Public Lighting Systems

Public Lighting Systems Aalborg University Esbjerg 2013 B312 Aleksandrs Levi Andrei-Viorel Radulescu Mike Castro Lundin Minik Nathanielsen Olsen Kris...
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Public Lighting Systems Aalborg University Esbjerg 2013

B312 Aleksandrs Levi Andrei-Viorel Radulescu Mike Castro Lundin Minik Nathanielsen Olsen Kristian Brogaard Kristensen Rolan Abdulrazzak Ossi Rolf Magnus Roos

Public Lighting Systems Title: Public lighting systems Printing Year: 2013 Group ID: B312 University: Aalborg University Esbjerg Authors:

Aleksandrs Levi

Andrei-Viorel Radulescu

Mike Castro Lundin

Minik Nathanielsen Olsen

Kristian Brogaard Kristensen

Rolan Abdulrazzak Ossi

Rolf Magnus Roos

Abstract: Street lighting is a huge part of a city infrastructure, and the technology involved in it accounts for a great amount of energy used. We will attempt to tackle the inefficiency in power consumption and other factors of HPS Street lighting by replacing these for LED lamps. The problem will be approached in a theoretical investigation. Many important factors will be considered when analysing the effects of possible implementation of our solution, including power consumption, economical factors, brightness and others. We conclude that although some issues were found, the implementation of LED would bring numerous advantages.

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Public Lighting Systems Preface The project “Public lighting systems” has been made by 7 students of the Electronics and Computer Engineering in the first semester, part of the Project 0 or P0. It was developed in the time frame of 16 days, with the supervision of professors Akbar Hussain and Torben Rosenørn. Additional supervision and help was given by Jóhann Sigurdarson. This report and project is targeted at engineering students of our level and our supervisors, however also taking into consideration what would be desired of a project of this nature in a business context. For this report the Vancouver citation style has been used, using a number to organize references. All references are contained in the last section, named references.

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Public Lighting Systems Table of Contents Abstract: ........................................................................................................................................ 1 Preface .......................................................................................................................................... 2 Introduction .................................................................................................................................. 4 Problem Analysis ........................................................................................................................... 4 Definition of Task .......................................................................................................................... 5 Relevance .................................................................................................................................. 6 Interested parties ...................................................................................................................... 6 Municipal Authorities ............................................................................................................ 6 Power companies ...................................................................................................................... 6 Technicians/companies maintaining streetlight ................................................................... 6 Public ......................................................................................................................................... 6 Tech companies..................................................................................................................... 7 Solution ......................................................................................................................................... 7 Evaluation of Solution ................................................................................................................... 8 From light bulbs to HPS: Brightness and lifetime...................................................................... 9 Lifetime: .............................................................................................................................. 10 LED and HPS comparison ........................................................................................................ 11 Other LED Advantages and the future of LED lighting ............................................................ 13 Evaluation of Sources and accuracy of research ......................................................................... 15 Conclusion ................................................................................................................................... 16 References................................................................................................................................... 18

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Public Lighting Systems Introduction Our modern civilisation needs light. This might be an understatement, because we humans are drawn to the light, it is in our instincts. So it makes sense to try to improve lighting systems. Moreover, some systems that we use today have some issues. Some systems might use way more power than what is necessary and some parts of the infrastructure are old. So how can we improve the already existing systems? Before we chose what problem to run with we had a lot of other suggestions on the table - we were talking about sensors in the street lamps based on the GSM network or Wi-Fi. The point of the sensors was the ability to dim the lights when there were not any people around in need of light. The theory behind the sensors was to have something static inside the lamps rather than to have something exposed to the elements and to vandalism. There were of course some problems with this type of sensor. For people to be “seen” by the sensor they had to have a mobile phone with them for the lights to turn on. We also talked about fitting a solar panel on top of the street lamps to reduce the power required to run the lamp. We had some ideas of how to implement and design the lamp. However, there were some problems as well. If the solar panel was placed on top of the lamp the elements like leaves and rain would cause algae and moisture to form in and on the solar panel that would cause it to lose some of its effectiveness. The lack of power would require cleaning of the lamps.

