whether natural aeolian processes on Mars would sweep off the settled dust. The ... the length of the journey, even initially astronautswill probably stay on the.
NASA
TechnicaA
Aeolian.
Memorandum
........
_
Removal
Photovoltaic
of Dust_ From Surfaces__on. Mars
James R._Gaier and Marla Lewis Researc_h__Cente__r Cleveland,
102507
E. Perez-Davis ...................
..... .....
.........
Ohio
and Mark
Marabito
Cleveland Cleveland,
State University Ohio
February
1990
_(NASA-TM-1025OI) FROM PHOTOVOLTAIC 19
p
AEOLIAN SURFACES
REMOVAL ON MARS
OF
Nq0-19299
_UST (NASA) CSCL
? - _S---L7
L._.
1OA
Unclas 03120
0271126
Y:
4dr
Aeolian
Removal
of Dust
From
Photovoltaic
James R. Gaier and Maria E. Perez-Davis, NASA Lewis Research Center, Cleveland, Mark Marabito Cleveland State
University,
Cleveland,
OH
OH
Surfaces
on Mars
44135
44115
SUMMARY it is well documented Martian
year.
dust storms system
Dust
elevated
photovoltaic
Using
surfaces, whether
natural
of wind
velocity,
coating
material
were
photovoltaic
witfi
an
Although
the
angular
significantly
studies covered
it appears have
of attack dependence velocities
that
the arrays that
winds
on Mars would sweep
surface
which
to clear
effects
chemistry
for protective
coating
or abrasion
of Martian
45*
sharp,
show
quite
resistance.
the
near
to guide
most
dust.
The
and surface the
design
clearing.
arrays
required
the ground, such
but
of dust previous
arrays
to be
Providing
that
to our test dust, the materials
velocity
of
arrays
the perspective
to cause
to
efficient
from the ground.
for other considerations
The static threshold
surface,
mounted
From
can be expected
in an attempt
Most importantly,
horizontally
off the dust.
dusts is comparable
may be optimized
help
were uncovered.
may be erected
saltation
can
power
dust simulated
off the settled
off of the Martian
approaching is not
wind
suggested
to Martian-like
height
each local
photovoltaic
to uniformly
Principles
for the Martian
hamper
technique
by sand if they are set up less than about a meter
the surface
chemical
of attack,
nearly
or in any of the numerous
and seriously
developed
processes
in huge dust storms
dust storms,
subjected
investigated.
angle
higher
aeolian angle
arrays bound
mounted
were
engulfed
surfaces
a recently
samples
effects
clearing
in these global
could settle on photovoltaic
performance.
determine
that Mars is totally
such as transparency, is low enough
that
there
used and are
regions on Mars which experiencewinds strong enoughto clear off a photovoltaic array if it is properly oriented. Turbulence fencesproved to be an ineffective strategyto keep dust cleared from the photovoltaic surfaces.
INTRODUCTION In the past few yearsthere hasbeen a growing consensusthat the United Stateswill, perhaps in the next twenty years, send a manned mission to land on the surface of Mars. Because of the length of the journey, even initially astronautswill probably stay on the surfacefor an extendedperiod of time, perhaps severalweeks. During their staythere will be power requirements which will exceedthose of present spacecraft1,and an important component of that power will no doubt be supplied by photovoltaic arrays. Photovoltaic arrays will be subjectedto an environment unlike those in which they haveheretofore been used. The atmosphereof Mars consistsof CO2 (95.3percent), N2 (2.7 percent), Ar (1.6 percent), O z (0.13 percent), ppm
or less of 03, Ne, Kr, and Xe 2. Natural
high velocity winds, dust, ultraviolet and
CO (0.07 percent),
atmospheric
Results
condensates
radiation,
(H20
oxidizing
from the Viking
species landers
in the soil 3. Although showed
to move at higher velocities that on occasion temperatures
there
range
from
a threat
99.9 percent
on Mars
soil composition,
suggest
of the wind
or less 4, dust storms
aeolian
and
such as
to photovoltaic
the Viking landers
of 20 m/s
(up to 32 m/s) 5, and
changes,
arrays.
the presence measurements
were
observed
features
(sand dunes,
etc.) suggest
at low pressure
(5 - 8 torr).
