BIPV Technology: Equipping Buildings with "Green Power Plants"

I. The "Wonderful Union" of Photovoltaic Power Generation and Buildings

In today's era of pursuing green environmental protection and sustainable development, the combination of photovoltaic power generation and buildings has become a hot topic. Among them, distributed photovoltaic power generation is the most mainstream application form, with broad development prospects. According to data from the China Photovoltaic Industry Association, by 2023, this combined form of distributed photovoltaic power generation projects accounted for 80% of the total distributed photovoltaic power generation projects nationwide, and future demand will continue to increase.

There are two main ways to combine photovoltaic power generation and buildings. One is to install a photovoltaic power generation system on existing buildings, which is BAPV. This method is like putting a power-generating "coat" on the building; it does not affect the original function of the building, but it cannot replace some of the functions of the building itself. The other is Building-Integrated Photovoltaics (BIPV), which we will focus on today. It integrates photovoltaic components and buildings during design, construction, and installation, making photovoltaic components part of the building's external structure. It can generate electricity and be used as a building material, like injecting power-generating "superpowers" into the building. This is an important means of achieving ultra-low energy consumption, near-zero energy consumption, or even energy production in buildings and is considered the main development direction of future distributed photovoltaic power generation.

 

BIPV technology has many advantages. First, it is energy-efficient and environmentally friendly. Using photovoltaic components to generate electricity reduces reliance on fossil fuels and lowers environmental pollution. Second, it saves land resources by combining power generation and building functions, making it particularly suitable for densely populated areas with limited land resources. Moreover, BIPV technology can also meet the aesthetic requirements of buildings. Photovoltaic components of suitable colors can be selected according to the surrounding environment, allowing the building to blend perfectly with its surroundings.

Next, let's delve deeper into the key photovoltaic components in BIPV technology, looking at their types and their practical applications.

II. Three Types of Photovoltaic Components: Each with its own "Special Abilities"

According to the development of photovoltaic power generation technology, photovoltaic components can be roughly divided into three categories: crystalline silicon photovoltaic components, thin-film photovoltaic components, and new photovoltaic components represented by perovskite photovoltaic components and dye-sensitized photovoltaic components. In recent years, the photoelectric conversion efficiency of thin-film photovoltaic components and perovskite photovoltaic components has improved significantly, and their applications in the BIPV field are increasing, becoming a research hotspot.

(1) Crystalline Silicon Photovoltaic Components: "Veteran Players" with Advantages and Challenges

Crystalline silicon photovoltaic components are considered "veteran players" in the photovoltaic field. In 2023, the best photoelectric conversion efficiency of monocrystalline silicon solar cells in the laboratory reached 26.7%, and commercially available ones reached 24.0%; the best laboratory efficiency of polycrystalline silicon solar cells was 24.7%, and commercially available ones reached 22.4%. Such high conversion efficiency gives it certain application advantages in the BIPV field. Taking the European BIPV market as an example, 90% of photovoltaic roof building materials use crystalline silicon photovoltaic components; 56% of building facade structures also use them. However, crystalline silicon photovoltaic components also have some problems. Their light transmittance is relatively low, their colors are monotonous, and it is difficult to achieve lightweight design. These shortcomings limit their further development in the BIPV field. To enable large-scale application in BIPV buildings, these problems need to be addressed.

 

(2) Thin-Film Photovoltaic Components: "Rising Stars" with Huge Potential

Thin-film solar cells are "rising stars" with huge potential in the BIPV field, with many outstanding advantages. For example, they have a high light absorption coefficient, perform well in low-light environments, and can be made into flexible photovoltaic components. Currently, cadmium telluride (CdTe) thin-film solar cells and copper indium gallium selenide (CIGS) thin-film solar cells have been successfully commercialized.

 

CdTe thin-film solar cells have a relatively simple structure and manufacturing process, and the manufacturing cost is relatively low. First Solar, a leading company in thin-film photovoltaic products, has repeatedly broken world records for the photoelectric conversion efficiency of CdTe thin-film solar cells, reaching 22.1% in 2023, with an average industrial photoelectric conversion efficiency of 19.9% and a manufacturing cost of less than $0.25 per watt, and high production capacity. Globally, companies such as First Solar in the United States, and Kaiseng Group and Longyan Energy Technology (Hangzhou) Co., Ltd. in China, can mass-produce CdTe thin-film photovoltaic components on a large scale.

