LIGHTING ECONOMIST

How do LED tubes measure up against the conventional fluorescent sources they aim to replace? Dave Tilley does the maths so you don’t have to

I discussed LED tubes in an earlier article, but only as part of a broader examination of alternatives to fluorescent sources – and I considered only energy efficiency and relative ROI. This time, I will focus on the LED tube as an apparent rival to fluorescent.

There are many reasons to choose an LED tube as a retrofit source – they are the same reasons that other energy-efficient technologies are specified:

  • rising energy prices,
  • legislation,
  • technology and
  • compliance.

In the past, choosing a replacement source would be the responsibility of a property or maintenance manager, perhaps with some input from the finance director. Today there is another voice to be heard, that of the energy manager. Clearly their focus will be on energy reduction, but at any price?

Back to basics

Before we compare LED and fluorescent tubes, we should consider the characteristics of LED sources:

  • they strike a balance between power consumption, lumen output and life;
  • construction is complex and the weakest component will determine quality;
  • thermal management is essential to quality and performance; and
  • standards are not generally available.

Bear these facts in mind as we compare LED tubes with the conventional fluorescent variety.

The table below has been compiled from data from seven LED tube makers and the four leading UK manufacturers of T8 and T5 fluorescent tubes.

Clearly, the characteristics of LED tubes are difficult to pin down, so how do we specify them? The following evaluation is based on 100 units operating for 4,200 hours a year with an electricity cost of £0.10 per kilowatt-hour. The LED tube data is averaged.

The baseline analysis shows the obvious: use fewer watts and you will save energy. First round to the energy manager. However, if costs are factored into the equation, the picture looks a little different.

Then there is the impact of long-life fluorescent sources.

Without getting too technical, the life of a particular type of fluorescent tube is described by a normal distribution and will be subject to the rules of standard deviation. Therefore there is a significant probability that an early failure will occur.

That’s the science over with, now on to the maths.

Consider the following:

  • Maintenance savings have a greater impact than energy use.
  • The age of the installation is a key factor.
  • The type of maintenance regime will influence the financial analysis.

With return on investment achieved after 2.5 years for an LED tube operating 24/7, it appears to be a real contender. But there are other factors that must be considered: lumen depreciation, colour rendering index, service life, power factor correction, gear loss and switching cycles.

Of these, power factor correction is probably the most important, particularly when evaluating the power consumption of an LED source. So, before the final analysis, what should you think about if you are considering LED tubes?

  • Is the performance data credible?
  • Remember that thermal management is critical.
  • Independent testing will provide validation.
  • Ensure you know where the source was designed, manufactured and tested.

LED tubes are ideal for a number of applications. The key, as always, is to understand the application. The effect of cold on LEDs is well documented, but there are other applications where LED tubes should at least be considered.

  • Corridors and warehouse aisles where controlled light is of benefit.
  • 24/7 activities were maintenance costs are critical.
  • Food areas where enclosed sources are needed.

I am confident there are other applications where the characteristics of the LED tube will have benefits. There is one fundamental thing to remember; do not enclose the LED tube. Thermal management will be compromised and cause early failure.

Energy managers must realise that energy reduction is important, but not at any price. The application must be taken into account, and failure to respect this will result in poor lighting schemes that will have a negative impact on business performance.

THE LIGHTING ECONOMIST’S VERDICT

I started making the assumption that LED tubes are at best a novelty; something to talk about and show those people who love LED.
However, like most light sources there are applications in which an LED tube will have benefits, not only for the client but for the application. The important thing – as always – is to understand what you are buying. davidt@lo2ltd.com

 

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