NASA/TM--2002-211728 IECEC-2002-20038

A Systems Model for Power

Technology Assessment

David J. t--ioffman

Glelm Research Center, Cleveland, Ohio

July 2002
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NASA/TM--2002-211728 IECEC-2002-20038

A Systems Model for Power

Technology Assessment

David J. Hoffman

Glelm Research Center, Cleveland, Ohio

Prepared for the

37fl_ In.tersociety Energy Conversion Engin.eering Con:ference

sponsored by the Institute of Electrical and Electronics Engineers,

Electron Devices Society

Washington, DC, July 28-A ugust 2, 2002

National Aeronautics and

Sp ace Ad.minis tration

Glelm Research Center

July 2002
Trade names or manufacturers' names are used in this report for

identificatio_ only. This usage does not constitute a_ official

endorsement, either expressed or implied, by the Natio_al

Aeronautics and Space Administration.

NASA Center for Aerospace Information

71121Standard Drive

Hanover, MD 211076

Available frorn

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Springfield, VA 22100

Available electronical] y at http://gltrs.grc.nasa.gov/GL,TRS
A SYSTEMS MODEL FOR POWER TECHNOLOGY ASSESSMENT

David J. Hoffman

National Aeronautics and Space Administration

Glenn Research Center

21000 Brookpark Road

Cleveland, Ohio, 44135

Phone: 216-433-2445

Fax: 216-433-2995

Email: david.j.hoffman@grc.nasa.gov

ABSTRACT

A computer model is under continuing

development at NASA Glenn Research Center that

enables first-order assessments of space power

technology. The model, an evolution of NASA Glenn's

Array Design Assessment Model (ADAM), is an Excel

workbook that consists of numerous spreadsheets

containing power technology performance data and

sizing algorithms. Underlying the model is a number

of databases that contain default values for various

power generation, energy storage and power

management and distribution component parameters.

These databases are actively maintained by a team of

systems analysts so that they contain state-of-art data

as well as the most recent technology performance

projections. Sizing of the power subsystems can be

accomplished either by using an assumed mass

specific power (W/kg) or energy (Wh/kg) or by a

bottoms-up calculation that accounts for individual

component performance and masses. The power

generation, energy storage and power management

and distribution subsystems are sized for given

mission requirements for a baseline case and up to

three alternatives. This allows four different power

systems to be sized and compared using consistent

assumptions and sizing algorithms. The component

sizing models contained in the workbook are modular

so that they can be easily maintained and updated. All

significant input values have default values loaded

from the databases that can be over-written by the

user. The default data and sizing algorithms for each

of the power subsystems are described in some

detail. The user interface and workbook navigational

features are also discussed. Finally, an example

study case that illustrates the model's capability is

presented.

IMPORTANCE OF SYSTEMS ANALYSIS

System analysis provides the basis for sound

technology program planning. The results of the

tradeoff studies and technical assessments performed

by systems analysts help provide the rationale to

advocate and direct technology development

programs. In the past, a successful case for

developing new technology could be made by simply

stating that it would cost less, weigh less or be more

reliable than the baseline. Now, in order to justify the

investment, technology program planners demand to

know the magnitude of the technological benefit for

specific missions or applications.

A well-planned, defendable technology

development program needs accurate, consistent,

unbiased "honest broker" technology assessments

that temper the enthusiasm of advocates and fairly

quantifies the benefits of competing technologies for

specific missions and applications. It is also desirable

to eliminate duplication and repetition in developing

models and performing the systems analysis in order

make the best use of human resources.

The power and on-board propulsion technology

development program managers at NASA Glenn

Research Center consider systems analysis to be

extremely important and have formed a Systems

Assessment Team to coordinate power and on-board

propulsion systems analysis (Hoffman, 2000).

ANALYSIS FIDELITY AND CONSISTENCY

The fidelity and consistency of space power

systems analysis results can vary widely for a number

of reasons.

Regarding fidelity, there are top-level, first-order

assessments that make high-level assumptions

regarding technology performance, such as an

assumed mass specific power (W/kg) or efficiency of

a given subsystem, intended to quickly illuminate

global trends and guide overall program planning. The

feasibility of the assumed level of performance for the

component or subsystem technology must then be

substantiated, a point often either neglected or

glossed over by "hand-waving."

