With the advent of solid state lighting technology like LED lamps, the need for lighting fixtures need redefinition. In the case of earlier technologies, requirements for ingress protection and heat dissipation were different. Now, requirements for the lighting fixtures have to take into account the flat dispersed nature of LED placements.
The cost of the lighting fixtures and their efficiency plays a major role in the successful deployment of LED lighting solutions as retrofits/replacements to the conventionally existing lighting systems. Ingress protection, which is essential to protect the lamp, results in temperature build up inside the fixture – due to conduction being the only mode of heat transmission to the environment. This is a design challenge. The other major design challenge is the design the fixtures as retrofits without sacrificing the light transmission (beam distribution) characteristics such as cosine efficiency and batwing efficiency. The effectiveness of heat sinks plays a major role in determining the operating efficiency of the lamp due to the operating temperature. Thus, the lighting fixture affects the performance of the lamp that must be kept in mind during the design.
Light Emitting Diodes (LEDs) are miniature bulbs of directional or pointed beams with UV resistant, weather proof, unbreakable plastic super-lamination. Basically, the mode of building up of a luminaire of large size is through clustering of miniature sources into a lamp. An individual LED gives a maximum output of 1200 lumen. Luminaires are configured out of around 3 to 60 LEDs.
Solid state, opto white semiconductors LEDs in conjunction with solar PV have the potential for wiping off the dependence on kerosene lanterns from the country. It is a continually evolving technology, financially becoming cheaper and is an ideal energy saving solution in the lighting sector as a portable as well as in-situ technology. The energy efficiency of LEDs (laboratory), which was around 100 lumen/W in 2008 went up to 132 lumen/W in 2009 and 231 lumen/W in 2011 and 275 lumen/W in 2016. The field efficiencies are presently around 140 lumen/W. The difference in the laboratory and field conditions is the operating temperature and other test conditions like operating current, etc. CPRI, Bangalore is one of the premier LED lantern and home lighting system testing agency in India.
A major advantage of LED lighting technology is its ability to be integrated into smart grid configurations in the electricity distribution sector because it is a solid state technology and can be digitally controlled for illumination level, on/off operations, etc. The building lighting load and street lighting load when using LED systems would be of much lower electric rating and can also be controlled with ease. It is an ideal solution for a demand side management for minimisation of the grid evening peak created because of lighting loads. Dimming, without drop in energy efficiency is a distinct feature of LED lighting.
The Government of India through the EESL under the Ministry of Power has launched a massive programme to replace conventional CFL and incandescent lamp technologies with LED lamps.
Challenges in Lighting Accessories
The massive replacement program involves the replacement of conventional lamps by LED lamps. The mechanism is through replacement using the same 230 V AC pin type holder. The challenge here is to make the LED lamp compact and fit into the same old pin type holder. The volume and mass of the LED lamp must confirm to this holder size. This requires innovative designs to ensure that mass and volume are within limits.
While the main thrust of the Indian program is on the 9-10 W LED lamps with pin type holders using 230 V AC, the lighting fixtures and accessories play a critical role in the acceptability and technical performance of larger size fixtures of larger sizes of LED lamps from around 25 W to 100 W to 500 W.
The outdoor fixtures can be classified as:
- Linear strips
• Lay in Troffers
• Down light
• Wall packs
• Flood lights
• High bay
Configurations with different degrees of freedom:
- Track light
The indoor fixtures can be classified as:
- Pin type fixture
• Recessed fixtures
• Wash down
• Wrap around
• Sensor fixtures (also for outdoor)
• Vapour proof (globe) (also for outdoor)
• Temper proof (also for outdoor)
• Explosion proof (also for outdoor)
Requirements of Lighting Fixtures
The requirements for the enclosures of lighting fixtures are:
- Lumen distribution
• Ingress protection
• Heat dissipation
The lighting fixtures have an impact on the energy efficiency of the lamp since the fixtures has a role in determining the operating temperature depending on its heat dispersion characteristics.
The light guides of LEDs are generally rectangular to provide cosine beam distribution efficiency of above 60%. The batwing distribution (polar distribution) efficiency must be over 70%.
The outdoor LED lighting systems have requirements of IP 65:
- 6- Total protection against dust (no ingress of dust) and complete protection against contact. It must also satisfy ingress under negative pressure up to 8 hours of air flow.
- 5-Protection against low pressure water jets from all directions. Limited water ingress is permitted. Water (12.5 l/min) at 100 kPa projected by a 6.3 mm nozzle for 3 mins. should not result in ingress.
For indoor LED systems or home LED systems (semi indoor also included) should satisfy IP 22:
- 2- Protection against solid object over 50 mm, e.g., accidental touch by persons.
