In many lighting applications the LED device must fit in a fixed space to accommodate a variety of customer requirements which usually neglect thermal management considerations. A common example is luminaire design where the ceiling or wall fixtures are based on a pre-existing design using non-LED technologies. These designs commonly have both restricted space for heat dissipation through conduction and limited air flow to remove heat via convection. In cases where there is space to remotely dissipate the heat, heat pipes can be used to transport the heat from the device to a heat sink located elsewhere. This is called the remote sink.
The remote sink solution has a heat pipe in direct contact with the LED device at one end, which serves as the evaporator. At the other end the heat pipe is connected to the heat sink, the condenser. A sketch of a conceptual design can be seen in Figure 1. Here two heat pipes are in direct contact with both the LED at the bottom and heat dissipating fins at the top. A wall or other enclosure can be placed in between the LED and heat sink to separate the two.
Aluminum has a thermal conductivity of about 180 W/m K, while the thermal conductivity of copper is only 400 W/m K. In contrast, the effective conductivity of a heat pipe can range from 10,000 to 100,000 W/m K. This high effective thermal conductivity allows the heat sinks to be located remotely from the LED.
Figure 2 shows a photograph and an infrared (IR) image of a heat pipe transporting heat to a remote sink. The heat pipe heat sink is operating at natural convection conditions with 30 Watts of applied heat. The heat pipe clearly demonstrates the transport of heat isothermally from the heat source to the heat sink and the even distribution of heat to the heat sink. A slight increase in temperature is measured across the heat sink (<0.5 °C), due to the sensible heating of air rising through the heat sink. Heat pipes can efficiently transfer heat approximately 8 inches with minimal thermal gradient, and over even greater distances when the heat pipe is gravity aided. Note that the number, size shape and location of heat pipes would be specific to the design.