Development of High Flux Solar Simulators for Solar Thermal

Solar Energy Materials & Solar Cells 141 (2015) 436–446

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Solar Energy Materials & Solar Cells

journal homepage: www.elsevier.com/locate/solmat

Review Development of high ﬂux solar simulators for solar thermal research

Ben M. Ekman n, Geoffrey Brooks, M. Akbar Rhamdhani

Faculty of Engineering and Industrial Science, Swinburne University of Technology, Hawthorn, VIC 3122, Australia article info abstract

Article history: High flux solar simulators, used to produce controlled high temperature experiments are a valuable tool Received 16 March 2015 for the research and development of high temperature material processes. As an alternative to a direct Accepted 9 June 2015 external solar concentrator where the sun's radiation is concentrated using a parabolic dish, an indoor Available online 2 July 2015 solar simulator uses an array of high intensity discharge lamps attached to ellipsoidal mirrors to focus Keywords: their light at a secondary focal point where temperatures in excess of 2000 °C can be reached. To mimic, Solar energy as closely as possible, the spectrum of the sun, a novel high flux solar simulator design using metal halide Solar simulator lamps has been constructed. The 42 kWe simulator consisting of seven 6 kWe metal halide lamps fl Concentrated heat ux delivered a peak thermal flux of approaching 1 mW/m2 to the secondary focal plane of a closely coupled ellipsoidal reflector. A comparison of different designs and their performance is also presented in this paper. Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.

Contents

1. Introduction ...... 436 2. Design...... 437 3. Arc modelling and optical characterisation ...... 441 4. Fabrication...... 443 5. Flux measurement...... 443 6. Performance...... 444 7. Summary...... 446 Acknowledgement...... 446 References...... 446

1. Introduction resemble as closely as possible the spectrum of the sun and in the main has been a high intensity arc lamp coupled to an optical The application and design of solar simulators falls into two reflector resulting in the delivery of an intense beam of con- main classifications, non-concentrating uniformly distributed light centrated radiation with wavelengths in the UV, visible and used in the testing of photovoltaic (PV) cells and solar hot water infrared regions of the spectrum. As early as 1973 researchers [1] collectors and high flux concentrators used to generate high inserted a tungsten–halogen heat source positioned at one focus of temperatures exceeding 1000 °C used for a variety of research a suitably shaped ellipsoidal reflector to heat small objects placed applications including material processing, thermo-chemical at the secondary. High flux solar simulators first began to appear reactions, the production of solar fuels and in simulated solar in the early 1990's in the USA using xenon lamps [2]. Argon arc thermal power generation. The radiation heat source needs to lamps and high powered xenon lamps were used in 2003–2007 at ETH Zurich [3,4]. By 2012 high intensity solar simulators were