The surface
135 to 300 K 7, and daily temperature
swings ranging
from 20 to
8.
One of the possible nearly
velocities
are very high winds 6, albeit
50 K are not uncommon
the planet
on board
(0.03 percent),
conditions
rapid temperature
and CO2) may pose
from the soil analysis experiments
of highly
environmental
H20
annually.
threats
comes from local and/or
Infrared
spectra
global
from the Mariner 2
dust storms
9 spacecraft
which engulf suggested
that
the
dust
is a mixture
andesite
of many
or montmorillonite),
2 _ m. 9 A significant storm
1° which
It is not
known
tenuous
but
array
The removed
designed
from
AND
There
sample
of this
array
on Mars.
wind (2.54
substrates.
or diamond-like
which
was The
which (see
by natural
affect
this process.
of the
aeolian
surface
is about a dust
of the array.
might
Perhaps
array
quartz,
during
the performance
array?
how
we
square,
are
were
across Samples
left
evaluated
the
from
bare
the
be.
the
Will
the
photovoltaic
to be self-clearing.
likely
it is that
processes,
and
thin,
two corners, held
both
of surface
thick
surface,
the
dust
will
how
the
from
a photovoltaic
shape
be and
sputter
deposited
with
of SiO 2 and
down
at a tilt angle
the
of the sample
coatings.
These
coatings drag, dusted holders
45, 67.5,
array,
for the
a coating
indium
of
tin oxide coatings
for photovoltaic and
for low mass,
substrates. by means
by a foil tab attached of 0, 22.5,
used
PTFE,
low aerodynamic
the
turbulence. were
determinations
held
and
on
coverslips
for protective
designed
coating
glass
I summarizes
to present
in specially and
effects
mixture
materials
weight
dust removal
or ion beam
Table
candidate
effect
planetary
50 percent
(DLC).
mounted
were
could
5 mil (.13 mm)
for accurate were
which
height
were
they
important samples
tests
carbon
substrates
1).
ability
array
obsidian,
atmosphere
accumulation
off of the
the
glass,
in the
on the
dust
is to determine
(PTFE),
because
stretched Figure
cm)
These
(ITO),
The
dust
size
degrade
a problem
arrays
"velocity,
polytetrafluoroethane
arrays.
be deposited
the
of variables
In these
inch
chosen
can
a variety
SiO2,
were
study
particle
and significantly
blow
basaltic
MATERIALS
are
of attack,
may
serious
winds
photovoltaic
METHODS
One
how
basalt,
the average
so as to maximize
purpose
of the
angle
the light
velocity
orientation
surface
occlude
(granite,
that
of dust
at thi_ point
high
can be
and
amount
could
minerals
of foil tabs
to a removable
or 90 degrees
from
pin the
floor.
The
sample
holders
could
also
be held
horizontally
for
optical
transmittance
measurements. The subjected
sample
to a dusting
The method
dust Optical
significant
amounts
It also contained (0.6 percent).
Company.
from
those
in angle and height Because holders
samples
was difficult
study
Although
from the surface
to control,
13 dusting
imposed
pressure, runs
of the dusted
at once.
samples
was much lower.
The
winds
on
at NASA
to a few torr)
wind tunnel
and
the
as 0.82.
The
was
simulated
Ames Research 14 m in length
Center.
using
no more
the
had
samples
uniformity
four
on the
to settle out.
is shown in Figure
The MARSWlT
are
of dust in the
samples
of the pristine
Martian
than
accumulated
the amount
resulting
spatial
a meter
and the
for larger particles
(Td) to transmittance
as high
about
apparatus,
upon
(III) oxide
of magnitude
of dust which
The Td/T o for each sample
Mars
(MARSWlT)
by the dusting
respectively).
on Mars
to be similar.
required,
with
wind velocities
are expected
dependent
(89 percent)
from this powder,
than
the order
The amount
from
that the chemistry
different
greater
11.
powder
and chromium
for dust clearing
and the time allowed
were
grinding
experiments,
being critically
as low as 0.18 and others
operation
the values
of a dust storm.
