An investigation of the feasibility of using a solar water heater for a lunar base is described. During the investigation, computer codes were developed to model the lunar base configuration, lunar orbit, and heating systems. Numerous collector geometries, orientation variations, and system options were identified and analyzed. The results indicate that the recommended solar water heater could provide 88 percent of the design load and would not require changes in the overall lunar base design. The system would give a 'safe-haven' water heating capability and use only 7 percent to 10 percent as much electricity as an electric heating system. As a result, a fixed position photovoltaic array can be reduced by 21 sq m

Haynes, R. Daniel

Somers, Richard E.

English

NASA Technical Reports Server

597
SOIA WATER HEATING SYSTEM
FOR A LUNAR BASE
- 1
Richard E. Somers and R. Daniel Haynes
RF.MTECH, Inc.
3304 Westnaa Drive
Huntsvale AL 35805
This paper describes an investigation of the feasibtli O, of using a solar u_ter heater for a lunar base.
During the investigation, computer codes u_re dezeloped to ,uglel the lunar base configuration, lunar
orbit, and heating systems. Numerous collector geometries, orientation variations, aml system optious
u_e identified and analyzed. The results imP'care that the recommended solar u_ter heater could
provide 88% of the design load and wotdd not require changes in the oteraa lunar base design. The
system would give a "safe-haven" u_ter heating capabat'_, and use only 7% to 10% as much electr_" O,
as an electr_" heating system. As a result, a fixed position photovoltaic array can be reduced by 21 m 2.
INTRODUCTION
Hot water will be needed at a lunar base for various sanitation
requirements such as dishwashing, clothes cleaning, bathing, and
food preparation. The environmental control and life-support
system (ECLSS) will also require a continuous and significant
amount of heat for processing waste water. Typical hot water
usage temperatures range from 40°C to 80°C. Electric water
heating using a photovoltaic or solar dynamic array can be
expensive and inefficient and can require intensive maintenance.
Another important goal is to prevent loss of equipment or life
during an emergency by providing a "safe-haven." This is an
independently operating portion of the base (typically the habitat
module) to which the personnel can retreat. Electric water
heating systems could require unacceptably high power levels
from limited auxiliary power systems during an emergent3'.
To avoid these problems, a solar water heater similar to a
terrestrial _tem is proposed ( Fig. l ).
FLAT PLATE
SOLAR COLL
HABITAT
PHOTOVOLTAIC
POWER ARRAY
COMMUNiCATiONS
ALTERNATE SOLAR -
DYNAMIC POWER
LABORATORY
Fig. 1. Lunar ba._e ,,_)lar collector.
https://ntrs.nasa.gov/search.jsp?R=19930004817 2020-03-17T08:50:06+00:00Z
598 2nd Conference on Lunar Bases and Space Activities
THERMAL MODEL
To investigate the proposed concept, computer programs were
developed to assess the performance of candidate systems and the
effects of system options. The external influences and internal
processes of the lunar base are illustrated in Fig. 2 and listed in
Table 1. TRASYS (.]ensen et al., 1977) was used to model the
external effects, and a REMTECH program, QCOLL, written
specifically for this purpose, was used to model the internal
thermal processes.
External Influence Model
As indicated in Fig. 3, .several types of collectors were modeled.
A fixed position, flat plate collector and two types of wariable
position, concentrating or focusing collectors (dish and trough)
were investigated. For reference during the investigation, the lunar
base was modeled as two half-cylinders aligned east-west, similar
to Fig. 2. The orientation of a collector to the sun determines the
amount of incident energy. Obviously, the greatest incident energy
occurs when the collector is perpendicular to the sun. In
terrestrial applications, the collector must track the sun in two
axes to obtain the maximum amount of energy because of daily
and seasonal changes in the sun's position. For a lunar base, the
situation is simpler because the Moon's equatorial plane is only
tilted 1.5 ° to the ,solar ecliptic plane and therefore has no
"seasons." By tilting a collector at an angle equal to the latitude
of the lunar base, the collector can track the sun by rotating about
only one axis during the lunar day. Traditional terrestrial
applications, however, have shown that tracking the sun is
unnecessary and that fixed position, fiat plate collectors work
well. Terrestrial applications have also shown that even when fiat
plate collectors are positioned relatively far from the optimal tilt
and due .south (northern hemisphere) azimuth, they still perform
well. To determine effects of tilt and azimuth, the following cases
were run
Case ID latitude Tilt Azimuth
L 000" 0 ° 0 ° 0 °
L 400 45 ° 0 ° 0 °
L 430 45 ° 30 ° 0 °
L 440* 45 ° 45 ° 0 °
L 434 45 ° 30 ° 45 °
L 444 45 ° 45 ° 45 °
* Denotes optimum condition for that latitude.
