Maximize Solar Collector Efficiency: The Critical Role of the Solar Absorber

Maximizing the efficiency of solar collectors: The key role of solar absorbers

In the field of solar thermal utilization, efficiency is the ultimate yardstick for measuring the value of technology. In the journey of pursuing ultimate efficiency, solar absorbers undoubtedly play the most crucial role. It is the "heart" of the solar thermal collection system and the core place where light energy is converted into thermal energy. Understanding, optimizing and innovating solar absorber technology is the fundamental way to unleash the potential of the entire system and maximize energy output. This article will delve into the working principle of solar absorbers, comprehensively examine their extensive application scenarios, objectively analyze the current development status, and look forward to their innovative and promising future prospects.

I. Working Principle: The exquisite conversion from photons to thermal energy

A solar absorber is a precise energy conversion interface, and its performance depends on how it processes the incident solar radiation. This process is not merely a simple "black object absorbing heat", but rather an efficient collaboration involving materials science, thermodynamics and optics.

1. Energy capture: Spectral selective absorption

Sunlight is a wide-spectrum electromagnetic wave, with its energy mainly concentrated within the wavelength range of 0.3 to 2.5 micrometers. A common black surface absorbs most of the sunlight, but it also re-releases a large amount of heat in the form of long-wave infrared radiation. The primary task of a solar absorber is to break this balance and achieve "spectral selective absorption".

This relies on the special coating on the surface of the absorber, which is usually only a few micrometers thick - the selective absorption coating. This coating is designed as:

The "greedy devouring" of shortwave radiation: It has an extremely high absorption rate (α ≥ 0.94) for the solar spectrum (0.3-2.5µm), converting the vast majority of photon energy into lattice vibrations, that is, thermal energy.

The "stingy guardian" of long-wave radiation: It has an extremely low emissivity (ε ≤ 0.06) for the infrared radiation (>2.5µm) generated by its own heat, and strives to suppress the loss of heat in the form of radiation.

This characteristic of "high absorption and low emission" is the key for the absorber to achieve and maintain high temperatures. The performance of the coating is measured by the selectivity (α/ε ratio), and the higher the ratio, the more outstanding the coating performance.

2. Energy transfer: Efficient heat conduction and minimal heat loss

Capturing energy is just the first step. How to effectively transfer it and reduce losses is equally crucial.

Heat conduction: Absorbers are typically made of materials with high thermal conductivity, such as copper and aluminum, to form heat-absorbing plates or tubes. The generated heat is rapidly conducted from the coating to the entire metal substrate, preventing local overheating.

Heat exchange: In a liquid circulation system, metal flow channels (such as tubes or plates) are closely combined with heat-absorbing plates, and heat is transferred to the heat transfer medium (water or antifreeze) within the flow channels through convection. The optimization of structural design, such as the layout of flow channels and welding techniques, directly affects the efficiency of heat exchange.

Heat loss suppression: In addition to suppressing radiation loss through low-emissivity coatings, the system also reduces heat loss in other ways. In flat-plate collectors, glass covers and side insulation layers are used to reduce convection and conduction losses. In vacuum tube collectors, the entire absorber is placed in a vacuum interlayer, almost completely eliminating convective and conductive heat loss.

Ii. BTE Solar Absorbrs 

Laser-welded, high performance, copper or aluminium absorbers

We manufacture pre-finished full-surface absorbers in 1-minute cycles from copper pipe and aluminium sheet with harps and meandering shapes. If the client desires it, we also process aluminium pipes and copper sheet. All standard metal coatings can be processed.

The self-developed manufacturing process guarantees an intact absorber area because the connection of metal and pipe is only made from the back of the absorber. Due to the purely metallic connection, an optimal heat transfer, and therefore good efficiency, is achieved. Our process enables a gapless connection within a very short period of time.

The CAD-based absorber design, the CNC-controlled pipe processing, and the fully automatic laser welding plant allow for highly precise, cost-optimised production sequences. The mature laser welding technology at KBB for the industrial production of larger quantities ensures an optimal connection of the pipes to the absorber metal, with a constant production quality.

Full-surface absorbers, manufactured at KBB with a high-selectivity coating, yield the highest performances. Upon client request, they are manufactured up to a size of 3.00 m². Each absorber is tested for pressure resistance and pressure tightness.

We give the secure packaging of the absorbers special attention. We want the high quality absorbers to get to you in a fault-free condition.

Iii. Application scenarios: from domestic hot water to the core engine of industrial power

With its highly efficient energy conversion capacity, solar absorbers have become the universal core of numerous solar thermal utilization systems, and their application scope is extremely wide.

1. Heating for civil and commercial buildings

Domestic hot water supply: From vacuum tube water heaters on the rooftops of every household to split systems on the balconies of high-rise residences, absorbers are the cornerstone for providing low-cost domestic hot water.

Space heating: Combined with floor heating or fan coil systems, solar absorbers provide comfortable auxiliary heating for residential buildings, office buildings, schools and other structures in winter, significantly reducing the consumption of fossil energy.

Building Integrated Solar System (BIST) : Absorbers are directly designed as part of the building's exterior walls, roofs or balcony railings, fulfilling their functions while meeting aesthetic requirements, thus becoming the "active skin" of the building.