(6.6 and 3.0 percent
substantially
and
in detail elsewhere
It is recognized
to altitudes
simulation
be dusted
the elevation
transmittance were
could
is probably
horizontally,
oxide powder
dioxide
from .1 to 25/_m.
elevated
of size limitations
sample
this
become
in these
an aluminum
optical
of iron (III) oxide (0.6 percent)
size ranged
held
are discussed
1800 grit
and titanium
soil, while not known, which
was
were
in the aftermath
dust distribution
experiments
of silicon dioxide
the samples
dust accumulation
It was principally
in this size range.
chamber,
so that
simulates
in these
The particle
the particles
may differ
tilted
a small amount
of the Martian
trends
which
used
American
probably
were
of dusting and the resulting
The
but
holders
ratios
For of
(To) which
of each
dusting
2.
Surface
Wind
Tunnel
is a low pressure
with a 1 by i.1 by 1.1 m test section
located
(down 5 m
from the tunnel entrance. This flow-through wind tunnel is located within a 144,000ft3 vacuumchamber which wasback-filled with CO2. Its characteristicsare describedin detail elsewhere12. The samples were placed in the MARSWlT and tested under the wind conditions listed in Table II. The samples were weighed before dusting, after dusting, and after MARSWlT exposure. However, the weight of the dust added to the optical surfaceswas below the sensitivity of the balance used (0.1 mg). Optical transmittance measurements were made by sliding the transmittance measurementdevice (TMD) over the sample. In the TMD a white light sourceis suspended above the sample, and the sensinghead of a Coherent Model 212 Power Meter is beneath the sample. transmittance dusted
Absolute
transmittance
measurements.
Measurements
(T Oand T d respectively),
MARSWIT
parameter,
exposure
samples
of the value
particle Assume
on the surface. is stronger number increases
converted
into
percent
and after the samples
were subjected
than
as the ratio
were
to winds in the
('If) is a function
of the transmittance
that can be exerted will increase
up until a monolayer 5
upon
dusting
is, unfortunately,
angle to the wind, surface of dust deposited
solely from particles
by the dynamic
on wind
on this parameter.
of the amount
are sufficiently
as the total number
There
using a dust
change
change
dustings
of wind velocity,
of Tf from T Oarises
For the most part these particles
is, as T O decreases)
sample
It may also be a function
that the degradation
the forces
was evaluated
to vary from zero to one.
of T d used in different
size, and time.
of these particles (that
from the samples
('If - Td) to that of the transmittance
is constrained
The final transmittance
initially.
and after the dusted samples
which was defined
This function
a dependence
chemistry,
were made before
of dust which was cleared
of the dusted
(T O - Ta).
were
(T,).
The amount clearing
measurements
remaining
small that surface pressure
adhesion
of the wind.
of particles
dusted
is built up.
Beyond
The
on the sample that there
is
only particle-particle cohesion. Thus, Tf will be a function of T d until the monolayer established,
and beyond
that it will not.
low T d, the dust clearing effectiveness.
For
independent
parameter
dusting
would
runs
at two
different
Samples
were placed
at about
50 cm, which should
A turbulence It was thought
heights
fence was constructed
horizontal
of attack,
which
and coating
there
clearing
parameter
should
be
floor
of the
wind
tunnel
were
be within the floor's boundary
tested.
layer, and
it. to increase
the wind turbulence at clearing
at the sample.
the dust at wind speeds array of eight.
