is a silicon or gallium arsenide photovoltaic cell having bandgap energies in this same wavelength region for optimum cell efficiency

Chubb, D. L.

English

NASA Technical Reports Server

.United States Patent [191 
Chubb 
[54] THERMIONIC-PHOTOVOLTAIC ENERGY 
CONVERTER 
[75] Inventor: Donald L. Chubb, Olmsted Falls, 
Ohio 
1731 Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. NO.: 580,573 
[22]  Filed: Feb. 15, 1984 
.................................... HOlL 31/00 
.......................................... 136/253 
................................ 136/253, 254 
~561 References Cited 
U.S. PATENT DOCUMENTS 
3,433.676 3/1969 Stein 136/253 
3,462,310 8/1969 Rubenste 136/253 
3,751,303 8/1973 Kittl ................. 136/253 
3.929.510 12/1975 Kittl .................... 136/247 
4234,352 11/1980 Swanson ............. 136/253 
[ i i ]  Patent Number: 4,528,417 
[45] Date of Patent: Jul. 9, 1985 
4,313,425 2/1982 Crackel et al. ...................... 126/438 
4,316,048 2/1982 Woodall .............................. 136/253 
OTHER PUBLICATIONS 
A. E. Von Doenhoff et al., I R E  Trans. Military Elec- 
tronics, Apr. 1959, pp. 46-51. 
Primary Examiner-Aaron Weisstuch 
Attorney, Agent, or Firm-Gene E. Shook; John R. 
Manning 
P I  ABSTRACT 
A thermionic-photovoltaic energy conversion device 
comprises a thermionic diode 10 mounted within a hol- 
low tubular photovoltaic converter 12. The thermionic 
diode maintains a cesium discharge for producing ex- 
cited atoms that emit line radiation in the wavelength 
region of 850 nm to 890 nm. The photovoltaic converter 
is a silicon or galium arsenide photovoltaic cell 22 hav- 
ing bandgap energies in this same wavelength region for 
optimum cell efficiency. 
18 Claims, 4 Drawing Figures 
2 
r 20 I - QEL 
https://ntrs.nasa.gov/search.jsp?R=19850026128 2020-03-22T20:23:55+00:00Z
U.S. Patent Jui. 9,1985 Sheet 1 of2 4,528,417 , 
US. Patent Jui. 9,1985 Sheet 2 of 2 4,528,417 
f 
4 
QEL- 
FIG. 2 
FIG. 4 
FIG. 3 
1 
THERMIONIC-PHOTOVOLTAIC ENERGY 
CONVERTER 
4,528,417 
DESCRIPTION 
ORIGIN OF T H E  INVENTION 
The invention described herein was made by an em- 
ployee of the United States Government and may be 
manufactured and used by or for the Government for 
governmental purposes without the payment of any 
royalties thereon or therefor. 
TECHNICAL FIELD 
This invention is concerned with the direct conver- 
sion of thermal energy to electrical energy. More par- 
ticularly, the invention is directed to an improved en- 
ergy converter which utilizes both thermionics and 
photovoltaics. 
Thermionic energy converters and photovoltaic de- 
vices have been used separately in the past. A conven- 
tional thermionic diode converts the thermal energy of 
emitted electrons in a cesium plasma directly into elec- 
trical energy. A photovoltaic device converts radiant 
energy into electrical energy. 
It is, therefore, an object of the present invention to 
provide a thermionic-photovoltaic energy conversion 
device having a photovoltaic cell that is isolated from 
the high temperature portion of the system so that elec- 
trical energy is produced at low temperatures. This 
results in long life and reliability. 
PRIOR A R T  
Rubenstein U S .  Pat. No. 3,472,310 discloses appara- 
tus for the conversion of solar energy into electrical 
energy using a cesium atmosphere. The solar energy is 
used to maintain a cesium plasma in which charge sepa- 
ration is maintained so that useful electrical power can 
be obtained from the charge separation electric field. 
Cesium plasma is not produced thermionically and ce- 
sium line radiation is not used to drive a photovoltaic 
cell. 
Stein U.S. Pat. No. 3,433,676, Kittl U.S. Pat. Nos. 
3,751,303 and 3,929,510, Swanson U.S. Pat. No. 
4,234,352 and Woodall U.S. Pat. No. 4,316,048 describe 
thermophotovoltaic converters of various types. Such 
converters generally employ sunlight and, in some in- 
stances, a fuel burning cell as an energy source. These 
converters provide for the conversion of the radiant 
energy so produced into a form that can best be utilized 
by a photovoltaic (silicon) cell. In thermophotovoltaic 
devices, thermal input energy is used to heat a solid 
emitter. The purpose of the emitter is to produce radia- 
tion at wavelengths where photovoltaic cells are effi- 
cient. Either blackbody or broad-band emitters are 
used. 
DISCLOSURE O F  THE INVENTION 
The thermionic-photovoltaic energy conversion de- 
vice of the present invention uses electron thermal en- 
ergy to produce excited atoms which then emit line 
radiation. A photovoltaic cell converts this line radia- 
tion into electrical energy. 
The thermionic-photovoltaic energy conversion de- 
vice comprises two interdependent portions. The first is 
in the form of a discharge region in which a cesium 
discharge is maintained by thermionically emitted elec- 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
2 
trons similar to a conventional thermionic diode. The 
second portion comprises a photovoltaic cell. 
The performance of this thermionic-photovoltaic 
energy conversion device is superior to  that of a ther- 
mophotovoltaic energy conversion device because the 
line radiation from the cesium discharge is in the wave- 
