8.8 Energy savings with dimmable ballasts

So if you want to save energy you will try to reduce the lighting level automatically, dependent on the level of available daylight. As you have learned in Section 8.4, the reduction of the voltage fed into magnetic ballasts, although it does save energy, does not reach far enough to call it a »dimming technique«, so you will try with dimmable electronic ballasts. But again, the question was how far the savings potential would go. Measurements were commissioned with an independent certified lighting laboratory19 by the German Copper Institute DKI and the company M&R Multitronik to complement the measurements on magnetic ballasts described in Section 8.4. In order to obtain objective, comparable results compliant with the existing measurements reported in Section 8.4, a twin electronic ballast together with two commonplace, readily available T5 lamps (triphosphor, colour rendering index 840) were used, since it has turned out in Section 8.3 that a twin electronic ballast usually has lower losses than two single-lamp ones. As for the lamps, the lowest wattage of the biggest available size (1449 mm) was chosen because the greatest efficiency could be expected from these. This led to a rating of 2*35 W.

The T8 lamps had been tested before with an ambient temperature of 25°C according to the standard where they usually perform their best efficacy. The T5 lamps were additionally measured with an ambient temperature of 35°C, deviating from the standard, since for some good reasons they are optimized to this ambient temperature.

Fig. 8.16: Light outputs of different systems employing T5 and T8 fluorescent lamps, plotted against the absolute electrical systems power input

The results were summarized in Fig. 8.16, where the systems’ light outputs were plotted against the respective electrical power intake. Further, a line was included in the plot, representing a constant efficacy of η = 80 lm/W, which should represent a guideline for the efficiency in today’s lighting installations. In this way the following becomes evident:

• The efficacy of any T8 system increases during input power reduction. Generally speaking, the values in the lower segment lie above the 80 lm/W »guideline«, while in the upper half they lie below, and especially in the overload range they strongly tend to flatten out.
• The T5 lamps exhibit the inverse behaviour: Efficiency decreases during dimming. Values in the upper range tend to lie above the »guideline«, while values in the lower range will rather lie below.
• The improved efficiencies of the T5 lamps at 35°C against the values measured at 25°C become quite obvious.
• But unfortunately this type of plot is not very adequate for a direct comparison of either system against the other one because there are not any two lamps T5 and T8 with equal electrical power ratings available.

Fig. 8.17: Light efficacies of different systems employing T5 and T8 fluorescent lamps, plotted against the relative electrical systems power input

It was therefore successfully tried to find a different method to compare both of the systems to each other by plotting the light efficacy against the relative system power (Fig. 8.17). In this type of graph a direct comparison of different systems should be possible when keeping the following remarks in mind:

• For the T8 systems, what is meant by relative systems power is the ratio of the measured systems power at the respective voltage divided by the systems power measured at rated voltage of the same system (for instance, with the old magnetic ballast class EEI=C the re-ference point representing 100% is 69 W, that of an improved magnetic ballast class EEI=B1 is 61.4 W, which represent the respective systems values measured at 230 V).
• For the T5 system, what is meant by relative systems power is the ratio of the measured systems power at the respective dimming level divided by the systems power measured when set to full light output (100%, i. e. same system with dimmer set to full power).
• For ease of orientation, the minimum requirements for class A1 are plotted in the chart in stroke-dotted lines once for a reference ambient temperature of 25°C and once for 35°C.
• The non-dimmable electronic ballast also included in the measurements could not reasonably be displayed in this format, since its power intake, along with the light output, is invariable and would have yielded only a dot.

Hence, the above description facilitates the following observations:

• The T5 system under test by far exceeds the minimum requirements.
• It becomes even clearer now that the efficacy of the T8 system increases due to power re-duction (and accordingly drops inadequately in the overload range), while the efficacy of the T5 system is best at full power and drops during dimming.
• At full load and 25°C ambient temperature the T5 system is about equally efficient as the best T8 magnetic system (EEI=B1).
• At full load and 35°C ambient temperature the T5 system is ≈10% more efficient than the best T8 magnetic system is at 25°C.
• At ≈75% of their respective electrical power input measured at 230 V or, respectively, of the undimmed lamp, the efficacy of the best T8 magnetic system is about equal to that of the T5 system at 35°C.
• When reducing, respectively dimming, the systems power to ≈60%, the efficacy of the T5 system even drops below that of a T8 system with an ancient class D magnetic ballast which was rescued from a scrap metal container back around 1986.
• When reducing to ≈50% input power the possible range of application for the voltage re-duction technique ends. Otherwise the lamps will go out completely. A greater dimming range can be implemented with dimmable electronic ballasts only.

This facilitates the following conclusions:

• Dimmable ballasts provide only a rather limited energy savings potential. Who wants to save energy should reasonably employ a combination of voltage reduction and subsequent grouped automatic switching (e. g. from the aisle side to the window side in an office) after exploiting the (limited) »dimming« potential of voltage reduction – optionally, wherever possible, applying a technique which comes without any need for stand-by consumption and using electronic starters7, which spare on the lamp life as well as on the employees’ nerves wherever switching occurs more frequently than once a day.
• The voltage reduction technique is no replacement for dimming. Who wants to dim has to use dimmable electronic ballasts. On the background of today’s knowledge all techniques for dimming magnetic ballasts that have ever been around are makeshift solutions and do not satisfy modern needs. They should therefore not be considered any longer.