Abstract: This article, through engineering case studies, discusses the feasibility of integrating a grid-connected photovoltaic power generation system into the design of an underground wastewater treatment plant. The system is planned to power lighting and ventilation systems within the plant. It details the application of grid-connected photovoltaic technology in the project, covering system design, calculations, and cost considerations. Additionally, it analyzes the challenges encountered when applying the system in similar projects.
Keywords: Grid-connected Photovoltaic Power Generation System; Underground Wastewater Treatment Plant; Benefit Analysis
Introduction
Wastewater treatment, as an energy-intensive industry, consumes a significant amount of electricity, accounting for over 40% of its total cost. According to relevant data, the annual electricity consumption for urban and rural wastewater treatment in our country has exceeded 10 billion kW·h, accounting for approximately 2% of the total social energy consumption. With the acceleration of urbanization, the increased volume of wastewater necessitates the construction or expansion of new wastewater treatment plants, while planning departments have set higher requirements for land resource conservation in these plants. Guided by the "ecological pollution control" concept, developing underground wastewater treatment plants in areas with relatively scarce land resources has become the goal for the development of new urban wastewater treatment facilities. However, underground plants have higher requirements for ventilation and lighting, which translates to a higher energy consumption per unit of treated water. Actual project calculations show that this portion of energy consumption accounts for 7% to 10% of the total electricity consumption of underground plants, and the load in this section requires a coefficient significantly higher than that in conventional plants. Therefore, in the engineering design of underground wastewater treatment plants, how to reduce the additional operational costs compared to conventional plants has become a critical issue to consider.
During the project design phase, after comparing multiple options (such as photovoltaic power generation and light-guided lighting), it is proposed in this project to integrate the above-ground buildings and landscape design, set up photovoltaic arrays, construct a photovoltaic system, and organically combine the photovoltaic system with the above-ground buildings and landscape of the wastewater treatment plant. This will supplement power sources for underground space lighting, ventilation, and other equipment, effectively utilize the above-ground space of the wastewater treatment plant, and reduce the operational costs of this project.
I. Grid-connected Photovoltaic Power Generation System Design
Project Overview
The project adopts an underground wastewater treatment plant construction plan as per the approved plan. The current scale is 75,000 m³/d, with a future scale of 150,000 m³/d, occupying approximately 68.8 acres of land. The plan has high requirements for the above-ground landscape of this project, but the actual usable above-ground space is limited. The above-ground landscape effect is shown in Figure 1.
In line with the overall layout and landscaping requirements, a comprehensive design for a photovoltaic power generation system is being implemented on the rooftop of the ground-level integrated building. The photovoltaic modules are arranged in arrays using a 20-series, 4-parallel connection method. After the arrays are fixed on racks, they are laid flat on the southern-facing rooftop. The arrays face due south (azimuth angle of 0°) with a tilt angle of 20°, measuring 33.19m in length east-west and 3.76m in width north-south, with a total floor area of approximately 125 square meters.
2. Photovoltaic Power Generation System Design Calculation
(1) Load Requirements:
After calculation, the lighting load of the underground chamber in this project is approximately 48kW, and the ventilation load is about 133kW. Based on the available area for the photovoltaic array, the lighting load of the underground chamber is determined as the load for the photovoltaic power generation system.
- Meteorological and Geographical Data:
The National Meteorological Bureau has researched the multi-year weather data for the region where this project is located, as shown in Table 1.
(3) System Design Coefficient:
Based on the above meteorological data and in conjunction with various on-site conditions, the system parameters are as follows.
Design Coefficients for Photovoltaic Battery Design
1) Photovoltaic Array Loss Coefficient
The array loss coefficient of photovoltaic modules refers to the loss caused by component mismatch during the process of combining them into an array. During component configuration, the voltage mismatch control value is ±2%, the current mismatch control value is 4-1%, and the power mismatch control value is 4-1%.
2) Environmental Coefficient
The grid-connected power generation system is undergoing optimized installation, achieving excellent environmental adaptation with an environmental coefficient of 100%.
3) Attenuation Coefficient
As the usage time progresses, photovoltaic battery components may experience a decrease in power generation due to physical reactions caused by ultraviolet radiation. The selected components have an actual degradation rate of ≤10% over 10 years.
② Design coefficients related to system operation and security
The system's power availability rate exceeds 99.99%.
2) MTBF > 100,000 hours.
3) Photovoltaic battery支架 wind coefficient: 30 m/s.
4) Voltage and current loop safety factor ≥ 1.5.
