Power Systems Specification Sheet

Greg Merritt
gmerritt@mail.utexas.edu
John Opiela
opiela@ccwf.cc.utexas.edu
August 1995

Requirements

The electrical power subsystem provides power generation, storage, distribution, regulation, and control. The system will provide power for average and peak load periods. The system must allow operation of the science sensors, and transmission of their data, for a minimum of ten hours after lunar sunset. Additional data could be gathered by telemetering the power system status to Earth. This would allow better characterization of the system's degradation in the lunar environment. The design should preserve the option of continuing operations for more than one lunar day. Also, the design should incorporate a power budget, scheduling individual loads throughout the mission to prevent the combined loads from exceeding the power supply.

Information Interface

The design and operation of the power system requires information from the other subsystems. The power system must also send information to the other systems, as well as the required electrical power.

Information from Subsystem	Subsystem	Information to Subsystem
Power requirements (actuators).	Structure	Mass. Maximum launch loads. PV array position req.
Power requirements by mission phase.	Thermal Control	Energy dissipation. Thermal limits. PV array position.
Power requirements by mission phase	Navigation, Guidance and Control	PV array pointing requirements
Power requirements by mission phase	Propulsion
Power requirements by mission phase. Load scheduling	Command and Data Handling (Computers)	Load scheduling. Subsystem status
Power requirements by mission phase. Ground-control direct commands	Communication
Power requirements	Oxygen Plant
Power requirements by mission phase	Science Sensors

Power System Sizing Example

The power system is designed to meet the unique requirements of the mission. The properties of the system can be estimated using a standard algorithm and the properties of the selected components. Sample sizing calculations for a low-Earth orbit are shown below.

Step	Reference (Wertz and Larson)	Examples
1. Determine requirements and constraints for power subsystem solar array design. a. Avg. power required during daylight & eclipse b. Orbit altitude and eclipse duration c. Design lifetime. (Oversize the beginning of life (BOL) power to compensate for end of life (EOL) degradation; solar is poor for missions over 10 years.)	10.1, 10.2 (Input parameter) Input parameter Input	500 W during daylight & eclipse 800 km; 35.1 min. 10 years
2. Calculate amount of power that must be produced by the solar arrays, Psa	Psa=((PeTe/Xe)+(PdTd/Xd))/Td Pe and Pd are the power requirements during eclipse and daylight. Xe and Xd are the efficiency of the paths from the arrays to the batteries and the loads (typically Xe = 0.6 and Xd = 0.8). Te and Td are the times in eclipse and daylight.	Pe = Pd = 500 W Te = 35.1 min Td = 65.9 min Xe = 0.6, Xd = 0.8 Psa = 1069 W
3. Select type of solar cell and estimate power output with sun normal to surface of cells	Si: Po = 0.141358 W/m^2 = 190 W/m^2 GaAs: Po = 0.181358 = 244 (0.14 and 0.18 are the efficiencies for the cells; 1358 is the solar flux at Earth distance)	If we choose Si, Po = 190 If we choose GaAs, Po = 244
4. Find BOL and EOL power, per unit of array. If we configure the s/c well, its appendages will shadow few cells and shadowing losses will be slight. (Shadowing causes temp. variations and degrades the cells.) For Si cells, the ref temp is about 28C. Efficiency falls about 0.5% per degree above 28C.	Table 11-35 Eq. 5-6 PBOL = PoIdcos q	Id = degradation=0.77 q = sun incidence angle = 23.5 degrees PBOL = 134
5. Determine EOL power for the array. Radiation can cause a great amount of damage. Si cells protected by coverslides lose 15% of their voltage and current when exposed to 10^15 MeV for 4 to 5 years.	Degradation for Si = 3.75% per year, GaAs = 2.75% per year. Life degradation is given by Ld = (1-degrad/yr)^sat life PEOL = PBOLLd	For Si, Ld = 0.68 over 10 yr mission. For GaAs, Ld = 0.76 PEOL = 91.4 for Si PEOL = 102 for GaAs
6. Estimate solar array size	Asa = Psa/PEOL	For Si, Asa = 11.7 m^2; for GaAs Asa = 10.5 m^2
7. Estimate mass	Mass = 0.04*Psa	Mass= 42.8 kg
8. Iterate?

Battery design example

(Design Handbook, UT)
Assume the following:
1) One independent bus
2) Constant bus voltage of 28.6 V
3) Constant load of 500 W
4) Depth of discharge of 70%
5) Length of eclipse is 90 minutes
6) Two NiH batteries are used.

