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8.9 Energy savings with dimmable ballasts

By: 
Stefan Fassbinder
/ Published on: 
Thu, 09/04/2008 - 19:22


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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 laboratory 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.

Still, these considerations do not yet include the following circumstance:

Dimmed operation of fluorescent lamps represents permanent cathode heating operation. The position »Lights off« is usually identical with the position »Dimmed down to 0«, where no more light is generated, but the filaments continue to be heated (Fig. 8.18), albeit these states are often confused with each other. After all the lights are off in any case. As far as only the daylight sensor exerts control over the electronic ballasts in this way but over the rest of the time it is made sure that e. g. in an office the light is turned off properly after work and on weekends, this is not yet a tragedy. It has not proven a good practice, however, to completely turn off the supply voltage to the lighting e. g. by switch timers. After all, the cleaners are yet to come somewhat later on, and nobody knows exactly how much later, and at some point in time there will be someone working right through into the night or on a Saturday – usually the boss himself. When the lights go out this will rouse trouble, and this trouble will happen only once. The facility manager, being the target of the corresponding complaints, but not in charge of coming up for the electricity bill, will immediately sabotage the timer switch. So the lights will always be »dimmed down to 0« whenever the light is »off«. This also sabotages the underlying energy savings efforts carried out during the planning stage and also the payback calculations which these were based upon. In cases of favourable exceptions, this is taken into regard, and the installation is configured adequately, so that users do not lose their daily savings at night.1 This is the case, for instance, when a daylight sensor and a presence indicator work together on the »project« of dimming but only the daylight sensor really dims, while the motion indicator releases a signal equal to »turn lights off completely« (including cathode heating). Subsequently solely the stand-by consumption of the ballast is left, which at present is limited to 1 W, from 2012 onwards only more 0.5 W, which limits the loss.

As a refurbishment, each sensor and each actuator in such a control system will actually require a power supply of its own to draw power from the mains. The net DC power requirement of any one such device may be as low as a few milliwatts, but a power supply unit including a transformer will be needed in every case. The smallest commercially available transformers, however, have a power rating of ≈1 VA and a no-load loss rate of ≈1 W. While the load loss is irrelevant at such a low degree of loading, it is the no-load losses in the great number of such devices adding up to the bulk of the stand-by power demand in such an installation. Modern control systems, which can be refurbished fairly easily if the commensurate cables or at least empty installation tubes have been fitted already during the building’s construction stage, use one single centrally located power unit and supply both DC power distribution and the signals via the same cable. This technique incurs the potential to reduce the stand-by consumption down to a fraction, although, by principle, a permanent demand for power also during periods without demand for light remains for all of the time. Hence, it remains to be considered in each individual case whether the implementation of a less sophisticated savings technique together with latest magnetic ballasts, which shuts off the entire lighting completely or, for instance, groups of it, could not be the cheaper and at the same time less power consuming approach.

Irrespectively of this, advertisements by the lighting industry continue devising a scenario promising an energy savings potential of, say, 80%.2 The mere transition from a magnetic to an electronic ballast on the same lamp is supposed to bring about 25% of energy savings. But, first of all, the worst and oldest magnetic system you can find in existing installations is always plotted against the best and most modern electronic solution. The existence of »improved« energy efficient magnetic gear is ignored and the differences between these blurred. Further, it remains totally unclear which system is compared to which one, but assuming the base case was a 58 W lamp being converted here to use the best electronic ballast classified A2, this would yield a maximum permissible systems power intake of 55 W. Then the old magnetic system, against which a savings potential of 25% was tapped, must have had a power intake of over 73 W. This matches the stone old magnetic ballast mentioned earlier which was recovered from a scrap metal container already back in 1986. It yielded a light flux of 5300 lm, while the same lamp performed only 4700 lm with an electronic ballast, but this difference is dropped unmentioned.

Fig. 8.18: Behaviour of a dimmable electronic ballast according to manufacturer’s documents

The transition to a T5 lamp even claims for a savings potential of 50%. So this can only refer to a 35 W HE lamp, with a maximum permissible power intake of 39 W with an electronic A2 ballast. However, this system has a light output of only 3300 lm – which again does not appear worth any mention. Of course you can save energy when you replace a Greyhound bus with a Volkswagen Rabbit and keep silent about the number of seats available. Additionally, the »cut-off« feature is mentioned as a further advantage here, which comes as an inevitable inherent constituent of the magnetic system, but there it finds no mention.

Finally, the introduction of a dimming technique is supposed to raise the savings to 80% – without any explanation of the circumstances and assumptions under which this was calculated or measured.

The comparison to the measurements described in the previous sections, however, does not show any similarity of any kind to these claims of the advertisements (Fig. 8 19).

