LED Streetlights Cooling Design

When it comes to LED lights, many people focus on lumens and pay less attention to cooling.  In fact, LED lumens are increasing rapidly.  In 2006, lumens per watt in mass production reached 50 and it is still going up fast.  Heat transfer theory has matured, its channels are clear: conduction, convection, radiation and phase change (such as heat pipes).

According to equivalent between luminous flux (lm) and radiant flux (watt), Km = 683 lm/w is the scale unit, which mean ideally (black body radiation), 1W will generate 683lm.  Even LED reaches 200lm/w, not all energy will be converted into lights and some of it will become heat.  Cooling will be a long-lasting issue to LED lights.  

Major cooling technologies in LED streetlights include: convection, fans, heat pipes and loop heat pipes.  Cooling with fans is complicated and unreliable.  Heat pipes and loop heat pipes are not cost-effective.  Streetlights are used outdoors in the evening in small sizes and cooling areas are at the upper part, so it is suggested to adopt convection in LED streetlights.

The possible problems include:
1. Unreasonable cooling fin size.
2. Unreasonable distributions: Without taking how lights operate into concerns, cooling effects will be reduced from inappropriate cooling fins arrangements.
3. Heat conduction without heat transfer: Although many manufacturers are introducing various measures including heat pipes, loop heat pipes, and silicon grease, heat is eventually released through surface areas.
4. Heat balanced overlooked: If heat is unevenly distributed on fins, it will be less effective or not effect at all.

Current LED streetlight cooler technology is mostly using heat plate, which is a 5mm thick copper plate, as heat spread.  Some are using cooling fins, but it becomes heavy.  Weight is such an important concern to streetlights because they are 9 meters high.  If heavy, it would be dangerous facing typhoons and earthquakes.  Maybe when LED is widely adopted in streetlights in the future, coolers as a module will effectively solve the current problems.
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