Abstract: This paper elucidates the basic components and working principle of a solar photovoltaic (PV) power generation system, proposes the concept of building-integrated photovoltaic (BIPV) systems, and analyzes its development advantages. Then, it presents a design concept for constructing a solar PV power station in the new plant area of Kyushu Electric, and finally predicts the development prospects of this project.
Keywords: solar energy; photovoltaic power generation system; building-integrated photovoltaics
1. Introduction
Energy is the fundamental driving force of social development. Modern buildings rely excessively on traditional energy sources such as coal, oil, and natural gas. High-energy-consuming and inefficient buildings not only increase energy consumption but also pollute the environment. Solar energy, as an inexhaustible and clean energy source, is safe, pollution-free, renewable, and widely distributed. It is the most ideal renewable and green energy source, the source of all fossil fuels and many renewable energy sources, and the most sustainable way to alleviate the energy crisis. It is an energy product that countries around the world are vying to develop. Solar photovoltaic power generation, as a new form of energy, has developed rapidly in recent years. Promoting and using solar photovoltaic power generation is of great significance for energy conservation and emission reduction.
Solar photovoltaic (PV) power generation is a form of power generation that directly converts sunlight into electrical energy. Due to its advantages such as safety, reliability, cleanliness, noiselessness, pollution-free operation, short construction period, and simple maintenance, it is widely used in modern urban buildings, municipal public works, and lighting projects. In recent years, with increasing demands for building energy efficiency, building-integrated photovoltaic (BIPV) systems have become the development trend for PV power generation.
2. Solar photovoltaic power generation technology
2.1 Basic Components of a Solar Photovoltaic Power Generation System
A solar photovoltaic power generation system mainly consists of solar cell arrays, controllers, battery banks, inverters, and other equipment. The functions of each component are as follows:
(1) Solar cell array: A solar cell array consists of solar cell panels and an array support. Because the voltage of a single solar cell is generally low, they are usually connected in series and parallel to form a solar cell panel with practical value as an application unit. Then, according to the power supply requirements, multiple application units are connected in series and parallel to form a solar cell array.
Solar panels (made of certain semiconductor materials, currently mainly polycrystalline silicon, monocrystalline silicon, and amorphous silicon, assembled using specific processes) are the most important component of a solar photovoltaic system and also the most valuable part of the system. When sunlight shines, the solar cells absorb light energy, and opposite charges accumulate at the two ends of the cell, generating a "photovoltaic voltage," also known as the "photoelectric effect." Under the influence of the photoelectric effect, an electromotive force is generated at the ends of the solar cell, converting light energy into electrical energy; it is an energy conversion device.
(2) Battery bank: Its function is to store the electrical energy generated by the solar array when it is exposed to sunlight and to supply power to the load at any time. In a grid-connected solar power generation system, a battery bank is not required.
(3) Controller: A device for regulating and controlling electrical energy.
(4) Inverter: It is a device that converts the DC power provided by the solar cell array and the battery into AC power. It is a key component of the photovoltaic grid-connected power generation system. Since the solar cell and the battery are DC power sources, the inverter is essential when the load is an AC load. According to the operation mode, the inverter can be divided into stand-alone inverter and grid-connected inverter. Stand-alone inverter is used for stand-alone solar cell power generation systems to supply power to stand-alone loads. Grid-connected inverter is used for grid-connected solar cell power generation systems. This article mainly introduces the solar photovoltaic grid-connected power generation system [1]. As shown in Figure 1, the grid-connected inverter is composed of power switching devices such as IGBT. The control circuit makes the switching elements turn on or off in a certain regular manner, so that the polarity of the output voltage alternates between positive and negative, and converts the DC input into AC output. According to the output waveform, the inverter can be divided into square wave inverter and sine wave inverter. The square wave inverter has a simple circuit and low cost, but the harmonic components are large. It is generally used in systems with a power of several hundred watts or less and with low requirements for harmonics. Sine wave inverters are expensive, but they can be used with a variety of loads.
Figure 1: Main circuit topology of grid-connected inverter
In addition to the functions of a regular inverter, a grid-connected inverter should also have the following functions:
1) Pure Sine Wave Synchronous Grid-Connected Power Transmission: Instantaneous current control is achieved through a DC/AC voltage-source inverter, controlling the current to a 50Hz sine wave, which is then automatically synchronized with the grid before being fed into the grid. Grid-connected power transmission using a sine wave current will not generate harmonic interference or excessive reactive power components to the grid.
