MEET THE TEAM: Discover the Work Packages 4, 7, and 8


Discover the people and work done behind POLYPHEM in our blog articles series!

For this fourth episode, meet the Fraunhofer ISE (Germany) team composed of Shahab Rohani, Peter Schöttl, Nicholas Chandler, Maitane Ferreres, and Tom Fluri involved in work packages 4, 7, and 8 dealing with variety of research questions from challenges in designing the plant to simulation-based optimization and benchmarking.

Shahab Rohani is a researcher in Solar Thermal Power Plants and High Temperature Storage at Fraunhofer ISE. With a B.Sc. in Mechanical Engineering and an M.Sc. in Sustainable Energy Resources, he has been contributing to various R&D projects in the past eight years including simulation-supported design and optimization, system evaluation, and water saving solutions. After finishing his Ph.D. study in 2021, he is now leading projects focusing on hybridization and application of artificial intelligence in CSP plants.

Peter Schöttl is an Energy and Process Engineering graduate of the Technical University of Munich (Germany), and holds a Ph.D. in optimization of Solar Tower receivers from ETH Zurich (Switzerland). He joined Fraunhofer ISE in 2013 in the group Concentrating Collectors and Optics. He supervises the continuing development of the optical simulation software Raytrace3D and develops algorithms for the efficient and flexible simulation and design optimization of heliostat fields and Solar Tower receivers.

Nicholas Chandler is a researcher in Solar Thermal Power Plants at Fraunhofer ISE. He has a B.Sc. in Mechanical Engineering and an M.Sc. in Renewable Engineering, and Management. His research is primarily focused on concentrating solar power simulation, techno-economic optimization strategies and CSP hybridization with other renewable technologies such as PV with battery storage.

Maitane Ferreres is a researcher at Fraunhofer ISE. She has a B.Sc. in Physics, a B.Sc. in Electronic Engineering, and an M.Sc. in Renewable Energy with a specialization in Solar Thermal Technology. Her research area covers the simulation and optimization of CSP power plants and the development of new measurement techniques for soiling assessment.

Tom Fluri is leading Fraunhofer ISE’s research group Solar Thermal Power Plants and High Temperature Storage. The focus of his group is on the development of system simulation tools for CSP and the development of high-temperature thermal energy storage technologies. With a Ph.D. in Mechanical Engineering, he is currently involved in research projects covering the topics of systems evaluation and optimization, storage development and integration, hybridization, collector development, and optimization of operation and maintenance.


Virtual performance models and digital twins are crucial parts of an R&D project. In POLYPHEM, they support the development of the project in the right direction and contribute to its success in different ways:

  • Simulation-supported design in early stages;
  • Virtual extended tests of components using verified models;
  • Identify weaknesses in the operation of the real plant through Digital Twin;
  • Annual yield and performance assessment, cost and market analysis of the technology, and benchmarking;
  • Optimization of plant parameters dependent on the specific location and demand profile;
  • Life Cycle Analysis through multi-year simulation.

Design of the plant layout and specification of the components

The assessment of the design and operation of the technologies prior to construction of the demonstration plant (Work package 4) is a key task of the POLYPHEM project. A virtual model of the entire POLYPHEM plant allows us to study and analyse the behaviour of the plant. This enables the project to detect weaknesses in the early stages and accordingly adapt the design of the innovative components and better understand the constraints of the plant. Therefore, a simulation-supported design can significantly improve the performance of the future plant while saving a substantial amount of time of examination and avoiding possible design failures.
Fraunhofer ISE leads the modelling and simulation activities within the project and develops both static (steady state) and dynamic (transient) simulation models (digital twin).
Using the static model of the concept created in EBSILON®Professional, the overall plant layout was assessed, and the technical specification of the components was specified.
Furthermore, the operation modes and the interface between the sub-systems were defined. The dynamic models have been also partly deployed within the design phase of the plant.
Namely, the Fraunhofer ISE in-house ray tracing simulation tool (Raytrace3D) has been used for the assessment of the Heliostat field and the receiver in terms of optical efficiency and losses. Particularly, the optical assessment based on Raytrace3D directly led to relevant insights that affected the prototype design process.

Figure 1: Visualization of the THEMIS (where the POLYPHEM prototype is being integrated) ray tracing model, with detailed views of one heliostat with 9 sub-facets and the POLYPHEM absorber module at the backside of the MINI-PEGASE cavity

Technology evaluation and component assessment

The validated performance models can be also used to extend the testing and evaluation of the technology beyond the physical tests and thereby also saving time and reducing expenses.
Moreover, virtual tests provide the opportunity to study the performance of the plant (e.g., flexibility and dispatchability) under a variety of conditions that may not be set in the demonstration plant due to practical limitations such as the size of the components and local weather conditions. The following figure illustrates an example of verification of the simulation model through measurement from an experiment of the stratified storage tank. For more details, check our scientific publication!

Figure 2: Comparison of experimental data and TES model simulations for a charge process from 180 °C to 250 °C with mass flow of 0.2 kg/s

Performance evaluation, optimization, and failure detection

The target of the POLYPHEM technology is to deliver clean and renewable power and/or heat. Therefore, its cost and competitiveness with other renewable technologies should also be investigated and that is only possible through annual yield simulations using the virtual model. The following graph represents the simulated power production of the plant on two consecutive days, in which a virtual demand curve is introduced to the model as target production.