TABLE 1. External and internal parameters.
External Influences Internal Processes
Configuration
• Shapes and position of collector and adjacent structures
• View factors to adjacent
Or_,ntation
• Orbital mechanics of Moon
• Location, tilt, and azimuth on Moon
• Angle of collector at any time to sun
Energy
• Direct solar and albedo
• Direct and reflected IR
CoUector
• Type of system (plate or concentrator)
• Size
• c_ and _
• Pumped fluid or heat pipe
• Flow properties (rate, laminar flow)
• Piping size and material properties (r, Cp, p)
• Heat balance (solar absorption-IR emission)
Storage
• Volume
• Heat exchanger (pmnf_d fluid)
• Heat balance (solar energy added-energy to load)
/X, mand Load
• TUne and duration of loads
• Simultaneous multiple loads
• Temperature of loads
DIRECT SOLAR
J__" RE RADI&TION I ///ENERGY
/ 'Y0o.Eo,oR
/ // \\ ilW
f J ALSEDO
._gll:.I;.o" ..... _c...OER
Fig. 2. External and internal conditiorts modeled.
(a) _ (b_ (c__/
[ {,,r,,'''¸
0 FIlE N TA"r tO N
Fig. 3. Types of collectors modeled: (a) flat plate (pumped liquid or heat
pipe; (b) two-axks tracking dish; (e) one-axis tracking trough.
The effect of tilt can be seen in Fig. 4 where the incident energy
for each due south (0 ° azimuth) orientation is plotted. The plots
start at lunar midnight, so no energy is received until approxi-
mately seven terrestrial days later. The incident energy increases
until solar noon (approximately 14 days) and then decreases as
the sun sets. For tilt, the worst case shown is the L 400 where
the collector at a 45 ° latitude is flat on the surface. Approximately
60% of the ideal case incident energy is incident on this collector.
By tilting the collector to 30 ° (L430), the incident energy is
increased and is only about 5% less than the optimum L 440. The
optimum at 45 ° latitude, L 440, and the optimum at 0 ° latitude,
L 000, are equal because they both have the same orientation to
the sun.
Figure 5 shows the effect of azimuth. Here, the collectors
pointed 45 ° to the east peak two to four days sooner and received
95% as much energy as the collectors pointed due south
(northern hemisphere is assumed). It is seen, therefore, that the
lunar fixed position, flat plate collectors follow the .same trend
as terrestrial collectors in that they collect energy at near
optimum levels even when positioned considerably off the
optimum tilt and azimuth.
400
r-
300
I--
x
200
kl_
-r
100
O9 f
0
A
14
Time (Days)
LEGEND
_= L4oo
/k = L430
0 = L440
l
21 28 35
Fig. 4. Effect of collector tilt on incident solar energy.
1.26
E
0.95
0.63 u_
O32 "_
000
400
e-
_300
m
,7 200
"1-
100
CO
i-
i +.:l
7 14 21
Time (Days)
I
] LEGEND
= L430
_= L440
z_ = L434
C)= L444
28 35
1.26
O.05 E
o63 ,'7
"1-
0.32
09
000
Somers and llaynes: Solar water heating s],stem 599
The incident energy for the dish and trough concentrators is
constant, since they track the sun. Es,sentially, the)' receive the
solar constant, 1352W/m 2 (429Btu/hr-ft 2) during the daylight
hours, which is the maximum possible amount of solar energy
awailable. The integrated total energy x-alue is approximately twice
that of the fixed position, flat plate at optimum oricntation.