125 in (3.2
every .375 in (9.5 mm).
are expected
that there
factors
shape,
and surface
even
total
leave
clearing.
the surface
that which it is believed under
calmer
In these
to become conditions.
to be the
they will be discussed
first, and
to clear photovoltaic
above The without
given
static
threshold
impact
experiments
suspended Particles 6
less
including
the particle
than
time
velocity upwind
during a global
more
efficiently.
for the wind velocity
sufficient
from
velocity
on the effects.
surfaces
value
which,
which will effect the static threshold chemistry.
were found
as small perturbations
will be a threshold
and
efficiency,
Accordingly,
will be discussed
will be no clearing, perhaps
to dust clearing
of the wind.
material
wind velocities
dust particles
out
runs of
for the same dust clearance
It was made up of an vertical
variables
and the velocity
several
settle
the
flow might be effective
rods spaced
It might also be suspected
significant,
dust
for dusting
AND DISCUSSION
Higher
which
from
be well above
that the turbulent
diameter
turbulence
T d, the
at about 2.5 cm, which should
The two most important angle
take a higher value
of high
lower than those in the free stream.
RESULTS
of T d then,
of T d.
Winds
mm)
If T_ is a function
is
is that
particles. particle
1 _m
will be
velocity There
at are
size, particle
size was chosen
dust storm,
about
there
below
to match
but which would in size
will stay
suspendedfor very long periods of time, and those larger than about 50 _ m will never be transported
far from the site where
experiment
mimic the Martian
however,
is likely to be quite
According
they first become
from that found
thought,
superoxides
bombards
which
may be generated
the surface s . With
whatever
exotic surface
of much more
water
the present
chemistry
vapor
shown to dust the samples
should
starting
in the Earth
Figure simulated
3 shows
Martian
some indication
and that trends should
hope
constantly to duplicate
the presence
the surface
polishing
chemistry
powder
has been
11. Thus, this material
in angle, height,
be repeated
of peroxide
that
In addition,
change
aggregation
the dust The
clearing amount
of the experimental
as a function that
error.
value was near 45*.
Samples
dust clearing
at velocities
about
below
transmittance
still less clearing,
with velocities
varying
of the samples
were cleared
of the samples
cleared
is a
turbulence,
with dust of different
There
value
of angle
of the data
for various
points
is a clear indication
with an attack
etc.
surface
while
velocities
lie below
those
at wind velocities
at 45 ° cleared
of
zero
from Figure
angle of zero showed
slightly less efficiently
give 3 that
virtually
no
to about
92
as low at 35 m/s.
Samples
than those'at
Samples
45°.
but more than those held at 0 ° . This trend was found
from 30 to 85 m/s. comparably.
appreciably.
some
100 m/s,
held at angles of 22.5° and 67.5° cleared held at 90* showed
would
and are
its effect.
wind.
of their original
soil.
evenly with little particle
the optimum
percent
we cannot
The optical
studies,
to be basaltic,
radiation
soil.
point for these
to evaluate
ultra-violet
environment
Martian
of the particles,
results infer the possibility
state of knowledge
still be valid, but the experiments
chemistries
by the
are likely
might exist in the Martian
even if we did know how to simulate
reasonable
the Viking
chemistry
used in this
on Mars.
the soils on Mars
known to be rich in iron oxides 9. Further, and
The particles
dust size and shape, u the surface different
to current
airborne.
Note
In the test with a higher In the test with a lower that the time exposed 7
velocity velocity
(124 m/s) (10 m/s)
all
none
to the wind was not the
samein all cases (see Table I), but the angular dependence is not expected
of samples
configuration
dependence
the measured 85 m/s,
wind
samples
consistent
an order
predicted
of magnitude
layer of spherical
particles
less than a monolayer
smooth
expect
(90*)
a vertical
higher
the threshold
This should
below
(see Fig. 3).
velocity
assumptions.
on various
to be smaller
be an
and White is considerably
experiments
by Iverson
were
angular
than predicted.
particles
velocity
axis.
holders
The
of somewhat
less than
The experimental
conditions,
Iverson
assumed
laying on a bed of similar particles.
of non-spherical
sample
on 0 ° , 30 ° , 60 ° , and 90 ° tilts.
were not the same as the theoretical
one might
around
with the other
clearing velocity
test, vertical
values 13. Using the 0 ° data we find a threshold
about
however,
to having
was indeed
The threshold
and White
In the experiment,
substrates.
there
Intuitively,
in the experiment
a
was
however,
because
of the
substrate. Given
mechanism
the
angular
remaining showed
dependence
of detachment
most part, however,
would
on dusted
confirmed
conditions.
