DESIGN TOOL FOR RELIABLE SOLAR SYSTEMS

Nuchida Suwapaet and John Duffy University of Massachusetts Lowell 1 University Ave. Lowell, MA 01854 John_Duffy@uml.edu ABSTRACT A design algorithm for remote communication system was developed to meet an availability goal. The algorithm takes into account three modes of possible failure: individual component breakdown, energy deficiency (loss of load probability), and radio transmission loss. Least cost designs options are selected through tradeoffs among the three modes of possible failure. Design choices include component redundancy, more solar energy collection (photovoltaic) modules, more energy storage (batteries), higher radio power, larger antennas, and more reliable components. The algorithm was applied to the design of a satellite ground station system and radio network station systems in a remote mountainous area of Peru. Mean times to repair are long there. Nevertheless, a goal of 90% availability was set and achieved. This algorithm represents a true design tool in which a design goal (constraint) is specified and then the tool chooses the number and size of components to minimize cost (or some other objective function). 1. INTRODUCTION Reliability analysis has been considered as an important step in any system design process. In solar power systems, a reliable system means having sufficient power in the system or in the other word, having a small loss of load probability (LOLP), which is defined as the expected fraction of load not met by the photovoltaic system during its lifetime. However, the solar power system could fail not only from the lack of the power but also from the component failure. A design method that takes into account LOLP due to both weather-dependent causes and component failure has been developed based on the reliability model (Wijessooriya and Duffy 1993). This algorithm allow designers to size array and storage, configure parallel components, and add redundant systems (if necessary) to meet a combined component availability and LOLP goal and to minimize overall system cost. The design algorithm here takes into account three modes of possible failure in remote communication systems: individual component breakdown, energy deficiency (LOLP), and radio transmission loss. The cost optimization is also a part of the algorithm. This algorithm is a true design tool in which a design goal (constraint) is specified and then the tool chooses the number and size of components to minimize cost (or some other objective function). Most other so-called tools are really simulations in which the components are chosen first and then the system is simulated to see if meets the design goals. 2. OVERVIEW 2.1 Reliability Analysis Reliability is defined as the probability that a system will perform properly for a specified period of time t under a given set of operating conditions. The failure rate, λ, is the probability of failure given that the component has survived up to time t. The failure rate is assumed to be constant. The exponential distribution is employed when constant failure rates adequately describe the behavior of continuously operating systems. Then the reliability may be written as