Research PaperExperimental study of the energy and exergy performance for a pressurized volumetric solar receiver
Introduction
With rapidly increasing energy prices and globalization, process industries seek opportunities to reduce production costs and improve energy efficiency. Among the energy-efficient technologies, Concentrated Solar Power (CSP) system is considered as one of the most attractive ways to solve the energy crisis in the future [1], [2]. Many developed countries like the United State and the European Commission have been devoted to the solarized Brayton micro-turbines system over the past decades [3], [4], [5].
Compared to the traditional gas turbine, solarized Brayton turbines use a solar receiver to replace the combustion chamber in the traditional gas turbine [6]. The solar concentration part which is used to provide high temperature air is very crucial for the entire solar power system. The system efficiency and the cost of the power generation are highly depended on the solar concentration conversion efficiency from solar radiation to the thermal fluid. Thus, the solar concentration part has to be well designed in order to achieve high efficiency and low pressure loss. Many studies have been devoted to the design and performance of the receiver. Neber and Lee [7] designed a high temperature cavity receiver using silicon carbide. Then a scaled test section was placed at the focal point of a parabolic dish collector and reached a maximum temperature of 1248 K. Lim et al. [8] designed a tubular solar receiver with a porous medium and found the optimal design point of the proposed solar receiver concept to heat up compressed air. The results of this study offer a valuable design guideline for future manufacturing processes. Wu et al. [9] developed a novel particle receiver concept for concentrating solar power (CSP) plants. Special attention was paid to the effect of rotation on convective flow in a cylindrical cavity with heated side walls for solar applications. Buck et al. [10] introduced a receiver module consisting of a secondary concentrator and a volumetric receiver unit which was closed with a domed quartz window to transmit the concentrated solar radiation. Hischier et al. [11], [12] proposed a novel design of a high-temperature pressurized solar air receiver for power generation via combined Brayton–Rankine cycles. It consists of an annular reticulate porous ceramic bounded by two concentric cylinders. The heat transfer mechanism was analyzed by the finite volume technique and by using the Rosseland diffusion, P1, and Monte–Carlo radiation methods. It was found that, for a solar concentration ratio of 3000 suns, the outlet air temperature can reach 1000 °C at 10 bars, yielding a thermal efficiency of 78%.
It is recognized that the flow and heat transfer processes in the solar receiver are very complicated. Over the past years, many studies have been devoted to the optimization of the design using theoretical and numerical methods. Tu et al. [13] studied a saturated water/steam solar cavity receiver with different depths by adopting a combined computational model. Various trends of thermal efficiency and heat loss with depths were obtained. A suitable cavity depth was finally found for the receiver. Wang and Siddiqui [14] developed a three-dimensional model of a parabolic dish-receiver system with argon gas as the working fluid to simulate the thermal performance of a dish-type concentrated solar energy system. Wu et al. [15] presented and discussed temperature and velocity contours as well as the effects of aperture position and size on the natural convection heat loss. Their study revealed that the impact of aperture position on the natural convection heat loss is closely related to tilt angle, while the aperture size has a similar effect for different tilt angles. Hachicha et al. [16], [17] proposed a numerical aerodynamic and heat transfer model based on Large Eddy Simulation (LES) modeling of parabolic trough solar collectors (PTC), and verified the numerical model on a circular cylinder in the cross flow. The circumferential distribution of the solar flux around the receiver was also studied. von Storch et al. [18] proposed a process for indirectly heated solar reforming of natural gas with air as heat transfer fluid. Different solar receivers were modeled and implemented into the reforming process.
On the other hand, many numerical research works are also conducted to simulate the detail heat transfer process. Flesch et al. [19] numerically analyzed the impact of head-on and side-on wind on large cavity receivers with inclination angles ranging from 0° (horizontal cavity) to 90° (vertical cavity) and compared with the data published in the open literature. Yu et al. [20] performed a numerical investigation on the heat transfer characteristics of the porous material used in the receiver of a CSP with different structure parameters. The effects of different boundary conditions were revealed. Tu et al. [21] proposed a modified combined method to simulate the thermal performance of a saturated water/steam solar cavity receiver. Capeillere et al. [22] numerically studied the thermomechanical behavior of a plate solar receiver with asymmetric heating. The numerical results showed that the choice of the shape and levels of the solar irradiance map is crucial. The distribution of the most relevant incident solar flux and the geometry compromise were determined. Wang et al. [23] conducted a numerical study focusing on the thermal performance of a porous medium receiver with quartz window. Their results indicated that the pressure distribution and temperature distribution for the condition of fluid inlet located at the side wall was different from that for the condition of fluid inlet located at the front surface. Roldan et al. [24] carried out a combined numerical and experimental investigation of the temperature profile in the wall of absorber tubes of parabolic-trough solar collectors using water and steam as the heat-transfer fluids. A good agreement between the measured and computed thermal gradient was achieved.
Exergy analysis has been applied in various power studies. In the authors’ earlier studies [25], [26], a coiled tube solar receiver had been designed and tested in the real solar radiation condition. But due to the limitation of the tube material, the coiled tube solar receiver could not achieve very high temperature. Thus, a pressurized volumetric solar receiver using metal foam as thermal absorbing core is designed in this work. It appears from the previous investigation that the key point for the solarized Brayton micro-turbines is to develop solar receivers which have exemplary performance on the pressure loss and heat transfer. To the best of the authors’ knowledge, there is a lack of available experimental data under real concentrated solar and variable mass flow conditions especially for the cases of extremely high heat flux and high temperature. To this end, the present research is aimed to experimentally analyze both the efficiency and heat loss of a pressurized volumetric solar receiver under real solar radiation and variable mass flow conditions in more detail.
Section snippets
Experimental apparatus
The experimental study was conducted at a location with the geographical position of 30°36′ latitude and 120°22′ longitude, Hangzhou, China. The whole system, shown schematically in Fig. 1, mainly consists of three components: dish, compressor and receiver. The dish used for the experimental tests of the developed solar heat receiver is shown in Fig. 2. All 40 trapezoidal, pre-bent mirrors are resin molded and laminated. The reflective surface is applied as an adhesive foil. At the bottom of
Uncertainty analysis
The uncertainties of the measurements in the experiment are dependent on the experimental conditions and the measurement instruments. An uncertainty analysis is performed on the receiver power and the receiver exergy , which are the most important derived quantities from the measurements when using the propagation of error method described by Moffat [31]. The uncertainty of the receiver power could be calculated by the following equation:
Results and discussion
Fig. 6 shows the variation of the solar irradiance (G) during a test period from 10:00 am to 13:30 pm. The experimental data were collected on November 6th, 2015, which falls in the local autumn season in Hangzhou, China. According to Fig. 6, it is shown that the solar irradiance fluctuates around 600 W/m2. The solar irradiance data increases slowly with time except two fast drops observed in the afternoon for about 15 min. The reason could be due to the fact that two short periods of passing cloud
Conclusions
This paper performed an experimental study to investigate the thermal performance of a pressurized volumetric solar receiver under real solar radiation conditions. In order to design a high efficiency solar receiver, some important parameters such as different porous material, the size of the quartz window, the shape of the cavity, should be selected carefully. In the current work, a parabolic dish with solar tracker system is well designed and the obtained results are analyzed using energy and
Acknowledgement
The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 51206164).
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