This article integrates forward thinking in efficiency with renewable energy generation to illuminate a large commercial or industrial building with zero net energy use, which of course means zero net CO2 emissions from lighting. For a more thorough exposition, you may download our white paper here.
The Four Critical Subsystems
To achieve net zero energy use over one calendar year, one must integrate four critical subsystems: luminaires (electric lighting), daylighting, photovoltaics (PV), and system controls. We will touch on each subsystem below and show how they can work together to provide a beautiful, well-lit building that consumes a net of zero energy.
Luminaires: For net zero buildings, LED technology is superior to T5 or T8 fluorescent technology. The essential reasons are higher light output per Watt, longer life, no maintenance, plus truly excellent system integration with intelligent controls and daylighting. LEDs can auto-dim or brighten as needed in response to the variability inherent from daylight.
Daylighting: Using a high efficiency sky dome for daylighting takes 1/10th of the rooftop area to illuminate the building interior with zero emissions than using PV to power electric lights. The data supporting this statement is beyond the scope of this summary, but can be found in our white paper. This leaves more rooftop area available for HVAC systems and for more PV to offset other building electrical loads. This is only helpful however if the sky dome is designed for good optical efficiency, no leaks, has high R-value, and is protected against long-term yellowing of the outer dome. When properly implemented, daylighting contributes to a dramatically more human-friendly interior environment.
Photovoltaics (PV): Large commercial and industrial buildings are often ideal candidates for rooftop PV solar arrays with simple ballasted fixed mounting over low-slope membrane roofs, at a scale which is very cost-effective. Yet it is often true that insufficient rooftop area exists to achieve net zero unless building efficiency measures are used in combination with PV generation. Preserving available rooftop area through the use of daylighting can help solve this problem.
Intelligent Controls: Intelligent controls are required to meet our system objectives. Intelligent lighting control includes: motion sensing, photo-sensing, data logging, auto-dimming, and wireless communications.
Keys to Successful System Integration
The daylighting system and the LED lighting system must emit similar distributions of light (called a candela distribution). If they do not then the pattern of illumination within the building will be constantly shifting as the system varies from daylit to LED lit. However if the daylighting and LED lighting systems do have similar candela distributions, then it becomes possible to operate on full daylight, full electric light, or in mixed mode with equally pleasing results.
The LED lighting system must constantly adjust output in response to the inherent variability from daylighting. The fact is that the intensity of light available from the sky is constantly changing and varies from 10,000 to 0 foot candles (107,000 – 0 lux). Furthermore the LED lighting system must be sized to provide adequate illumination at night time, and must reduce or shut off power consumption whenever natural light is available to illuminate the structure. This automatic control function is known as “daylight harvesting”.
The PV system must be sized large enough to generate enough energy on an annual basis to offset the energy used by the lighting system. Employing daylight harvesting can drastically reduce energy use during daylight hours. LEDs reduce energy used at night time compared to other, less efficient electric lights. Deploying intelligent controls ensures that energy is not wasted, but is sparingly used only as needed to make up for the variability of natural light. Combining these elements together enables the PV system to remain a small percentage of the roof area, making net zero operation more technically and economically feasible.
Design Algorithm for Net Zero Lighting
Step 1: The system is designed to operate at night with only the LEDs for illumination. Define spacing of luminaires using industry proven simulation software for this purpose. For an integrated lighting solution such as the hybrid LED-Whole Sky Dome from Replex, this also defines the required spacing of the skylights. (We stress once again the importance of the sky dome and LED systems having well-matched candela distributions) Compute the total instantaneous power draw of the entire system at 100% output at night time full-on operation. This is simply done by adding up the power draw of all luminaires and controls assuming 100% full-on conditions.
Step 2: Define the expected operating hours for the building. Is this a 10 hour/day 5 day/week facility, or multi-shifts, or even a full 24/7 operation? Take into account daylight harvesting by subtracting operating hours where the LEDs are completely off. Don’t forget to estimate the number of hours per day operating in mixed mode at fractional power. At night, assume the LEDs operate at 100% power. With this information, calculate the annual electrical energy usage in KWH to illuminate the building.
Step 3: Size the PV array to produce enough KWH per year to offset the KWH per year consumed by the lighting system. Well proven PV system models are used in combination daylight resource data for the building’s location published by the National Renewable Energy Lab (NREL).
One Example: 112,000 sqft (10,400 m2) building located in Ohio, 40 ft (12m) ceiling height, designed for 45 fc (484 lux) average illumination at the work plane, operating 24 hours/day 5 days/week. Simulation suggests spacing skylights/luminaires on a 20×25 foot (6×7.6 m) array, resulting in 3.2% of roof area for skylight aperture. The proposed system employs 213 Replex Whole Sky Domes, each integrated with 4 Digital Lumens LLE-6 luminaires and 1 DLA-LLE intelligent control module. Annual energy consumption is estimated at 187,391 KWH/year, which can be completely offset by a 151 kW photovoltaic array, using a ballasted fixed mount facing south at a 10 degree tilt angle. Such an array consumes approximately 9% of the roof area for PV modules, plus another 10% of the roof area for spacing between rows and buffer space around the PV. Altogether this system leaves approximately 70% of the roof area still available for HVAC equipment and for additional PV that can be used to offset other building electrical loads.
For more information, please check out our white paper.
Posted on 10/28/2015 at 12:00:00 AM
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