LED grow lights are the standard for modern horticulture, offering precise spectrum control. While highly efficient, LEDs produce heat, but they distribute it differently than High-Intensity Discharge (HID) lamps. Understanding this distinction is crucial, as heat management impacts plant health and requires a new approach to climate control.
The Physics of Heat Generation in LED Fixtures
The heat generated by an LED fixture is an unavoidable byproduct governed by the law of energy conservation. A Light Emitting Diode (LED) converts electrical energy into light through electroluminescence within a semiconductor chip. This process is not 100% efficient; for typical high-power horticultural LEDs, only about 30 to 40% of the input power is converted into light.
The remaining 60 to 70% of the electrical energy is lost as heat, primarily at the diode’s P-N junction. This heat must be efficiently removed from the fixture. If the junction temperature rises too high, it causes light decay, meaning the light output gradually diminishes. Poor thermal management shortens the operational lifespan and reduces efficiency, requiring effective heat removal for longevity.
Radiant vs. Convective Heat Transfer
The difference between LED and HID lighting lies in how they transfer waste energy. Heat transfer occurs through conduction, convection, and radiation. Traditional HID lamps dissipate a large percentage of heat energy as radiant heat, which is infrared (IR) energy that travels in a straight line and directly warms the plant canopy.
LED fixtures transfer most waste heat away from the diodes into an aluminum heat sink. This heat then transfers to the surrounding air through convection. HID fixtures direct approximately 55% of heat energy as radiant heat toward plants, while LEDs emit around 27%. LEDs primarily warm the ambient air of the grow room, while HID lights primarily warm the plant surface.
Impact of LED Heat on Plant Canopy and Environment
The shift from radiant to convective heat transfer has consequences for the plant canopy and the controlled environment. Traditional HID lighting creates a high Leaf Surface Temperature (LST) because radiant infrared energy directly warms the leaves. Under LEDs, the LST is significantly cooler, sometimes by as much as 7 to 9°F, requiring growers to adjust their climate strategy.
The cooler canopy temperature affects the Vapor Pressure Deficit (VPD), which measures the difference between air moisture and saturation. VPD indicates a plant’s transpiration rate. To maintain optimal transpiration and photosynthesis, the ambient air temperature in an LED grow room often needs to be slightly warmer, sometimes 80–85°F, compared to an HID room. While lower radiant heat minimizes the risk of leaf burn, maintaining proper air temperature and humidity is necessary for efficient metabolic processes.
Strategies for Thermal Management
Effective thermal management involves fixture design and environmental controls. Manufacturers use large, finned heat sinks, often made of aluminum, to move heat away from the LED chips and increase the surface area exposed to the air. Some fixtures utilize active cooling, such as integrated fans, while others rely on passive cooling through natural air convection.
Managing convective heat added to the room air requires robust environmental controls. Proper air exchange, ventilation, and exhaust systems are necessary to remove warmed air and maintain a consistent ambient temperature. Because LEDs emit less radiant heat, they can generally be placed closer to the plant canopy than HID lights. However, maintaining the recommended hanging distance is important to prevent localized heat buildup and ensure uniform light distribution.