Fig. 8.19: Light outputs of different systems employing T5 and T8 fluorescent lamps, plotted against the absolute electrical systems power input

On top of all, the value measured at 230 V had to be used for the system with the ancient ballast still rated 220 V – which, after all, is only realistic, for the line voltage rating today is 230 V. But it makes the system’s power intake rise over-proportionally to 80 W, dragging the efficacy further down. Only in this way could the efficacy of the old system, which was already lousy, be deteriorated once more so as to leave a tiny little margin for improvement towards the T5HO system, or else the »savings potential« of the T5HO lamp versus a system dating back to ≈1965 would have turned out negative, and we wanted to spare the T5 lamp such an intrusion.

The individual measurement results and calculations derived from these have been compiled in Table 8.13.

An office room was considered here where

  • for 1000 h/a the lights are on at full power,
  • for 1000 h/a half of the light suffices, for which purpose the magnetic system is stepped down to 64%, because a lower reduction is not possible, so that it actually produces more light than necessary,
  • for 1000 h/a no artificial light is needed, so that the automatic dimming function dims down to the minimum value of 2%, but where, on account of permanent cathode heating, the power intake still lies at 14%,
  • for the rest of the year absence is detected by the presence indicator, and the stand-by loss of the electronic ballasts drops down to the future maximum value of 0.5 W.

For the magnetic system the according states »dimmed down to zero« and »lights off« are identical and both match a full separation from the supply. Since also the control gear of the »EnOcean« brand does not draw any power from the mains the power intake is equal zero here.

4 of such magnetic systems, as described in section 8.5, generate the same quantity of light as 3 of such electronic systems do, as described in section 8.7, i. e. with 6 lamps rated 35 W each. This allows for a direct comparison of both systems. It only requires the quantification of the total light energy in lumen hours. While electric power is measured in kilowatts and light power in lumens, electric energy is measured in kilowatt hours. So accordingly the light energy generated by this number of kilowatt hours has to be measured in »kilo lumen hours« or »mega lumen hours«. This is exactly what was done in Table 8.13.

Table 8.13: A magnetic system with a complete switch-off function is energetically superior to an electronic system with a commensurate dimming function

The sobering resume is that there is not much left of the energy savings benefit of dimming, but rather the opposite: Along with an automatic, daylight and presence dependent control the magnetic systems generate more light energy at the same peak lighting power and even use a bit less electric energy for this than the electronic systems do. Their mean annual efficiency is thereby better. This is so thanks to the following circumstances:

  • The efficiency advantage even of T5HE lamps is only marginal.
  • The efficiency advantage of electronic ballasts is similarly marginal, other than generally assumed and propagated.
  • The efficacy of electronic dimmed systems deteriorates during dimming, whereas
  • also at the minimum level of the dimming range with practically no light output a substantial »no-load« consumption of 14 W remains flowing and
  • even when the lighting is switched »off« dimmable ballasts need to remain in operation, which, though representing only a minor fraction of the power intake, still stands for a fraction of the overall energy consumption that is no longer negligible.

After all it would also provide quite a lot of comfort and convenience to park one’s car with the engine running in the evening, since a modern diesel engine uses barely any more than 0.3 litres of fuel per hour when idling – apparently not worth the talk, hardly any more than 1% of the top speed value, and in the morning the car would already be warm and not even frozen after a cold winter’s night. But nobody does that, because at the filling station it would soon become obvious that a night of idling is equivalent to an 80 km trip the next day and would hence no less than double the average consumption for such a distance!

Electricity, however, you purchase blind, paying once a year without a clue when you have consumed how much for what. This situation leaves you dependent on half-way correct indications by fabricators and installation planners about their ratings. If this is not the case nobody will realize – until the day someone takes the trouble and undergoes the expenses to measure.

In this context it needs to be pointed out once more that the measurements presented here would absolutely not compare the best available magnetic system against the poorest electronic solution, as is always the case vice versa – and has to be done in this way to elaborate any advantage for the electronic option at all. Rather, we did not only select the most efficient electronic system according to catalogue data but

  • a rounded figure was used for the annual usage time of an office – in the according studies carried out by the EU less than 3000 hours per year are assumed for an average European office
  • measured values of an excellent electronic ballast were used which consumed »only« 35% of its rated power at 25% of the maximum light output, instead of just calculating with the permissible maximum of 50% input power at 25% brightness »for reasons of simplicity«
  • the measurement on the T5 lamps was carried out at 35°C ambient temperature because for some good reasons these lamps are optimized to operate at such temperature, rather than measuring at 25°C, as the standard requires it for historical reasons.

So if someone should intend to run town the electronic ballast – or at least its dimmable version – these »slip holes« could and should still be used. In a description of the facts which only intends to shed the correct light upon some lightly made statements in lighting advertisements this is not necessary, for it goes without saying that electronic gear performs some excellent features the traditional magnetic technique cannot offer. As mentioned earlier, all dimming techniques that have ever existed before the invention of the electronic ballast behaved very much like makeshift solutions, while sometimes the option of a wide range of brightness regulation is just required, such as in conference rooms. Of course it also provides more of a feeling of luxury and comfort when the light dims down continuously and mostly unnoticed as daylight arises than when it is switched off abruptly. But who believes to be able to buy in more energy efficiency than a magnetic ballast with an electronic starter can offer along with the increased luxury is moving on thin ice.

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