Capturing Warmth: The Science of Solar Collectors and Absorbers

By Abdul Ahad NazakatReviewed by Laura ThomsonJan 22 2026

Solar collectors capture solar radiation and convert it into thermal energy. While solar photovoltaics often dominate public discussion for their role in electricity generation, solar thermal technology serves a different - and equally vital - purpose: supplying heat. In fact, thermal energy accounts for nearly 50 % of global final energy consumption.¹ Solar collectors and absorbers play a central role in meeting this demand, converting sunlight into heat for residential, commercial, and industrial applications.

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What Are Solar Collectors and Absorbers?

Solar collectors are heat exchangers designed to capture solar radiation and transfer it to a working fluid, such as water, air, or oil. The core component of these systems is the absorber, the surface material responsible for photothermal conversion.²

The technical performance of an absorber depends on two physical properties: high absorptivity within the solar spectrum (visible and near-infrared) and low emissivity in the thermal infrared spectrum. This spectral selectivity allows the material to capture sunlight while limiting the amount of heat re-radiated back into the atmosphere. Collectors are categorized based on their design and temperature range:

1. Non-concentrating systems: These include flat-plate and evacuated-tube collectors. They remain in a fixed position and capture both direct and diffuse sunlight, generally for applications requiring temperatures below 100 °C.
2. Concentrating systems: These use mirrors or lenses to focus sunlight onto a smaller absorber area. These systems can achieve temperatures ranging from 400 °C to over 1000 °C for specialized industrial requirements.³

Current Applications and Industries

Solar thermal technology provides heat across various scales. In residential settings, flat-plate collectors are standard for domestic hot water and space heating. In commercial sectors, transpired air collectors are utilized to preheat ventilation air in large-scale facilities, with some systems reaching solar conversion rates of approximately 90 %.²

The industrial sector is an expanding market for this technology, specifically through Solar Heat for Industrial Processes (SHIP). By the end of 2023, approximately 600 large-scale solar heat systems were in operation, representing a total installed capacity of 2.3 GWth.⁴ Key industries currently utilizing these systems include:

• Food and Beverage: Utilized for pasteurizing, blanching, and crop drying.
• Textiles and Chemicals: Providing heat between 80 °C and 250 °C for dyeing, finishing, and distillation.
• Mining and Desalination: Supplying the thermal energy required for mineral separation or seawater purification.
• Energy Utilities: Concentrating Solar Power (CSP) plants focus heat to drive steam turbines for grid-scale electricity generation.^{1, 4}

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Technical Importance and Efficiency

The operational importance of solar collectors is found in their high energy conversion efficiency. Although standard photovoltaic (PV) panels typically convert 15 % to 20 % of sunlight into electricity, solar thermal systems can convert more than 70 % of solar energy into usable heat.⁵

This efficiency makes solar thermal a practical option for decarbonizing sectors that are difficult to electrify. In many industrialized nations, manufacturing process heat accounts for roughly one-third of energy-related carbon emissions.

Replacing fossil-fuel-fired boilers with solar collectors provides a method for reducing these emissions without the energy losses inherent in converting electricity back into heat. Furthermore, thermal energy can be stored in materials such as water, molten salts, or stone more economically than electricity can be stored in chemical batteries, allowing for heat delivery during non-sunny periods.⁴

Recent Research and Developments

Recent scientific efforts have focused on increasing the temperature limits and durability of absorber materials.

• High-Throughput Material Screening: Researchers recently identified Zintl-phosphide (BaCd2P2) as a high-performance absorber material through the computational screening of 40,000 candidate compounds.⁶ This material possesses an energy bandgap of 1.45 eV and shows stability in environmental conditions, which may address the degradation issues found in previous thin-film absorbers.
• Thermal Trapping: A 2024 study conducted at ETH Zurich demonstrated the use of semitransparent quartz for thermal trapping. By placing a quartz rod in front of the absorber, researchers reached temperatures of 1050°C. The quartz acts as a thermal shield, allowing solar radiation to pass through while preventing infrared heat from escaping. This method increased efficiency from 40 % to 70 % at high concentration levels.⁷
• Hybrid PV/T Systems: Photovoltaic-Thermal (PV/T) systems are a developing area of research. These units generate electricity and heat simultaneously. The thermal component cools the PV cells, which improves their electrical performance while capturing the excess heat for building services.⁵

Challenges and Technical Constraints

Widespread adoption of solar thermal technology is restricted by several technical and economic factors. Thermal degradation is a primary concern; absorber coatings must withstand decades of extreme temperature cycles and UV exposure. Data from long-term solar installations indicate that aging can reduce optical efficiency by approximately 10 % to 15 % over a decade due to surface oxidation and the failure of vacuum seals.⁸

Economic constraints also exist. The initial capital expenditure for high-temperature concentrating systems is higher than that of established natural gas infrastructure. Thermal energy is less efficient to transport over long distances than electricity, so collectors must be located near the end-use point. There is a documented need for standardized, modular components and more cost-effective Thermal Energy Storage (TES) media to manage the intermittency of solar radiation.

The Future of the Technology

The future development of solar thermal technology is focused on greater integration and the expansion of high-temperature applications. As thermal trapping and advanced selective coatings transition from experimental to commercial stages, solar energy may become a viable replacement for fossil fuels in high-heat processes such as cement and steel production, which require temperatures exceeding 1000 °C.⁷

There is also an increasing focus on the circular economy. Future collector designs are being evaluated for the recyclability of their copper, aluminum, and glass components. As environmental regulations and carbon-pricing mechanisms evolve, the integration of solar collectors with digital grid management and large-scale thermal storage will likely increase, providing a more consistent energy supply for industrial operations.

References and Further Reading

1. Weiss, W., & Spörk-Dür, M. (2024). Solar Heat Worldwide 2024. International Energy Agency Solar Heating & Cooling Programme. https://www.iea-shc.org/solar-heat-worldwide
2. Kalogirou, S. A. (2014). Solar Energy Engineering: Processes and Systems (2nd ed.). Academic Press. https://doi.org/10.1016/C2011-0-07038-2
3. U.S. Department of Energy. (2024). Solar Thermal Power and Industrial Processes Basics. Office of Energy Efficiency & Renewable Energy. https://www.energy.gov/eere/solar/solar-thermal-power-and-industrial-processes-basics
4. IEA SHC. (2025). Solar Heat for Industrial Processes (SHIP) Database. https://www.iea-shc.org/Data/Sites/1/publications/Solar-Heat-Worldwide-2025.pdf
5. Pang, W., et al. (2020). Analysis of a photovoltaic–thermal solar hybrid system with a focus on efficiency. Applied Energy. DOI: 10.1016/j.apenergy.2019.114328
6. Qu, J., et al. (2024). BaCd2P2: A promising earth-abundant solar absorber. Chemistry of Materials. DOI: 10.1021/acs.chemmater.3c02901
7. Casati, E., et al. (2024). Solar thermal trapping at 1,000°C and above. Device. DOI: 10.1016/j.device.2024.100399. https://doi.org/10.1016/j.device.2024.100399
8. Bouhal, T., et al. (2018). Technical Review of Solar Collector Aging. Journal of Solar Energy Engineering. DOI: 10.1115/1.4038831 https://asmedigitalcollection.asme.org/solarenergyengineering/article/140/3/031001/382024/

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