n used in research centres [5–8] that were capable of generating Corresponding author. Tel.: þ61 411604711/þ61 392144857. E-mail address: [email protected] (B.M. Ekman). intense fluxes and concentrations of over 4000 suns all using http://dx.doi.org/10.1016/j.solmat.2015.06.016 0927-0248/Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved. B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446 437 xenon lamps that were close coupled to a precision ellipsoidal The visible light emission constitutes about 25% of the total mirror. light output, with most of the energy falling into the infrared Optical configurations based on parabolic-shaped mirrors are spectral region. Approximately 70% of the xenon arc lamp output commercially available for large-scale collection and concentration occurs at wavelengths longer than 700 nm, while less than 5% of of solar energy for the generation of electrical power. The total the output consists of wavelengths less than 400 nm. Similarly amount of radiated power collected by any of these systems is with the argon arc lamp in Fig. 3 below showing low energy proportional to the projected area of the mirrors. Their arrange- emission in the visible light region of 380–700 nm and high ment depends mainly on the concentrating system selected and energy emission spikes in the 700–1000 nm range. the latitude of the site [9]. The most common configurations used By contrast the metal halide lamp spectral distribution is very for concentration of the sun's energy in solar thermal applications similar to that of natural sunlight shown in Fig. 4. The distribution are linear concentrators such as parabolic troughs and Fresnel profile is quite similar to that of sunlight except for a spike at 850– concentrators, or point concentrators such as the central solar 950 nm. The high energy component of the radiation is contained towers and parabolic dishes. An alternative reliable research tool is within the visible range of the spectrum from 400–800 nm. The required that is capable of providing an artificial source of con- metal halide lamp is an artificial light source that emits a radiation centrated energy with a spectral distribution as close as possible to spectrum that closely mimics that of natural sunlight. The intense that of natural sunlight. A high flux solar simulator will create the infrared energy spikes that xenon lamps emit require either forced constant conditions required for controlled high temperature air cooling for low wattage lamps or water cooling for higher experimentation. At the same time the design objectives must be powered lamps. In addition the reflectors are more prone to to simulate the parameters that would be encountered by high damage and may require forced air cooling to their surface. concentration solar systems capable of processing materials on With metal halide lamps by contrast approximately, 90% of the commercially viable scale including the spectral distribution and a electrical energy supplied to the lamp is converted to and radiated large focal hot spot of the reflected sun image. Metal halide lamps out as energy, and the remaining 10% is lost through ohmic effects are unique in how their spectral distribution closely replicates to the foils and electrodes. About 75% of the power consumed is sunlight and their larger arc length results in less intense regions radiated by the discharge arc itself. This radiation output, illu- of high flux [3]. strated in Fig. 5, is split up into the 10% UV, 45% visible and 20% This paper describes the design and fabrication of a high con- infrared radiation. Around 15% of the energy is emitted by the centration solar simulator using an array of seven high powered electrode and the bulb which can reach temperatures of more than metal halide lamps each coupled to a precision ellipsoidal 900 °C [10]. reflector. As the arc size within the lamp increase, the transfer efficiency of radiative energy originating at the arc that reaches the target is reduced [3,11]. This negatively affects the magnitude and dis- 2. Design tribution of the radiative flux in the target plane. For this reason even though the metal halide lamp emits a spectral distribution Factors that were taken into consideration during the design that more closely replicates sunlight, xenon lamps have been stage included costs, the emission spectrum of the lamp, the lamp chosen over metal halide. However the luminous efficacy, which is efficiency, lamp cooling, reflector size, shape and quality, reflector described as how efficiently a lamp converts electrical energy into surface coating, the positioning of the lamp within the reflector, visible light, has in the past been very limiting with little choice reflector support and safety. From a comparison of the lamps used but to choose a xenon lamp, this is no longer the case [10]. Xenon in high intensity solar simulators, the xenon lamp has clearly been lamps that have a power range of 4–6 kW have a luminous flux the preferred choice for two main reasons, the radiation spectrum range of 155,000–280,000 lm and an efficacy of 39–47 lm/W. The and the size of the arc light source. The spectral distribution of the power equivalent in the metal halide lamp has a luminous flux suns radiation is shown in Fig. 1. range of 380,000–600,000 lm and an efficacy of 95–100 lm/W The bulk of the energy is within the visible light wavelength of making the metal halide lamp more efficient in converting elec- 350–700μm sharply falling off in the UV region and a more gra- trical power to light in this size range. In addition xenon lamps dual fall in the near and far infrared. By contrast the xenon lamp operate under very high pressure and are dangerous in the event emission spectrum shown in Fig. 2 shows a flat low energy dis- of any exploding bulb. Metal halide lamps on the other hand tribution profile in the visible light region with a number of operates at much lower pressures and has a protective outer bulb intense spikes in the infrared [10]. for added protection. There is a direct relationship between the

Fig. 1. Solar spectral distribution incident on the earth's surface. (Reproduced from ASTM Terrestrial Reference Spectra). 438 B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446

Fig. 2. Spectral distribution of radiant energy emitted by the xenon lamps (reproduced from National High Magnetic Field Laboratory).

Fig. 3. Spectral distribution of radiant energy emitted by the argon arc lamp [reproduced from 4].