2. Industrial process heating

This is a "blue ocean" market with huge potential for solar thermal utilization. In industries such as food processing, textile printing and dyeing, chemical engineering, plastics, and brewing, a large amount of medium and low-temperature thermal energy ranging from 80°C to 250°C is required. Large-scale solar thermal fields based on high-performance absorbers can directly provide clean heat sources for these industrial processes, achieving "solar steam" and effectively replacing gas or coal-fired boilers.

3. Agriculture and Special Applications

Greenhouse heating: To provide supplementary heat for agricultural greenhouses in winter and extend the planting season.

Seawater desalination: Utilizing the heat energy collected by absorbers to evaporate seawater is one of the sustainable technologies to address the shortage of fresh water.

Swimming pool heating: Even in a simple uncovered system, weather-resistant absorbers can effectively increase the water temperature of the swimming pool.

4. Concentrated solar power generation

In trough-type, tower-type and other concentrated solar power stations, the absorber plays the role of a "receiver". It is heated to several hundred degrees of high temperature by highly concentrated sunlight, and then the heat is transferred to heat-conducting fluids such as molten salt, and is ultimately used to drive steam turbines for power generation. The absorbers here must remain stable and efficient under extreme high temperatures and high heat flux density.

Iv. Development status: mature technology and market challenges coexist

After decades of development, solar absorber technology has reached a high level of maturity, but it also faces fierce market competition and transformation pressure.

Diversification and maturity of technical routes: Mainstream technologies include vacuum tube absorbers (all-glass and U-tube metal heat absorbers) and flat plate absorbers. Magnetron sputtering PVD coating has become the standard process for high-performance absorbers, with a lifespan of over 20 years. The technology is highly mature and reliable.

Performance improvement enters a plateau period: For conventional medium and low-temperature applications, the absorption rate and emissivity of the absorption coating have approached their physical limits. Further marginal improvement requires huge R&D investment, and the cost-effectiveness is gradually decreasing.

Cost pressure and market competition: The production cost of high-performance absorbers, especially that of PVD processes, remains relatively high. This makes it face competition from both ordinary coating products and electrochemical coating products in price-sensitive markets.

The cross-industry squeeze from photovoltaic: The continuous sharp drop in the price of photovoltaic modules poses the most severe challenge to the market of medium and low-temperature thermal utilization in solar energy. In many scenarios, the "photovoltaic + electric heat pump/resistance heating" solution poses a direct competition to traditional solar thermal systems due to its flexible installation and initial cost advantages.

V. Development Prospects: The Future of Innovation-driven and System Integration

Despite the challenges, the future of solar absorbers is not stagnant but is evolving towards higher performance, greater intelligence and deeper integration.

1. Revolutionary breakthroughs in materials and coatings

Medium and high-temperature coatings: The research and development focus is shifting towards coatings that can remain stable for a long time at temperatures above 250°C in the air. Selective coatings based on new materials such as aluminum chromium nitride and diamond-like carbon will strongly promote the medium-temperature industrial application of solar energy and the development of efficient CSP.

Smart materials and adaptive coatings: Explore materials with thermochromic properties that can automatically adjust their emissivity at different temperatures, maintain a low emissivity during heat collection, and increase the emissivity when overheated for self-protection, thereby enhancing system safety and adaptability.

2. Optimization of structural design and manufacturing process

Microchannels and enhanced heat transfer: Designing microscale flow channels inside the absorber can significantly increase the heat exchange area, reduce thermal resistance, and improve heat transfer efficiency. It is particularly suitable for compact and high-power-density applications.

Additive manufacturing (3D printing) : By using metal 3D printing technology, complex and integrated heat-absorbing structures that cannot be achieved through traditional processes can be manufactured, achieving the optimal topological combination of flow channels and heat-absorbing bodies and maximizing thermal performance.

3. Core position in photovoltaic and solar thermal systems

Solar photovoltaic and thermal systems are an important direction for the future. In this system, the absorber directly serves as a heat sink for the photovoltaic backsheet, efficiently collecting the waste heat generated by the solar cells. This not only significantly reduces the temperature of photovoltaic modules (enhancing their power generation efficiency and service life), but also simultaneously obtains valuable thermal energy, achieving a leapfrog improvement in the comprehensive utilization efficiency of solar energy per unit area.

4. Integration of digitalization and intelligence

Absorbers in the future will no longer be "dumb" components. By integrating micro temperature and flow sensors and connecting to the Internet of Things platform, its working status can be monitored in real time, performance degradation can be evaluated, maintenance needs can be predicted, and it can be intelligently coordinated with other energy systems (such as heat pumps and heat storage tanks) to achieve the optimization of system-level energy efficiency.

Conclusion

The solar absorber, this seemingly simple component, is actually the pearl on the crown of solar thermal utilization technology. Its performance directly determines the energy output and economy of the entire system. Under the grand goals of global energy transition and carbon neutrality, the pursuit of higher efficiency is endless. Through continuous innovation in materials science, refinement in structural design, and deep integration with digital and intelligent technologies, solar absorbers will continue to break through their limits and consolidate their irreplaceable core position as efficient solar energy collectors. It is not only the key to maximizing the efficiency of solar collectors, but also a powerful engine driving the entire society towards a clean and low-carbon energy future.