Innovative design of heat dissipation structurefor LED street lamp based on AD and TRIZ
R. Ma, L. Liu & T. YuShanghai Key Lab of Intelligent Manufacturing and Robotics, School ofMechatronic Engineering and Automation, Shanghai University, China
Abstract
To solve the problems of low heat dissipation, low reliability, and complex struc-ture of a light-emitting diode (LED) street lamp, we study the integration theory ofaxiomatic design (AD) and Teoriya Resheniya Izobreatatelskikh Zadatch (TRIZ),and the FloEFD simulation model. We have established an integrated design modelon the basis of AD and TRIZ and applied to the design of heat dissipation struc-ture of an LED street lamp. We also analyze the function of the heat dissipationstructure, as well as determine the design conflict, and solve it. Then, we obtain anew type of a heat dissipation structure for LED street lamps and adopt the thermalsimulation of FloEFD to verify the rationality of the heat dissipation structure. Itcan improve the performance and reliability of LED street lamps.Keywords: AD, TRIZ, LED street lamp, heat dissipation structure, innovativedesign.
1 Introduction
Light-emitting diode (LED) lamps are a new generation of lighting products withthe following characteristics: high efficiency, long life, simple structure, smallvolume, light weight, rapid response, good seismic performance, and full colorspectrum. They have introduced a new field of lighting technology in the traditionallighting industry and provided a new direction for designing energy-saving light-ing. In many applications of LED products, LED street lamp is a very promisingapplication. Owing to the complex environment of outdoor lighting, many factorsaffect the reliability of LED street lamps; the junction temperature of an LED chiphas a great influence on the life, reliability, and failure mode of an LED [1]. Whenan LED is connected to the power supply, the temperature of the positive–negative
junction raises. Once a certain threshold is reached, it affects LED color, lumin-ous flux and voltage, etc. At the same time, the high temperature can also causethe thermal expansion of the packaging material, which leads to the failure of theLED, so the heat dissipation design of LED street lamps is very important [2].
The traditional method has not been able to solve the technical prob-lems considering the heat dissipation in the process of the LED street lampsdesign. In this paper, we use axiomatic design (AD) and Teoriya ResheniyaIzobreatatelskikh Zadatch (TRIZ) to guide the design and design a heat dis-sipation structure of LED street lamp, which is beautiful and good in heatdissipation. Finally, we achieve the objectives of reducing the junction temper-ature of lamps, as well as prolonging their service life and improving theirreliability.
2 Integrated design model based on AD and TRIZ
2.1 Integration theory of AD and TRIZ
Axiomatic design is a structural design method proposed by Professor Nam P.Suh of Massachusetts Institute of Technology in the 1980s [3]. Axiomatic designincludes functional independence axiom and minimum information axiom. Inde-pendence axiom maintains independence between functional requirements. It isthe basis for judging whether a design is reasonable. It is used to analyze the rela-tionship between functional requirements and design parameters. This relationshipcan be expressed as a design matrix A (Fig. 1):
{FRs} = |A| {DPs}, (1)
Figure 1: Three forms of design matrix.
When the design matrix A is a diagonal matrix, the design is a noncouplingdesign. When the design matrix A is an upper triangular matrix or a lower tri-angular matrix, the design is a quasicoupling design. If the design matrix A is ageneral matrix, a variation in the design parameters has a mutual effect on the twofunctional requirements. It does not satisfy the requirements of the independenceaxiom. There may be design conflict; further analysis will determine whether thereis a conflict.
Axiomatic design can help designers judge the rationality of a design, but it doesnot provide the right method for a correct design. Researchers, such as R. A. Shir-waiker, G. E. Okudan, and Tan Runhua proposed the use of TRIZ to solve theconflict problem in the process of AD analysis [4, 5]. TRIZ is the solution to
the problem of invention. It is based on summarizing a large number of patentsand innovation, summed up the tool that to solve specific conflicts in the designprocess. It consists of 40 inventive principles and a separation principle, 39 gen-eral parameters and a conflict matrix, the evolution law of a technology system,etc. The aim is to solve conflicts in design and obtain innovative solutions.
