In this paper we present a dynamic three-dimensional volume element model (VEM) of a parabolic trough solar collector (PTC) comprising an outer glass cover, annular space, absorber tube, and heat transfer fluid. The spatial domain in the VEM is discretized with lumped control volumes (i.e., volume elements) in cylindrical coordinates according to the predefined collector geometry; therefore, the spatial dependency of the model is taken into account without the need to solve partial differential equations. The proposed model combines principles of thermodynamics and heat transfer, along with empirical heat transfer correlations, to simplify the modeling and expedite the computations. The resulting system of ordinary differential equations is integrated in time, yielding temperature fields which can be visualized and assessed with scientific visualization tools. In addition to the mathematical formulation, we present the model validation using the experimental data provided in the literature, and conduct two simple case studies to investigate the collector performance as a function of annulus pressure for different gases as well as its dynamic behavior throughout a sunny day. The proposed model also exhibits computational advantages over conventional PTC models-the model has been written in Fortran with parallel computing capabilities. In summary, we elaborate the unique features of the proposed model coupled with enhanced computational characteristics, and demonstrate its suitability for future simulation and optimization of parabolic trough solar collectors.

The Combustion and Solar Energy Laboratory (C&SEL) at San Diego State University is developing a Small Particle Heat Exchange Receiver (SPHER) to absorb and transfer heat from concentrated solar radiation to a working fluid for a gas turbine. The SPHER is to be used with a Concentrated Solar Power (CSP) system where a heliostat field highly concentrates solar radiation on the optical aperture of the SPHER. The solar radiation is volumetrically absorbed by a unique carbon nanoparticle gas mixture within the cavity of the SPHER. This research focuses on comparing a Computational Fluid Dynamics (CFD) model using the ANSYS FLUENT Discrete Ordinates (DO) Model and a program developed by the C&SEL which uses a Monte Carlo Ray Trace (MCRT) method to calculate the spatial and directional distribution of radiation for an idealized solar receiver geometry. Previous research at the C&SEL has shown successful implementation of the MCRT method to calculate the spatial and directional distribution of radiation for an idealized solar receiver geometry. The MCRT method is highly accurate and will serve as the benchmark solution for this research. However the MCRT code takes several days to run, is inflexible to geometry changes, and is cumbersome to implement as the MCRT code needs to be rewritten for each new receiver geometry being considered. These factors necessitate the need to find an alternate method that accurately calculates the spatial and directional distribution of radiation for a solar receiver and can be efficiently implemented for various receiver geometries being studied. The Discrete Ordinates method is a new method for solving the Radiative Transport Equations (RTE) using a FORTRAN program, developed by the C&SEL, and the ANSYS FLUENT Discrete Ordinates model for calculating the RTE. The FORTRAN program calculates the proper inlet radiation boundary conditions that ANSYS FLUENT uses for calculating the RTE. The methodology used for determining the correct CFD mesh, radiative boundary conditions, optimal number of DO theta and phi discretization, as well as the optical properties of the working fluid will be presented in this paper. The main focus of this research is to compare two different methods for solving the Radiative Transport Equations within the idealized SPHER. The solution data for several cases using the previous coupled MCRT method and the ANSYS FLUENT Discrete Ordinates method is presented for both a collimated and diffuse gray radiation approximation. The case studies focus on researching how the MCRT method and Discrete Ordinates method differ when comparing critical receiver parameters such as the mean outlet temperature, wall temperature profile, outlet tube temperature profile, and total receiver efficiency while keeping the total inlet radiation flux of 5 MW and inlet mass flow rate of 5 kg/s constant. This research also presents a study on the optimal Discrete Ordinates angular discretization, as well as a study to determine the solution’s dependence on the number of inlet boundary conditions imposed on the window.