Problem Analysis In our project, we must take into consideration several conditions that will be factors in determining the success of the project and must be taken into account when considering implementation of the proposal. These conditions are the following 1. Light pollution: One of the issues that are important nowadays is regarding light pollution. Light pollution is defined as any adverse effect of light caused by society (1). This includes, but is not limited to, effects on nocturnal animals, problems it causes for astronomers, phenomena such as skyglow (2), and there are even studies linking it to certain types of cancer (3). 2. Energy consumption: This factor has been growing as environmentalist movements have grown. Organizations such as Greenpeace have been promoting energy efficiency (4) and as a consequence the public has become more aware about this. It is desired that the new system uses less energy; however, we must also consider what effects this might have on the power infrastructure.

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Public Lighting Systems 3. Light intensity: This, opposite to the light pollution factor, considers light intensity as a positive thing. As public lighting is mainly used for safety issues, it is desired for it to cover the biggest area possible. This factor also would include studies into the breaking reaction taken by drivers as a result of the light spectrum and intensity. 4. Cost: It must be taken into consideration to the cost of implementing such system, including initial purchase of the proper infrastructure, as well as personnel required to implement it and further studies required before implementation. 5. Maintenance: This issue should take into account how often the new system will require maintenance or replacement of parts, as well as how much this would cost.

Definition of Task Our task in this project is to find a solution to the public lighting system that will provide advantages in at least two of the factors mentioned in the problem analysis. The project will be carried out as a theoretical study into these factors. Other factors will also be mentioned and the extent with which they affect our solution will be considered. We shall limit our project to the consideration of a change in the bulbs, specifically from High Pressure Sodium (HPS) lamps into LED lamps, and consider the implications caused by this action. The following report will deal with the idea of using LED street lighting as opposed to HPS lights. Our initial idea was also to investigate the possibility of supplying the street lights with solar panels and batteries to supply at least part of the power consumption as well. However, as we have a fairly limited timeframe to work on this, and as all the ideas we had concerning solar powered street lights had been tried before, we have limited the focus for our report. In the following pages we will compare LED light and HPS light, both in terms of quality of lighting as well as power consumption. We will also investigate whether there are other aspects which we can use to compare the two. Finally, we will also investigate the impact of the reduced power consumption, both financially, but also whether it will have an impact on the power grid in general.

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Public Lighting Systems Relevance The idea of changing old HPS street lights with newer LED street lights is not new; it has been investigated, and even implemented in some places. There are, however, a few things that make full implementation complicated: -

Cost: in many cases, it is not just a matter of changing a light bulb, but it involves change of major parts of the infrastructure. Some people oppose the change because they feel the LED light is colder in respect to light temperature, in comparison to HPS-lamps.

However, the newest LED lights are both warmer in colour temperature and the electricity savings can amount to upwards of 70% (5). It is this project’s goal to help show that LED lights are the future of street lighting. Because of the topic, the report is mostly of relevance to municipal authorities and companies in charge of maintaining the street lights or lighting in large areas.

Interested parties Municipal Authorities As in the later years it has been a stated goal from both national and international authorities that CO2-emission should be reduced, switching to cheaper lighting is an easy way to achieve at least a step in the right direction. This is helped even further by the fact that technology is here, and that it is proven to be a financially viable option (6).

Power companies Seeing as these companies make their living off of consumers of electricity, these might possess an interest in keeping more expensive lighting in the streets. Especially since street light consumes power at a time of day, when most of their customers decrease their power consumption. Technicians/companies maintaining streetlight

As there is also companies that have to do the work of switching the bulbs and luminaires, these will also have an interest in this. On one hand, it is a lot of work right now, which will be a good thing. On the other hand, LED lights have quite an increased lifetime compared to HPS lights. So in the long run, it might be of more interest to them to keep the old HPS bulbs.

Public As many feel LED light is inferior, this might pose a problem when suddenly all street light would be switched to LED. Therefore, it would have to be made very clear to people what advantages switching to LED brings financially and ecologically. 6

Public Lighting Systems Tech companies

The companies producing both Luminaires and light bulbs, be it HPS or LED will probably also have a certain interest in the project, as it would be quite large orders to switch all street lights to LED. For example, Esbjerg municipality has decided to switch to LED lights, which means that 27 835 luminaires will have to be changed (7).

Solution For considering our solution of replacing HPS lamps with LEDs, we shall do the following theoretical work:     

Analyse the impact of this change in the power grid, and whether there would be an issue. Compare a normal incandescent light bulb to HPS lamps regarding brightness and lifetime. Compare LED lighting systems with other incandescent lights. Consider implementation and prices, as well as development of the cost through time. Consider and compare power consumption, money spent, beam angles and nature of light emitted.

