A Novel Iris Mechanism for Solar Thermal Receivers

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.status: publishe

Two-Dimensional Transient Heat Transfer and Optical Analysis of a Solar Receiver

Transient nature of solar radiation creates challenges in operating solar reactors requiring semi-constant cavity temperatures. A promising approach to overcome this problem is to implement a variable aperture. This paper presents a heat transfer model of a solar reactor featuring a variable aperture. The model is an in-house developed code comprising transient two-dimensional heat transfer analysis. The model is coupled with an optical analysis based on the Monte Carlo Ray Tracing (MCRT) or the Radiosity Net Exchange (RNE) method to simulate radiation within the receiver. The model is also coupled with a fluid dynamics analysis based on implementing a staggered grid system and the SIMPLE algorithm to obtain the velocity field of the fluid flow. Validation of the model was made through experimental results obtained using a 7 kW high flux solar simulator with an input current of 155 A and a fully opened aperture at 60 mm radius. Results showed satisfactory accuracies given an experimental maximum uncertainty of ± 8.6°C. However, the MCRT method provided more accurate results. The difference between the experimental and the model’s steady state temperature values had an average of 6.3°C and 11.8°C for the MCRT and RNE methods, respectively. Based on the temperature distribution results, it was noted that the RNE method fails to predict accurate radiation distribution where two surfaces meet at perpendicular angles. Finally, the in-house code was used to determine an optimum aperture radius per highest possible exhaust temperature value which was 52.5 mm for the operating conditions outlined above.status: Published onlin

Monte Carlo Ray Tracing Coupled CFD Modeling and Experimental Testing of a 1 kW Solar Cavity Receiver Radiated via 7 kW HFSS

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Experimental performance comparison and optical characteristics of aperture mechanisms for solar cavity receivers

© 2019 International Solar Energy Society Dynamic control of solar thermal processes is a critical bottleneck demanding to be addressed for the development of viable alternatives to traditional industrial processes that rely on high temperature process heat from fossil fuel combustion. Current technologies possess various concentrated solar power (CSP) control strategies to compensate for natural fluctuations of solar irradiation while some of the dynamic control problems are addressed by creative reactor design to capture solar energy more efficiently. In this paper, a solar receiver with two light entry controlling aperture mechanisms is proposed as a promising control method for manipulated radiation entry into the cavity. Design and motion characteristics of these aperture mechanisms govern the amount of flux intercepted by the receiver. Performance of these apertures in maintaining semi-constant temperature inside the receiver was tested numerically and experimentally for comprehensive explanation of the physics behind the results. An optical analysis was done by implementing two different Monte Carlo Ray Tracing (MCRT) approaches for comparison and verification of complex aperture designs. Total radiative losses were determined by considering the energy balance on the quartz window by taking the spectral-dependent optical properties of the window into account. A thermal analysis was done in conjunction with the optical analysis to demonstrate thermal-optic performance of both mechanisms. A 7 kW high flux Xenon bulb was optically modeled according to experimentally obtained radiation maps. The optical model was coupled as an input to a thermal computation with lumped parameter assumption. Numerical results were validated by comparison with experimental results for both steady and dynamic responses. Estimation of a high flux scenario at steady state as well as for a given DNI pattern were numerically predicted and comparisons with conventional control methods were made. Results demonstrated the potential of the variable aperture in terms of temperature control and receiver efficiency.status: Published onlin

Numerical and experimental investigation of heat transfer in a solar receiver with a variable aperture

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Optimization of Design and Process Parameters for Maximized and Stable Solar Receiver Efficiency

Maintenance of stable process conditions and optimal solar thermal reactor efficiency demand direct response to transient behavior of solar radiation. This study presents a thermodynamic model including an explicit correlation between the feedstock flow rate and receiver aperture size to achieve optimal receiver efficiency under stable process conditions. Optical analysis of the system was made using TracePro software package yielding an optical model with absolute deviation for peak flux and half width of 7.6 W/m 2 and 0.375 mm respectively. Two different types of receiver aperture mechanisms were designed and manufactured to experimentally test the impact of aperture size, and the impact of different design concepts. The first one features translational motion, whereas the other concept uses a rotational disc mechanism. An experimental proof of both concepts has been studied using a 7 kW high flux solar simulator using these two variable aperture mechanisms. The TracePro model was used to determine the aperture opening factor (AOF) for both mechanisms, which was used to develop the thermodynamic model. In this model, maximum achievable reactor temperatures were observed for iris and rotary aperture and found to be 251°C and 325°C respectively, at 135 A of solar simulator input current. Optimum aperture size and optimum flow rate were identified for desired steady state reactor temperature at different flux levels.status: publishe

An Iris Mechanism Driven Temperature Control of Solar Thermal Reactors

In spite of their attraction for clean production of fuels and commodities; solar thermal reactors are challenged by the transient nature of solar energy. Control of reactor temperature during transient periods is the key factor to maintain solar reactor performance. Currently, there are few techniques that are being used to accommodate the fluctuations of incoming solar radiation. One of the commonly practiced methods is to adjust the mass flow rate of the feedstock which is very simple to implement. Another method is focusing and defocusing of the heliostats which requires careful control of the heliostat field. Although these techniques are very convenient and widely being practiced successfully, there are several drawbacks associated with each of them. For example, although the temperature inside a solar reactor can be easily controlled by varying the mass flow rate of the feedstock, it disturbs the flow dynamics inside reactor. This is a major problem for cases where the flow pattern must be maintained constant. Therefore, an alternative temperature control inside the reactor should be developed. This paper presents a promising approach where an iris mechanism is driven by a closed loop control system to adjust the area where solar energy enters the reactor.status: publishe

An experimental study on temperature control of a solar receiver under transient solar load

© 2019 International Solar Energy Society 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.status: Published onlin

A critical assessment of present hydrogen production techniques: is solar cracking a viable alternative?

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