Fig. 4. Spectral distribution of radiant energy emitted by the metal halide lamp. The white line is the intensity distribution of the sun [reproduced from 10]. electrode gap, lamp voltage, operating pressure and luminous at the target is concentrated into a small spot, are difficult to efficacy of a discharge lamp [10] and as a general rule lamps with contain thus creating strong thermal gradients resulting in mate- small electrode gaps generally have low efficiencies. Our design rial thermal stress. Efforts are now being made by some [11], using uses an array of commonly focused lamps, each comprising of a optical mixers such as polished tube flux guides, to defocus the truncated ellipsoidal reflector closely coupled to a metal halide rays and produce a preferred more uniform flux density distribu- high intensity discharge lamp. An array of seven, 6000 W lamp/ tion. The metal halide lamps used in our design, because of the reflector modules, are arranged in a circular pattern. Although the longer arc length, will have a more uniform flux density dis- arc gap is significantly larger in a metal halide lamp, researchers tribution without the need for post defocusing equipment. using xenon lamps have found [11] that the temperatures pro- The lamp chosen as the radiation source is the OSRAM HMI duced by these high intensity fluxes generated by short arc lamps 6000 W/SE, Hydrargyrum Medium-arc Iodide metal halide lamp. resulted in a shorter life for the reflectors and because the energy The lamp dimensions and the technical details are shown in Fig. 6. B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446 439

15% radiation losses As shown in Fig. 7, the metal halide lamp consists of an inner and outer quartz glass envelope. The rounded 4 mm diameter ends of the cathode and anode are separated by a 23 mm gap. Although larger metal halide lamps are commercially available these would require auxiliary cooling which would add significant complexity to the design and increase the overall cost of the simulator. Each of the seven lamps was closely coupled to an ellipsoidal reflector. Having a multiple array of lamp/reflector units provides the flexibility required to allow varying power intensities at the target depending on the test circumstances. In order to concentrate the light emitted by the artificial light source, we utilise the fact that rays originating from a point source can be collected 5% ohimic losses 5% ohimic losses (foil + electrode) in their entirety on to a target point by placing the source and (foil + electrode) 10% UV 20% IR wavelength wavelength target points on the foci of an ideal highly reflective ellipsoid of 45% Visible (VIS) range range wavelength revolution. Unlike other geometric reflector shapes which have a <380nm >780nm range single focal point such as parabolic or spherical reflectors where 380-780nm collimated light may be focused, the ellipsoidal reflector has two Fig. 5. Energy conversion in a metal halide lamp as a percentage of the power input focal points. The ellipsoid shown in Fig. 8, takes light that is gen- [10]. erated at the primary focal point F1 and reflects it to F2 the secondary focal point. Precision made elliptical mirrors are therefore a highly efficient means of transferring radiation from the source to a target point. However since the source is not a point source but has a finite volume, the image at the secondary focal point is magnified to a degree depending on the source dimensions as well as the eccentricity of the reflector. In addition reflectors with higher eccentricity result in the arc location being closer to the reflector surface with potentially damaging consequences due to the higher thermal loads on the surface of the reflector [11]. Fur- ther, the focal length of the reflector is to be minimised to avoid magnification of specular errors, the orientation of the lamp is to be confined to within vertical 7135°, as per manufacturers spe- fi fl Fig. 6. HMI 6000 W/SE XS metal halide lamp [3]. ci cations, the distance between re ectors is to be minimised but within practical operational limits and the truncation diameter is to be maximised in order to capture the maximum amount of light. The effect of the truncation angle and the tilt angle are less pronounced [3]. One design challenge faced by this project was to mount each lamp at the focal point of the ellipsoidal reflector and to include a degree of focus adjustment. At the same time each reflector module must be fully adjustable to allow the beams to converge at the focal plane. Each lamp/reflector module must be supported by the frame and allow heat to dissipate. For the reflector geometry chosen and considering a point light source located at the primary focal point, the portion of light emitted by the lamp but not reflected by the elliptical reflector was calculated to be 37%, this is made up of rear opening losses of 19% plus front aperture opening losses of 18%. The portion of light emitted by the Fig. 7. Photo of the installed 6 kWe metal halide lamp showing the inner and outer quartz glass envelopes and the electrode gap. lamp which makes contact with the surface of the reflector is 63%.