length region of 850 nm to 890 nm. At such a wave- 
length silicon and gallium arsenide photovoltaic cells 
have their maximum efficiency. In contrast, a thermo- 
photovoltaic energy conversion device produces radia- 
tion over a broad wavelength range. A large portion of 
this radiation cannot be utilized by the photovoltaic 
cell. Also the thermionic-photovoltaic devices do not 
require highly efficient optical filters necessary in the 
thermophotovoltaic devices. 
BRIEF DESCRIPTION OF T H E  DRAWING 
The advantages and novel features of the invention 
will be more fully apparent from the following detailed 
description when read in connection with the accompa- 
nying drawings wherein 
FIG. 1 is a longitudinal section view of a thermionic- 
photovoltaic (TEP) energy conversion device con- 
structed in accordance with the present invention, 
FIG. 2 is a transverse section view of an alternate 
embodiment of a TEP energy conversion device em- 
bodying the features of the present invention, 
FIG. 3 is a longitudinal section view taken along the 
line 3-3 in FIG. 2, and 
FIG. 4 is a longitudinal section view taken along the 
line 4-4 in FIG.  2. 
BEST MODE FOR CARRYING O U T  THE 
INVENTION 
Referring now to FIG. 1 there is shown a thermionic- 
photovoltaic energy conversion device constructed in 
accordance with the invention. This device combines a 
thermionic diode portion 10 with a photovoltaic con- 
verter portion 12. 
The thermionic diode 10 comprises an emitter elec- 
trode 14 and a collector electrode 16 spaced therefrom. 
The emitter and the collector are mounted within a 
closed tubular housing 18 containing cesium. A lead 20 
electrically connects the emitter and collector in a man- 
ner well known in the art. 
The emitter electrode 14 is preferably tungsten while 
the collector electrode 16 is preferably molybdenum. 
The housing 18 is of a transparent material, such as glass 
or sapphire, that has high optical transmission in the 850 
nm-890 nm wavelength region. 
The photovoltaic converter portion 12 is a silicon or 
gallium arsenide photovoltaic cell 22 having a tubular 
configuration. The electrical power output (Qei) of the 
photovoltaic cell 22 is connected to a load in a manner 
well known in the art by a suitable lead 24. The thermi- 
onic diode portion 10 is mounted within the photovol- 
taic converter portion 12 so that the photovoltaic cell 
22 surrounds the chamber formed by the housing 18. 
An axial magnetic field is provided by a suitable mag- 
net 26 to reduce losses from electron thermal diffusion 
to the housing 18. This magnetic field can be produced 
by an electromagnetic coil or by a set of permanent 
magnets. It is further contemplated that the magnet 26 
may be unnecessary in certain applications. 
As input power (Q;J heats the emitter electrode 14, 
electrons are emitted from its surface. These electrons 
move within the chamber formed by the housing 18 to 
the molybdenum collector electrode 16, which is main- 
4,528,4 17 
3 4 
tained at a temperature lower than the emitter. These 
thermionically emitted electrons maintain a cesium dis- 
charge within the chamber. The heat (QioSs) that must 
be removed to maintan the collector temperature is the 
major loss in the system. In order to attain efficient 5 vice comprising 
operation, the emitter must be at a temperature at least 
1800" K. The collector must be maintained at approxi- 
mately 1000" K. wavelengths, and 
I,, the discharge region, between the emitter elec- 
trode 14 and the collector electrode 16, electron colli- 10 
sions with ground state atoms will produce excited 
atoms. These excited atoms radiate power (Qr) in a 
wavelength band. This invention relies on the 
fact that cesium's lowest excited states emit radiation in 
the 850 nm-890 nm wavelength region.  hi^ range 15 
matches the bandgap energies of both silicon (si) and 
gallium arsenide (GaAs). Because both silicon and gal- 
in this wavelength region, the energy conversion occurs 
at the cell's highest efficiency. Therefore, the discharge 
region is surrounded by either a Si or GaAs photovol- 
taic cell to convert the photon energy to electrical en- 
ergy. 
ergy that results in atomic excitation is a loss. This loss 
plus ionization and gas heating losses are minimized by 
keeping the gap between the diode electrodes small. 
Conversely, for an efficient TEP converter, it is neces- 
as possible to atomic excitation energy. Therefore, a 
large gap between the emitter electrode 14 and the 
collector electrode 16 is essential. As a result, related 
shorting and mechanical problems are eliminated. This 
promotes long life of the thermionic-photovoltaic en- 35 
ergy conversion device of the present invention. 
DESCRIPTION OF A N  ALTERNATE 
EMBODIMENT 
embodiment of a thermionic-photovoltaic energy con- 
verter constructed in accordance with the present in- 
vention. This embodiment is preferred in Certain aPPli- 
cations where the geometry is more suitable for cou- 
P h  to a heat Source ( Q i d  Also, a higher output Power 45 
density (Qel) may be attained with this embodiment. 
The principle of operation of the annular device 
shown in FIG. 2 is the same as that of the planar device 
