LONG-TERM MEASUREMENTS OF ENERGY PRODUCTION OF SOLAR PHOTOVOLTAIC PANELS

This article deals with the forecast of solar energy in Latvia. Latvia characterizes with rather low irradiances and high nebulosity, but long days in summer in comparison to traditional solar energy using in the southern countries. Two stationary mounted and two tracking to sun solar photovoltaic (PV) panels Solet 60.6-WF-250, the surface area 1.6 square meters for each, are studied in this article. Measurements are done in Ulbroka, Latvia, on the roof of the Ulbroka Scientific Centre. Energy produced by panels is entered into the public electricity grid using the solar micro invertor Enphase with MPPM function and registered by Envoy. The Enphase Micro inverter converts the DC output of the PV module into grid-compliant AC power and continuously adjusts electrical parameters for the maximum power gain. Daily energy sums are registered from April 1 till December 31 of 2020. Daily energy sum from the mentioned system can reach 10 kWh on a clear summer day, but in November and December the daily energy sum produced is near to zero. Total energy gain from the mentioned system (with total area of solar panels 6.4 square meters) in the whole mentioned time period is approximately 940 kWh. Measurements are compared to theoretical daily energy sums of solar radiation taking into account also nebulosity. Data on the nebulosity are taken from the public homepage of the Latvia Environment, Geology and Meteorology Centre. Such comparison allows to estimate real efficiency of panels. We obtained the efficiency 12-13%, what is little bit less than 15% given in the technical characteristics of the panel given by the producer. Described calculations can be used for forecast of received solar energy if nebulosity forecast (real or many years mean for every day) is known. The method can be used for any location worldwide.