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Lecture 7: Power Systems and Thermal Management

Lecture 7: Power Systems and Thermal Management. Electrical Power Subsystem. Power Source. Energy Storage. Power Distribution. Regulation & Control. Power System Structure and Requirements. Typical Requirements Supply continuous electrical power to s/c loads during mission

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Lecture 7: Power Systems and Thermal Management

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  1. Lecture 7:Power Systems and Thermal Management

  2. Electrical Power Subsystem Power Source Energy Storage Power Distribution Regulation & Control Power System Structure and Requirements • 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

  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- 238 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

  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:

  8. 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:

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

  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

  19. The Preliminary Design Process - Continued

  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. 11-22 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. Wicking material Gas Heat in - evaporation Liquid flow via wick Heat out - condensation 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 flat 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 +300 0C. • Resistance Thermisters: Uses a pure platinum conductor. Very accurate and expensive

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