5) The equipment capacity safety factor is set at a low value of 120%.
③ System Power Generation Statistics
System-generated electricity statistics are as shown in Table 2.
3. Grid-Connected System Design
In this project, under normal circumstances with a functioning public power grid, electricity is generated by the photovoltaic array during clear daylight hours. It is then regulated by an inverter for grid connection and supplied to the power load at the photovoltaic grid connection point—the underground space lighting at the sewage treatment plant. Due to the power consumption of the underground space lighting load generally exceeding the power generation capacity of the battery array, the lighting equipment's power is provided by both the battery array and the grid, with the solar battery array's electricity used first. In special circumstances, such as when the underground space lighting at the sewage treatment plant employs a specific scene combination, if the power generation capacity of the photovoltaic array exceeds the power consumption of the lighting equipment in certain specific scenarios, the excess electricity is uploaded to the public grid and then supplied to other power load equipment through the grid.
The underground wastewater treatment plant is designed with a secondary load capacity, and backup power sources have been considered for critical loads, including underground space lighting. Therefore, the solar system in this project generally will not experience "islanding" effects. However, to prevent any unforeseen incidents, over/under voltage and over/under frequency protection schemes are adopted: if the grid-connected inverter's output power (active power, reactive power) does not match the load demand power, voltage or frequency will deviate. Once it exceeds the normal range, the system's hardware and software-defined over/under voltage protection settings and over/under frequency protection settings can be utilized for detection. This will trip the inverter's grid connection switch, causing the inverter to stop operating, thereby preventing the formation of an island.
4. Computer Data Collection Device
To visually demonstrate the operation of the photovoltaic power generation system and highlight the effective utilization of clean energy, the project designed a computer data collection device for the photovoltaic power generation system. The device connects to the power regulator via an RS485 interface, processes the collected data, and presents it in graphical display interfaces and data tables. It also calculates and statistically summarizes the itemized and aggregated data of various parameters daily, monthly, and annually, storing it in the form of data curve charts.
5. Profitability Analysis
Social benefits
The grid-connected photovoltaic power generation system of this project generates approximately 15×10 MWh of electricity annually.3kW.h, which is equivalent to saving 5.4 tons of standard coal annually, and reducing CO...2The emissions are approximately 12.2 tons, SO2The emissions are approximately 0.135t, with a reduction in nitrogen oxide emissions of about 0.066t.
(2) Economic Benefits
The photovoltaic power generation system operates during peak daylight hours each day. Assuming an average electricity cost of 1.1 yuan/kWh in the project area, and without considering electricity price increases or government subsidies, the project is estimated to save approximately 165,000 yuan in electricity costs annually, with a static payback period of about 3 years.
Ankore Distributed Photovoltaic Monitoring System
1. Overview
AcrelCloud-1200 Distributed Photovoltaic Operation and Maintenance Cloud Platform monitors inverters, meteorological equipment, and camera devices at photovoltaic sites to assist users in managing their dispersed photovoltaic sites. Key features include: site monitoring, inverter monitoring, power generation statistics, inverter one-line diagram, operation logs, alarm information, environmental monitoring, equipment files, operation and maintenance management, and role management. Users can access the platform via the WEB and APP interfaces to promptly grasp photovoltaic power generation efficiency and revenue.
2. Application Sites
Currently, China has two types of distributed application scenarios: residential rooftop photovoltaic systems in rural areas and industrial and commercial rooftop photovoltaic systems. Both types of distributed photovoltaic power stations have seen rapid development this year.
3. System Architecture
Inverters and multi-functional power metering instruments have been installed in the photovoltaic substation. Data collected is uploaded to the server via a gateway and centrally stored and managed. Users can access the platform via PC to obtain real-time operation status of the distributed photovoltaic power stations and the performance of each inverter. The overall structure of the platform is as shown in the figure.
4. System Features
The AcrelCloud-1200 Distributed Photovoltaic Operation and Maintenance Cloud Platform Software employs a B/S architecture, allowing any authorized user to monitor the operational status of photovoltaic power stations across various buildings within the area through a web browser, based on their permission level. This includes information such as the geographical distribution of the power stations, station details, inverter status, power generation curve, grid connection status, current electricity generation, and total electricity generation.
(1) Photovoltaic Power Generation Comprehensive Dashboard
● Display the number of photovoltaic power stations, installed capacity, and real-time power generation output.
Cumulative daily, monthly, and annual electricity generation and revenue.
Cumulative social benefits.