NASA/TM--2002-211728 l
Alternatively, given more time and information, a

more detailed "bottoms-up" analysis can be

performed using higher-fidelity models to more

accurately simulate the performance and/or determine

the mass of a component or subsystem. The question

in this case is whether or not the extra time and effort

is worth the result. Ultimately, however, detailed highfidelity models are required in order to verify the

performance initially estimated by the top-level

systems analysis.

Regarding apparent inconsistencies, systems

studies performed by different analysts can lead to

apparently contradictory results because the analysts

make slightly different assumptions for a number of

parameters for different reasons, or use different

criteria to assess the results. Analysis results can be

further skewed if they are based on over optimistic

predictions of the performance of newer, less mature

technologies. Frequently, and for a variety of reasons,

the actual performance of a newer technology in a

specific application is much more conservative than

initial optimistic expectations. In the end, simply

understanding the motive of one organization or

analyst performing the systems analysis versus

another can go a long way in explaining their

seemingly inconsistent results.

BENEFITS OF A STANDARD MODEL

Recognizing the issues associated with the varying

levels of model fidelity and consistency, there is some

appeal to developing a standard model, or set of

models, to perform systems analysis. The kind of

systems analysis that will be addressed for the

remainder of this paper is that needed to be

performed in a relatively quick fashion with a

moderate degree of fidelity in order to discern

meaningful trends in space power system technology

performance in a subsystem, system or even

spacecraft and mission-level context.

It is recognized that for this type of systems

analysis, there are many competent engineers that

could create spreadsheet level models to do the job.

While each of their models may not violate the

currently understood laws of physics, the level of

fidelity in terms of the phenomena they choose to

model and at what detail will not typically be the

same. Also, it is a waste of an organization's human

resources to have a new analyst create a new model

for a similar study that was performed by a previous

analyst who did not share their model and has since

moved on.

For the efficient use of human resources and to

ensure accuracy and consistency of analytical results,

a standard model using consistent input parameters

derived from a common database in a model that

sizes space power systems with common algorithms

which account for all relevant and significant

phenomena that affects system performance and

sizing is needed.

MODELING APPROACH

Because of their ease-of-use, ease-oftransmission (via email) and general ubiquity,

spreadsheets are typically the software chosen for the

top-level systems analysis models being discussed.

NASA Glenn's initial approach at developing a model

to assess solar arrays will be discussed next, followed

by the continuing efforts to evolve the initial model

into a standard space power system assessment tool.

In the Beginning: ADAM

In 1999, NASA Glenn contracted the development

of an Excel TM spreadsheet model with SAIC,

Schaumburg, IL, that would enable a consistent

comparison of solar array options using emerging

thin-film solar cells versus those using traditional

crystalline solar cells. While much less efficient at

present, thin-film cells are much lighter (-4x) than

multijunction crystalline cells. Thin-film cells also hold

the promise of being much cheaper to manufacture.

The model sought by NASA Glenn would enable a

consistent comparison of solar arrays in order to

quantitatively determine the performance

characteristics that the emerging thin-film cells must

attain in order to be a viable option to the entrenched

crystalline cell rigid array baseline most often flown in

space. SAIC enlisted the support of Spectrum Astro,

Gilbert, AZ, to create the model they dubbed ADAM,

for Array Design Assessment Model. The ADAM

model was first used in a thin-film array parametric

assessment presented at the 2000 IECEC (Hoffman,

et al., 2000).

Then Came: EVE

While ADAM performed well for its initial purpose

of assessing solar arrays, it was incomplete from the

perspective of assessing the performance of energy

storage and power management and distribution

technologies. Extending ADAM's approach to these

other subsystems, a more comprehensive spacecraft

power system assessment tool was created: EVE, or

EValuation Engine.

EVE keeps the best features of ADAM and

expands on them to enable comprehensive "bottomsup" assessment of not only solar array power

generation, but also energy storage using a variety of

chemical battery, regenerative fuel cell and flywheel

energy storage technologies and more detailed power

management and distribution components including

solar array shunt regulators, battery charge/discharge

units, power switching and distribution units and

power distribution cables.

MODEL DESCRIPTION

EVE is a "top-down/bottoms-up" medium-fidelity

Excel workbook that uses a structured approach to

size a baseline solar photovoltaic space power

system and three alternatives side-by-side for a given

performance point. As such, it enables rapid,

consistent comparisons of the impacts of space

power technology at the system and spacecraft level.