Internal Processes Model
The first step in modeling the internal pr(_'esses was to
establish the hot water usages and express them as a profile of
the demand heating load. The plot shown in Fig. 6 is based on
the loads shown in Table 2, which gives each assumed load and
delivery temperature for an eight-person operation. Thcse h)ads
may' change in the future, but the plot shown is the current
estimate.
Two internally different collector .systems were investigated and
were modeled ,separately. The first, shown in Fig. 7a, rises a
pumped liquid as the heat transfer medium. Since the liquid must
not boil or freeze in the collector, the direct use of water was
ruled out. Therefore, the liquid is pumped through a .separate,
collector/heat exchanger closed loop. In the meantime, w_ter is
pumped fi'om storage through the heat exchanger and returned.
The collector can be either a plate or a concentrator.
In the second system (Fig. 7b), the collector is a plate to which
heat pipes have been attached. A heat pipe is a closed tube filled
with a substance in its liquid and vapor phases. Heat on one
portion of the tube evaporates the liquid and causes a higher
vapor pressure in that portion. The vapor flows to the lower
pressure areas of the tube where it is cooled and it conden_s,
thus perpetuating the low pressure. The liquid thus condensed
flows by capillary action back to the heated region in small tubes
placed in or under the heat pipe or meshes and grooves cut insidc
the heat pipe. In the heat pipe system model, the storage water
is pumped from storage through a heat exchanger; this portion
of the model coincides with the pumped fluid model.
m
o
400
300
200
100
4 8 12 16 20 24
Time-hours
7.03
5.27
3.52
1.76
0.00
Fig. 5. Effect of collector azimuth on incident energy. Fig. 6. Demand heating load.
600 2nd Conference on Lunar Bases and Space Activities
TABLE 2. Individual demand loads for lunar base.
Demand Occurrence Amount Ttme Temperature
Handwashing'*: 24 0.61 kg (1.35 Ib) I rain 41°C (I05°F)
Showers "t++ 4 3.60 kg (8 lb) 18 rain 41°C (I05°F)
Potable" 3 6.89 kg ( 15.2 ib) 10 min 74°C (165°F)
Dishwashing** 1 43.50 kg ( 96 lb) 20 min 60 ° C ( 140 ° F)
Clothes Washing °_ 2 49.90 kg ( 110 lb) 20 min 66°C (150°F)
Dish Drying_ 1 150 W 1.5 hr 38°C (IOO°F)
Clothes Drying_ 1 750 W 3 hr 66°C (150°F)
Water Recovery, Continuous 300 W 54°C ( 1300 F)
Water Recovery Continuous 300 W 82°C (180°F)
R. Bagdigion, personal communicatio_l.
_ ASHRAE, 1984.
: 1_ Garcia, personal communication.
D. Schiller, personal communication.
(a)
tco]J
tsto
21 C
tcol
COLLECTOR _' tcolo
eft + pump 1
_[utA_ EXCHANGER] _tstoi
tsto
v tmax tsto ? pump 2
--[ STORAGE TANK t
RECLAMATION •
pump 4 _
q.
|
21 C
tsto
pump 3
--t i
(b)
plate_
tcol
I evaporator
.... section
condensor
=...._ = _ _section
HEAT PIPE ttstoiv tmax tsto pump 1
STORAGE TANK 1
tsto
RECLAMATIONpump3_
qi USAGE
-.t.
|
Ftg. 7. System models for the internal processes: (a) pumped fluid or concentrator system; (b) heat pipe collector system.
The computer models for each system perform energy balances
on all components at approximately two-minute intervals in the
lunar day. Normally the run time simulated 1440 hr of activity on
the lunar base. The first 720 hr allowed the system to reach steady
state, and a normal operation cycle could then be observed for
the second 720 hr. Generally, the pumps are predicted to run con-
tinuously during the lunar day (-14 terrestrial days). However,
ff the storage temperature becomes 93°C, the pumps are "turned
off" by the model and excess heat reradiates to ,space.
PERFORMANCE
The methodology used for determining ,system performance
and comparing options consisted of three steps. Initially, a
baseline system was established. This was based on terrestrial
experience and iteratively running the program for some of the
options. A pumped fluid, flat plate collector ,system that heated
100% of the demand hot water with ,solar energy was chosen.
