Case study: daylight-harvesting variables
By studying the variables of daylight harvesting under various conditions, lighting designers can determine what works best.
To complete this study, IES-VE software was used as the energy and daylighting simulation tool. It has Radiance, commonly used for daylighting analysis, as a built-in tool. Radiance calculates the amount of the light that penetrates a building's façade based on several inputs, such as building geometry, glazing percentage, and average historical sunny/overcast information, and allows the user to carry over the daylighting information into the energy model directly. It also allows the user to locate daylighting sensors within the space.
Compared with a standard building with daylight dimming, the lighting designers needed to understand four different scenarios. Each iteration answers the following:
- What is the difference in lighting energy consumption without daylighting controls (minimally compliant with ASHRAE Standard 90.1-2007)?
- What is the difference in lighting energy consumption if the building were minimally compliant with ASHRAE Standard 90.1-2010?
- What is the difference in lighting energy consumption for the same building in a different location?
- What is the difference in lighting energy consumption when selecting glazing with a greater ability to transmit light?
With the understanding that it is impossible to discuss these questions under every scenario of climate, building usage, and glazing construction, assumption have been made for the energy model under which observations can be applied to a majority of buildings.
To answer these questions, we will start with a simplified, hypothetical building with a "standard" daylighting configuration. The base case building is an approximately 100,000-sq-ft, 4-story square commercial office building composed of 90% office area and 10% core. Each floor is divided into four 15-ft-deep perimeter open-office zones, two interior open-office zones, and a central core. The building is modeled with a 12-ft floor-to-floor height and glazing is modeled as 40% of exterior wall area. Office lighting density was 0.98 W/sq ft. All lighting in a perimeter zone is controlled via one sensor, placed approximately at the center of the zone.
With four perimeter zones per floor and four floors, there are 16 sensors modeled within the building. The lighting is controlled to continuously dim from 100% total lighting load down to 10% as the illuminance at the work plane reaches 50 lux. All office light fixtures are modeled with automatic lighting controls. The simulated building is located in New York City. The images below illustrate building geometry, zoning, and sensor placement.
The four questions will be answered with four different tests, changing only one parameter against the base case. Test 1 removes the daylighting controls from the base case. Test 2 follows the criteria for a minimally compliant building with ASHRAE 90.1-2010, which includes minimizing the sidelighted area, modeling automatic sensors, and controlling daylight dimming with stepped control. Test 3 shifts the location of the building to Miami. Finally, test 4 increases the variable light transmission (VLT) to 75%. Inputs and results are shown in Table 2.
Results show that our base case building consumes approximately 753 MMBtus of electricity annually. Test 1 demonstrates that daylighting can be incredibly effective in reducing lighting consumption.
So how does the base case compare to a minimally compliant ASHRAE 90.1-2010 building? Test 2 shows the base case building still exceeding mandatory lighting requirements. ASHRAE only requires dimming control for fixtures 8 ft from the perimeter wall based on the 3-ft sill height and 5-ft window height. It also requires, at minimum, stepped dimming control. Auto-shutoff is also modeled as a requirement. Stepped dimming control has lower costs, but it doesn't capture all the savings a fully dimmable ballast can provide. Modeled automatic sensors are expected to have higher savings, particularly on weekends, because ASHRAE default schedules normally assume a fixed 5% lighting load even when offices are not occupied. However, the base case building still exceeds an ASHRAE minimally compliant building by 5%.
The results in test 3 demonstrate only slightly greater savings when considering a similar building constructed in a warmer climate. This proves that similar lighting energy savings can be achieved in both colder and hotter climates, and daylighting effect is not directly correlated with climate.
Finally, in test 4, although we have nearly doubled the VLT percentage, we observe that increased VLT of the glazing only provides a slight lighting energy savings improvement. In a climate like New York City's, daylighting is sufficient for designers to consider low-VLT glazing systems. However, a cloudier climate may be more sensitive to the VLT percentage and requires buildings to increase this value to capture sufficient daylighting for savings.
Christian Paunon is a mechanical engineer at Arup and enjoys working on projects that deliver high value for clients. His experience in energy modeling includes developing new-building and renovation energy models for energy-code compliance and LEED. His work extends throughout the building lifecycle, from providing preliminary energy studies to aiding in the design process to constructing energy models for use in understanding energy-conservation measures in existing buildings.