Transcript Document

Space Based Solar Power
Satellite Conceptual Design
for Retrodirective Control
Space Engineering Institute
Spring 2009
Overview
• Creating a satellite module that will be
attached to a Japanese experimental satellite
in a Low-earth orbit.
• Our team objective is to create a satellite
module that can test the retrodirective beam
control method of sending microwave power
back to Earth.
• The module must provide its own power, and
have its own thermal management systems.
Module System
Sandwich Design
Space
Environment
& Orbit
Thermal
Management
Photovoltaic
Cells
Structures &
Materials
Energy Storage
& Power
Conversion
Retrodirective
Control logic
Antenna Array
Environment Analysis
Semester Goals and Expectations
• Become familiar with Satellite Tool Kit software
• Model a cubic satellite and evaluate solar energy
collected at each of its six faces to identify the
optimum location for the solar panels
• Determine the maximum, minimum, and average
solar energy collected on the optimum location during
one orbit
Satellite Orientation
•Orange-North
•Green- South
•White- Nadir
•Yellow-Zenith
•Purple-Leading
•Teal- Trailing
Satellite Orbit Options
•
•
Geostationary
• Altitude: 35,786 km
• Inclination: 0º
Low Earth Orbits
– Critically Inclined Sun Synchronous:
• Perigee altitude: 400 km
• Retrograde inclination: 116.565º
– Circular:
• Altitude: 500 km
• Inclination: 45º
– ETS-VII: Japanese satellite with similar initial conditions
• Altitude: 550 km
• Inclination 35º
Energy and Power Received
•
•
Power α cos(θ)
– θ= angle between the sun vector
and the vector pointing normal to
the face
– Units: [W/m2]
Energy=Power*Time
– Units: [J/m2]
– Also dependent on the cosine of θ
http://solar.mridkash.com/wp-content/uploads/cosine-law.jpg
Approach
• STK provides angular data for each face of the cubic
satellite
• Angular data are converted to power [W/m^2]
• Power data are converted to energy data [J/m^2]
Data from STK
Solar Flux (W/m^2)
Data from STK
Solar Flux (W/m^2)
1600
1400
1200
1000
800
600
400
200
0
Date
The Zenith, Nadir, Leading and Trailing faces
have approximately the same exposure to the
sun. The antenna will be located on the Nadir
face, so it reasonably follows that the solar
panels be placed on the Zenith face, directly
opposite the antenna.
A circular orbit with an altitude of 500 km and
inclination of 45° was chosen, because with
the options available, this orbit allows the solar
panels to receive the most sunlight.
Average Solar Flux: One Year
385.75 W/m2
Average Solar Flux: In Sunlight
833.83 W/m2
Minimum Solar Flux:
0 W/m2
Maximum Solar Flux:
1366 W/m2
Thermal Management
Thermal Management
Objective
• To perform thermal analysis of the satellite and
ensure a suitable operating environment for the
payload.
• Tools
– Thermal Desktop software
Research Topics
• Low earth orbit environment
• Temperature requirements for internal components
• Cooling/heating methods
External Environment
In LEO, the satellite will be heated by:
• Direct sunlight
• Earth’s albedo
• Earth’s IR emittance
The total heat absorbed by the satellite will not remain
constant. Fluctuations occur due to:
• Entering/exiting Earth’s shadow
• Varying surface conditions on Earth
Satellite Interior
The interior environment of the satellite must be kept at
a proper temperature range. Most electronic equipment
onboard must operate in a surrounding temperature
range of 0 to 50 degrees Celsius.
Factors to consider for the internal energy balance:
• Fluctuating external heat rates
• Heat released by electronic equipment
-Low level baseline operation
-High level during periodic transmission
• Thermophysical properties of structural material
Cooling/Heating Methods
External
• Radiators: Do not require energy. Release heat without re-entry
(thermal diode)
Internal
• Thermoelectric Coolers/Heaters: Require energy. Can
absorb/emit heat by reversing polarity
• Mechanical cooling: Expander, compressor, or heat
exchanger. Takes up space and weight.
• Resistive Heating: Requires energy but elements are compact
in size.
• Heat Pipes: Passive
Thermal Desktop
Objectives
• Develop a model for the satellite module.
• Use the orbital information from STK to determine thermal
environment of the satellite.
Progress
• In process of creating models.
Thermal Desktop, Continued
Example of absorbed flux from sun, earth’s
albedo and IR emittance.
Materials and Structures
Structural Requirements
The satellite must have ability to:
• Withstand launch loads
• Provide desired rigidity
• Protect sensitive payload components from
extreme temperatures.
Material Selection
Currently evaluating two different materials:
Ti6Al4V Titanium alloy VS.
Aluminum Alloy( 7075T651)
Although Titanium is 60%
heavier than Aluminum, it is
over twice as strong.
Possibility of having titanium
based honey comb exterior;
joined by a smaller portion
of aluminum interior.
