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An efficient 3.168 kW solid-state solar thermal simulator (SSSTS), capable of supplying ~30,000 suns at a focal plane via a fibre optical delivery, has been developed. The source consists of 41 diode lasers, each operated at a wavelength and power of ~915 nm and ~80 watt, respectively. The SSSTS provides a semi-top hat radiation profile and can be focused to a diameter of ~10.5 mm. The electro-optical power efficiency of the SSSTS was evaluated to be 55%, where the maximum value of the uniform radiation flux exceeds 36.6MW/m2. As such, the present technology is relevant to solar thermal applications that are not wavelength-sensitive or where narrow line-width is desirable. Additionally, the fibre optical delivery feature enables ease of direction onto a suitable target, without the need of large ellipsoidal reflectors usually employed for the conventional arc solar simulators. To demonstrate the new SSSTS, ZnO:Zn particles were introduced into the path of the radiation to investigate the change in their own temperature by using the laser induced phosphorescence technique (LIP). The temperature of the averaged particles was measured at a different radiation flux over a ~87 mm2 cross sectional area. A change in the average particle temperature of up to 225°C was detected within the measurement volume when the SSSTS was operated at a flux of ~30,000 suns. The unique characteristics of the SSSTS, namely, the uniformity, high power flux, efficiency, ease of delivery, and precise control of the radiation flux responds to the current demands of solar thermal research.
© 2016 Optical Society of America
1. Introduction
Devices designed to simulate concentrated solar have been developed to deliver high flux radiation suitable for high temperature thermal processing. These devices typically employ xenon-arc lamps because the relatively short arc enables a higher flux to be generated than is possible from the metal halide counterpart, although this is gained at the expense of a spectrum that is further from their real solar spectrum [1]. Electric-arc solar simulators are useful for development of solar reactor technology as they achieve relevant fluxes, where recent technologies were able to achieve fluxes in the order of 11 MW/m2, while being much more reliable and easy to operate than real solar concentrators [2, 3]. They are also useful for investigating the net effect of CSR on a specific phenomenon where a close match with the solar spectrum is important [4]. However, they suffer several disadvantages relative to laser sources for the fundamental study of the interactions between high flux radiation and combustion-related phenomena. For example, the broadband nature of the solar spectrum (real or simulated) makes it impossible to isolate the wide range of potential mechanisms that can be excited by high flux radiation. Furthermore, the lamps do not generate a true point source, which makes it extremely difficult to obtain either a detailed distribution of the flux intensity within a given volume or to perform any corrections for phenomenon such as flux attenuation. Lasers offer wavelength specificity and coherence, as well as well-defined distributions, which can be useful for these investigations. In addition, the ongoing cost reductions of light emitting diodes (LED) has now result in the cost of solid-state lasers to be competitive with solar simulators as a heating source. In recent years, new laser technologies have been developed with the potential to combine the above-mentioned advantages [5], though none have been capable of achieving power outputs that can rival the electric-arc simulator of Petrasch [3]. However, the present work describes the development and application of a new fully solid-state radiation source coupled to a fibre optical head to provide easy delivery of radiative power equivalent to ~30,000 suns. It also demonstrates the system experimentally through heating of particles in suspension. The uniformity of the current radiation profile provides distinctive advantages when compared to conventional xenon-arc lamps which typically have “hot-spots” due to the way the radiation are overlapped. These “hot-spots” have the potential to cause damages to solar reactors when operated at high flux, of which the current Solid-State Solar Thermal Simulator (SSSTS) eliminates.
2. Experimental setup
The experimental section consists of three main parts - the solid-state laser source, the thermal solar system arrangement and the particle temperature measurement setup.
2.1 The solid state laser source
The schematic diagram of the radiation source is shown in Fig. 1. The system consists of 41 fibre-coupled diode laser modules (Nlight, Model e06.0900915200) and one pilot guide laser, resulting in a total of 42 lasers. Each nLight diode laser produces ~80W with a power efficiency of 45-55%. The guide-laser power and wavelength are 16mW and 658 nm respectively. The 42 Laser-Fibre-Modules (LFM) are divided into six groups, I-VI, with each group consisting of 7 LFMs. The seven LFM (200μm core and 220μm clad diameter) of each group are then spliced into a 400μm combiner, with typical splicing efficiency better than 99%. The 6 combiners outputs were then combined into a custom-built fiber combiner with a 1500μm fibre output, which was then tapered to 1350μm fiber to maximize the brightness potential. The fiber face was then coupled into a high power end cap for robustness and waste heat management. The typical efficiency of the splicing and bundling is better than 99% and 97% respectively. The multi-laser source employs integrated cooling plates for the Modules and for the fiber splices/combiners. A water chiller (PolyScience 6500 Series Chiller) with working temperature between −10°C and 40°C was used to maintain the cooling plates at relatively low temperature by ensuring operating water temperature was 18°C.
