Digital computer program for minimizing weight of fuel cell power systems - prelaunch startup and instrumentation - mathematical model of static moisture removal process - fuel cell mode

Desai, R.

Monette, G.

Williams, C.

English

NASA Technical Reports Server

. 
NAS 8-5392-QPR.004 
c" 
FOURTH QUARTERLY PROGRESS 
REPORT FOR PERIOD ENDING 
AUGUST 15, 1965 
.-" ~ _ _ - -  - 
RESEARCH & DEVELOPMENT 
O F  OPEN CYCLE 
FUEL CELLS 
I C q t r a c t  No. NAS 8-5392 I 
Prepared for: 
National Aeronautics and Space Administration 
I 
GPO PRICE $ George C. Marshall Space Flight Center 
CFSTI PRICE(S) $ 
Hard copy (HC) 
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ff 653 July 65 
Huntsville, Alabama 35812 
DDC-IRA E 
RESEARCH DIVISION 
ALLIS-CHALMERS 
MI L W A U K E E, W I SCO N S I N 5320 1 
SEPTEMBER 15, 1965 
https://ntrs.nasa.gov/search.jsp?R=19660001312 2020-03-16T22:01:05+00:00Z
unx** 
NAS 8-5392-QPR-004 
Fourth Quarterly Progress Report 
4 2  Contract NAS 8-5392 
Ending August 15, 1969 
PREPARED FOR 
NATIONAL A E m A J J m  AND S PAC E ADMIMS T RATION 
George C. Marshall Space Flight Center 
Huntsville, Alabama 
._ - _-_ - *---- - 
PREPARED B Y  
R. Desai, G. Msnette, C. Williams 
Research Division, Department 3341 
Allis-Chalmers Manufacturine ComDanv 
Milwaukee 
A p p r o v e > w : w  
J. R. HurPey, Manag 
Systems Research and 
Approved: 
3: L. h a t n e r ,  Program Manager 
T A B L E  OF C O N T E N T S  
Page 
INT RODU C T ION . * .  i 
SUMMARY e e . . . . . . . . . . . e . . . . . . . . . . iii 
1.0 COMPUTER PROGRAM FOR FUEL CELL PARArVIETER 
OPTIMIZATION e e . . . e e . . . . . . 1 
1. 1 Effects of Adding a Water Recovery Subsystem . e . . . 1 
1.2 Discussion e . e e . . . e . . . , . . . 2 
2,O FUEL CELL STARTUP TECHNIQUES AND 
PRELAUNCH INSTRUMENTATION . . . . . . . 3 
2 .  1 System Evaluation Criteria.  . . . . . a . . . . . e 3 
2.2 Instrumentation . . . . e . . a e . . . . 3 
2 , 3  System Startup 0 D D P 0 0 0 0 0 . * 0 . 0 0 . . 0 . * 4 
2 .4  Sea Level Operation ., e . . . e . e . . 5 
3.0 MAT'PXEMATECAE MODEL OF STATIC ItfOISTUfPE 
REMOVAL PROCESS . e . . . - . . . 7 
3,  1 Discussion . . e . e . e . . . . . e 7 
4.0 RELIABILITY MODEL OF FUEL CELL SYSTEMS ., e e 8 
4. 1 A Study to Compare the Performance of Various Systems . 8 
I N T R O D U C T I O N  
This report is the Fourth Quarterly Progress Report issued under 
Modification Number 4 to Contract NAS 8-5392. Progress during 
the period of May 16, 1965 through August 15, 1965 is covered 
he rein. 
A B S T R A C T  
Modification and extension of the existing digital computer program for 
minimizing the weight of fuel cell power systems has been completed and 
design charts a re  presented. The study of prelaunch startup and instru- 
mentation has been completed. 
representation of the static vapor pressure method of water removal has 
The computer program for the mathematical 
been debugged. 
se rie s parallel 
to compare the 
The results of a study to compare the reliabilities of various 
An analysis fuel cell configurations a re  given in chart form. 
performance of series versus parallel systems i s  presented. 
