STOP & SHOP, NEW PALTZ, NY

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1 Daylighting Case Studies STOP & SHOP, NEW PALTZ, NY Project Highlights Skylights with 50 degree splayed light wells Dimming T5 luminaires Hybrid open loop and closed loop photocontrol system Stop & Shop is a chain of almost 400 grocery stores located throughout New England, New York, and New Jersey. This case study evaluates the performance of innovative skylights used to provide daylight illumination in Stop & Shop s 70,000 square foot store in New Paltz, NY. The skylights are designed to capture sunlight more effectively than conventional skylights when the sun is low in the sky, at the beginning and end of the day. Daylighting in stores during these times is highly desirable because it creates a bright and inviting environment for shoppers who stop by on their way to or from work in the morning and evening. New Paltz, NY, is located 80 miles north of New York City, at 42 north. Summers are warm and humid with an average daily high temperature of 82 F. Winters are cold with an average daily minimum of temperature of 34 F and frequent snow. Precipitation and cloud cover occur intermittently at all times of year, meaning that the skylights have to perform effectively under both clear and cloudy skies. NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011

2 DAYLIGHTING DESIGN Skylights are located above the shopping aisles, but not over the produce area or over the checkouts because the designers preferred to illuminate fruit and vegetables with electric spotlights, and to avoid checkout screen reflections caused by overhead skylights. The skylights are spaced on a 20 x22 grid. The 5 square skylights have 1 high domes, and are triple glazed with clear prismatic acrylic. A 3 tall light well finished with reflective foil connects the skylight to a prismatic diffuser flush with the finished ceiling. The skylight to floor area ratio is 5.7%, which gives an effective aperture of 1.5%, using transmittance factors as calculated by SkyCalc 3.0. The skylights are mounted at the 17 roof plane. A 50 degree splayed well (formed with ceiling tiles) passes through the 3 deep plenum to a 10 square opening at the 14 suspended ceiling height. The splayed light wells provide three benefits. First, they increase the efficiency of light transmission by shortening the skylight well at this store the well is 2 6 shorter than it would otherwise be, which improves overall skylight photometric efficiency by approximately 10% 1. Second, the splayed light wells improve the quality of the light by creating a transitional brightness from the brighter skylight to the dimmer ceiling, which softens its appearance from below, reducing glare. Third, the light wells increase the height of the diffusers above the floor, which allows the skylight rows to be spaced further apart while maintaining uniformity of illuminance. This means that fewer rows of skylights are needed. In this store, the increase in height also allowed the skylight rows to be spaced directly in line with the shopping aisles, resulting in a very clean architectural appearance. Skylights with splayed light wells improve the uniformity of lighting and soften the appearance of the bright skylights 1 Source: Skylight Design: Photometric Characteristics. Owen Howlett, Jon McHugh & Lisa Heschong. IESNA Annual Conference, Tampa, Florida, July 25 28, NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011

3 LIGHTING CONTROLS Photocell mounted on the light well creates a hybrid open loop and closed loop control system. VISUAL ENVIRONMENT Daylighting Case Studies To ensure that all the luminaires are dimmed to the same level for a consistent appearance, only one photocell is used for the entire sales floor The photocell is located on the underside of one of the splayed light wells, looking down at 45 degrees. This means that the photocell is receiving reflected light from the sales floor (which would make it a closed loop system), but is also receiving some light directly from the skylight itself. This makes it a hybrid closedloop and open loop system. In a sidelit classroom school or office, a similar hybrid system could be achieved by locating a ceilingmounted photocell close to a window. Studies have shown that photocells in these locations create very effective and robust photocontrol systems. The maintenance staff can view the setpoints of the photocontrol system using a central graphical interface. The interface allows them to control dimming levels, thresholds and fade times, and to create an unobtrusive and comfortable environment for shoppers. The store s interior visual environment is diffuse, open and shadow free in the shopping aisles, and warmer and more focused in the produce area. The contrast between the diffuse and focused lighting creates a clear visual boundary between those two spaces. The ceiling over the aisles is white with 85% reflectance 2 x4 suspended tiles. It is illuminated from below by continuous row direct/indirect T5 fixtures on 4 suspension cables, which creates a very uniform brightness on the ceiling. The shopping aisles are 7 wide with 7 high shelves, and the floors are highly reflective (57%) to maximize reflected light to items on the lower shelves. The electric lighting provides between 40 and 45 footcandles of horizontal illuminance along the centerline of the aisle. The photocontrol system dims the luminaires to a minimum of 50% of full output. Dimming the luminaires to less than 50% of full output would result in darker ceilings during daylight hours and would potentially create a glare effect from the bright skylights, making the stores less visually comfortable. NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011

