Possible meteorological effects arising from the existence and operations of a solar power satellite (SPS) system rectenna are examined. Analysis and model simulations in some chosen site situations and meteorological conditions indicate that the meteorological effects of the construction and operation of a rectenna are small, particularly outside the boundary of the structure. From weather and climate points of view, installation of an SPS rectenna seems likely to have effects comparable with those due to other nonindustrial land use changes covering the same area. The absorption and scattering of microwave radiation in the troposphere would have negligible atmospheric effects

Lee, J.

English

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_- _ RECTENNA-RELATED ATMOSPHERIC EFFECTSJ. Lee, Energy and Environmental Systems Division
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i. INTRODUCTION
The primary interest in the assessment of possible environmental effects
arising from the existence and operations of the satellite power system (SPS)
rectifying antennas (rectennas) is meteorological in nature. The presence of a
rectenna covering an area of approximately 100 km 2 would be expected to alter
the aerodynamic characteristics of the surface in its immediate vicinity. A
change in surface roughness affects the vertical fluxes of momentum and thermal
energy, and a change in radiative properties (albedo and emissivity) affects
the surface energy budget. The operation of a rectenna would add an additional
heat source at the surface. The consequences of these changes would be ex-
pected to alter the wind velocity profile and stability of the planetary bound-
ary layer and hence to alter the local cloud population.
The possible influence of microwave transmission through the troposphere
would be due to the absorption, eapeclally in clouds, of microwave energy along
the beam path, causing local heating. On the other hand, the presence of con-
vective or turbulent air motions and the existence of hydrometers cause refrac-
tion, scattering , and absorption of microwave and can lead to beam wanderlng
and spreading. The scope of this discussion is limited to the effect of SPS on
tropospheric atmosphere. Many issues of concern regarding the effects of
atmospheric conditions on SPS beam propagation can be found in the proceedings
of a workshop held in August, 1978, * and will not be discussed here.
2. PREVIOUS STUDIES
A preliminary assessment based upon the maxlmummlcrowave-beam power
density of 230 W/m 2 and an a_erage waste heat release rate of 7.5 W/m 2 from a
rectenna covering approximately i00 km 2 was conducted in 1977 by the Johnson
Space Center. 2 The findings were that the effects of an SPS rectenna on
weather and climate would be small compared to the direct environmental con-
sequences of construction, and that the rectenna's influence would be similar
to that of an average suburban development. Microwave heating of the lower
atmosphere through gaseous absorption would be negligible. Any actual effects
of microwave heating inside a cloud would not be detected in the presence of
the natural variance of cloud and storm phenomena. Scattering by particles,
even in heavily-polluted atmosphere, would also be negligible.
3. CONSENSUS OF AUGUST 1978 WORKSHOP 1
The above study was reviewed at the August 1978 workshop and the con-
clusions were updated. Three main topics were discussed: the effects of
waste-heat release on the atmosphere at the rectenna site; microwave inter-
actions with the atmosphere; and the possible effects of the microwave beam on
atmospheric electrification processes. The following brief summary highlights
the most important issues.
a. Rectenna Waste Heat and Structure. Construction of a rectenna would
modify the thermal and radiative properties of the ground on which it is built;
operations would introduce a heat source at the surface. Although tb_ magnitude
of the perturbation of the average surface heat budget would be on the order of
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10%, microwave beam wandering and spreading due to atmospheric refraction may
occasionally give rise to larger effects.
It is possible to investigate the effects of the rectenna by studying
the effects of land-use changes. Small temperature changes (of the order of
I°C) can be expected under light wind conditions. Changes in cloud populations
can also be expected. Somewhat larger man-made dissipation rates over compar-
able areas have been associated with apparent anomalies in the distribution of
rainfall.
In hilly terrain, on scales smaller than the rectenna dimensions, di-
urnally varying changes occur in the surface energy budget that are larger than
the projected rectenna waste heat. It is therefore expected that the meteoro-
logical effects of a rectenna would vary from site to site, and the central
maximum heat dissipation (approximately 16 W/m 2) might become important in
augmenting a naturally occurring topographic effect.
Assessment of possible weather and climate effects over areas larger
than the mesoscale should not be confined to the influence of the rectenna
alone -- it is necessary to consider the whole satellite power system in the
context of the energy demand it is designed to meet. The overriding feature of
the system is that the major inefficiency, the rejection of waste heat, is in
space. Furthermore, there are no significant emissions of material into the
troposphere during operation.
b. Microwave Propasation. The atmospheric absorption of microwave
