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Optik 126 (2015) 4658–4664

Contents lists available at ScienceDirect

Optik

jo ur nal homepage: www.elsev ier .de / i j leo

ynamic mesh-based analysis of dynamic irradiance characteristicsf solar simulator

inglong Menga,b,∗, Yanpeng Lia, Yaxiu Gua

Chang’an University, School of Environmental Science and Engineering, 126 Yanta Road, Xi’an 710054, ChinaZhejiang University, The State Key Laboratory of Fluid Power Transmission and Control, 866 Yuhangtang Road, Hangzhou 310058, China

r t i c l e i n f o

rticle history:eceived 2 August 2014ccepted 19 August 2015

eywords:olar simulatoromputational fluid dynamicsynamic mesh

a b s t r a c t

The unsteady motion of a solar simulator was simulated using dynamic mesh technology in Fluent soft-ware. The dynamic irradiation characteristics of the simulator were studied under various conditions.Mesh updates were achieved using a dynamic layering method, and the periodic lifting motion of thesimulator was defined using user-defined functions (UDF). Detailed dynamic irradiance characteristicswere obtained for comparison with experimental results. The results showed that the simulator heightand the number of light sources used were the main factors that affected the irradiance. The irradiancehas a linear relationship with the simulator height, which means that the irradiance non-uniformity

ynamic layering decreases with decreasing solar height; in addition, the sum of the irradiances under the various oper-ating conditions matches the superposition of the irradiance. The dynamic irradiation numerical resultsare consistent with the experimental results at typical points, which verifies the reliability of the mov-ing mesh numerical model. The validated model can be used to study various simulator conditions andprovides forecast data for diurnal variation simulation of solar radiation.

. Introduction

Environment simulation technology (EST) is a comprehensivengineering technique that was defined by Wang [1]; it applieshe theories of numerous fields, including thermology, mechanics,lectricity, biology, optics, physics and automation science, alongith many practical methods such as refrigeration, calefaction, vac-um, air-conditioning, control and measurement [2]. EST is nowidely applied in a variety of scientific research fields, and it expe-ites the completion of experiments because of the merits of EST,

n that time is fully controllable, the experimental conditions canasily be repeated and the results will be accurate [3]. In addition,eld-based experiments can be very difficult to perform because ofhe long periods and high costs involved, and thus it is necessaryo create a simulated environment to assess new technologies. ESTs widely applied in various fields [4,5], including military, agricul-ural, automotive, weapons, biological and weather applications.he most famous example of EST is Biosphere 2; full details of Bio-

phere 2 can be obtained in [6–9]. A desert environment simulationaboratory was built in 2006 in Xi’an Jiaotong University. In this lab-ratory, there is a solar simulator that can simulate both static and

∗ Corresponding author at: Chang’an University, School of Environmental Sciencend Engineering, 126 Yanta Road, Xi’an 710054, China. Tel.: +86 18229017219.

E-mail address: mql19@163.com (Q. Meng).

ttp://dx.doi.org/10.1016/j.ijleo.2015.08.085030-4026/© 2015 Elsevier GmbH. All rights reserved.

© 2015 Elsevier GmbH. All rights reserved.

dynamic irradiance. The static characteristics were given in Refs.[10] and [11]. In this paper, the dynamic characteristics of the solarsimulator will be investigated.

To realize an experimental simulation of diurnal irradiancevariation, the solar simulator characteristics must be understoodsufficiently. The dynamic mesh model is therefore introducedusing Fluent commercial computational fluid dynamics (CFD)software. The dynamic irradiance distribution characteristics of aspecified irradiation plane are studied when the solar simulatorin the desert environment simulation laboratory is set accord-ing to the expected requirements. Detailed reference data forexperimental studies of simulated solar radiation intensity can beobtained through numerical simulations. The solar simulator canprovide an accurate and stable experimental research platform forsolar thermal energy utilization.

The remainder of this paper is organized as follows. Section 2describes the experimental platform and its function. Section 3provides a detailed description of the dynamic mesh models. Ourresults and their experimental validation are provided in Section 4,and our conclusions are presented in Section 5.

