Giving Industrial Plant Roofs a Green Future: The Wonderful Application of BIPV Mode

I. New Opportunities for Photovoltaic Buildings under the "Dual Carbon" Wave

Under the guidance of the "14th Five-Year Plan", China is vigorously encouraging the development of the photovoltaic industry and green buildings. Behind this is the important goal of achieving carbon peaking and carbon neutrality. To achieve this goal, green industries, including Building-Integrated Photovoltaics (BIPV), have ushered in unprecedented development opportunities. In 2022, many provinces introduced policies to support the development of BIPV, such as increasing project subsidies, which injected a "strong heart" into the development of the BIPV model. In the past, the BIPV model developed relatively slowly due to its high cost. But now it's different. With the rapid development of the photovoltaic industry, the cost of photovoltaic power generation systems is decreasing year by year, and coupled with policy subsidies, the BIPV model is about to enter a stage of rapid development.

However, at present, most of the research on the BIPV model focuses on the cost, and there is not enough research on its specific application in the roofs of industrial plants. In terms of the application of distributed photovoltaic power generation technology, the market is mainly dominated by the Building-Applied Photovoltaics (BAPV) model, which is attached to buildings. Many investors believe that the cost of the BIPV model is much higher than that of the BAPV model, so they are reluctant to choose the BIPV model. Today, let's delve into the application of the BIPV model on the roofs of industrial plants and see what advantages and challenges it has.

II. BIPV and BAPV: Different Models

Currently, the main design schemes in China that combine buildings and photovoltaic power generation systems are the BIPV and BAPV models. Let's talk about the BAPV model first. It's like putting a photovoltaic power generation "coat" on a building. Photovoltaic components are usually arranged on the roof of the building to form a rooftop photovoltaic power generation system. This is also the main application form of the whole county (city, district) distributed photovoltaic power generation projects that the country is vigorously promoting, and its application range is quite wide. The BAPV system mainly includes photovoltaic components, photovoltaic brackets, and some metal pressure blocks, clamps, weight blocks, and bolts used for fixing. When selecting the material for the photovoltaic bracket, 6063 aluminum alloy, Q235 ordinary carbon structural steel, or Q355 low-alloy high-strength structural steel will be used according to the specific situation. During the design, it must be ensured that the entire BAPV system will not affect the stability of the main building structure under various loads, such as constant loads, wind loads, snow loads, and earthquake effects.

《BIPV Project Technical Solution.pdf》

The BIPV model is different from the BAPV model. In the BIPV model, photovoltaic components are directly used as the roof. It generates electricity while also acting as a building material, becoming part of the building. This is a new type of rooftop photovoltaic power generation system that can save a lot of land resources and may become the main form of distributed photovoltaic power generation in the future. Currently, the BIPV model is mainly used in newly built industrial buildings, such as the roofs of factories and warehouses in industrial parks. Its application in commercial and residential buildings is relatively less common, generally used on the roofs or walls of large comprehensive supermarkets and high-rise office buildings. The composition of the BIPV system is also quite complex, including photovoltaic components, waterproof brackets, Z-shaped steel photovoltaic component mid-support structure, transparent inspection walkways (this can be selected for installation according to the building function), inspection sheet metal walkways, color steel plate walkways, and various water-cutting water-stop plates, waterproof sealing strips, and ridge covers designed according to the actual building conditions. In order to reduce the load burden on the main building structure, the waterproof bracket generally uses Q355 low-alloy high-strength structural steel, and the Z-shaped steel uses Q235 ordinary carbon structural steel. When installing photovoltaic components, the BIPV model has particularly high requirements for the construction of the waterproof structure, and it must ensure that the entire roof is waterproof.

 

III. Influencing Factors of BIPV Mode Application on Industrial Plant Roofs

(I) Load-Bearing Capacity: A "Good Helper" to Reduce Roof Burden

Although the country is vigorously promoting rooftop distributed photovoltaic power generation projects, many factories are unable to successfully install them due to insufficient structural load-bearing capacity. Currently, the proportion of light steel structure factories in industrial parks is considerable. In recent years, the standards and specifications of the construction industry have been continuously updated, such as the "Steel Structure Design Standard" in 2017, the "Unified Standard for Reliability Design of Building Structures" in 2018, and the "General Specifications for Engineering Structures" and "General Specifications for Steel Structures" in 2021. The new specifications have increased the design parameters and safety factors of buildings, which has resulted in many industrial plants designed according to old standards that no longer meet the load-bearing requirements and need to be reinforced.

 

The roofs of common light steel structure factories are generally composed of steel beams, purlins, and color steel plates. The self-weight of a common 0.6mm thick single-layer color steel plate is about 0.13kN per square meter. If a photovoltaic power generation system is installed on the roof, the weight of the photovoltaic components, waterproof brackets, and supporting auxiliary materials is approximately 0.20kN per square meter. In this calculation, the total weight of the roof after installation is approximately 0.33kN per square meter. However, if the BIPV model is used, and the photovoltaic components directly replace the color steel plates, the constant load of the steel structure roof can be significantly reduced.

 

Let's take an industrial plant as an example. The total constant load of the color steel tiles and purlins on the roof of this plant is 0.20kN per square meter, the live load on the roof is 0.50kN per square meter, the basic wind pressure is taken as 0.30kN per square meter according to the design requirements (50-year recurrence period value), the basic snow pressure is taken as the 100-year recurrence period value, which is 0.45kN per square meter, and the single-span span is 8m. In the BAPV mode, the load of the photovoltaic components plus the photovoltaic bracket is taken as 0.15kN per square meter. Researchers used the structural design software PKPM to calculate the constant load arrangement, reinforcement envelope, and steel structure stress ratio on the steel beams under the BAPV and BIPV modes, respectively.