The design of a secondary concentrator for the Small Particle Heat Exchange Receiver (SPHER) using a Monte Carlo Ray Tracing (MCRT) method is discussed in this paper. Applying basic MCRT rules, a modular solver logic for secondary concentrators is established. The logic is coded into FORTRAN subroutines to be compatible with MIRVAL, a ray trace code for heliostat fields created by Sandia National Laboratories. Based on a 3D Compound Parabolic Concentrator (3D-CPC) example the power of the simulation based on the Sandia heliostat field calculations is demonstrated. The results of the simulations are used to calculate the solar flux distributions in the ideal 3D CPC inlet and outlet planes as well as the incident ray distribution hitting the secondary concentrator. Code modifications to account for surface irregularities and spectral reflectivity are implemented in the appropriate FORTRAN subroutine. Using the automated simulation routines first the optimal receiver tilt angle and secondly the secondary concentrator acceptance angle are determined. These parameters combined with the fixed secondary concentrator outlet radius — which is determined by the SPHER window diameter — fully constrain the 3D CPC geometry. The flux maps generated using MATLAB post processing on the derived concentrator results clearly indicate the strengths and weaknesses of the specific concentrator and heliostat field combination. The influence of the secondary concentrator on the window incident flux distribution and window transmission, absorption and reflection properties is evaluated. Early findings using the code suggest higher yearly average power entering the receiver when compared to a non-secondary case. The reason for this effect is found in increased heliostat efficiency towards the edges of the heliostat field. At the same time the peak power hitting the window is found to increase very slightly only. This means the maximum window design specifications do not need to be adjusted dramatically to be able to accommodate the average power increase. First indications using the MCRT output in preliminary receiver simulations suggest increased receiver efficiency and a boost in receiver outlet gas temperature. The combined effect of these improvements is expected to raise overall power generation efficiency by improving the gas- / steam turbine combined cycle efficiency.

A new type of high temperature solar receiver for Brayton Cycle power towers is being designed and built in the Combustion and Solar Energy Laboratory at San Diego State University under a DOE Sunshot Award. The Small Particle Solar Receiver is a pressurized vessel with a window to admit concentrated solar radiation that utilizes a gas-particle suspension for absorption and heat transfer. As the particles absorb the radiation that enters the receiver through the window, the carrier fluid (air in this case) heats which oxidizes the particles and the flow leaves the receiver as a clear gas stream. After passing through an in-line combustor if needed, this hot gas is used to power a turbine to generate electricity. The numerical modelling of the receiver is broken into three main pieces: Monte Carlo Ray Trace (MCRT) method (written in FORTRAN), ANSYS Fluent (CFD), and the User Defined function (written in C code) for oxidation. Each piece has its advantages, disadvantages, and limitations and the three pieces are coupled to finalize the calculation. While we have successfully demonstrated this approach to obtaining the velocity and temperature fields, one big challenge to this method is that the definition of the geometry is a time consuming programming task when using MCRT. On the other hand, arbitrary geometries can be easily modelled by Computational Fluid Dynamics (CFD) codes such as FLUENT. The goal of this study is to limit the use of MCRT method to determining the appropriate input boundary condition on the outside of the window of the receiver and to use the built-in Discrete Ordinates (DO) method for all the radiation internal to the receiver and leaving the receiver due to emission. To reach the goal, this paper focuses on the DO method implemented within FLUENT. An earlier study on this subject is based and advanced. Appropriate radiation input for the DO method is extensively discussed. MIRVAL is used to simulate the heliostat field and VEGAS is used to simulate a lab-scale solar simulator; both of these codes utilize the MCRT method and provide intensity information on a surface. Output from these codes is discretized into DO parameters allowing the solution to proceed in FLUENT. Suitable benchmarks in FLUENT are used in a cylindrical geometry representing the receiver for the comparison and validation. This method will allow FLUENT to be used for a variety of problems involving concentrated solar energy.

