Definition and Use of a Daylight Coolness Index

This paper examines the relationship between lighting energy savings from daylighting and daylight-imposed cooling loads. From DOE2.1C simuIation results for specific commercial buildings and glazing options, we show that for any daylighted or electrically lighted space, the cooling load component from natural or artificial lighting can be broken into three component factors. Each of these factors can be expressed mathematically as constants that are a function of glazing characteristics and illumination levels within the space. These three constants can then be combined into a single daylight coolness index. Thus, from (model or simulation) illuminance measurements for a proposed daylighted space, a designer can quickly compare the relative cooling load impacts of different daylighting designs.