We would also propose that practical work be done in the future if this project would be taken into consideration for implementation. This could include:   

Testing endurance of LED lamps in adverse weather and extreme temperatures. Testing of different kinds and sizes of LED lamps to decide the most appropriate. Further testing of the selected lamp, taking into consideration the affected parties, including drivers, local fauna, and security issues around it.

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Public Lighting Systems Evaluation of Solution To analyse the impact of the change in the power grid and compare power consumption, we contacted Sydenergi for information. We e-mailed the questions to Stig Christensen ([email protected]) who answered our questions with another representative from Sydenergi: Jens Ammitzbo ([email protected]). Following interview was conducted in two parts. The answers for the first part of the interview was conducted with Mads Peter H. Sørensen ([email protected]) via an e-mail correspondence conducted Wed. 09/11-13 and Thur. 09/12-13, an engineer from the Tech & Environment department (the Roads & Traffic subdivision to be specific) of Esbjerg Kommune. Our main topic was the power grid in Esbjerg. In his answers, Sørensen explains that power lines are dug into the ground except for a small stretch (appr. 1-2%). But they are planning to move them underground within the next few years. Streetlights hang in wires above in the old part of downtown area, an approximate total of 2-3%. The lights are fed by ca. 500 meter cupboards. Maintenance is outsourced to a private contractor that has won the compulsory competitive tendering through the EU. They will maintain the task between 4 to 6 years. Their task is described in the tender documents available. The document also determines the economic aspect of the project. On the question of installing solar cells in top of the streetlights, Sørensen continued: [Translated from Danish]“There are no plans to install solar cells to reduce the power consumption as of yet; it simply is not profitable. To do so, it requires better solar cells, better batteries and better LED’s in the sockets - AND additional wired backup if the light intensity has to be the consistent all night during the dark winter. The investments will never be profitable as of today.” 1. Are there any areas of town where streetlights has a higher power-consumption that other i.e. businesses and commercial buildings compared to homes and apartment buildings? Answer: The inner town has a higher lightclass. Normal streets need at least 2.5 lux. The inner town needs 5.0 lux. Main roads need even more lux. 2. What kind of lights/lightbulbs are used for streetlights?

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Public Lighting Systems 2a. If you are using different types of lightbulbs i.e. LED's and HSP-bulbs: What are the pros and cons for using LED-lights vs. HSP-lights for streetlights? 2b. Is the brightness of the lamp regulated after the daylight? If so, how do you manage that task? Are you using light meters, or do you employ a schedule of sunrises and sunsets, and weather reports in regards of overcast etc. issued by DMI or some other weather report service? Answer: We are now changing about 75 % of our 25.000 lights. From HPS to LED. The reason is the economy. Changing to LED we save up to 75 % energy. Energi are definity [sic] the most expensive part of running the streetlights. The light starts at sundown (lightmeter). At 22.00 hour the LED’s dim to 50 %. At 6.00 o clock the light turns to 100 %. At sunrise (lightmeter) the lights turn off. The last part of the interview was conducted with Jens Ammitzbo ([email protected]) and aforementioned Stig Christensen from Sydenergi. 3. Does Sydenergi produce the same amount of power 24/7? Answer: SE do not produce power. Powerstations, windmills, hydropowerstations deliver the power and Energinet.dk controls the Power in the Network. When the powerdemand is reduced during the night, Energinet.dk shut down a Powerstation or perhaps Windmills so produced Energy equals demand. 4. We imagine there's an outburst of consumption when daytime offices open between 8 A.M. and 9 A.M. and around dinner-time when people gets home after work to cook and watch the news, surfing the net, etc. Is that correct? Answer: Yes. 5. Do you keep records of power consumption? Answer: Yes kWh per customer, not how many kW. Try and look at Energinet.dk homepage.

From light bulbs to HPS: Brightness and lifetime The first electric lamp, invented by Edison, had a lifetime of 40 days, and used cotton thread. When analyzing brightness of a lamp the context must be taken into account, factors like the height from the light source to the place that needs lighting and the overall luminous efficacy of the light source, which describes the amount of lumens produced by a certain amount of watts. Since HPS is the most common street light source, we have used that as our benchmark to compare LED lights with. There are, however, lots of other kinds of lighting and here we will try to compare HPS to those light sources.

