2Universidad Politécnica de Madrid – Instituto de Energia Solar

ABSTRACT: Good performance of a stand-alone system is related to correct sizing. The purpose of this paper is to present an analytic method for sizing stand-alone systems in Brazil. The task of sizing a PV system consists of finding the PV-array area and the battery capacity that fit in well with the energy demand and the behaviour of solar radiation for a specific loss of load probability (LLP). To achieve this goal, a computer program was developed to simulate a stand-alone system with constant demand during the year. The system was simulated during ten years in order to find the slope of the modules that leads to the lowest PV-array cost for LLP=10-1 and LLP=10-2. The procedure was then applied to obtain the isoreliability curves, i.e. the PV-array capacity as a function of the storage capacity, for both LLPs. This approach was carried out for 144 Brazilian cities. Equations were fitted to the values of slope of the modules found for each place as a function of the latitude and clearness indexes. The isoreliability curves can be obtained if two parameters, denominated a and b, are estimated. Two methods are presented. The method A consists of finding the parameters a and b as a function of the latitude and in the method B these parameters are obtained as a function of the latitude and clearness indexes. In this way, a stand-alone system can be more easily sized in Brazil, starting from a set of equations. The proposed analytic methods were applied to size a stand-alone system in Torres (latitude =- 29.3o). For LLP=10-2, the difference between the simulated and estimated values is lower than 10 % and 6 % when method A and method B are used, respectively