Electrical Power Subsystem Dr Andrew Ketsdever MAE 5595 Lesson 11

Outline Electrical Power Subsystem –Introduction –Types of Power Sources –Design Considerations –Nuclear Power Nuclear Reactor Radioisotope Thermoelectric Generator (RTG) –Solar Arrays Types Sample Calculation: Array sizing –Batteries Types Sample Calculation: Battery storage Depth of Discharge –Degradation of Solar Arrays –Radiation

Introduction

Power Sources Chemical: Battery, Fuel Cell –Converts chemical energy into electrical power Stored energy Chemical reaction Solar: Photovoltaic –Coverts solar radiation into electrical power Nuclear: Fission –Coverts nuclear fission energy into electrical power through conversion of heat Dyanamic: Heat energy –Stirling, Rankine, Brayton cycles (15-35% efficient) We will mainly talk about solar array and battery systems

Power Sources From Space Vehicle Design, by Griffin and French

Design Considerations

Nuclear Reactor

Reactor Schematic

Nuclear Reactor Reflector –Reflects neutrons produced in the reaction back into the core –Prevents neutron leakage –Maintains reaction balance –Can be used to reduce the size of the reactor –Typically made of Beryllium

Nuclear Reactor Moderator –Slows down neutrons in the reactor –Typically made of low atomic mass material LiH, Graphite, D 2 O H 2 O absorbs neutrons (light water reactor) Slow (or Thermal) Reactor –Uses moderator to slow down neutrons for efficient fissioning of low activation energy fuels Fast Reactor –No moderator. Uses high kinetic energy neutrons for fissioning of high activation energy fuels

Nuclear Reactor Fuel Element –Contains the fissile fuel –Usually Uranium or Plutonium –Contains the propellant flow channels High thrust requires high contact surface area for the propellants Heat exchange in the flow channels critical in determining efficiency and performance of the system

Nuclear Reactor Control Rods –Contains material that absorbs neutrons Decreases and controls neutron population Controls reaction rate When fully inserted, they can shut down the reactor –Configuration and placement is driven by the engine power level requirements –Typically made of Boron –Axial Rods Raised and lowered into place. Depth of rods in the reactor controls the neutron population –Drum Rods Rotated into place with reflecting and absorbing sides

Fission Fission is a nuclear process in which a heavy nucleus splits into two smaller nuclei –The Fission Products (FP) can be in any combination (with a given probability) so long as the number of protons and neutrons in the products sum up to those in the initial fissioning nucleus –The free neutrons produced go on to continue the fissioning cycle (chain reaction, criticality) –A great amount of energy can be released in fission because for heavy nuclei, the summed masses of the lighter product nuclei is less than the mass of the fissioning nucleus

Fission Reaction Energy The binding energy of the nucleus is directly related to the amount of energy released in a fission reaction The energy associated with the difference in mass of the products and the fissioning atom is the binding energy

Defect Mass and Energy Nuclear masses can change due to reactions because this "lost" mass is converted into energy. For example, combining a proton (p) and a neutron (n) will produce a deuteron (d). If we add up the masses of the proton and the neutron, we get –m p + m n = u u = u –The mass of the deuteron is m d = u –Therefore change in mass = (m p + m n ) - m d = ( u u) - ( u) = u –An atomic mass unit (u) is equal to one-twelfth of the mass of a C-12 atom which is about 1.66 X kg. So, using E=mc 2 gives an energy/u = (1.66 X kg)(3.00 X 10 8 m/s) 2 (1eV/1.6 X J) which is about 931 MeV/u. So, our final energy is 2.24 MeV. The quantity 2.24MeV is the binding energy of the deuteron.