NASA/TM--2002-211728 2
The phrase "top-down/bottoms-up" refers to the

feature of the model that allows the analyst to simply

assume the performance of power subsystems or

components (e.g., specific power or energy, areal

density and efficiency, etc.) when performing a "topsdown" analysis to quantify the system benefits in a

"what-if' fashion. In "bottoms-up" mode, the analysts

defines the mission requirements and selects

component/technology types then lets the model's

scaling algorithms calculate the component or

subsystem performance.

Pertaining to EVE's fidelity, it is considered

between high and low, hence medium, for the

following reasons. For mass estimation, a high fidelity

model would need to account for every component (or

even piece-part) in a system and have accurate

estimates or scaling rules for those components. For

electric performance, a high-fidelity model would need

to simulate the detailed electrical performance of

components, subsystem and system as a function of

time accounting for all significant environmental

factors. At the opposite extreme, a low fidelity model

may represent a power system only by its three main

subsystems_power generation, energy storage and

power management and distribution (PMAD)_and

simply assume top-level performance parameters and

simple, linear scaling relationships.

EVE lies between these extremes, as will become

apparent in the following sections.

Model Orqanization and Confiquration Control

The EVE Excel workbook is a collection of 56

individual worksheets of six basic types: Configuration

Control, Workbook Navigation, Database, Input

Definition, Calculation, and Output/Results Display.

Movement between the worksheets is facilitated by

numerous macro buttons that use visual basic to

rapidly change the active worksheet.

Within a worksheet, a cell's background and

alphanumeric text is color coded to indicate whether it

is a user-input (bold blue text/blue background), a

default value resulting from a calculation (purple

text/yellow background), an intermediate result (black

text/light green background), or a calculation result

used on another sheet (black text/bright green

background). All input parameters have default values

derived from database entries or calculations. These

default values can be overridden in user-defined input

cells for almost every input parameter. Pull-down

menus are used wherever possible to present the

user with a valid set of data selections. Named cells

are used extensively to pass parameter values

between worksheets efficiently.

On every input, calculation and output worksheet,

the first column (Column A) contains the name or

explanation of the parameters used in that sheet. The

next column to the right (Column B) contains the

values of those parameters for the Baseline power

system option. The next three columns (Columns C,

D and E) contain the values for the three power

system options being compared. These features

increase the usability of the model not only for the

model creators, but also for analysts unfamiliar with

the model. All too often, the creator of a complex

spreadsheet is the only one who can use it.

Because they are so easy to work with and

transmit, spreadsheets can be difficult to place under

configuration control. In the evolution of the ADAM,

now EVE model, a deliberate attempt was made to

keep track of changes and versions. The first

worksheet in the EVE workbook is a "Change Control

Log" to keep track of the version number of the

model, a description of the latest changes and who

made them, and the date and time. When the model

is used for a specific analysis, the analysts are

encouraged to keep the version number in the

filename so as to easily indicate which version of the

model was used when files are stored in a directory

(e.g. the name of the Excel file used to illustrate this

paper is IECEC02_EVE_vlp0.xls indicating EVE

version 1.0 was used.)

Main Naviqation Worksheet

The "Model Main Menu" worksheet should be the

first sheet seen when opening the workbook, or at

least the sheet to operate from when setting up an

analysis. It contains links to every database, input,

intermediate calculation and results output sheet. It is

divided into four vertical and four horizontal sections.

Horizontally, the top section looks like a block

diagram of a space power system, as it is intended to

be. Each block is actually a macro button that will take

the user to the worksheet used to define a particular

subsystem, such as solar arrays or energy storage.

The next horizontal section contains macro buttons to

take the user to the component parameter databases

(e.g., solar cells). The third horizontal layer contains

macro buttons to take the analyst to the intermediate

calculation and component sizing worksheets. The

bottom layer contains macro buttons to jump to the

results display sheets.

The four vertical sections of the Main Menu sheet

divide the four horizontal sections just described into

Mission Definition, Solar Array, PMAD and Energy

Storage areas.

Figure 1 - Model Main Menu Worksheet

NASA/TM--2002-211728 3
Databases

There are seven databases contained in the

workbook: Missions, Environments, Materials, Solar

Cells, Array Structures, PMAD and Energy Storage.

These databases store default values of all model

input parameters.