2. Hardware and its function

2.1. Dimensions

A three-dimensional drawing of the laboratory is shown inFig. 1. The laboratory is 10,280 mm long, 6950 mm wide and

Q. Meng et al. / Optik 126 (2015) 4658–4664 4659

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1000 1200 1400 1600 1800 2000 2200 24008006004002000.0

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distance is limited by the cable lengths, while the maximum dis-tance is limited by the ceiling height. A forced-air cooling systemwas constructed to carry away the heat energy that surrounds the

ig. 1. Schematic three-dimensional diagram of the simulation laboratory environ-ent.

pproximately 4800 mm high. The lab itself is divided into twohambers: the device chamber and the test chamber. The deviceoom comprises a refrigeration device, supply fans, a heatingevice, the solar simulator, a humidifier and a dehumidifier. Thebservation room/control room, in which the experimental per-onnel operate the control equipment during the experiments andbserve the test chamber conditions through the double-layerlass, is located above the device room. The usable floor area is0 m2, while that of the test chamber is 53 m2. In the center of theest chamber, there is a sand pit, which is 6440 mm long, 5040 mmide and approximately 1350 mm deep; the sand originated in the

aklimakan Desert, and thus offers a real parcel of sand physiog-omy and the subterranean desert environment. Three 7.5 kW axial

ans are used as supply fans and three 11 kW axial fans are used aseturn fans. The test chamber door is insulated with 150 mm of ply-ood. Wall heat preservation (i.e., thermal insulation) is provided

y 120 mm of polyvinyl ester/polyurethane foam on the interior of70 mm of brick and concrete (the constructed structure is shown

n detail in Fig. 2), and the ceiling consists of a 5 mm-thick dou-le layer of glass affixed with gum water and a color steel plateith 100-mm-deep plastic foam. There is a 50-mm-wide space

etween the glass and the color steel plate. The color steel platean be inserted/removed as necessary.

.2. Solar simulator system

The solar simulator is sometimes used for collector testing, buts more often used for durability tests for the automotive and elec-ronics industries. In this paper, it is used to model experimentalesting of indoor airflow characteristics.

From previously reported documents, it is known that severalinds of lamps have often been used as light sources for solar sim-lators, including xenon arc or mercury xenon arc lamps, metal

alide lamps, high pressure sodium vapor lamps, mercury vapor

amps and incandescent spotlights. While the metal halide lamputputs more energy in the ultraviolet range and lower energyn the visible range than natural solar light, it is suitable for

Fig. 2. Floor plan of the laboratory, (a) lower (b) upper.

Wavelength/nm

Fig. 3. Comparison of spectrum energy distributions for the RSDL lamp and AM1.5.

simulation of full spectrum applications. Taking our research objec-tives and the economic aspects into account, we adopted a typeof metal halide gas discharge lamp (called the Reflector SunlightDysprosium Lamp, or RSDL, by the manufacturer; a comparison ofthe spectrum energy distribution for the RSDL lamp with that ofAM1.5 illumination conditions is shown in Fig. 3) with rated powerof 400 W. The RSDL lamp is a sealed beam unit with a quartz arcbulb placed at the focus of a parabolic reflector. Chemical elementgases, such as dysprosium, thallium, iodide and mercury gases, fillthe bulb to produce a special dense light that has a spectral profilethat is very similar to that of natural solar light. The lamp is simpleand convenient to operate electrically, and does not require com-plicated cooling. When the preset charge voltage is reached, thelamp is ignited.

A photograph of the multiple-lamp solar simulator is shown inFig. 4. The completed simulator facility consists of 188 lamps, a steellamp mounting frame (4.5 m × 3.88 m) and four variable speed ele-vators that can be manually controlled and computer-controlled.Fig. 5 shows the layout of the solar simulator. The lamps are pos-itioned in 15 rows, where each row consists of 12 or 13 lamps. Thereare eight columns of 13 lamps and seven columns of 12 lamps.The lamp face has a radius of 0.10 m. The lamp base-to-lamp basespacing is 0.295 m, so the lamp-to-lamp spacing remains at onlyat 0.095 m. Below the simulator, there is a cuboid-shaped sandbunker. The sand surface to be used as the target area is irradi-ated by the simulator. The spacing between the lamp face and thesand surface is adjustable in a range from 1.0 m to 2.5 m, dependingon the irradiance desired at the irradiated surface. The minimum

Fig. 4. Multiple-lamp solar simulator (shown before the construction of the forcedcooling system).