2.2 Thermal simulation based on FloEFD
FloEFD is a general fluid heat transfer analysis software [6] mainly used forthermal simulation of LED lamps, as well as for determining whether the junc-tion temperature of an LED chip is within the scope of regulation and determiningwhether the design of a heat dissipation structure is reasonable. The main steps inthermal simulation of lamps are as follows:
(1) Establishing the model of LED lamps and importing into the FloEFDsoftware
(2) Creating an “LED lamp” project and setting the relevant parameter prop-erties including unit, analysis type, gravity direction, fluid type, solidmaterial, and mesh
(3) Examining the model and setting the boundary condition(4) Adding material properties to each component of an LED lamp, adding
contact resistance on the contact surface of lamp parts, establishing thedouble resistance model to replace a chip as the heat source, and setting thetarget of the components
Finally, the temperature distribution of an LED lamp can be obtained by post-processing. And we can determine whether the junction temperature of a chip isreasonable.
2.3 Integrated design model based on AD and TRIZ
By comparing the TRIZ and AD theory, we find that each has its own advantages.Designing a heat dissipation structure for an LED street lamp involves the op-timization and integration of two theories and combining with FloEFD thermalsimulation. Then, we build the integrated design model of a heat dissipationstructure for the LED street lamp on the basis of AD and TRIZ (Fig. 2).
The integrated design model mainly includes the following steps:(1) Establishing the function-structural model for the heat dissipation structure
of the LED street lamp – on the basis of the characteristics of the heatdissipation structure and user’s requirements.
(2) Determining the coupling of the design matrix and rearranging the designmatrix. If the design matrix is a diagonal or a triangular matrix, it is anoncoupling design On the basis of the independence axiom, the designcan be accepted for the next step or rejected.
(3) Using TRIZ’s correlation tool to continue conflict resolution, the innovativeprinciple solution of the heat dissipation structure is obtained to determine
Figure 2: Integrated design model of heat dissipation structure based on ADand TRIZ.
the integrated design model of the heat dissipation structure for the LEDstreet lamp.
(4) Establishing the model of the heat dissipation structure of the LED streetlamp and then carrying out thermal simulation based on FloEFD to determ-ine whether the junction temperature of the LED chip is within a reasonablerange.
(5) Determining, on the basis of the simulation results of FloEFD, whetherdesign requirements are met. If they are not meet, TRIZ’s correlation toolis used to carry out the conflict resolution again. And the structure modelis established for simulation. Otherwise, the next step is followed.
(6) Determining the scheme of the heat dissipation structure on the basis ofthermal simulation based on FloEFD. Finally, a high reliability of the heatdissipation structure of the LED street lamp is obtained.
3 Innovative design of heat dissipation structure for LED streetlamp based on AD and TRIZ
3.1 User requirements analysis
As temperature greatly influences luminous flux, color, and reliability of lamps, itrequires LED street lamp working continuously. This research on heat dissipationof LED street lamps, natural convection is mainly adopted; the parameters of finshape, thickness, fin pitch, etc. mainly depend on the experience of engineers, aseffective methods for determining these factors are lacking. The structure of the finand heat sink, which are parts of the lamp, will affect the appearance of the lamp.The purpose of this paper is to design a new type of heat dissipation structureon the basis of a 400 W LED street lamp and control the junction temperature ofthe LED chip within 85◦C. The street lamp has the following requirements: goodthermal performance, high reliability, beautiful appearance, simple structure, easyto manufacture, low cost.
3.2 Analysis of heat dissipation structure of LED street lamp based on AD
First, user requirements need to be summarized: FR0, which is an LED street lampthat can work continuously and have a beautiful appearance. The correspondingdesign parameter is summarized as DP0, which is the heat dissipation structure ofthe lamp. Function decomposition is FR0; FR1 is the reasonable structure of heatdissipation; FR2 is simple and beautiful appearance of the lamp; and FR3 is lightweight.
We introduce some user demands as constraints on the design parameters of theheat dissipation structure, namely C1: good performance in heat dissipation, C2:low cost of lamp, C3: high reliability of lamp, C4: convenient maintenance of lamp,C5: beautiful appearance of lamp, and C5: simple structure of heat dissipation.
