Phased Progress in Tackling Three Key Challenges of the Solar Flat-Plate Collector Industry

Solar flat-plate collectors, a core type of solar water heater, are gaining rising attention worldwide thanks to their reliable operation, architectural adaptability, and aesthetic appeal. Although vacuum tube systems have historically dominated the Chinese market—with sales of 38 million square meters in 2015 (87.36% market share)—flat-plate systems accounted for only 5.5 million square meters (12.64%). By contrast, in Europe and North America, flat-plate collectors hold around 90% of the market and are the undisputed mainstream product.

This contrast underscores both the challenges and opportunities for flat-plate technology in China. With energy utilization efficiencies above 76%, higher safety standards, better durability, and longer lifespans, flat-plate solar systems are positioned to capture a growing share of the domestic market. Yet, despite these advantages, the Chinese flat-plate collector sector continues to wrestle with long-standing issues: sluggish technological development, insufficient innovation capacity, and persistent low-quality, price-driven competition.

Industry discussions and expert insights suggest that meaningful progress has been made in addressing these issues, particularly in three interrelated areas: product design, water mist management, and sealing technology.

1. Product Design: Moving Beyond Imitation Toward Innovation

Product design represents the foundation of collector performance. As emphasized by industry experts, collector design has long been a weak link in China’s flat-plate sector. Too many enterprises rely heavily on imitation, leading to slow technological progress and widespread product homogeneity.

To break this cycle, companies must enhance their R&D and innovation capabilities. Central to performance improvement is the development of advanced selective coatings, which directly influence thermal efficiency. Coatings must meet market demand by balancing key parameters such as solar absorptance and thermal emittance.

BTE Solar, one of the world’s top two solar thermal and material manufacturers, is currently the only Chinese enterprise with dual-layer coating production capability. The company provides both blue selective and black selective coating for flat-plate collectors. While black coatings cost over 20% less than traditional blue coatings, they deliver nearly the same efficiency, offering customers a cost-effective yet high-performance solution. This dual portfolio allows flexibility to match different market needs—whether prioritizing durability, aesthetics, or affordability.

For instance, blue coatings have traditionally faced challenges with salt spray resistance. However, through hermetic cavity design with controllable breathability, BTE Solar’s membranes achieve both stability and long-term performance in demanding environments.

Key concerns include:

1. Supporting materials: Polyester fiber, often used in back sheets, loses insulation performance above 120°C, yet even lower-grade collectors may surpass this temperature during stagnation.

2. Seals and gaskets: Common EPDM rubber materials degrade above 150°C, falling short of strict reliability standards.

3. Structural integrity: Excessive idle temperatures can cause deformation, leaks, and delamination of back sheets.

The solution lies in developing designs that maintain hermetic cavity sealing combined with controllable breathability, ensuring that materials can withstand idle exposure while preventing irreversible structural damage.

2. Water Mist: Achieving Controlled “Breathability”

In advanced markets, flat-plate systems dominate solar heating projects, especially in high-rise urban applications. Yet, water mist formation remains a universal problem that undermines efficiency and service life. Moisture accumulation reduces heat absorption, corrodes absorbers, and degrades insulation over time.

A failure mode and effects analysis (FMEA) has identified the root causes of water mist:

1. Excessive moisture absorption in insulation materials.

2. Insufficient hermetic sealing.

3. Inadequate ventilation pathways.

4. Moisture within the collector originates from three main sources:

5. Residual moisture left during production.

6. Infiltration through external seepage.

7. Moisture absorption from ambient humidity.

Traditional “non-hermetic” designs leave gaps that act as breathing channels. While such gaps may allow vaporized moisture to escape under heat, the gap size is inconsistent and uncontrollable—sometimes too small to discharge water vapor, sometimes too large, creating permanent leakage pathways.

To address this, the industry is increasingly adopting the principle of “sealing first, controlled venting second.”

One-way air vents allow moisture to evaporate naturally as internal temperatures rise.

Low-absorption insulation materials minimize water sources within the collector.

PTFE (expanded polytetrafluoroethylene) membranes combined with bamboo fiber materials create a dustproof, waterproof, and breathable system. These structures discharge internal moisture while maintaining pressure balance, preventing both seepage and condensation.

This approach enables collectors to maintain a dry, stable internal environment, effectively reducing the risk of corrosion, insulation degradation, and performance losses. By preventing the accumulation of internal moisture and condensation, the overall service life of the product is significantly extended, while thermal performance remains stable and reliable even under challenging operating conditions. Many enterprises in the industry now regard hermetic cavities with controllable breathability as not only a technical upgrade but the inevitable direction for the future development of flat-plate collectors.

Building on this innovation trend, BTE Solar has become one of the leading factories specializing in high-quality solar thermal collectors. Through advanced membrane technology and optimized structural design, the company is able to manufacture collectors that are effectively free from moisture-related problems, ensuring long-term durability and consistent performance across different climates. This allows BTE Solar’s products to be applied widely in various markets, from residential buildings to commercial projects and large-scale government programs.

