Electric power system utilizing thermionic plasma diodes in parallel and heat pipes as cathode

Byrd, A. W.

English

NASA Technical Reports Server

HEAT PIPE THERMIONIC DIODE POWER SYSTEM 
Filed Sept . '67 2 Sheets-Sheet 1 
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https://ntrs.nasa.gov/search.jsp?R=19710001580 2020-03-17T02:33:50+00:00Z
A. W. BYRD 
HEAT PIPE THERMIONIC DIODE POWER SYSTEM 
Filed Sept. 6, 1967 2 Sheets-Sheet a 
INVENTOR.(SI 
AMBROSE W. BYRD 
BY 
from a high temperature emitting material, such as tung- 
sten or molybdenum. Such a thermionic cathode might 
then be utilized by surrounding it with an anode on the 
radiating end, to form a diode. 
when used with radioactive isotopes, particularly alpha 
emitters in the transuranium group, because of the low 
specific power of the isotopes (5 watt per gram). The 
power density of the isotopes is too low to heat an 
optimum size cathode (1 inch diameter x 2 inches long) 
to a temperature level high enough for electron emission. 
However, if a large amount of the isotope is encapsulated, 
ma diodes in parallel. The diodes each incorporate a the heat generated will be great enough to operate the 
cathode which is integral to a heat pipe and is heated by heat pipe thermionic diodes. In this case, however, suf- 
an isotopic heat source through the highly efficient heat 15 ficient insulation, such as pyrolytic graphite, must be 
transfer function of the heat pipe. The system employs a used to direct the isotopic heat to the heat pipes. The 
circulatory cooling system utilizing a liquid as a heated end of the cathode could be inserted into the 
coolant of a nuclear reactor or any other heat source coolant. 
and properly do its job. 
Past isotopic thermionic diodes have been constructed 
3,509,386 
~~~O~~~ 
Hnnisville, Ah., assignor to the United 
States of America as represented by the AdmirPistraPoa 
of the National AerQnantics and Space A d m ~ & & i @ n  5 The proposed heat pipe cathode be advantageous 
U.8. CII. 310-4 2 Claims 
A power system utilizing a number of thermionic 
20 BACKGROUND OF THE 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 royal- 
ties thereon or therefor. 
Field of the invention 
The invention relates to an electric power system and 
more particularly to an electric power system utilizing 
thermionic diodes which are heated by heat pipes. 
Description of the prior art 
A heat pipe is a device normally used to transport 
heat from its source and transfer the heat to a specific 
environment. It is composed of a pipe sealed at each 
end, enclosing a wick and a heat transfer fluid. The 
wick lies along the inner periphery of the wall and 
extends the length of the pipe. At one end of the device 
heat is absorbed by vaporization of a heat transfer fluid. 
The vapor then expands and permeates the pipe. Since 
a portion of the pipe is being used as a radiating fin, the 
vapor condenses along the length of the tube, giving up 
its latent heat of vaporization in the process. The con- 
densate is returned to the vaporizer by means of capil- 
lary flow through the annular wick. 
The thermionic emission of electrons from a heated 
metal is a well known physical phenomenon. This phe- 
nomenon is made use of in ail electron tubes for radio, 
television, amplifiers, etc. The most recent application 
has been in the direct conversion of heat energy to elec- 
trical energy and is most promising for space power use 
since it eliminates the need for rotating machinery in the 
conversion process. 
How the cathode of a thermionic diode is heated 
makes no difference. Any method of heating is adequate 
as long as the temperatures are high enough for electrons 
to be driven from the emitter surface. Use is made of 
the cathode (emitter) by surrounding it with an electron 
collector, hereafter called an anode, which must operate 
at a lower temperature than the cathode. If an external 
electric load is connected between the anode and the 
cathode a current will be made to flow. The cathode 
and anode must be physically and electrically separated. 
Physical separation is accomplished by spacing the ele- 
ments by a small gap on the order of .002 inch to .015 
inch, while electrical separation is accomplished through 
ths use of electrical insulation. 
