# question about LED street light design



## seanzh (Jun 21, 2011)

I have two questions about the design of LED street light. We have a sealed shell of fixture, but it was for HPS bulb and it is sealed. 

Q1: In order to retro-fit the shell for LED, do we need to punch holes on the shell for the air flow? Where should the heatsink be placed for proper heat radiation? 

Q2: Now we have LED arrays, optical part, driver in hand. Is there any other parts we need in order to assemble a street light fixture? Which area of specilist should we hire for the design?

Thanks for sharing.


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## Harold_B (Jun 21, 2011)

We have designed LED street lights, High and Low Bay light, and parking lot lights. You have chosen the most challenging and most competitive market. You will need to define several performance criteria before you engage a design house (and it sounds like you should engage a design house).

First, where do you intend to sell these products? In the US, North America, Asia, world-wide? This will determine power requirements, lighting patterns (type I, II, III, IV, medium, etc), cooling requirements, etc.

The engineering you will likely need in my opinion based on projects we have worked on like yours will be optomechanical, optical, and electrical with the thermal studies, lighting simulations and optimization, mechanical packaging, and electronics included.

Hope that helps!

Harold


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## seanzh (Jun 22, 2011)

Harold_B said:


> We have designed LED street lights, High and Low Bay light, and parking lot lights. You have chosen the most challenging and most competitive market. You will need to define several performance criteria before you engage a design house (and it sounds like you should engage a design house).
> 
> First, where do you intend to sell these products? In the US, North America, Asia, world-wide? This will determine power requirements, lighting patterns (type I, II, III, IV, medium, etc), cooling requirements, etc.
> 
> ...


 
Thanks for the detailed explanation, Harold.


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## SemiMan (Jun 22, 2011)

seanzh said:


> Thanks for the detailed explanation, Harold.



seanzh -- can you give a little more details about who you are ... and why you would want to do this?

To make the worlds best streetlight is not easy. To make a relatively good one is not quite as hard as it once was given the available optics, power supplies, etc. Feel free to PM me if you do not want to post publicly.

Semiman


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## jtr1962 (Jun 23, 2011)

seanzh said:


> Q1: In order to retro-fit the shell for LED, do we need to punch holes on the shell for the air flow? Where should the heatsink be placed for proper heat radiation?


The absolute very _last_ thing you want to do is make holes for airflow. That will virtually invite insects which in turn will eventually cause the electronics to fail. Remember that insects consume, infest and destroy everything they come in contact with. You want to have a completely sealed fixture to keep them out. A passive heat sink is what you want here but of course you need to expose it to the outside air. You can do this by replacing the original streetlight diffuser with a new one which incorporates the heat sink on the top third or so (you really don't need to emit light from this area anyhow). Orient the fins vertically for best cooling effect.

Second recommendation is to put the driver electronics in the base of the streetlamp for easy replacement. It's likely that the LEDs will last far longer than the driver. You avoid issues of needing a crew and lift by keeping the driver electronics near ground level.

Third recommendation is to underdrive the emitters. This means using more emitters which will drive up the cost. The upside is the entire unit will last long enough to justify the replacement cost. Just the power savings alone from underdriving would likely cover the cost of the additional emitters. The only question is what sort of lifetime to aim for. If I were doing this I would aim to match the streetlight fixture design life (200 years?). At ~13 hours per day that implies underdriving the LEDs enough so you can obtain about 1 million hours life to 70% of initial output. If on the other hand these fixtures will be replaced in 50 years, then a 250,000 hour lifetime goal would be adequate.


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## Steve K (Jun 23, 2011)

jtr1962 said:


> Third recommendation is to underdrive the emitters. ...


 
This raises a question... what's the primary determinant of LED lifetime? Is it just the current through the LED, or is it the temperature? For electronics in general, the rule of thumb is to keep semiconductors as cool as possible, and derate them by some amount (i.e. stay below the limits on voltage, current, etc.). 

Do LED manufacturers publish what the relationship between temperature and lifetime is? Is it linear, or exponential, or what? As a rule of thumb, I've always assumed that if the junction temperature can be kept down to 50 or 60 degrees C, it would be fine. 

Reducing current would increase the LED's luminous efficacy, so maybe it's worth doing for that reason alone. It kinda depends on whether you can convince customers to pay the higher initial cost and wait for the savings in the operating costs later. 

regards,

Steve K.