The band gap of CIGS thin-film solar cells can be optimized by adjusting the ratio of In and Ga elements, thereby improving the photoelectric conversion efficiency. Since its initial research and development, its photoelectric conversion efficiency has made great progress. In 2023, the laboratory photoelectric conversion efficiency of CIGS thin-film solar cells developed by Michigan State University reached 24.8%, and the efficiency of commercially available photovoltaic components developed by First Solar reached 23.4%. Companies such as Solar Frontier in Japan, First Solar in the United States, and Kaiseng Group in China have achieved large-scale production of CIGS thin-film solar cells and their photovoltaic components, with a bright market outlook.

 

However, after thin-film solar cells are made into photovoltaic components, due to lamination, encapsulation, and other processes, problems such as multi-layer interface reflection, resistance loss, and uneven illumination may occur, leading to a decrease in photoelectric conversion efficiency. This is a problem that needs to be solved for further promotion in the BIPV field.

(3) New Photovoltaic Components: "Potential Stocks" Full of Hope

Dye-sensitized photovoltaic components are one of the emerging photovoltaic component technologies. They have advantages such as translucency, low manufacturing cost, simple preparation process, and good weak light performance, and are gradually emerging in commercial applications. The large-scale application effects of dye-sensitized photovoltaic components have been verified in famous buildings such as the Swiss Technology Convention Center in Lausanne, Switzerland, and the Science Tower in Graz, Austria. They are less sensitive to the angle of incident light and may even have higher photoelectric conversion efficiency at higher temperatures. However, in the process of transitioning from the laboratory to industrialization, some obstacles have been encountered, such as poor long-term stability and difficulty in expanding the size of solar cells to meet the output power requirements of photovoltaic components.

 

Perovskite photovoltaic components are the most promising emerging technology in the photovoltaic field in recent years. Since relevant research reports in 2012, its photoelectric conversion efficiency has been continuously breaking through, reaching 25.7% in 2023, developing much faster than other new types of solar cells. Moreover, it can be combined with commercially available solar cells to develop tandem solar cells to further improve photoelectric conversion efficiency. Currently, the photoelectric conversion efficiency of perovskite/crystalline silicon tandem solar cells has reached 33.7%. However, to achieve large-scale commercial production in the BIPV field, some problems still need to be solved, such as improving the stability of solar cells, finding a substitute for the toxic Pb element, and preparing large-area solar cells.

 

III. Diverse Applications of BIPV Technology: Transforming Buildings into "Power Plants"

To achieve a seamless integration of photovoltaic power generation technology with buildings, designers need to consider many aspects, such as building appearance, photovoltaic component installation requirements, power demand, power generation efficiency, and architectural aesthetics. When applying BIPV technology, the optimal installation angle, size, and position of the photovoltaic components should be determined based on the geographical location of the building to maximize power output. Furthermore, reducing the building's power load is crucial, which can be achieved through optimizing the building's envelope structure, increasing daylight hours, and utilizing natural ventilation. BIPV technology has a wide range of applications, primarily focusing on building facades, roofs, windows, fences, and awnings.

 

(1) Photovoltaic Facades: The "Gorgeous Transformation" of Building Exterior Walls

Photovoltaic facades replace traditional exterior wall materials or decorative materials with BIPV components. They offer the functionality of traditional curtain walls while generating electricity, giving buildings a unique style. A typical photovoltaic facade involves fixing photovoltaic components between two high-transmittance glass panels, then installing them on the building's exterior. Outdoor air enters from the bottom of the curtain wall and exits from the top, carrying away the heat generated by the photovoltaic components, reducing the surface temperature, improving power generation efficiency, and extending service life. In some special cases, fans and air ducts are added between the photovoltaic components and the building wall to form a photovoltaic/photothermal building integrated system (BIPV/T), delivering heat indoors to reduce winter heating loads.

