Lighting Research Center Lighting Research Center
    Volume 9 Issue 3
July 2006    
application - The use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications. ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. compact fluorescent lamp (CFL) - A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. grid - The combination of electric power plants and transmission lines operated by an electric utility. lamp - A radiant light source. lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. halogen lamp - An incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. luminance - The photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (square feet or square meter). glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. illumination - The process of using light to see objects at a particular location. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. fluorescent lamp - A low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light. inverter - Also known as “power inverter.” A device used to convert direct current (dc) electricity into alternating (ac) current. irradiance - The density of radiant flux incident on a surface. light-emitting diode (LED) - A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement. photon - A small bundle or quantum of electromagnetic energy, including light. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels.
What is the process to determine the appropriate size of PV panels for a particular application?

Two examples are given below to demonstrate the process involved in determining the size of photovoltaic (PV) panels for a particular PV lighting system. The first is a parking lot luminaire that provides approximately 10-lux illuminance on the ground. The second is a post-top luminaire that will provide about 0.5-lux illuminance (assuming a luminaire that has a single light source and an optical efficiency of 50%, which uniformly distributes all light output on a circular area with a radius equal to the pole height).

These are the same examples used in the previous section (see "How does solar radiation vary by location?"); however, in this example they are specified for a particular location: San Diego, California.

Example 1:

The first example uses the following assumptions:

  • The lighting system is a PV-powered parking lot luminaire.
  • The light source is an 11-watt compact fluorescent lamp (CFL) powered by a dc ballast with a total system wattage of 13 watts.
  • The CFL will be turned on for eight hours of operation at night per day.
  • PV panel conversion efficiency is 10%.
  • A flat PV panel is used.
  • Electronics (including charge controller and dc ballast) efficiency is 80%.
  • Battery charge-discharge efficiency including conduit loss is 60%.
  • The location is San Diego, California (33° N latitude).

The wattage of this example system (13 W), was determined through lighting calculation, considering parameters such as the required illuminance on the ground, size of area to be lighted, and luminaire efficiency. This same calculation is needed for grid-powered lighting systems. A CFL was selected for this application because it is the most efficient light source, considering the required light output, to produce the illuminance needed on the ground from the mounting height selected. Once the light source wattage has been determined, the following steps should be performed:

Step 1 - Calculate the daily energy consumed by the light source in watt-hours.
EDaily Consumed = Lamp Wattage × Daily Operating Hours
                     = 13 watts × 8 hours/day
                     = 104 watt-hours/day

Step 2 - Calculate the electric energy that the PV panels need to produce each day.
Assume the battery capacity of the system is large enough to allow necessary charging and discharging for powering the lamp.

EPV Produced = EDaily Consumed / (Electronics Efficiency × Battery Charge/Discharge Efficiency)
                 = (104 watt-hours/day) / (80% × 60%)
                 = 217 watt-hours/day

Step 3 - Calculate the amount of solar radiation that the PV panels need to collect each day.
ESolar Radiation Needed = EPV Produced / (PV panel conversion efficiency)
                             = (217 watt-hours/day) / 10%
                             = 2170 watt-hours/day

Step 4 - Find the average daily solar radiation at the location for the seasons in which this lighting system will be used, or for the season with the lowest amount of solar radiation if designing for use year-round.
In December, the 30-year-average of monthly solar radiation on a horizontal, flat panel in San Diego is 2900 watt-hours/square meter/day. For a flat panel tilted to an angle of latitude plus 15 degrees (facing south), it is 5000 watt-hours/square meter/day.