In the development work leading to the present inven- 
tion, the inventor believed that the heat pipe could be 
made into a thermionic cathode (emitter). In order to 
do so, it would be necessary to construct the heat pipe 
with the isotope in the form of a cylinder with one end 
and the periphery insulated in order to direct the heat 
flow to the uninsulated end which formed the emitting 
surface. This was necessary in order to raise the tem- 
25 perature at this end due to the low specific power (watts 
per gram) of the isotope. In the proposed combination of 
a heat pipe with a thermionic cathode, however, such 
a configuration would be unnecessary in order to utilize 
an isotopic heat source to heat a cathode. 
SUMMARY OF THE INVENTION 30 
Accordingly, it is an object of this invention to utilize 
a heat pipe to heat the cathode of a thermionic diode. 
A further object of the invention is to construct a heat 
35 pipe. made of a high temperature emitting material so that 
the heat pipe itself may be utilized as the cathode of a 
thermionic diode. 
Yet another object of this invention is to develop a 
power system utilizing thermionic diodes having heat 
40 pipes as cathodes and utilizing a low power density heat 
source. 
These and other objects we accomplished in the pres- 
ent invention which provides an electrical power system 
utilizing a plurality of thermionic diodes each of which is 
45 heated by a heat pipe. The system includes a heat 
source which is enclosed in a container. The diodes are 
disposed on the outer surface of the heat source con- 
tainer. Both the heat source container and the diodes are 
enclosed in a pressure vessel which is connected to a 
BRIEF DESCRIPTION OF THE DRAWTNGS 
The invention will be more fully understood by the 
following detailed description when taken together with 
55 the accompanying dravings in which: 
FIGURE 1 is a perspective view of the power system 
with portions of the pressure vessel and the heat source 
container cut away. 
FIGURE 2 is a longitudinal sectional view of the heat 
FIGURE 3 is an end sectional view of the heat pipe 
50 cooling system. 
60 pipe diode. 
diode. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
With continued reference to the accompanying figures 
wherein like numerals designate similar parts throughout 
the various views and with initial attention directed to 
FIGURE 1, there is illustrated a typical embodiment of 
70 the heat pipe thermionic diode power system designated 
generally by the numeral 10. A heat source 12 which is 
preferably composed of radioactive isotopes is enclosed 
65 
3,509,386 
3 4 
in a container 414. The heated ends of the heat pipe heat pipe (500 times greater than copper) and the lower 
plasma diodes 16 are embedded in the heat source con- weight of the heat pipe. Under actual test conditions, 
tainer 14. The heat source 42, container 14, and heat heat pipes have been operated with the heated end eleven 
pipe diodes 16 are surrounded by a pressure vessel 18 degrees higher than the condensing end, pumping against 
for total containment. The heat pipe diodes I6 are con- the normal gravity field. Therefore, heat pipes will pump 
nected in parallel so that the total power output of the in a zero-G condition. 
power system PO may be connected to an external load As far as has been determined experimentally to date 
(not shown). the heat transfer process is nearly isothermal with meas- 
A circulatory cooling system is used to cool the heat ured temperature differences about .04 degree K per 
source 12 and the heat pipe diodes 16. Liquid metal centimeter along the heat pipe length. 
coolant 20 is circulated by electro-magnetic pump 22 As mentioned previously, a principal application of the 
through the pressure vessel 18, out of the top 26 of the heat pipe thermionic diode lies in its being used with an 
pressure vessel 48, through output cooling line 28, through isotope heat source, such as Pu23*, Cm242, Cm244, and 
radiator 30, through input cooling line 32, and back into Poz1*, each of which is an alpha emitter, to produce elec- 
the bottom 24 of the pressure vessel 18. 15 trical power for deep space voyages. 
Referring again to FIGURE 1, one cycle of op- Lithium has been chosen for the coolant 20, as well 
eration of the heat pipe thermionic diode power as the heat pipe working fluid 40, because of its low 
system 18 is as follows: heat flows from radioactive heat density, low vapor pressure at elevated temperatures, 
source 12 to cause current to flow in thermionic heat pipe high specific heat, and the attendant low pumping power 
diodes 16 which are embedded in the heat container 14. 20 requirements. The operating temperatures are: 1800" C. 
