Understanding the Luminous efficacy of Solar Street Lights

Understanding Luminous Efficacy

Luminous efficacy is a measure of how efficiently a light source produces visible light, defined as the ratio of luminous flux (measured in lumens) to power (measured in watts), expressed in lumens per watt (lm/W). A higher luminous efficacy indicates more visible light output for a given power input, signifying better lighting effects. It’s an essential metric for evaluating the performance and efficiency of a lighting device.

The light efficacy of LEDs pertains to how effectively individual light sources, such as LED chips, convert electrical energy into visible light, reflecting the brightness produced per unit energy. Conversely, the light efficacy of luminaires encompasses the overall energy utilization of the entire device, including components like reflectors and lenses. Addressing various factors is key to improving the efficacy of luminaires.

Impact of Luminous Efficacy on Solar Street Lights

• Energy Efficiency Improvement: High-luminous-efficacy sources significantly improve the overall energy efficiency of solar street lights. For example, a 10-watt LED lamp with 100 lm/W can provide 1000 lumens of light output, whereas an incandescent lamp of the same power might only yield 100-170 lumens.
• Cost-Effectiveness: Although high-efficacy light sources may have a higher initial cost, they save substantial energy and maintenance costs in the long run. For example, LEDs can last over 30,000 hours, more than five times longer than traditional high-pressure sodium lamps, reducing the frequency of bulb replacements.
• Environmental Adaptability: High-efficacy sources maintain high output efficiency under low-light conditions, which is critical for solar street lights. For instance, LEDs can maintain high efficacy even during overcast days or deep discharge conditions, while incandescent and fluorescent lights are more easily affected by such conditions.

Luminous Efficacy Selection in Practical Applications

• Major Roads and Highways: These roads have high traffic volumes and require high luminous efficacy and illuminance; LEDs with efficacy ranging from 120 to 150 lm/W are recommended. For example, a 100-watt LED can provide 12,000-15,000 lumens to meet high illuminance demands.
• Secondary Roads and Roads: These roads have lower traffic volumes and slightly lower requirements for luminous efficacy and illuminance; LEDs with efficacy ranging from 100 to 120 lm/W are suitable. For example, a 50-watt LED can deliver 5,000-6,000 lumens to meet basic lighting needs.
• Sidewalks, Commercial Pedestrian Streets, and Residential Areas: These areas require higher color rendering and visual comfort, recommending LEDs with efficacy between 100 to 120 lm/W, along with high CRI. For instance, a 30-watt LED can provide 3,000-3,600 lumens to ensure an excellent visual experience for pedestrians and residents.

Optimization Measures for Luminous Efficacy

• Optimal Tilt and Orientation of Solar Panels: Proper tilt and orientation can maximize solar panel efficiency, thereby enhancing overall luminous efficacy. For example, calculating the best tilt angle based on local latitude can significantly improve solar panel output.
• Maximum Power Point Tracking (MPPT) Technology: Solar controllers using MPPT technology can ensure that solar panels operate at maximum power output, improving photovoltaic conversion efficiency. For example, MPPT can increase current during cloudy days or after deep discharges, comparable to increasing the panel’s capacity by 1.3 times.
• Heat Management Design of Light Sources: Choosing fixtures with good heat dissipation ensures that LED lights operate at safe temperatures, extending their lifespan and maintaining high luminous efficacy. For example, using aluminum substrates and heat sinks can effectively reduce operating temperatures and enhance efficacy.

Real-World Case Studies

• Tsinghua University (Zhuhai) Science and Technology Park Solar LED Street Lights: This project utilized 120 high-brightness white LEDs, with a total power of 6.5 watts and efficacy around 100-150 lm/W. The brightness of the streetlight sources is equivalent to a 25-watt incandescent lamp, providing approximately 200-300 lumens with excellent results.
• A Secondary Road in a City: This secondary road employed 70-watt ceramic metal halide lamps with an efficacy of about 100 lm/W. The daily lighting time is 8 hours, resulting in a daily electricity consumption of 560 Wh, with a system designed to sustain continuous power for three consecutive overcast days.

