COMPARISON STUDY OF AIR AND THERMAL OIL APPLICATION IN A SOLAR CAVITY RECEIVER

Nowadays, solar dish collector with a cavity receiver is accounted as an efficient and compact system for converting solar radiation energy into thermal energy. All of the incoming solar irradiation to the dish aperture area, is concentrated at the dish focal point where the solar receiver is located. In the current study, the thermal performance of the dish collector with a rectangular cavity receiver was evaluated. Air and thermal oil were examined as the solar working fluids. The performance of the solar dish collector was evaluated at different values of the mass flow rate ranging from 0.002 to 0.06 kg/s as well as different solar irradiation ranging from 600 to 1200 W/m. The results revealed that the collector efficiency improved with increasing the mass flow rate and solar irradiation. The thermal performance of the solar dish collector improved with application of the thermal oil as the solar working fluid compared to the air in the investigated solar system. The results indicated the higher cavity surface temperature could be achieved by using air as the solar working fluid compared to the thermal oil.


INTRODUCTION
Recently, the renewable energy sources are the important subject for research due to the serious environmental problems such as the fossil fuel depletion, emissions of CO, CO2, global warming, and ozone depletion. Solar energy is accounted as a favorable renewable energy. Parabolic dish concentrator is a kind of efficient solar collector for converting the solar radiation energy to thermal energy or power producing. There are different kinds of absorber in the dish concentrators [1]. The cavity receivers due to their special structure are more efficiently compared to other kinds of dish absorber [2]. The solar cavity receiver absorbs the concentrated solar heat flux from the dish concentrator. The thermal heat losses from the cavity receiver are included the conduction, convection, and radiation heat losses [3].
Some researchers have investigated the performance of thedish collector using the cavity receiver numerically and experimentally. Taumoefolau et al. [4] modelled a cavity receiver with the fluent software. They investigated the effect of the cavity inclination angle on the convection heat losses. Good agreement was found between the measured and predicted values of the convection heat loss for the investigated cavity receiver. Steinfeld and Schubnell [5] proposed a semi-empirical method to determine the optimum aperture size as well as to optimum operating temperature of a solar cavity receiver. Bammert et al. [6] suggested the calculation principles of the cavity optimum dimensions for achieving the highest performance. Stine and Harrigan [7] considered the optical errors as well as the inaccuracies of tracking systems as typical errors in the solar concentrators. Sendhil and Reddy [8] investigated the performance of a fuzzy focal solar dish concentrator using three kinds of cavity receivers. They concluded that the modified cavity receiver is the preferred receiver type. Khalsa et al. [9] presented a method with CFD codes. This technique allows the incoming radiation to interact with participating media such as falling solid particles in a high-temperature cavity receiver.
The flow and heat transfer research of the cavity receiver can help to estimate the thermal performance and to optimize the design of the receiver significantly [10][11][12][13][14]. Harris and Lenz [15] investigated the thermal performance of a solar concentrating system with different shapes of the cavity receiver. Xiao et al. [16] were examined the thermal performance of a solar cavity receiver with and without glass cover. They considered the influence of mass flow rate as well as the flow direction of the air as the working fluid. Loni et al. [17,18] analytically investigated different shapes of cavity receivers. They presented the optimum structure of the cavity receivers. In another works, Loni et al. [19,20] thermodynamically and exegetically studied a solar ORC system using cavity receivers. They reported the effect of different structural and operational parameters on the ORC performance. Pavlovic et al. [21,22] numerically and experimentally examined a spiral cavity receiver as the solar receiver in the dish concentrator. The effect of wind speed was numerically and experimentally considered by [23]. They presented some models for prediction of the forced convection in a hemispherical cavity receiver. In some studies, researchers have studied the application of different nanofluids in cavity receivers [24,21]. Loni et al. [25] predicted the cavity thermal performance using ANN method. They showed a good prediction of the thermal performance using ANN method. In this study, a dish concentrator using a rectangular cavity receiver was thermally modeled. Air and thermal oil were used as the solar working fluid. A dish concentrator with aperture diameter of 1.8 m and 84% reflectivity was chosen in this research. A cavity receiver with the height equal to 1.5a, inner tube diameter equal to 5 mm, and the inlet temperature of the working fluid equal to 120°C were assumed. The performance of the solar dish collector was evaluated at different values of the mass flow rate ranging from 0.002 to 0.06 kg/s as well as different solar irradiation ranging from 600 to 1200 W/m 2 . The thermal performance of the dish concentrator using a cavity receiver was considered using two investigated solar working fluids. The solar dish performance was considered under variation of the mass flow rate as well as the solar irradiation. The results of the current study elicited which kind of working fluid is proper for different applications.