4660 Q. Meng et al. / Optik 126 (2015) 4658–4664

ltfnhemilo

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3

3

pmvst

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st

wt

Fig. 5. Layout of the large-scale solar simulator collocation.

amp surfaces. The air velocity flowing through the solar simula-or is adjustable in a range from 0.1 to 0.5 m/s using an inverteran driven by a transducer. The fan speed is adjusted based on theumber of lighting lamps in use. The attitude of the simulator to theorizon can be adjusted from 0◦ to 30◦ for specific requirements,.g., when an angled beam is necessary for application testing. Thiseans that the system has an angular range of variation for the

ncident light. The simulator is driven by four elevators. The angu-ar variation can be implemented by adjusting the vertical speedsf any two of the four elevators.

To adjust the irradiance, the solar simulator is mounted on theoveable steel frame in the upright direction, and the lamps in the

imulator are then divided into four groups: A, B, C and D (wherehe numbers of lamps are 42, 56, 48 and 42, respectively). The fourroups can be operated individually or in combination to produceifferent amounts of radiation. The hexagonal lamp configurationf the simulator and the arrangement of the lamp groups is alsohown in Fig. 5. The irradiance can be adjusted by using differ-nt group combinations and light heights. The irradiation contoursor the minimum and maximum average pyranometer readingsf approximately 150 and 1100 W/m2 are produced without anyignificant changes in the homogeneity of the irradiation field.

. Dynamic irradiance characteristics of solar simulator

.1. Method: dynamic mesh

In ANSYS Fluent [12], the dynamic mesh can be used to simulateroblems with boundary motion. The motion can be a prescribedotion (e.g., where the user can specify the linear and angular

elocities with time) or an unprescribed motion, where the sub-equent motion is determined based on the solution at the presentime.

Several different mesh rebuilding schemes, including layering,moothing and remeshing, can be used for different moving partsithin the same simulation as required.

.1.1. Dynamic mesh conservation equationsThe integral form of the conservation equation for a general

calar, ˚, on an arbitrary control volume, V, which has a boundaryhat is moving, can be written as

d∫

�˚dV +∫

��(−→u − −→ug)d−→A =

∫∇˚d

−→A +

∫S�dV (1)

dtV ∂V ∂V V

here, � is the fluid density, −→u is the flow velocity vector, −→ug ishe grid velocity of the moving mesh, � is the diffusion coefficient,

Fig. 6. Sketch of the regional map of the dynamic mesh.

and S� is the source term of �. ∂V represents the boundary of thecontrol volume V.

The time derivative term in Eq. (1) can be written, using a first-order backward difference formula, as

ddt

∫V

��dV = (��V)n+1 − (��V)n

�t(2)

where n and n + 1 denote the respective quantity at the currentand subsequent time levels. The n + 1 th time level volume Vn+1 iscomputed from

Vn+1 = Vn + dV

dt�t (3)

where, dV/dt is the volume time derivative of the control volume.To satisfy the grid conservation law, the volume time derivative ofthe control volume is computed using

dV

dt=

∫∂V

−→ug · d−→A =

nf∑j

−−→ug.j · −→

Aj (4)

where, nf is the number of faces on the control volume, and−→Aj is the

j face area vector. The dot product−−→ug.j · −→

Aj on each control volumeface is calculated using

−−→ug.j · −→

Aj = ıVj

�t(5)

where, ıV is the volume swept out by the control volume face jduring the time step �t.

3.2. Dynamic simulation with dynamic mesh method

3.2.1. Mesh gridIn GAMBIT (a software package designed to help analysts and

designers build and mesh models for computational fluid dynamics(CFD) and other scientific applications), the size of the laboratorymodel is 7370 × 6950 × 4900 (H) mm3. To reduce the grid numberupdates and guarantee the computing speed, the model is dividedinto two parts: the static and dynamic grid areas. The size of thedynamic grid region is 4500 × 3880 × 2500 (H) mm3. The remainingregion is the static mesh region. The two parts are linked using anon-regular interface. To use the corresponding layer algorithm,the dynamic mesh regions near the top and bottom boundariesand near the interface must be hexahedral meshes. A sketch of theregional map of the dynamic mesh is shown in Fig. 6.