Figure 3: Visualization of simulation results for assessment of yield and dispatchability of the technology

Furthermore, the verified model can also be used to identify weaknesses in the operation of the real plant by comparing the measured operational data to simulation results (ideal condition), as well as, to carry out an optimization of plant parameters dependent on the specific location, demand profile, and market conditions.

Figure 4: Example of effect of Interest Rate on POLYPHEM LCOE Price. Created through preliminary simulation models of Fraunhofer ISE

Through multi-year simulation and yield assessment considering e.g. degradation and increasing Operation & Maintenance (O&M) cost”, a more accurate Life Cycle Assessment (LCA) can be carried out ensuring that the environmental footprint of the project is minimized. The LCA covers the Global Warming Potential and damage potential to the ecosystem, human health, and resources over the lifetime of the project. Fraunhofer ISE carries out an LCA study with the simulation software SimaPro using the plant performance information from the performance model.

The tool chain OPTIPHEM

The OPTIPHEM tool chain has been developed by Fraunhofer ISE for the design, simulation, and techno-economic evaluation and optimization of a potential commercial scale of the POLYPHEM plant.
Precisely, the tool chain assesses the best solution for a given specific location and ambient conditions, providing the optimal size of the plant and each component. The tool also considers different operating strategies, adjusting the sizing of multiple components accordingly. The optimization process is based on a sensitivity analysis using a parametric variation approach.

Figure 5: The OPTIPHEM tool chain

Dynamic Simulation Model for the POLYPHEM Prototype – Digital Twin

For performance predictions of the POLYPHEM system based on annual yield assessment, but also for short-term forecasting for the optimization of prototype operation, dynamic simulation models for the various plant components and the entire system are necessary. They should be highly accurate and with a sufficient level of detail, but also computationally fast enough to be used for annual simulations – a Digital Twin of the real plant.

At Fraunhofer ISE, this simulation tool chain has been created based on our two powerful in-house software tools:

  • Raytrace3D for the optical assessment of the Solar Island (heliostat field and receiver) through ray tracing;
  • and ColSim CSP for thermo-hydraulic system simulations of the POLYPHEM cycle, including the receiver, gas turbine, ORC power block, and thermocline storage.

Over the duration of the project,

  • a simulation model for the entire plant has been developed,
  • very detailed models have been developed for some key components of the POLYPHEM system: gas turbine and air receiver (see an example in figure 8 of flux concentration and temperature maps),
  • a control approach covering different operation modes has been integrated,
  • and a techno-economic optimization of the system with different cost scenarios has been performed.

The compressor and micro-gas turbine (topping cycle) are two key elements of the combined cycle as the exhaust gas from the turbine satisfies the heat requirement of the storage and the ORC power block (bottoming cycle) and thus need to be modelled properly for an accurate performance prediction. Therefore, detailed dynamic models for the compressor and the micro gas turbine have been created based on performance characteristic curves. The curves are generated through an adapted methodology prior to system simulation.

Figure 6: Created performance characteristic curve of the micro gas turbine in different shaft speeds for annual system simulation
Figure 7: Micro Gas Turbine Performance Map created through the developed tool designed for characterization of Micro Gas Turbines
Figure 8: Flux concentration and temperature maps on the POLYPHEM absorber panel, derived from ray tracing and a detailed thermo-hydraulic receiver model in ColSim CSP

Raytrace3D & POLYPHEM

Raytrace3D is a Fraunhofer ISE in-house software suite that is used for the assessment and optimization of concentrating collectors. Through continuous development, it is adapted and constantly improved to fit the needs of projects like POLYPHEM. The simulation engine itself and the pre-/post-processing tools are implemented with a performant yet flexible combination of C/C++ and Python. A comprehensive library of geometries, materials, volumetric materials, and light sources is included that suits the needs of heliostat field/receiver modelling in POLYPHEM.
An exemplary ray tracing scene for the heliostat field is displayed in the picture below.

Figure 9: Exemplary visualization of Raytrace3D model of a heliostat field

Additionally, hereafter some notable features relevant for the simulation of heliostat fields in POLYPHEM:

  • Automatic bounding box algorithm for hierarchical pre-structuring of the ray tracing scene =>This accelerates the simulation by several orders of magnitude.
  • Light sources on the heliostat surfaces instead of above the scene =>This accelerates the simulation by avoiding the simulation of obsolete rays. Shading is taken into account with an initial ray tracing step from the heliostats towards the sun.
  • Detailed evaluation of ray history =>This allows for quantification of separate loss contributions (cosine losses, shading, absorption on heliostats, blocking, atmospheric attenuation, spillage, absorption on secondary surfaces, and reflection from absorber surfaces) both for the entire field and individual heliostats.
  • Spatially discretized flux maps on the absorber surfaces
  • Full integration with dynamic system simulation in ColSim CSP based on a sky discretization and a load level interpolation approach
  • Detailed models including sub-geometries for heliostats, tower and receiver

ColSim CSP

ColSim CSP is the Fraunhofer ISE in-house system simulation software that performs transient yield assessment based on annual simulations. It was initially designed by Prof. Christof Wittwer in 1999 and continuously developed and used at Fraunhofer ISE subsequently.
ColSim CSP can perform quasi-dynamic hydraulic simulations and allows for the integration of complex control strategies. Based on modules of C and C++ codes, it presents an adaptable and flexible interface for the modelling and simulation of different technical systems. From the ColSim CSP library the necessary components, such as pumps, heat exchangers, pipes, or solar receivers can be chosen and linked together in line with the project outline. Moreover, there is the possibility of using a wide range of working fluids in ColSim CSP.