Solar thermal systems have increasingly become popular for harnessing solar energy for various applications. For instance, engineers are shifting from conventional fossil fuel-based systems to parabolic trough collectors (PTC) for industrial heating applications and power generation.

While this renewable energy technology is contributing to meeting the 2050 net-zero emissions goals, its intermittent nature and higher capital costs compared to conventional fossil fuel systems are some of the challenges hindering its widespread use today.

Therefore, research and development (R&D) activities have been undertaken over the years to improve the overall performance and reduce the costs of PTC systems. These efforts have focused on several key aspects affecting the performance and cost of PTC systems, such as the geometry, working fluids and choice of engineering materials.

Research and development (R&D) activities have been undertaken over the years to improve the overall performance and reduce the costs of parabolic trough collectors. Source: topten22photo/Adobe Stock

Latest trends in PTC geometry

The conventional parabolic trough solar collector features a parabolic-shaped mirror that reflects and focuses incident sunlight onto a receiver. The conventional receiver is typically made of a stainless steel absorber tube and glass cover, with the annular space between them evacuated to very low pressures to minimize heat losses.

One of the latest trends in PTC geometry is to design these collectors to have a large aperture width while reducing the manufacturing complexity and costs associated with large aperture systems. For instance, Abengoa Solar developed the SpaceTube8.2 collector to have an aperture width of 8.2 m while optimizing the manufacturing of the collector substructures.

By increasing the aperture width, engineers can achieve increased solar energy collection and energy absorbed by the working fluid, which could lead to higher thermal efficiency. However, highly precise manufacturing and sun tracking technologies are often needed in large aperture systems to ensure the reflected sunlight does not miss the receiver and cause energy loss.

Trends in working fluids

A desirable heat transfer fluid used in parabolic trough collectors must have high thermal transport properties, low viscosity, high heat storage capacity, low melting point, high boiling point and low cost. However, it is oftentimes challenging to get a working fluid that satisfies all of these requirements.

Thermal oils are quite popular working fluids in PTC systems due to their good thermal transport properties, high thermal stability (often up to 400° C), freeze resistance (due to their low melting point), and reduced chance of corrosion compared to water or steam. For instance, the Noor I power plant in Morocco and Andasol I power plant in Spain work with thermal oils in the solar field block.

Liquified salts, also called molten salts, are another class of working fluids used in PTCs. These salts are typically a mixture of various sodium and potassium nitrates, and they offer several advantages over other types of working fluids. For instance, they typically have higher thermal stability (up to 600° C) compared to thermal oils, making them suitable for higher-temperature applications. In addition, their high heat capacity makes them very effective for thermal storage applications. However, engineers will typically find them to be more expensive than conventional thermal oils.

Other common fluids used in PTC systems include steam and compressed gases, such as carbon dioxide. In addition to being stable at extremely high temperatures, these gases are abundant, readily available, and affordable. However, these gases typically have lower thermal transport properties compared to thermal oils and molten salts at the same Reynolds number.

Nanofluids are also gaining considerable attention as potential fluids for PTC systems. A nanofluid is simply a class of engineered fluids that consists of a base fluid and nanoparticles suspended within it. The use of these nanoparticles increases the thermal conductivity and heat capacity of the working fluid, which in turn helps to increase the PTC’s overall thermal performance.

Learn more about how nanofluids boost heat exchanger efficiency on GlobalSpec.

Trends in materials

The efficiency, reliability and cost-effectiveness of PTC are heavily influenced by the materials used in their construction. Advances in material science have led to several emerging trends that aim to improve performance and reduce the costs of PTCs.

For instance, advanced reflective coatings using nanomaterials, silver, or aluminum with protective layers are helping to enhance the reflectance and longevity of mirrors, reducing energy losses, and increasing efficiency. In addition, engineers are shifting from conventional absorber tube coatings (such as simple black coatings and chrome plating) to high-performance multi-layer selective coatings to maximize solar absorption and minimize infrared radiation losses.

Innovative support structures, typically made of lightweight steel and composite materials, are also helping to achieve lighter, cheaper and more durable support structures.

Learn more about solar power systems on GlobalSpec.

Conclusion

The advancement in PTC design symbolizes a significant leap toward a more sustainable future. By innovating in geometry, working fluids and material science, engineers are paving the way for a solar energy landscape that not only competes with conventional energy sources but also leads in efficiency and environmental stewardship. Therefore, collaborating with solar thermal systems manufacturers and investing in continual research and innovation is not just a recommendation but a necessity for a world striving to meet its energy demand while preserving the planet.