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This paper presents characterization of a new high flux solar simulator consisting of a 10 kW Xenon arc via indirect heat flux mapping technique for solar thermochemical applications. The method incorporates the use of a heat flux gauge (HFG), single Lambertian target, complementary metal oxide semiconductor (CMOS) camera, and three-axis optical alignment assembly. The grayscale values are correlated to heat flux values for faster optimization and characterization of the radiation source. Unlike previous work in heat flux characterization that rely on two Lambertian targets, this study implements the use of a single target to eliminate possible errors due to interchanging the targets. The current supplied to the simulator was varied within the range of 120–200 A to change the total power and to mimic the fluctuation in sun's irradiance. Several characteristic parameters of the simulator were studied, including the temporal instability and radial nonuniformity (RNU). In addition, a sensitivity analysis was performed on the number of images captured, which showed a threshold value of at least 30 images for essentially accurate results. The results showed that the flux distribution obtained on a 10 × 10 cm2 target had a peak flux of 6990 kWm−2, total power of 3.49 kW, and half width of 6.25 mm. The study concludes with the illustration and use of a new technique, the merging method, that allows characterization of heat flux distributions on larger areas, which is a promising addition to the present heat flux characterization techniques.

Variable aperture mechanisms are being used in many fields including medicine, electronics, fluid mechanics, and optics. The main design characteristics of these aperture concepts are the use of multiple blades regulating aperture area and consequently the incoming medium flow. Manufacturing complexities primarily depend on the concept geometry, material, and the process application requirements. Design of a variable aperture demands meticulous methodology and careful consideration of the application field. This paper provides an in-depth methodology on the design of a novel iris mechanism for temperature control in high temperature solar thermal receivers and solar reactors. Such methodology can be used as a guideline for iris mechanisms implemented in other applications as well as in design of different apparatuses exposed to high temperature. Optical simulations in present study have been performed to demonstrate enhanced performance of the iris mechanism over conventional Venetian blind shutter serving as optical attenuators in concentrating solar power systems. Results showed that optical absorption efficiency is improved by 14% while reradiation loss through the aperture is reduced by 2.3% when the iris mechanism is used. Correlation for adaptive control of aperture area was found through computational surface area measurement. Experimental testing with a 7 kW solar simulator at different power levels demonstrated the performance of the mechanism to maintain stable temperature under variable flux.

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Current state-of-the-art development of concentrated solar power (CSP) applications target cost-effective and highly efficient processes in order to establish commercialization of these technologies. The design of solar receivers/reactors and their respective flow configuration have a direct impact on the operational performance of the solar thermochemical processes. Thermal efficiencies, reaction kinetics and other key output metrics are the intrinsic result of the chosen configuration. Therefore, reactor design optimization plays a crucial role in the development of solar thermochemical applications. In this study a computational fluid dynamics (CFD) model of a directly-irradiated cavity receiver has been developed. The CFD-domain is coupled with incoming radiation that is obtained by using Monte Carlo Ray Tracing (MCRT). Experimental campaigns of the cavity receiver were carried out using a 7 kW High Flux Solar Simulator (HFSS) as radiative source. Temperature readings were obtained at different locations inside the cavity receiver for both wall and gas temperatures. In order to mimic naturally changing insolation conditions, the HFSS was run at different power levels. Heat flux at the aperture of the solar receiver was experimentally characterized. The acquired heat flux maps validated the intermediate results obtained with the MCRT method. The coupled computational model was validated against the measured temperatures at different locations inside the receiver. Computed temperature contours inside the receiver confirmed the experimentally observed non-uniformity of the axial temperature distribution. The validated analysis presented in this paper was then used as a baseline case for a parametric study. Design optimization efforts were undertaken towards obtaining temperature uniformity and achieving efficient heat transfer within the fluid domain. Enhanced flow circulation was achieved which yielded temperature uniformity of the receiver at steady state conditions. The outcome of this parametric analysis provided valuable insights in the development of thermal efficient solar cavity receivers. Hence, findings of this study will serve as a starting point for future solar reactor design. For example, it was found that reversing flow direction has an adverse effect on the temperature uniformity inside the receiver. Similarly, increasing the inlet angle does not positively affect the temperature distribution and hence should be chosen carefully when designing a solar reactor.