Radioisotope Thermoelectric Generator (RTG) Heat released by radioactive decay is converted into electrical energy Half-life of the radioactive material must be long enough to insure a relatively constant power level Half-life must be short enough to insure enough power is produced US uses Pu-238 –86.8 yr half-life –0.55 W/g

Radioactivity In 1899, Ernest Rutheford discovered Uranium produced three different kinds of radiation. –Separated the radiation by penetrating ability –Called them   -Radiation stopped by paper (He nucleus, )  -Radiation stopped by 6mm of Aluminum (Electrons produced in the nucleus)  -Radiation stopped by several mm of Lead (Photons with wavelength shortward of 124 pm or energies greater than 10 keV)

 -Particle Decay The emission of an  particle, or 4He nucleus, is a process called  decay Since  particles contain protons and neutrons, they must come from the nucleus of an atom

Ulysses RTG Pu-238 Decay Branch leads mostly to the emission of  -particles –Easily shielded kg 4400 W or heat P BOL = 285 W P EOL = 250 W Efficiency ~ 6.5 %

Solar Arrays A solar array is an assembly of individual solar cells connected to provide direct current power –Power range: Few W to 10kW –First array launched on Vanguard 1 in 1958 Certain wavelengths of light are able to ionize silicon atoms An internal field is produced by the junction separates some of the positive charges ("holes") from the negative charges (electrons) The holes are swept into the positive or p-layer and the electrons are swept into the negative or n-layer Most can only recombine by passing through an external circuit outside the material because of the internal potential energy barrier.

Solar Flux –Maximum solar energy flux (normal to solar beam) variation is quite significant at Earth orbit, between 1422 W/m 2 at perihelion to 1330 W/m 2 at aphelion, a 6.7 % annual change –Typically a value of 1358 W/m 2 is used

Eclipse LEO –Once per orbit typically, except high inclinations GEO –Equatorial plane is 23.5º inclined relative to the ecliptic plane –Two eclipse “seasons” centered around equinoxes 45 days Longest eclipse of about 70 minutes

Eclipse

Configuration

Deployables

Cylindrical –Projected area of spinner is 1/  of surface area of cylinder sides Must account for orientation with respect to the sun Solar Array Configurations

Omnidirectional –Equal projected area from any direction (sphere) –Used by many small-sats or low power S/C (attitude doesn’t effect power generation) –Projected area is ¼ of total surface area Solar Array Configurations

Inherent Degradation – loss of power from perfect case Shading of cells Temperature differential across solar array Real estate required for connections between cells Solar Array Configurations

Solar Array Design What solar cell material we choose Considerations: – Efficiency – Cost – Lifetime (radiation hardness) – Operating temperature – Ease of manufacturing (lay-up panels) – … Choice is application specific

Solar Array Design

From Spacecraft Systems Engineering, by Fortescue and Stark Solar Array Design

Solar Array Characteristics

EPS—Design Effect of Temperature on Solar Cell Performance From Space Vehicle Design, by Griffin and French

Solar Array Characteristics From Space Vehicle Design, by Griffin and French

P sa = power generated by solar array P e and P d = S/C power loads during eclipse and daylight T e and T d = times each orbit spent in eclipse and daylight X d = efficiency getting power from S/A directly to loads (typically is 0.85) X e = efficiency getting power from S/A to charge battery and then from battery to the load (typical value is 0.65) (1&2) Calculate power output of Solar Arrays EPS—Design Solar Array Design Process

P o = power density output for cells (watts/m 2 ) –Flux (or P i ) = input solar power density (watts/m 2 ) –  (or  ) = efficiency of solar cell material P BOL = power density S/A’s generate at beginning of life (watts/m 2 ) P EOL = power density at end of life (watts/m 2 ) I d = inherent degradation  = sunlight incidence angle (3&4) Determine size of arrays needed to generate power EPS—Design Solar Array Design Process

P EOL = power density generated at end of life (watts/m 2 ) L D = lifetime degradation A process will be defined later in this lecture for determining L d (5) Account for degradation due to exposure to the space environment EPS—Design Solar Array Design Process

(6) Find size of solar array needed at end of life Substituting in previous equations: EPS—Design Solar Array Design Process

Solar Array Design Process Example Problem…

Energy Storage: Batteries

Primary Batteries

Secondary Batteries

Equation for battery capacity: C r = total S/C battery capacity P e = average eclipse load (watts) T e = eclipse duration (hr) DoD = depth of discharge (0  DoD  1) N = number of batteries (need at least two if want some partial redundancy) n = transmission efficiency between battery and load (typical value is 0.9) EPS—Design Battery Design Process