Only
turbulence
direction
involve
glass surfaces
no directionality
further
of the dust
this did not appear
by the
photograph angle
at the surface
which is approximately
models
of Bagnold
surface
at the threshold
subjected
14in which
must
winds
became
very
act to aerodynamically
aerodynamic
to the surface.
with an attack
subjected
low was there
velocity. 8
For
the
angles angle of
This was
to the
appreciable
This view is supported
same
streaking.
lift the particles
lift plays a key role in particle
the
attack
of the wind arrival.
sample
that
of the dust layer
at different
Only on the samples the direction
suspect
of dust particles.
Photomicrographs
to 35 m/s
of a half-round
normal
one might
or sliding
to be the case.
from the photographs
as the attack
clearing,
the rolling
to the dust removal.
22.5° could it be discerned
Thus,
in 85 m/s
at 0 ° , 30 ° , 60 ° , and 90* from the wind
equivalent
of dust clearing
to be time dependent.
In one series angled
of the efficiency
out
in a
by classical
motion
from
a
Given the cautions above, the static threshold velocity to remove the surface
was determined.
us the minimum
static
The data taken
threshold
threshold
value was between
maximum
wind speed
some parts
of the Martian
from
turbulence.
two
pressure
MARSWIT. velocity
Figure
differences
placed
at a 45* angle
the minimum
than the average
daily on
velocity)
height
at several
two heights.
to maximize
small boundary
different
dynamic
the boundary there
7 shows
observed,
(where
the of
layer, and the
was no appreciable
to the floor.
Figure
of the
and the height
in a 55 m/s
nearer
effects
layer
velocities
6, however,
In one experiment,
layer
induced
50 cm from the floor
of the boundary
the dust clearing.
was
local velocities.
were within
from Figure
artificially
(and so a lower mean
3 cm and about
the approximate
and
Turbulence
wind, The
a sample holder
was
that in this extreme
with the lower
samples
slightly less clearing. Turbulence
lower
turbulence,
on end so as to fix the samples
may have been
that will give
9 m/s 15, it is not uncommon
mean velocity
run at about
As can be noted
was placed
samples
boundary-layer
It can be seen that the lower samples
holder
showing
were
these
that
of dust from surfaces?
but it may result in higher
the flee-stream
between
case there
in the clearing
sources,
5 shows
ones were not.
this is higher
sites of about
will result in a lower
samples
becomes
the samples. upper
different
to move the particles) Identical
4 it can be seen
Although
landing
because
from
surface s .
is turbulence
Turbulence
In Figure
30 and 35 m/s.
at the Viking
How important studied
value.
at 45" is of most interest,
dust particles
was also induced
at a wind speed the threshold
slightly (see Figure
by placing
near the threshold.
wind
speed,
a "fence" of cylindrical
rods in front of the
The hope was that the turbulence
but the fence
was found
to actually
fence would
hinder
the clearing
8).
A wide variety would be most effective
of photovoltaic
cell coatings
in shedding
the dust. Because 9
was tested
to determine
of the probable
which
differences
coatings in surface
chemistrybetween the test material and actual Martian soils this is risky, but perhapssome general surface principles can be determined. Even though there was a wide variety of materials both conducting and insulating, hard and soft, and high and low coefficients of friction, there were only slight differencesamongthe ability of the coatingsto shedthe dust. For a eachangle of attack (0°, 22.5°, 45° , 67.5°, and 90*) andfor the wind velocities of 55, 85, and 124m/s, each coating was ranked on the basis of dust clearing parameter from highest (1) to lowest (3 or 6, depending on the number of samples). The average ranking over all of the anglesat a given wind speed for each of the coatings is shown in Table III. The last column in Table HI showsthe averageranking for eachcoating over all of the anglesand all of the wind speeds. Although the error is probably large, there may be somevalidity to the rankings. Glassand SiO2havenearly equal scores,as do PTFE and PTFE/SiO 2. ITO wasthe easiestto clear, and DLC the hardest. Surfaceadhesion testsare planned to test the validity of the ranking.