Variable aperture can assist in maintaining semi-constant temperatures within a receiver's cavity under transient solar loading. An in-house code has been developed to model a receiver and effectively control its components to achieve semi-constant temperatures under transients. The code consists of a full optical analysis performed via the Monte Carlo ray tracing method in addition to a transient two-dimensional heat transfer analysis. The system studied consists of a cavity type solar receiver with 60 mm radius fixed aperture on the cavity body, a variable aperture mechanism mounted on the receiver's flange, and a 7 kW Xenon arc solar simulator. A composite shape consisting of a hemisphere attached to a cylinder is proposed to model the Xenon arc. The in-house code has been experimentally validated through experimental tests for different input currents to the solar simulator, volumetric flow rates, and aperture's radii. The optical analysis was validated based on heat flux measurements, where it had percentage errors of 0.8, 0.5, 1.1, and 3.2% for the peak power, total power, half width, and half power. For the heat transfer model, percentage errors of 3.2, 2.9, and 5.3% at the inlet, center, and outlet sections of the receiver were determined for different flow rates using maximum input current and opening radius. The aperture mechanism was capable of maintaining an exhaust temperature of 250°C based on actual Direct Normal Irradiance data. Results showed that the variable aperture is a promising apparatus even in applications where the maximum temperatures are desired based on an observed optimum radius of 57.5 mm.

Solar thermochemical process technology has great potential to store solar energy as chemical fuels, however, there remain several challenges that have hindered its industrial commercialization. One of the main challenges is the transient nature of solar energy which causes instability and reduces the efficiency of the process. A promising solution to cope with this problem is to use a variable aperture mechanism to regulate the light entry into solar receiver. A robust control algorithm is required to automatically adjust the aperture size and keep the temperature semi-constant under various transient conditions including short or long cloud coverage and natural variation of solar radiation from sunrise to sunset. In our previous work, an adaptive predictive controller was developed for aperture size adjustment and its performance was evaluated by computer simulations.The present work takes our previous study to the next level by experimental evaluation of the proposed controller on a solar receiver radiated by a 7 kW solar simulator. Two different variable aperture mechanisms, namely as iris mechanism and rotary aperture, are used for adjustment of the light entry into the receiver. Experimental results indicate that both of the mechanisms have reasonable performance in response to changes to a setpoint and disturbance in incoming solar radiation. However, the iris mechanism exhibits superior performance due to its capability of continuously changing the aperture size. For an elaborated evaluation of the iris mechanism, a real day of solar irradiation was simulated in the lab by changing the power level of the solar simulator based on a normal irradiance profile of a sunny day. According to the experimental results, the required temperature control was achieved with a maximum error in the temperature setpoint less than 1.95°C.

In solar thermal research, there is a lot of attention has been paid on optimal design of solar reactors and cavity receivers in order to overcome inherent technical challenges. While many of these studies focus on the geometry and flow optimization of the reactor/receiver, few have focused on control of light entry into the cavity via auxiliary embedded mechanisms. Because the natural fluctuation of solar radiation affecting the thermal behavior of these reactor/receivers during daytime, it is important to address this challenge. In order to cope with transient nature of solar energy, several modular devices have been conceived. They are designed based upon concepts featuring control of set of blades or interchanging apertures to adjust light entry. These techniques yield promising performance on maintaining the desired reactor temperature and solar to fuel efficiencies. Towards that effort, an extensive fundamental background in designing, manufacturing, and testing of such mechanisms has been accumulated by our research group. This paper provides an overview of three successful mechanical apertures developed by our group to regulate flux entry into solar thermal receivers. The overview covers methodological aspects commonly encountered during the design process. A thorough comparison of performance records of these mechanisms are given per their actuation methods. Effect of each aperture mechanism and their individual change of cross-sectional shape is being evaluated through extensive optical simulation. The simulation results give insights on their respective optical performance by quantifying their radiative energy gain and losses. The paper also presents a numerical method coupling optical model to a thermodynamic analysis yielding accurate estimation of reactor temperature per experimental validation. Finally, numerical simulation showed that closed loop control of aperture size can efficiently regulate the temperature throughout a sunny day. Results of this research highlight the advantage of adopting variable apertures in solar cavity receivers.