Turning down the lights
Retrofitting parking structure lighting with automatic controls and more energy-efficient light sources can reduce parking facility operational costs and minimize the need to expand power plants.
By Donald R. Monahan, PE, Walker Parking Consultants, Denver -- Consulting-Specifying Engineer, 12/1/2008 1:00:00 AM
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Parking structures are significant users of electrical energy for lighting and can be a political hot-potato because they typically are illuminated 24 hours a day and, due to their open nature, are quite visible to the public. Due to the visibility and long operational hours, conservation of energy for lighting systems can contribute to both a savings in operating costs and a reduction of greenhouse gas emissions from the power plants that generate the power to operate these lighting systems. Another perk: tax deductions and rebates from local utilities for lighting retrofits.
Conservation of energy for parking garage lighting systems involves the use of energy-efficient light sources and automated controls to turn off lights when they are not needed. Automatic controls to detect daylight contributions can turn off the lights on the perimeter of the garage or, if adequate daylighting penetrates the garage, turn them off entirely to save energy.
Motion sensors may be used to turn the parking garage lights to full output when cars or people enter the garage and automatically turn off or turn down the lights when the garage is not occupied. Motion sensors are not practical for completely extinguishing metal halide or high-pressure sodium lamps because they take 10 to 20 minutes to restrike, but they may be used to dial down the light output by approximately 50% when the space is unoccupied. Photocells may be used for the top decks of the parking garage to turn the upper garage lights on at dusk and off at dawn.
Reducing the lights to 50% of their nominal lighting output when the facility is unoccupied could save 25% to 40% on lighting operational costs. For above-ground, open parking garages (20% of perimeter wall area is open at each level), lights near perimeter wall openings could turn off when there is adequate daylight infiltration. Computer calculations of daylight infiltration indicate that light fixtures within 30 ft of a perimeter wall opening could be turned off entirely, even on cloudy days. Light fixtures in the area of 30 to 60 ft from a perimeter wall opening could be turned off if a photocell mounted on an interior column at 5 ft above the floor measures adequate daylight infiltration (more than 20 foot candles). Those lights would turn back on if the daylight infiltration fell below 5 foot candles.
Lighting selection
Parking structures are typically lighted 24 hours per day, 7 days a week, or 8,760 hours per year, which makes the electrical utility cost the first or second largest expense for parking structure operation. The typical parking facility lighting system consists of 150 W metal halide or high-pressure sodium fixtures with a total input wattage of approximately 190 W per fixture, including the ballast power consumption. These metal halide fixtures may be placed on a spacing of approximately 30 ft by 30 ft (one every 900 sq ft). A 1,000-car parking structure will have approximately 360 light fixtures in the general parking areas. The total annual power consumption for this sample parking facility would be approximately 600,000 kWh. With an energy cost of $0.08 per kWh, the annual utility cost would be about $48,000 and could vary considerably in areas where energy costs are higher, such as on the East and West Coasts where utility costs are nearly double the national average.
Substituting more energy-efficient light sources also can reduce lighting operational costs. A light fixture with four 4-ft-long T8 lamps or two T5HO lamps will have nearly equivalent maintained illuminance as a 150 W metal halide or high-pressure sodium fixture. The energy consumption of the fluorescent fixture is 112 W for the T8 fixture or 123 W for the T5HO fixture, compared to 190 W for the metal halide or high-pressure sodium fixture. Therefore, there is a 41% energy savings for the T8 fixture or 35% savings for the T5HO fixture. However, fluorescent lamps lose light output in cold weather and can be used only in enclosed fixtures equipped with low temperature ballasts, where the temperature is frequently below freezing.
LED light fixtures are not impacted by cold weather and use 128 input W, resulting in an energy cost savings of approximately 33% compared to 150 W metal halide or high-pressure sodium fixtures. The unit cost of LED fixtures is three to four times the cost of a high-intensity discharge fixture, which generally precludes their widespread use.
Retrofitting parking structure lighting with automatic controls and more energy-efficient light sources not only reduces parking facility operational costs, but it also minimizes the need to expand power plants that contribute to global warming.
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