Foreword
After years of development and accumulation, LED lighting technology has entered the fast lane of commercial development. As an important part of the lighting industry, road lighting accounts for about 20% to 30% of the lighting power consumption in the whole country, and its status is very important. Therefore, the development and promotion of semiconductor lighting technology must inevitably consider the application of semiconductor lighting technology in road lighting. Generally, in the development and promotion of LED street lamps, the following three problems are encountered: secondary optical design, thermal design and photovoltaic system design. This article will make a specific elaboration on this, in order to play a role in attracting jade.
1, secondary optical design
1.1 Choice of optical design software
In the field of lighting, the optical software commonly used in China is mainly divided into two categories: lighting engineering design software and optical device design software. The function of the lighting engineering design software is to design a set of solutions for the users based on the standardized luminaires determined by the existing light distribution, and provide corresponding visual requirements. For example, DIALux, OxyTech, Agi32, etc. are commonly used lighting engineering design software; the function of optical device design software is to design corresponding optical devices according to the characteristics of the light source and the desired optical effect, such as reflectors and lenses for LEDs. Currently, commonly used software for optical device design are ZEMAX, TracePro, ASAP, LightTools, and so on.
When choosing software, decide according to your needs. The municipal design institute and road lighting engineering company mainly use lighting engineering design software, while the lighting manufacturers mainly use optical device design software. Moreover, even the same kind of design software has different focuses. For example, TracePro is used for reflectors and reflectors, but it is difficult to design lenses. ZEMAX is good at lens design.
In actual design, sometimes different software needs to be combined.
1.2 Advantages and Disadvantages of Two Optics
Common LED optics are available in the form of reflectors and lenses, each with its own advantages and limitations, and should be selected as needed. According to the author's design experience, if the spot is required to be circular, the use of the reflector is less than the loss of the lens; if the spot is non-circular, the reflector itself will have a certain loss of light. In addition, when dealing with the light distribution requirements of the beam angle below 20o, the reflector will be much more awkward than the lens. However, there is a problem of dispersion in the lens. This problem is difficult to solve fundamentally, and only measures can be taken to alleviate the degree of dispersion. It can be seen that which optical device is selected depends on the characteristics of the light distribution requirement.
1.3 LED radiation type selection
Generally, the radiation patterns of the LEDs include a Lange type, a side shot type, a batwing type, and a condensing type. In the field of road lighting, according to the design experience, the author prefers the Langer type and the batwing type.
1.4 Some inappropriate views
There may be some inappropriate views in the current road lighting field, the authors summarized as follows:
(1) Overemphasizing the lumens of lamps and high illumination of the working surface, neglecting the application of lighting and the design of light distribution
The result of this is that the street lamp center illumination is too high and the uniformity is not perfect. Both waste energy and have not achieved the desired results;
(2) Incorporate lighting physiology into lighting design or lighting engineering design
Road lighting is mainly for bright lighting, involving lighting physiology, requiring sufficient brightness, uniformity and glare of the working surface. Therefore, the function of the street light is not only to meet the brightness requirements.
(3) Mixing of different light distribution requirements
The arrangement of street lamps includes single-sided cloth lamps, cross-border lamps on both sides, opposite cloth lamps on both sides, T-junction cloth lamps, cross-road cloth lamps and corner cloth lamps. Different arrangement modes of street lamps have different corresponding light distribution requirements. If all applications use the same light distribution design, it is not appropriate.
(4) Lack of cooperation with optical software designers
At present, domestic luminaire manufacturers are less cooperative with optical software designers. For example, Germany's famous lighting calculation software DIALux develops and provides related technical services with the support of many world famous lighting manufacturers (such as Philips, BEGA, THORN, ERCO, OSRAM, BJB, Meyer, Louis Poulsen). This in itself is a platform for standardizing and internationalizing its own business, through which it can feed its product information to the widest range of potential users as quickly as possible. However, only a few companies in the country have done this work.
(5) did not consider that you can make a fuss directly on the lens of the LED itself.