 

Numerous studies have shown that optimizing ventilation conditions can improve the performance of photovoltaic facades. For example, some researchers have improved the power output of BIPV buildings by optimizing airflow; others have found that forced ventilation can enhance the photoelectric conversion efficiency of polycrystalline silicon solar cells. Furthermore, different types of photovoltaic facades are constantly being innovated. For instance, double-layer photovoltaic facades with artificial ventilation can improve building thermal comfort and energy collection efficiency; CdTe thin-film photovoltaic components not only reduce glare but also prevent hot spot effects, making them suitable for BIPV buildings; and photovoltaic precast concrete (PVPC) facades can flatten uneven concrete exterior walls. There is also a photovoltaic curtain wall composed of vacuum insulated semi-transparent thin-film photovoltaic glass and low-emissivity coated glass, which has a very low heat transfer coefficient and excellent insulation and power generation performance.

 

Photovoltaic facade products on the market are increasingly focusing on appearance design, incorporating patterns, textures, and colors, and can be customized to create creative CdTe thin-film photovoltaic glass according to customer needs. The photovoltaic glass industry, represented by CdTe and CIGS thin-film photovoltaic components, is driving the rapid development of China's BIPV technology. For example, the 400kW photovoltaic facade project of Kaiseng Robot Intelligent Equipment R&D Center uses the new generation of thin-film photovoltaic glass independently developed and produced by Kaiseng Technology Group. The office power consumption of the entire R&D center can be met by this green electricity, setting a good example for the construction of green and low-carbon industrial parks.

 

(2) Photovoltaic Roofs: Roofs That Can Also "Generate Wealth"

Integrating photovoltaic components with roofing materials creates photovoltaic roofs. Depending on the building materials, photovoltaic roofs can be divided into photovoltaic skylights, photovoltaic tile roofs, and metal photovoltaic roofs. Photovoltaic roofs not only save on roofing material costs and reduce upfront construction costs but also extend the lifespan of the roof.

 

Research has found that increasing ventilation during the photovoltaic component power generation process can improve power generation efficiency. For example, some researchers have enhanced the performance of polycrystalline silicon solar cells by increasing ventilation, optimizing ventilation rate, and air gap; others have studied the stepped arrangement of photovoltaic components on sloped roofs, verifying the impact of ventilation rate on component performance.

With the increasing maturity of BIPV technology, many demonstration projects have been implemented. For example, the 8.5-generation TFT-LCD ultra-thin float glass substrate production line factory project is the largest single thin-film BIPV application demonstration project in China. It adopts a "self-generation and self-use, surplus electricity to the grid" model, significantly reducing industrial energy consumption indicators and setting an example for achieving building energy transformation and building a green and smart energy pilot city.

(3) Photovoltaic Windows and Shading Components: The "Perfect Combination" of Daylight and Ventilation and Power Generation

Daylight and ventilation are important functions of windows. Traditional photovoltaic windows using opaque crystalline silicon photovoltaic components prevent light transmission in the installation area; while semi-transparent crystalline silicon photovoltaic components allow light transmission, the effect is limited. Thin-film photovoltaic components allow photovoltaic windows to appear transparent, improving indoor lighting. Some researchers have studied the optimal solar cell coverage of polycrystalline silicon photovoltaic components in semi-transparent photovoltaic windows, and the influence of parameters such as different window-wall ratios, transmittance, and solar incidence angles on the performance of photovoltaic windows.

 

Photovoltaic components can also be combined with building shading components to form various photovoltaic awnings, such as exterior window photovoltaic awnings, eaves photovoltaic awnings, and corridor photovoltaic awnings. Combining BIPV technology with Venlo-type glass greenhouses can create photovoltaic greenhouses that generate electricity, transmit light, and collect heat, achieving energy saving and efficiency improvement, increased production and income.

 

4. The "Competition" Between Two Types of Photovoltaic Components in BIPV Scenarios

Currently, mass-produced crystalline silicon photovoltaic components have a higher photoelectric conversion efficiency than thin-film photovoltaic components. They can achieve a larger installed capacity in the same area, making them more economical in centralized ground-mounted photovoltaic power plants.