In this study, the vertical boundary motion of the solar simulatorleads to the computation of a wide range of grids and large-scaledeformation, so the volume mesh in the deforming regions that issubject to the motion defined at the boundaries is updated usinglocal remeshing building schemes. The volume mesh is updatedautomatically. Following the introduction of a fixed grid update

method, the model grid is divided into static and dynamic grid areasin two parts. The solar simulator is used for the motion region andthe entire region of space that is occupied by the grid is the fluiddynamic region. The moving grid is shown in Fig. 7.

Q. Meng et al. / Optik 126 (2015) 4658–4664 4661

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udvls

dvzs

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Table 1Irradiance information on the target when all lamps are on.

Time (s) Maximum(W/m2)

Minimum(W/m2)

Average(W/m2)

Nonuniformity(%)

t = 0 1077.37 889.25 946.27 9.57t = 240 1189.34 982.38 1043.83 9.53t = 480 1277.17 1062.95 1150.17 9.15t = 720 1378.13 1169.46 1262.49 8.19t = 960 1454.84 1260.71 1358.52 7.15t = 1200 1658.75 1452.87 1536.31 6.62t = 1440 1447.77 1265.7 1304.15 6.71t = 1680 1394.64 1171.62 1229.47 8.69t = 1920 1274.11 1067.09 1157.26 8.84

Fig. 7. Moving grid.

During the numerical simulation, 188 small cylinders repre-ent the 188 lamps of the solar simulator. The surface area of eachylinder is equal to that of each dysprosium lamp. The boundaryonditions of the simulation mode are defined as follows. The roomalls and the ground are set as adiabatic surfaces. The sand plane is

et as the third kind of boundary condition, where the heat trans-er coefficient is taken to be k = 0.58 W/(m2 ◦C). The air speed at thenlet is set as the entrance speed, and the return and exhaust out-et air speeds are set as the outflow. The radiation model discreterdinates (DO) in Fluent were used to define the radiation of theolar simulator [13].

.2.2. User-defined functions (UDF)The motion of the solar simulator is described using UDF

user-defined functions). When the solar simulator has a liftingovement in the vertical direction, it is not related to the mesh

eformation, and the mesh nodes have no relative movement. Here,he DEFINE-CG-MOTION UDF [14] was used to specify the motionf the dynamic zone for the solar simulator by providing Fluentith the linear velocity at each time step. The DEFINE macro and

ts argument are as follows.DEFINE CG MOTION(name, dt, vel, omega, time, dtime)Here, the argument name is the UDF name specified by the

ser. The argument dt is a pointer to the structure that stores theynamic mesh attributes that the user has specified. The argumentel represents the linear velocity. The argument omega is the angu-ar velocity ω. Finally, time is the current time, and dtime is the timetep.

The movement of the solar simulator is only in a vertical liftingirection with linear speed. The horizontal velocities x-componentel[0] and xy-component vel[1] are zero, and the vertical velocity-component vel[2] is not zero. The UDF DEFINE-CG-MOTION ishown as follows:

#include “udf.h”DEFINE CG MOTION(moving,dt, vel, omega, time, dtime){if (time < = 1200)vel[2] = −1/1200;elsevel[0] = 0.0;vel[1] = 0.0;

el[2] = 1/1200;NV S(omega, = 0.0);}In the UDF, the solar simulator motion cycle is set for 40 min

ithin a lifting distance of 1 m, where the mobile speed is a con-tant 1/1200 m/s. During the simulation, a data file is saved every40 s until the end of the movement process. The irradiance on anrbitrary plane can be read directly from the data file. The irradi-

nce changes can also be acquired from the saved data. Simulationf unsteady irradiance can also be achieved by the dynamic meshpproach. The motion of the solar simulator is defined as a rigidody movement. The positions of the dynamic grid generation and

t = 2160 1199.29 998.31 1109.58 9.15t = 2400 1105.67 910.51 960.51 9.68

disappearance are on the top and bottom of the overall dynamicmesh region, respectively.

