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 1 and the second one c 2 are 100 and 1000 mm, respectively, so that we can obtain the eccentricity of ellipse e, the value of which is 0.818. For convenient calculation, the solar thermochemical reactor is substituted with a circular incident surface, so that we can show the incident heat flux more clearly.

Figure 1

Schematic diagram of light convergence process.

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 2 O 3 ceramic thermal insulation for protecting the reactor and reducing heat loss during the progress of thermochemical reactions. The concentrated solar energy will be transmitted into the reactor inner cavity through the transparent quartz glass window installed on the front surface of the reactor, and the reactant gas will be led into the reactor inner cavity by four opposite inlets in the meanwhile, so that the oxidation-reduction reaction occurs under the action of catalyst in the lumen. In order to obtain the reaction temperature, two thermocouples are installed in the light incident area and reaction area, respectively.

Figure 2

Structure diagram of solar thermochemical reactor.

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: Tf=300K

Outside wall: h=5W/(m2·K)T air =300K

Outlet: ∂Tf∂x=∂Tf∂y=0

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.

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.

Figure 3

Contour (left) and x-axis energy distribution (right) of incident heat flux.

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.

Figure 4

Temperature and velocity distribution contours of solar themochemical reactor.

Analysis of thermal stress

This section will discuss the thermal stress of quartz window and Al 2 O 3 ceramics thermal insulation, respectively. To simplify the calculation, the way to add displacement load was adopted in this article. Figure 5 indicates the von Mises stress of quartz window. Note that only the edge portion of the quartz window was loaded, because only this portion is connected to the fastener. As shown in this figure, the maximum stress appears in the junction of the load portion and free portion, followed by the central area. When the thermal stress reaches 128 MPa, the quartz window will be broken, so it is safe under the current conditions.

Figure 5

Von Mises stress distribution of quartz window.

Figure 6

Von Mises stress distribution of Al 2 O 3 ceramics thermal insulation.

The result of von Mises stress distribution of Al 2 O 3 ceramic thermal insulation is shown in Figure 6. It can be observed that most of the thermal stress is concentrated on the front of the reactor, especially in the incident radiation areas. Obviously, the maximum stress of 12.5 MPa occurs in the middle of the gas inlet and thermocouple socket. Due to the smallest thickness in this part, it is more likely to break. This phenomenon was also observed in previous experiment. Thus, it is recommended to avoid the situation, where two holes are too close to each other. Moreover, the edge of radiation entrance, as well as the vicinity of the aperture, is also the region of stress concentration. Although the compressive strength under ideal conditions is 850 MPa, the ceramics still can be broken because of the high temperature environment, excessive local concentration thermal stress, etc.