Selecting the right source
New light sources offer great potential in the right applications but it’s also important to keep track of developments in established sources to get the best results.
Phil Croker of Venture Lighting explains.
In the last few years we have seen some major advances in lighting technology and some exciting new light sources entering the market. Inevitably, these receive a great deal of publicity around the benefits they offer to the end user.
However, what is often missed is that some well-established lighting technologies have continued to evolve so that they too offer significant benefits to the end user. This can lead to inappropriate selection of light sources for certain applications that fails to deliver maximum benefits to the building operator.
High bay lighting is a case in point. Traditionally, because of the mounting heights, the light source of choice has been HID (high intensity discharge) – either metal halide or high pressure sodium. Now, though, there are attempts to achieve comparable results with both LED and T5 fluorescent lighting despite the recent advances in metal halide lighting technology. Clearly, both LED and T5 have a great deal to offer in an office lighting application, and other applications with relatively low ceilings, but are they really the most suitable light sources for high bay lighting?
To put this into perspective it’s interesting to look at both of these scenarios, starting with LED as the newest comer to the high bay arena.
To assess this situation we looked at the cost of lighting an open area of 25m x 25m with luminaires mounted at a height of 5m to achieve an illuminance of 300 lux. The light sources were a 150W metal halide lamp compared to a 143W LED luminaire, running for 10 hours per day, six days per week over a five year period (see figure 1).
The received wisdom with LED lighting is that the capital cost is considerably higher than with other light sources but this is offset by the lower running costs. And certainly the capital cost is higher. In the situation described above, 30 LED fixtures would be required, at a cost of £300 each, resulting in a capital cost of £9,000. In contrast, just 18 metal halide luminaires at £100 each would achieve the same light levels at a capital cost of £1,800.
Clearly a 143W LED fixture will consume less electrical energy than a 150W lamp but the need for nearly twice as many fixtures to achieve the required light levels offsets this. So over the five year period, the LED fixtures would consume 13,770kWh at a cost of £1,377 (@ £0.10/kWh), while the metal halide installation would consume just 8,812.8 kWh, costing £881.
The combined total of capital and energy costs, therefore, comes to £15,885 for LED lighting over five years, and £6,206 for the metal halide lighting. This is not to detract from the concept of LED lighting, but simply to illustrate the point that comparisons need to be made during the specification process, taking account of how metal halide light sources have improved in recent years.
This point is further illustrated by the use of multi-tube T5 fluorescent lighting, which can certainly deliver a sufficiently high light output for high bay applications. Unfortunately, in many such cases the decision to use T5 has been based on inaccurate information and meaningless comparisons, resulting in lighting installations that disappoint in terms of both lighting performance and energy performance.
To a very great extent, the reasons for this are historical. T5 systems have made a name for themselves on the back of the combination of a good lamp, high efficiency electronic control gear and high quality reflectors. However, not all T5 systems are equal, but there is a tendency to apply the top of the range reputation to systems that very definitely fall within the mid to bottom range.
In parallel with this situation, when specifiers are considering the most appropriate choice of light source, they may well compare a top of the range T5 system to the metal halide lighting of four to five years ago rather than the top of the range metal halide systems that are currently available.
To understand this, it’s important to bear in mind that a T5 system is more than just a lamp. Those fittings that provide the high performance described above also make use of high performance reflectors to achieve a good light output ratio (LOR). They also incorporate high efficiency control gear, which is essential in ‘nurturing’ the lamp and providing the extended lumen maintenance associated with such T5 systems. These high end systems are certainly capable of providing an LOR of 90% with an efficacy of 85lm/W.
However, much has changed since the T5 system was first introduced to the market. As is the case with so many innovative and successful products, it has become a price-driven commodity item with many poor imitations of the original. Typically, such imitations achieve their competitive pricing by compromising on the quality of reflectors and control gear. So while the lamp itself may maintain an efficacy of 85lm/W, the LOR in many systems has plummeted to 40-50%, with a shorter useful lamp life.
In contrast, metal halide lamps and their control gear have come a long way during this same time period and are streets ahead of the old systems that T5 systems are frequently compared to. Consequently, an efficacy of 110lm/W with an LOR of 90% is now readily achievable with metal halide. Not only is this LOR comparable to that of the best T5 systems; the efficacy is considerably higher.
In fact, a metal halide system with electronic control gear, using the latest reflector technologies, is more than capable of out-performing the very best T5 systems in high and low bay applications. And if a lower priced T5 system is being considered to stay in budget, the performance differences will be even more marked.