Comparing Tubular LEDs and Tubular Fluorescents
First published: 23rd October 2014
Commercially-available energy-efficient lighting has progressed in the three years since I wrote, Comparing Tubular Fluorescents and the two years since I wrote, Comparing LED and Tungsten Halogen Lamps. This test compares a newly-available LED tubular replacement for a T8 fluorescent tube to an equivalent T5 conversion fitting.
Materials and Methods
The same equipment as described in Comparing Tubular Fluorescents was used but KWh meter measurements were not taken.
The luminaires and lamps tested were:
|oldT5||OSCxx T5/28W electronic ballast with old 28W T5 fluorescent tube. It was noted that some darkening of the tube was visible, so newT5 was added to the test.|
|newT5||OSCxx T5/28W electronic ballast with new 28W T5 fluorescent tube.|
|LED||Forcast LED-418D 6500K 18W tubular LED.|
|Distance from light fitting to PV cells||45cm|
Please refer to the table above for the abbreviation key.
|Lamp||Light intensity (PV cell output, mV)||Current (Clamp meter, A)|
|Lamp||Light output1||Power (W)2||Power Saving3||Lighting efficiency4|
- 1 Light output is expressed as a percentage of the light output from oldT5
- 2 The power is calculated from the current and voltage. This ignores the effect of the power factor of the luminaries. These figures are lower than expected, given the nominal power ratings of the components.
- 3 The power saving is the percentage less than the power used by oldT5
- 4 The lighting efficiency takes into account the light produced as well as the power used. Expressed as a percentage of the least efficient case, oldT5.
From these figures, and adding the cost of the new equipment, it is possible to calculate the number of hours of operation before the savings have paid for the equipment. A one-for-one replacement is assumed, this results in a change in lighting levels, as noted in the Light output column of the first table above, but a change of 10% or less is probably not noticeable.
The cost for electricity is assumed to be HK$1.16
|Description||Cost (HK$)||KWh to break-even||Operation hours to break-even|
|oldT5 (original situation)||0|
Sources of Error
There are many potential sources of error:
- Measurement of light intensity. The response of the PV cells to light intensity may not be linear. The response may vary according to the colour temperature of the light source - the tubes used had different colour temperatures.
- Measurement of current. The use of a clamp meter with ten turns of cable may provide inaccurate current readings. The resolution of the meter (as increased by the ten turns) is 0.01A, which is 14% of the lowest reading.
- Power Factor. It was assumed that, because the fluorescent ballast and the LED power supply are modern, electronic designs, they will both have a good power factor. This might be incorrect, meaning that the assumption that the current and power are directly related would be wrong too.
Conclusions and Areas for Further Study
- T5 fluorescent tubes produce less light as they age. In this case, a new tube boosts light output by 45%. At what point does it become advisable to replace a still working T5 tube?
- The tubular LED is clearly better than the T5 fluorescent in this test, but replacing working T5 fittings with tubular LEDs cannot be justified by cost savings alone. The time to break-even would be over 8 years for an office-hours usage pattern.
- LED lamps are supposed to have a far longer life than fluorescent tubes, but this is unproven. Early failures could wipe out the projected lifetime savings.
- The deterioration of light output over the lamp lifetime for LEDs is unknown.
- The light output of LEDs have improved substantially in recent years, and further improvements are predicted. An LED lamp bought next year could be a significant improvement over an LED lamp bought this year.
Tubular LEDs are clearly now the replacement of choice when upgrading old T8 and T12 luminaires. They are probably the best choice when a T5 luminaire fails, but if a T5 tube fails, it is simpler to replace the tube. Bulk replacement of existing, working T5 luminaires is not recommended.