Depending on the functional requirements FR1, FR2, FR3, and their correspond-ing constraints of C1, C2, C3, C4, C5, C6, the design parameters are obtained underthe condition of the constraints. They are DP1: convection cooling of fin, DP2:surface appearance of lamp, and DP3: overall structure shape of lamp.
Through analysis, it is found that design parameters that affect the functionalrequirement FR1 are DP1 and DP2; design parameters that affect the functionalrequirement FR2 are DP1, DP2, and DP3; and design parameters that affect thefunctional requirement FR3 are DP1 and DP3. The design matrix is obtained onthe basis of the application of the independence axiom:⎡
In the matrix, 1 represents the two correlation, 0 means the two is not relevant, inthe following process the symbol meaning is the same. Equation (2) shows that thedesign matrix is a general matrix, which does not satisfy the independence axiom.The design is coupled with the need for further decomposition of the function:
(1) FR1 is decomposed into FR11, the heat generated by an LED chip that israpidly transmitted to the fins, and FR12, the quantity of heat emitted intothe atmosphere from the fins. The corresponding design parameters areDP11, the shape of the radiating fin, and DP12, the heat convection channelfor a lamp. The formation of the design matrix is:
[FR11
FR12
]=
[1 01 1
] [DP11
DP12
]. (3)
(2) FR2 is the simple and beautiful appearance of a lamp. We have no specialstructure on the surface, in line with the aesthetic requirements of people.So there is no need for further decomposition.
(3) FR1 is decomposed into FR31, and FR32. The corresponding design para-meters are DP31, the connected structure between the heat dissipationstructure and the power cavity. The formation of the design matrix is:
[FR31
FR32
]=
[1 01 1
] [DP31
DP32
]. (4)
According to design matrixes (3) and (4), it can be known whether thedesign matrix is a diagonal matrix, satisfying the independence axiom.Through the “zigzag mapping transformation,” we obtain the hierarchymodel of a function and the hierarchy model of a design parameter, that is afunction-structural model of the heat dissipation structure (Fig. 3). Throughthe analysis of the model, we find that there is a conflict between light andheat, as well as between the structure and the heat. To optimize the design ofan LED street lamp, we must solve these conflicts and optimize the designparameters of the coupling.
3.3 Solution of design conflict and principle solution based on TRIZ
Axiomatic design has advantages in determining the design of lamps, but it can-not solve the conflict problem in the design process [7]. According to designmatrix (2), this design is a quasicoupling design, which can be accepted by theindependence axiom. But if the heat generated by an LED chip cannot be passedto the fins for heat dissipation, and the heat from the fins cannot be effectively
Figure 3: Function-structural model of heat dissipation structure for LED streetlamp.
released into the atmosphere, it directly affects the normal operation of an LEDstreet lamp. Through the analysis of design constraints, it is found that this is ac-tually a technical conflict. By TRIZ, we convert the above functional requirementsinto TRIZ standard parameters, checking the following table [8]:
FR11: the heat generated by an LED chip, which is rapidly transmitted to thefins, which can be converted into standard parameter 12: showing the shape.
FR12: the quantity of heat emitted into the atmosphere from the fins, which canbe converted into standard parameter 6: the area of the stationary object.
To improve the above two characteristics, which will lead a decrease in the fol-lowing characteristics: standard parameter 8: for the volume of stationary object;standard parameter 32: for manufacturability.
According to TRIZ standard parameters, with checking table [8], the availableconflict matrix is shown in Table 1.
Combined with the actual situation of an LED street lamp, inventive principlesthat we can use are the inventive principle 1: segmentation. And through analysis,we can know that the thermal performance of the structure of an LED street lampwill be promoted, which will lead the fins’ increasing heat dissipation. But theweight of the structure will be also increased. This is actually a physical conflict,and we can use the principle of separation of space, the corresponding inventiveprinciples being 1, 2, 3, 4, 7, 3, 17, 24, 26, 30. Combined with the actual situationof an LED street lamp, the inventive principle that we can use is the inventiveprinciple 24: mediator.
According to research, 80% of innovation is based on existing technology in-novation [9]. So it is very important to study the heat dissipation structure of atypical LED street lamp. An LED street lamp uses the integrated design of shelland the heat dissipation structure; the driving power is placed inside. The powercavity and the heat dissipation structure are integrated. The structural model of thewhole lamp is shown in Fig. 4.