Fig. 1 Solid-State Solar Thermal Simulator (SSSTS) controller system comprised of 6 groups (I-VI) of 6 individual Laser Fibre Modules (LFM), where the LFMs labelled 1-6 are power-delivering, and the centre module in group VI is an in-built visible pilot laser to envision the output radiation form the Fibre-Optic Head.
To generate a controlled source of radiation, the output of the 1350μm fibre was collimated by an adjustable three-lens system, with effective focal length and diameter of 80.2mm and 50mm respectively. The translation distance between the fibre-face and the nearest lens face was ± 4mm. With the fibre-face placed at 64.8mm away from the nearest face lens, it is possible to produce a beam waist of near-constant 10.5 mm diameter at a distance of ~500 mm away from the front of Fibre-Optic Head (FOH). The current supplied to each group is achieved via a PC. Once the desired current is input to a group, the group temperature and the voltage will be displayed by the PC.
2.2 Description and characterisation of the solid-state solar thermal simulator (SSSTS)
The Fibre-Optic Head (FOH) was integrated into a metal enclosure (740mm x 780mm x 1200 mm) with a front metal roller door, as shown in Fig. 2. A UV silica plate (Diam = 52.8 mm) was placed at 300 mm away from the front of FOH to reflect 8% of the incident radiation to a water-cooled power meter (Gentec model HP100A-4KW-HE). The small reflected power was used as an active radiation power monitoring. The main radiation was directed to pass through a fluidised bed and stopped by a second water-cooled power meter (Gentec model HP100A-4KW-HE).
Fig. 2 Schematic diagram of experimental setup to characterise the Solid-State Solar Thermal Simulator (SSSTS). P: polarizer, λ/2: half waveplate, I: iris, M: mirror, ThC: thermocouple, L: lens, FC: fibre cable, BS: beam splitter, BD: beam dump, MFC: mass flow controller, FB: fluidized bed.
When operated at high power, it was observed that the temperature of the FOH could approach temperature of 70°C. A constant flow of air was therefore provided to the outside of the FOH surface to maintain operating temperature between 18°C and 30°C, as measured with a thermocouple attached to the outside of the FOH housing. The radiation profile of the SSSTS was recorded using a digital SLR camera, (Nikon D5500 with Tamron f = 90 mm Macro lens) by viewing the beam at the surface of the power meter. The resulting beam profile was observed to have a near top-hat profile, as seen in Fig. 3.
2.3 Application of the solid-state solar thermal simulator
To demonstrate the capability of the new source, the SSSTS was employed to heat particles suspended in a fluidised bed as particle-heating by radiation is the central most important component in solar particle receivers [6, 7]. A mix of ZnO:Zn thermophosphor particles and CaSO4·2H2O particles were fluidised in a 32mm diameter bed, designed to transport particles to within a chamber in which radiation is directed through two circular apertures, as shown in Fig. 4. Two additional circular apertures, aligned orthogonal to the radiation propagation axis, were used to monitor the temperature of the particles. The outlet of the fluidised bed was connected to a particle trap before being connected to an exhaust line. A constant dry airflow of 5.5 L/min was required to maintain stable particle fluidisation.
Fig. 4 Close-up of particles heated by 910nm SSSTS beam and excited by 355 Nd:YAG laser sheet through optically-accessible fluidised bed, the phosphorescence signal of which is collected via fibre-optics cable.