Next, three variations of each option were simulated using the
Somers and IIaynes: Solar water heating system 60 1
appropriate computer model. In the final step, t_o, SFE, 7, and
other parameters were compared to the baseline to determine
whether the options being studied had a serious impact on system
output.
Figure 8 is an example of a comparison between various values
of a and e. The tst o was computed for the baseline (_--0.80,
e ---- 0.20), for improved values (a=0.96, e----O.12), and for a
near-black body (a--0.96, e--0.96). Note that in an ideal
collector a= 1.O0 and e--0.00, which means all energy is
absorbed and none is reradiated. Only minor improvement is seen
using the improved values, but a marked decline in tst o and SFT
is evident for the near-black body. The effect of all options is
summarized qualitatively in Table 3.
Once the influence of each option was understood, no more
work was done with those found to be negligible, and only the
collector size and storage volume impacts were studied further.
This study produced a series of "design curves" by which a
designer can make quick trade-off studies.
Figure 9 shows the design curves for a pumped fluid collector.
The SFT has been plotted as a function of collector area and
storage volume. As expected, SFT increases as the collector size
increases. This occurs regardless of the storage size up to -2 m 2.
The explanation for this phenomenon, which was seen in the
design curves for all systems, is that the collector is too small to
supply the total load. The collected energy is used almost
immediately, leaving little or none to be stored; thus, little storage
is required. Beyond the 2 m 2 point, additional energy is available
after the demand has been satisfied; therefore, storage must be
larger to accommodate it. If the collector size is increased beyond
4.5 m 2, little change occurs in SFT unless the storage volume is
increased.
It can be seen that the storage volume is very large compared
to the collector area. This large volume is required because of
the exceptionally long night during which no solar energ 3' is
available to replace tank energy losses caused by water osage.
RECOMMENDED SYSTEM
To provide -100% of the demand load with a solar system
would require a minimum area of 4.5m 2 (50fi 2) of pumtx'd
liquid or heat pipe, flat plate collector, or 2.3 m 2 (25 fiz) (ff dish
or trough concentrator. Bec'ause the concentrators track the sun
and thus could cause maintenance problems, and because the flat
plate area is so modest, it is recommended that the flat plate be
used. The heat pipe collector offers a major advantage in that a
micrometeoroid strike would only rupture one of the heat pipes,
but the others would continue to function since they are closed
units. Rupture of a pumped liquid line would cause a shutdown
of the entire system until the line was repaired. The heat pipes
in a collector, however, must be carefully leveled horizontally to
prevent a gravity gradient or they will not function properly. The),
also have a major disadvantage, since under no-load, high-solar
input conditions they will boil, creating pressure that can burst
the pipes. The pumped fluid system operates the same regardless
of tube alignment and can be drained to aw)id boiling or freezing.
Therefore, the recommended _stem collector is a 4.5m 2,
pumped liquid, flat plate. The coating would have c_ = 0.80 to
0.95 and e = 0.10 to 0.20.
To achieve _ 100% solar contribution, our model predicts that
the storage must contain a minimum of 30,0001 (8000gallons)
of water (Fig. 9). Although water may well exist as ice on the
Moon (Staehle, 1983) or may be produced by burning hydrogen,
300
_" 250
o
200
¢-t
E
b.- 150
o 100
5O
L
LEGEND
F] a=0.80 (=0.20
,:*=0.96 _=0.96 __
/_ a=0.06 (=0.12]
120
.,_^_^^,
,C;_7_y_._ ,.i...... _ _._: "-i....... '*I...........
r
!
.........i..................i
240 360 480 600 720
Time (Hours)
Fig. 8. Effect of absorptivity and emissivity.
149
t-
._O 1.0
t_
121G uT.
o.. _ 0*
93 t_
Q. :,=
E (o
_o "3 0.6
66 V-- E
D
,
0 V-- 0.4
3S _
10
,/,S
0.00
I
8000 GMI D
2000 GM
• 1ooo G_
500G_l
1
1
3.72 7.43 11.15
J
LEGEND
[] = 30285 L
<>= 7570 L
El= 3785 L
O = 1890 L
1486
Collector Area (m 2)
Fig. 9. Design curve fi}r flat plate pumped fluid s'}_tem.
