1. Lecture 7: Power Systems and Thermal Management

2. Power System Structure and Requirements
Electrical Power Subsystem
Energy Storage
Power Source
Power Distribution
Regulation & Control
Typical Requirements
Supply continuous electrical power to s/c loads during mission
Control and distribute electrical power
Handle average and peak electrical load
Provide ac, dc power converters
Protect against failures in the EPS
Suppress transient voltages and protect against faults

3. Power System Design Process
Step
Info. Required
Derived Requirements
1. Identify requirements
Top-level requirements, s/c configuration, mission life, payload definition
Design requirements, average and peak power
2. Select power source
S/c configuration, average load requirements
EOL power required, type of solar cell, mass and area of solar array, solar array configuration
3. Select energy storage
Orbital parameters, average and peak load
Battery capacity required, battery mass, volume and type
4 Identify power regulation and control
Power source selection, mission life, regulation and thermal control requirements
Peak power tracker or direct energy-transfer system, thermal control requirements, bus-voltage quality, power control algorithms

4. Power Sources Power sources
Photovoltaic
Static
Dynamic
Planar
Concentrators
Thermionics
Thermoelectrics
Brayton
Stirling
Rankine
Photovoltaic solar cells convert incident solar radiation directly to electrical energy
Static power sources uses a heat source, typically plutonium or uranium-235 for direct thermal-to-electrical conversion
Dynamic sources also use a heat source – concentrated solar, plutonium-238, or enriched uranium – to produce power via Brayton, Stirling or Rankine cycles

5. Comparison of Power Sources
Design Parameters
Solar Photovoltaic
Solar Thermal Dynamic
Radio-
isotope
Nuclear Reactor
Power range (kW)
Specific power (W/kg)
9 - 15
8 - 10
Specific cost ($/W)
16K – 18K
Hardness to natural radiation
Medium
High
Very high
Stability and maneuverability
Low
Degradation over life
Storage required for eclipse?
Yes No
Sun angle sensitivity
None
Sensitivity to shadowing
Low (with bypass diodes)
Fuel availability
Unlimited
Very low

6. Solar Array Design Process
Determine requirements and constraints
Av. Power needed during daylight and eclipse
Eclipse durations
Design lifetime
Calculate power that must be produced, Psa

7. Solar Array Design Process
3. Select type of solar cell and estimate power output, P0 , with the sun normal to the surface of the cells
4. Determine BOL power production per unit area, taking account of inherent degradation:
And the cosine loss and life degradation:
Elements of inherent degradation
Nominal
Range
Design and assembly
0.85
Temperature of array
Shadowing of cells
1.00
Inherent degradation, Id
0.77

8. Secondary Battery Couple Specific Energy Density
Energy Storage
Primary batteries have higher specific energy densities but cannot be recharged. Thus, they typically apply to short missions.
Characteristics of some secondary batteries:
Secondary Battery Couple
Specific Energy Density
(W-Hr/Kg)
Status
Nickel-Cadmium
25-30
Space-qualified, extensive database
Nickel-Hydrogen
(individual pressure vessel)
35-43
Space-qualified. Good database
(common pressure vessel)
40-56
Space qualified for GEO and planetary
(single pressure vessel)
43-57
Space-qualified
Lithium-Ion
70-110
Sodium-Sulfur
Under development

9. Energy Storage

10. Black Body Radiation Model
A sea of photons is surrounded on all sides by high temperature atoms. These particles randomly absorb or emit photons, permitting all possible energy transitions compatible with conservation of overall energy