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Public Lighting Systems

(8) From this table it becomes obvious why HPS has been the standard light source chosen for street lighting. As a first factor it has the potential to provide the best light output per energy usage, meaning it will be cheaper to run. The second factor which is also quite relevant is the life expectancy of the bulbs, and HPS is also in the top in this factor, meaning less working-hours spent maintaining the infrastructure. Finally, the direction control of HPS lamps means less light will be thrown in direction where you either do not need it, or even worse - do not want it. As can be seen, the HPS lamp actually loses when it comes to colour rendition, and this is probably why we do not see a lot of HPS lamps installed in homes. However for street lighting it is fully acceptable, especially when we take into account all factors. Lifetime:

The lifetime of a lamp is highly dependent on the temperature. Basically, with increasing temperature the lifetime of a lamp is reduced. Depending on the watts of the lamp, we will receive different amounts of hours (lifetime). o 40 watt can give 2300 - 7200 hours o 60 watt can give 1800 – 5100 hours o 100 watt can give 1300 – 3200 hours These are the average lifetimes of the respective watt values for incandescent light. (9)

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Public Lighting Systems Some of the characteristics of high-pressure sodium (HPS) lamps are that they have high effective lighting, this meaning that they are efficient and have a long lifetime, as well as it does not attract insects. These lamps have the ability to spread light along far distances. Regarding current and electric potential used by HPS lamps, the following table outlines the change of these as the watt output increases Type

Energy(W)

Electric current(A)

Electric potential(V)

SON-T70W

70

0.98

98

SON-T100W

100

1.20

100

SON-T150W

150

1.80

100

SON-T250W

250

3.00

100

SON-T400W

400

4.60

100

SON-T600W

600

5.80

100

SON-T1000W

1000

10.30

110

(10) As stated before, sodium lamps have a long lifetime compared with most other light sources. The lifetime of a sodium lamp can be up to 24,000 hours (11), and their power varies between 35 and 180 watts. This in comparison to incandescent light bulbs is approximately 20 times more time per light bulb.

LED and HPS comparison It is considered nowadays that LED street lights are cheaper, more eco-friendly and available than HPS (High-pressure sodium lamps), and that is why they are so wide spread all over the world. All those factors become even more obvious after thoroughly comparing LED lamps with HPS lamps – the topic that is going to be covered in this chapter. There are many reasons for the majority of people to prefer LED lamps over HPS. First of all, power consumption of those two types is significantly different. LED lights have low power consumption and their lifetime is rather long and, furthermore, very predictable. (12) The lifetime of those lamps is 10 years, which leads us to the

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Public Lighting Systems conclusion that maintenance expenses are not very high, hence LED lamps are much cheaper when talking about long periods of time – they do not have to be replaced by new ones so often. HPS lamps, in turn, have a quite short lifespan of 2-3 years, leading to the need to periodically change the lamps, which, of course, makes those lamps pretty expensive. Talking about power consumption – a LED lamp with power of 67W emits the same amount of light as the 90W HPS lamp, making LED 26% better in terms of light intensity per power used. (12) The other thing that is drastically different in LED lamps compared with HPS is the speed of turning them on. In order to turn on an HPS lamp you actually have to wait some time until their brightness reaches its full potential, while LEDs reaches full brightness right away. That is why LEDs are really easy to turn them back on after a power failure. Of course, changing lamps in a big city is very expensive and it cannot be done in a short period of time. But it is undisputable fact that even in 10 or 20 years our cities will need light (probably even more than they do now), so all these expenses will ultimately pay off on a larger time scale, leading to a better economic situation. Light, emitted by LED lamps, is also much more suitable for driving conditions than that of HPS. It is white and very similar to usual daylight that people are used to. Standard yellow light, emitted by many other light sources, including HPS lamps, in turn, usually makes people feel warm and even sleepy, which is not very good for normal driving conditions, since it may lead to road accidents. Pictures of HPS and LED street lighting respectively. (13)

The beam angle of examined lamps is quite important too. It is known that HPS lamps have 360-degree lightning angle, which means that some part of emitted light is just wasted. LED lamps, on the other hand, does not have that kind of problem, due to the fact manufacturers use special lenses to make the lamps light up a specific area. (14) Of course that does not mean their beam angle is always perfect, and there are

