A method for the Dynamic Characterisation of Daylighting in Mediterranean Climates

The traditional daylighting calculation methods and the metrics used for its quantification have some limitations. For example, these methods do not take into account, effectively and realistically, the consequences of the dynamic characteristics of natural light and in particular those associated with its energy-related impacts and with the influence of the occupants of the buildings in the final daylighting performance.

In view of that, an integrated method for characterization and realistic prediction of the dynamic performance of daylighting was developed. The new method takes into account: i) the characteristics of the luminous climate in the southern European regions, where non-overcast sky conditions prevail; ii) the guarantee of an adequate indoor visual environment and minimization of visual discomfort; iii) the energy-related impacts of daylighting; iv) the influence of environmental management systems and their control strategies in the final daylighting conditions and energy use; and, v) the expectations, preferences, attitudes and behaviours of individuals towards their indoor environmental conditions and control systems at their disposal.

The development of the new method included the following tasks: i) the mathematical modelling and development of new evaluation parameters for the dynamic characterization of daylighting, ii) the experimental characterization of selected case studies including their shading systems, iii) the evaluation, through post-occupancy evaluation surveys, of the preferences and behaviours of individuals towards their luminous environment and control systems and subsequent proposal of a behavioural model for daylighting.

The new method is based on an improved version of the Daylighting-Hours Indicators Method. As improvements, it incorporates the impact of the electric lighting and shading systems (types, zoning, controls and patterns of use) in the energy use for lighting and final daylighting conditions, as well as the information from a behavioural model that was also developed. The method determines the value on an indicator called daylighting-hours indicator that can be applied to an interior space. Daylighting-Hours are the average number of hours of the period of use of that space during which daylighting illuminance levels exceed a reference level and ensures visual comfort. The indicator is determined for a point of a plane which is representative of the space.

Particularly important were the findings of the post-occupancy evaluation surveys that lead to the development of the behavioural model. Some of the most interesting results of that model were the following:

  • The occupants prefer bright light environments with abundant natural light and, in general, if the daylighting levels are not too low (larger than 100 lux), and if they have a high degree of freedom in controlling the artificial lighting and shading systems, they tend to use daylight instead of artificial light throughout most parts of the day. The typology and functional characteristics of the interior spaces are some of the most important constraints to the maximisation of daylight use;
  • There is a greater tolerance to visual discomfort problems due to daylight than to artificial lighting. Also related with these findings, the illuminance threshold for the occupants to consider the luminous environment as adequate is lower for daylight than for artificial light;
  • When comparing the measured illuminance levels with the occupants’ subjective appraisal of the “amount of daylight” in interior spaces, a significant spread of judgement was observed. Although the preferred daylight illuminance levels were to be found between 300 and 500 lux, the 100 – 300 lux and the 500 -1000 lux ranges were closely ranked;
  • Depending on the perceived individual degree of freedom in adjusting the shading devices and on their perceived efficiency, in general, individuals consciously and consistently operate these shading devices on a daily basis. Also, the main reasons referred for having to adjust the shading devices were: the need for more daylight, to establish contact with the outer environment, the necessity to control glare from the sun and the need to control the solar heat gains (particularly in summer). Moreover, it also identified a larger trend for the active dynamic control of the shading systems than for the control of the electric lighting.
  • The study carried out an interdisciplinary approach and was based on the assumption that the objective characterizations of indoor luminous environments have not, by itself, the ability to capture the subjective sensory experiences of individuals, which in turn influence the actual daylight performance of buildings. Therefore, the study highlighted and characterized the influence of individuals on the final daylighting conditions, through their preferences, attitudes and behaviours towards the environmental control systems and related control strategies. This approach proved to enhance the explanatory and predictive capacities of more traditional daylighting analysis tools and can be applied to the design of new and existing buildings.

    Figure 1: The dynamic characteristics of daylight can contribute to the comfort and well-being of individuals in buildings.