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Possibilities of savings in some lighting applications and some
problems with LED luminaries
Daniel Todorov, Nicolay Mihailov
Abstract: Reducing power consumption and increasing energy efficiency is important in the
background of increased electricity prices and carbon dioxide emissions. LEDs seem to be good alternative
to the traditional artificial light sources. LEDs are replacing the conventional incandescent, HPS, mercury
vapor and other luminaries, due to the better luminous and energy efficiency. Furthermore the small size and
the versatile chemical substances in the die provide flexibility in luminary’s construction as well as in
chromatic spectrum. Aim of this study is to make a brief overview on some of the applications, which can be
beneficial to horticulture and municipality. During the review some important points that need special
attention are reviewed – proper heat dissipation and LED power supply as a determining factors for the
lifetime.
Key Words: LED lamp, Energy saving.
INTRODUCTION
Having in mind the high electricity prices and the pollution produced by fossil fuels
used by power plants, reducing consumption is a priority task. Lighting is a must in the
developed countries and for any industry. The percentage of electricity used for lighting
from the total electricity consumption, varies from 5% to 15% depending on the country –
for Bulgaria it is estimated to be 10% [5]. It is well known that only 10% of the electricity
consumed by an incandescent lamp is transformed into visible light – the rest is spent on
heating. It is better with the halogen lamps but still most of the energy is lost. Furthermore
halogen luminaries are often blamed to be responsible for many household fires, because
they get heated about 500°C. Compact fluorescent lamps (CFLs) are with better efficiency
– 4 times less energy consumed when compared to incandescent - but they have
limitations in the number of cycles. CFLs are suspected to be dangerous for the skin –
because of the UV radiation emitted by the luminary due to cracks in the internal
phosphorous coating [6]. High pressure sodium lamps are also widely used to replace the
mercury vaporized lamps, but HPSs have too small overall light efficiency because of the
small CRI and light losses caused by diffusion. Overall efficiency of HPS ranges from 50 to
65 lm/W [11] Best alternative to all these previously used luminaries is the LED. LEDs
produce light thanks to the photoelectric effect. The efficiency of LEDs is a matter of
constant improvements – nowadays its easily achievable efficiency above 80 lm/W and
some of the main suppliers announce that in laboratory conditions achieved 250 lm/W.
Other advantages include high values of CRI – little more than CFLs; wide chromatic
spectrum achieved by different chemical substances, that is condition for LEDs’ wide
application. However LEDs are semiconductor devices and as such they are sensitive to
high temperatures. High working temperatures can reduce the efficiency and can shorten
the life of a single LED twice or more. Also there should be paid special attention on the
drivers, because improperly driven LEDs can undergo decrease in luminous flux and life
time.
APPLICATIONS
LEDs are in fact crystals, that due to the received electrical energy, emit
electromagnetic energy. This EM energy has different wavelength depending on the
chemical compounds inside the chip. Of interest is the EM energy with wavelength inside
the visible light spectrum 390 nm to 700 nm. This versatility in chip’s chemical compounds
determines the versatility in applications according to wavelength or color. On the other
hand the development of semiconductor industry also made possible to have a single LED
with power exceeding 1 W. As a result LEDs have been used wherever light and efficiency
is needed. LED luminaries advance over the traditional luminaries in many aspects:
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



There are no hazardous materials;
Several times greater lifetime see Table 1 – LEDs have lifetime about 100 000
hours (but due to improper heatsink calculations and high ambient temperature can
reduce to 25 000 h depending on many factors). In comparison the lifetime of
durable incandescent lamp is 8000h, for HPS it is 10 000 to 24 000 hours, CFLs are
rated to have lifetime about 6000h till 15000 for the expensive industrial ones, but
their lifetime is strongly affected by many factors, the greatest of which is the
number of cycles. For example several years ago in the traffic light service of Ruse
an experiment was conducted. The results showed that even expensive CFLs live
no more than 3 days - less than a working week. This is about 2500 – 3000 cycles
which similar to the ratings of the manufacturers;
Better focusing (at least 75%) is achieved when using LED luminaries, compared to
other lamps – HPS (50%) [8], HPM, CFL and incandescent;
Better color rendering index (CRI) almost the same as CFLs. This describes the
color reproduction compared to daylight. The higher – the closer it reproduces the
real color. It is also investigated that Higher values of CRI help in faster object
recognition by an observer, even in dimmer light conditions. Luminaries with higher
CRI are recommended in places where precision in work is necessary. In places
lighten with luminaries with low CRI, ametropia and astigmatism can occur in one’s
eyes.
