Peter Schwarzbözl

A New Fast Ray Tracing Tool for High-Precision Simulation of Heliostat Fields

A completely new ray tracing software has been developed at the German Aerospace Center. The main purpose of this software is the flux density simulation of heliostat fields with a very high accuracy in a small amount of computation time. The software is primarily designed to process real sun shape distributions and real highly resolved heliostat geometry data, which means a data set of normal vectors of the entire reflecting surface of each heliostat in the field. Specific receiver and secondary concentrator models, as well as models of objects that are shadowing the heliostat field, can be implemented by the user and be linked to the simulation software subsequently. The specific architecture of the software enables the provision of other powerful simulation environments with precise flux density simulation data for the purpose of entire plant simulations. The software was validated through a severe comparison with measured flux density distributions. The simulation results show very good accordance with the measured results.

Optimization of Heliostat Aim Point Selection for Central Receiver Systems Based on the Ant Colony Optimization Metaheuristic

The optimization of the selection of heliostat aim points in a solar power tower plant with the objective of an increased overall efficiency represents a NP-hard optimization problem of high dimension. This paper presents a universal procedure for the purpose of aim point optimization based on the ant colony optimization metaheuristic that uses the principles of swarm intelligence. The applicability of the developed aim point optimization procedure to central receiver systems is demonstrated on a test case, for which the electrical power of a concentrated photovoltaic (CPV) receiver is maximized for a selected operating point. The example of a CPV receiver was chosen due to its nonlinear and nonmonotonous dependency of efficiency and flux density. It is shown that the optimization result is very close to the theoretical maximum

Annual Yield Analysis of Solar Tower Power Plants With GREENIUS

An existing software tool for annual performance calculation of concentrating solar power and other renewable energy plants has been extended to enable the simulation of solar tower power plants. The methodology used is shown and a demonstrative example of a 50 MWe tower plant in southern Spain is given. The influence of design power and latitude on solar field layout is discussed. Furthermore, a comparison of the tower plant with a 50 MWe parabolic trough and a Linear Fresnel plant at the same site is given.

The Solar Power Tower Jülich — A Solar Thermal Power Plant for Test and Demonstration of Air Receiver Technology

The open volumetric receiver technology allows the use of air as heat transfer medium at high temperatures in solar thermal power tower plants. It combines porous ceramic or metallic absorber structures with a strictly modular receiver design. Highly concentrated solar radiation is used to produce hot air as ‘firing’ for a steam rankine cycle. The advantages of this technology are simplicity and scalability, the ability to include a thermal storage, the low thermal capacity and a high efficiency potential. This receiver technology was developed in various joint projects of research and industry over the past years. It was tested and qualified in the worlds largest test center for concentrating solar power, the Plataforma Solar de Almeria (PSA) in Southern Spain with a nominal power of 3 MW incident radiation. In June 2006 it was decided to build a tower power plant with thermal storage in Julich, Germany, with a design power of 1,5 MWe. The objectives of this plant are to test and demonstrate the solar air technology as a complete system, to develop control and plant management strategies and to improve the overall performance and reliability. The location in Germany was chosen as it is close to the research institutions involved and it allows the investigation of the system performance under fluctuating irradiation conditions. The Solar Power Tower Julich is scheduled to start operation by the end of 2008. The five year project comprises design, construction and a two year test operation phase, accompanied by an intensive R&D program. The experiences of this project will be a vital step towards a successful market introduction.

Optimizing Heliostat Positions with Local Search Metaheuristics Using a Ray Tracing Optical Model

Optimizing heliostat positions of solar tower power plants The life cycle costs of solar tower power plants are mainly determined by the investment costs of its construction. Significant parts of these investment costs are used for the heliostat field. Therefore, an optimized placement of the heliostats gaining the maximal annual power production has a direct impact on the life cycle costs revenue ratio. We present a two level local search method implemented in MATLAB which is using the Monte Carlo raytracing software STRAL with a specific annual time scheme for the evaluation of the annual power output. The algorithm was applied to a solar tower power plant with a heliostat field of size 624. Compared to former work of Buck, we were able to improve both runtime of the algorithm and quality of the output solutions significantly. Using the same environment for both algorithms, we were able to reach the best solution of Buck with a speed up factor of 10.

Dynamic modeling of molten salt power towers

A detailed understanding of the transient behavior of a receiver using molten salt as heat transfer fluid is of great importance for an efficient and safe operation. To analyze the transient operation a dynamic model for the flow in the receiver is currently under development, which will be capable to analyze the one-phase flow during normal operation and the two-phase flow during filling and draining. The model can be coupled to raytracing simulation in order to use a realistic flux density distribution as input for the model. In the paper the modelling approach for the receiver model is described shortly and validation results are discussed. This includes a detailed discussion of the heat transfer during the filling procedure, where an interesting phenomenon was discovered. Finally, the results for a parameter variation of the filling procedure and the simulation results for the impact of certain cloud events on the operation of the receiver are presented.

An automated model-based aim point distribution system for solar towers

Distribution of heliostat aim points is a major task during central receiver operation, as the flux distribution produced by the heliostats varies continuously with time. Known methods for aim point distribution are mostly based on simple aim point patterns and focus on control strategies to meet local temperature and flux limits of the receiver. Lowering the peak flux on the receiver to avoid hot spots and maximizing thermal output are obviously competing targets that call for a comprehensive optimization process. This paper presents a model-based method for online aim point optimization that includes the current heliostat field mirror quality derived through an automated deflectometric measurement process.

4.17 Solar Tower Systems

Solar tower systems are an emerging renewable energy technology, offering cost-effective storage for daily load cycles. This enables full decoupling of collection of solar energy and production of electricity. The technology of solar tower systems is described in detail, including specific performance characteristics and their dependence on external conditions. An overview over the current status of the technology is given, with an outlook on future development options. Cost aspects are discussed as well. Another important feature is the high local content, supporting economic development in the deployment regions.

Dynamic Modelling of Molten Salt Central Receiver Systems

Molten salt central receiver (MSCR) systems are a very promising option for the large-scale production of electricity from solar radiation. A central receiver system consists of a field of thousands of mirrors which reflect the sunlight to the top of a tower, at which the receiver is located. The receiver is built of tubes where molten salt is flowing through as the heat transfer fluid. The salt is heated up due to the concentrated solar radiation. The hot salt can be easily stored in large unpressurized tanks driving a convectional steam plant afterwards. The efficiency and the easy storage option have made the MSCR system to the predominant concentrating solar technology in the recent years.