2) Maximum Power Point Tracking (MPPT) Technology for Solar Cells: Solar cells based on crystalline silicon have different IV characteristic curves under different irradiation intensities and temperatures. Since there is a maximum power output point corresponding to the IV characteristic, tracking the maximum power output point of the solar cell (MPPT) has become one of the key points to improve the efficiency of the entire system.
3) Anti-islanding operation technology: When the grid is running and there is a power outage due to an unexpected situation, the grid-connected equipment should be able to detect the power outage in a timely manner, disconnect from the grid, and stop supplying power to the grid in order to protect personal and equipment safety.
2.2 Working principle of solar photovoltaic grid-connected power generation system
Solar power generation systems produce electricity through the photoelectric effect. The generated electricity is converted from direct current (DC) to alternating current (AC) by an inverter, and then regulated and controlled by a controller, as shown in Figure 2. Under sunlight, solar cell modules generate a certain electromotive force (EMF). These modules are connected in series and parallel to form a solar cell array, ensuring the array voltage meets the system input voltage requirements. This EMF is then converted back to AC by the inverter. A portion of the AC power is sent to a distribution cabinet, where it is switched to supply power for purposes such as lighting. The remaining power is fed into the power grid.
Figure 2: Solar photovoltaic grid-connected power generation system
2.3 Characteristics of Solar Photovoltaic Systems
(1) Energy saving: Solar photovoltaic conversion provides electricity, which is inexhaustible;
(2) Environmentally friendly: no pollution, no noise, no radiation;
(3) High quality: High-tech products and green energy; users attach importance to technology and environmental protection, which enhances the image and class of the company.
(4) Wide applicability: Solar energy originates from nature, so it can be used wherever there is sunshine.
(5) Flexibility: Flexible access to and from the power grid, which is beneficial to improving the load balance of the power system and reducing line losses.
3. The concept and trend of building-integrated photovoltaic (BIPV) power generation
Building Integrated Photovoltaics (BIPV) is a new concept in solar power generation. Simply put, it involves installing solar photovoltaic arrays on the exterior surface of a building's building envelope to provide electricity. This system seamlessly integrates solar power generators into the building's walls or roof. Its working principle is exactly the same as a regular photovoltaic system; the only difference is that the solar modules serve as both the system's generator and the building's exterior wall material.
Building-integrated photovoltaic (BIPV) systems typically consist of photovoltaic arrays, walls or roofs, cooling air ducts, and support structures. A complete BIPV system also requires additional equipment loads, batteries, inverters, and system controllers. BIPV systems can operate as independent power sources or be connected to the grid. When a BIPV system is grid-connected, batteries are not required, but grid connection is necessary, and grid-connected power generation is a current trend in photovoltaic applications. Installing photovoltaic modules on the roof or exterior walls of a building, with the output terminals connected to the public power grid via controllers and inverters, and supplying power to users through parallel connections between the photovoltaic array and the grid, constitutes a residential grid-connected photovoltaic system. Because this type of system requires almost no batteries, it significantly reduces system costs. Furthermore, in addition to power generation, it offers multiple functions such as environmental protection and the substitution of certain building materials; therefore, BIPV systems have become a hot topic in research and development.
4. Design Analysis of Harbin Jiuzhou Electric 3MW Rooftop Photovoltaic Power Station Project
4.1 Project Design Concept
The new plant of Harbin Jiuzhou Electric is located in Songbei District, Harbin. This phase of the project is the rooftop photovoltaic power generation project of Jiuzhou Electric's research building, with a planned installed capacity of 3MW. The building construction highlights the theme of energy conservation and environmental protection. A photovoltaic array is installed on the roof, and the output end is connected to the public power grid through a controller and inverter. In order to save costs, a battery bank is not installed. Instead, the photovoltaic array and the grid are connected in parallel to provide power to the company. When the photovoltaic power generation system is working, the generated electricity is converted for public lighting, testing and production. When there is excess electricity generated, it is fed into the grid.
