Concentrating solar thermal systems (CSTs) are gaining attention as one of the renewable technologies capable of harnessing the power of the sun to produce heat and electricity. These systems are typically made of mirrors that reflect sunlight onto a receiver that contains a working fluid. The energy from the sun is converted into thermal energy in the working fluid, which can be used for domestic water heating and power generation applications.

However, as simple as this operating principle might seem, engineers spend a great deal of time designing these systems to operate and perform desirably. Among the several considerations that go into their design, optical analysis remains one of the key processes performed during the early stages of research and development of CSTs.

Optical analysis involves simulating various optical interactions (such as absorption, reflection and refraction) of solar radiation on different components of the CST. The end goal of this process is to determine the optical efficiency, potential losses and heat flux distribution of the receivers of CST systems.

Optical analysis is one of the key processes performed in the early stages of research and development of concentrating solar thermal systems. Source: Esteban Martinena/Adobe stock

Why optical analysis?

To better understand the process of optical analysis, consider a parabolic trough collector, which is made of a parabolic-shaped mirror and a receiver tube, as shown in Figure 2. The parabolic mirror is made of a material with high reflectivity that reflects the incident solar radiation to the focal line of the mirror. The receiver tube, which consists of an absorber tube and a glass cover, is placed along the focal line of the parabolic mirror so that it receives a large portion of the reflected radiation.

Figure 2. Parabolic trough collector. Source: Andrew Buck/CC[SA][3.0]

In an ideal case (where the mirror is perfect and has a reflectivity of 1), all of the incident radiation will be reflected to the mirror’s focal line. However, this is not usually the case in reality, as imperfections in the mirror’s surface, manufacturing errors and limitations in material properties will cause some of the incident rays to be transmitted through the mirror and some of the reflected rays to miss the receiver tube. Moreover, misalignment of the receiver tube from the mirror’s focal line can also lead to some of the reflected rays missing the receiver.

Optical analysis allows engineers to account for different optical properties and optical errors in the optical performance of concentrating solar thermal systems. Moreover, given that the geometrical configuration significantly affects the performance of CSTs, engineers can analyze the effects of different geometrical configurations on performance, allowing them to save time and cost that would otherwise be used in creating and testing prototypes.

Some of the optical errors affecting the performance of concentrating solar thermal systems

The errors considered in the optical analysis of parabolic trough collectors can be categorized as:

1. Random errors
2. Non-random errors

As its name implies, random errors are those that occur without a predictable pattern and vary in an unpredictable manner each time the measurement is taken. Random errors in the optical analysis include slope errors and material errors, such as the specularity of the reflective mirror.

Slope errors take into account the irregularities on the surface of the mirror, such as roughness, deformation and distortion of the mirror surface. These irregularities affect the direction of the reflected rays from the mirror surface. They are typically associated with the manufacturing accuracy of the mirror and microscopic irregularities.

Non-random errors (also called systematic errors) include misalignment of the receiver tube from the focal line of the parabolic mirror. Given that most of the reflected rays from the parabolic meet at the mirror’s focal line, it makes sense to place the receiver along this focal line. However, installation errors, thermal expansion of the receiver materials and mechanical stresses (usually due to wind loads) oftentimes cause the misalignment of the receiver with the focal line.

Optical analysis using statistical ray tracing

The most commonly used method for performing optical analysis is statistical ray tracing. This method involves specifying the number of rays to be traced through a defined CST. These rays encounter various optical interactions based on the defined properties of the system (such as reflectivity, transmissivity and absorptivity) and the optical errors (slope errors and mirror specularity errors). This allows engineers to determine parameters such as the optical efficiency and heat flux distribution.