A solar oven heated by concentrated solar radiation as the heat source of a liquid metal magnetohydrodynamic (LMMHD) power generation system is proposed. The design allows the production of electric power in space, as well as on Earth, at high rates of efficiency. Two types of the solar oven suitable for the system are discussed

Hohl, F.

Lee, J. H.

English

NASA Technical Reports Server

NASA Technical Memorandum 81965 
SOLAR-DRIVEN LIQUID METAL 
MAGNETOHYDRODYNAMIC CENERATOK 
Ja H. Lee and Frank Hohl 
(HASA-Tfl-81965) S O L A B - D R I V E P  L I Q U I D  METAL 
MAGWETOiiPDBODPIAMTC G E N E L I A I O B  (NASA) 16 p 
dC A 0 2 / H E  A01 CSCL 201 
N81-27926 
Unclas  
G3/75 26694 
MAY 1981 
National Aeronautlcs and 
Space Adminlstratlon 
langley Research Cerrler 
Hampton. Virgrnla 23665 
https://ntrs.nasa.gov/search.jsp?R=19810019388 2020-03-21T12:54:22+00:00Z
T a b l e  o f  C o n t e n t s  
SU.ARY .................................................................. 1 
INTRODUCTION ............................................................. 2 
SPACE POWER SYSTEMS ...................................................... 3 
L IQUID METAL MHD GENERATORS .............................................. 4 
SOLAR-DRIVEN LMMHD GENERATOR ............................................. 5 
ADVANTAGES OF SOLAR-DRIVEN LMMHD GENERATOR .............................. - 7  
CONCLUSION ............................................................... 8 
REFERENCES ............................................................... 9 
FIGURES ................................................................. 10 
SOLAR-DRIVEN LIQUID METAL MAGNETOHYDRODYNAMIC GENERATOR 
J a  H .  Lee 
Vanderbi 1 t Univers i ty  
and 
Frank Hohl 
NASA Lana1 ey Research Center 
SUMMARY 
A solar oven heated by concentrated solar r a d i a t i o n  as the heat source 
o f  a l i q u i d  metal magnetohydrodynamic (LMMHD) power generation system i s  
proposed. The design a l l w s  the production o f  e l e c t r i c  power i n  space, as 
we l l  as on Earth, a t  high ra tes  o f  ef f ic iency.  Two types of so la r  ovens 
sui tab le  f o r  the system are discussed. 
I NTRODUCT ION 
The development of two-phase l i q u i d  meta l  (LM) W D  power generators 
has been underway t o  c a p i t a l i z e  on t h e i r  advantages over the plasma MHD 
systems which requi re extremely high temperatures I >  3000 K ) . ’  The LMWD 
can be operated a t  a temperature down t o  450 K, r e s u l t i n g  i n  s i g n i f i c a n t l y  
reduced requirements on materials. However, the d r i v i n g  heat sources con- 
sidered f o r  previous LMMHD were coal, nuclear, and other  conventional 
sources. For space applications, E l  1 i o t t  o f  the J e t  Propuls ion Laboratory 
studied a nuclear reactor-driven LMMHD ear ly  i n  the 1 9 6 0 ’ ~ . ~  
A new concept, so lar -dr iven L M D  generators, which u t i l i z e s  a 
;pecially designed so la r  oven as the heat source and which enables large- 
s a l e  power production i n  space a t  a h igh e f f i c iency ,  i s  proposed i n  t h i s  
report. This concept may be implemented f o r  near-term space app l ica t ions  
before the development o f  new mater ia ls  and unproven technology requ i red  
f o r  the plasma ;&ID systems. The concept may a lso  be appl ied t o  developing 
t e r r e s t r i a l  cent ra l  power s ta t ions  dr iven by a largc-  scale so la r  rece iver  
on Earth. Another app l i ca t ion  o f  the new concept i s  the conversion o f  
space power laser  energy i n t o  e l e c t r i c a l  energy for which the working 
f l u i d s  are heated by the  high power laser. 
2 
SPACE POWER SYSTEMS 
Prior methods of producing electric power from solar energy i n  space 
are classified as follows: (1) photovo1t;ic (solar ce l l s ) ;  (2) thermoelec- 
t r ic ;  and ( 3 )  piasma MHD generators. There also has been a proposal f o r  a 
l i q u i d  metal MHO generator driven by a nuclear reactor i n  space. 
