Comparative Analysis of All-Sky Luminous Efficacy Models Based on Calculated and Measured Solar Radiation Data of Four Worldwide Cities

Donghyun Seo

doi:10.1155/2018/8180526

Abstract

Luminous efficacy model uses solar radiation data to generate illuminance data, and its performance also depends on the quality of solar radiation data. Various luminous efficacy models are reviewed and evaluated to select a universal luminous efficacy model. Since most luminous efficacy models are fitted with specific local climate characteristics, the model that has the least locality as well as accuracy is a mandatory requirement. Three representative luminous efficacy models are selected and evaluated with measured solar radiation and illuminance data from four worldwide cities. It was found that all the evaluated models provide good predictions (over 0.96 R2 value) for both global and diffuse illuminance. Among them, the Perez luminous efficacy model shows the highest performance in terms of accuracy and bias. However, illuminance data prediction based on estimated solar radiation data is more common practice rather than those from measured solar data. The performance of the selected luminous efficacy models is evaluated when recently proposed universal solar radiation model supplies predicted solar radiation data. The result indicates that the quality of estimated solar radiation data has a much deeper impact on the performance of the luminous efficacy model. Within the current limited technology and measured data resource, the consecutive processing of the modified Zhang and Huang solar model and Perez luminous efficacy model could provide the best option to predict both global and diffuse solar radiation and illuminance. But, users of the model-based illuminance data should interpret their simulation results with the error (30%~40% in RMSE and ±6% in MBE) in predicting global and diffuse illuminance.

Highlights

Luminous efficacy in daylight is defined as the ratio of illuminance to solar radiation that is expressed in Lm/W
EnergyPlus adopts the Perez luminous efficacy model [1] and DOE-2 [2] uses the average value model to artificially generate outdoor illuminance data when the data is missing in input weather file
Littlefair model yields the worst performance among the considered models; but still, this model provides high R2 values in the range of 0.96~0.99 for both global and diffuse illuminance

Summary

Introduction

Luminous efficacy in daylight is defined as the ratio of illuminance to solar radiation that is expressed in Lm/W. Luminous efficacy model is one of the essential parts in detailed building energy simulation tools to evaluate the performance of daylighting systems and their controls because measured illuminance data are scarce and are even more difficult to obtain than measured solar radiation data in most cities in the world. The vast majorities of building energy and daylighting simulation tools rely on modeled illuminance data rather than measured data. There are currently no comprehensive detailed building energy simulation tools that have a credible luminous efficacy model for worldwide cities because most of the models use site-dependent correlation coefficients. The development of universal models for predicting solar radiation and illuminance levels is an important part of the development of a comprehensive building energy simulation tool

Results

Discussion

Conclusion