The surface temperature of the heated ZnO:Zn particles were measured by a non-intrusive laser diagnostic technique namely, laser induced phosphoresce LIP [8–10]. It should be noted that although the fluidised bed contained a combination of ZnO:Zn and Gyprock particles, the temperatures of only ZnO:Zn particles were able to be measured due to its thermo-phosphorescent properties. A third harmonic Nd:YAG laser (Quantel Q-smart 850) laser operated at 355nm was then directed into a polariser, half-waveplate, and subsequently a three-lens system to form an excitation sheet of 20 mm height used to excite the ZnO:Zn thermophosphor particles. The position of the Nd:YAG laser sheet was arranged such that it was at a 7° angle with respect to the SSSTS beam, intercepting at the centre of the fluidised bed, as shown in Fig. 4. A two-lens light-collecting channel, connected to a 7 fiber round-to-linear bundle (Thorlabs, BFL200HS02) was used to direct the emission to a 0.5 m spectrometer equipped with a 300 grove per mm grating (Acton Spectra pro Princeton Instruments). The resolved spectra were recorded using an ICCD camera (Princeton Instruments-Max4). Throughout the investigation, temperature from three thermocouples (Type K) connected to the outside of the FOH, the body of the fluidised bed and the inside metal box respectively were monitored. The three thermocouples were connected to data logger (Pico Technology USB TC-08) to a PC for storage.
3. Results and discussion
The exact laser wavelength of the diode lasers used has a weak dependence on the diode temperature [5]. As such, it was necessary that the SSSTS wavelength be properly characterised accordingly. Figure 5 presents the change in wavelength with the laser temperature. It can be seen that the wavelength increases with increasing operating temperature. It should be noted that the “operating temperature” in this case is defined as the temperature collected via an in-built thermocouple within the SSSTS system and is function of both the input current and time of which the SSSTS is turned on for. In the present case, the peak wavelength of the emitting radiation is compared against the operating temperature of Laser-Fibre-Module (LFM) Group IV, TIV. Figure 5 shows that, within the operating temperature range of the SSSTS, the peak wavelength ranged from 904nm to 918.5 nm. The wavelength change of the SSSTS can thus may be characterised by the equation:
Fig. 5 Change in the wavelength of the emitting radiation as function of the diode laser surface temperature.
The characterisation of the operation of the solid-state solar thermal simulator (SSSTS) can be defined by three parameters. These are the optical power output, Qout,optical, the temperature of optical fibre head (OFH), TOFH, and the SSSTS efficiency, 𝜂, which is defined by,
where Qin,elec is the power calculated by multiplying the current and the voltage supplied. It is important to monitor the TFOH to ensure it does not surpass 30°C during operation. To calculate 𝜂, the voltage and optical power have been recorded as function of the electrical current, I. Fig. 6 presents the electro-optical conversion efficiency, η, laser power output, Qout,optical, and F.O.H. temperature, TFOH, of the SSRS as a function of I. It can be seen that each LFM group behaves slightly differently, i.e. they are not completely identical. Fig. 6(a) shows the calculated η of the different LFM groups when I was varied from 2A to 17A. The η of each diode laser is between 45% and 55%. The efficiency is seen to reach a maximum at Iin = 6A, with Group VI having highest overall maximum efficiency of 55.34% and Group I having the lowest at 47.18%. Figure 6(b) depicts the laser power output, Qout,optical, from the different LFM groups as a function of I. The power output increment was found to increase with I consistent with expectation. The power outputs from LFM Groups I – V are observed to be consistent with each other, as they each have 7 nLight diode lasers supplying an average of ~80W to the system. In contrast, the LFM Group VI which only has 6 power-suppling nLight diode has a much lower power output. This is caused by the difference in FOH temperature, as can be observed from Fig. 6(d).Fig. 6 The Solid-State Solar Thermal Simulator (SSSTS)’s (a) electro-optical efficiency, η, (b) optical power output, Qout,, (c) electrical input power, Qin, elec, and (d) fibre optical head temperature, T, as a function of the input electrical current, I. 
The calculated radiation flux as a function of I and laser diameter, dlaser is presented in Fig. 7. It can be seen that at the minimum possible dlaser of 10.5mm, the maximum heat flux was recorded to be 36.6 MW/m2, equivalent 30,000 times the maximum direct solar irradiation. The diameter of the 3.168 kW maximum output radiation is a factor of five smaller than that of a typical lamp-based solar simulator, while the area is smaller by a factor of 25 [3, 11]. The diameter of the region of uniform radiation flux, equivalent to ~30,000 suns, is 10.5 mm, which is a factor of 4.1 times the highest reported for a lamp-based simulator [3].
Fig. 7 The calculated radiation flux as a function of current and three different diameters, circle: 10.5 mm, square: 20 mm; diamond: 40 mm.