This research expands on previous work by coupling the in-house Monte Carlo Ray Trace (MCRT) radiation model with the more sophisticated fluid dynamics modeling capabilities of ANSYS FLUENT. This allows for the inclusion of more realistic inlet and outlet geometries in the receiver, as well as a turbulence model and much finer grid sizing. Taken together, these features give a more complete picture of the heat transfer, mixing, and temperature profiles within the receiver than previous models. This flow solution is coupled to the MCRT code, by using the in-house MCRT radiation solver to provide the source term of the energy equation. The temperature data output from FLUENT is then fed back into the FORTRAN MCRT code, via a User Defined Function written in C#, and the two models iterate until convergence. The solar input has been modified from the previous model to provide a Gaussian fit to a calculated flux distribution, which is more realistic than a uniform flux. Initial results for a 5 MW solar input agree with the trend identified in Ruther’s work regarding the influence of particle mass loading on heating in the receiver. The maximum outlet temperature reached is 1430K, which is on target for driving a Brayton cycle gas turbine. Cylinder wall temperatures are consistently below those of the gas boundary layer, and significantly below the maximum gas temperature in the receiver cavity.

A blow-down facility for experimental analysis of real gases is under construction at Politecnico di Milano (Italy), in collaboration with Turboden s.r.l. and in the frame of the research project named Solar . Experiments are meant to characterize flow fields representative of expansions taking place in Organic Rankine Cycle (ORC) turbine passages. Indeed, ORC power plants represent a viable technology to exploit clean energy sources, but ORC turbines design tools still require accurate experimental data for validation. A significant improvement of turbine efficiency is expected from detailed investigations on vapour streams; in fact, ORC turbines design tools still require accurate experimental data for validation. The facility is equipped with a straight axis supersonic nozzle as a test section and a batch-closed loop plant has been designed in order to reduce investment and operational costs. Due to the batch operation, the evaluation of the time evolution of main processes involved in the cycle is of great importance. To this purpose a dynamic simulation of the test rig has been carried out using a dynamic simulator based on an object-oriented modeling language, Modelica , allowing an easy development of component models structured with a hierarchical approach. Models include control loop devices, strongly influencing processes duration. This paper presents how the test rig has been modelled, with particular emphasis on the models framework and on simulation procedure; the calculation results are finally discussed. With a lumped parameter approach, a first scheme of the facility has been built by modelling each of the three main plant section (heating, test, condensation) using components included in a self-made library. Several models, not embedded in the Modelica standard libraries, have been created using Modelica code; among them the most important has been the supersonic nozzle. In order to better describe the facility behaviour and the thermal losses, a plant calculation refinement has been carried out by the development of finite volume based one-dimensional models of ducts and reservoirs, either in radial or axial direction; in particular, a novel distributed-parameters model has been built for the heating section. All simulations have been performed using Siloxane MDM and Hydrofluorocarbon R245fa as reference fluids and FluidProp ® to calculate thermodynamic properties. A quasi 1-D steady nozzle flow calculation has also been carried out by implementing FluidProp ® routines in a dedicated Fortran software. Since the unsteady nozzle expansion is well approximated by a sequence of steady states, the computation provides all thermodynamic properties and velocity along the nozzle axis as a function of time. Simulation results have given a fundamental support to both plant and experiments design.

The operation of solar energy systems is necessarily transient. Over the lifetime of a concentrating solar power plant, the system operates at design conditions only occasionally, with the bulk of operation occurring under part-load conditions depending on solar resource availability. Credible economic analyses of solar-electric systems requires versatile models capable of predicting system performance at both design and off-design conditions. This paper introduces new and adapted simulation tools for power tower systems including models for the heliostat field, central receiver, and the power cycle. The design process for solar power tower systems differs from that for other concentrating solar power (CSP) technologies such as the parabolic trough or parabolic dish systems that are nearly modular in their design. The design of an optimum power tower system requires a determination of the heliostat field layout and receiver geometry that results in the greatest long-term energy collection per unit cost. Research presented in this paper makes use of the DELSOL3 code (Kistler, 1986) which provides this capability. An interface program called PTGEN was developed to simplify the combined use of DELSOL3 and TRNSYS. The final product integrates the optimization tool with the detailed component models to provide a comprehensive modeling tool set for the power tower technology.