The Missions database contains end-of-life (EOL)

power requirements, mission duration, power

subsystem and spacecraft description (mass,

dimensions, etc.) defaults for six representative

missions: LEO Earth Science, GEO Communications,

Jupiter Orbiter, Near-Earth Deep Space (1.5 AU),

Venus Orbiter, Mars Communication Orbiter.

The Environments database currently contains

direct solar radiation, planetary infrared radiation,

planetary radius, mass and average albedo values for

Venus, Earth, the Moon, Mars and Jupiter.

The Materials database contains density, Poisson's

ratio, Young's modulus, absorptivity and emmisivity

data, as appropriate for the various materials that

comprise solar cells, solar array support structures,

and other space power system components.

The Solar Cells database contains beginning-oflife (BOL) cell efficiency at a user-defined reference

temperature along with cell temperature coefficient

and the material layers and thicknesses that comprise

each solar cell listed in the database, among other

data.

The Array Structures database contains data for

sizing rigid honeycomb solar array panel substrates

and representative areal density values for various

flexible array support structures.

The PMAD and Energy Storage databases contain

"top-down" specific power (W/kg), specific energy

(Wh/kg) and efficiency data for various PMAD and

energy storage technology options.

While the information in these databases can be

edited, it is intended that the existing database entries

would be left relatively static. New entries can easily

be added to any of the databases so that the

information can keep pace with technology

development. If there is a need to override any of the

default data loaded into the model from the

databases, this can be done on the appropriate input

worksheet, as described next.

Input Worksheets

There are six input worksheets: Mission Definition,

Solar Arrays, PMAD, Cabling, Loads and Energy

Storage. Default values for the input parameters

defined on these sheets are fed by the databases just

described or intermediate calculations performed on

the sheet itself. So as not to destroy the database

look-up formula that loads the default values, there is

a separate cell where the user can enter values that

the model will use instead of the defaults.

In the Mission Definition worksheet, the user must

select the mission type and operating environment

from a pull-down menu and define the mission lifetime

in years. The default circular orbit altitude is based on

the mission type selected, obtained from the Missions

Database. The user can define any altitude desired by

entering a value in the "User-Defined Orbit Altitude"

Cell. Calculated orbit period and maximum eclipse

period for the central body selected by the mission

type are given on this sheet as well.

Reflecting its heritage as an array design

assessment model (ADAM), EVE's Solar Arrays input

worksheet contains 43 user-defined parameters that

fully describe a solar array from the bottom up: solar

cell performance and material construction (material

type and thickness for up to five layers), solar cell

efficiency knockdown factors, solar array blanket or

panel material construction (material type and

thickness up to 3 layers), and solar array support

structure type (rigid panel or flexible deployable) and

characteristics (e.g., number of array wings, array

wing aspect ratio, cell packing factor, rigid panel hinge

stiffness or flexible array deployed first fundamental

frequency).

Figure 2 - Solar Cell performance input data for

Baseline case.

The PMAD input worksheet allows the user to

define values for operating efficiency and specific

power for four classes of components: Array

Regulator Units, Battery Charge Regulator Units,

Power Switching and Distribution Units, and DC-DC

Converters. Efficiency and specific power values can

be default values obtained from the PMAD Database,

or a user-entered value. In the present version, the

user can also use values calculated from more

detailed "bottoms-up" sizing algorithms for the Power

Switching and Distribution Unit and the DC-DC

Converter Units. Detailed sizing models for the Array

Regulation and Battery Charge Units have recently

become available, but have not yet been integrated

into the workbook.

Figure 3 - PMAD Array Regulation Unit input data

for Baseline case.

There is a separate worksheet to calculate array

wiring harness and power distribution cable masses

based on user-defined cable transmission efficiency,

NASA/TM--2002-211728 4
cable lengths, conductor type (AI or Cu) and crosssection (flat or round), level of redundancy and

iteratively-calculated cable operating temperature.

In the present version, the worksheet for defining

the required load power consists of a cell to define the

total required EOL power (W) and a cell to define the

required load voltage. In future versions, the model

could allow the definition of a distributed set of loads

at user-defined voltages.

The Energy Storage input worksheet is similar to

the PMAD input worksheet. It contains a table where

the type of energy storage is selected from a pulldown menu displaying the technology options

contained in the Energy Storage Database. Based on

this selection, default values of roundtrip efficiency,

maximum depth-of-discharge and specific energy are

displayed. The user can then enter user-defined

values for any of the parameters, if desired. The user

can also select an option to have parameter values

calculated from detailed "bottoms-up" worksheets for

flywheels and regenerative fuel cells.

