Solar Collectors and Concentrators

Modeling Solar Concentrating Systems in Thermal Desktop

Parabolic trough solar field

C&R Thermal Desktop®, CRTech TD Direct®, RadCAD®, and SINDA/FLUINT offers best-in-class thermal radiation solutions, which are critically important to both space vehicle and solar power system analyses. This tool suite also uniquely offers single- and two-phase thermohydraulics, which means an entire solar thermal energy system can be modeled from the collectors to the steam power cycle or feedwater heat exchangers.

The latter strategy was recently implemented near Grand Junction, Colorado by Xcel Energy. In that system, a parabolic trough system is used to preheat feedwater to a coal burning power plant, thereby reducing the amount of coal required.

Paraboloids and Parabolic Troughs

Many solar concentrators are paraboloids, offset paraboloids, or parabolic troughs, as pictured below. These shapes are included in Thermal Desktop as parametric objects. Parametric objects are predefined shapes that can be easily changed by changing a few values. More importantly, parametric shapes have the correct shape regardless of discretization. This fits the idea of thermal-centric: discretize for the solution and not for the shape.

Solar collector receivier temperature

Arbitrary shapes with curved elements

Parametric shapes work well for some systems, but a reflector's shape may be not be well represented by the predefined shapes. An example is a compound parabolic concentrator (CPC), or Winston cone. A CPC is an axisymmetric body formed by rotating a parabolic section about an axis such that any light that enters within the designed acceptance angle is captured at the base of the body. Curved mesh elements can be generated by TD Direct to accurately represent just about any shape of reflector with fewer nodes than possible with flat finite elements.

Example of Compound Parabolic Concentrator  

Behavior of Compound Parabolic ConcentratorRay Trace for Winston Cone

 

Additional Thermal Desktop, RadCAD, and FloCAD Capabilities for Concentrating Solar Power Systems

  • Accurate curved surfaces for radiation calculations
  • Transparent surfaces and solids with refraction
  • Mapping of thermal results to structural models
    • Receiver pipe
    • HCE supports
  • Parameterized analyses
    • External convection to ambient
    • Internal convection between receiver pipe and glass envelope
    • Optical properties of the reflector and receiver
  • Free molecular heat transfer for near-vacuum in glass envelope
  • Diffuse solar load and diffuse sky IR radiation
  • Solar tracking for surfaces or groups of objects
  • Psychrometrics for condensing air heat exchangers
  • Condenser, evaporator, and boiler sizing and simulation
  • Phase change materials for thermal energy storage
  • Turbomachine components
    • Cycle-level analysis of power generation cycles
    • Performance map-based descriptions of single- or multi-stage turbomachines
    • Steam turbines
    • Heat transfer fluid pumps

These capabilities may be used separately for component-level analyses or together for plant-level analyses.

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

FloCAD model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar is designed to be attended in the order they were presented. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.