the influence of daylight control on led service life

23.11.2017 | Lighting knowlege

LEDs age more slowly in partial load operation. The Arrhenius equation enables the correlation between partial load operation and degradation of luminous flux to be very precisely calculated. However, for use in daily lighting design practice this complex approach is not very practical. An alternative is the simple linear relationship: Dpart load = 1 – (1-Dfull load) x part load/full load

With LED luminaires the luminous flux decreases with advancing operating life. The degradation D must be taken into account when planning a lighting system so the requisite illuminance levels can still be achieved at the end of the operating life. If the luminaire is controlled via daylight control, the effective operating duration is less – and therefore also the luminous flux degradation. But how can the effect of this partial load operation on luminous flux degradation be specifically calculated? The theoretical fundamentals for this are provided by the Arrhenius equation that describes the dependency of temperature on physical and chemical processes. This exponential function is however not practical for daily use. There is a linear alternative though that can be derived from the total energy saving.  

 

influence of partial load operation

Seen precisely, this is the nominal value of the partial load across the complete operating duration. This can be roughly estimated for example, and is logically derived from the energy savings achieved by e.g. a light management system. In offices for example with a high / nominal daylight component the partial load can be calculated with an approx. 50% / 40% saving (partial load = 50% / 60%). With the linear approach the behaviour of luminous flux across the operating duration is represented linearly. For reasons of simplification the actual service life in real partial load operation is replaced by a reduced service life in full load operation (figure 1, L80 = 50,000 h, reduced service life 30,000 h).


calculation of luminous flux degradation

full and partial load operation

The degradation in luminous flux (proportionate residual luminous flux after expiry of the real service life) in partial load operation can now be determined using the following linear equation:

The degradation in luminous flux for full load can be taken from the specification of the luminaire manufacturer – see the calculation example below. According to the manufacturer specification, luminous flux at full load reduces after 50,000 h to 80% of its initial value (L80 = 50,000 h). With an annual operation duration of 2,500 h, this value is therefore achieved after 20 years of full load operation (50,000 h /2,500 h/year = 20 years).


calculation of maintenance factor

full and partial load operation

The maintenance factor (MF) of a lighting system is a measure of the reduction in illuminance over the course of the system service life. The luminous flux degradation (D) in the form of the lamp luminous flux maintenance factor (LLMF) flows directly into the formula MF = LLMF x LSF x LMF x RMF with LLMF = D. The other sub-maintenance factors can be taken from the appropriate CIE tables:

  • LSF = 1,00     (luminaire survival factor for LED luminaires with L80 = 50,000 h)
  • LMF = 0.96   (luminaire maintenance factor for very clean LED luminaires)
  • RMF = 0.97  (room maintenance factor for very clean rooms, maintenance interval 3 years)

 

Conclusion

  • If light management is used, the LED degradation reduces significantly compared to full load operation. In practice, the degradation in luminous flux can be linearly calculated in a simplified way.
  • In the calculation example the 60% partial load operation increases the maintenance factor by 10%. As a result, around 10% less luminaires are needed to ensure the stipulated illuminance at the end of system service life (50,000 h).

All further information about the topic of daylight control can be found in the free webinar "Influence of daylight control on LED service life".

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