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## jtr1962 (Jun 23, 2011)

Steve K said:


> This raises a question... what's the primary determinant of LED lifetime? Is it just the current through the LED, or is it the temperature? For electronics in general, the rule of thumb is to keep semiconductors as cool as possible, and derate them by some amount (i.e. stay below the limits on voltage, current, etc.).
> 
> Do LED manufacturers publish what the relationship between temperature and lifetime is? Is it linear, or exponential, or what? As a rule of thumb, I've always assumed that if the junction temperature can be kept down to 50 or 60 degrees C, it would be fine.


Cree actually put out a nice paper detailing lifetime versus current at different junction temperatures. It turns out both junction temperature and current affect lifetime. Interestingly, ambient air temperature around the LED dome also has an effect.

Unfortunately, they don't have data at currents less than 350 mA. If you keep junction temperatures at 60°C, then lifetime to 70% initial output approaches 140,000 hours ( 30 years in streetlamp duty ). The lifetime versus current curve though seems to be exponential. I'll hazard a guess that you can exceed 300,000 hours at 200 mA, and perhaps 500,000 hours at 100 mA. Not much point going under 100 mA. 80 to 100 mA is around where efficiency starts to peak for most power LEDs.

As for efficiency versus number of emitter tradeoffs, I recall running the numbers once. It appears to be an economically viable decision to increase the number of emitters in many cases but you really have to do the numbers. For example, decreasing current from 350 mA to 100 mA increases efficiency by 13% according to my testing of the XP-G. However, output goes down to 30%, so you need 3.333 times as many emitters. Let's do the math for a 20,000 lumen streetlight. You would need about 140 emitters at 350 mA, and 470 at 100 mA. That's 330 more emitters. At say $2 each, this adds $660 to the cost of the project, plus whatever the associated optics cost. Let's say $1000 even ( $3 per emitter with the optics ). Let's also assume 100,000 hours life. At 350 mA you use 145 watts (not including driver losses). This drops to 128 watts at 100 mA. Power savings is 17 watts. This is 1700 kW-hr over the lifetime of the streetlight. At 15 cents/kWhr this comes to $255 saved in energy costs. However, at 100 mA lifetime is easily doubled relative to 350 mA, so basically then you're buying the emitters twice at 350 mA compared to 100 mA. The real comparison then is 140x2 =280 emitters versus 330 emitters. The surplus emitter cost then might only come to 50 x $3, or $150. Subtract the $510 you save in energy costs over the 200,000 hour life of the 100 mA design, and you're way ahead, not even including the labor costs to change out the 350 mA version at 100,000 hours.

What clouds the decision unfortunately is that LED efficiency is still a moving target. In 5 years time you might have LEDs which get better efficiency at 700 mA then an XP-G gets at 100 mA. I'm going to go out on a limb then and say it might not pay to design with drive currents much under 350 mA until LED efficiency plateaus. Further, since much LED research is geared towards reducing the efficiency drop at higher currents, in the future LEDs may be nearly as efficient at 350 mA or 700 mA as they would be at 100 mA. That basically means you only need to consider lifetime when making the calculations to determine drive current.


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## Steve K (Jun 23, 2011)

interesting analysis! I think I'd modify the underlying assumptions and consider using some of the LED arrays (like Bridgelux.. I think there's a thread about a Cree array that is 40v @ 1 amp) instead of 3W LEDs. Just reducing the number of solder joints should improve the reliability, especially if they are hand soldered. The corresponding reduction in hardware, optics, etc. would certainly reduce the cost of assembly.

Steve K.


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## John_Galt (Jun 24, 2011)

Some very good thought and analysis being stated here.

With all of the above in mind, another thing to juggle would be modularity. 

Basically, you could design it so that you have (small) groups of LED's and optics that can be replaced individually if damage does occur. 

As the current trend of higher efficiency LED's is a larger foot print (larger die area) and lower/equal surface intensity, make sure these modules are designed to accomodate an expansion if area needed for each LED and optic. This increase in footprint may be offset by the increased output at the same drive level, but that is something for you guys to juggle.

