High-Power LED Streetlight Thermal Design in Phoenix, Arizona

Scenario

Urban infrastructure project for smart street lighting in Phoenix, AZ — a desert climate with summer ambient temperatures regularly exceeding 45°C. The design must ensure 10-year reliability for 150-W COB LED modules mounted on aluminum extrusion heat sinks. Constraints include no forced airflow (passive cooling only), strict height/weight limits (<8 kg per fixture), and compliance with IES LM-80 thermal lifetime requirements.

Given Data

• Maximum Junction Temperature (t_j_max): 135°C (per LED manufacturer’s derating curve at 100k-hour L70 life)
• Ambient Temperature (t_a): 48°C (design worst-case summer daytime ambient, per ASHRAE RP-1462)
• Power Dissipation (p_diss): 150 W (total thermal load from LED + driver losses)

Calculation

Using the thermal resistance formula derived from Fourier’s law:

$$ R_{\text{th}} = \frac{T_{j,\text{max}} - T_a}{P_{\text{diss}}} = \frac{135^\circ\text{C} - 48^\circ\text{C}}{150,\text{W}} = \frac{87}{150} = 0.58,^\circ\text{C}/\text{W} $$

The Thermal Resistance Calculator confirms: r_th = 0.58 °C/W (precision 2).

Result and Decision

A custom anodized aluminum heat sink with 32 vertical fins (120 mm tall × 220 mm wide × 35 mm base thickness), optimized via CFD simulation, achieved 0.56 °C/W in lab testing under natural convection (48°C ambient, horizontal mounting). This met the 0.58 °C/W target with 3.4% margin. TIM selection was upgraded from standard silicone grease to phase-change pad (0.12 °C·cm²/W contact resistance) to minimize interfacial resistance.

Lesson

Passive heat sinks in high-ambient environments require margin beyond theoretical Rth — real-world fin efficiency drops sharply above 45°C ambient due to reduced air density and buoyancy-driven airflow; always validate with empirical testing at worst-case ambient, not just room temperature.