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Stanford CEE 215 - Lecture Notes

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Optimizing Your DesignUnderstanding Daylight PatternsDesign Target IlluminanceFig. 5-1: Representative Illuminances for Typical TasksDaylight SaturationFig. 5-2: Daylight Saturation with Varying Skylight AperturesFig. 5-3: Optimization Graphs - San Bernardino Grocery StoreYearly Illumination PatternsFig. 5-4: Yearly Illuminance PlotsMonthly Illumination PatternsFig. 5-5: Ave. Monthly IlluminationWeekly Illumination PatternsFig. 5-6: Weekly Illuminance PatternsDesigning for SavingsHeating and Cooling EffectsFig. 5-7: Annual Energy and Cost Savings by End UseSavings by Climate ZoneFig. 5-8: Annual Energy and Cost Savings by Climate ZoneSavings by Building TypeFig. 5-9: Annual Energy and Cost Savings by Building TypeSavings by Glazing TypeFig. 5-10: Annual Energy and Cost Savings by Glazing TypeSavings by Control TypeFig. 5-11: Annual Energy and Cost Savings by Control TypePutting It All TogetherOnce you understand all of the options for skylights and photocontrols, it is important to beable to select products and design a system that will optimize the performance of yourskylighting 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 illuminancelevels?• What is the optimum skylight system to reduce energy consumption and cost, andto provide good daylighting amenity?In this chapter we discuss how different variables and choices in skylighting design affectthe energy performance of the building, and look at examples that allow you to comparethe 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 ofa 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 providea secondary effect on the savings achieved with a skylighting system.• Building Type: The hours of operation, the lighting power density, the internalheat gains, and the heating and cooling setpoints are all important determinants forskylighting savings.• Glazing Materials: Choices of glazing materials determine how much daylightgets into a building, and also how much heating or cooling requirements areaffected.• 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 ofthese variables changing at a time. The resulting graphs give a sense of the relative impactsof each variable; they are not, however, absolute answers for any system.Optimizing Your Design5.5-1optimizing your designOnce you have an idea how these variables interact, you are ready to work on your ownbuilding design. SkyCalc allows you to compare alternate designs for your particularbuilding and location to generate specific savings values for your situation.The relationship between daylight illumination and the desired illumination in a buildingis the single greatest determinant of skylight savings. Heating and cooling impacts areimportant, but they are generally a second- or third-order effect. Therefore, we begin witha discussion on how much electric light and daylight are appropriate in a space.5.1.1. Design Target IlluminanceThe first step in designing a daylighting system is to determine the desired illuminance forthe tasks to be performed. The desired level of illuminance in a room will depend on thegeneral 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 generalcategories of illuminance, ranges of illuminance values, and a procedure for selectingilluminance levels depending on the type of activity in a space. Figure 5-1 summarizes theIESNA recommendations for a number of common tasks for a specific type of activity andoccupant. These midrange values, given in footcandles and lux, are representative only.The IESNA procedure for determining recommended illuminances specifies a range of illu-mination values for any given task. There are a number of factors which should be takeninto account to determine which part of the range should be used. The assumptions usedin Figure 5-1 are noted in its caption.Building uses change over time and it would be wise to provide enough flexibility toaccommodate potential changes. Remember also that illuminance levels do not necessarilycorrelate to lighting quality, which depends on the properties of the entire visual environment.Many occupancies, like retail, manufacturing, schools, and churches, require special con-sideration of the specific tasks, the interior design, and equipment and/or aisle layout tounderstand appropriate illumination levels. Refer to the IESNA Handbook, and to otherRecommended Practice documents published by IESNA, for more detailed recommendationson illuminance values for specific applications and space use.IESNA-recommended illuminances are usually considered the minimum illuminancerequired for a given task. Additional illumination is not generally considered a problem, andis often a bonus. However, energy codes have functioned to set a maximum lighting powerdensity for spaces, based on the implementation of these recommended illuminances andUnderstanding Daylight Patterns5.1.skylighting guidelines5-2on the application of reasonably efficient technology and design. The interaction of IESNArecommendations with energy codes has narrowed the range of expected interior lightlevels from electric lighting systems.Once you have determined the desiredilluminance levels for the primary tasks inyour daylit space, the next question is, nat-urally, how to size and design the skylightsystem to provide that illuminance. Thisis not the same kind of undertaking assizing and designing an electric lightingsystem to provide a given illuminancelevel on a task surface. Electric lights pro-duce a nearly constant light output, dayand night. Daylighting systems provide aconstantly changing light input to thebuilding, which varies hour by hour andseason by season.The question of how to size the skylightsystem does not have any one answer.Even a very small skylight can provide theminimum desired task illuminance someof the time; a very large skylight system may provide more than the desired


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Stanford CEE 215 - Lecture Notes

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