An Experimental and Numerical Study on the Heat Transfer Driven Dynamics and Control of Transient Variations in a Solar Reactor

University of Minnesota M.S.M.E. thesis.July 2019. Major: Mechanical Engineering. Advisor: Nesrin Ozalp. 1 computer file (PDF); xiv, 156 pages.There is a major challenge in utilization of concentrating solar power for stable energy conversion processes due to fluctuating nature of available irradiation. This challenge can be handled by development of a robust control system that is capable of absorbing fluctuations in the sun’s irradiance without significantly changing the flow dynamics. In this thesis, a heat transfer driven model predictive control system is developed for advanced control of changes in solar flux by implementing a heat exchanger coupled variable aperture mechanism to capture spilled radiation. Experimental testing of the system was performed using a new 10 kWe xenon arc high flux solar simulator which was fully characterized in this thesis. Experimental results showed that the aperture mechanism can maintain the temperature of the reactor within ± 5 °C under severe radiation fluctuations during a cloudy day. Therefore, the aperture mechanism is a promising alternative to current traditional temperature control methods

Solar-Thermal Production of Hydrogen and Graphitic Carbon via Methane Decomposition

Current hydrogen and carbon production technologies emit massive amounts of CO2 that threaten Earth’s climate stability, especially as demands for these materials continue to grow. Compared to alternative clean hydrogen production technologies, solar methane pyrolysis has lower energy requirements, produces carbon materials of commercial interest, and provides higher process efficiencies. In this work, a new solar-thermal methane decomposition process involving flow through a fibrous carbon medium to co-produce hydrogen gas and high-value graphitic carbon product with zero CO2 emissions is presented and thoroughly investigated. A 10 kWe custom-designed and built solar simulator is used to instigate the methane decomposition reaction with direct irradiation in a custom solar reactor. In contrast to prior work on solar methane pyrolysis, the present process reaches steady-state thermal and chemical operation from room temperature within the first minute of irradiation due to localized, direct solar heating of fibrous medium. Additionally, the present approach provides enhanced thermal transfer and efficiency, delivers graphitic carbon product in an easy to handle and extract form, and prevents undesired carbon deposition within the reactor that would otherwise lead to process interruption. These aspects are ongoing challenges reported in prior literature. In contrast to similar methane decomposition prior work that reports production of amorphous carbon black, this work produces high-quality graphite with production rates that are order(s) of magnitude higher. Parametric variations of methane inlet flow rate (10-2000 sccm), solar power (0.92-2.49 kW), operating pressure (1.33-40 kPa), and medium thickness (0.36-9.6 mm) are thoroughly presented, with methane conversions as high as 96% and graphite Raman D/G peak ratios as low as 0.06. A pathway to process scale-up for continuous production is presented by implementing a roll-to-roll processing method, which was effective in achieving continuous processing with methane conversion enhancements up to 1.5 times higher. The optical design of the solar reactor was then optimized using a secondary concentrator, by which solar-to-chemical efficiencies increased by up to 62% to reach a maximum demonstrated efficiency of 6.5%. Realizing this process at scale would avoid emissions of 10 kg of CO2 per kg of H2 and 5 kg of CO2 per kg of graphite

Solar-Thermal Production of Hydrogen and Graphitic Carbon via Methane Decomposition

Current hydrogen and carbon production technologies emit massive amounts of CO2 that threaten Earth’s climate stability, especially as demands for these materials continue to grow. Compared to alternative clean hydrogen production technologies, solar methane pyrolysis has lower energy requirements, produces carbon materials of commercial interest, and provides higher process efficiencies. In this work, a new solar-thermal methane decomposition process involving flow through a fibrous carbon medium to co-produce hydrogen gas and high-value graphitic carbon product with zero CO2 emissions is presented and thoroughly investigated. A 10 kWe custom-designed and built solar simulator is used to instigate the methane decomposition reaction with direct irradiation in a custom solar reactor. In contrast to prior work on solar methane pyrolysis, the present process reaches steady-state thermal and chemical operation from room temperature within the first minute of irradiation due to localized, direct solar heating of fibrous medium. Additionally, the present approach provides enhanced thermal transfer and efficiency, delivers graphitic carbon product in an easy to handle and extract form, and prevents undesired carbon deposition within the reactor that would otherwise lead to process interruption. These aspects are ongoing challenges reported in prior literature. In contrast to similar methane decomposition prior work that reports production of amorphous carbon black, this work produces high-quality graphite with production rates that are order(s) of magnitude higher. Parametric variations of methane inlet flow rate (10-2000 sccm), solar power (0.92-2.49 kW), operating pressure (1.33-40 kPa), and medium thickness (0.36-9.6 mm) are thoroughly presented, with methane conversions as high as 96% and graphite Raman D/G peak ratios as low as 0.06. A pathway to process scale-up for continuous production is presented by implementing a roll-to-roll processing method, which was effective in achieving continuous processing with methane conversion enhancements up to 1.5 times higher. The optical design of the solar reactor was then optimized using a secondary concentrator, by which solar-to-chemical efficiencies increased by up to 62% to reach a maximum demonstrated efficiency of 6.5%. Realizing this process at scale would avoid emissions of 10 kg of CO2 per kg of H2 and 5 kg of CO2 per kg of graphite

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

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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

Characterization of a New 10 kW(e) High Flux Solar Simulator Via Indirect Radiation Mapping Technique

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Solar-thermal production of graphitic carbon and hydrogen via methane decomposition

This work reports a process in which concentrated irradiation from a simulated solar source converts methane to high-value graphitic carbon and hydrogen gas. Methane flows within a photo-thermal reactor through the pores of a thin substrate irradiated by several thousand suns at the focal peak. The methane decomposes primarily into hydrogen while depositing highly graphitic carbon that grows conformally over ligaments in the porous substrate. The localized solar heating of the porous substrate serves to capture the solid carbon into a readily extractable and useful form while maintaining active deposition site density with persistent catalytic activity. Results indicate a strong temperature dependence with high decomposition occurring in the central heating zone with concentration factors and temperatures above 1000 suns and 1300 K, respectively. Even with a large flow area through regions of lower irradiation and temperature, methane conversion and hydrogen yields of approx. 70\% are achieved, and 58\% of the inlet carbon is captured in graphitic form