CONCLUSIONS Even in this first preliminary study principles have been found which can help to guide the design of photovoltaic arrays bound for the Martian surface. Most importantly, if an array is to be self-cleaningit should be tilted at an angle approaching 45*. Although there is wide latitude with this requirement, it seemsmost important that the arrays are not erectedhorizontally. Most importantly, arraysmounted with an angle of attack approaching 45° show the most efficient clearing. Although the angular dependence is not sharp, horizontally mounted arrays required significantly higher wind velocities to clear off the dust. From the perspectiveof dust clearingit appearsthat the arraysmay be erected quite near the ground, but saltation can be expected to cover the arrays if they are set up less than about a meter from the ground16.Providing that the surfacechemistryof Martian dusts is comparable to our test dust, the materials used for protective coating may be optimized 10
for other considerations the same assumption, clear
such as transparency, there
where
off a photovoltaic
are regions
array
some other clearing
to be an ineffective
which
technique
strategy
and chemical
on Mars which
is properly
or abrasion
experience
oriented,
though
will have to be employed.
to keep dust cleared
resistance.
winds strong there
enough
are other
Turbulence
from the photovoltaic
Given
fences
to
regions proved
surfaces.
ACKNOWLEDGEMENTS The support
authors
like to thank
staff at NASA Ames
essential
assistance
contributions State
would
preparing discussions
Center
the
MARSWlT
of S.K. Rutledge
of NASA
University, the
with
Research
and
M. Kussmaul
optical
coatings.
and expertise
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Res. 84 (1979), pp. 2853-63.
R. Hahn, C.E. Carlston,
T.Z. Martin, A.R. Peterfreund, B.M. Jakosky, J. Geophys. Res. 82, (1977) pp. 4249-91.
11
and D. Pidek, J. Geophys.
E.D.
Miner,
and F.D.
8.
J.E.
Tillman,
The
Astronautical 9. O.B.
Case
for
Society,
Toon, J.B. Pollack, and the Martian Dust Storm
10. R.E.
Arvidson,
E.A.
Mars
1985)
II,
Vol
62,
Science
and
Technology_
C. Sagan, "Physical Properties of the of 1971-1972", Icarus 30 (1977).
Guinness,
(American
pp. 333-42.
H.J.
Moore,
J. Tillman,
Particles
and S.D. Wall,
Composing
Science
222 (1983)
pp. 463-8. 11.
M.E.
Perez-Davis,
J.R.
Gaier,
R.
Kress,
and
J.
Grimalda
(1989)
manuscript
in
preparation. 12. R. Greeley, B.R. Memorandum
White, 78423
13. J.D.
B.R.
14. R.A.
Iverson
and
Bagnold,
White,
Philos.
15. J.B. Pollack, 479.
J.D.
Iverson,
Sedimentology
Trans.
C.B. Leovy,
16. R. Greeley and J.D. Tita_.....__n, (Cambridge
J.B. Pollack, (1977).
Royal
Y.H.
Mintz,
Soc., and
Ser.
and
R.N.
29, (1982),
pp.
A, 249,
W. Van
Iverson, W___indas a Geological University Press, 1985).
Table I -- Photovoltaic
Array Coatings
Coating
Thickness
none
..............
(1956)
Camp,
Process
Leach,
pp. 239-97.
Geophys.
Res.
on Earth,
Tested Deposition
Substrate glass
650 A
ion beam
glass
PTFE
= 1000 A
ion beam
glass
2 ._ 1000 A
ion beam
glass
ITO
-_1000 A
ion beam
glass
DLC
_ 1000 A
ion beam
glass
12
Technical
111-9.