The photovoltaic generation of electric power has been uti1 ited for many 
years w i t h  solar z ; ; s  on various spacecraft. The efficiency of the solar 
cells has been improved considerably due to intensive research efforts. 
However, the solar cells are diff icul t  to operate a t  temperatures greater 
than 500 K because of sharp decreases i n  efficiency and useful l i fe .  Con- 
sequently¶ solar cells require large-area panels for many practical appli- 
cations.  Thus, the u n i t  cost fo r  electric power from the solar cel ls  i s  
very high. For example, to produce 25 kilowatts of electric pawer from 
solar cells w i t h  10-percent efficiency the effective area of the solar 
cell panel iliist be approximately 180 rn’. In addition to  the high  cost of 
solar cells ,  the electrical c i rcui t  for power collection becomes a costly 
task for such a large b u t  diffuse power source i n  space. As the demand f o r  
higher power levels i n  space will undoubtedly continue i n  the future, i t  i s  
diff icul t  t o  expect that such a demand could be satisfactorily met w i t h  
solar cell s. 
Thermodynamic cycles of electric power generation from solar radiation 
i n  space have been extensively investigated since these cycles require 
minimal modifications of the we1 l-developed technology for conventional 
steam power p l a n t s  on Eartn. However, the efficiency (typically less than 
35 percent) of these methods is limited by the rather low temperature of 
steam generated w i t h  solar energy. 
On the other hand, plasma MHD generators operated a t  temperatures 
greater than  2000 K give a h i g h  efficiency for electric power production. 
However, continuous operation a t  such high tempzratures results i n  severe 
material problems yet t o  be solved. The maximum duty cycle tested for a 
high-temperature plasma MHD lasted only a few days w i t h  coal gas as the 
working fluid. No experimental work has been made on a solar plasma MHD 
generator to date, although i t  i s  under consideration by some 
i nves t i ga tor s . 
3 
LIQUID METAL MHD GENERATORS 
A l i q u i d  metal FHD generator was o r i g i n a l l y  proposed by D. E l l i o t t  
( J e t  Pro u l s ion  Laboratory) as p a r t  o f  a space power system using a nuclear 
reactor.' Two metals, cesium and l i t h i m ,  were considered as the working 
f l u i d s  of the generator. Figure 1 shows the system proposed by E l l i o t t  i n  
the 1960's. The cesium (Cs) leav ing the r a d i a t o r  as a condensate i s  punped 
through the regenerative heat exchanger t o  the nozzle where i t  vaporizes as 
i t  comes i n  contact  w i t h  the l i q u i d  metal l i t h i u m  ( L i )  from the l i q u i d  
loop. The cesiwn accelerates the l i t h i m  i n  the nozzle, thus impart ing an 
increased k i n e t i c  energy t o  the separator and then passes back t o  the 
radiator.  The l i t h i u n  leaves the separator a t  a r e l a t i v e l y  high v e l o c i t y  
( a  15C m/s) and f lows through the MHO generator. The cooled L i  i s  reheated 
i n  the heat source and pumped back t o  the nozzle. M. Pet r i ck  a t  the 
Argonne National Laboratory (ANL) l a t e r  proposed a new conversion cyc le  
tha t  would be compatible w i t h  the l i q u i d  metal f a s t  breeder reactor. '  The 
disadvantages o f  E l l i o t t ' s  cyc le  ( i .e.,  a f i x e d  and high operat ing tempera- 
ture range ( >  1700 K),  and the d i f f i c u l t y  i n  handling the l i q u i d  flow i n  
the lcMD channel) were a l l e v i a t e d  by the adoption o f  a two-phase generator 
cycle.' The basic idea was t o  u t i l i z e  the f a c t  t h a t  a two-phase mixture i s  
a compressible f l u i d  and thus i s  an e f f e c t i v e  thermodynamic working f l u i d  
tha t  could be expanded d i r e c t i y  through the MHD generator l i k e  a gas 
expanding through a turb ine from which e l e c t r i c  power i s  extracted 
( f i g .  2). The mixture, as 't leaves the generator, i s  f u r t h e r  expanded i n  
a nozzle t o  increase i t s  k i n e t i c  energy and i s  then sent t o  a separator. 
