THE DESIGN OF LOW-COST STRUCTURES FOR EXTENSIVE GROUND ARRAYS H. A. Franklin and R. S. Leonard Bechtel National, Inc., San Francisco, California There is a great deal of similarity between the functional requirements for support structures for flat plate photovoltaic arrays and for the Satellite Power System (SPS) rectenna panels. Much work relevant to the SPS rectenna design effort has been done on developing design criteria and structural designs for low-cost support structures for terrestrial photovoltaic power plants. This paper reports on the work done by Bechtel National, Inc. for Sandia Laboratories to develop conceptual designs of solar array support structures and their foundations including considerations of the use of concrete, steel, aluminum, or timber. Some cost trends were examined by varying selected parameters to determine optimum configurations. Detailed civil/structural design criteria were developed during this work. Using these criteria, eight detailed designs for support structures and foundations were developed and cost estimates were made. Cost estimates for array supports and foundations were shown to vary between $2 to $3 per square foot of supported panels (deflated to 1975 dollars). A result of this study was to identify wind as the major loading experienced by these low-height structures, whose arrays are likely to extend over large tracts of land. The proper wind load estimating is essential to developing realistic structural designs and achieving minimum cost support structures. Existing building codes are not directly applicable for determining the wind loads on these structures. Consequently, wind tunnel testing of a conceptual array field was undertaken and some of the resulting wind design criteria are presented in this paper. SPS rectenna system designs may be less sensitive to wind load estimates, but consistent design criteria will remain important. Concepts: In developing low cost support concepts for either the terrestrial photovoltaic power plants or SPS ground stations the functional requirements must be well understood. Some of these are: • spacing requirements to avoid shading • construction and maintenance access requirements • environmental restrictions and construction materials (rusting, wood rot, degradation due to UV) • size limitations due to transportation • reflection/vibration limits Concepts considered ranged from panels placed directly on the ground to having the energy collection system integrated into the sloping roof of a large building structure which also houses office and condominiums. Various foundation concepts were also reviewed. After a preliminary screening, the main study effort concentrated on simple structures made up of posts and beams. The posts were supported on caissons or footings, or were directly embedded in the ground. Several of these concepts are shown in the following pages. Design Criteria: These low (close to the ground) light-weight structures are not governed by any of the existing categories of building codes such as the Uniform Building Code (UBC) or American National Standards Institute (ANSI) A.58.1-1972 "Building Code Requirements for Maximum Design Loads in Buildings and Other Structures." Yet for studies whose results will be widely used to
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