Fig. 8. Complete ellipsoid geometry of the reﬂector design [12]. 440 B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446

A custom designed reflector with a smaller rear opening and a the light emitted by the lamp making contact with the reflector, longer truncation length would improve on this however, bespoke the result is that only 57–60% of the light from each lamp will designs incur significant costs and although produce improved reach the secondary focal point region at the target receiver. The results were cost prohibitive. remainder of the light will be scattered. By comparison the xenon The largest available elliptical reflector design produced was lamp generates 70% of its energy in the over 700 nm range where used for this simulator with a truncated diameter of 510 mm, a the reflectivity performance is 10% less at an average of 80% focal length of 1585 mm, a rear opening diameter of 140 mm, a (Table 1). fl re ector height of 267 mm and an eccentricity of 0.92 as shown in A comparison of installations of high flux solar simulators in Fig. 9. date order is listed in Table 2. Low flux simulators commonly used fl Elliptical re ectors can be made in a number of ways, most to test photovoltaic cells or other low to medium temperature commonly via the spinning of aluminium. A more accurate concentrators have not been included. Clearly the xenon lamp is method is by the electro-forming of nickel onto a precisely the most favoured light source used. machined and polished mandrel to form the highly accurate A point source of light located at the primary focal point will be ellipsoidal shape followed with surface coatings which results in reflected by the elliptical reflector to the secondary focal point different coefficients of reflectivity and wear resistance depending however all arc light sources have a dimension and the greater the on the coating material [12]. In this case the reflectors were made arc dimension, the greater the distance from the primary focal by electro forming of nickel and coated by electro vacuum point (located at the centre of the arc) to the end of the arc. This is deposition of aluminium followed by a surface coating of silicon demonstrated in Fig. 11 below. The resultant geometric flux at the oxide. With the thermal coefficient of expansion of aluminium target plane will be related to the light angle (from γ to α) which is nearly double that of nickel, heat induced reflector deformation of nickel electro-formed reflectors during operation is halved. The dependant on the size of the light source and the focal length of fl reflectivity of the surface as a function of wavelength is shown in the ellipsoidal re ector. Light at the secondary focal plane that falls Fig. 10. With the reflector losses encountered, the percentage out of the size of the receiving aperture would be additional lost reflectance curve in the 400–700 nm where most of the energy of scatter and result in further reduction to the concentration the metal halide lamp occurs averages 85–95%. With only 63% of efficiency.

Fig. 9. Cross-section of elliptical reﬂector [12].

Fig. 10. Reﬂectivity vs wavelength chart for polished aluminium with silicon oxide coating [12]. B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446 441