The LED itself is a complete luminaire system, including lenses, illumination sources, heat sinks, lead frames, and more. If the light distribution needs to be achieved by the lens design of the LED itself, it is no longer necessary to use a secondary optical system such as a reflector or a lens, because the light loss is caused by both the reflector and the lens.
2, LED street light cooling design
2.1 Long-term heat dissipation problem
At present, people put their main eyes on the lumens of LEDs, and pay less attention to the heat dissipation of LED lamps. In fact, the lumens of LEDs are rapidly increasing. In the past 2006, the single-watt lumens of mass-produced LEDs have reached 50 lumens, and this number is growing rapidly. The corresponding heat transfer theory system has matured, and the heat transfer methods we can use are also basically clear: conduction, convection, radiation and phase change heat transfer (heat pipe is a classic example of phase change heat transfer). For this reason, the measures we can take on heat transfer or heat dissipation are visible and limited.
According to the equivalent relationship between luminous flux (lumen) and radiant flux (Watt):
Among them, Km = 683 lm / w, is the scale of the luminous flux, is the scale of the radiant flux. That is to say, the 1W radiant flux may produce 683 lm of luminous flux in the most ideal case (blackbody radiation).
It can be seen that even if the luminous efficacy of the LED reaches 200 lm/w on one day, only less energy is converted into light energy output, and the rest is converted into heat energy. Therefore, in the long run, the heat dissipation problem of LED lamps will be a long-standing problem.
2.2 Ways of cooling design of street lamps
At present, the main heat dissipation methods are: natural convection heat dissipation, forced cooling by a fan, heat pipe and heat pipe heat dissipation. Adding a fan to force the heat dissipation system is complicated, the reliability is low, and the heat pipe and the loop heat pipe heat dissipation method are high in cost. The street lamp has the advantages of outdoor nighttime use, the heat dissipation surface is located on the side and the body shape is restricted, which is beneficial to the natural convection heat dissipation of the air. Therefore, the LED street lamp is recommended to select the natural convection heat dissipation method as much as possible.
2.3 Misunderstanding in heat dissipation design
Domestic lighting companies have fewer specialized personnel and departments to do heat transfer design, which will be a wrong system in the LED lighting era, which will bring the company into some misunderstandings. Specifically, possible problems are:
(1) The heat dissipating fin area is set freely
Some lighting companies do not have a quantitative concept when adding fins to their luminaires, but just design their area by feeling. This is a serious mistake.
(2) The arrangement of heat dissipating fins is unreasonable
The arrangement of the heat dissipating fins of the lamp does not take into account the use of the lamp, which affects the effect of the fin.
Taking the LED street lamp as an example, the heat dissipation surface is on the side of the lamp, and the lamp head has a mounting inclination angle. Therefore, it is generally recommended that the heat dissipating fins be arranged along the C90 direction of the luminaire to facilitate the flow of air between the fins, rather than arranging the fins along the C0 direction of the luminaire.
(3) Emphasis on heat conduction and neglect of convection heat dissipation
At present, this problem may be a general problem. Numerous manufacturers have considered various measures: heat pipes, loop heat pipes, thermal grease, etc., but have not realized that heat ultimately depends on the external surface area of ​​the lamps.
(4) Neglecting the balance of heat transfer
This issue is a deeper problem. If the temperature distribution of the fins is severely uneven, it will cause a part of the fins (the lower temperature part) to have no effect or a limited effect.
3, photovoltaic system design
The combination of low power consumption, low operating voltage and low output power and low output voltage of solar modules is a perfect match. Therefore, when discussing LED street lights, we must not discuss solar LED street lights. In the process of developing and promoting solar LED street lights, the author has learned some problems in current solar street lights. These issues will be addressed below and some new concepts will be proposed.