4. Results and experimental validation

4.1. Dynamic mesh simulation results

The dynamic mesh simulation results (for the temporal and spa-tial irradiance distributions when all lamps are on) are shown inFig. 8. From Fig. 8, we see that the spatial distribution of the irra-diance in the target area is rectangular, and that there is higherintensity in the central region. Over time, the irradiance intensityon the surface first gradually increases, and then decreases (thesolar simulator is near the irradiated surface, and is then furtheraway from the irradiated surface). As the distance between theirradiated surface and the simulator decreases, the effective areaexpands, and the average irradiance at the target area increasesas a linear function of the distance. From other simulations (theseirradiance distribution results are not shown here), we can see thatwhen the solar simulator is at a certain height, the groups in whichthe rectangular radiation area changes from large to small in turnare Groups B, C, and A.

The irradiance information (including the variations of themaximum and minimum average irradiance, and the irradiancenonuniformity) in a single period on the target when all lamps areon is shown in Table 1. Here, the nonuniformity is defined by Eq.(6).

Nonuniformity(%) = Emax − Emin

Emax + Emin× 100% (6)

Here, Emax is the maximum irradiance measured by the detec-tor(s) over the target test area, and Emin is the minimum irradiancemeasured by the detector(s) over the target test area, and both aremeasured in W/m2.We consider the irradiance time variation in a3.0 m × 2.5 m area under six illumination conditions: group ABCD,group ABC, group AB, group A, group B and group C. The averageirradiance variation vs. time is as shown in Fig. 9. From Fig. 9, weknow that at a certain time (i.e., when the solar simulator is at acertain height), the higher the lamp light source number is, thenthe larger the irradiance will become. From these results, we alsofound that the average irradiance produced by any combination oflamp groups is equal to the total irradiance produced by the sepa-rate groups, i.e., the total irradiance distribution is a superpositionof that for each group. The results indicated that the irradiance isinversely proportional to the lamp height. As expected, the irra-diance intensities decrease as the simulator moves away from the

target test plane. From the simulation results, we also see that theradiation intensity distribution of an irradiated surface is mainlyaffected by the height of the solar simulator and the number oflamps used.

4662 Q. Meng et al. / Optik 126 (2015) 4658–4664

Fig. 8. Irradiance distribution on the target vs. ti

iut

Fig. 9. Average irradiance vs. time.

The irradiance nonuniformity vs. time characteristics are shown

n Fig. 10. The characteristics show that when more lamps weresed at the same time (or distance), then the irradiance distribu-ion at the target area became more uniform. During lowering of

0 240 480 720 960 1200 1440 1680 1920 2160 2400 26406

8

10

12

14

16

18

Non

unifo

rmity

/%

T/s

ABCD ABC AB A B C

Fig. 10. Irradiance nonuniformity vs. time.

me when all lamps are turned on (W/m2).

the solar simulator, the irradiance uniformity increases continu-ously, i.e., the irradiance non-uniformity is reduced: from 9.57%to 6.62% with all group switched on, from 13.58% to 11.48% withgroup ABC on, from 15.41% to 12.72% with group AB on, and from17.46% to 14.01%, from 16.59% to 13.57% and from 16.73% to 13.88%with groups A, B and C switched on, respectively. The irradiancenon-uniformity in turn increases when the solar simulator rises.This shows that when a certain number of lamps is switched on, asthe height that the solar simulator is above the irradiated surfacedecreases, then the irradiance distribution becomes more uniform.

4.2. Experimental validation

To verify the dynamic mesh model, an irradiance test was car-ried out in a uniform solar simulator lifting period.

One statistical indicator has been used to test the efficiency ofthe model: the root mean square error (RMSE) [15]. It is expressedas follows:

RMSE = 100⟨Mi

⟩√√√√ 1

N

N∑i=1

(Si − Mi)2 (7)

The RMSE parameter represents the deviation of the simulated(Si) values vs. the measured (Mi) values. N is the number of datapoints used in the validation process and

⟨Mi

⟩is the mean value

of the measured values at the surface. A smaller absolute value ofthis parameter indicates a better model.

A secondary standard EP09 Pyranometer (Middleton SolarInstruments, manufactured by Carter-Scott Design, Victoria,Australia) with an electronic data logging system was used to mea-sure the global solar radiation at the test surface. The irradiancewas measurable from 0 to 2000 W/m2 for light in the wavelength

range from 300 to 3000 nm. This sensor (shown in Fig. 11) has afast (less than 10 s for 95% signal response), stable (less than 0.6%drift per year) and linear (less than ±0.25% nonlinearity) platinumresistance thermal sensor.