When using the calculated values for PMAD and

energy storage devices, the user must go to those

specific worksheets to further define a further set of

input parameters.

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Figure 4 - Energy Storage input definition

indicating calculated flywheel values have been

selected.

Calculation/Component Sizinq Worksheets

There are twelve intermediate calculation or

component sizing worksheets that size the power

subsystems to meet the mission requirements at EOL

and calculate their corresponding masses, either with

the tops-down user defined or defaulted specific

power, energy and efficiency assumptions or with

values for those parameters calculated by the more

detailed bottoms-up component sizing algorithms, or

a mixture of the two if so desired.

For the power generation subsystem, there is an

overall Solar Array Sizing Calculation worksheet, a

Flexible Array Deployment Mast Sizing sheet, a Rigid

Array Structure Sizing sheet and a Cell/Array Panel

Thermal Calculation sheet. The Flexible Array

Deployment Mast Sizing sheet uses a curve of

deployable lattice boom radius versus stiffness in

order to size the array to meet a user-defined first

fundamental deployed frequency requirement (for a

more complete description, see Hoffman, et al.,

2000). The Rigid Array Structure Sizing sheet also

takes into account array stiffness requirements. The

Cell/Array Panel Thermal Calculation sheet uses a

single-node thermal model to estimate the cell

operating temperature of a sun-tracking solar array at

the sub-solar (orbit noon) point in a circular orbit

about a central body.

For the PMAD subsystem, there is an overall

PMAD Sizing Calculations sheet that calculates

PMAD component masses sized to meet the mission

requirements, either using top-down specific power

and efficiencies or bottoms-up values calculated by

the Array Regulation Unit, Battery Charge/Discharge

Regulation Unit, DC-DC Converter Unit, and two

types of Power Switching and Distribution Unit Sizing

sheets.

Similarly for the Energy Storage subsystem, there

is an overall Energy Storage Sizing Calculations

sheet that calculates energy storage mass sized to

meet the mission requirements using top-down

specific energy and efficiencies. Alternatively,

bottoms-up values calculated by Flywheel sizing or

Fuel Cell Sizing sheets can be used. A bottoms-up

chemical battery sizing sheet is planned.

In addition to the sheets dedicated to sizing and

calculating the mass of the power system just

described, there is an additional worksheet that

estimates the effect of deployed solar arrays on the

attitude control system. Specifically, the worksheet

estimates the amount of propellant required for

reboost due to atmospheric drag and desaturation of

momentum wheels. This feature is extremely

important when comparing the spacecraft-level mass

impacts of larger-area lightweight thin-film arrays

versus much smaller but heavier crystalline cell

arrays. One such use of this feature in the ADAM

model has been reported by Hoffman, et al. (2001).

Main Results Summary Worksheet

The main results summary worksheet contains

seven sections of tabular data that allow rapid

comparison of key input assumptions and output

calculations: Mission Requirements & Load Summary,

System Design Summary, Power System Mass and

Performance Summary, Array Performance Metrics,

Solar Array Size Summary, Solar Array Mass

Summary, and Array Structural Characteristics. These

tables are illustrated in Figures 5 and 6.

There is also a worksheet that gives a detailed

power subsystem mass breakdown and subsystem

mass fractions.

MODEL USAGE

At this point, it should be evident that the EVE

model greatly facilitates rapid definition, assessment

and comparison of a baseline space power system

and three alternatives (although only one alternate

option is shown in the figures to save space).

Although easy and quick to use in the hands of an

experienced power systems analyst, the moderate

fidelity of the model is aimed at allowing meaningful

trends to be discerned that could be validated by

higher-fidelity models. The model is ideally suited for

NASA/TM--2002-211728 5
1.0 Mission Requiremenls & Loads Summary

Mission Type LEO Earth Science

Operating Envi[onmen! LEO _....................ii_

End of Life (EOL} Load Power, Waits 5001]

Mission Lifetime, yrs 7

Altitude, km 500

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Figure 5 - Mission and System Design

Input/Output Summary.

sensitivity studies to determine the systems-level

effect of improvements in the technology of space

power systems components, either singly, or in

combination.

While originally intended for use as a systems

analysis tool to guide technology program planning,

the EVE model would also do well in a so-called

"design center" environment where rapid conceptual

system sizing is performed.

