Estimating Solar Irradiance and Components

-The performance of models to estimate solar irradiance and its components is assessed using data for six Canadian stations for nine years (1968-76). Greatest emphasis is placed on a model, the MAC model, which uses cloud information from different layers. Here, effects on model estimates of using observed cloud layer opacities instead of amounts is examined because after 1976 the latter quantity is no longer recorded. Effects of aerosol are also examined. Two other models are considered here; a version of the MAC model which uses total cloud information rather than layer information, and Rietveld's[1] sunshine-based model. In general, the layer model yielded the best results. This is in agreement with a previous Canadian study[2]. RMSE values are between 11 and 15 percent for global irradiance on a daily basis and decrease below 10 percent for averaging periods greater than 2--4 days. Daily RMSE values for the direct beam and diffuse components are 25 per cent. These decrease below 10 per cent for averaging periods larger than 10-15 days. Errors increase when layer opacities are used but results are still acceptable. Aerosol effects can be neglected for much of the country except in Montreal and Toronto. Rietveld's model gives results of lower accuracy than the MAC model but may be useful for quick, easy estimates. The MAC model using total cloud amount performed poorly. INTRODUCTION Measurement networks do not provide solar radiation data with sufficient spatial resolution and temporal continuity to define the climatological potential for solar energy utilization. Calculation procedures are therefore needed to fill spatial gaps where measurements are not made and to fill temporal gaps at stations where measurements are made. Several methods for calculating global irradiance will be discussed in this paper and their performance assessed with lengthy records of measured irradiance at six Canadian stations. Attention is confined to methods which use readily available meteorological data. Since cloud is the major modulator of irradiance these methods depend on available cloud measures. Canadian stations measure hourly sunshine and observe total cloud amount and total opacity, and amount, opacity and cloud type in each of four layers. If these data can be used to provide satisfactory estimates of global irradiance the present measurement network of 51 stations in Canada can be supplemented by over 100 stations. (a) Overview Models were selected to utilize the cloud information collected at Canadian stations. All models are non-spectral. The most physically-based models, termed "layer models", include those developed at the Center for Environment and Man--CEM[3], McMaster University--MAC [4] and the University of British Columbia-CLS[5, 6]. Their general form for global irradiance G n G = Go ~ ' t , J ( a ) (1) where Go is the cloudless sky irradiance, ~ is the ith cloud layer transmittance and [ (a) is a function of surface albedo a to incorporate multiple reflections between ground and atmosphere. Direct beam irradiance on a horizontal surface can be stated