SiO 2
50% PTFE/SiO
NASA
Let. 3 1976)
Mars,
Venus
p.
and
Table II -- Wind
Conditions
Velocity
Press0.re
Within the MARSWIT Dynamic
Pressure
Tcmpera.ture
Time
10 m/s
7.6 torr
1.8 Pa
290 K
10.0 rain
23
7.6
9.8
290
10.0
30
7.6
16.6
290
10.0
31
7.6
17.7
290
15.0
35
7.6
22.6
290
5.0
55
7.6
55.8
290
2.0
85
7.6
134.
290
.50
124
7.6
283.
290
.75
Table III -- Relative
Ease of Dust Clearence
From Photovotaic
Coatings
Coating
55 m/s
85 m/s
124 m/s
Overall
ITO
1.0
1.6
2.5
1.9
1.0
1.8
3.0
2.2
PTFE
2.0
2.3
2.3
2.3
SiO 2
3.0
1.9
3.6
2.8
Glass
2.0
2.4
3.8
2.9
DLC
3.0
2.1
4.3
3.2
PTFE/SiO
2
13
ORIGINAL PAGE IS OF POOR QUALITY ORtGINAIC PAGE BLACK AND WHITE PHOTOGRAPH
Figure
1. - Sample
pholovoltalc
1,0
0,8
o
0.6
P
0.4
_
holder
designed
to test
I
1
I
-J
I '
Z
8
rl
f_
I
t
from
I
I
_
|
w -
8
I
1
t
I
1
L
I
1
2
3
4
5
6
7
MARSWIT Figure
removal
II
l
--
-
dust
I1
0.2
aeolian
surfaces.
2, - Uniformity
of dust
14
8
RUN
deposition
each
MARSWlT
run
ORIGINAL PAGE IS OF POOR QUALITy 1.0
0.8
_"
0.6
I-.-
_
(1.4
Fo
& 0.2
8 8
B
_
8 O
_
0
.
-0.2
I
I
1
[
(a) Dust cleating
I
1
1
I
I
from a I0 rrffs wind.
I
1
I
1
I__L_J__[__
(b) Dust clearing from a 30 m/s wind.
f.0 8
o
0.8 o _" I.o 1-.
_.
0.6
8
o 0 o
o
8
o
o
0,4
"El
_
0.2
I I I I I II_l_kl .... ._L_L .....L__I
-0,2
(c) Dust clearing from a 35 m/s wind,
1 [
L_ I
(d) Dust clearing from a 55 m/s wind.
1.0 0 0
0.8
o o
_" l,-
o
o
o
@
0
o
0
o o
o o
g 8 0
0.6
0 o
o
g _
0.4
I--
&
_
o.2
I
---0.2 0
10
1 20
I
I
[
I
I
I
I
30
40
50
60
70
80
90
I I I I I I I I I 100
10
20
30
40
50
60
70
80
ANGLE TO WIND, deg
ANGLE TO WIND, deg
(e) Dust clearing from a 85 mls wind.
(f) Dust clearing from a 124 m/s wind.
Figure 3. - Dust clearing as a function of angle for several different martian wind speeds.
15
90
100
1.0 0
0 0
0.8
8
o8
P-- 0.6 o I--
_. I--'
i
0.4
8 8
o.2 m o 8
_
I
I
20
40
--0.2 0
I
I
[
60 80 WIND VELOCITY,
I
100
120
140
m/s
Figure 4, - Dust clearing from a smooth 45" angle surface.
_oo F 80 60
m/s
55
85
124
40
20
10
°! 0.8 0.6 0.4
0.2 1.0 I
0: 0
I 20 40 6o
I I I I _o I00 _20 140
FREE STREAM VELOCITY,
mls
Figure 5. - Nominal boundary layer profiles.
16
National Space
Aeronautics
Report Documentation
and
Page
Administralion
1. Report
2. Government
No.
NASA
Accession
No.
3. Reclpient's
Catalog
No.
