Tag Archives: Parabolic troughs

Simulation-based analysis of parabolic trough solar power plants around the world

Fig. 1: 3D heat map of the Keahole Plant in Hawaii
Fig. 2: SEGS-8 in California and NOOR-1 in Morocco
In Version 7.1.7 of Energy3D, I have added the basic functionality needed to perform simulation-based analysis of solar power plants using parabolic trough arrays. These tools include 24-hour yield analysis for any selected day, 12-month annual yield analysis, and the 3D heat map visualization of the solar field for daily shading analysis (Figure 1). The heat map representation makes it easy to examine where and how the design can be optimized at a fine-grained level. For instance, the heat map in Figure 1 illustrates some degree of inter-row shadowing in the densely-packed Keahole Solar Power Plant in Hawaii (also known as Holaniku). If you are curious, you can also add a tree in the middle of the array to check out its effect (most solar power plants are in open space with no external obstruction to sunlight, so this is just for pure experimental fun).
Fig. 3: Hourly outputs near Tuscon in four seasons

Fig. 4: Hourly outputs near Calgary in four seasons
As of July 12, I have constructed the Energy3D models for nine such solar power plants in Canada, India, Italy, Morocco, and the United States (Arizona, California, Florida, Hawaii, and Nevada) using the newly-built user interface for creating and editing large-scale parabolic trough arrays (Figure 2). This interface aims to support anyone, be she a high school student or a professional engineer or a layperson interested in solar energy, to design this kind of solar power plant very quickly. The nine examples should sufficiently demonstrate Energy3D's capability of and relevance in designing realistic solar power plants of this type. More plants will be added in the future as we make progress in our Solarize Your World Initiative that aims to engage everyone to explore, model, and design renewable energy solutions for a sustainable world.
Fig. 5: Hourly outputs near Honolulu in four seasons

An interesting result is that the output of parabolic troughs actually dips a bit at noon in some months of the year (Figure 3), especially at high altitudes and in the winter, such as Medicine Hat in Canada at a latitude of about 51 degrees (Figure 4). This is surprising as we perceive noon as the warmest time of the day. But this effect has been observed in a real solar farm in Cary, North Carolina that uses horizontal single-axis trackers (HSATs) to turn photovoltaic solar panels. Although I don't currently have operation data from solar farms using parabolic troughs, HSAT-driven photovoltaic solar arrays that align in the north-south axis work in a way similar to parabolic troughs. So it is reasonable to expect that the outputs from parabolic troughs should exhibit similar patterns. This also seems to agree with the graphs in Figure 6 of a research paper by Italian scientists that compares parabolic troughs and Fresnel reflectors.

The effect is so counter-intuitive that folks call it "Solar Array Surprises." It occurs only in solar farms driven by HSATs (fixed arrays do not show this effect). As both the sun and the solar collectors move in HSAT solar arrays, exactly how this happens may not be easy to imagine at once. There doesn't seem to be a convincing explanation in the Internet forum where folks discussed about it. Some people suggested that the temperature effect on solar cell efficiency might be a possible cause. Although it is true that the decrease of solar cell efficiency at noon when temperature rises to unfavorable levels in the summer of North Carolina can contribute to the dip, the theory cannot explain why the effect is also pronounced in other seasons. But Energy3D accurately predicts these surprises, as I have written in an article about a year before when I added supports for solar trackers to Energy3D. I will think about this more carefully and provide the explanation later in an article dedicated to this particular topic. For now, I would like to point out that Energy3D shows that the effect diminishes for sites closer to the equator (Figure 5).

Modeling parabolic troughs in Energy3D

Fig. 1. The absorber tube of a parabolic trough
A parabolic trough is a type of concentrated solar collector that is straight in one dimension and curved as a parabola in the other two, lined with mirrors. Sunlight that enters the trough is focused on an absorber tube aligned along the focal line of the parabola, heating up the fluid in the tube (Figures 1 and 2). If the parabolic trough is for generating electricity, the heated fluid is then used to vaporize water and drive a turbine engine. A power plant usually consists of many rows of parabolic troughs.

Fig. 2. A view from the absorber tube.
Parabolic troughs are another common form of concentrated solar power (CSP), in addition to solar power towers that Energy3D has already supported (there are two other types of CSP technologies: Dish Stirling and Fresnel reflectors, but they are not very common). According to Wikipedia, there are currently more parabolic trough-based CSP plants than tower-based ones.

In the latest version of Energy3D (V7.0.6), users can now add any number of parabolic troughs of any shape and size to design a solar thermal power plant.

Fig. 3: Parabolic troughs at different times of the day

Parabolic troughs are most commonly aligned in the north-south axis so that they can rotate to track the sun from east to west during the day. This kind of trackers for parabolic troughs works in a way similar to the horizontal single-axis tracker (HSAT) for driving photovoltaic solar panel arrays. You can observe their motions when you change the time or date or animate the movement of the sun in Energy3D. Figure 3 illustrates this.

Like photovoltaic solar panel arrays, parabolic troughs have the inter-row shadowing problem as well. So the distance between adjacent rows of parabolic troughs cannot be too small, either. But unlike solar power towers, parabolic troughs do not have reflection blocking issues among mirrors. Figure 4 shows this.

This new addition greatly enhances Energy3D's capability of modeling CSP plants, moving the software closer to the goal of being a one-stop shop for exploring all sorts of solar solutions. In the coming weeks, we will start to build 3D models for parabolic troughs in the real world.
Fig. 4: Inter-row shadowing in parabolic trough arrays