MODEL DESIGN AND METHODOLOGY Dish Collector Modeling
A rectangular cavity receiver is investigated in this study (see Figure 1). The working fluid flows from the bottom to the top of the investigated cavity receiver. According to previous research, the contributing parameters to the temperature profile and the heat flow on the receiver wall can be separated into two components: geometrydependent and temperature-dependent [17,18,21]. Their research has shown that the effects of the geometrydependent factors can be found with SolTrace software as an optical analysis tool. The temperature-dependent factors including radiation, convection, and thermal conduction losses can be calculated using a thermal model.

Optical Modeling
The optical analysis is conducted using the commercial software SolTrace. This tool uses the Monte Carlo ray tracing method to perform the optical analysis. The heat flux rate over each coil of the absorber is found separately and finally, the total absorbed heat rate is found by adding the absorbed heat rate of each coil. Table 1 gives more detail about the optical analysis of the rectangular cavity receiver.

Thermal Modeling
The rectangular cavity receiver is covered with insulation. The heat loss from the receiver consists of convection, radiation, and conduction heat losses. The height and inner tube diameter of the investigated cavity receiver are equal to 1.5a, and 5 mm, respectively.
The net heat transfer rate at the receiver tube is [27]: While the receiver efficiency is defined as = . .

Numerical Methods for Receiver Modeling
The surface temperature ( , ) and the net heat transfer rates ( , . ) at different elements of the tube were determined by solving Eqs. (6) and (7) using the Newton-Raphson Method [21,26].
The receiver surface temperature at different elements of the tube and the net heat transfer rate depending on the aperture size, the height of the cavity receiver, the mass flow rate of the solar working fluid, the receiver tube diameter, the working fluid inlet temperature and the dish reflectivity. The view factors for different tube sections are shown in Tables 2 which determined via the view factor relations available at [27]. Note that, for the analysis, the receiver tube of the rectangular cavity is divided into some sections as determined by Eq. (8) Also, the properties of the air as the working fluid were obtained by [19]. Figure 2, and Figs. 3a and 3b show the variation of the receiver efficiency, outlet temperature, and maximum cavity surface temperature versus the variety of the mass flow rate ranging 0.002 to 0.06 kg/s for air and thermal oil as the solar working fluid, respectively. From Figure 2, it could be resulted that the collector efficiency increased with increasing the mass flow rate of the working fluid. This result of the collector efficiency is verified by the literature [28,29]. As seen from Figure 2, the collector efficiency shows higher values using the thermal oil compared to the air as the solar working fluid. Also, it concluded form Figure 3, the outlet temperature of the working fluids and cavity surface temperature decreased by increasing the mass flow rate of the working fluid. The maximum surface temperature and outlet temperature using air as the solar working fluid shows the higher values compared to the application of the thermal oil as the solar working fluid. This is because the specific heat of the thermal oil is higher than the specific heat of the air.

Solar Irradiation
Figs. 4, 5 and 6 show the variation of the cavity heat gain, collector efficiency, and outlet temperature of the working fluid versus the variation of the solar irradiation ranging 600 to 1200 W/m 2 , respectively. The air and the thermal oil were used as the working fluid. As resulted from Figs. 4, 5, and 6, cavity heat gain, the collector efficiency and the outlet temperature increased with increasing the solar irradiation. This issue is due to higher heat flux absorption with increasing the solar irradiation. It can result from Figs. 4, and 5, the heat gain and collector efficiency for the thermal oil have higher values compared to the air as the solar working fluid. On the other side, as seen from Figure 6, the outlet temperature of the working fluid increased by application of the air as the solar working fluid compared to the thermal oil. This is due to the thermal oil has higher thermal capacity than the air. So, it could be 226 recommended that the thermal oil can be used as the solar working fluid for achieving higher thermal efficiency in the solar power systems as well as the air can be applied to achieving higher surface temperature in the Bryton cycles. Air as the working fluid Thermal oil as the working fluid Figure 6. Variation of the outlet temperature versus the solar irradiation for the air and the thermal oil as the working fluid at the mass flow rate of 0.01 kg/s CONCLUSION In the current study, a dish concentrator with a rectangular cavity receiver is thermally investigated. Air and thermal oil were examined as the solar working fluid. The performance of solar dish collector was evaluated at different values of the mass flow rate ranging 0.002 to 0.06 kg/s as well as different solar irradiation ranging 600 to 1200 W/m 2 . The results were concluded as followings: • The collector efficiency increased with increasing the mass flow rate of the working fluid.
• The outlet temperature of the working fluids and cavity surface temperature decreased by increasing the mass flow rate of the working fluid. • The collector efficiency showed higher values using the thermal oil compared to air as the solar working fluid. Whereas, the higher cavity surface temperature and the outlet temperature can be achieved by application of air compared to the thermal oil as the working fluid. • The collector efficiency and the outlet temperature of the working fluids increased with increasing the solar irradiation. • Finally, it could be recommended that the thermal oil can be used as the solar working fluid for achieving higher thermal efficiency in the solar power system as well as the air can be applied to achieving higher surface temperature in the Bryton cycle.