S U M M A R Y  
The digital computer program for  fuel cell system parameter optimization 
has been modified and extended to include the use of batteries for supplying 
a portion of peak power. 
radiator as functions of the system operating parameters was modified. 
effects of including a water recovery subsystem are  now considered by the 
expanded program. 
The subroutine for computing the properties of a 
The 
The criteria for evaluating fuel cells at the launch area  and the required 
instrumentation a re  listed. The methods of rapid startup and prelaunch 
operation a re  presented. 
The results of a computer runmade to simulate the transient operation 
of fuel cells at 200 ASF are presented. 
The study to compare the reliabilities of various series-parallel fuel cell 
configurations has been completed. 
producing 2 KW are  presented. 
performance of series versus parallel systems a re  also described. 
Rtlia5ilities of various c~&igurations 
The results of an analysis to compare the 
-iii- 
1.0 COMPUTER PROGRAM FOR FUEL CELL PARAMETER OPTIMIZATION 
The existing computer program (IBM 704) for  fuel cell parameter optimi- 
zation has been modified and extended to include the use of batteries for 
supplying a portion of peals power. Modification of the subroutine which 
computes the properties of the radiator as functions of the system operating 
parameters has been completed. 
considered by the expanded program. 
The effects of collecting water.'are also 
A complete description of the program has been prepared for publication, 
1. 1 Effects of Adding a Water Recovery Subsystem 
The effects of adding a water recovery device to the present fuel cell 
system are  as follows: 
(1) The amount of heat to be dissipated will  become greater and 
hence radiator weight wil l  increase, 
( 2 )  The weight of a water recovery device itself wil l  have t~ 
be considered. 
\' 
\\Since general information regarding the second effect is not available, 
. detailed consideration has been given only to the first effect. 
of the callection device itself can be included with the auxiliaries to the 
fuel cell. 
The weight ' Y  .\ 
'3, is the control character in the program to determine the weight of a 
radiator. If Qt, is set equal to  1100 Btu/hour, the program wil l  calculate 
the optimum system weight when no water is collected. If % is zero, 
system weight wi l l  be calculated when the entire amount of generated water  
-1- 
I 
is collected. If Q is w ( 1  loo), then weight wi l l  be calculated for a system 
collecting (1-w)100 percent of generated water, where w is a fraction, so 
that 0 < w < 1 . 
1.2 Discussion 
This program calculates the minimum weight of alkaline hydrogen-oxygen fuel 
cell systems supplying power within set voltage limits. 
mating the minimum system weight for various power levels a re  presented in 
Figures 1 and 2. The weights are  determined on the basis of present Allis-  
Chalmers capabilities (January 1964 technology), 
storage, a radiator with a 0.999 probability of survival, and a voltage regu- 
lation requirement of 2 9 t  2 volts. 
Nomograms for approxi- 
a supercritical reactant 
Minimum system weight can be further reduced by using either primary o r  
rechargeable batteries to supply a portion of peak power. The reduction in 
weight can be fifty percent o r  more if the peak power is four to five times 
the base load. 
system if the peaks occur repeatedly. 
Rechargeable batteries wi l l  provide the minimum weight 
-2- 
2.0 FUEL CELL STARTUP TECHNIQUES AND REQUIRED PRELAUNCH 
INSTRUMENTATION 
The specific cri teria for evaluating a fuel cell system at the launch area 
haw been developed, and the required instrumentation for this purpose is 
listed. Also considered a re  problems associated with the rapid startup 
and ground (or sea level) operation of a system. 
2. 1 Svstem Evaluation Criteria 
The criteria for evaluating a fuel cell system are: 
( 1 )  Leakage Rates - There should be negligible leakage of any 
reactant or  coolant. 
(2) Control System Operation - The control system of a typical fuel 
cell system consists of sensors, valves, and a master contrbl. 
Each part of the system should operate within specified limits, 
(3 )  Stack Operation - The fuel cell stack should produce required 
power within specified voltage limits. 
2. 2 Instrumentation 
A list of measuring devices needed to establish the cri teria developed in  the 
previous paragraph is given below. 