From Space Vehicle Design, by Griffin and French Battery Design Process

Quantifying Solar Array Degradation Big Picture: Trying to go from environment to performance From Spacecraft Systems Engineering, by Fortescue and Stark and NASA JPL Pub 96-9, GaAs Solar Cell Radiation Handbook New values for P max, V mp, I mp

Effect of Charged Particles on Solar Cells High energy protons & electrons collide with the crystal lattice structure Collisions displace atoms from their lattice sites Eventually, the displaced atoms form stable defects Defects change the propagation of photoelectrons in the lattice structure

Radiation Shielding Coverglass tends to protect solar cells (from solar and physical handling), but adds weight to design Depending upon exact environment, more coverglass can actually cause more damage to cells (because of high damage caused by lower energy protons embedding in lattice) Find optimum coverglass thickness for orbit – tedious calculations Assumptions/material properties: Ratio of the solar cell coverage area to solar panel area: about 0.85 Areal density of the solar array (before adding coverglass): about g/cm 2 Density of coverglass: about 2.2 g/cm 3

Damage Equivalency: Electrons Solar cells damaged when struck by 1 MeV electrons –Solar cells can be tested easily to characterize the effects of radiation by using a stream of 1 MeV electrons (damage vs number of electrons) Strategy: compare the damage done by a particle at a particular energy level (E) to the damage done by a 1 MeV electron. –Relationship is captured by damage coefficient [D(E,t)] –Function of the energy of the particle and the thickness of the protective shield in front of the solar cell (cover slide) Equivalency allows the damage done by all electrons to be “Normalized” to equivalent 1 MeV electrons

Damage Equivalency: Electrons Particle energy distribution chart may be broken into ‘bins’ of energy levels The particles from each bin cause a certain level of damage – equivalent to some number of 1 MeV electrons The total degradation (damage) to the arrays may be found by summing the equivalent # 1 MeV electrons and reading experimental performance charts

Damage Equivalency: Protons Equivalency also allows damage done by protons to be “Normalized” to equivalent 1 MeV electrons –For Electrons: Table of damage coefficients converts the damage done by one electron at an energy E to the damage done by a number of equivalent 1 MeV electrons AND for various solar cell cover slide thicknesses –For Protons: Similar table converts protons to equivalent 10 MeV protons…BUT…The 10 MeV protons are then converted to equivalent 1 MeV electrons: (i.e. damage to Voc from one 10 MeV proton equals damage from MeV electrons) ParameterFactor P max 1000 V oc 1400 I sc 400

Solar Cell Performance: Max Power From NASA JPL Pub 96-9, GaAs Solar Cell Radiation Handbook

Quantifying Solar Array Degradation: Process Break up total particle environment into energy bins Equivalent # 1 MeV electrons/cm 2  sec Equivalent # 10 MeV protons/cm 2  sec Damage equivalency

Simplified Approach Process Equivalent # 1 MeV electrons/cm 2  sec due to electrons (solar max & min) Damage equivalency Equivalent # 1 MeV electrons/cm 2  sec due to protons (solar max & min) Total Equivalent no. of 1 MeV electrons/cm 2  sec (solar max & min) Add Select worst case (solar max or min) Integrate over lifetime Total Equivalent no. of 1 MeV electrons/cm 2  sec Total # 1 MeV electrons/cm 2 Power out/cm 2 +

Simplified Approach Example From NASA JPL Pub 96-9, GaAs Solar Cell Radiation Handbook

Simplified Approach Example From NASA JPL Pub 96-9, GaAs Solar Cell Radiation Handbook

max min same From NASA JPL Pub 96-9, GaAs Solar Cell Radiation Handbook Simplified Approach Example

Solar maxSolar min Electron fluence (e/cm2/yr) 3.24E E+12 Proton fluence (e/cm2/yr) 2.03E+15 Total (e/cm2/yr) 2.033E E+15 Worst case annual fluence is: 2.033E+15 (e/cm2/yr) Multiply by number of years for the mission Then use the appropriate chart in GaAs Handbook to figure area needed for solar panel For 0 , 3000 nmi orbit with 30 mils coverglass, annual fluences are: EPS—Special Topics Simplified Approach Example (cont’d)

Solar Cell Performance: Normalized Max Power From NASA JPL Pub 96-9, GaAs Solar Cell Radiation Handbook