Output current flows to output terminals 34 for connec- for the emitter and 800" C.  for the anode. These tem- 
tion to an external load (not shown). Electro-magnetic perztures give a net thermal efficiency of approximately 
pump 22 cools the heat source 12 and heat pipe diodes 10 percent. In order to reach this efficiency, Cesium 
16 by pumping liquid metal coolant 20 through pressure gas at 300"-400" C. will be introduced from a common 
vessel 18 and in sequence through output cooling line 25 heated reservoir (not shown) through piping into the in- 
28, radiator 30, cooling line 32 and back to pressure terelectrode space 54 to reduce the space charge. 
vessel 18. If the power system described above is used in space 
Looking now to FIGURE 2 of the drawings, the struc- power applications, helium gas formed by alpha particle 
tural details of the heat pipe plasma diodes 16 may be decay during the mission will be vented to space through 
seen. The heat pipe 36 is a pipe which i s  sealed at each 30 a known type of pressure relief valve with a micronic 
end and encloses a wick 38 and heat transfer fluid 40. filter (not shown). 
At the heated end 42 of the device, heat is absorbed by From the foregoing, it may be seen that applicant has 
vaporization of the heat transfer fluid 40. The vapor 44 invented a novel electric power system utilizing a spe- 
forms from fluid 40, then expands, and permeates the cifically designed heat pipe which may be used as a 
heat pipe 36. Since a portion of the heat pipe 36 is 35 thermionic emitter. Because of the extremely high heat 
being used as a radiating fin, the vapor 44 condenses transfer efficiency of the heat pipe diodes the system 
along the length of the heat pipe 36, giving up its latent makes possible an efficient electric power system utiliz- 
heat of vaporization and condensing to fluid 40 in the ing low specific power isotopes as a heat source for its 
condenser section 46 of the heat pipe 36. The condensed thermionic diodes. The system described is a low voltage, 
fluid 40 is returned to the vaporizer section 48 in the 40 high current power system. This type of power output 
heated end 42 of the heat pipe 36 by means of capillary is dictated by the fact that a number of diodes are 
flow through the annular wick 38. The heat pipe 36 is embedded in a common material containing the heat 
utilized as a thermionic cathode by constructing it from source, thereby precluding series connection and forcing 
a high temperature emitting material such as tungsten or Parallel connection of the diodes for power use. There- 
molybdenum. Then, when heat is furnished to the heat 45 fore, this power system may 'be used in space power 
pipe 36 at its heated end 42 the opposite end or con- applications to drive a magneto-hydynamic thruster, 
denser section 46 emits electrons. An anode 50 is posi- which requires a high current source. However, the 
tioned adjacent to the heat pipe condenser section (cath- Power system may also be used with power conditioning 
ode) 46. Preferably, the anode 58 is a cylindrical cap in other applications, if desired. 
which fits over the end of the heat pipe 36. The anode 50 50 Obviously, numerous modifications and variations of 
i s  insulated from the heat pipe condenser section (cath- the Present invention are possible in the light of the 
ode) 46 by annular insulating rings 52 made of a suit- above teachings. It  is therefore to be understood that, 
able insulating material such as, for example, alumina. within the scope of the attendant claims, the invention 
The sectional view of the heat pipe diodes shown in may be practiced otherwise than is specifically described. 