To measure leakage from each subsystem and leakage between subsystems, 
the following devices wil l  be needed: 
(a) Pressure Measuring Device 
-3 -  
(c)  A source of inert gas (e.&. Helium) 
Since the volume of each portion of the system is known, leakage rates 
could be inferred from pressure decline with time for each closed off 
portion of the system. 
The sensors can be checked by testing their output for a known condition. 
For  example, the temperature measuring devices can be checked if the 
module is known to be at ambient temperature. 
be checked by supplying a set of input signals simulating the output of the 
sensing transducers and observing the command signals which are  sent to 
the valves. 
their reaction to these same signals. 
The master control can 
The valves can be tested by observing, via supervieory contacta, 
For checking the satisfactory operation of the stack, voltage and current 
measuring devices w i l l  be needed. Several voltage measurements a t  pre- 
determined currents would establish the performance against the known 
V-A characteristics of a normally functioning system. 
2. 3 System Startup 
The heat required to bring a fuel cell system up ta its normal operating tem- 
perature can be supplied by any of the following sources. Approximately 20 
Btu per O F is required to  heat the typical 2 KW system. 
(a) Electrical energy: 
(1) Applied within the coolant duct by resistance heaters 
-4- 
L b 
( 2 )  Applied to the outer w a l l  of the canister by electrical 
resistance heaters 
(b) The heat of chemical reaction inside the fuel cell, i. e., to operate 
the fuel cell and not allow the excess heat to  escape. 
data indicates that typical Allls -Chalmers cells can sustain current 
densities of 30 ma/ cm 
Preliminary 
2 at temperatures down to 0" F. 
Of these sources, electrical energy applied to the coolant duct has-been 
chosen for the present Allis-Chalmers cexterline module. This heating 
method tends to induce a temperature gradient within the fuel cell stack, 
which gives rise to a KOH concentration gradient. 
to study this problem indicated that the predicted gradients fall within the 
acceptable operating limits of the stack. 
An analysis performed 
2.4 Sea Level Operation 
Sea level operation of a fuel cell system requires that a method of water 
removal be provided. 
small vacuum pump wil l  be needed to remove non-condensables from the water  
vapor cavities, otherwise a vacuum pump with sufficient capacity to handle the 
produced water vapor will  have to be provided. 
If the system hail its awn water recovery device, a 
It is possible to operate a fuel cell fo r  a short period with no provision for 
removing the product water. 
which results fo r  various loads and durations. 
cell is such that a dilution to 25% concentration from 40% will  cause flooding. 
Figure 3 shows the dilution of the electrolyte 
The capacity of a typical 
Other possibilities for water removal exist, such a s  operation at elevated 
temperature, with product water vapor being exhausted above ambient pres- 
sure; continuous purging of reactants, etc., but these were not studied. 
-5- 
At present the use of a vacuum pump is considered to be the most desirable 
method for water removal since the stack would operate in the normal range 
of temperature and pressure. 
-6 - 
3.0 MATHEMATICAL MODEL O F  STATIC MOISTURE REMOVAL PROCESS 
The mathematical model of static moisture removal was  completed and the 
computer program far  it has been debugged. It is now porsible to determine 
the transient and equilibrium water concentration throughout the cell for any 
temperature, gas pressure, vacuum cavity pressureo current, cell dimensions, 
and initial conditions. 
in the hydrogen, oxygen, and vacuum cavities may be assumed. 
Any current distribution and temperature distribution 
3. 1 Discussion 
A computer run was  made to simulate transient operation of a 0.2 f t 2  cell 
at 200 ASF. 
in the gas cavities, electrolyte, and water removal capillary matrix after the 
cell had reached an equilibrium condition, Elements i = 1 to i = 10 
represent the length of cell in the direction of gas flow and elements j 
to j = 12 represent the distance between the oxygen cavity and the vacuum 
cavity, 
respectively, a r e  caused, primarily, by the dry inlet gas, which evaporates 
water from the matrix a t  the inletend and then deposits this wa€er near the 
outlet end. 