11. Black Body Radiation Model

12. Black Body Radiation Model
UV & Vis
Infrared
Microwave
Wien’s law

13. COBE (Cosmic Background Explorer) satellite data precisely verifies Planck’s radiation law

14. Black Body Radiation Model

15. Thermal Equilibrium of an Isolated Body in Space
Electronics

16. Spherical Spacecraft Equations

17. Spherical Spacecraft Equations

18. Putting the Equations to Work: The Preliminary Design Process
Step
Notes
1. Determine requirements and constraints
Identify temperature limits – see Table 11 43, L&W
Estimate electrical power dissipation
2. Find the diameter of a sphere with the
same surface area as the spacecraft
Make first-order estimates assuming an isothermal, spherical spacecraft (using the above equations).
3. Select radiation surface property values
Initially assume white paint with S=0.6 and IR=0.8
4. Compute worst-case hot and cold temp.s
for the spacecraft
Upper limit: Use high-side values of all power input terms
Lower limit: Include only the IR emissions.
5. Compare worst-case hot and cold temp.s
with temp. limits found in step 1.
If worst-case hot temperature is > required upper limit, use a deployed radiator with a pumped-looped system. Otherwise, use body-mounted radiators
6. Estimate required area for body-mounted
radiator.
Use upper temp. limit for radiator temp., assume no heat inputs and max. heat dissipation – see equation 11.23, L&W
7. Estimate radiator temp. for worst-case cold
conditions
Use the area from step 6 and min. heat dissipation

19. The Preliminary Design Process - Continued
Step
Notes
8. If temp. in step 7 is less than the lower
limit, determine heater power required to
maintain radiator at lower temp. limit
Assume radiator temp. is at the lower limit
9. Determine if there are special thermal
control problems
Identify components with narrow temp. ranges, high power dissipation or low temp. requirements. See thermal control options in section , L&W.
10. Estimate subsystem weight, cost and
power.
I f no special problems, use 4.5% of spacecraft dry weight, 4% of the total spacecraft cost, and heater power from step 8.

20. Thermal Control Devices and Strategies - If special thermal control problems are encountered in step 9
Materials and Coatings
Optical Solar reflectors
Silver-Coated Teflon
MultiLayer Insulation
Electrical Heaters
Thermostats
Space radiators
Cold-Plates
Doublers
Phase Change Devices
Heat Pipes
Louvers
Temp. Sensors
Adhesive Tapes
Fillers
Thermal isolators
Thermoelectric Coolers
Cryogenic Systems
Active Refrigeration Systems
Expendable Cooling Systems

21. Thermal Control Devices and Strategies
Materials and Coatings: paints, silverized plastics, special coatings – all with special absorptivity & emissivity values– See Table 11-44
Optical Solar Reflectors (OSRs):
Highly reflective surface mounted on a substrate and overlaid with a transparent coating.
Reflects most incoming radiation back to space, IR emissivity = 0.8, solar absorptivity = 0.15
Expensive and fragile.
Silver-Coated Teflon - Cheaper alternative to OSRs.
MultiLayer Insulation (MLI):
The primary spacecraft insulation device.
Alternate layers of aluminized Mylar or Kapton, separated by net material, e.g. nylon, Dacron or Nomex
See Fig for the effective emmitance of MLI
Electrical Heaters
Used in cold-biased systems to bring selected components up to proper temp.
Thin electrical resister between two Kapton sheets
Typical power densities  1 W/cm2
Thermostats
Switches to turn heaters on/off
Typical operating range: -50 to 1600C

22. Thermal Control Devices and Strategies
Space radiators
Heat exchanger on the outer surface of the spacecraft that radiates waste heat
Can be structural panels or flate plates mounted on the spacecraft
Cold-Plates
Heat dissipated by electrical equipment is conducted across the interface to the cold plate. Fluid circulating through the cold plate Carries the heat to a space radiator.
Heat Pipes
Lightweight devices used to transfer heat from one location to another, e.g. from an electrical component to a space radiator
Temp. Sensors
Thermisters: Semiconductor materials that vary their resistance with temperature. They operate around -50 to C.
Resistance Thermisters: Uses a pure platinum conductor. Very accurate and expensive
Heat in - evaporation
Wicking material
Gas
Heat out - condensation
Liquid flow via wick