Materials
Properties
Titanium
(Ti6Al4V)
Aluminum
Alloy( 7075T651)
Units
Density
4.43
2.81
g/cm3
Tensile
Strength
880
572
MPa
Thermal
Conductivity
6.7
130
W/m.K
Modulus of
Elasticity
114
71.7
GPa
Thermal
Expansion
8.6
23
*10-6/ºC
Weight Comparison
Thickness
of
Natural
Titanium
Frequency( (Ti6Al4V)
hertz)
panel
(cm)
Thickness of
Aluminum
Weight
Weight (per
Alloy( 7075(per unit
unit area) Ti
T651) panel
area) Al
(kg/m*s^2)
(cm)
(kg/m*s^2)
100
1.413
1.528
613.99
421.20
200
2.826
3.056
1227.97
842.40
300
4.238
4.584
1841.96
1263.61
400
5.651
6.112
2455.95
1684.81
Honeycomb Layer
Planned use of
“Honeycomb” design:
•Hexagonal Structure
• Uses the least amount of
material to create a lattice
of cells within a given
volume
• Maintains strength
Preliminary Sandwich Structure
Layered design that takes
advantage of each materials
different thermal properties.
Energy System
Goal & Requirements
•
Collect Solar Energy and store it to power the RF amplifiers
– Collect power from 1m2 solar panel.
– Store energy in a medium that can withstand high drain current.
– Energy storage mediums must have a wide operating temperature
range.
DC to RF Converter Options
Tube magnetron @ 5.8 GHz
• Can output 650W with 65% efficiency
• Heavier than solid state options
– (1.1kg vs .6g)
• Produces more heat than solid state converters
• Requires a high voltage power supply
to excite the electrons.
GaN HEMT solid state converters @ 5.8 GHz
• Fujitsu converter can output 320W theoretically
• Cree converter can output 35W, commercially
available now.
• Lightweight (.6 g) and extremely small size
relative to the magnetron.
Energy Storage – Li-ion
• For storing energy from the photovoltaic cells Liion and Li-S batteries are being considered.
– Li-ion batteries have an energy density of 110 Wh/kg.
– Saft MPS space series batteries that are already
thermally insulated and autonomously heated.
– Have a wide operating temperature range ( -5o F to
140o F for charging and -40o F to 140o F in operation)
– Built in over current and charging circuits into the
module.
– 17 Ah capacity per battery @ 28V.
Energy Storage - Continued
• Lithium sulphur batteries are being considered
for their higher energy density (350 Wh/kg vs the
110Wh/kg for Li-Ion)
– Experimental, expensive technology.
– No history of satellite use.
• Ultracapacitors
– High energy density capacitor used for powering the
Solid state microwave converters when transmitting a
signal.
– Ultracapacitors can handle 20A continuous current.
– Will be used in conjunction with the Li-ion batteries to
power the GaN HEMT amplifiers at their maximum
capacity
Current Concept
• Maximum Power Point Tracker monitors the voltage and current
of the Solar Panel and tracks the peak point on the power curve.
• Battery Management System tracks the charge rate, voltage and
current.
Antenna and Retrodirective
Control
Retrodirective Beam Control
• The implementation of retrodirective beam
control is critical to accurate beam pointing,
as well as the overall safety of the system.
• The key objective is to have the power beam
of the solar power satellite’s transmitter
pointed only in the direction of a received
pilot beam, which provides a phase reference
• Retrodirective beam control ensures that
microwave power transmission is both safe
and insusceptible to accidental misalignment.
Proposed Retrodirectivity
Method
• The 2.9 GHz incoming pilot signal is
received at a Frequency of ω1 and
Phase φ1
• To conjugate, the received signal is
next mixed with a reference source
of Frequency 2ω1 and Phase φref
• The conjugated signal is then mixed
itself to produce a signal with
Frequency 2ω1 and Phase -2φ1
• After conjugated and doubled, the
signal is transmitted from a different
transmitting subarray
• The complete phased array
transmits a 5.8GHz beam in the
direction of the incoming pilot signal
Proposed Phased Array Antenna
Concept
• Linear Microstrip Patch
Phased Array Antenna
• The Microstrip Patch Antenna
will operate at a Frequency of
5.8 GHz, and will have an
Input Impedance of 50Ω
• The Antenna’s design
features a 4x4 Phased Array
consisting of 15 Transmitting
Elements and 1 nested
Receiving Element each
spaced 0.5λ apart
• The 4 subarrays are
expected to be at different
phases prior to power
transmission
5.8 GHz 4x4 Linear Microstrip Patch Phased
Array with nested 2.9 GHz Receiving Element
Antenna and Transmitter
Interface
• Magnetron
• Solid-State
‒ RF power is split to feed
fed to each antenna
subarray
‒ Negligible power loss
may occur during
energy feed
‒ Loss expected from
phase shifter
‒ Facilitates electronic
beam steering
‒ Power amplifier and
phase shifter are placed
behind each
transmitting element
‒ Microwave filters are
required to countervail
amplifier-spawned
noise
Advantages of Proposals
Microstrip Patch Antenna
Retrodirectivity Method
• Advantages
• Advantages
‒ Low cost to manufacture
‒ Light weight and low
profile
‒ Supports both Linear and
Circular Polarization
‒ Conjugates pilot signal
directly at RF
‒ Reduction in the number of
electronics per antenna
subarray
‒ Less power consumption