Key Formulas for Solar Street Light Design

This article summarizes essential formulas commonly used in solar street light design, integrating national standards and practical case studies from various papers:

1. Average Road Illuminance Calculation

Formula:
Eavg = (N × Φ × U × K) / A

• Parameter Description:
  • N: Number of fixtures
  • Φ: Total luminous flux per lamp (lm)
  • U: Utilization factor (0.4-0.6)
  • K: Maintenance factor (0.7-0.8)
  • A: Road area (m2) = Road width × Lamp spacing

Example:
6m wide road, lamp spacing 30m, using 10,000 lm LED, one-sided lighting:
Eavg ≈ (1 × 10,000 × 0.5 × 0.75) / (6 × 30) ≈ 20.8 lx

2. Solar Panel Power Calculation

Formula:
Ppv = Qday / (Hpeak × ηsys)

• Parameter Description:
  • Qday = PLED × Twork (Daily energy consumption, Wh)
  • Hpeak: Local annual average peak sunlight hours (check meteorological data, e.g., Beijing 4.5h)
  • ηsys: System efficiency (0.6-0.75, including line losses, controller losses)

Example:
Load power 80W, daily operation 10h, Shanghai Hpeak=3.8h:
Ppv ≈ (80 × 10) / (3.8 × 0.65) ≈ 324 W

3. Battery Capacity Calculation

Formula:
C = (Qday × D) / (DOD × ηbat × Vsys)

• Parameter Description:
  • D: Number of consecutive cloudy days (usually 3-5 days)
  • DOD: Depth of discharge (0.5 for lead-acid batteries, 0.8 for lithium batteries)
  • ηbat: Charge/discharge efficiency (0.85-0.95)
  • Vsys: System voltage (12V/24V)

Example:
Daily consumption 800Wh, 24V system, 3 days backup, lithium battery:
C ≈ (800 × 3) / (0.8 × 0.9 × 24) ≈ 138.9 Ah → Choose 150Ah battery

4. Solar Panel Installation Angle

Formula:
θ = φ + (5° to 15°)

• Parameter Description:
  • φ: Local geographical latitude
  • Winter optimization: latitude +10°~15°, summer optimization: latitude -5°

Example:
Nanjing latitude 32°, fixed bracket tilt angle set at 37° (32°+5°) to improve winter power generation.

5. Wind Pressure on Solar Panels

Formula:
F = 0.61 × v2 × A

• Parameter Description:
  • v: Maximum wind speed (m/s)
  • A: Wind-facing area of the photovoltaic panel (m2)

Example:
Panel area 2m2, design wind speed 30m/s:
F = 0.61 × (30)2 × 2 = 1098 N
Need to verify the wind resistance of the lamp pole and foundation.

6. Component Operating Voltage Correction (Temperature Effect)

Formula:
Vmp = Vmp(STC) × [1 + α × (T – 25)]

• Parameter Description:
  • α: Temperature coefficient (approximately -0.35%/°C for monocrystalline silicon)
  • T: Actual operating temperature (°C)

Example:
Nominal component voltage 18V, operating temperature 60°:
Vmp ≈ 18 × [1 – 0.0035 × (60-25)] ≈ 15.3 V

7. Voltage Drop Compensation Due to Temperature

Formula:
ΔV = Nseries × α × ΔT × Vmp(STC)

Example:
3 series-connected components, each Vmp=30V, temperature difference 35°:
ΔV ≈ 3 × (-0.0035) × 35 × 30 ≈ -11V
Need to adjust the MPPT voltage range.

8. Solar Panel Capacity Optimization Design

Empirical Formula:
Ppv(opt) = 1.2 × Ppv

• Consider shadowing, dust loss (efficiency reduction of 10-20%)
• When paralleling multiple components, increase bypass diodes to reduce hotspot effects.

9. Typical Design Parameter Comparison Table

Note: Actual design should be combined with PVsyst simulations and DIALux lighting simulations, and validated through field tests.