PLANS

Systems analysts at NASA Glenn plan to

collaboratively evolve the EVE model (or whatever it

ends up being called), adding increasing levels of

fidelity and capability without compromising its easeof-use. The component databases will need to be

updated to maintain pace with technology

development. The bottoms-up component sizing

models will be expanded and validated. Consideration

will be given to adding time-dependent performance

simulation, at least to some degree.

CONCLUSION

The comprehensive space power system sizing

spreadsheet model described in this paper provides a

standardized approach for performing systems

analysis in order to quantify the impact of space

power technology performance and discern

meaningful trends to guide program planners.

REFERENCES

Hoffman, David J., 2000, "Power and On-Board

Propulsion System Benefit Studies at NASA GRC,"

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iiiiiiiiiiiiiiiiiiiiiN_iiiiiiiiiiiiiiiiiiii _i_i_i_i_i_i_i_i_i_NNiiiiiiiiiiiiii

iiiiiiiiiiiiiiiiiNiSNiiiiiiiiiiiiiiiiiiiii _N_iiiiiiiii

Figure 6 - Power System Sizing Output Summary.

Proceedings, of the 2000 SAE Power Systems

Conference, Society of Automotive Engineers,

Warrendale, PA, pp. 173-178.

Hoffman, et al., 2000, "Thin-Film Photovoltaic Solar

Array Parametric Assessment," Proceedings, of the

3_ h Intersociety Energy Conversion Engineering

Conference, American Institute of Aeronautics and

Aeronautics, pp. 670-680.

Hoffman, et al., 2001, "Thin-Film Solar Array Earth

Orbit Mission Applicability Assessment", Space

Photovoltaic Research and Technology Conference,

NASA Glenn Research Center.

NASA/TM--2002-211728 6
Form Approved REPORT DOCUMENTATION PAGE

OMB No. 0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3, REPORT TYPE AND DATES COVERED

July 2002 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

A Systems Model for Power Technology Assessment

& AUTHOR(S)

David J. ttoffman

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Resem'ch Center at Lewis Field

Cleveland, Ohio 44135 - 3191

WU-755-1A- 16-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E---13482

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

National Aeronautics and Space Administration

Washington, DC 20546- 0001 NASA TM--2002- 211728

IECEC-2002-20038

11. SUPPLEMENTARY NOTES

Prepared for the 37th Intersociety Energy Conversion Engineering Conference sponsored by the Institute of Electrical

and Electronics Engineers, Electron Devices Society, Washington, DC, July 28-August 2, 2002. Responsible person,

David J_ ttoffman, organization code 692(I, 216-433-2445_

12a, DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 20 Distribution: Nonstandard

Available electronically at Ntp://gltrs.grc.n;_sa.gov/GIiI'RS

"l-his publication is available from the NASA Center for AeroSpace Im%rmation, 301-621-0390.

12b. DISTRNBUTION CODE

13. ABSTRACT (Maximum 200 words)

A computer model is under continuing development at NASA Glenn Research Center that enables first-order assessments of space

power technology. The model, an evolution of NASA Glenn's Array Design Assessment Model (ADAM), is an Excel workbook that

consists of nmnerous spreadsheets containing power technology perfommnce data and sizing algorithms. Underlying the model is a

number of databases that contain deliault values Ibr various power generation, energy storage and power management and distribution

component parameters. These databases are actively maintained by a team of systems analysts so that they contain state-of-art data as

well as the most recent technology performance projections. Sizing of the power subsystems cau be accomplished either by using an

assumed mass specific power (W/kg) or energy (Wh/kg) or by a bottoms--up calculation that accounts l_r individual component

performance and masses. The power generation, energy storage and power management and distribution subsystems are sized for given

mission requirements for a baseline case and up to three alternatives. This allows four different power systems to be sized and compare, d using consistent assumptions and sizing algorithms. The component sizing models contained in the workbook are modular so

that they can be easily maintained and updated. All significant input values have default values loaded from the databases that can be

over-written by the user. The default data and sizing algorithms for each of the power subsystems am described in some detail. The user

interface and workbook navigational features are also discussed. Finally, an example study case that illustrates the model's capability

is presented.

14. SUBJECT TERMS

Computer programs; Spreadsheets; Spacecraft models; Spacecraft power supplies;

Electric power

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

15. NUMBER OF PAGES

12

16. PRICE CODE

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OF THIS PAGE OF ABSTRACT

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