4 DAYLIGHTING PERFORMANCE In addition to providing basic illumination and conserving electric lighting energy, the daylighting system illuminates the vertical displays of goods. One of the key performance metrics for store lighting is whether it can illuminate the lowest display shelf effectively. This is often difficult because the lowest shelf is furthest from the electric light source, and the displayed items are at an oblique angle to the incoming light. Electric lighting systems overcome this by using carefully positioned luminaires which use reflectors to direct as much light as possible to the bottom of the display. But because daylight changes constantly, it cannot be controlled as accurately as electric lighting. As shown in the graph to the right, daylight typically provides only about two thirds as much light as the electric lighting system provides, in terms of vertical display illuminance for this particular display 2. In the graph, to provide a fair comparison, both the daylight and electric light are providing 50fc horizontal illuminance at floor level. Therefore, setting the photocontrols to simply maintain a constant horizontal illuminance in the store would result in reduced illuminance at the shelves. One of the benefits of an open loop control combined with dimming ballasts is that it can be set to trade off daylight against electric light more gradually, so that the combined illuminance from daylight and electric light is higher than the design illuminance from electric lighting alone. 2 To allow a fair comparison with electric lighting, illuminance levels are normalized to 50fc in the center of the aisle. The graph is based on spot measurements of vertical illuminance taken at the front of the shelving units. NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011

5 Daylighting Case Studies The second graph shows average daylight illuminances at floor level, taken at the midpoint between four skylights. The colored lines show how the average illuminance varies hourly and monthly. This graph illustrates that skylights provide much more light during the summer than in the winter, a typical daylight design problem that is not easily solved. Increasing the skylight area to augment winter illumination increases summer heat gains, but reducing skylight area sacrifices winter daylight levels. Getting this balance right is essential to good design, and can be improved by using advanced sun tracking skylights or skylights that are shaped to capture low angle sunlight. Daylight autonomy 3 was measured for six points in the store (all at center aisle), and the values ranged between 34% and 61%, with an average of 44% daylight autonomy. ENERGY PERFORMANCE We recorded illuminances at two locations directly beneath the skylight and at the center point between four skylights. In both cases, the logger was mounted downward facing on the ceiling, and was calibrated on site to the illuminance at floor level. Illuminances were recorded for six months to ensure that a range of sky conditions were included. Because the electric lighting is dimmed to the same level throughout the store and daylight levels vary relative to skylight position, the relationship between daylight and electric light levels differs slightly across the store. The graph shows that directly beneath a skylight, the daylight illuminance reached 50fc before the electric light began to dim, and that electric light reduced to a minimum output (50%) when daylight reached 70fc. At the midpoint between skylights, where daylight levels are lowest, the electric lights began to dim when daylight reached 30fc, and stopped dimming by 50fc. Taken together, these graphs show that the photocontrol system setpoints are fairly aggressive because they begin to dim the electric lights as soon as daylight levels exceed the design illuminance (30fc), but are fairly conservative because the luminaires only dim to 50%. The table below shows the percentage lighting savings due to the photocontrol system, for each month studied. The savings are shown 3 Figures quoted are DA300, i.e., the percentage of the time that daylight levels exceed 300 lux during the period 8:30am to 5:30pm, accounting for daylight savings time. NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011

6 relative to a typical 8am 6pm working day and a 24 hour baseline. (Stop & Shop stores are typically open 24 hours/day.) The installed lighting power density is 1.35 W/sf over the sales area. The annual lighting energy use of the building is 11.3 kwh/sf annually, which is a savings of 0.52 kwh/sf annually (4%) attributed to dimming the electric lighting when sufficient daylight is available. June July Aug Sept Oct Nov Dec Savings 8am 6pm 16% 17% 16% 10% 2% 0% 0% Savings over 24 hours 7% 8% 7% 5% 2% 0% 0% The average annual lighting energy reduction is 4.3%, which is low compared to the 14% savings predicted by the SkyCalc 3 software using a model of this building. This may be because the weather in New Paltz (which combines frequent cloud cover and winter snow cover) was unusually cloudy for the monitored period. Also, SkyCalc does not take into account the fact that snow may settle on the skylights, blocking the daylight completely. The measured savings in November and December and most of October were zero, so we believe that accumulated snow accounts for much of the difference. A close analysis of the data showed that the luminaires were dimming up and down repeatedly during the day. If the photocontrol setpoint were set slightly lower, the luminaires would remain dimmed for longer periods, which would significantly increase savings. And more importantly, if minimum electric lighting thresholds were reduced to 10%, large increases in savings could be achieved. NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011

7 Daylighting Case Studies STAFF COMFORT SURVEYS Two online surveys one conducted in the summer and one in the winter to see whether the performance of the system differed over the course of the year resulted in 32 completed responses. The results of four of the survey questions are reported relative to a benchmark of high performance daylit buildings from a recent study 4. The overall response to the lighting quality was strongly and consistently positive. The Stop & Shop responses were close to the average responses from the benchmark study. 4 Lisa Heschong. Heschong Mahone Group Daylight Metrics Project: Final Report. California Energy Commission. Publication Number: TBD, PIER program. NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011

8 LESSONS LEARNED The splayed light wells allowed the skylights to be spaced farther apart and to match the layout of the sales floor, enhancing the overall design aesthetic of the store. The splayed light wells allowed the skylight well to be shortened, which improved photometric efficiency The savings from the photocontrol system were low (4%), but could be increased by changing the system setpoints to dim luminaires to 10% output, or turned off under full daylight conditions, such as midsummer. The daylight threshold could also be lowered to 30fc value to increase savings (however, this would reduce illuminances below the 40fc design level for some of the day). Measured levels of daylight in the store are lower than expected. This may be due to unusually cloudy or snowy climatic conditions during the monitoring period. In general, it is important that skylights are kept clean and unobstructed, to maximize the achieved daylight levels. NYSERDA Daylighting Case Studies Stop & Shop, Prepared by the Heschong Mahone Group 2011