Figures 4 and 5 show the concentration gradients that developed 
= 1 
Relatively dr ier  and wetter elements near the reactant inlet and outlet, 
The concentration gradient in the electrolyte is due to the chemical reaction 
which results in production of water at the hydrogen electrode and consumption 
of water at the oxygen electrode. 
is necessary for removing water from the cell. 
gradients show the effect of temperature on concentration. 
perature distribution is listed in Figure 4. 
The gradient in the water removal matrix 
The steps in the concentration 
The assumed tem- 
-7 - 
4 - 0  RELIABILITY MODEL O F  F U E L  CELL SYSTEMS 
The reliability study of various series, parallel and series-parallel fuel 
cell configurations has been completed. 
for two types of systems as shown in Figure 6. Type I systems a re  
designed to produce 2 KW at 27 volts, and 0.8 KW at 31 volts. 
systems are designed to produce 2 KW at any voltage. A voltage converter 
must be used in conjunction with Type I1 systems if specified voltage limits 
are  to be met. 
sidering 1, 2, and 3 parallel strings and also by cross-connecting these 
strings. 
without a converter, the operating current density per section must be 
smaller and more sections must be added to the system as  more parallel 
s t r ings a re  added. 
The reliabilities were compared 
Type I1 
In both cases, the power requirements were met by con- 
Since Type I systems are  designed to meet the voltage requirement 
The results of the study are  shown in Figure 7. 
based on an individual cell section reliability of 0.98. 
the more parallel strings that the system has, the more reliable it is. 
connecting these strings further increases the reliability because the failure 
of one section does not render an entire string useless. 
The calculations a re  all 
Note that, in general, 
Cross 
4. 1 A Studv to Compare the Performance of Various Svstems 
A study has been made to compare the power output, voltage, and current 
of n ce 11s connected in ser ies  (series system) with n cells connected 
in parallel (parallel system). 
It is necessary to compare the two systems under equivalent conditions, 
which means that the systems operate at the same percentage of maximum 
power o r  short circuit current, 01 at a voltage such that the average voltage 
per cell for the ser ies  system equals that of the parallel system. If the 
-8- 
comparison is made under equivalent conditions, the following conclusiuns 
can be drawn: 
(1) The parallel system will  produce higher power. 
(2) The average voltage per cell for the series system will equal 
that for the parallel system. 
( 3 )  The average current for the parallel system wil l  be greater 
than the common series current. 
It was found that the current distribution fox the parallel system will  be 
skewed such that: 
sP 1 - =  
K Is sS 
where: 
- 
sP Standard deviation of the reciprocals of 
the parallel currents. 
Standard deviation of the section voltages 
for the series system. 
Average V-A slope of the n cells 
The common series current. 
. 
5 . 0  CONCLUSIONS 
The computer program f o r  fuel cell system parameter optimization has 
been modified and expanded. 
the minimum system weight for various energy and power Levels. 
minimum system weight can be further reduced by using batteries to supply 
a portion of the peak loads. 
o r  more if the peak power is four to five times the base load. 
I Nomograms are  presented for approximatiag 
The 
The reduction in weight can be f i f ty  percent 
Reliability studies of various series-parallel fuel cell systems show that 
in general, a parallel system is more reliable that a ser ies  system. 
analysis far comparing the performance of ser ies  versus parallel systems 
indicated: 
The 
(1) The parallel system will produce higher power. 
(2 )  Average voltage per cell for ser ies  system wil l  be equal to 
that for parallel system, 
(3 )  Average current for parallel system wil l  be greater than that 
for seriel3 system, and 
(4) The current distribution for a parallel system wil l  be skewed. 
