Thermal Modeling of LED Lights
Using computational fluid dynamics software to model different product assessment configurations, including the details of an LED, UL is seeking to develop better ways to evaluate LED safety and performance and to update the safety standards.
WHY THERMAL MODELING OF LED LIGHTS MATTERS
LEDs (light-emitting diodes) are far more efficient than traditional incandescent lighting technology and thus have the potential to reduce energy use and greenhouse gas emissions. This, coupled with decreasing manufacturing costs, is driving explosive growth for the LED industry and attracting the aggressive development of new applications and products that are projected to help the industry expand, particularly in the automotive and general lighting sectors.1 UL is using thermal modeling to rapidly evaluate and refine tests as well as to validate products, all of which helps us safeguard LED innovations.
Today, lighting accounts for 19% of the world’s energy use and produces carbon dioxide emissions equivalent to what 70% of the world’s passenger vehicles produce each year.2 LED lights use at least 75% less energy than incandescent bulbs and last 25 times longer,3 providing an opportunity to reduce light-related energy consumption by 50% and global carbon dioxide emissions by 10% by 2025.4 To realize these sustainability benefits, the LED lighting industry must achieve its projected growth rate, which will increase LED market share globally from 10% in 2010 to 60% in 2020.5
Some of the LED industry growth is being driven by governmental regulation around the world pushing for energy efficiency; lighting companies are responding by pursuing the development and enhancement of LED technologies that are poised to transform the industry.6 Heavy investment by these companies is cutting LED manufacturing costs by 30% each year.7
Today, LED technology is creating entirely new lighting possibilities. These include dynamically changing light color temperature and flexible designs, as seen in cars, that challenge the standard interfaces that have been in place for decades.8 New growth opportunities are also emerging in the area of intelligent lighting systems, which result from the greater controllability of LED-generated light.9 The growth of LED lighting will lead to a fundamental disruption of the lighting industry along its entire value chain.10 The new LED products and applications that emerge will create a need for new and updated standards and, in turn, new ways to evaluate safety and performance. And the pace of change inherent in the projected 500% increase in LED market share from 2010 to 202011 means that standards and testing methodologies will need to be updated or developed quickly.
WHAT DID UL DO?
We used thermal modeling to enable us to simulate, explore and understand LED performance and safety characteristics, and to validate test methods. Today we use computational fluid dynamics (CFD) software specifically to help us create new tests and modify existing LED tests. Our CFD modeling tools help us determine which tests are most appropriate, given a specific set of criteria. The optimal test will simulate realistic worst-case conditions to properly assess safety performance.12
One example is our use of CFD software to model different LED testing configurations, including the details of an LED, to understand the impact of various parameters on actual airflow and temperature patterns within the LED created by a test box (i.e., a test chamber designed to determine the thermal performance of an LED light engine, or LLE). This is important because one of the key challenges in the design and performance of LEDs is thermal management. Unlike incandescent lighting, which dissipates heat via infrared radiation mostly through the front, LEDs dissipate two-thirds of generated heat via conduction to heat sinks attached to their backs. As such, thermal management is critical in maintaining low juncture temperatures within the LED that ensure long product life and safety.13
Specifically, we employed thermal modeling to determine whether the temperature test box proposed to update the Standard UL 1993 could realistically represent worst-case thermal luminaire designs. These would allow a typical LLE to operate within its rated limits. We developed a detailed thermal simulation of an LLE enclosed in a test box, and we also modeled the same test for a redesigned test box (having some space between the LLE and the box) to understand the effect of vertical clearance in temperature distribution. UL had to also build three-dimensional numerical models and carry out conjugate heat transfer analyses to predict the temperature and flow profile for each test box configuration.14
However, before carrying out the actual thermal analysis, we compared our CFD model with experimental test results. With confidence in the model predictions, we performed thermal analyses of the same LLE model placed inside different test boxes.15
The results of this work helped us establish that the initial test box design in the UL 1993 CRD (Certification Requirement Decision, a document that addresses a potential change in the application of safety requirements included in a Standard) could be improved. The results also highlighted the sensitivity of the LED performance to the vertical clearance between the LLE and test box in determining the temperature.
UL’s CFD models successfully predicted the temperature limits of an LED light engine enclosed inside a box. Given the assumptions used for the study and the actual results obtained, we are confident that our CFD tool can accurately predict temperature by modeling an LED luminaire and LLE system.16
Ultimately, computational modeling is an important tool we use to gain insight into how a product functions, how it malfunctions and the risks involved. With the data our modeling generates, UL can help product design engineers develop better ways to evaluate LED safety and performance and provide technical justification to the Standards Technical Panel (STP) and other interested stakeholders to accelerate test development times internally and eventually adopt them into standards. Overall, reduced development times help UL customers who are looking for certification that is relevant to their product. Accelerating the process of safeguarding LEDs is critical to supporting the growth of the industry and the adoption of this highly sustainable lighting technology.17