It is clear from Fig. 11, that the size of the light source and the are contained within a reactor. Initially these are small scale pro- focal length have a significant effect on the magnification of the totype reactor experiments however one of the goals would be to focused beam at the secondary focal plain.. The light souce mag- prove the viability for commercial size operations. Although there nification of a 15 mm arc with a 3000 mm focal length would are currently no commercial high temperature solar reactors that result in the equivalent magnification to a 23 mm arc with a are operational and processing material on a large scale, the solar 1500 mm focal length. In addition one can conclude that the outer simulator must mimic the paramaters of such commercial facility. region of a reflector (with smaller γ) generates a smaller and In this scenario the sun's rays are not parralel but have a finite sharper image of the source. The ellipsoidal reflector used is angular size of the suns disc of about 33 min of an arc or about express by Eq. (1) and with the known arc gap of the Osram metal 9.6 mrad.Therefore the reflected rays form an image of finite size halide lamp used of 23 mm, angles γ and α were 12° and 3.5° centred about the focus. The width or beam spread of the reflected respectively. sun's image is Δr on a plane normal to the reflected beam. Fig. 13 describes the geometry of the angle of the suns ray reflected from x2 y2 1 a parabolic mirror. 2 +=2 a b ()1 The width of the reflected beam at the focal plane is given by where, a¼34.0 and b¼13.5 and Eccentricity, e is 0.92. Eq. (2) For a lamp containing a 9 mm arc gap, angles γ and α were 4.5 ° ⎛∈ ⎞ Δ=rp2tan⎜⎟ and 2 ° respectively. Focal lengths of 1585 mm and 3000 mm, were ⎝ 2 ⎠ ()2 used to determine the size of the arc image magnification shown fi in the graph in Fig. 12. where p is the focal length. In addition to the nite angle of the Although focal length has a significant effect on the size of sun, additional errrors are introduced that include shape errors, fl image at the secondary focal plane and a shorter focal length is non-specular re ection, tracing errors and receiver alignment preferred, it is the arc length that has the most significant effect. A errors. In addition the suns intensity is not evenly distributed receiver aperature diameter of less than 330 mm will result in a across the disc.The combined effect of the errors was statistrically significant amount of radiation spillage and radiation shielding is determined (harris & duff) and found that 68% of the errors fall s s s required to protect the surrounding receiver hardware from within 1 standard deviation(1 ), 95% within 2 and 99% within 3 . damaging heat fluxes. A shorter focal length is limited as this Eq. (2) can now be rewritten as Eq. (3) creates other problems including a reduced working space and ⎛ σ ⎞ Δ=rp2tan⎜⎟ n greater tilt angles that are restricted by the lamp manufacture as ⎝ 2 ⎠ ()3 well as the increased difficulty of heat dissipation. A compound þ s þ s þ s parabolic concentrator (CPC) with an inlet diameter of 350 mm, where n equals 2 for / 1 , 4 for / 2 and 6 for / 3 with an acceptance angle of 31°, matching the rim angle of the For a focal length of 20 m the sun will have a beam spread of simulator, would enhance the flux concentration by a factor of 2 approximately 800 mm at the focal plane. This has been shown to [3]. be the case at the PROMES research facility in Odeillo France where the worlds largest 1830 m2 parabolic mirror with a 18 m Another important aspect that must not be overlooked is that focal length concentrates 1 MW of solar power onto a 800 mm the solar simulator is an in-door research tool that is to be used as diameter spot to produce about 10 MW/m2 at its focus [13]. Irre- an alternative to a large parabilic dish where the sun's rays are spective of the intensity of the hot spot which is relative to the collected and directed to the focal point and the high temperatures helliostat collection mirror area, the long focal length will produce a large hot spot diameter. Commercial solar reactors of the future Table 1 Specifications of the HMI 6000 W/SE lamp [3]. will similarly require large focal lengths for operational constraints and to enable the processing of commercial volumes of materials. Rated lamp wattage W 6000 Table 3 lists the parameters of the lamp/reflector array chosen Rated lamp voltage V 123 and how these compare to other published high flux solar simu- Rated lamp current A 55 Ignition voltage [cold/hot] kV s 10/40, Max 60 lator installations. Luminous flux lm 600,000 Colour temperature kV s 6000 Electrode gap mm 23 3. Arc modelling and optical characterisation Lamp length-l1 mm 360 Bulb diameter-d mm 75 LCL-a mm 210 Illumination systems depend greatly on the source character- istics. The design of a solar simulator requires the accurate

Table 2 Comparison of high ﬂux solar simulators – in date order [2–8].

Simulator Date Lamp type Power (kW/ Number of Total electric Delivered radiative Peak ﬂux Stagnation tempera- lamp) lamps power (kW) power (kW) (kW/m2) ture(K)

BERKELEY CA -KUHN 1991 Xenon 20 1 20 3 16,000 (4000) NASA RES CENTRE- 1996 Xenon 30 9 270 NR NR NR JAWORSKE ETH ZURICH-HIRSCH 2003 Argon 200 1 200 75 4250 2900 ETH ZURICH-PETRASCH 2007 Xenon 15 10 150 50 11,000 3700 UNI OF MINNESOTA- 2011 Xenon 6.5 7 45.5 7.5 3700 2740 KRUEGER UNI OF FLORIDA-ERICKSON 2012 Xenon 6 7 42 NR 4230 2942 DLR-GERMAN AEROSPACE 2012 Xenon 6 10 60 NR 4200 (2930) CENTRE NR – not reported (calculated) 442 B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446

Fig. 11. Scheme of an elliptical reﬂector and arc source size.

Fig. 12. Size of arc image at the secondary focal plane for an arc length of 9 mm and 23 mm and a focal length of 1585 mm and 3000 mm.

Fig. 13. Reflection of the sun's rays from a parabolic mirror. modelling of the source parameters in order to have the fabricated the asymmetry fully accounted. The difficulty is to obtain a true system agree with the design. For an arc source, often a spherical image of the arc. Fig. 14a is a photo of the lamp using a DIN 13 or a generic cylinder can be used to model the emission however shade glass filter with what appear to be a large spherical shaped arc sources tend to be deformed or bowed so the generic source arc. Fig. 14b shows the arc under a variable neutral density filter model is a poor representation of the actual arc source. In order to that permits the cylindrically shaped arc to be visible and distinct effectively model the simulator design with optical design soft- from the over exposed glow surrounding it. The volume of the ware, the most crucial parameter is the source model. Many lamp 23 mm long, 4 mm diameter cylindrical image of the arc source designs have existing ray files that have been determined either by under ND filtered light is significantly smaller than that of a the manufacturer or researchers which accurately model the light 23 mm diameter spherical source. source of the arc however there were no ray files available for the The centre of the arc is positioned at the focus of the ellipsoidal 6000 W metal halide lamp used. One method to model the arc and reflector and ray tracing is performed. The lamp/reflector design was generate a ray set or ray file is to capture the image of the arc with entered into FRED optical software [14] including the orientation of B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446 443