4.2 Feasibility Analysis
The sun delivers 800,000 kilowatts of energy to the Earth's surface every second. If the energy from just one hour of sunlight could be accumulated, it could meet humanity's energy needs for a year. Solar energy is inexhaustible. Most regions in my country have the conditions to develop solar photovoltaic power generation. This project is located in Harbin, situated between 125°42′ and 130°10′ east longitude and 44°04′ and 46°40′ north latitude. Harbin has a mid-latitude continental monsoon climate, characterized by four distinct seasons: spring brings lush greenery and fragrant lilacs; summer offers cool and pleasant weather for relaxation and respite from the heat; autumn brings clear skies and vibrant colors; and winter is a winter wonderland. The average temperature in January is approximately -19°C, and the average temperature in July is approximately 23°C. The average annual precipitation is 569.1 mm, with summer accounting for 60% of the total annual rainfall. The average daily irradiance on the horizontal surface is 3.63 kWh/m², and the average daily irradiance on the 45° inclined surface is 4.5 kWh/m², indicating relatively abundant solar energy resources. Furthermore, the absence of tall buildings obstructing the south side of the factory building provides favorable conditions for project implementation. Based on a conservative 75% system efficiency (25-year average efficiency), the 3MW rooftop power generation project could generate approximately 3,695,625 kWh annually. Without considering national, provincial, or municipal grid-connected electricity price subsidies, and assuming a current industrial electricity price of 1.0 yuan/kWh, this would result in a cumulative annual reduction of 3,695,625 yuan in grid electricity purchases. Therefore, this project is feasible.
4.3 Design of Main Components
Harbin Jiuzhou Electric's 3MW rooftop photovoltaic power generation system plans to adopt a segmented power generation and centralized grid connection scheme. As shown in Figure 3, the system is divided into three 1MWp photovoltaic grid-connected power generation units, and each 1MW power generation unit uses four 250KW grid-connected inverters. The battery modules of each photovoltaic grid-connected power generation unit are connected in series and parallel to form a solar cell array. The solar cell array is input into the photovoltaic array combiner box and then connected to the DC distribution cabinet. Then, it is connected to the transformer distribution device through the photovoltaic grid-connected inverter and the AC lightning protection distribution cabinet, ultimately realizing the grid-connected power generation scheme of the entire photovoltaic grid-connected system.
Figure 3. Block diagram of the design scheme for Kyushu Electric's 3MW solar grid-connected power generation system
(1) Solar cell array
1) Selection of solar photovoltaic modules
Compared with monocrystalline silicon photovoltaic modules, polycrystalline silicon photovoltaic modules have a service life of up to 25 years and a power degradation of less than 15%.
Although polycrystalline silicon has a slightly lower conversion efficiency than monocrystalline silicon, it has the advantages of high production efficiency and low cost, and polycrystalline silicon modules are generally used in high-power photovoltaic grid-connected power generation systems.
Based on cost-effectiveness calculations, this plan proposes to use 165Wp solar photovoltaic modules, with a peak power of 165Wp, a peak voltage of 24V, and a peak current of 7A.
2) Efficiency calculation of grid-connected photovoltaic system
The overall efficiency of a grid-connected photovoltaic power generation system mainly consists of three parts: the efficiency of the photovoltaic array, the efficiency of the inverter, and the AC grid-connected efficiency.
Photovoltaic array efficiency η1: The ratio of the actual DC output power to the nominal power of the photovoltaic array under a solar radiation intensity of 1000W/m2. Losses in the energy conversion process of the photovoltaic array include: module matching losses, surface dust shading losses, unusable solar radiation losses, temperature effects, maximum power point tracking accuracy, and DC line losses, etc., and are calculated using an efficiency of 85%.
Inverter conversion efficiency η2: The ratio of AC power output by the inverter to DC input power, calculated with an efficiency of 95%.
AC grid connection efficiency η3: The transmission efficiency from the inverter output to the high-voltage grid, mainly the efficiency of the step-up transformer, which is calculated as 95%.
The total system efficiency η<sub>total</sub> is: η<sub>total</sub> = η<sub>1</sub> * η<sub>2</sub> * η<sub>3</sub> = 77%.
3) Calculation of solar radiation on the surface of the tilted photovoltaic array
The data obtained from the weather station are all solar radiation on a horizontal surface, which needs to be converted into the radiation on the tilted surface of the photovoltaic array before the power generation can be calculated.
For a photovoltaic array fixed at a certain tilt angle, the solar radiation energy received is related to the tilt angle. A simple empirical formula for calculating the radiation amount [2] is:
RD=S×[sin（α+β）/sinα]+D
Where: RD—total solar radiation on the tilted photovoltaic array surface;
S—Direct solar radiation on a horizontal surface;
D—Scattered radiation;
α — Solar altitude angle at noon;
β – tilt angle of photovoltaic array.
By substituting the data obtained from the meteorological station into the formula to calculate a series of data, it was found through analysis that the solar radiation energy received throughout the year is the largest when the sun shines at a 45° tilt angle in Harbin city. Therefore, it was determined that the photovoltaic array should be fixedly installed at a 45° tilt angle.