If this adds to the cost, and doesn't decrease efficiency or lifespan, then the increased cost could be touted as a boon to the group/entity purchasing these units. Something along the lines of "these units are modular. If damage occurs to a unit, it can be repaired here, in house, saving the cost of replacement of an entire unit. This modularity also means that as there are significant gains in efficiency, you (the entity purchasing the lights) can pay a (smaller than complete new unit) cost and have these units upgraded in the future."


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## SemiMan (Jun 24, 2011)

I forgot to answer one of the original questions:

You DO NOT retrofit an HPS fixture with LEDS .... ever. Let me repeat.... NEVER! It is never a good idea. You will not save money, you will not have better lighting, your LEDS will not last .... Start with a new fixture or not at all.

Semiman


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## jtr1962 (Jun 24, 2011)

SemiMan said:


> I forgot to answer one of the original questions:
> 
> You DO NOT retrofit an HPS fixture with LEDS .... ever. Let me repeat.... NEVER! It is never a good idea. You will not save money, you will not have better lighting, your LEDS will not last .... Start with a new fixture or not at all.


I tend to agree here but maybe the municipality isn't budgeting for brand new fixtures.


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## seanzh (Jun 24, 2011)

jtr1962 said:


> I tend to agree here but maybe the municipality isn't budgeting for brand new fixtures.


 
Yes, this is kinda my reasons here. The current design is very popular that the manufacturer don't want to introduce another model. Of course a new design with similar look and style would also be good.

Really appreciate everyone's suggestions.


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## seanzh (Jun 24, 2011)

SemiMan said:


> I forgot to answer one of the original questions:
> 
> You DO NOT retrofit an HPS fixture with LEDS .... ever. Let me repeat.... NEVER! It is never a good idea. You will not save money, you will not have better lighting, your LEDS will not last .... Start with a new fixture or not at all.
> 
> Semiman


 
Hi Semiman,

I sent you a PM. Please check.

Thanks.


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## deadrx7conv (Jun 24, 2011)

Is this for cobra-head type retrofit for streetlight, or post-mounted for sidewalk/park lighting? 

Q1: The heatsink needs to be outside your fixture. 
Q2: You need engineering support from a light manufacturer, which will most likely try to sell you their completed product. 

Municipality's also shouldn't enter manufacturing. 

Seanzh, what driver, LED, HPS fixture, heatsink.... do YOU have that you want to mix together? spec's pictures....?


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## AB8XL (Jun 24, 2011)

*Hella Eco StreetLine Retrofit Module.*

See datasheet: http://www.hella.com/produktion/Hel.../HellaUSA/Datasheet_Hella_Cobrahead48-425.pdf

http://www.hella.com/produktion/Hel...E/LED_Street_Lighting/LED_Street_Lighting.jsp

*Hella and Hubbell to develop non-automotive LED lighting*.

http://www.ledsmagazine.com/news/8/6/19

In my opinion this is the industries best design for a cost effective solution for cobra head retrofits.


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## deadrx7conv (Jun 24, 2011)

Enclosed heatsink?


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## znomit (Jun 25, 2011)

deadrx7conv said:


> Enclosed heatsink?


:sick2::sick2::sick2:


Or not:


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## Th232 (Jun 25, 2011)

A heatsink radiates heat to the air in contact with it, which then carries the heat away via air movement (convection, wind &c). If you enclose a heatsink, this can't happen because the heatsink and the air is (surprise, surprise) enclosed. That already makes me suspicious of that design.

I find it interesting that there're images which show the heatsink not enclosed (like the one in znomit's post), while the diagram deadrx posted and this one show it being enclosed.


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## AB8XL (Jun 25, 2011)

It is enclosed, the other image is virtual and modified so it looks like your seeing in to the housing. You do get some air flow around the hinges, seems and where the mast is connected.


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## SemiMan (Jun 26, 2011)

I make my living off designing and selling products that incorporate LEDs including area lights. So right off I can state a bias.

However, I can state that if you are trying to "replace" an existing fixture, it is NOT easy.

1) HPS is a very efficient light source -- in terms of lumens / watt even after degradation. It is also a point source so controlling it is not impossible.

2) Whether N.A. or Europe for legal reasons you want to try to stay with standards for lighting levels. These do not currently give much advantage to the LED spectrum over HPS but that is a whole different issue.