Light Source
CRI
HPS
Mercury Vapor
CFL
LED
30
50
60-90
70-90
Table 1
Comparison between widely used lighting sources [3]
Efficiency, in lm/W
Lifetime,
in
hours
(x1000)
60-120
10 - 24
50
10
50 - 75
6-15(1)
90 -150(2)
50 -100
(1) – these values are for durable street lighting CFLs
(2) – it is subject to improvements
Below some of the possible usages are listed and described.
Horticulture
Energy consumed from a greenhouse is 10% - 30% of the total production costs [7].
The two major consummators are Lighting and heating. Supplemental lighting is necessary
for increasing the crop production. It is used whenever ambient sunlight is not sufficient –
mostly in colder seasons and regions. That’s why energy efficiency of lighting equipment
is important – by reducing the consumption, the overall price of production is reduced as
well.
At this time supplemental lighting is provided by high intensity discharge lamps
(HID) like HPS and metal halide lamps (MH) in greenhouses, and fluorescent T8
luminaries in growing chambers. But as already mentioned this is not the most efficient
lighting. There are also other problems like – HPS and MH generate lots of heat that is
unwanted close to the crops, that’s why these luminaries should be placed distantly. This
imposes making higher greenhouses. In comparison when using LED luminaries
generated heat can be removed separately. Only small amount of this heat is emitted in
the spectrum, the other can be removed through active heatsinking. Thus the emitting
surface of the LED can be placed closer to the plants without any risk of overheating or
stressing the plants [7]. This makes possible building low-rise greenhouses, and even
greenhouses at several floors.
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Other characteristics of LEDs, is that they can emit electromagnetic energy with
certain wavelength according to the chemical substance of the emitting crystal. Thus LEDs
can be selected, according to wavelength, in order to match the absorbance peaks of
plants’ pigments [7]. This can save energy lost in non-absorbable wavelengths/colors, by
the plant.
Fig. 1. LED lighted plants in greenhouse [18]
Unfortunately there is not much experience with LED greenhouses. Tests are being
conducted now [7, 13, 14], because several years ago the efficiency of LEDs was about 50
lm/W and the price was too high, making LEDs incomparable to HPSs and CFLs.
According to [7] plants grown by using LED supplemental lighting are comparable to plants
grown using HPS luminaries. However there are still many unknowns about the exact
absorption spectrum of plants, possible plant changes, exact savings and etc. Many other
researches should be done in order to analyze fully this possible use of LEDs .
Street lighting
Traditionally used luminaries in street lighting are HPS and CFLs, they replaced the
inefficient mercury vapor lamps. LEDs are getting popular in this field of application since
the mentioned improvements of their efficiency. Replacement of HPS with LED luminaries
is fact from several years in the developed countries and is being carried out in developing
countries nowadays. The delay in improvement is because of the high prices of LED
luminaries.
The expected energy savings, taken from other reports are 2-3 times (50-70%) [3,
12].
Here proper luminary design should be taken into account, because there is
danger of disability glare [9], which is inadmissible, when talking about street lighting. In
[10] Threshold Increment is defined as “the percentage increase in contrast required
between an object and its background for it to be seen equally well with a source of glare
present” or a measure of the loss of visibility caused by disability glare from a lighting
source. The main purpose of street lighting is precisely the safety on the road, that’s why
obstructive light is unwanted and should be carefully avoided by proper design of the
luminary.
