Fragaki, A.

Markvart, T.

Southampton (e-Prints Soton)

DOES GLOBAL WARMING AFFECT THE DESIGN OF STAND-ALONE PV SYSTEMS ?A. Fragakia, T. MarkvartbMaterials Research Group, School of Engineering Sciences at Southampton University, Highfield, Southampton SO17 1BJ, UK.aE-mail:af8@soton.ac.uk, bE-mail:tm3@soton.ac.ukFigure 1: System sizing curveFigure 2: Variation of the Yearly Mean RadiationFigure 3: Increase in the requirement for storageDiscussionDuring the time period 1974-1990, there is an increase in the minimum number of storage days required in order that the system operates without shedding load (Figure 1). In fact, by the end of the 25 years we need almost twice the number of storage days than during the first years of this period.The corresponding decrease in the yearly mean solar radiation values during this period is not so pronounced (Figure 2), which means that it is the change in the correlation of the day-to-day radiation values, which lead to deeper discharges of the battery. Therefore, as it appears from our data, although the average irradiation changes little, there may be a systematic change in the ‘persistence’ of low solar radiation values over longer period than hitherto.Conclusions• A new methodology may be needed to predict the future battery state of charge for the design of PV systems. •The use of longer radiation data series is expected to show a variation in the profile of the battery state of charge, which can be possibly characterized statistically and incorporated in sizing.References1. Barra L, Catalanotti S, Fontana F, Lavorante F. An analytical method to determine the optimal size of a PV plant. Solar Energy 1984; 33:509-514.2. Chapman,R.N. ‘ A Simplified Technique for Designing Least Cost Stand-Alone PV/Storage Systems’,19th IEEE Photovoltaic Specialist Conference,New Orleans, 1987; pp.1117-1121.3. Chapman, R.N.Sizing Handbook for Stand-Alone Photovoltaic/Storage Systems, SAND87-1087,Sandia National Laboratories, Albuquerque, New Mexico, April 1987.4. Bucciarelli LL. Estimating loss-of power probabilities of stand-alone photovoltaic solar energy systems. Solar Energy 1984; 32(2):205-209.5. Bucciarelli LL. The effect of day-to-day correlation in solar radiation on the probability of loss of power in a stand-alone photovoltaic energy system. Solar Energy 1986; 36(1) 11-14.6. Sidrach-de-Cardona M, MoraLl. A general multivariate qualitative model for sizing stand-alone photovoltaic systems. Solar Energy Materials and Solar Cells 1999;59:185-197.7. Sidrach-de-Cardona M, MoraLl. A simple mode for sizing stand-alone photovoltaic systems 1998;55:199-214.8. Egido M, LorenzoE. The sizing of stand-alone PV systems: a review and a proposed new method. Solar Energy Materials and Solar Cells 1992;26: 51-69.9. Narvarte L, Lorenzo E. On the usefulness of stand-Alone PV Sizing Methods. Progress in Photovoltaics: Research and Applications, Prog. Photovolt: Res. Appl. 2000;Vol8:391-409.10. Markvart T. (ed). Solar Electricity. John Willey & sons. Chichester, 19941 2 3 4 5 6 7 8 9 101234567891011SIZING CURVEBattery size (Days of storage)Generator size (number of modules in parallel)70 72 74 76 78 80 82 84 86 88 90 92 942.72.82.933.13.23.33.4YEARLY MEAN RADIATION (JULY 1970-JUNE 1993)YearRadiation (kWh/m2)70 72 74 76 78 80 82 84 86 88 90 92 9412345678VARIATION OF REQUIRED BATTERY CAPACITY (JUL 1970-DEC 1994) YearDays of storage requiredIntroductionVirtually all PV system-sizing algorithms today are based on the statistics of solar radiation in the past. The sizing methods1,2,3,4,5,6,7,8, either use long-term monthly average radiation values or daily series of solar radiation data. However, meteorological evidence together with solar radiation series9 are beginning to emerge that indicate that past data may not provide all the clues to the future patterns. The aim of the present work is to examine the validity of this assumption, and demonstrate the sensitivity of the system output to the solar radiation conditions along the years. Objectives•To introduce a new sizing approach• To examine the effect of the long-term solar radiation profile, on the corresponding profile of the battery state of chargeMethodologyIn place of the traditional methodology based on LLP, an alternative methodology is suggested which determines the reliability of supply in terms of the duration of system operation without loss of load.ResultsThe profile of the battery state of charge of the system was obtained using 25 years of daily solar radiation data from 1974-1990 as an input in a long-term model, and assuming constant system efficiency. 

Does global warming affect the design of stand-alone PV systems

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