There the l i q u i d  metal i s  separated from the gas and i s  returned v i a  a d i f -  
fuser through the heat source t o  the mixer. The gaseous working f l u i d  i s  
then handled as i n  a normal Brayton cycle; i t  i s  passed through the regen- 
e ra t i ve  heat exchanger t o  the heat sink and i s  then compressed and sent 
back t o  the mixer v ia  the heat source. The gaseous component i s  tk  ther-  
moc!yr!amic working f l u i d ,  and the l i q u i d  metal, (which remains i n  a closed 
loop) i s  the electrodynamic working f l u i d .  A t  ANL, a Nak-N LMMHD gene- 
r a t o r  has been tested and an e f f i c i ency  of greater than P 0 percent a t  
1500 K was estimated. However, the heat sources considered by ANL 
researchers were f o s s i l  combustion, high-temperature gas-cooled nuclear 
reactors (HTGCNR), fusion reactors, and 1 i q u i d  metal f a s t  breeder reac- 
tors. Solar heat sources were no t  considered. U t i l i z a t i o n  o f  the so la r  
rad ia t i on  as the heat source of LMMHD f o r  space power production has been 
proposed for  the f i r s t  time i n  1978, and a U.S. patent i s  n w  pending on 
t h i s  invention. Subsequently, the t e r r e s t r i a l  appl icat ions o f  the so la r  
LMMD generator have been studied by Pierson, e t  al. a t  the Argonne 
National Laboratory. 
4 
SOLAR-DRIVEN LMMHD GENERATOR 
The solar-dr iven LMMHD system3 consists o f  the fo l l ow ing  subsystems: 
(a) a large solar co l l ec to r ;  (b) an oven heated by so lar  energy; ( c )  a 
mixer f o r  mixing the gas and l i q u i d  metal; (d)  a MHD generator i nc lud ing  a 
magnet, inverters,  and a power transmission c i r c u i t ;  (e)  a gas / l i qu id  metal 
separator; ( f )  a punp f o r  recyc l i ng  the l i q u i d  metal and a compressor f o r  
the working gas; and (g) 3 space r a d i a t o r  f o r  cool ing the gas. 
The operation o f  the system i s  as fo l lows (see f i g u r e  3 ) .  The sun- 
l i g h t  photons (1) are co l l ec ted  by the large-area so la r  c o l l e c t o r  (2)  and 
are re f l ec ted  t o  the focusing m i r ro r  ( 3 )  i n  f ron t .  The r e f l e c t e d  photons 
are t ransmit ted through the transparent windaw ( 4 )  o f  the so la r  oven (5). 
The window i s  constant ly cooled and maintained clean by the r a d i a l  f l o w  o f  
the working gas (6).  The working gas consists o f  one o f  the noble gases 
such as hel iun or  argon. The working gas molecules p a r t i a l l y  absorb the 
photons whi le  they f l o w  through the conical  volume o f  the so la r  oven ( 5 )  i n  
which heating and compression o f  the gas takes place. The ,311 o f  the oven 
i s  made o f  r ings o f  r e f l e c t i v e  mater ia l  which can withstand a temperature 
greater than 1000 K. The l i q u i d  metal i n j e c t e d  throlrgh nozzles (7)  i s  a l so  
heated i n  the so lar  oven. The l i q u i d  metal and the d r i v i n g  gas are mixed 
i q  the mixer (8) where the so lar  energy i s  focused t o  the minimun area. 
Thus the two-phase working f l u i d  i s  heated t o  the maximum temperature. 
The two-phase f l u i d ,  mixed by the mixer a t  the entrance, f lows through 
the MhD channel (9) o f  the generator. The MHD channel i s  surrounded by the 
magnet (10) ( n o t  shown), and the power condi t ion ing u n i t  (11) ( n o t  shown). 