- 10- 
7 
6 :i NOMOGRAM FOR APPROXIMATING MINIMUM WEIGHT OF HfOg FUEL CELL SYSTEM 600 VOLTAGE LIMITS = 29*-2 VOLTS REACTANTS STORED SUPERCRITICALLY RADIATOR PROBABILITY OF SURVIVAL= 500i 0.999 < 3:l MAXIMUM POWER MINIMUM POWER - 
ALLIS - CHALMERS 1965 CELL DATA 
APPROXIMATE 
WEIGHT, LBS 
250 \ 
200 1 
150 \ 
1 1 1  I ‘IO 
Figure 1 
100 
NOMOGRAM FOR APPROXI MATING M IN I MUM WE1 GHT 
VOLTAGE LIMITS: 29 f 2 VOLTS 
REACTANTS STORED SUPERCRITCALLY 
RADIATOR PROBABILITY OF 
SURVIVAL = 0.999 
MAXIMUM POWER 
MINIMUM POWER 
ALLIS - CHALMERS 1965 CELL DATA 
APPROXIMATE - 
- 
-500 - 
d 
700- 
- - - - - - 
I '  \ '100 
Figure 2 220 
0 
00 cu a CD e cu 0 rr) M YC) rr) rr) 
HOY lH913M % - NOIlV&llN33N03 
FIGURE 3 
CONCENTRATION G R ~ E N T S  ALONG CELL LENGTH FOR 
ELEMENTS IN ELECTROLYTE AT 200 ASF 
46 
45 
4 4 ,  
43 
n 
I s 42 
I# 
- 
- 
- 
- 
- \ 
- T=362O K @ i ,=l , lO 
- T=363O K @ t=2,3,4,7,8,9 
- T =364@ K @I 1 =5,6 
- - 
- 
- 
' 
- - ADJACENT TO 0, CAV. 
- - 
- 
Figure 4 
5 
0 
c5 - 
-J 
41- I - - 
- 
I - 
40 
39 
38 
- 
- 
- 
- I 
' 
- I I I - ADJACENT TO H, CAV. 
L - 
REACTANT INLET DEAD END 
CONCENTRATION GRADIENTS ALONG CELL LENGTH FOR 
ELEMENTS IN WATER REMOVAL MEMBRANE 4 GAS 
43 
42 
n 
S 
0 
Y 
6.0 41 
Y 
s 
0 
2 
0 
0 
0 
-I 
40 - 
39 
(ADJACENT TO H, CAV,) - 
OXYGEN 
2 3 4 5 6 7 8 9 IO i 
SYSTEM CONFIGURATIONS 
TYPE I 
35 cel l  sections in s e r i e s  
dJ-+-----ul 
,iii) 
i v) 
2 paral le l  s t r ings of 32 sections 
2 paral le l  cel l  s t r ings of 32 s e l l  sections 
( c r o s s  connected) 
3 paral le l  s t r ings of 32 cel l  sections 
3 paral le l  s t r ings of 32 cel l  sections 
( c r o s s  6ennec ted) 
37  c e l l  sections (redundant system) 
TYPE I1 
42 cel l  sections in  s e r i e s  
21 paral le l  s t r i ngs  of 21 ce l l  sec- 
tions 
2 paral le l  s t r i ngs  of 21 ce l l  sections 
( c ros s  c o h e c t e d )  
3 paral le l  s t r i ngs  of 14 ce l l  sections 
3 paral le l  s t r ings of 14 ce l l  sect ions 
( c r o s s  connected) 
a n c 3 ~ u t l c f p r t D  a0nanoana 
'00 h 0 Ogf f  
qKJaC3U== 
42 c e l l  sections (redundant system) 
Figure 6 
I 
TABLE OF RELIABILITIES 
C i r c u i t  
Based on Individual Section R e l i a b i l i t y  of 0.98 
Sys tem producing 2 kw a t  Sys tem producing 1 k.w 1 
27 volts  1 22 volts  a t  any voltage 
'TYPE I 
1 
2 0 . 2 7 4  0.773 
3 0.274 0.988 
4 0. 536 0. 892 
5 0. 968018 0.999744 
6 0. 9939716 
1 
0.4930746 0.4930746 
I o.999994 
0.493074b 
0 . 7 7 3  
0.988 
0.896 
0.999744 
0.999994 
T Y P E  I1 
System producing 2 kw a t  any voltage 
0.428 
0. 428 
0 . 4 2 8  
0. 847898 
0. 985 - 1.0 
l- C i r c u i t  
a t  any voltage 
System producing 1 k w  
--i 0 .428 
0.88124 
0. 9912 
0.985 
0.9999 ! 
-1.0 
Figure 7 


Research and development of open-cycle fuel cells Fourth quarterly progress report, period ending Aug. 15, 1965

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