Modeling and simulation of 1 MWe solar tower plant’s solar flux distribution on the central cavity receiver

Abstract

The solar flux distribution rule inside a central cavity receiver is of great significance to the safe operation of solar tower power plants. In this paper, a heliostat field model was fully developed to simulate the solar flux distribution on the inner surfaces of a cavity receiver of a solar tower power plant by means of the Monte-Carlo ray-tracing method. In addition, the mathematical modeling process that starts from the incident solar rays to the absorbed energy by the inner surfaces of the cavity receiver was presented in detail. According to the final layout of the heliostat field, a dynamic simulation of the solar flux inside the cavity receiver during the vernal equinox was performed. The results indicated that the incident energy reflected by the heliostat field was mainly distributed on the rear and lateral surfaces throughout the day. Moreover, at different time points, the solar flux distribution rule inside the cavity receiver was also analyzed in detail. In order to verify the validity of this model, the simulation results were taken to compare with the experimental data of a random heliostat. Furthermore, to further testify the accuracy of our model, the simulation results obtained by inputting the coordinates of the CESA-I’s heliostat field into our model were also taken to compare with the published experiment data. Ultimately, both of the comparative results show that they can be good references for the safe design of the whole system.

Introduction

The accelerating trends of global warming and energy crisis are forcing current governments to pay more attention to renewable energy sources. Gradually, more and more countries have recognized concentrated solar power (CSP) as a clean, highly efficient and large-scale renewable energy technology. The DAHAN, 1 MWe demonstration solar power tower plant in China, supported by the Ministry of Science and Technology (MOST), is a key listed project of the 11th Five Year Plan of China National Hi-Tech R&D (863 Plan) [1]. The objective of the demonstration is proving the feasibility of solar tower technology. The success of this plant will have a far-reaching influence on the development of solar thermal power technology in China. Because this plant is still under construction, a simulation of the whole system by modeling each part of this system, which can be used to verify the feasibility of its design and study future system characteristics, is necessary.

In the DAHAN plant, the heliostat field, which mainly plays a role in concentrating and reflecting low-density solar irradiation onto a central cavity receiver to heat the working medium flowing in the heating panels, is a critical solar tower plant equipment. In addition, its solar flux distribution rule on the inner surfaces of the cavity receiver has a great impact on the safe operation of the whole system. Therefore, research on the distribution rule of incident solar flux on the receiver heating panels is crucial.

Previous scientific studies about the heliostat field model propose many calculation tools and analytic methods. In general, with respect to the developed tools, these investigations can be approximately grouped into two types. The first group is simply based on the energy balance necessary to develop the heliostat field model [2], [3], [4], [5], [6]. The most representative tool is the “STEC” module of the “TRNSYS” platform. In these studies, the heliostat field model of solar power tower plants was roughly expressed as the product of DNI (Direct Normal Irradiation), concentrating area as well as heliostat field efficiency. Although the research results could provide the amount of concentrated energy for the central receiver system, they failed to obtain detailed solar flux distribution information on central receiver heating panels. The second group mainly refers to some special calculation codes such as DELSOL3 [7], WinDELSOL1.0 [8], SOLTRACE [9], MUEEN [10], and SENSOL [11]. While the adopted methods of some tools also belong to the method of energy balance (such as ray-tracing method), they are quite different from those mentioned above. In fact, the chief function of these codes is to perform the design and optimization calculation of the heliostat field layout. Relying on these codes, the static distribution of the concentrated solar flux on the target or the aperture of cavity receiver at some time points can be calculated after a long computing interval. However, the dynamic variation of solar flux varying with time inside the cavity receiver is difficult to obtain. Moreover, these calculation codes cannot meet the demand of real-time simulation. With regard to the analytic methods, the computational process of the solar flux reflected by a single heliostat can be formulated in four views [12], [13], [14], [15]: the incoming ray formulation; the mirror plane formulation; the solar disk formulation and the pin-hole formulation. Nevertheless, these methods usually lead to a slowly converging numerical problem.

Besides, although some relative papers have been published regarding the same “DAHAN” power plant, such as Yao et al. [6], Wei et al. [16], [17] and Xu et al.[18], there are some differences worth noting. As mentioned in the first group, the “TRNSYS” software is adopted as the main simulation platform in Yao’s paper, and the heliostat field model is simply expressed as the product of DNI, concentrating area as well as the total efficiency. Therefore, information about cavity receiver solar flux distribution is hardly obtained. With respect to Wei’s papers, the main purpose of the papers is to provide a new method for the design and optimization of the heliostat field layout, and a new tool named “HFLD” is finally developed. As such, solar flux distribution is not the focal point in his papers. Furthermore, although Xu et al. [18] have also done important work regarding the modeling and simulation of the heliostat field, this paper mainly focuses on the whole system, which means the modeling process for the heliostat field is mentioned casually, while detailed information about solar flux distribution on the inner surfaces of the cavity receiver is also not revealed. However, in the current study, in order to study cavity receiver solar flux distribution, a heliostat field model was fully and clearly developed by means of the Monte-Carlo ray-tracing method to simulate the dynamic variation rule of the incident solar flux.