3) Though us and competitors would like you to believe that LEDS are wickedly better that HPS currently while meeting existing light levels, they are not "wickedly" better, but they are better when implemented properly:
- Optical pattern to match the intended requirement - which currently is where the most benefit is from
- Newest GEN LEDS - another big benefit now
- High quality power supply
- Good thermal design for long life - The last two being where the rest of the economic benefit comes from


No retrofit is going to provide all these benefits and in fact many of them provide none of these benefits. Hence why retrofits are NOT the way to go. Some things to consider:

- Installation cost of a "proper - and being kind here" retrofit can take as long as installing a new LED fixture
- New purpose built LED fixture will last 1.5 - 2.5x the life
- Purpose build fixture will not have optic limitations

It's an easy decision. If you cannot afford to do the upgrade properly, then you definitely cannot afford to do it improperly.

Semiman


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## Oznog (Jul 4, 2011)

Yeah, there's a huge cooling problem. Other light sources don't require cooling. In fact, AFAIK, an incan LOSES efficiency as heat escapes through the bulb wall. 

Streetlights can't employ cooling holes, as stated. Insects will quickly move in and clog the fins or jam fans stuck. Road dust will build up, too. Enclosing a heatsink where it can only recirculate air inside the tube body is "mostly" pointless, although, in theory, the length of the steel pole may have enough surface area for cooling even without fins if the heat was homogeneously distributed down the length (that would be a trick!).

A filter in front of an intake port has to be cleaned, or it, too, will inevitably clog. 

Exposing a finned aluminum heatsink can be subject to the same problem as the holes. Insects, birds, and dust can clog it. Larger fins are less prone to this, but do less cooling, and the idea's just not compatible with a "drop in replacement" in an HPS fixture.

I just don't see the capacity for drop-in. By replacing the ENTIRE head with a coarse-finned aluminum body thermally connected to the die, it'll work just fine. Drop-in sounds like a scam. Don't be surprised in seeing companies advertise them online. Sturgeon's Law: 90% of Everything Is Crap. 

Don't overlook the specific environment issues. Dust changes from place to place. The species of insect and bird in different areas have widely differently sizes and habits and that'll change what's possible by quite a bit. Another serious regional issue- LED traffic lights freeze over in the winter, and I assume streetlights may too. It actually requires a heater to clear it. But note that heating the metal body is not entirely helpful. The problem is the ice on the output glass, and glass is an insulator, and you can't simply bolt a resistor to glass to heat it up, glass is an insulator. You'd probably want to heat the air cavity between the LED and glass.

Maintenance is a huge cost to clean out a heatsink way up a pole. It's really what the major make-or-break issue here is. A city might save 10%-20% on electricity, but the key is how often they need to be serviced. It's not even important whether that requires _replacement_. Going up the pole every year to clean LED heatsinks is vastly more expensive than replacing HPS bulbs every 3 years, even if the LED fixture in theory lasts 5x longer.

I agree HPS is HARD TO BEAT. The lumens/W are excellent. The best LEDs are only marginally better, and subject to derating for temp and optics losses. An LED might have better CRI or color temp to improve aesthetics over HPS, but that has nothing to do with being "green".

The main selling point why cities get into LED streetlights now is because "it looks cool". In a small capacity, it probably increases tourism and investment by giving the city a unique look. Oddly enough, to go to such great expense without saving power is almost the polar opposite of being "green".


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## SemiMan (Jul 5, 2011)

Oznog said:


> Yeah, there's a huge cooling problem. Other light sources don't require cooling. In fact, AFAIK, an incan LOSES efficiency as heat escapes through the bulb wall.
> 
> Streetlights can't employ cooling holes, as stated. Insects will quickly move in and clog the fins or jam fans stuck. Road dust will build up, too. Enclosing a heatsink where it can only recirculate air inside the tube body is "mostly" pointless, although, in theory, the length of the steel pole may have enough surface area for cooling even without fins if the heat was homogeneously distributed down the length (that would be a trick!).
> 
> ...


 
The latest GEN streetlights which are hitting 90 lumens/watt at the fixture level (with all losses) and with reasonable cost are finally able to show good improvements over HPS in terms of lifetime costs ... however, this is also not just about the cost points, but finally being able to get output that allows reasonable pole spacing.

If you are designing for the Chinese market, then Bridgelux may be a good option. With their current parts, I just do not see it NA and Europe. We actually expect our light to be on the road, not the trees, houses, sky, etc.

Semiman


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