Q. Meng et al. / Optik 126 (2015) 4658–4664 4663

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imta2icruapisn(ms

swaTggsoa

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u

Fig. 13. Irradiance test results at typical points, (a) Group A (b) Group B (c) Group C

ig. 11. Test site and the pyranometer sensor, (a) Measurement site (b) Sensor:yranometer.

The irradiated area below the solar simulator is larger than therradiated test area that is required to meet the specific test require-

ents. From Ref [10], it is shown that the average irradiance overhe 2.0 m × 1.5 m rectangular test area at the center of the irradi-ted plane, lying 2.0 m from the lamp array, is 1000 W/m2. In the

m × 1.5 m rectangular irradiated area, the nonuniformity of thellumination is 4.27%. Fig. 12 shows the points (marked numeri-ally) at which the irradiance was measured over a 1.8 m × 1.5 mectangular test area. The irradiance distribution was based on val-es measured in a rectangular grid at intervals of 0.6 m and 0.75 m,s shown. The nodes of the grids correspond to the measurementoints. The total number of nodes was 13. The center point on the

lluminated area was located at the projection point of the solarimulator. For a symmetrical irradiance distribution on the illumi-ated area, the irradiance results from four measurement pointspoints 1–4) are given when a single group is switched on. When

ultiple groups are switched on to verify the irradiance superpo-ition, the irradiance on the center point is measured.

The measurement points at which the irradiance was to be mea-ured over the test area were established before the measurementas implemented; the solar simulator was also turned on for half

n hour in advance, so that the light source reached a stable state.wo different experiments were implemented: in the first, group A,roup B, and group C were each opened singly; in the second, tworoups (groups A and B) are opened at the same time. Before thetart of each measurement, the solar simulator was again turnedn for half an hour in advance to ensure that the simulator was in

stable state.Fig. 13 shows the irradiance time-dependent results under four

ifferent conditions when the solar simulator was lifted uniformly.

t is obvious from the figures that the irradiance in Fig. 13 and thatn Fig. 9 have the same change trend.

Further comparisons of the simulated and experimental resultsnder the different conditions show that when groups A, B and

Fig. 12. Measurement points.

(d) Group AB.

C were switched on, their average RMSEs were 4.13%, 4.21% and3.95%, respectively, while that of group AB was 5.1%. The data indi-cate that the simulation results are in good agreement with theexperimental results, which further proves the reliability of thecalculations of the moving mesh numerical model. In addition,Fig. 13(a), (b) and (d) show the irradiance superposition charac-teristics, i.e., where the sum of the individual irradiances of groupA and group B on the surface is equal to that of group AB. Thisexperimental result is also consistent with that of the numericalsimulation. It was also found that the simulation results were lowerthan the experimental results. The reason for this is that the mea-surement results are for the irradiance at a height of 50 mm (i.e.,where the sensor base itself is at a height of 50 mm), while the dis-tance between the simulator and the sensor is actually 1950 mm.In addition, the solar simulator is arranged with reflecting platesat both sides. The reflecting plates partially affect the measure-ment results, which cause the actual results to be higher than thesimulation results.

5. Conclusions

From the results, we drew the following conclusions.

(1) There is a linear relationship between the irradiance andthe height of the solar simulator. The non-uniformity of theirradiance decreases when the height of the solar simulatordecreases. The irradiance on the irradiation surface presentssuperposition characteristics, which provides us with infor-mation references to realize different irradiance lamp groupcombination schemes;

(2) comparison of the simulated and experimental results showsthat the numerical simulation of the irradiance distribution is

reliable. The measurements of the actual irradiance and thedynamic irradiance characteristics of the solar simulator aretime-consuming and laborious. Numerical simulations couldprovide detailed reference data for artificial dynamic irradi-ance reproduction (particularly for diurnal irradiance variationsimulations).

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664 Q. Meng et al. / Opt

cknowledgments

This work was supported by the National Natural Science Foun-ation of China (Grant no. 51208059), the Special Fund for Basiccientific Research of Central Colleges, Chang’an University (Granto. 0009-2014G1291074), and the Open Foundation of the Stateey Laboratory of Fluid Power Transmission and Control (Grant no.ZKF-201215).

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