energy at the proposed SPS frequency is negligible in clear air for the pro-
Jected tropospheric path lengths of about 20 km. However, some absorption by
condensed water (clouds and precipitation) would occur when storms entered the
beam path.
c. Atmospheric Electricity. Direct interactions with the atmospheric
electric fields are not thought to be important at the proposed frequency.
However, the mere physical presence of the rectenna might have some modifying
influence on the occurrence and electrical behavior of thunderstorms over and
around the rectenna.
4. MODEL CALCULATIONS
In order to examine further the rectenna effects on local meteorological
variables, a trial simulation using a three-dimensional, turbulence-closure
model S was made for a daytime, planetary-boundary-layer condition (constant
potential temperature up to 650 m in height, then increasing with height at a
rate of 3.5@C/km) with logarithmic wind profile up to 4 m/s and remaining con-
stant above that. It was found that the increased roughness over the rectenna
would considerable increase the surface heat flux (by a factor of 3.5) and
friction velocity (by a factor of 1.9) at the center of the site in comparison
with values located at the upstream boundary. The numerical values given in the
parentheses are valid for the case of dry convection and little temperature
contrast between the rectenna and the surrounding surface. Inclusion of 8 W/m 2
of waste heat would cause a surface temperature perturbation of less than 0.1°C.
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A more realistic simulation was performed for a potentially unstable
boundary layer with light winds over moist, flat ground. 4 Such a situation is
conducive to the natural formation of cumulus clouds without precipitation. The
simulation indicated that, excluding the effects of albedo changes, the major
cause of the perturbation is again the change in surface roughness rather than
the release of waste heat. Air and soll temperature decreased during the day-
time and increased only marginally at night. The increased mechanical mixing
resulted in increased evaporation and absolute humidity, increased cloud amount,
and decreased cloud-base height. The decrease in solar radiation resulting from
the increase in cloud amount is greater than the waste heat term. Cloud modifi-
cation would be expected to be quite different if the roughness had not been
changed. The results found in this case (moist convection) are considerably
different from those in the dry convection case above. A preliminary analysis
of the problem of rectenna albedo and its diurnal variation indicated that dif-
ferences between the rectenna albedo and that of the surrounding surface may be
most significant factor. 4 These effects need to be quantified in future work.
Information regarding the amount of microwave absorption per unit path
length as a function of rainfall rate is available. 5 With the most extreme
rainfall rate of 254_n/hr as an example, the attenuation at the proposed 2.45-GHz
frequency is estimated to be about 0.063 dB/km. At the proposed maximum power
density of 230 W/m 2, the absorbed microwave power inside the storm would be
approximately 3.2 x i0-3 W/m S, which is approximately two orders of magnitude
smaller than the release rate of the buoyant energy of a typical cumulus cloud.
It is reasonable to conclude that the absorption of SPS microwave power by a
storm will have no significant influence on cloud dynamics and thermodynamics
and the associated precipitation.
5. CONCLUSIONS
Analyses and model simulations in some chosen site situations and meteoro-
logical conditions indicate that the meteorological effects of the construction
and operation of a rectenna are small, particularly outside the boundary of the
structure. From weather and climate points of view, installation of an SPS rec-
tenna seems likely to have effects comparable with those due to other nonindust-
rial land-use changes covering the same area. The absorption and scattering of
microwave radiation in the troposphere would have negligible atmospheric effects.
It seems clear that rectenna-related meteorological effects are not a
critical factor in the overall environment impact of the SPS. However, there
are some remaining areas of detail that should be investigated; they are con-
cerned with the radiative properties of the rectenna structure, the possible
"triggering" of convective instability u_der certain meteorological conditions,
and the nature of the terrain at and near the structure.
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6. REFERENCES
1. Lee, J., orsanlzer, "Proceedings of the Workshop on Ideteorologlcal
Effects of Satelllte Power System Rectenna Operation and Related Microwave
Transn_Lsslon Problems," CONF-7808114, Office of Energy Research, U.S. Dept. of
Energy, Washington, DC 20545, Dec. 1979.
2. Allen, N.C., "Preliminary Study of Possible Weather Effects Due to
Solar Power Satellite Rectenna Operation," Internal Note No. JSC-12519, Johnson
Space Center, National Aeronautics and Space Administration, Houston, TXp 1977.
3. Yamada, T., "A Three-dimenslonal, Second-order Closure Numerical
Model of Mesoscale Circulation in the Lower Atmosphere," Argonne National Labora-
tory, Report No. ANL/EER-78-1, 1978.
4. Robinson, G.D., The Center for the Environment and Man, Inc.,
unpublished information, Jan. 1980.
5. Skolnlk, M.I., ed., "Radar Handbook," McGraw-Hill Book Co., New York,
NY, 24-1 to 24-40, 1970.
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Rectenna related atmospheric effects

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