Optimizing Your Design 5 Once you understand all of the options for skylights and photocontrols it is important to be able to select products and design a system that will optimize the performance of your skylighting system and produce the most cost effective design The essential questions are How much of the time will daylight levels meet or exceed the desired illuminance levels What is the optimum skylight system to reduce energy consumption and cost and to provide good daylighting amenity In this chapter we discuss how different variables and choices in skylighting design affect the energy performance of the building and look at examples that allow you to compare the relative effects of each We will examine each of the following variables in turn Climate Climate is the most important influence on the energy performance of a skylighting system especially how the availability of illumination varies daily seasonally and between locations Heating and Cooling Heating and cooling energy losses and or savings provide a secondary effect on the savings achieved with a skylighting system Building Type The hours of operation the lighting power density the internal heat gains and the heating and cooling setpoints are all important determinants for skylighting savings Glazing Materials Choices of glazing materials determine how much daylight gets into a building and also how much heating or cooling requirements are affected Photocontrols Controls are essential for any energy savings from skylights Switching and dimming systems produce different patterns of savings We have used SkyCalc to run multiple calculations for a generic building with only one of these variables changing at a time The resulting graphs give a sense of the relative impacts of each variable they are not however absolute answers for any system optimizing your design 5 1 Once you have an idea how these variables interact you are ready to work on your own building design SkyCalc allows you to compare alternate designs for your particular building and location to generate specific savings values for your situation 5 1 Understanding Daylight Patterns The relationship between daylight illumination and the desired illumination in a building is the single greatest determinant of skylight savings Heating and cooling impacts are important but they are generally a second or third order effect Therefore we begin with a discussion on how much electric light and daylight are appropriate in a space 5 1 1 Design Target Illuminance The first step in designing a daylighting system is to determine the desired illuminance for the tasks to be performed The desired level of illuminance in a room will depend on the general function of the space and on the visual requirements of the task to be performed The Illuminating Engineering Society of North America IESNA has established general categories of illuminance ranges of illuminance values and a procedure for selecting illuminance levels depending on the type of activity in a space Figure 5 1 summarizes the IESNA recommendations for a number of common tasks for a specific type of activity and occupant These midrange values given in footcandles and lux are representative only The IESNA procedure for determining recommended illuminances specifies a range of illumination values for any given task There are a number of factors which should be taken into account to determine which part of the range should be used The assumptions used in Figure 5 1 are noted in its caption Building uses change over time and it would be wise to provide enough flexibility to accommodate potential changes Remember also that illuminance levels do not necessarily correlate to lighting quality which depends on the properties of the entire visual environment Many occupancies like retail manufacturing schools and churches require special consideration of the specific tasks the interior design and equipment and or aisle layout to understand appropriate illumination levels Refer to the IESNA Handbook and to other Recommended Practice documents published by IESNA for more detailed recommendations on illuminance values for specific applications and space use IESNA recommended illuminances are usually considered the minimum illuminance required for a given task Additional illumination is not generally considered a problem and is often a bonus However energy codes have functioned to set a maximum lighting power density for spaces based on the implementation of these recommended illuminances and 5 2 skylighting guidelines on the application of reasonably efficient technology and design The interaction of IESNA recommendations with energy codes has narrowed the range of expected interior light levels from electric lighting systems Once you have determined the desired illuminance levels for the primary tasks in Figure 5 1 your daylit space the next question is nat Representative urally how to size and design the skylight Illuminances for system to provide that illuminance This Typical Tasks is not the same kind of undertaking as Assumptions sizing and designing an electric lighting Occupants aged 40 55 system to provide a given illuminance room surface level on a task surface Electric lights pro reflectances 30 70 duce a nearly constant light output day demand for speed and night Daylighting systems provide a and or accuracy constantly changing light input to the important This building which varies hour by hour and information based on season by season recommendations in the IESNA Handbook The question of how to size the skylight 1993 system does not have any one answer Even a very small skylight can provide the minimum desired task illuminance some of the time a very large skylight system may provide more than the desired illuminance most of the day If the skylight system is too small it may not save enough energy and money to warrant the investment If it is too large it will provide more light than is needed and could unnecessarily increase heating or cooling energy consumption The variability of daylighting is one of its most appealing aspects and one of the reasons people respond so well to skylights The task illuminance provided by a skylight system may range from zero at night to three to four times the task minimum during the day Usually the extra light is a positive benefit provided illuminance levels are not so high as to cause visual discomfort problems Daylighting illumination per se is not regulated by energy codes
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