However, the situation is different in BIPV scenarios. Photovoltaic components in BIPV scenarios must first meet the shape and functional requirements of the building, unlike traditional photovoltaic power plants where the orientation and installation angle of the components are prioritized. This weakens the power generation efficiency advantage of crystalline silicon photovoltaic components. Moreover, crystalline silicon photovoltaic components have a monotonous appearance, low light transmittance, and affect the aesthetics and lighting of the building. In addition, they are prone to micro-cracks under vibration, and local dirt blockage can cause hot spot effects, affecting the reliability and safety of BIPV buildings.

In contrast, thin-film photovoltaic components have their own advantages in the BIPV market. They can respond to a wider range of wavelengths of sunlight, have a longer effective working time, generate more electricity per watt of power, and have a more obvious advantage in weak light power generation under non-optimal installation angles. Their appearance color can be customized, and the light transmittance can be adjusted, making them more flexible to apply. Moreover, thin-film photovoltaic components have good hot spot resistance, and the power generation loss caused by local shading is less, and hot spots will not pose a safety hazard to the building. However, the current economic efficiency of thin-film photovoltaic components is still not as good as that of crystalline silicon photovoltaic components, and some raw materials required for their preparation are rare metals, which are difficult to obtain, and rare earth reserves also limit their production.

With the requirements for energy efficiency renovation of existing buildings proposed in China's 14th Five-Year Plan, the scale of new building construction and existing building renovation will continue to expand in the future, and the demand for the BIPV market will also continue to increase, which is of great significance to achieving the "carbon peak" goal.

V. Future Prospects of BIPV Technology: Opportunities and Challenges Coexist

Currently, photovoltaic power generation technology is becoming increasingly mature, and national policies are strongly promoting the application of BIPV technology. In June 2022, the Ministry of Housing and Urban-Rural Development and the National Development and Reform Commission jointly issued a plan proposing that by 2025, the photovoltaic coverage rate of newly built public institution buildings and newly built factory rooftops should strive to reach 50%. This indicates that the conditions for large-scale application of BIPV technology are in place, and the timing is ripe. However, as an emerging industry, there are still areas that need improvement.

 

In terms of standardization, although some regulations have been issued, national standards, industry standards, and regulations related to BIPV technology, including design, construction, and architecture, as well as architectural standard atlases, are still not perfect, which will affect the further promotion of the BIPV market. In terms of market acceptance and recognition, there is also room for improvement. On the one hand, when developing BIPV products, their architectural attributes should be fully considered, and material properties should be incorporated from the design stage to address the technical gap between the construction and photovoltaic industries and provide a complete set of BIPV services. On the other hand, starting from the application end, analyze the needs of different scenarios, improve the building material level of BIPV products, and increase publicity through applications in high-quality demonstration projects.

In the future, BIPV technology should pay more attention to the integration of the photovoltaic and construction industries. BIPV products must not only improve the photoelectric conversion efficiency but also meet the requirements of building materials, such as airtightness, water tightness, wind pressure resistance, and in-plane deformation performance. The adaptability of BIPV products and power electronic products such as inverters and power optimizers should be optimized to improve the safety performance of BIPV buildings. Moreover, the application of BIPV technology cannot be limited to the unification of photovoltaic components and building shapes. It should also use energy storage technology and intelligent information platforms to achieve the balance of photovoltaic power supply and demand, minimize the building's consumption of fossil energy, and promote the development of buildings towards low-carbon and zero-carbon.

VI. Summary: The Development Path of BIPV Technology

BIPV technology combines photovoltaic components and buildings, with huge development potential and advantages. We have learned about three types of photovoltaic components that can be used in BIPV technology, each with its own advantages and disadvantages, and all are constantly developing and improving. At the same time, BIPV technology has a wide range of applications in building curtain walls, roofs, windows, etc., and there have been many successful demonstration projects. Although BIPV technology currently faces challenges such as imperfect standardization and low market acceptance, with technological advancements and policy support, it will surely have broader development space in the future, making greater contributions to achieving the "dual carbon" goals and promoting the sustainable development of the construction and energy industries.