Design daylight availability for greenhouses using supplementary lighting

An analysis is presented of daylight availability data in terms of suitable design criteria for greenhouse supplementary lighting systems. “Percentile exceedance” values for the daily photosynthetically active radiation (PAR) integral are proposed as the relevant design criteria, and a method is presented for using these data to determine the target supplementary light level in a greenhouse. A large set of daily radiation data were analysed to generate percentile data for 201 locations in the USA varying in latitude from 18.4° to 71.3° and elevation from 1 to 2300 m. Analyses were carried out to assess the dependence of these percentile data on site latitude and elevation, and a mathematical model was fitted to the data. Results indicate that percentile data vary negatively with latitude and positively with elevation in a manner that can be calculated using these two inputs and a percentile-related coefficient with a resulting r2 of 0.982. The strength of relationship is greatest for moderate and high percentiles, with a mean absolute error (MAE) of 2.0 mol m−2 d−1 for percentiles of 70% and above. These results indicate that percentile exceedance data can be used as a basis for assimilative lighting system design, both for locations near areas where recorded data are available, and for locations where only latitude and elevation data are known.