Traffic lights
The next table shows the consumption of incandescent durable traffic light bulbs in
comparison with the LED traffic light modules manufactured & used in Ruse:
Table 2
Comparison between incandescent laps and LED modules used in traffic lights
Type
of Wattage, W
Lifetime, h
luminary
Traffic lights, Traffic
lights, Pedestrian
diameter = 300 diameter = 200 traffic lights
Incandescent
100
70
70
8000
LED
12
8
5
70000 – 90000
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The realized reduction of consumption for Ruse is 8-10 times. These high values
are achieved because the illuminated area is small. After modernization, we observed
the LED modules: The first LEDs that were mounted in 1999 were with chemical
compounds GaAlAs, promoted to be with 100000 hours lifetime. Their luminous flux
dropped to 60-50% after 3 years of service, which is equal to about 12000 hours. Later
Different supplier was used – with LEDs chemical compounds – AlGaInP. They lasted
minimum 5 - 6 years. The two LED modules with the different LEDs (GaAlAs & AlGaInP)
were at the same working condition. It appears that GaAlAs LEDs are less durable in
outdoor conditions -25 to +70°C. There are as well similar reports [15]. The green LEDs
used in our traffic lights are with composition of InGaN, they are observed to last, at least
10 years. A conclusion can be made that the chemical substance also plays certain role
for the overall lifetime.
The main disadvantage of introduction of LEDs into lighting applications is the high
initial investment. The price of LEDs is reducing with the mass production and the
electricity prices are being raised, which makes this investment reasonable.
Maybe the greatest advantage of LED luminaries is that with such a small
consumption, Photo Voltaic (PV) stand alone luminaries can be used.
LIFETIME AS THE MOST IMPORTANT CONCERN
Lifetime of LED depends mainly on junction temperature. Considering the
Shockley’s diode equation, the LED’s voltage-current relationship can be described as
[16]:
 Vd

 nVT

I  Is  e
 1 ,




VT 
kT
,
q
(1)
(2)
where I is diode forward current;
IS - reverse bias saturation current;
Vd - diode forward voltage;
n - diode ideality factor;
VT - thermal voltage;
k - Boltzman's constant;
q - charge on an electron;
T - temperature.
The saturation current is:
I s (T2 )  I s (T1 ).e ks .(T2  T1) .
(3)
It is visible, that saturation current and forward voltage are temperature-dependent.
Voltage-current graphs are drawn at a given temperature of the junction for particular LED,
then n, k, q and Is are constants. Example of voltage-current graphs is shown on Fig. 2 a.
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Fig. 2. Voltage-current graph for Cree’s XLamp [17]
Together with voltage-current graphs, suppliers give the luminous flux as function of
the current Fig 2 b. Increase in current increases the flux, but also generates more heat in
the crystal, which increases the junction temperature and the consequences have contrary
effect on the flux – it’s shown on Fig. 3 a.
a
b
Fig. 3. Relative luminous flux as function of junction temperature, at Ic=700mA (a);
Thermal design (b) [17]
In order to dissipate the generated heat in the crystal, heatsink is used. This
heatsink dissipates the heat to the ambient space, but as ambient temperature increases
less heat can be dissipated, thus heat production should be decreased in order to prevent
thermal damages. On Fig. 3 b it is shown supplier’s recommendation for current derating
as function of ambient temperature increases.
On Fig. 4. the lifetime, as function of junction temperature, at different currents is
shown.
Fig. 4. Lifespan depending on junction temperature and forward current for Cree XLamp
LEDs
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All this showed the connection between temperature, current and voltage and
temperature’s main role in LED’s lifespan.
Driving LEDs
Proper control is needed to maintain stable values of current and voltage. There are
several approaches in driving LEDs. Using the dependence of voltage and current – it’s
possible to drive a LED by either voltage or current. The simplest way of driving is by using
ballast resistor, but this is ineffective further more small voltage changes lead to big
current changes (see Fig.2 a) and as it was shown the voltage is negatively affected by the
temperature. When, for example, voltage changes from 3 V to 3.2 V, the current changes
from 1 A to 1.8 A – this can be dangerous for the lifespan of the LED or LED array. This
leads to the idea that constant current driver should be used. Keeping the forward current
constant has another advantage – array of several LEDs can be connected in series with
only one constant current driver. It is also possible to use constant voltage regulator that is
precise enough because is comparing constantly the output voltage with its reference
voltage.