FIGURE 3 illustrates how the anode 50 encloses the heat 55 What is claimed is: 
pipe 36 with an interelectrode space 54. Inside the heat 1. A device for generating electrical energy compris- 
pipe 36 is the wick 38 and the open center area 56 of the ing: 
condenser section 44 (FIGURE 2). (a) a pressure vessel; 
One cycle of operation of the heat pipe thermionic (b) an isotopic heat source; 
diode follows: heat is absorbed at heated end 42 of heat 60 (C) a container for said heat source, said container 
pipe 36. Heat transfer fluid 40 absorbs heat and is being positioned within said pressure vessel; 
and moves under vapor pressure to (d) a plurality of plasma diodes disposed on the 
of the heat pipe 36 where it con- outer surface of said heat source container, each 
denses back to fluid 40. In condensing, vapor 44 gives said diode comprising: 
up heat which is absorbed by condenser section 46. The 65 (1 1 a heat pipe, one end of said heat pipe being 
heating of condenser section 46 causes it to emit elec- embedded in the surface of said heat source 
trons, which flow to anode 58. The cathode 46 and anode cmtainer, and the opposite end extending di- 
external load 58 in series with meter rectly away from said heat source container, 
condenses back to fluid 40, the fluid said heat pipe being capable of acting as a 
(condensate) 40 returns to the vaporizer section 48 by 70 cathode when heated; 
means of capillary flow through the annular wick 38, (2) an anode positioned adjacent to said heat 
thus completing the heat transfer cycle. pipe at said opposite end of said heat pipe, 
The heat pipe thermionic diodes described above said anode comprising a cylinder, having one 
weigh less per electrical watt than other diodes presently closed end, which fits over the said opposite 
in use because of the high thermal conductivity of the 75 end of the said heat pipe; 
3,509,386 
(3) at least one electrical insulator positioned 
between said heat pipe and said anode, for 
providing an interelectrode space; 
(4) a gas positioned in the interelectrode space 
between said heat pipe and said anode, for 
reducing the effect of space charge; 
(e) a layer of heat insulation, disposed on the outer 
surface of said heat source container, lbeeween said 
diodes, for directing the isotopic heat to said diodes; 
(f) a cooling system connected to said pressure ves- 
sel, for cooling said diodes, said cooling system 
comprising: 
(1) an input pipe connected to one side of said 
(2) an output pipe connected to an opposite side 15 
(3) a pump connected in series in said system; 
(4) a radiator connected to said input and out- 
(5) a liquid coolant flowing through said other 20 
pressure vessel; 
of said pressure vessel; 
put pipes; 
elements of said cooling system and also flow; 
ing through the pressure vessel around said 
diodes to complete its cycle and cool the said 
diodes. 
2. The device for generating electrical energy of claim 25 
(a) a cylindrical section of pipe having two closed 
ends; 
(b) a fluid contained inside said pipe, for removing 
heat from the pipe at its end embedded in said heat 40 
source container, through vaporization of said fluid, 
1 wherein each said heat pipe comprises: 
6 
and discharging heat to the pipe at its said opposite 
end, through condensation of said fluid; 
(c) a wick positioned along the inner surface of said 
pipe for returning said fluid from said opposite end 
to s a 2  embedded end; 
(d) the heat discharged to the pipe at the said o p  
posite end being effective to cause the said opposite 
end of the heat pipe to act as a cathode. 
3,054,914 
3,093,567 
3,113,091 
3,201,619 
3,234,412 
3,243,613 
3,252,O 15 
3,279,028 
3,3 02,042 
3,368,084 
3,378,449 
3,229,759 
3,426,220 
References Cite 
UNITED STATES PATENTS 
9/1962 Hatsopoulos et al. _ _ _ _ _  3 1 0 - 4  
6/1963 Jablonski et al. _ _ _ _ _ _  3 1 0 4  X 
12/1963 Rasor et al. _ _ _ _ _ _ _ _ _ _  31- 
8/1965 Gleason et a1 _ _ _ _ _ _ _ _ _  3 1 0 4  
2/1966 Sankovich et al. _ _ _ _ _ _  3 1 0 - 4  
3/1966 Grover _ _ _ _ _ _ _ _ _ _ _ _ _ _  31- 
5/1966 Johnson _ _ _ _ _ _ _ _ _ _ _ _ _  3 1 0 - 4  
10/1966 Hall et al. _ _ _ _ _ _ _ _ _ _  29-25.13 
1/1967 Grover et al. _ _ _ _ _ _ _ _ _ _  3 1 0 4  
2/1968 Hall _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  31- 
4/1968 Roberts et a1. _ _ _ _  165-1050 X 
1/1966 Grover _ _ _ _ _ _ _ _ _ _ _ _ _  165-105 
2/1969 Black et al. _ _ _ _ _ _ _ _ _ _  3 1 0 - 4  
MILTON 0. HIRSHFJELD, Primary Examiner 
D. F. DUGGAN, Assistant Examiner 
U.S. C1. X.R. 
165-1 05 


Heat pipe thermionic diode power system Patent

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