● Bar chart displays monthly electricity generation
(2) Power Plant Status
●The power station status display shows the basic parameters of the photovoltaic power station, including the power generation capacity, subsidy electricity price, peak power, etc.
●Calculate the daily, monthly, and annual electricity generation and revenue for current photovoltaic power stations.
● The camera monitors the on-site environment in real-time and integrates parameters such as irradiance, temperature, humidity, and wind speed.
● Display the current number of photovoltaic power station inverters connected and their basic parameters.
(3) Inverter Status
● Basic parameters of inverters are displayed.
● Display of daily, monthly, and annual electricity generation and revenue.
● Display inverter power and environmental irradiance curves through graphical charts.
● DC side voltage and current inquiry.
● Voltage, current, active power, frequency, and power factor inquiries.
(4) Power Plant Generation Statistics
● Display the statistical reports of the selected power station's hourly, daily, monthly, and annual electricity generation.
(5) Inverter Power Generation Statistics
● Display the time, date, month, and annual generation statistics report for the selected inverter
(D) Power distribution diagram
● Real-time display of inverter's AC and DC side data.
● Display the current number of inverters connected components.
● Display current environmental parameters such as irradiance, temperature and humidity, wind speed, etc.
● Display inverter models and manufacturers.
(7) Inverter Curve Analysis
● Display curves for AC/DC voltage, power, irradiance, and temperature.
(8) Incident Record
● Operation Log: User Login Inquiry.
● SMS Log: Check SMS push time, content, sending result, and replies.
● Platform Operation Log: Check仪表 and gateway offline status.
● Alarm Information: Categorize and process alarm divisions, record the content of the alarm, the time of occurrence, and the confirmation status.
(9) Operating Environment
● Video Surveillance: With video cameras installed on-site, real-time monitoring of the photovoltaic station's operation is possible. For cameras with hardware capabilities, recording playback and pan-tilt control functions are also supported.
Conclusion
An underground wastewater treatment plant, despite the added underground space lighting and ventilation loads, can effectively utilize photovoltaic power generation technology as a supplementary power source. Moreover, since this portion of the load is commonly used in underground wastewater treatment plants, the electricity generated by the photovoltaic power system can be used immediately without the need for energy storage devices. This simplifies the system configuration and avoids issues related to the recycling and pollution of spent batteries.
Due to the limitations of the overall project plan, above-ground structures, and landscape design, it is challenging to extensively utilize above-ground space for constructing photovoltaic arrays. After determining the load requirements, the distribution and grid connection systems should be reasonably designed in conjunction with the capacity of the photovoltaic power generation system. To achieve building-integrated photovoltaics in an underground wastewater treatment plant, it is necessary to intervene early in the planning and approval phase of the project, combining the functions of above-ground buildings with architectural and structural expertise to determine the parameters of the photovoltaic power generation system.
In recent years, with the maturity of photovoltaic power generation technology and the reduction in costs, the application of photovoltaic power generation systems in wastewater treatment plants has gradually increased, and successful large-scale application cases have been reported from time to time. However, there are relatively few cases of large-scale photovoltaic power generation systems in underground wastewater treatment plant construction. The author believes that this is mainly due to limitations in the planning, overall layout, landscaping, and effective usable space of underground wastewater treatment plants. To effectively utilize renewable energy in underground wastewater treatment plants, while achieving wastewater treatment, it is necessary to effectively reduce the increased energy consumption and operation costs compared to conventional wastewater treatment plants. Industry designers need to conduct research, analysis, and design from multiple perspectives. The analysis in this article, based on engineering examples, is intended to initiate discussion and is open to correction.
Reference
Zhang Yong. Application of Grid-Connected Photovoltaic Power Generation Technology in Underground Sewage Treatment Plants. Tianjin Municipal Engineering Design and Research General Institute, Tianjin 300392
Zhang Yong, Ji Hongrui. Smart Control Technology Boosts Wastewater Treatment Concept Plant [J]. Automation & Exhibition, 2017(1): 50-52.
[3] Dong, Wei-xiao Zhang, Fu-qiang Liu, et al. Application of Solar Power Systems in the West-to-East Gas Pipeline Project [J]. China Construction Dynamics (Sunshine Energy), 2004(2): 34-43.
[4] B/T 19939-2005 Technical Requirements for Grid-Connected Photovoltaic Systems [S]. Beijing: China Quality Inspection Press, 2005.
Ankore Enterprise Microgrid Design and Application Manual. 2020.06 Edition.