 

The calculation results show that the BIPV mode can effectively reduce the roof constant load and reduce the rigid frame stress ratio. For some light steel structure factories with a relatively light degree of excess, the use of the BIPV mode eliminates the need for additional reinforcement of the building structure, directly meeting the load-bearing requirements, saving reinforcement costs. For projects with more obvious excess, the BIPV mode can also reduce reinforcement costs.

(II) Cost: Short-Term and Long-Term "Economic Accounts"

Cost is a key factor to consider when industrial plants choose the BIPV model. Let's take the distributed photovoltaic power generation project of the Jiangxi Provincial Expressway Electric Power New Energy Co., Ltd. invested in the Jiangxi Traffic Emergency Maintenance Base as an example to carefully calculate this economic account.

This project plans to use the roofs of 5 warehouses in the maintenance base, with a total area of approximately 12949 square meters, and a total installed capacity of photovoltaic components of 1.68MW. The project is located at No. 2 Keji Avenue, Gongqing City, Jiujiang City, Jiangxi Province. Using the BIPV model on the roofs of the warehouses in this maintenance base has many advantages. It can fully utilize the existing roofs and generate "green" electricity. In the long run, compared with the BAPV model, it can also reduce project construction costs and achieve comprehensive land use.

 

From the perspective of investment cost per watt, based on past design experience, if the BAPV model is adopted, excluding reinforcement costs, the investment cost per watt is approximately 3.6 yuan. This project uses the BIPV model, and the owner has high requirements for aesthetics. After calculation, the investment cost per watt is approximately 4.1 yuan. Looking at this number alone, it seems that the BIPV model is not cost-effective, and the rate of return does not meet the owner's requirements, making it seem to have little investment value. However, in reality, after communication between the designer and the owner, a solution was found, which is a cost transfer business model. During the factory design phase, the economic feasibility of the BIPV model and the BAPV model is compared. The roof construction and maintenance costs saved by the BIPV model are used to offset the additional waterproof bracket costs of the project. In this way, the project's rate of return can meet the requirements of the owner.

 

Looking at the overall cost, under the same type of photovoltaic components, comparing a newly built 10,000-square-meter color-steel roof photovoltaic power generation system, the cost difference between the BAPV model and the BIPV model is quite large. Using the BIPV model not only increases the installed capacity of photovoltaic components by 30%, but also includes the initial color-steel roof construction cost and later maintenance costs. The BIPV model can save 1.26 million yuan compared to the BAPV model. From this perspective, the BIPV model still has a significant advantage in construction costs. However, it should be noted that the cost data here is for reference only. In actual projects, careful calculations must be made based on the specific situation.

IV. Real Challenges of the BIPV Model

Although the BIPV model has many advantages, compared to the relatively mature BAPV model, it also has some problems.

(1) High Short-Term Construction Costs

In the short term, the unit construction cost of the BIPV model is higher than that of the BAPV model, at least 0.5 yuan more per watt. The costs in terms of materials, technical design, and installation and construction need to be reduced. However, in the long run, the roof maintenance cost of the BAPV model will be much higher than that of the BIPV model, so from a long-term economic perspective, the BIPV model still has a great advantage.

(2) Building Ventilation Affects Power Generation Efficiency

Photovoltaic components generate heat during power generation. If the building's ventilation performance is poor, this heat cannot be dissipated, which will reduce the power generation efficiency of the photovoltaic components, increase the indoor temperature, and greatly reduce the energy-saving effect of the building. Currently, the BIPV model is mostly used on the warehouses of industrial plants. Therefore, before deciding to adopt the BIPV model, the building's ventilation performance must be evaluated first. If the ventilation is poor, it is best to modify the building first, re-evaluate it after the modification, and then make a decision.

(3) Strict Construction Requirements

BAPV project construction is relatively simple and can generally be operated on the original roof. However, BIPV projects are different because there is no existing roof layer. During construction, special cranes, trailers, and other equipment may be needed. Moreover, when installing the main waterproof structure, the construction process requirements are particularly high. Even a slight mistake may cause water leakage indoors on rainy days, resulting in economic losses. Therefore, it is particularly important to find a professional team with experience in BIPV project construction, which can greatly reduce the risks brought about by construction.

 

V. Summary: The Future of BIPV is Promising

Through research and analysis of actual cases, we have found that the BIPV model has many advantages in the application of industrial plant roofs. It can reduce the permanent load of the steel structure roof of industrial plants. If the plant is not seriously overloaded, the BIPV model may not even require reinforcement, saving reinforcement costs; even if it is significantly overloaded, it can reduce reinforcement costs. In terms of cost, under the same construction conditions, the BIPV model can reduce roof construction costs. In the long run, for every 10,000 square meters of factory building, the BIPV model can save 1.26 million yuan in construction and maintenance costs compared to the BAPV model.

Although the BIPV model currently has some problems, such as high short-term construction costs, requirements for building ventilation, and high construction difficulty, with the increasing attention paid by the government to the BIPV model and the successive introduction of relevant electricity price subsidy policies, these problems will gradually be solved. It is believed that in the near future, the BIPV model will be more widely used in industrial plants and even more building fields, contributing greater strength to achieving the "dual carbon" goal.