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Virtual Solar Field (VSF)

Virtual Solar Field (VSF) is a software application developed by DLR for simulating parabolic trough solar fields. In parabolic trough power plants, hundreds of rows of parabolic mirrors (referred to as collectors) focus sunlight and concentrate it onto a heat transfer medium, thus generating temperatures of up to 550 degrees Celsius. The economic feasibility of such systems can be further improved through, for example, improvements in solar field control and operation strategies. A possibility for achieving such improvements is by increasing the amount of the collected solar energy through the reduction of defocusing events during plant operation. Furthermore, temperature drops and pressure fluctuations in the pipe network and in the actuators play a critical role.

To develop and evaluate the benefits of such improvements, accurate representation of the behavior of the solar field under realistic conditions is needed. This can either be achieved through testing on existing power plants or through detailed modelling and simulations. The operators of commercial power plants are often reluctant to test new strategies during the normal operation in order to not affect the planned power output and energy yield of the plant. In these cases, the Virtual Solar Field (VSF) offers unique possibilities to test new procedures to improve the plant yield before their implementation in commercial power plants.

The VSF simulation software can simulate large solar field systems with more than 1000 collectors. It models the multiple pipes and collectors separately. Time-dependent mass and energy balances are computed for each flow element and the individual mass flow distribution in the parallel loops is computed by a hydraulic system solver.

Overview about the main components of the hydraulic network. Credit: DLR

The Virtual Solar Field is able to consider, both, spatial and temporal variations in solar irradiation data and delivers accurate representation of the solar field behavior, for example loop outlet temperatures, under such conditions. With such a tool, control and operation concepts can be tested and the plant can be virtually observed under realistic solar conditions.
The virtual solar field model can be coupled to a field control software via TCP/IP connection in order to test the control strategies.

Heat map showing loop outlet temperatures (lines) and SCA middle temperatures (boxes) during cloud passage. Credit: DLR

The model can also be used as a virtual test platform with the following most important features:

• Scope of the model
• Representation of a whole solar field with an arbitrary number of collector loops.
• Flexible arrangement of the loops for mapping of asymmetric collector fields
• Detailed modelling of the distributor and collector systems
• Applicable to single-phase heat transfer fluids
• Collector performance data may vary temporally and spatially
   
• Physical modelling principles
  • Transient mass and energy balance for fluid volumes in all piping sections (one-dimensional axial discretization)
  • Energy balance for the absorber tube walls
  • Calculation of the pressure losses in all piping sections and fixtures
  • Calculation of the mass flow distribution in the hydraulic network depending on the actual thermal  condition of the collector loops and headers and on valve settings
  • Optical and thermal balancing on the receivers and isolated header pipes
  • Spatially discretized direct irradiance over the whole field according to user-defined resolution of the input data
     
• Control options
  • Individual sun tracking signals with user-defined characteristics for each collector assembly
  • Valve travel/flap position for flow control in the subfields
  • Valve travel at the entrance for each loop (either static or controlled)
  • Pressure gain across solar field pumps
  • TCP/IP interfacing with external simulation management tools

Software facts:

• Simulation program in C++
• Depicts several hundred solar collectors arranged in individually modelled sections
• Coupled hydraulic and thermal equation solver
• Several spatially discretised elements for each collector
• Calculation speed up to 100 times faster than real time
• Can be connected to various other programs

The VSF is being upgraded to allow for its public distribution. A free trial version of the software is also foreseen. For this, the software interfaces are updated and implemented and full documentation is made available. The upgrade is based on the requirements of potential users compiled from direct interviews and on-site testing. In addition, the usability of the software will be verified through test cases with selected customers, which also give potential customers the first hands-on experience.

Possible applications:

• Investigating the behaviour of larger collector fields over time
• Analysing the effect of cloud movements on system operations
• Development of control and operational management concepts
• Testing of solar field control systems
• Configuration of control parameters and components prior to commissioning
• Simulation in system monitoring applications