• 2-Protection against direct water sprays equivalent to 3mm of rainfall/min. for 10 min. up to 15° from the vertical.
Figures 1-12 show the designs of lighting fixtures from those available in the market. The obverse and reverse are shown to indicate the design.
The rate of increase of temperature in the LED is given by:
The heat generation is from the LED system (inclusive of driver) and the heat dispersion is from the heat sink and from thermal dispersion. While the thermal dispersion is a small quantity (15% of the heat generation) the bulk of the heat is to be dispersed through the heat sink. Inadequate or inefficient heat sink will result in rise in temperature which will cause loss of performance as discussed below.
The increase in junction temperature is on account of either excessive generation of heat or inadequate withdrawal of heat.
The heat generation or thermal load (q) (W) of the LED emitter/source/engine of electric power input of P (W) is given by,
For a typical LED with a lumen efficiency of 100 lumen/W the thermal load is 85% of the input electric power to the emitter (after subtracting of the thermal loss in the driver). Heat sinks must be designed of conditions of higher power and lower lumen efficiency.
The thermal load (q) (W) of the driver of rating P (W) is given by,
The driver thermal load is the fraction of the electric power absorbed in the driver as heat which is typically between 2% to 15% of the input electric power.
Inadequacy in the heat sinks in the LED emitter and increase in temperature of the electrolytic capacitors at the output stage of the LED drivers – also lead to increase in junction temperature (over the design temperature).
The fixture design must take into consideration the heat sinking capability. While the material cast aluminium and powder coating affect the heat transfer characteristics, their optimisation is called for to keep in mind not only the structural considerations but also the heat transfer considerations.
Figures 13-15 show the temperature profiles at the reverse (finning area) of the light fixtures after certain lengths of operation.
Challenges for the Lighting Fixtures
Figure 1: Fixture for 9 LED 9 W-obverse…
Figure 2: Fixture for 9 LED 9 W-reverse…
Figure 3: Fixture for 12 LED 24 W-obverse…
Figure 4: Fixture for 12 LED 24 W-reverse…
Figure 5: Fixture for 32 LED 72 W-reverse…
Figure 6: Fixture for 50 LED 100 W-reverse…
Figure 7: Fixture for 44 LED 90 W-obverse…
Figure 8: Fixture for 96 LED 180 W-reverse…
Figure 9: Fixture for 96 LED 180 W-obverse…
Figure 10: Fixture for 96 LED 180 W-reverse..
Figure 11: Fixture for 96 LED 210 W-obverse..
Figure 12: Fixture for 96 LED 210 W-reverse…
The major challenge for the LED lighting fixtures is the performance under retrofitting. The massive replacement program calls for replaceme of different types of ballasted and gas filled lamps with LED. The retrofitting calls for placing the new fixture in the place of the same old one. The retrofitting must be accomplished at the maximum efficiency (the performance must be maximum and must not deteriorate after placement of the fixture due to retrofit constraints), minimum cost and minimum displacement of the positional requirements. The ingress protection also results in heat trapping in the system and there will be internal temperature rise in the system. The heat sinking fins do not give uniform temperature conduction in both the x and y directions resulting in temperature concentration as indicated in Figures 14 & 15. This calls for innovative designs such as heat pipes for dispersion of heat from the interior of the fixture to the outside.
Figure 13: Outside temperature (reverse) of 18 W LED lamp fixture after 5 hours…
Figure 14: Outside temperature (reverse) of 100 W LED lamp fixture after 2 hours…
Figure 15: Outside temperature (reverse) of 100 W LED lamp fixture after 2 hours…
The main conclusions are:
i. Integration of LED lighting systems with either locally or centrally based SPV into street lighting systems and building lighting systems with intelligence and controllable features, has a fair share of potential as a substitute/retrofit for conventional technologies.
ii. The LED sector is poised for an increase in its market share from the present 3% to 55% in 2020. The composite growth rate of LED technology may be around 45-55% compared to the present sale unless the scenario is altered by newer more efficient technologies. This calls for efforts at cost reduction through improved designs.
iii. One of the main deterrents to penetration of LED technology is the capital cost. The cost of the bare LED is over 40% of the system cost. Significant cost reduction is expected in LED costs following the present trends and factors like innovative manufacturing processes, indigenous manufacturing capability and volumes. The lighting fixtures also add to the capital cost as well as affect the performance of LEDs via the temperature and the transmittance efficiency.
The cost of the lighting fixture, their efficiency and their design plays a major role in the successful deployment of LED lighting solutions as retrofits to the conventional systems. The designs have to be configured keeping in mind the constraints of the original lamps to integrate with the lighting system and aesthetics.
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