Energy yield analysis and evaluation of solar irradiance models for a utility scale solar PV plant in South Africa

Date
2016-12
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: It is critical for electrical utility providers to be certain of the field performance of their systems in order to realize returns on project investments. Due to the actual output of installed solar PV modules being far more involved than in standard test conditions, some real world systems outperform simulations. The output of a solar PV system depends on various factors like local weather, system design characteristics, etc. Various energy forecasting simulation software exist and PVsyst is one of the most reliable when given the right input data. It is vital that software simulated energy yield values closely correlate with the actual field performance. In this study, the cause of the seasonal differences in measured and simulated energy yield for a solar PV power plant in South Africa is investigated. It was observed that a fixed tilt 75 MWP operational system significantly outperformed the software simulations in winter months during its first year of operation. Normally it is expected that simulations perform better than real world systems. In order to achieve our goals, an initial yield assessment was done in PVsyst and simulation results were evaluated against measured data. Differences in actual and estimated weather conditions were avoided by using irradiance and ambient temperature values recorded at the solar PV plant site. Due to the overestimation of losses in the PVsyst model, the simulated results did not correlate with the measured values of energy yield. In the simulations, the real world system was underestimated by a yearly average value of 6.4% and 5% using the Hay and Perez model respectively. Understanding the performance drivers and losses within a solar PV system is crucial for ensuring reliable and high performance solar PV systems. Due to the overestimation of losses in the default PVsyst loss model, system loss values were then derived from the data recorded on the solar PV plant site. Findings from the calculated losses were then used to improve the PVsyst model for the solar PV plant. After using 30 minute average input data and the improved PVsyst loss model, the real operational system was slightly underestimated by the improved simulation model by a yearly average value of 0.78%. Seasonal differences between measured and simulated energy yield were still observed. These differences were larger in winter than in summer and they were also in correlation with the variances in the measured and PVsyst estimated Plane-Of-Array irradiance. Due to the seasonal differences between the improved PVsyst model and the measured energy yield values in winter months, irradiance models that estimate the Plane-Of-Array irradiance from Global Horizontal Irradiance measurements were evaluated for South African climatic conditions. The most common irradiance decomposition models namely Orgill and Hollands, Erbs, Louche, Reindl1, Reindl2, DISC and Dirint were evaluated as well as combinations of the Isotropic, Sandia, Klucher, Hay, Reindl and Perez transposition models with the best performing decomposition models. Based on the root mean square values, the best performing decomposition models were the Dirint, Louche and Disc whereas the Perez, Reindl and Hay transposition models performed the best for South African sites. On a monthly basis, simulations done using measured Plane-Of-Array irradiance were within 2% of the field recorded values. It was then concluded that the choice of irradiance data, transposition and loss models has a significant effect on the short and long term energy predictions.
AFRIKAANSE OPSOMMING: Dit is belangrik dat maatskappye uitvind hoe goed hulle sisteme in die praktyk presteer sodat hulle verseker kan wees van ’n goeie opbrengs op hulle belegging in die projek. Omdat die werklike uitsette van son fotovoltaïese (FV) modules baie meer kompleks is as in standaard toetstoestande, vertoon sommige sisteme onder werklike toestande (in die praktyk) beter as in die simulasies. Die uitsette van ’n son FV sisteem hang af van verskillende faktore soos die plaaslike weer, die ontwerpkenmerke van die sisteem, ens. Daar bestaan verskillende soorte simulasie sagteware wat voorspellings kan maak, waarvan PVsyst een van die mees betroubaarste is as die regte data ingevoer word. Dit is veral belangrik dat die sagteware-gesimuleerde energie opbrengs waardes met die werklike prestasies in die veld moet ooreenstem. Tydens hierdie studie is daar ondersoek ingestel na die oorsake van seisoenale verskille in die gemete en gesimuleerde energie opbrengs van ’n son FV aanleg in Suid-Afrika. Daar is gevind dat ’n 75 MWP operasionele sisteem die sagteware simulasies gedurende die wintermaande gedurende die eerste jaar nadat dit in werking gestel het, ver oortref. Gewoonlik word daar verwag dat simulasie beter sal vaar as sisteme in die praktyk. Om ons doelwitte te bereik is daar ’n aanvanklike opbrengs assessering in PVsyst gedoen en die bevindinge is teen gemete data evalueer. Verskille in werklike en geskatte weerstoestande is vermy deur om stralings waardes en omringende temperature wat op die terrein self gemeet is, te gebruik. Aangesien die verliese in die Psyst model verkeerd geskat is, het die gesimuleerde resultate nie met die gemete energie opbrengs ooreengestem nie. In die simulasie is die werklike sisteme met ’n gemiddeld van 6.4% en 5 % onderskat gebruik van die hooi en Peres model onderskeidelik. Dit is baie belangrik om die prestasie drywers en verliese binne ’n son FV sisteem te verstaan om sodoende die betroubaarheid en hoë opbrengs van die sisteem te verseker. Weens die oorskatting van verliese in die verstek PVsyst verlies model, is verlieswaardes verkry uit data wat by die aanleg self opgeteken is. Die berekende verlies syfers is toe gebruik om die PVsyst model van die aanleg te verbeter. Nadat gebruik gemaak is van 30-minuut gemiddelde inset data en die verbeterde PVsyst verlies model, is die werklike operasionele sisteem met ’n jaarlikse waarde van 0.78% deur die verbeterde simulasie model, effens onderskat. Seisoenale verskille tussen die gemete en simuleerde energie opbrengs is egter nog-steeds opgemerk. Hierdie veskille is in die winter groter as in die somer en het ooreengestem met die verskille in die gemete en deur die PVsyst geskatte Plane-Of-Array straling. Weens die seisoenale verskille tussen die verbeterde PVsyst model en die gemete energie opbrengs in die wintermaande, is stralingsmodelle wat die "Plane-Of-Array-straling van Global Horizontal Irradiance skat vir Suid-Afrikaanse toestande ge-evalueer. Die volgende modelle is ge-evalueer: Orgill en Hollands, Erbs, Louche, Reindl1, Reindl2, DISC en Dirint, asook kombinasies van Isotropic, Sandia, Klucher, Hay, Reindl en Perez transposisie modelle. Gebaseer op die wortel van die gemiddelde kwadraat waardes, is die modelle wat die beste presteer die Dirint, Louche en DISC modelle en die transposisie modelle wat in Suid-Afrika die beste presteer is Perez, Reindl en Hay. Op ’n maandelikse basis is simulasies wat met Plane-Of-Array straling gedoen is, binne 2% van die waardes wat in die praktyk opgeteken is. Daar is tot die slotsom gekom dat die keuse van stralingsdata, transposisie en verlies modelle ’n belangrike effek het op lang- sowel as kortstermyn energie voorspellings.
Description
Thesis (MEng)--Stellenbosch University, 2016.
Keywords
Photovoltaic power generation, Energy harvesting, Solar energy, Photovoltaic power systems, Photovoltaic cells, UCTD
Citation