At the same time, BTE Solar has achieved multiple international certifications, including ISO quality management, CE compliance, and the prestigious Solar Keymark certification. These credentials not only demonstrate the company’s strong commitment to product safety, reliability, and environmental standards but also create added value for customers. In many regions, governments and utility companies provide financial incentives or subsidies for certified high-efficiency solar collectors. By choosing BTE’s systems, project developers and distributors are able to qualify for these programs more easily, ultimately gaining higher returns on investment while supporting the transition to clean energy.

In short, with certified high-efficiency flat-plate solar collectors that combine advanced sealing technology, excellent thermal output, and long-lasting reliability, BTE Solar helps its partners maximize both heating performance and access to policy-driven benefits, strengthening competitiveness in today’s global renewable energy market.

3. Sealing Technology: Strengthening Structures for Reliability 

Sealing remains one of the most decisive factors in ensuring the long-term performance and safety of solar thermal systems. Even with highly efficient absorbers and advanced selective coatings, the overall reliability of a solar collector can be compromised if the sealing is inadequate. Poorly executed frame joints or weak bonding between the backsheet and the enclosure frequently result in water vapor penetration, corrosion of metallic parts, thermal insulation degradation, and eventually a shorter service life for the entire system. For customers and manufacturers alike, mastering sealing technology is no longer optional—it is a core requirement for competitiveness. 

Traditionally, most frame structures have relied on a combination of adhesive coatings, double-sided sealing tapes, or lever-based pressure strips. In this widely used design, the backsheet is inserted into what is known as an “alligator mouth” structure, where it is pressed tightly by a specialized backsheet machine and further locked into place with internal angle brackets or corner connectors. Compared with earlier riveted frames, this approach does represent significant progress: it offers a more uniform fit, improves basic weather resistance, and helps streamline mass production. However, despite these improvements, such structures often fall short of meeting the increasingly demanding hermetic requirements of today’s solar thermal systems. 

The limitations become especially visible at two critical points: the 45° corner joints and the linear engagement areas of the alligator mouth. At these locations, microscopic gaps are difficult to eliminate completely. Over time, thermal expansion, vibration, and external stress can worsen these gaps, compromising overall flatness and allowing air or water vapor to infiltrate the collector interior. This risk is particularly evident in large-area flat plate panels, where even minor deformations are magnified. Furthermore, the mechanical design of the alligator mouth requires the use of thicker and heavier materials to maintain structural strength. While this improves rigidity, it inevitably increases both raw material consumption and manufacturing costs, putting additional pressure on producers seeking to balance performance and price.

To address these persistent challenges, forward-looking enterprises and R&D teams have been actively exploring more reliable sealing strategies. Several promising approaches are currently shaping the industry: 

1. External angle bracket designs: By moving the sealing point outside the primary absorber–backsheet interface, these designs simplify the geometric requirements of hermetic sealing and significantly reduce the likelihood of hidden micro-gaps at corner joints. 

2. Continuous adhesive application: Instead of assembling segmented frames with discrete bonding points, new production lines are beginning to apply adhesives in one continuous process across an integrated frame. This not only improves sealing uniformity and flatness but also ensures stronger alignment and eliminates weak junctions. The trade-off, however, is the need for major investments in precision equipment and automation. 

3. Advanced silicone adhesives: With exceptional elasticity and resilience, silicone-based adhesives maintain sealing effectiveness across an unusually wide operating temperature range—from as low as -55 °C to as high as 210 °C. This ensures reliable performance even in harsh climates with strong temperature fluctuations, sandstorms, or high humidity. 

4. Reinforcement ribs and absorber plate supports: By enhancing frame strength and preventing warping or deformation, these mechanical improvements directly contribute to long-term sealing integrity. A well-supported absorber plate distributes stress more evenly, reducing the chance of seal fatigue and leakage. 

Together, these innovations highlight the solar thermal industry’s continuous progress toward achieving a delicate balance between manufacturability, cost control, and hermetic reliability. For end users, better sealing translates into greater confidence in system durability, lower maintenance costs, and a longer service life. For manufacturers, mastering advanced sealing techniques is a way to differentiate their products, reduce warranty risks, and build stronger trust with international clients.

Looking Ahead: Toward a More Competitive Flat-Plate Industry

The evolution of China’s flat-plate solar collector industry is unfolding in phases. The sector has already moved beyond the stage of widespread imitation, making tangible progress in design innovation, water mist mitigation, and sealing technologies.

As a global leader in solar thermal, BTE Solar combines advanced selective coating technology with extensive international project experience. With its unique dual-layer coating production lines and flexible membrane options, the company is helping partners worldwide improve thermal stability, reliability, and cost-efficiency in their solar hot water solutions.

For businesses exploring high-quality flat-plate collector systems or seeking long-term collaboration opportunities, BTE Solar welcomes inquiries and partnerships to jointly expand the clean energy market.