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Public Lighting Systems also some restrictions based on the lamp shapes, but it still helps to reduce the amounts of energy consumed by the lamps. Among other things, there is so-called colour rendering index (CRI), which basically shows how “real” and “true” an object looks when lightened by a certain light source. Therefore, light emitted by LED lamps makes the objects it is aimed at look almost as they look in the real life, with a colour rendering index of over 75. CRI of the high-pressure sodium lamps is usually less than 23, and that is why the objects the light is aimed at appear to be yellow and unrealistic. (14) Of course, it is obvious that HPS lamps are not a bad choice – or rather, they were not a bad choice a few years ago. The main goal of our project is to reduce the amount of energy spent on lighting in cities– and that is exactly what LED lamps are really good at. Of course, we should not forget all other advantages that they give that made us only more eager to start exactly this type of project. That is especially important when thinking about how much energy is actually used on city lighting every year. It goes without saying that this information depends heavily on what city we are talking about. For example, in Los Angeles street lighting accounts for 60% of the total electrical bill, which is quite a lot. Furthermore, it has been announced that 4 years after changing street lighting system from HPS to LED lamps 63.3% electricity savings had been achieved, which also serves as a good illustration of the actual amount of energy that can be conserved using LED over HPS. (13)

Other LED Advantages and the future of LED lighting As previously mentioned LED Lighting is, in conclusion, cheaper than that of HPS lighting. But is LED really more efficient than other light sources? For instance, the basic LED advantages are following: (15)     



Energy Efficiency – at the current technology they can put out around 135Lumen/Watt Long lifetime – the medium time of usage is 50.000 Hours Rugged – As it is also called SSL ( Solid state Lighting) it is created from solid materials, with no filament No warm up period – they light and arrive at the full light in nanoseconds Not affected by cold temperatures – LED’s do not need a warm up time as they can start up even in sub-zero temperatures, which makes them very efficient for outdoors usage Directional – all the light is directed to one location, thus no light is wasted

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Public Lighting Systems 

Excellent colour rendering –They give out the “white” light which is easier for the human eye  Environmentally Friendly - not containing any mercury or other hazardous materials.  Controllable – the brightness can be controlled with greater ease. Besides all the above mentioned advantages LED’s are being used more and more because of their ideal use in frequent on/off cycling, unlike the fluorescent Lamps which burn out faster when cycled frequently. They also do not burn out at once like the fluorescent Lighting, but they simply dim over time. In the last year the LED has also gained a lot on the lighting market, thanks in particular to the advances brought to the production. The prices for a LED lamp have decreased exponentially as it can be seen from the following chart:

Price comparison of the different type of lamps. (18)

There are, however, also some disadvantages to the usage of LED’s. The main problem of a LED is that it has to be supplied with the correct voltage and the same current on a constant flow. LED performance largely depends on correctly engineering the fixture to manage the heat generated by the LED, which causes deterioration of the LED chip itself. Overdriving the LED or not engineering the product to manage heat in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. The most common design of a heat sink is a metal device with many fins, which conducts the heat away from the LED.

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Public Lighting Systems At this point the initial cost of a LED lighting system is already a lot cheaper in comparison with other incandescent lighting methods as it can be seen in the next graph.

Comparison based on the life of the bulb and the electrical cost of 10cent/kWh.(www.continentallighting.com)

Evaluation of Sources and accuracy of research In the duration of this project we have been doing a lot of research. It must be noted that not all sources agreed entirely on the effectiveness of LED street lighting opposed to the alternative, HPS and fluorescent lighting. If we go to EL:CON’s website, the contractor responsible for maintaining Esbjerg’s street lighting and the infrastructure required, we see a statement that LED provides savings of upwards to 70%(7). However, we have also found other sources that have more conservative statements. If we look at ClimaCare’s website, a company who is selling among other things LED products; they have a case where they have calculated savings for a real world street. They claim savings of 58% in energy (6). And finally we have the LED Lighting Watch which has compared LED with HPS and Sodium lamps, which found a saving of only 26%(12) compared to HPS lamps. Additionally there are doubts regarding how realistic it is to expect 50.000 hours of lighting out of LED lamps (16), since there are still some variables that are unaccounted in the calculations involving this technology.

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Public Lighting Systems We have, however, found that there is a trend of the efficiency of LED’s being rapidly increased these years, and that might go a ways of explaining some of the different opinions

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All sources agree that LED lighting will save power, and if the LED’s are kept at low enough temperatures they should also have a considerably longer lifetime than HPS lamps. This means the only problem is how long time it will take for the savings to repay the cost of implementation.