The MHD channel acts as a turb ine and e l e c t r i c  generator i n  one un i t ;  the 
gas dr ives the l i q u i d  across the magnetic f i e l d ,  and thus generates elec- 
t r i c a l  power. Since the l i q u i d  has a high heat capacity, the expansion 
occurs a t  almost constant temperature and provides avai lab le energy i n  the 
gas exhausting the MHD generator. The l i q u i d  acts as a l a rge  heat source 
f o r  the gas, and thermal energy i s  continuously exchanged frcm the l i q u i d  
t o  the gas and most o f  the enthalpy change i n  the generator depends on the 
l i q u i d .  The gas i s  separated from the mixture i n  the separator /d i f fuser  
(12)  and i s  recouped i n  the regenerative heat exchanger (13). The gas i s  
then returned t o  the so lar  oven (5) by way o f  the rad ia to r  (14) and c o w  
pressor (15) .  The l i q u i d  metal i s  returned t o  the mixer by the pump 16. 
Items (171, (181, (19), (201, (211, and (22) are thermally insu lated pipes 
for  re turn ing the working f l u i d s  as shown. I tem (23) i s  the valve attached 
t o  the reserve gas tank (24) t o  be used f o r  the i n i t i a l  s t a r t - u p  o f  the 
system. This completes the cycle and the gas and l i q u i d  metal are recycled 
i n  the system. 
An a l t e r n a t i v e  t o  the d i r e c t l y  heating so la r  oven i s  a heat exchanger 
s i m i l a r  t o  t h a t  used f o r  a coa l - f i r ed  LMMHD generator. The so la r  r a d i a t i o n  
i s  d i rected t o  any absorbing mater ia l  such as graphi te i n  which a heat 
exchanger i s  enclosed, o r  t o  an ensemble o f  heat-exchanging pipes w i t h  
m u l t i p l e  f i n s  made of absorbing material.  The thermal proper t ies o f  the 
mater ia l  used for the heat exchanger w i l l  d i c t a t e  the maximun temperature 
o f  the working f l u i d s  ( f i g .  4). 
5 
An a l t e r n a t i v e  cyc le  t h a t  may be considered i s  a Rankine cyc le  LMMHD 
generator. I n  t h i s  cycle, two types o f  l i q u i d  metal are used as the work- 
i n g  f l u ids .  The l i q u i d  metal w i t h  the l w e r  b o i l i n g  p o i n t  i s  vaporized i n  
the solar oven and i s  used as the dynailiic f l u i d  which i s  condensed i n  the 
rad ia to r  and i s  returned t o  the l i q u i d  phase before i t  i s  pumped back t o  
the solar oven through the regenerative heat exchan ler. The other l i q u i d  
metal w i t h  the higher b o i l i n g  p o i n t  remains i n  the i q u i d  phase throughout 
the cycle and acts as the electroconductive f l u i d  i n  the MHD generator. 
The systems described above can eas i l y  be modif 
The so lar  c o l l e c t o r  may be i n s t a l l e d  on Earth.2 
t rack the Sun continuously. 
ed f o r  appl icat ions on 
a gimbal mechanism t o  
Al ternat ive ly ,  a so lar  tower w i t h  a large number o f  m i r ro rs  on the 
ground may be used t o  provide energy t o  the so lar  oven. Numerous a l te rna te  
so la r  c o l l e c t o r s  w i t h  high concentrat ion r a t i o s  now under development could 
also be u t i l i z e d .  
The concept o f  the solar-dr iven LMMHD can be e a s i l y  extended t o  o the r  
sources o f  photons such as high power masers and lasers. The so la r  oven o r  
the so lar  heat exchanger can be used w i t h  l i t t l e  modif icat ion.  This a l t e r -  
nat ive may become important when high-power mas,?rs o r  l ase rs  are developed 
f o r  long distance power transmission. 
6 
ADVANTAGES OF SOLAR-DRIVEN LMWD GENERATOR 
The so lar -dr iven LMMtiD generator has important advantages as fo l lows: 
1. Abundant so la r  energy ava i lab le  i n  space i s  u t i l i z e d  instead o f  a 
nuclear reactor as the heat ing source f o r  a l i q u i d  metal MHO generator. 
2. As a r e s u l t  o f  the above, s i g n i f i c a n t  c a p i t a l  and payload savings 
are expectee f o r  the so la r  LMMHD system since the l i g h t e r  so la r  c o l l e c t o r  
rep1 aces the bulky nuclear reactor. 
