Reliability and maintainability evaluation of solar control systems

Control system problems affected the performance or reduced the effectiveness of 25 percent of the 47 DOE-sponsored solar heating and cooling sites that were reviewed. The reliability field information presented indicates that most of the control system problems that have been encountered so far have been design related. The second major reason some of the DOE-sponsored solar-energy systems experienced operating difficulties was improper sensor calibration, location, and installation. Defective components caused some operating problems, but the incidence of these problems was approximately one-third that of the design-related problems. After an introductory look at control systems and terminology, control problems are presented in detail. Since the major control problem is design-related, a Failure Modes and Effects Analysis (FME) on two existing systems is presented. While the FMEA cannot be used to improve the design of these operational systems, the results can be used to critique the system and to indicate where improvements can be made in future systems. Guidelines for design, sensor calibration, location, and installation are presented. In an appendix, the characteristics of various sensors that are applicable to solar-energy installations are presented.