25.0
>,
c}
r-
20.0
._o
e-,
15.0
CO
10.0
TABLE 3. Effect of optk)txs on s3_tem performance.
Significant Effect Negligible Effect
• Collector Size • Fluid "I'}T_e and Flow Rate
• Plate or Concentrator • Collector Tilt and Azimuth (within 20 * )
• Storage Size • Heat Pipe or Pumped Fluid
• Coating _ and e • Heat Exchanger Efficien,. T (within 0.5 to 0.8)
602 2nd Conference on Lunar Bases and Space Activities
an initial eight-man base would probably not have this ability. The
tank and water would have to be transported from Earth;
therefore, 30,000 1 of water and the tank for storing it are
probably impractical due to cost. The model also predicts that
if the storage volume is reduced by 75% to 7500 1 (2000 gallons),
the ,solar contribution only drops 12% (SFT = 0.88). This is still
a large contribution for a modest-sized system, ,so the smaller
volume is recommended.
The recommended system would have the following impact on
delivery to and installation at the lunar base
Mechanical
Collector Area: 4.5 m 2 ( 10 cm thick) Weight: 86 kg
Storage Volume: 7.6 m -_ Weight: 7557 kg
(See below)
Auxiliary Equipment 0.2 m 3 Weight: 80 kg
Volume:
Total Volume: 8.3 m _ Total Weight:
7723 kg
Electrical
Total power to operate equipment: 175W
Coefficient of performance
(Useful Energy/Operating Energy): 10.5
Reduction of fixed photovoltaic array (77 = 10%): 21 m 2
The above assumes that no hot water storage was necessary
before the addition of this system. This is not true, though the
exact amount is not known. An estimate would be 760 to 1180 1
(200 to 300 gallons). This amount should be subtracted from the
impact of the system storage volume and weight, reducing them
by 10% to 15%. Another consideration that partially offsets this
large storage volume and weight penalty is that backup and stored
electricity systems can be considerably decreased. The amount
would be dependent on the type of electric storage ,system, but
the reduction of weight and wflume of fuel cells, for example,
would be significant and make the solar system more attractive.
CONCLUSIONS
The thermal analysis has shown the following:
1. A heat pipe or pumped fluid flat plate collector of -4.5 m 2
is well suited for satisfying up to 100% of the assumed hot water
demand.
2. The volume of storage water required to allow significant
solar contribution would be very expensive if transported from
Earth.
3. A concentrator would need only 50% as much area ms a flat
plate; however, it could be more difficult to transport and
maintain.
4. The size of the photovoltaic array and backup and electricity
storage ,systems can be significantly reduced compared to an
electric water heater.
5. Except for storage volume the system has only a minor effect
on the base design.
The overall conclusion is that a solar water heater for a lunar
base is feasible and even desirable.
Cp =
eff =
k =
SFT --
too 1
tctfli 7_
tcolo =
t i =
tma x
tst o
tstoi
V =
p
NOMENCLATURE
Specific heat
Heat exchanger effectiveness
Conductivity
Heat load required by ith device
Solar fraction total (heat supplied to load by solar energy/
heat load)
Collector temperature
Fluid temperature entering collector
Huid temperature exiting collector
Delivery temperature required by ith device
Maximum temperature of storage
Water temperature in storage
Water temperature entering storage
Storage volume
Absorptivity in solar spectrum
Emissivity in infrared (IR) spectrum
Thermal efficiency of system (heat supplied to load by
solar energy/incident solar energy on collector)
Der_sity
REFERENCES
ASHRAE (1984) Hana_k of Systems.
Jensen C. L and Goble IZ G. (1977) _orma/Radiation Analys_ System
II-TRASYS H User Manual. Martin-Marietta Report MCR-73-105.
Staehlc R. L. (1983) Finding 'Paydirt' on the Moon and Asteroids.
Aeronaut. Astronaut., 44-49.


Solar water heating system for a lunar base

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