1. Introduction
Due to the increasing demand for clean energy, the application of solar energy has become an indispensable form of alternative energy use in modern society [1,2]. Nowadays, a solar-driven thermochemical reaction system using concentrated solar radiation for hydrogen and syngas (mainly H
In reaction process, the concentrated solar energy is transmitted into the reactor inner cavity through a transparent quartz glass window installed on the front surface of reactor, which provides the necessary energy for reaction process. However, the processes of cracking and reforming would not happen until the operating temperature reaches at least 1000 K [12]. During the experiment, we have found a significant problem that the quartz glass window might break due to high heat flux. In the meanwhile, the Al
In view of the aforementioned problems, the thermal stress of solar thermochemical reactor using concentrated solar radiation is investigated in this article to find out the broken conditions of solar thermochemical reactor and provide useful references for further study.
2. Methods
Solar radiation convergence model
Figure 1 shows the process of light convergence from the solar simulator to the solar thermochemical reactor under ideal conditions. This process including model building and numerical simulation was finished by the TracePro70 software with Monte Carlo ray tracing (MCRT). Based on the current experimental conditions and simulation requirements, the power of Xe lamp is set to be 6 kW. The first focal length c
Solar thermochemical reactor model
Figure 2 indicates the structure of solar thermochemical reactor. As shown in this figure, the whole reactor is made of Al
The heat transfer and fluid flow characteristics of solar thermochemical reactor were obtained by the FLUENT software. In order to simulate the incident radiation more realistically, DO model was selected to figure out the radiation heat transfer. Considering the small import fluid velocity of 0.005 m/s, the laminar flow model was chosen to calculate flow characteristics. The absorption and scattering coefficients of quartz window were ignored due to high transmission. The incident heat flux calculated by TracePro70 software is used as the thermal boundary condition for the heat transfer and flow simulation. The other boundary conditions are given as follows:
Gas inlet:
Outside wall:
Outlet:
Due to the high working temperature environment, the thermal capacity of air is defined as [13]:
(1)
The conductivity is [13]:
(2) Moreover, the thermal stress analysis of solar thermochemical reactor was accomplished by ANSYS Mechanical APDL 16.0 software after obtaining the results of heat transfer and fluid flow characteristics. Besides, the other physical parameters for simulation are given in Table 1.
Table 1
Thermal–physical and structural properties of materials [14–17].
3. Results
Incident heat flux of reactor
Figure 3 indicates the contour and x-axis energy distribution of incident heat flux on the quartz window front surface calculated by TracePro70 software, respectively. As shown in this figure, the spots at the center present several perfect concentric circles, which indicates that the amount of the light used in the simulation with MCRT is sufficient for the accuracy. According to the heat flux distribution data, a Gaussian-based energy density formula can be fitted by OriginPro8 software as follow format:
(3) When the luminous flux is set to be 6 kW, the aforementioned formula can be expressed as:
(4) According to the fitting result, the value of R-Square is 0.99986, which is accurate enough for the simulation.
Heat transfer and flow characteristics
The analysis of heat transfer and fluid flow characteristics of solar thermochemical reactor was finished by Fluent software, and the incident heat flux formula was added by Users-Defined Functions (UDFs). Grid independence tests have been carried out before the calculation. Figure 4 shows the temperature and velocity distribution contours of solar thermochemical reactor, respectively. As indicated in Figure 4, the incident radiation mainly focused on the areas near aperture due to the effect of light convergence.
Analysis of thermal stress
This section will discuss the thermal stress of quartz window and Al
The result of von Mises stress distribution of Al
4. Conclusion
According to the simulation results, the incident heat flux on the front surface of solar thermochemical reactor has a symmetrical Gaussian distribution, which can be programmed as heat boundary condition into the Fluent software by UDFs. Based on the analysis of thermal stress, it is recommended to increase the distance between gas inlet and thermocouple socket to prevent stress concentration. Besides, the sharp edge of ceramics is also one of the places that are prone to thermal stress.