TM-102507 5. Report
4. Title and Subtitle
Aeolian
Removal
of Dust From
Photovoltaic
Surfaces
on Mars
February
7. Author(s)
James
R. Gaier,
Maria
E. Perez-Davis,
and Mark
Date
1990
6. Performing
Organization
Code
8. Performing
Organization
Report
No.
E-5309
Marabito
10. Work
Unit
No.
591-14-41 g.
Performing
Organization
Name
and Address
National Aeronautics and Space Lewis Research Center Cleveland, Ohio 44135-3191 12.
Sponsoring
Agency
Name
National Aeronautics Washington, D.C. 15. Supplementary
James
16.
"x.
Contract
or Grant No.
13. Type
of Report
Technical
and Address
and Space Administration 20546-0001
14. Sponsoring
and Period
Covered
Memorandum Agency
Code
Notes
R. Gaier
University,
\
11.
Administration
and Maria
Cleveland,
E. Perez-Davis,
Ohio
NASA
Lewis
Research
Center;
Mark
Marabito,
Cleveland
State
44115.
Abstract
It is well documented that Mars is totally engulfed in huge dust storms nearly each Martian year. Dust elevated in these global dust storms, or in any of the numerous local dust storms could settle on photovoltaic surfaces and seriously hamper photovoltaic power system performance. Using a recently developed technique to uniformly dust simulated photovoltaic surfaces, samples were subjected to Martian-like winds in an attempt to determine whether natural aeolian processes on Mars would sweep off the settled dust. The effects of wind velocity, angle of attack, height off of the Martian surface, and surface coating material were investigated. Principles which can help to guide the design of photovoltaic arrays bound for the Martian surface were uncovered. Most importantly, arrays mounted with an angle of attack approaching 45*_sh6w the most efficient clearing. Although the angular dependence is not sharp, horizontally mounted arrays required significantly higher wind velocities to clear off the dust. From the perspective of dust clearing it appears that the arrays may be erected quite near the ground, but previous studies have suggested that saltation effects can be expected to cause such arrays to be covered by sand if they are set up less than about a meter from the ground. Providing that the surface chemistry of Martian dusts is comparable to our test dust, the materials used for protective coating may be optimized for other considerations such as transparency, and chemical or abrasion resistance. The static threshold velocity is low enough that there are regions on Mars which experience winds strong enough to clear off a photovoltaic array if it is properly oriented. Turbulence fences proved to be an ineffective strategy to keep dust cleared from the photovoltaic surfaces.
17.
Key Words
(Suggested
Mars; Dust; Durability
19.
18. Distribution
by Author(s))
Dust removal;
Photovoltaic
(of this
report)
20. Security
Unclassified NASA
FORM
1626
OCT
Unclassified-
surfaces;
Subject
Security Classif.
86
*For
Statement
Classif.
Unlimited
Category
(of this page)
20
21. No. of pages
Unclassified sale
by the
National
Technical
Information
Springfield,
Virginia
Price*
A03
18 Service,
22.
22161
1,0 1.0
Io
Io
I
0,6
o[]
o
0
0
m
I
I
I
I
I
I
o 0.6 0.4
HEIGHT,
0.4
I
i
u 8
l,,-
I
0.8
m
n
0.8
o
I
m
I
m
O
u
0 cm
0
m
02 m
0 -20
o 50 [3 3
0.2
m
I
I
]
I
I
0
20
40
60
80
m
lOO
I
I
I
1
I
I
I
1
l
1
2
3
4
5
6
7
8
9
ANGLE TO WIND, deg
HEIGHT FROM FLOOR, cm
Figure 6. - Dust clearing at different heights trom wind tunnel floor.
Figure 7. - Dust clearing in boundary layer at 55 m/s.
1.0 0.8 _"
o O
LAMINAR FLOW TURBULENT FLOW
0.6
0A
F,,i'
O
0.2
m
sa
0 -0.2
_
8
I
I
I
I
I
I
I
!
10
20
30
40
50
60
70
80
ANGLE TO WIND, deg Figure 8. - Dust clearing from a 30 m/s wind.
17
1 90
lOO
10