Maintenance and Lifespan

• Easy Maintenance: LED lights require minimal maintenance due to their long lifespan, drastically reducing the frequency of bulb replacements.
• Stable Light Color Over Lifetime: LEDs maintain good light color stability throughout their lifespan with slow luminous decay. For example, the color temperature of LEDs fluctuates by ±200K, resulting in better color consistency between sources.

Comparison of Luminous efficacy among Different Light Sources in Solar Street Lights

• Incandescent Lamps:
  • Light Efficiency: Typically only a few to a dozen lm/W.
  • Characteristics: Much of the electrical energy is converted into heat rather than visible light, resulting in low light efficiency and high energy consumption, making them unsuitable for solar street lights.
• CFLs (Compact Fluorescent Lamps):
  • Light Efficiency: Usually above 60 lm/W.
  • Characteristics: CFLs convert more electrical energy into visible light using phosphor materials, exhibiting higher efficiency but with a relatively shorter lifespan, suitable for low-power applications like garden lights.
• High-Pressure Sodium Lamps:
  • Light Efficiency: Typically above 100 lm/W.
  • Characteristics: Primarily used for outdoor lighting, high-pressure sodium lamps have excellent light efficiency but lower color temperature and poor color rendering, requiring inverters that complicate the system and increase failure rates.
• Metal Halide Lamps:
  • Light Efficiency: Typically above 100 lm/W.
  • Characteristics: Commonly used for high-intensity indoor and outdoor lighting, metal halide lamps have high light efficiency and good color rendering but longer startup times and shorter lifespans, suitable for cold regions and roads requiring accurate color recognition.
• LEDs (Light Emitting Diodes):
  • Light Efficiency: Significantly improved in recent years, now exceeding 100 lm/W and often higher.
  • Characteristics: LEDs are energy-efficient, environmentally friendly, long-lasting, and compact, making them the preferred light source for solar street lighting. The light efficiency of LEDs can be further enhanced through different packaging technologies and heat dissipation designs.

Factors Affecting Light Efficiency of Solar Street Lights

• Conversion Efficiency of Solar Panels:
  • Materials: Common solar panel materials include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide. Monocrystalline and polycrystalline panels, due to mature manufacturing technology, stable product performance, long lifespan, and relatively high photoelectric conversion efficiency, are widely used in solar street light systems.
  • Installation Conditions: The installation angle and orientation of solar panels significantly impact light efficiency. Generally, an installation angle equal to the local latitude is recommended for maximum sun exposure. Additionally, minimizing blocking from surrounding buildings is crucial to improving light efficiency.
• Battery Performance:
  • Types: Common batteries used in solar street light systems include lead-acid batteries, nickel-cadmium batteries, and nickel-hydrogen batteries. Sealed lead-acid batteries are the preferred storage unit due to their stable performance and moderate price.
  • Capacity: The battery capacity must meet nighttime lighting needs, typically reserving three days’ worth of system electricity. Undersized batteries can affect nighttime illumination, while oversized batteries can be wasteful and impact lifespan.
• Controller Functions:
  • Basic Functions: The controller features light control, time control, overcharge protection, and over-discharge protection functions, automatically managing lighting operation and protecting batteries during charging and discharging, thus extending their lifespan.
  • Advanced Functions: Modern controllers using MPPT (Maximum Power Point Tracking) technology can achieve higher charging efficiency during rainy days or deep discharges, improving the overall system’s energy efficiency.
• Fixture Design:
  • Photometric Distribution: The light distribution of the fixture determines how light is spread. Good photometric design can ensure even light distribution, reducing glare and enhancing lighting quality.
  • Installation Height and Spacing: The height and spacing of the fixtures also significantly influence light efficiency. It is generally advised to reduce spacing between fixtures and increase surface brightness to improve vertical uniformity. For instance, for cut-off fixtures, the installation height for single-side installations should be 50% of the effective road width, with spacing at three times the installation height.