The practical operation of the SSSTS is demonstrated through the heating of particles suspended in a fluidised bed, using laser induced phosphorus (LIP) as the temperature measurement technique. Continuous stream of dry air of mass flow rate of 5.5 L/min was fed into the fluidised be to maintain stable fluidisation. It should be noted that the usage of dry air was necessary to avoid particle agglomeration. A mix of two particles, namely CaSO4·2H2O and ZnO:Zn Thermophosphor, was placed in the fluidised bed, as seen in Fig. 4. ZnO:Zn Thermophosphor was used because its emission spectra profile is highly temperature-dependent [12, 13]. The CaSO4·2H2O particles used have diameters ranging from 100μm to 200μm, while the diameter of the ZnO:Zn particles used ranges from 1μm to 50μm, where sized by the Mastersizer 2000 (Malvern Instruments). The ratio of the ZnO:Zn to CaSO4·2H2O particles was maintained at 10:90 by volume. After the fluidisation within the fluidised bed had stabilised, the third harmonic from, a Nd:YAG laser (Quantel Q-smart 850) laser, operated at 355nm and energy of 4.25mJ, was used to excite the TPs. The SSSTS was then used to heat the particles within the fluidised bed cavity. To obtain accurate particle temperature measurement, it is important that the measurements be taken in situ and is well-resolved.
The investigation was performed when the fluidisation in the bed is stabilised by firstly collecting 400 single shot spectra when the SSSTS was switched off. The SSSTS was then turned on, at flux of ~0.28MW/m2, for ~1min to allow for thermal equilibrium within the fluidised bed before a separate set of 400 single shot spectra was collected. Next, the SSSTS was switched off again and allowed to cool over ~2mins, at which point another set of spectra was taken. The process was repeated with the flux varied from 0.27MW/m2 to 36.6 MW/m2, where a set of spectra at 0 W/m2 was performed at every interval. The recoded spectra were analysed to observe the wavelength shift of the peak. Figure 8 shows the on/off peak spectra wavelength of ZnO:Zn at flux 3.1 MW/m2, 20MW/m2 and 36.6MW/m2. This clear shift in emission spectra indicates a change in particle temperature.
Fig. 8 Peak wavelength of ZnO:Zn emission spectra subjected to heat flux of: (a) 3.1MW/m2, (b) 20MW/m2, (c) 36.6MW/m2. Each set of measurement consists of a case in which the SSSTS is turned off and one in which is it turned on.
In order to relate the emission profile to its temperature, a set of systematic temperature-dependence measurements were performed [14]. Based on this, a calibration curve in the range of 293K – 573K was obtained, and the particle temperatures were inferred from the emission spectra collected using the spectrometer. 400 single-shot spectra were recorded at each set, the average of which are presented in Fig. 9 as a function of laser heat flux. It can be seen that at a flux of ~34.8MW/m2, the average particle temperature observed is ~250°C. This flux exceeds the maximum possible value of 30,000 suns estimated for a parabolic dish concentrator, is twice as high as that measured in practice by Lovegrove et al [15] and is significantly higher again than that achieved from a heliostat field flux at the focal plane. This is important for research because it demonstrates a better-defined point source, which enables the more reliable development and validation of numerical models. Furthermore, this system possess a fibre optical delivery feature enables the ease of radiation projection onto a suitable targets.
Fig. 9 The average temperature of all particles within the irradiation volume measured with non-intrusive LIP technique.
4. Conclusions
A high power and efficient radiation source delivering 3.168 kW via a single fibre optical probe with a three lens collimating system. 41 diode lasers, each is operating at wavelength and power of ~915 nm and ~80 Watt respectively. It was found that it is possible to achieve a semi top hat radiation profile and can be focused to diameter down to ~10.5 mm, with a maximum power equivalent to 30, 000 suns. The maximum measured elector-optical power efficiency of the radiation source was 55%. The source was used to heat ZnO:Zn particles were introduced into the path of the radiation. When operated at flux of 30,000 suns, the average particles temperature of the entire particles was 250°C. This works clearly demonstrate the shift of particle emission. This was used to evaluate the average temperature of all particles with the viewing volume. Although there was no attempt addressing the heating and cooling processes of the irradiated particles, the finding clearly demonstrate the possibility of direct particle temperature measurements. The unique characteristics of the SSSTS namely, uniformity, high power flux, efficiency, accurate control of the flux level and ease of delivery, responds to the current demands of solar thermal research.
Funding
Australian Research Council (DP15012230).
References and links
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: Group I; 
: Group II; 
: Group III; 
: Group IV; 
: Group V; 
: Group VI; 
: Groups I – V; 
: Group VI.