The main advantages and disadvantages of the popular drivers are given below [2]:
 Ballast resistor – easy and inexpensive, but can’t control current accurately,
current varies with supply voltage, heat losses in resistor;
 Linear regulators – accurate current control and easy dimming, but is more
expensive than ballast resistor, heat is generated in the regulator =>
inefficiency;
 Switching regulator - more efficient than the above, wide input ranges,
dimming can be performed by PWM, but is more expensive, complicated
and can cause electromagnetic interference.
Today’s switching current regulators deliver high efficiency (about 90%) and their
price is dropping because of the mass production.
Five years ago we planted LED traffic light modules driven by constant current
drivers together with ballast resister driven LED arrays, on the same intersection. The
LEDs used are the AlGaInP that were experimentally proved to last longer. The results
now show, that LEDs driven by LED constant current drivers, have dropped to 90% of its
initial luminous flux. In comparison, the LED arrays driven by ballast resistors have
dropped to 80% of its initial luminous flux. It is obvious that those with specialized driver
will last longer that those with ballast resistor. We estimated, that each year, our
modernization brings about 53200$ savings for the municipality of Ruse, so it is cost
effective to be invested in more efficient LED drivers in order to prolong the lifespan of the
newly manufactured and installed LED modules.
Thermal management
As already explained this is the main problem with LEDs and the main lifetime
determining factor (see Fig. 4.). And the most frequent reason for early failure. The high
wattage LEDs are highly dependent on proper heat dissipation. That is why special
attention should be paid, whenever high power LED luminary is designed. There are 3
basic modes of transferring heat – conduction, convection and radiation. The thermal path
from the LED to the heatsink is through conduction. Convection and radiation is present in
the heatsink to ambient transfer.
Thermal calculations are easily done, since analogy with electricity is used [4].
Temperature T in K is corresponded to Voltage U in V,
Heat flow P in W is corresponded to Current I in A,
Thermal resistance Rth in K/W is corresponded to Resistance R in V/A,
Thermal capacitance Cth in Ws/K is corresponded to Capacitance C in As/V
-6-
Thus a model for heat transfer can be derived as shown on Fig. 5.
Fig. 5. Thermal model of heat generating electronic component
P(t) is power dissipation in the chip. For simplicity thermal resistance and
capacitance can be combined. A simplified model for calculating the heat sink’s thermal
resistance is presented on Fig.6.
Rthjc
P(t)
Tj
Rthch
Tc
Rthha
Th
Ta
Fig. 6. Simplified thermal model for heat transfer of power LED directly mounted on the
heatsink (without thermal PCB)
The junction to ambient resistance is:
R thja  R thjc  R thch  R thha 
Tj  Ta
P
.
(4)
From which, calculations for the heatsink’s maximum allowable thermal resistance
can be done.
CONCLUSIONS
This overview presents some of the application of LED lighting fixtures, which is just
a small part of the wide possible usages. LED luminaries are becoming cheaper enough to
replace the incandescent lamps in homes and are already well accepted in industry
applications.
The results from the modernization of the traffic lights of Ruse are briefly presented.
By this modernization reduction of the costs of electricity at average of 9 times was
achieved, which means, that we made possible the saving of roughly 50000$ per year
which is a good benefit to the municipality of Ruse. During this modernization,
experimentally we studied different luminaries and LEDs, a conclusion for better driving
was made, that can prolong the lifespan of each traffic light module.
The main problem of High Power LEDs was discussed – the proper thermal
dissipation as main factor determining lifetime.
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Paulauskas, A. Paulauskas, Possibilities of Energy Savings through Conversion to LED
Lighting in Western Region of Lithuania, 2012, ISSN 1392 – 1215.
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[15] http://www.lrc.rpi.edu/programs/futures/lf-leds
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ABOUT THE AUTHORS
M. Eng. Daniel Todorov, University of Ruse Angel Kanchev, Bulgaria, 8 Studentska
Str, Department of Electrical Power Engineering, e-mail: [email protected]
Prof. Nicolay Mihailov, DHC, University of Ruse Angel Kanchev, Bulgaria, 8
Studentska Str, Department of Electrical Power Engineering, tel.: +359 82 888 843, e-mail:
[email protected]
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