all 7 lamps together with their material properties. A selection is ) madeofthenumberofraystobetracedthroughtheoptical rim Φ

( mechanical system whilst encountering various optical interactions.

) Rim angle ﬁ ﬁ

e While the spectral de nition of the light source used has a signi cant effect on the accuracy of the simulation of the system, accuracy also increases with the number of rays traced however larger ray num- bers result in longer processing times. Over 1 million rays were Eccentricity( not reported (calculated) generated using FRED optical software. The results of ray tracing for a – single reﬂector are shown in Fig. 15a. NR The ray tracing modelling results shown in Fig. 15b indicate a ﬂ 2 Focal length (mm) peak ux of just under 700 kW/m . ) o 4. Fabrication α

Truncation angle( The schematic layout of the lamp/reﬂector array is shown in Fig. 16. The frontal view of 6 modules symmetrically arranged around a central module and a cross-sectional top view is shown. (mm) h The central module (no. 4) is ﬁx mounted and is only adjustable in ector

ﬂ the forward horizontal direction. Modules 3 and 5 are hinge Re height mounted allowing them to rotate sideways on a vertical axis. Modules 1, 2, 6 and 7 are able to be swivelled in both the vertical

(mm) and horizontal direction. d Fig. 17a, is an expanded 3D image of a single module showing ector dia- fl the support structure, the focus adjustment mechanism and the Re meter lamp socket. Each lamp/reflector module had a combined weight of 25 kg and required adequate support and strength to be safely fixed and held in position whilst maintaining their position during operation. Tem- peratures of the supporting structure were measured around the

3700 950 379 70 3000 0.87ﬂ 27.41 Stagnation temperature(K) lamp and re ector support after an hour of operation and reached 140 °C. The assembled array is shown in Fig. 17b. Other peripheral

) equipment includes the seven high voltage ignitors each wired to a 2 ux

ﬂ ballast or power supply. Each ignitor is required to be located in close proximity to the lamp socket and connected by specially insulate 6800 Peak (kW/m high voltage cable rated for an ignition voltage of 40,000 V during start-up. Each of the modules can be operated individually depending on the level of heat ﬂux required. In addition each of the modules can be positioned to target a single common focal point or arranged so as to not overlap but to cover a larger area within a volume

Delivered radiative power (kW) receiver. This would result in lower concentrated ﬂux intensities and amoreuniformﬂux distribution. . 7] , 6 Total electric power (kW)

, 5. Flux measurement [3 To avoid spectral errors in the measurement of the ﬂux distribution [15,16], ﬂux measurement involved a circular foil heat

Number of lamps transducer also known as a Gardon type water cooled (Model:

ux simulators Vatell TG1000-1) heat flux gauge. The thermal flux measured by fl the transducer is proportional to the temperature difference between the centre of the foil disc and the edge. The disk is 6 7 42 15.3 1122 2100 508 283 50 1585 0.918 31 Power kW/lamp bonded to a circular opening in a cylindrical heat sink. The foil is made of Constantan and the heat sink is Copper. Exposing the thermogauge transducer to heat flux produces a voltage that is type halide ector array of high proportional to the thermal flux. The thermogauge is coated with a fl high temperature colloidal graphite with an emissivity of 0.82 and 2011 Xenon 6.5 7 45.5 7.5 3700 2740 750 346 59.1 2032 0.89 37.7 2014 Metal 2012 Xenon 6 7 42 NR 4230 2942 800 456 50 1800 (0.87) 26 2007 Xenon 15 10 150 50calibration 11,000Av constant of 0.782 that compensates for the spectral absorptance of the coating [15,16]. The gauge, mounted on an aluminium plate, provided continuous heat flux readings while incremental movements were made in the x, y and z directions. Care was taken to ensure that the surface coating was not damaged as well as ensuring that convective heat loss was mini- PETRASCH KRUEGER ERICKSON BURNE UNI. mised to maximise gauge accuracy. The flux measurements taken ETH ZURICH- UNI OF MINNESOTA- Simulator Date Lamp UNI OF FLORIDA- EKMAN - SWIN-