Figure 4: Structural model of whole lamp: 1 – shell, 2 – LED chip, and 3 – glass.
An improved LED street lamp is still used in the integrated design of ashell and the heat dissipation structure. According to the inventive principle 1:segmentation, the original cavity, which places LED chips, is divided into twoparts to prevent the heat generated by LED chips from being highly concentrated.According to the inventive principle 24: mediator, four pieces of a heat pipe aresymmetrically arranged at the two ends of the heat dissipation structure, whichallows the heat generated by LED chips in the two regions to be transmittedrapidly to the fins for heat dissipation. At the same time, as the structure of the
whole lamp is made of ADC12, thermal-radiation ability is low. According to theconflict matrix, the thermal radiation ability of the street lamp can be improved.
According to the guidance of the innovation principle of TRIZ, we reestablishthe structural model of the LED street lamp (Fig. 5).
Figure 5: Improved structure model of the whole lamp: 1 – shell, 2 – heat pipe,3 – radiation material layer, and 4 – power cavity.
4 Thermal simulation and verification of LED street lampbased on FloEFD
The selected light source was OSRAM LCW CQAR.PC; the power of each lampbead was 2 W. In thermal simulation, the double thermal resistance model wasused to replace the light source of an LED and predict the junction temperatureof the LED chip. It considered all packages containing two parallel components(shell and knot) with a high-conductivity material. It needed to define two thermalresistance: junction – shell thermal resistance (Rjc) – and junction – board thermalresistance (Rjb), using these thermal resistance values to calculate the thermal con-duction through the package [10]. The double thermal resistance model is shownin Fig. 6.
Related parameter settings were as follows. Through the specification of OS-RAM LCW CQAR.PC, we obtained the value of Rjc as 3.8 K/W; the default settingof Rjb was 190 K/W. The ambient temperature was 30◦C, and the total power ofthe LED street lamp was 400 W. According to 37% of the electro-optic conversionefficiency, heat generated by LED chips was 252 W. Each light source module wasreplaced by a double thermal resistance model. We added contact resistance onthe contact surface of lamp parts. The metal layer material of printed circuit boardboard was Al6061, and the material of the heat dissipation structure was ADC12.The effective thermal resistance of the heat pipe was 0.3 K/W, and the radiationcoefficient was 0.98.
The thermal simulation result of the LED street lamp is shown in Fig. 7.The thermal simulation result of the improved LED street lamp is shown in
Fig. 8.From the results of thermal simulation, we knew that the junction temperature
of the LED chip was 95.11◦C before an improvement in the lamp structure, and
Figure 7: Thermal simulation result of LED street lamp.
the junction temperature of the LED chip was 82.95◦C after an improvement inthe lamp structure. The junction temperature of the LED chip that affected theperformance of the LED street lamp was controlled within 85◦C. The structure ofheat dissipation was reasonable, and it could ensure the normal operation of theLED street lamp.
5 Conclusion
In the design process of the heat dissipation structure of an LED street lamp, us-ing the integration theory of AD and TRIZ, as well as the FloEFD simulationmodel, we establish the integrated design model of the heat dissipation structure
Figure 8: Thermal simulation result of improved LED street lamp.
of an LED street lamp based on AD and TRIZ. Meanwhile, we analyze the designtask and functional requirements of the heat dissipation structure and establish afunction-structural model of the heat dissipation structure of the LED street lamp,clearing the design conflict of the lamp. And then, we establish the conflict matrixof the heat dissipation structure of the lamp by using TRIZ, as well as the applica-tion of the principle of separation, combined with the actual situation of the LEDstreet lamp, resolving the design conflict of the lamp. So the innovative scheme ofthe heat dissipation structure for an LED street lamp is obtained. Finally, thermalsimulation based on FloEFD is used to verify the rationality of the heat dissipation.It shows that the heat dissipation structure can improve the reliability of an LEDstreet lamp.
Acknowledgments
This work was supported by the National High Technology Research and Develop-ment Program of China (863 Program): 2013AA03A112, as well as the Absorptionof Imported Technology and Innovative Project of Shanghai City: 15XI-1-26. Theauthors are grateful for the financial support.
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