Solar Lighting: Making Steps Off-Grid towards a Sustainable Future

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Sponsored by SELUX Corporation
James Brigagliano

Another relevant item to compare when evaluating luminaires is the amount of lumens delivered. Delivered lumens refer to the total amount of lumens exiting a luminaire. Total input wattage, which considers the total watts consumed by the ballast or power supply, can be used when comparing two luminaires with a known delivered lumen value to identify how many delivered lumens are being produced for the amount of watts consumed. An example of this would be a luminaire delivering 3400 lumens and consuming 44 total watts would relate to 77.3 lumens per watt. Lumens per watt can be referred to as the efficacy of the luminaire; more lumens per watt is good. However, it is important not to use this as the only metric in which to compare luminaire performance because where and how the light is distributed is equally important.

LCS Example showing three primary angles for back-light, up-light and forward-light (yellow, blue & green) and ten secondary angles used to evaluate luminaire distribution in more detail

Source: IESNA Luminaire Classification System TM-15-07

 

System Sizing

To provide some additional insight into how systems are sized we will cover the effects of location and the basics of how panels and batteries are sized. PV systems can be sized by hand or with the use of a developed spreadsheet or computer software. All methods take into consideration the specific location of the site.

Location. Every location on Earth experiences an average of 12 hours of darkness per day for a year, but the actual number of hours of darkness on any particular day of the year varies from place to place. Locations around Earth's equator receive about 12 hours of darkness each day. In contrast, the North Pole receives 24 hours of darkness for a few months in winter. PV panels and batteries must be sized for a specific geographic location since the amount of time the system may need to operate fluctuates. System sizing based on a specific geographic location is necessary since the amount of available solar resource also fluctuates from place to place.

Peak sun hours is an equivalent measure of total solar irradiation in a day, 1 peak sun hour = 1,000w/m2 (e.g., a day with average irradiance of 600 W/m2 over 8 hours may only reach peak sun around noon for an hour or less. The total irradiation of 4800 Wh/m2 is equivalent to 4.8 peak sun hours). Solar radiation data sets include the amount of solar energy that strikes a surface at a particular location and time on earth. These data sets are based on 30 years of collected data. Organizations such as the National Renewable Energy Laboratories (NREL) as well as others make these data sets available. Data sets are based on peak sun hours average/max/min for a given month or year. They also consider tracking systems and array tilt angles.

 

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Originally published in Architectural Record
Originally published in November 2010

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