The loss of power supply probability as a technique for designing stand-alone solar electrical (photovoltaic) systems

Many solar electrical (photovoltaic) systems (SES) are designed and are in operation in many countries. Most of the analysis and design methods are based on the concept of power supply during a number of autonomous days (NAD). This NAD value refers to the number of consecutive days in which no power is supplied to the system from the solar cell array(SCA). The storage system capacity is thus based on the required energy during NAD. Though the NAD method is simple and in many cases will assure reliable power supply, it is uneconomical. With the relative high costs of solar cells and storage batteries, this means a system that may not operate at its optimum. A new method for analysis and design of SESs is presented in this paper, which enables to determine the minimum (and thus the economical) sizes of the SCA and storage system capacity. The reliability of the system is studied by the newly introduced “loss of power supply” (LOPS) probability concept. Some systems have been designed using the new method and compared to other design methods. One of the SESs has been in operation for over two years. The measured results verify the usefulness of the new method.