3.1 Inappropriate design ideas
(1) Use other light sources than LEDs
I have met a European customer and hope that we will develop a solar street lighting system with high-pressure sodium lamps as the light source. This is a mistake, which not only indicates the immaturity of the current solar lighting market, but also shows that our publicity and education for customers is not in place. If the solar street lamp system uses a light source other than the LED, it is inevitable to increase the inverter circuit in the charge and discharge circuit. The inverter circuit brings at least 20% loss, which means that 20% of the peak wattage of solar panels is wasted. At a time when solar panel prices are still high, this is unwise and out of the original intention of solar lighting.
(2) Blind pursuit of continuous rainy days
At present, some manufacturers claim that their solar LED street lights can guarantee 15 consecutive days of rainy days. According to the author's actual experience, 5 days of continuous rain guarantee days are more economical and reasonable. We assume that 15 days of continuous rain can be guaranteed, which means that the peak wattage of the solar panel is 50 to 80% higher than the number of watts required for the five days of continuous rain to ensure the number of days, and the battery capacity has to be doubled. This design method is not yet realistic at the moment.
(3) Fixed system parameters
The parameters of the solar LED street lamp system mainly include: solar panel peak wattage, solar panel tilt angle, battery capacity, and nominal power of the lamp head.
Now, many manufacturers give a fixed performance parameter table when pushing their products. In fact, this is not appropriate in solar LED street lighting systems. Among the many parameters, the nominal power of the lamp head can be relatively fixed, and the solar panel peak wattage, battery capacity and solar panel tilt angle should be determined according to the location of the solar LED street lamp system. Among them, the solar panel peak wattage should be determined according to the local solar radiation data, load characteristics (such as: winter load, summer load or annual balanced load), customer's working time requirements for the load; battery capacity mainly depends on The customer's working time requirements for the load; the solar panel tilt angle depends on the local latitude, local solar radiation data and other factors.
It can be seen that we cannot expect to design a solar LED street light system, which is suitable for all regions and all customers in the world. A truly economically sound approach should be to temporarily design system parameters based on user needs and characteristics.
(4) Battery capacity does not match solar panel parameters
Only the capacity of the battery is increased, and the peak wattage of the solar panel is not increased accordingly. This design does not increase the number of consecutive rainy days.
(5) Solar street light measured by the requirements of traditional street lamps
Due to the combination of the severe non-uniformity of solar energy distribution and the high cost of current solar panels, it is difficult to make solar street lamps have the same working capacity as traditional street lamps in an economical way. Therefore, it is neither objective nor fair to measure solar street lights by the standard of traditional street lamps.
3.2 Some new concepts
Based on the problems encountered in the actual design and the author's own understanding, the author puts forward the following concepts, and hopes to get the majority of PV system design experts to correct:
(1) Full load work
Definition: The actual number of hours worked per day of the load is equal to the number of design hours worked.
(2) Underload work
Definition: The actual working hours of the load per day are less than the design working hours. A special case of underloading is that the actual working hours are 0 hours.
(3) Full working days
Definition: The total number of days the load is at full capacity. In the days of full working, the average solar sun is not less than the annual average sunshine level.
(4) Underload working days
Definition: The total number of days the load is under load. In the days of underwork, the average solar sun is not greater than the annual average sunshine level.
(5) No guaranteed days
Definition: The total number of days in the year in which the working hours of the load are 0 hours is defined as the number of days that are not guaranteed. There is no guarantee that the number of days is an amount given for economic considerations. In the actual design, I recommend setting a value that does not guarantee the day. According to the author's understanding, this value can be designed to be 3 to 5 days.
4 Conclusion
This paper discusses in detail the three major problems that semiconductor lighting technology needs to face and solve in street lighting: secondary optical design, thermal design and photovoltaic system design. Point out some inappropriate design ideas that are common in current design, and give corresponding opinions based on the author's experience.
With the increasing global energy situation and the construction of a conservation-oriented society, semiconductor lighting technology, a new generation of energy-saving and environmentally-friendly lighting technology, will replace traditional lighting technology. At that time, road lighting, which is an important application direction of the lighting industry, must also be the world of semiconductor lighting.
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