shown in FIG. 1. However, this embodiment has an 
emitter 28 in the form of an annular tungsten tube. The 50 
heat input (Qin) flows down the centerline of the device. 
A molybdenum collector 30 is a series of rings separated 
from the emitter 28 by insulating rings 32. 
The collector rings 30 are held by four molybdenum 
tubes 34 which are enclosed in insulating tubes 36. Cool- 55 
ing fluid is circulated through the molybdenum tubes 34 
in order to maintain the proper collector temperature of 
about lOOO" K. An electrical lead 38 connects the four 
molybdenum collector tubes to the emitter tube. 
cesium discharge. The window 40 may be either glass 
or sapphire. Outside the window 40 is the Si or GaAs 
photovoltaic cell 42 which converts the line radiation to 
electrical power. from. 
been shown and described, it will be appreciated that 
various structural modifications may be made to the 
thermionic photovoltaic energy conversion device source of radiation comprising 
without departing from the spirit of the invention and 
the scope of the subjoined claims. 
I claim: 
1. A thermionic-photovoltaic energy conversion de- 
thermionic means for providing a discharge that pro- 
duces line radiation in a relatively narrow range of 
photovoltaic means isolated from said thermionic 
means and responsive to said wavelengths in said 
relatively narrow range for converting photons in 
said line radiation into electrical energy. 
2. A device as claimed in claim 1 wherein said thermi- 
Onic 
3. A device as claimed in claim 2 wherein the cesium 
discharge means includes means for thermionically 
emitting 
vice comprising 
a cesium discharge 
lium arsenide alloy photovoltaic cells are most efficient 4. A thermionic photovoltaic energy conversion de- 
20 a thermionic diode including 
a material for producing a discharge emitting a line 
radiation in a predetermined range of wave- 
lengths, 
a pair of spaced electrodes for maintaining said 
discharge, and 
a housing of a transparent material that has high 
optical transmission in said predetermined range 
of wavelengths for enclosing said spaced elec- 
trodes and said discharge, and 
In a conventional thermionic diode the electron en- 25 
sary to convert as much Of the energy 30 a photovoltaic converter of a material having band- 
gap energies matching the wavelengths in said 
predetermined range adjacent to and isolated from 
said thermionic diode for converting photon en- 
ergy of said line radiation that is transmitted 
through said housing to electrical energy, 
5. A device as claimed in claim 4 wherein the trans- 
parent housing contains cesium for producing the dis- 
charge. 
6.  A device as claimed in claim 5 wherein said spaced 
40 electrodes include an emitter electrode of tungsten and 
a collector electrode of molybdenum. 
7. A device as claimed in claim 6 including means for 
heating the emitter electrode to a temperature greater 
than 18000 K. 
8. A device as claimed in claim 6 including means for 
cooling the collector electrode and maintaining the 
Same at a temperature of about 1000" K. 
9. A device as claimed in claim 5 wherein the photo- 
voltaic converter is made of silicon. 
10. A device as claimed in claim 5 wherein the photo- 
voltaic converter is made of gallium arsenide. 
11. A device as claimed in claim 4 including means 
for providing an axial magnetic field between the 
spaced electrodes. 
12. A device as claimed in claim 4 wherein both the 
housing and the photovoltaic converter have tubular 
configurations with the housing being mounted within 
the converter. 
13. A device as claimed in claim 12 wherein the emit- 
14. A device as claimed in claim 13 wherein the col- 
lector comprises a plurality of spaced rings extending 
radially outward from the emitter and insulated there- 
15. In combination with a hollow tubular photovol- 
taic converter of the type responsive to line radiation 
having wavelengths in a predetermined narrow range, a 
Referring now to there is shown an 
D An optically transparent window 40 surrounds the 60 ter has a tubular configuration. 
While the preferred embodiment of the invention has 65 
5 
4,528,417 
6 
a S U D D ~ V  of atoms enclosed bv a transDarent housing 17. The combination as claimed in claim 16 including .. - I 
within said hollow tubular photovoltaic converter 
for producing line radiation in said ranges when 
excited, and 
a source of electron thermal energy within said trans- 
parent housing for exciting said atoms. 
16. The combination as claimed in claim 15 including 
a supply of cesium atoms within the transparent housing 
for producing the line radiation during discharge. 
a thermionic diode for maintaining a cesium discharge. 
18. The combination as claimed in claim 17 wherein 
said transparent housing is a closed tubular housing 
mounted within and spaced from the hollow tubular 
photovoltaic converter for containing the cesium atoms 
and the thermionic diode, said housing having high 
optical transmission in the predetermined wavelength 
range. * * * * *  
* 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 


Thermionic photovoltaic energy converter

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server