4) Series and parallel connection scheme for solar photovoltaic modules
This project plans to use a 250KW grid-connected inverter with a DC operating voltage range of 450VDC～880VDC and an optimal DC operating point of 560VDC.
The number of solar photovoltaic modules connected in series is NS=560V/24V=23. Considering the temperature change coefficient, we assume 18 solar modules are connected in series. The power of a single series column is 18×165Wp=2970Wp. A single 250KW inverter needs to be configured with approximately 85 columns of solar modules connected in parallel, which is NP=250KW/2970W≈85 columns. A 1MWp photovoltaic array unit needs to be designed with 85×4=340 columns of branches connected in parallel, for a total of 340×18=6120 solar modules.
Therefore, the actual number of 165Wp battery modules required for the entire 3MW photovoltaic system is M=3×6120=18360, with an actual power of 3.029MW.
According to the preliminary design plan of Kyushu Electric's 3MW photovoltaic power station, the project requires 18,360 165Wp polycrystalline silicon solar cell modules, arranged in an array of 18 modules in series and 1,020 parallel branch lines.
(2) Inverter selection
This solar photovoltaic grid-connected power generation system consists of three 1MWp photovoltaic grid-connected power generation units connected in parallel. Each grid-connected power generation unit requires four 250KW grid-connected inverters (DC operating voltage range: 450VDC～880VDC, optimal DC voltage operating point is 560VDC, maximum array input current 560A). The entire system is equipped with 12 photovoltaic grid-connected inverters of this model, forming a 3MW grid-connected power generation inverter system.
(3) Monitoring system
As shown in Figure 4, the monitoring system uses sunnyWebBox as the central control unit. This system can connect up to 50 grid-connected inverters and can monitor the daily power generation and cumulative power generation of each inverter. At the same time, the system is equipped with environmental sensors that can collect information such as irradiance, temperature, and wind speed to provide a basis for analyzing the system's power generation.
Figure 4: Monitoring System
(4) Environmental monitoring device
An environmental monitoring device is installed in the photovoltaic grid-connected power generation system to monitor environmental parameters such as solar radiation intensity, wind speed, wind direction, and temperature in real time. Its communication interface can be connected to the grid-connected monitoring system to record environmental data in real time.
(5) System lightning protection grounding device
To ensure the safety and reliability of this photovoltaic grid-connected power generation system and prevent damage to system components due to external factors such as lightning strikes and surges, a lightning protection grounding device is essential.
In addition, the AC side of the grid-connected inverter must be equipped with AC surge protection distribution cabinet, AC step-up transformer and other equipment.
By designing and implementing a 3MW photovoltaic power generation system integrated with the building structure, Jiuzhou Electric will achieve self-sufficiency in electricity consumption and sell surplus electricity to the power company to generate revenue. This not only saves on electricity costs but also reduces road excavation for cable laying and other construction work, lowering investment costs. After the project is completed, the company will be transformed into a demonstration and research base for building-integrated photovoltaics.
5. Conclusion
my country possesses abundant and widely distributed solar energy resources, indicating enormous potential for solar power development. Building energy consumption accounts for approximately 27.45% of total energy consumption in my country. Integrating solar photovoltaic (PV) power generation systems with buildings to meet their own electricity needs and achieve "zero energy consumption" can significantly alter the current high energy consumption situation in Chinese buildings. Building-integrated solar power systems (BISPs) is a pioneering undertaking. While the system itself doesn't present significant design challenges, the high cost of the photovoltaic conversion system necessitates policy support from relevant government departments. In July 2009, the Ministry of Finance, the Ministry of Science and Technology, and the National Energy Administration jointly issued the "Interim Measures for the Management of Fiscal Subsidies for the Golden Sun Demonstration Project," focusing on supporting PV power station projects.
This will strongly promote the integration of photovoltaic power generation systems with building engineering, and it is believed that in the near future, solar power generation will become an essential function of buildings.
References
[1] Lin Yong. Solar photovoltaic grid-connected power generation system. Shanghai Electric Power, 2005, (1): 49-53.
[2] Xu Liang. Calculation of radiation on a slope. Journal of Yunnan Nationalities University. 2010: 04.
About the author:
Zheng Wenying (1982-) is a female engineer currently working in the New Energy Business Department of Harbin Jiuzhou Electric Co., Ltd., where she is engaged in research on wind power converters and photovoltaic grid-connected inverters.
