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A parabolic trough system is a type of concentrating solar power (CSP) system that collects direct normal solar radiation and converts it to thermal energy that runs a power block to generate electricity. The components of a parabolic trough system are the solar field, power block, and in some cases, thermal energy storage and fossil backup systems. The solar field collects heat from the sun and consists of parabolic, trough-shaped solar collectors that focus direct normal solar radiation onto tubular receivers. Each collector assembly consists of mirrors and a structure that supports the mirrors and receivers, allows it to track the sun on one axis, and can withstand wind-induced forces. Each receiver consists of a metal tube with a solar radiation absorbing surface in a vacuum inside a coated glass tube. A heat transfer fluid (HTF) transports heat from the solar field to the power block (also called power cycle) and other components of the system. The power block is based on conventional power cycle technology, using a turbine to convert thermal energy from the solar field to electric energy. The optional fossil-fuel backup system delivers supplemental heat to the HTF during times when there is insufficient solar energy to drive the power block at its rated capacity.
The physical trough system model is a new parabolic trough model for Solar Advisor introduced in March 2010. The model approaches the task of characterizing the performance of the many of the system components from first principles of heat transfer and thermodynamics, rather than from empirical measurements as in the empirical trough system model. The physical model uses mathematical models that represent component geometry and energy transfer properties, which gives you the flexibility to specify characteristics of system components such as the absorber emissivity or envelope glass thickness. The empirical model, on the other hand, uses a set of curve-fit equations derived from regression analysis of data measured from real systems, so you are limited to modeling systems composed of components for which there is measured data. While the physical model is more flexible than the empirical model, it adds more uncertainty to performance predictions than the empirical model. In a physical model, uncertainty in the geometry and property assumptions for each system component results in an aggregated uncertainty at the system level that tends to be higher than the uncertainty in an empirical model. The empirical model can make more accurate performance predictions for systems similar to those on which the model was originally derived, but limits the range of components that you can include in the system. We've included both models in Solar Advisor so that you can use both in your analyses.
The following are some key features of the physical model:
| • | Includes transient effects related to the thermal capacity of the heat transfer fluid in the solar field piping, headers, and balance of plant. |
| • | Allows for flexible specification of solar field components, including multiple receiver and collector types within a single loop. |
| • | Relatively short simulation times to allow for parametric and statistical analyses that require multiple simulation runs. |
As with the other Solar Advisor models for other technologies, the physical trough model makes use of existing models when possible:
| • | Collector model adapted from NREL's Excelergy model. |
| • | Receiver heat loss model by Forristall (2003). |
| • | Field piping pressure drop model by Kelley and Kearney (2006). |
| • | Power cycle performance model by Wagner (2008) for the power tower (also known as a central receiver) CSP system model in SAM. |
For publications describing the subcomponent models, see References, Parabolic Trough Technology and Modeling.
To use the parabolic trough physical model:
| • | Open the sample parabolic trough sample template: On the File menu, click Open Sample Template and choose Sample Parabolic Trough Systems from the list, or |
The sample template contains four cases. The first three cases use the physical trough model, and the fourth case uses the empirical model. The first case represents a 100 MW baseline system with a medium temperature heat-transfer fluid and an indirect 2-tank thermal energy storage system. The second case represents a similar 100 MW system with dry cooling. The third case shows how to optimize the solar field thermal energy storage system size to minimize the system levelized cost of energy, and is described in Solar Multiple Optimization.
The parabolic trough input pages for this option described in this section are:
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