Table 3 Parameters of the lamp and re for each individual lamp are shown in Fig. 18a. 444 B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446

Fig. 14. (a) Image of a metal halide lamp using a Din 13 shade ﬁlter. Arc is not visible but appears to be spherical in shape. (b) Image of the metal halide lamp taken under a variable ND ﬁlter. Arc is now visible and cylindrical in shape.

Fig. 15. (a) Results of ray tracing for a single reﬂector. (b) Model of the distribution of ﬂux at the focal plane.

Fig. 16. Schematic of the positioning of the 7 module array, front and side views.

Each of the lamp/modules reached a peak flux ranging from 6. Performance 117 kW/m 2 to 148 kW/m2. The total thermal flux with all lamps focused to a common point is shown in Fig. 18b, reaching a peak of A theoretical calculation of the stagnation temperature based 2 927 kW/m2 and with an aperture diameter of 175 mm the input on a measured maximum heat flux value of 927 kW/m is given by power received is 12 kW. Eq. (2). The stagnation temperature is the highest temperature B.M. Ekman et al. / Solar Energy Materials & Solar Cells 141 (2015) 436–446 445

Fig. 17. (a) Expanded 3D image of a single module including mounting, support structure, lamp socket and reﬂector. (b) Solar simulator at Swinburne University consisting of seven metal halide lamps each coupled to a precision ellipsoidal reﬂector.

Fig. 18. Distribution of the thermal flux (a) each of the seven lamps measured at the focal plane. (b) with all lamps seven combined. Units are in watts/m2. that an ideal blackbody receiver would achieve when the energy is delivered through a 175 mm diameter aperture, an average being absorbed as fast as it is re-radiated reflectivity of 89%, a non-scattered light factor of 63% and only 50% of the arc light entering a 175 mm diameter aperture, correlates 4 qT= σ ()2 well to rated power of 6046 W per lamp. This results in a lamp to fi where q is the radiated flux and s is the Stefan–Boltzmann aperture (175 mm) transmission ef ciency of 25%. An installed CPC fi constant. would result in a transmission ef ciency of 51%. Experiments are fl Such high thermal fluxes would result in a corresponding currently being conducted where these high thermal uxes are directed into a hybrid designed receiver/reactor with varying stagnation temperature of 2011 K. By installing a CPC and configurations and the thermal effects are observed and measured. assuming this would increase the flux concentration by a factor 2 Average annual solar DNI or direct normal irradiation is the [3], the resulting stagnation temperature is 2390 K and an input quantity of direct solar radiation per unit area that is intercepted power of 24 kW. This demonstrates the effect of the T4 where large by a flat surface that is at all times pointed directly at the sun. fl increases of heat ux are required to produce relatively small Whilst on average the annual solar DNI for Melbourne are in range increases in temperature. of 1600–1800 kWh/m2, local hourly measurement were of parti- Individual lamp currents and voltages were not measured cular interest in this study. For purposes of comparison, random fi however according to Osram speci cation [10], the rated lamp measurements of solar flux using a TES 1333 Solar Power Metre voltage is 124 V and lamp current is 55 A (6820 lamp wattage). were taken on clear days during the months of August and Sep- Allowing 10% for fluctuations, power factor losses and ohmic los- tember from 10 am to 4 pm directed at the sun and ranged from ses at the electrodes (Osram specs), all the power can be 900 W/m2 to 1112 W/m2. On cloudy days this was substantially accounted for. Experimental measurements and calculations have less. From this it is fair to assume an amount of one solar constant verified that on average each lamp delivered 1714 W of power or 1000 W/m2 as the local Melbourne solar resource. Therefore to 446 B.M. 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Acknowledgement

The authors would like to thank the Australian Renewable Energy Agency, (ARENA) Australian Government (PhD Scholar- ship) for their ﬁnancial support.