Step 5 - Calculate the size of the PV panels needed.
If the PV panel is in a horizontal position:
Size of PV Panels = ESolar Radiation Needed / Daily Solar Radiation
                        = (2170 watt-hours/day) / (2900 watt-hours/square meters/day)
                        = 0.75 square meters
                        = ~8.2 square ft

If the PV panel is tilted with an angle of latitude plus 15 degrees (facing south):
Size of PV Panels = ESolar Radiation Needed / Daily Solar Radiation
                        = (2170 watt-hours/day) / (5000 watt-hours/square meters/day)
                        = 0.43 square meters
                        = ~4.6 square ft

It is advantageous to tilt the PV panel to an angle of latitude plus 15 degrees (facing south). Therefore, this lighting system in San Diego in December requires 4.6 square ft of PV panels (tilted as described above), in order to collect enough solar energy to power this parking lot luminaire, providing approximately 10 lux (100 moonlights) on the pavement throughout the night for one year.

Example 2:

This example uses the following assumptions:

  • The lighting system is a PV-powered post-top luminaire.
  • The light source is a one-watt light-emitting diode (LED), powered by an LED driver, with the system wattage of 1.5 watts.
  • The LED will be turned on for eight hours of operation at night per day.
  • PV panel conversion efficiency is 10%.
  • A flat PV panel is used.
  • Electronics (including charge controller and LED driver) efficiency is 80%.
  • Battery charge-discharge efficiency including conduit loss is 60%.
  • The location is San Diego, California.

The wattage of this system was again determined through lighting calculation, considering the same parameters as those in Example 1. In this case, an LED is the most efficient light source able to provide the required light output to produce the desired illuminance on the ground from the mounting height selected. CFLs are not available in small enough lumen packages (i.e., with low enough total light output ratings) to be used in this application.

Step 1 - Calculate the daily energy consumed by the light source in watt-hours.
EDaily Consumed = Lamp Wattage × Daily Operating Hours
                     = 1.5 watts × 8 hours/day
                     = 12 watt-hours/day

Step 2 - Calculate the electric energy that the PV panels need to produce each day.
Assume the battery capacity of the system is large enough to allow necessary charging and discharging for powering the lamp.
EPV Produced = EDaily Consumed / (Electronics Efficiency × Battery Charge/Discharge Efficiency)
                = (12 watt-hours/day) / (80% × 60%)
                = 25 watt-hours/day

Step 3 - Calculate the amount of solar radiation that the PV panels need to collect each day.
ESolarRadiationNeeded = EPV Produced / (PV panel conversion efficiency)
                           = (25 watt-hours/day) / 10%
                           = 250 watt-hours/day

Step 4 - Find the average daily solar radiation at the location for the seasons in which this lighting system will be used, or for the season with the lowest amount of solar radiation if designing for use year-round.
Using the same information as shown in Example 1, the amount of daily solar radiation is 2900 watt-hours/square meter/day for a horizontal, flat panel. For a flat panel tilted to an angle of latitude plus 15 degrees (facing south), it is 5000 watt-hours/square meter/day.

Step 5 - Calculate the size of PV panels needed.
If the PV panel is in horizontal position:
Size of PV Panels = ESolar Radiation Needed / Daily Solar Radiation
                        = (250 watt-hours/day) / (2900 watt-hours/square meters/day)
                        = 0.09 square meters
                        = ~1 square foot

If the PV panel is tilted with an angle of latitude plus 15 degrees (facing south):
Size of PV Panels = ESolar Radiation Needed / Daily Solar Radiation
                        = (250 watt-hours/day) / (5000 watt-hours/square meters/day)
                        = 0.05 square meters
                        = ~0.5 square foot

Again, it is advantageous to tilt the PV panel to an angle of latitude plus 15 degrees (facing south). Therefore, 0.5 square foot (0.45 square meter) of PV panels are needed (tilted as described above) for this lighting system in San Diego in December, in order to collect enough solar energy to power this 0.5-lux (5 moonlights) illuminance post-top luminaire. If designing for reliable year-round operation, then the appropriate size of PV panels for this system should be at least 0.5 square foot (0.45 square meter).


Previous
Previous
© 2006 Rensselaer Polytechnic Institute. All rights reserved. Next Next


Rensselaer Polytechnic Institute
LRC Intranet Web mail Lighting Research Center