Conclusion After comparing different light sources for street lightning, we can see that the LED is the future. It is also obvious there are some short-term problems that will need to be tackled, such as the initial investment fees, as they are quite steep. There is also the problem of it being a fairly new technology, and some concerns have been raised on how realistic the lifetime of LED’s will be in real world environments. As stated in the problem analysis we must consider: Light pollution, energy consumption, light intensity, cost and maintenance. In this report we have proven that LED lamps reduce light pollution compared to HPS. Furthermore, the energy consumption is also reduced significantly, and can even be expected to be improved further in the coming years. The lumen/watt ratio is better as well. Cost, however, can be a factor to implementation as the prices are still above that of HPS. And maintenance has been proven to be less expensive due to the fact that LED’s have up to 3 times the lifetime of HPS.

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Public Lighting Systems However, most of these problems are being worked on, and as more and more places are starting to implement LED, initial prices and maintenance costs are expected to decrease even further in the future. Therefore, we conclude that LED is the future of street lightning, and even though there are a few uncertainties, we would recommend a transition to LED Street lightning as soon as possible.

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Public Lighting Systems References Cover Photo: http://inhabitat.com/sustainable-city-street-lights-by-phillips/ , 2010 , Bridgette Meinhold

1: The California Institute of Technology Astronomy Department, Light Pollution and the Palomar Observatory, http://www.astro.caltech.edu/palomar/lp.html, 13-09-2013. 2: Mizon, B; Light Pollution, 2nd Edition, New York: Springer; 2012. Page 44-48 3: Haim, Abraham; Portnov, Boris A.; Light Pollution as a New Risk Factor for Human Breast and Prostate Cancers, Dordrecht: Springer; 2013. 4: Sven Teske, Energy Revolution, Greenpeace, 4th edition, 2012 5: EL:CON. LED-gadebelysning i Esbjerg commune. http://www.elcon-as.dk/nyhedermedier/nyhedsarkiv/2012/led-gadebelysning-i-esbjerg/ ,13-09-2013 6: ClimaCare. Beregning på tøndervej i Aabenraa. http://www.climacare.dk/da/beregning-paa-gadebelysning.asp,13-09-2013 7: EL:CON. LED-gadebelysning i Esbjerg commune. http://www.elcon-as.dk/nyhedermedier/nyhedsarkiv/2012/led-gadebelysning-i-esbjerg/ ,13-09-2013 8: Indoor Lighting Fixtures Classifications. ND http://www.electricalknowhow.com/2012/03/indoor-lighting-fixtures.html (Accessed: 19 September 2013). 9: S. J. Prais (1974) The Electric Lamp Monopoly and the Life of Electric Lamps, New Jersey, USA: Wiley. P. 153 – 155. 10: Ningbo Yamao Lighting Electric Appliance Company Ltd. () High Pressure Sodium Lamp, Available at: http://lampsinchina.asia/1-2-3-high-pressure-sodium-lamp.html (Accessed: 17 September 2013). 11 BL: Bulborama () High and Low Pressure Sodium Bulbs, Available at: http://www.bulborama.com/High-and-Low-Pressure-Sodium-Bulbs-c23/ (Accessed: 17 September 2013). 12: LED lighting watch, Street Lights Performance: LED Vs. High Pressure Sodium Vs. Mercury Vapor, http://www.ledlightsorient.com/watch/street-lights-performanceled.html, 11-09-2013.

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Public Lighting Systems 13: Forbes, Los Angeles Saves Millions With LED Street Light Deployment, http://www.forbes.com/sites/justingerdes/2013/01/25/los-angeles-saves-millionswith-led-street-light-deployment, 11-09-2013.

14: LED lighting blog, Led is Here and now more affordable than ever, http://initialled.blogspot.dk/2012/03/hpshigh-pressure-sodium-vs-led-lights.html, 11-09-2013.

15: “MRS BULLETIN “• VOLUME 33 • APRIL 2008 • p459-470 – “Solid-State Lighting” Colin J. Humphreys (Cambridge University, UK) 16: Klaus Ulrik Mortensen: Hold igen med LED-investeringer 26. October 2011 Available at: http://ing.dk/artikel/lysradgiver-til-kommunerne-hold-igen-med-ledinvesteringer-123394 (Accessed: 19September 2013). 17: LED Lighting: Massive Opportunity, Getting Better. 21. June, 2012 Available at: http://www.energycircle.com/blog/2012/06/21/led-lighting-massive-opportunitygetting-better (Accessed: 19 September 2013). 18: Price graph : http://www.ledmarketresearch.com/blog/Does_LED_Lighting_Have_A_Tipping_Point_ 270 report from IHS Electronics and Media, Market research company.

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