Lighting Efficiency

Are LEDs always more efficient that other lighting technologies?  Unfortunately it’s not that simple and as LCA practitioners helping design teams make good decisions we need to provide them guidance on how to lock in the environmental savings with a robust construction specification.  In the case of lighting there are three main factors that will drive the power consumption:

  • Light requirements – Lux (luminous flux per unit area)
  • Fitting Efficiency – Luminous efficacy (lumens output per unit power input)
  • Loss factors – Losses associated with ballasts, reflectors, soiling and utilisation factor

Light requirements

See our detailed post on Average Lighting Lux Levels for guidance on the recommended required light levels for different spaces and tasks.  There is a large range depending on the function of the area being lit and design teams should be very careful not to provide beyond the requirement.  It is extremely easy to inadvertently design a lighting layout that provides 50% more light that required.  But undoing that with efficient fittings, fixtures and controls is actually quite hard.  So meeting the lighting requirement without exceeding it is where the design should start.  The lighting design must also react to changes to fitting efficiency and loss factors to be effective.  For example, if a fitting is chosen that delivers more with the same (or less) energy demand the lighting design must be reconfigured to use less of these fittings.

Fitting Efficiency

To standardise the measurement of a light fitting’s effectiveness of producing visible light with a standard unit of power industry uses the term luminous efficacy.  Specifically this measure the light output (lumens) per unit of power used (watts), the higher the number the more effective a solution is at delivering light with less energy.

There are many lighting technologies available in the market and each of them have a plethora of options available.  As such it is not enough to simply specify “LED Light Fittings” because the lower end efficacy of LEDs require more energy than the upper end of some some of the more traditional technologies.  See the below chart for a comparison:

Sources: Compact Flourescent and LED from Energy Star Database 2020.  All other lighting technologies from Wikipedia

Note that luminous efficacy measures light output in all directions and therefor doesn’t account for the improved directionality of LED globes (light losses due to reflectors etc are largely eliminated with LEDs).

Not withstanding this it is important that the lighting design requirements go beyond simply specifying LEDs. The minimum efficacy of the globes+ballast (where applicable) should be specified to ensure that LEDs in the upper range of efficiencies are sourced which truly provide an energy savings against other technologies.  The below can be used as a general guide:

  • In residential applications LEDs should be specified with a minimum luminous efficacy of 90lm/W delivering a 10% reduction in energy for the same light output when competing with Compact Fluorescent globes.
  • In office and other commercial applications LEDs should be specified with a minimum luminous efficacy of 115lm/W delivering a 10% reduction in energy for the same light output when competing with T5 linear fluorescent globes.

Loss Factors

The light hitting the working plane is generally affected by a number of loss factors

Ballasts

This has been somewhat dealt with in the last section but is worth mentioning again as it is often a separate part of energy calculations for lighting.  Some lighting technologies require transformers or ballasts as part of the conversion of electricity to light.  Older style transformers and ballasts typically had quite high parasitic loads on the overall fitting (up to 30% was not uncommon).  Be sure to consider the ballast losses (if applicable) when running lighting scenarios in your LCA.

Reflectors

Other than LEDs, light globes generally disperse light in all directions from the source.  If a ceiling mounted light does not direct the light back down to the working plane, more fittings will be required to achieve the required lux levels.  So the effectiveness of the reflectors (or minimising losses due to poor reflectors) is important.  Reflectors should be both reflective as well as carefully designed to disperse light effectively on the working plane at the design height of the fitting (eg, light should not be concentrated in one area, providing too much light, whilst falling short of required levels in another area).

Example of T8 fluorescent light fitting with high reflectivity, minimal horizontal surfaces for soiling and parabolic design to effectively distribute light to the working plane.

LEDs excel in this area as they are directional in nature and hence don’t require reflectors.

Soiling

Over time dust will reduce the light hitting the working plane (known as soiling).  The design of fittings should minimise soiling by:

  • Reducing opportunity for ingress of dust whilst still dissipating heat.
  • Reducing the horizontal surfaces available for dust to collect (e.g. on globes and diffusers)

Utilisation factor

Once the light leaves a fitting it bounces from the working plane/s, floors, walls and ceiling to light other areas.  The reflectivity of surfaces in the room can actually have a significant effect on the lighting design.  For example, car parks with concrete or asphalt surfaces typically have a coefficient of utilisation of 45% (meaning that 55% of light is lost due to low reflectivity of surfaces).  Painting the car park surfaces white can increase the coefficient of utilisation to 65% equating to 30% reduction in the required fittings.

Similar affects can be achieved in office, residential, health care, retail, education facilities etc.  The reflectivity of surfaces, if carefully coupled with the lighting design can save energy.  This is particularly true for buildings with good access to natural light as the deeper into the floor plate the light can penetrate the less artificial light required.

Other Considerations

So it would be quite conceivable to achieve savings of 50% or more by applying some combination of the above knowledge to the lighting design.  Further than that, the design team should carefully consider lighting controls (lux and occupancy controls) that vary the light output in response to natural light levels and occupancy.  This article on occupancy sensors in underground car parks provides some indication on the huge potential savings associated with lighting controls.

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