We can use the following formula to determine the capacity needed per battery for the spacecraft:

Cr=Pe Te / ((DOD) N n) * (W - hr)

Pe = Average eclipse load = 500 W
Te = Maximum eclipse time = 1.5 hours
DOD = Limit on battery's depth of discharge = 70%
N = number of batteries = 2
n = Transmission efficiency between battery and load (typical .9)
Cr = Capacity per battery
These numbers lead to a battery capacity of 595.2 W-hr, or 20.8 A-hr at about 28 Vdc.
Now to find an approximate mass of the battery. We will assume the energy density is about 40 W-hr/kg. So the mass will be

Mb = Cr / Ed
Mb = Mass of each battery
Cr = Capacity, in W-hr, as calculated above = 595.2 W-hr
Ed = Energy density = 40 W-hr/kg
These numbers yield a battery mass of 14.9 kg. Note that we'll have two of these batteries. Redundant batteries ensure that not all storage capacity is lost if one battery fails.

Clementine System Reference

The Clementine mission consisted of a lunar orbiter only. Its systems used the latest technology, however, making it a good example. Properties of the Clementine battery and photovoltaic arrays are listed below.

Battery

Mission phase	No. of cycles	Cycle Duration (min.)	Load (W)	DOD (%)
LEO	144	36	146	21
Transfer	3	123	121	60
Lunar	370	75	226	65

Nickel-hydrogen common pressure vessel design. 22 cells with 1.64 V per cell.

The battery is mounted in a 1.19-kg graphite epoxy holder designed to withstand the launch loads.

Photovoltaic Arrays

The photovoltaic arrays provided power for operations and battery charging. Approximate power requirements were 270W in LEO and 344W in lunar orbit. Power distribution was by the direct energy transfer method. The arrays were manufactured by Applied Solar Energy Corp., City of Industry, CA.

The basic element is a 4x4 cm gallium-arsenide on germanium (GaAs/Ge) cell that is 5.5 mil thick, with a 3.5-mil coverglass. The cells were mounted on a graphite epoxy/aluminum honeycomb core substrate.

Array characteristics:

(Area and mass are compared to an estimated 4.3 square meters and 5.60 kg for silicon cells.)

The arrays were fully deployed after the solid rocket fired for TLI. Before this, supplemental power came from 18 parallel strings of 52 cells each, mounted on the interstage adapter. The adapter was jettisoned and left in Earth orbit.

The arrays were steered so that they faced the Sun. The array Sun sensor was a pair of angled solar cells whose voltage was monitored.

Power System References

Annarella, Cyril, et al., "Water and Oxygen Resources: A Lunar Discovery Mission", The University of Texas at Austin (7 December 1994).

Bents, D.J., et al., "SEI Power Source Alternatives for Rovers and Other Multi-kWe Distributed Surface Operations," NASA Technical Memorandum (TM) 104360 (Lewis Research Center: 1991?).

Chetty, P.R.K., "Satellite Technology and its Applications, 2nd edition", TAB Professional and Reference Books (Blue Ridge Summit, PA:1991).

Coates, D.K., and C.L. Fox, "Current Status of Nickel-Hydrogen Battery Technology Development," in "Proceedings of the 29th Intersociety Energy Conversion Engineering Conference", Aug. 7-11, 1994 [IECEC 1994], Monterey, CA, AIAA, p. 75-80.

Garner, J. Christopher, "The Clementine GaAs/Ge Solar Array," in IECEC 1994, p. 288- 293.

Garner, J. Christopher, William R. Braun, and Jeffrey Zagrodnik, "The Clementine Nickel Hydrogen Common Pressure Vessel Battery," in IECEC 1994, p. 124-129.

Linden, David. Handbook of Batteries and Fuel Cells. McGraw-Hill Book Company, (New York:1984).

"The 1990 NASA Aerospace Battery Workshop" (NASA CP-3119), NASA-Marshall Space Flight Center (Huntsville, AL:1991).

Rauschenbach, Hans S., "Solar Cell Array Design Handbook", Van Nostrand Reinhold Co. (New York:1980).

"Spacecraft Subsystems", Department of Aerospace Engineering and Engineering Mechanics, University of Texas, 1992.

"Surveyor Project Final Report,", Part I & II, Jet Propulsion Laboratory Technical Report 32-1265 (Pasadena, CA:1969).

Wertz, Wiley J. and James R. Wertz. "Space Mission Analysis and Design", Microcosm, Inc. (Torrance, CA:1992).

References on the WWW

Power Technology at Lewis - NASA-Lewis Research Center

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CSR/TSGC Team Web