These systems could be attractive alternatives to conventional solar arrays and associated power circuits.

Lyndon B. Johnson Space Center, Houston, Texas

Lightweight, flexible, thin, one-piece, solar-power packs are undergoing development. Each power pack of this type is a complete, modular, integrated power-supply system comprising three power subsystems that, in conventional practice, have been constructed as separate units and connected to each other by wires. These power packs are amenable to a variety of uses: For example, they could be laminated to the tops of tents and other shelters to provide or augment power for portable electronic equipment in the field, and they could be used as power sources for such small portable electronic systems as radio transceivers (including data relays and cellular telephones), laptop computers, video camcorders, and Global Positioning System receivers.

The three power subsystems in question are (1) an array of one or more photovoltaic cells (power-generation subsystem), (2) one or more storage batteries (energy-storage subsystem), and (3) electronic circuits to control the other two subsystems and the overall operation of the system [power-managementand-distribution (PMAD) subsystem]. The conventional approach to designing these subsystems results in impediments to the present goal of developing a highly integrated system: Conventional photovoltaic arrays are fragile and expensive; conventional storage batteries are bulky, must typically be able to withstand high pressures, and must be kept within fairly narrow ranges of operating temperatures; and conventional PMAD systems comprise high-power centralized conditioning and switching circuits that are formed on heavy, rigid printed-circuit boards and generate large amounts of waste heat. The structural, electrical, and thermal requirements that govern the design of a conventional power system usually make it necessary to mount each subsystem in a different location. For example, a photovoltaic array must be placed where it can face the Sun, the storage batteries must be mounted on a wall that can be kept at an acceptable temperature, and the PMAD subsystem must be mounted on a panel suitable for electronic circuitry.

A system of the type under development differs from a conventional power system in several notable ways:

* The power-generation subsystem is based on copper indium gallium diselenide (GIGS), which is a photovoltaic material well suited for flexible, thin-film photovoltaic cells and arrays. In comparison with conventional silicon-based photovoltaic arrays, thin-film GIGS photovoltaic arrays offer potential advantages of flexibility, durability, high specific power, and low cost.

* The energy-storage subsystem consists mainly of flexible, thinfilm lithium-based storage batteries. These are durable batteries that exploit long-life solid-state chemistry and can operate over a wide temperature range.

* The PMAD subsystem is implemented in thin, flexible electronic circuitry instead of conventional circuitry on rigid circuit boards.

* The integrated system can be made by laminating the three thin subsystems and forming vias (through-the-thickness electrical connections) among them.

* The weight and volume of the integrated system are much less than those of an equivalent conventional system that comprises three separate, wire-connected subsystems.

* Duplicates of the system, regarded as modules, can be combined into larger systems to satisfy larger power demands.