A higher e f f i c i ency  r e s u l t s  from the so lar  LMMHD system compared 
w i t h  so la r  c e l l s  and thennoelectr ic power generators i n  space. An e f f i -  
ciency o f  up t o  55 percent could be achieved by t h i s  system. 
3 .  
4. Near-term app l ica t ion  can be expected f o r  the so la r  LMMHD whi lc  
the plaaia MHD generators requi re breakthroughs i n  mater ia ls  and component 
development. This i s  because the so la r  LMMHD system i s  operated a t  much 
lower temperatures (1000 K )  than the  plasma MHD a t  > 2000 K, thus a l l e v i a t -  
i n g  the mater ia ls  problems. The so la r  LMMHD system can be constructed w i t h  
mater ia ls  f o r  which thermal cha rac te r i s t i cs  are already we1 1 known. 
7 
CONCLUSIONS 
The solar-dr iven LMMHD generator which u t i 1  i r e s  abundant so la r  energy 
co l lec ted  w i th  a 1 ightweight co l l ec to r  i n  space and e f f i c i e n t l y  generates 
high e l e c t r i c a l  power i s  proposed. Two types o f  so lar  ovens are considered 
to  create a two-phase f l a w  a t  a high speed i n  the LMMHD channel. The con- 
cept can read i l y  extend t o  t e r r e s t r i a l  appl icat ions and t o  a l ase r  energy 
converter. 
Langley Research Center 
National Aeronautics and Space Administrat ion 
Hampton, V i rg in ia  23665 
May 29, 1981 
REFERENCES 
1. Petr ick ,  M.; and Lee, Y. K.: Performance Character is t ics  o f  a L i q u i d  
Metal MHD Generator. Proceedings o f  the Symposium on MHO E l e c t r i c  
Power Generation, Paris, France, 1964. 
2. E l l i o t t ,  D. 6.: Performance Character is t ics  o f  Liquid-Metal MHD 
Generators. SM 107/41, I A E A ,  Vienna, J u l y  1968. 
3 .  Disclosure o f  invent ion "Solar-Driven L iqu id  Metal MHD Power Gene- 
ra tor . "  NASA Case No. LAR-12495, October 19, 1978. 
4.  Pierson, E. S., e t  al., Mechanical Engineering, - 102, 32 (1980). 
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__I- - -__ 
1 Hewet hu 2 Government Accession No 3 Recipient's Catalog No 
NASA TK-81965 I -- 
4 1,Itr dlld Subtltlr 
S O L A R - D R I  VEN LIQUID METAL MAGNETOHYDRODYNAMIC 
GENE RATOR 
d-thdris) 
5 Repon Oate 
May 1981 
6. Perforrnlng Organization Code 
5O6-55-13-03 
8 Prrtorminq Orgaruzdtioc Reprxr No 
NASA Langley Research Center 
Hamptc,), V i rg in ia  23665 
- 
7 Key  Words iSu ted by Author($)) 
Ma g ne t o h y r o d y  n ami  cs 
L iqu id  Metal 
Energy Conversion 
E 1 ec t r i  c Power P roduc ti on 
1 1  Contract or Grant No - 
18 Distribution Statement 
Unclassi f ied - Unl imited 
Subject Category - 75 
4 13 Type  of Repon and Perad G v e r e d  - 
9 Security Classif (of  this report] 
Uncl a s s  i f i ed 
14 Sponsoring Agency Code 
! 2  Siunroririy A y m c y  Name Jnd Address 
Ni.tional Aeronautics and Space Administrat ion 
Washington, DC 20546 
20 Security Classif (of this pagel 21. No. of Pager 22. Rice' 
Uncl ass i f i ed 15 A0 2 
-~ 
' 6  Aortrdct 
A so la r  oven heated by concentrated s o l a r  r a d i a t i o n  as the heat source 
o f  a 1 i q u i d  metal magnetohydrodynamic (LMMHD) power generation system i s  
proposed. The design al lows the production o f  e lec t i - i c  power i n  space, as 
well  as on Earth, a t  high ra tes  of e f f i c i ency .  Two types o f  the so la r  oven 
su i tab l?  f o r  the system are discus3ed. 


Solar-driven liquid metal magnetohydrodynamic generator

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