Actual Application Cases

• LED Street Lights:
  • A 9W DC energy-saving lamp installed at a height of 2.41 meters provides ground illuminance between 8 to 11 lx. Tests indicate that LED street lights perform well on fast roads and main roads, especially in cold and dusty environments, where their color rendering and light efficiency advantages are evident.
  • A 250W LED lamp installed at a height of 8.5 meters with a spacing of 30 meters achieves a ground illuminance of 19 lx, with illuminance between lamps at 11 lx and the darkest spot at 3.5 lx. While these values represent the initial illuminance of newly installed lamps, they generally meet the lighting needs of secondary and branch roads.
• HED Gas Discharge Lamps:
  • HED gas discharge lamps have a light efficiency of 100 lm/W and operate on DC power, making them a preferred light source for solar street lights. These lamps excel in low-power applications, meeting various usage conditions.

Methods for Measuring Light Efficiency of Solar Street Lights

• Brightness Measurement:
  • Brightness is typically measured using a brightness meter at a height of 1.5 meters above the ground. The longitudinal position of the measurement point should be 60 meters away from the first row of measurement points, with a longitudinal measurement length of 100 meters.
  • There are two ways to measure average brightness: integral brightness measurement and spot measurement. In integral brightness measurement, the average brightness is the average value of brightness measured below the lamp and averaged between two lamps. In spot measurement, the average brightness is the average of all measurement points’ brightness.
• Brightness Uniformity:
  • Road surface brightness uniformity can be divided into overall uniformity and longitudinal uniformity. Overall uniformity refers to the ratio of the minimum brightness to average brightness on the road surface; longitudinal uniformity refers to the ratio of minimum brightness to average brightness along a horizontal line from the observer’s position to the road axis.
  • It is generally recommended that the minimum longitudinal uniformity for major and minor roads is approximately 0.7 and 0.5, respectively. Reducing fixture spacing and increasing road brightness can improve longitudinal uniformity.
• Glare Limitation:
  • Glare can be categorized into disabling glare and discomfort glare. Disabling glare refers to extreme brightness or contrast that decreases visual performance; discomfort glare refers to extreme brightness or contrast that leads to discomfort.
  • The glare control grade (G) indicates that the higher the grade, the lower the discomfort level perceived by the human eye. Reasonable fixture design can effectively reduce glare and improve visual comfort.

Optimization Measures for Light Efficiency of Solar Street Lights

• Rational Selection of Lighting Equipment:
  • Selecting high-efficiency, long-lasting, and color-rendering light sources and fixtures can significantly improve road lighting performance. For instance, LEDs, due to their high efficiency and long lifespan, are increasingly becoming the mainstream choice for solar street lights.
  • Using energy-efficient ballasts with capacitance compensation can further enhance system efficiency.
• Optimizing Lighting Configuration:
  • Based on the specific conditions of the road and the photometric type of fixtures, choose an appropriate lighting configuration. For roads with substantial shading from trees, a transverse suspension lighting configuration can be selected where fixtures are suspended from cables crossing the road, perpendicular to the road axis.
  • For dual carriageways with central isolation strips, a symmetric lighting configuration is preferred, with fixtures installed on Y-type or T-type poles in the isolation strip. The height of poles should equal or exceed the effective width of one side of the road.
• Improving Control Systems:
  • Modern controllers have evolved from simple relay controls to microcomputer controls. By inputting the geographic coordinates and activation times, one can generate a seasonal lighting schedule for turning on and off the lights.
  • By using voltage regulation methods, controlling half-night and all-night lighting can be easily managed, leading to energy savings.

Conclusion

The selection of luminous efficacy in solar street lights should be based on specific application scenarios and requirements. For major roads and highways, LEDs with efficacy between 120 to 150 lm/W are recommended; for secondary roads and streets, LEDs with efficacy between 100 to 120 lm/W are suitable. For sidewalks, commercial pedestrian streets, and residential areas, LEDs with efficacy between 100 to 120 lm/W and high CRI are advised. By implementing reasonable system design and optimization measures, such as the optimal tilt angle of solar panels, MPPT technology, and good heat management, the luminous efficacy and overall performance of solar street lights can be significantly enhanced, meeting lighting needs, achieving energy savings, and extending lifespan.