1 Introduction
Moore Spring Plaza is a large shopping center in Suining City that uses a high-power ice storage central air conditioning system. This system stores excess electricity generated during off-peak hours at night as ice, which is then melted during peak daytime electricity demand to provide air conditioning service. Since nighttime electricity prices are much lower than daytime prices in most parts of my country, using ice storage central air conditioning can significantly reduce users' operating costs.
Ice storage central air conditioning systems require more equipment than conventional air conditioning systems and have a higher degree of automation. However, they can automatically utilize all the stored energy each day while meeting the building's 24-hour air conditioning requirements, thus maximizing cost savings.
2 Control System Structure
The control system consists of a lower-level computer (field control workstation) and a higher-level computer (central management workstation). The lower-level computer uses a programmable logic controller (PLC) and a touch screen, while the higher-level computer uses an industrial-grade computer and a printer. The system is equipped with necessary accessories such as communication equipment interfaces, network cards, modems, etc., to realize the parameterization and fully automatic intelligent operation of the cold storage system.
The lower-level computer and touchscreen can perform system control, parameter setting, and data display on-site. The upper-level computer performs remote management and printing, and it includes all the functions of the lower-level computer and touchscreen. The entire system uses the industrial-grade programmable controller of the lower-level computer as its core to achieve automated control. Control equipment and devices include: sensing elements, electric valves, frequency converters, etc.
2.1 Lower-level system (area workstation)
2.1.1 Touchscreen
The MT5000 color touchscreen is used as the operation panel, completely replacing conventional switches, indicator lights, and other components, resulting in a cleaner control cabinet surface. Furthermore, the MT5000 touchscreen can perform functions such as status display, system settings, mode selection, parameter settings, fault recording, load recording, time and date display, real-time data display, load curves, and report statistics on-site. The Chinese user interface is intuitive and user-friendly.
2.1.2 SIEMENS Programmable Logic Controller
The SIMATIC S7-300 series PLC is suitable for automation of detection, monitoring and control in various industries and occasions. Its powerful functions enable it to achieve complex control functions whether operating independently or connected in a network.
This product features opto-isolation and high electromagnetic compatibility; it has high industrial applicability, allowing ambient temperatures up to 60℃; and it possesses strong anti-interference, anti-vibration, and anti-shock properties, thus finding wide application in harsh working environments.
The free communication port mode is also a distinctive feature of the S7-300 PLC. It allows the S7-300 PLC to communicate with any other device or controller that has an open communication protocol. Specifically, the S7-300 PLC allows the user to define its own communication protocol (e.g., ASCII protocol), with a baud rate of 1.5 Mbit/s (adjustable). This greatly expands the range of communication capabilities, making control system configuration more flexible and convenient. Any peripheral with a serial interface, such as printers or barcode readers, frequency converters, modems, and host PCs, can be connected and used. Users can program communication protocols and exchange data (e.g., ASCII characters). Devices with RS232 interfaces can also be connected via a PC/PPI cable for free communication.
When the host computer is offline, the entire system can operate normally under the control of the slave computer.
2.2 Host Computer System (Central Management Workstation)
2.2.1 Host Computer
The host computer, also known as the graphic control center, mainly consists of an HP original multimedia computer and a laser printer. It uses the SIMATIC WINCC 5.1 software platform and features a user-friendly, all-Chinese interface. Administrators and operators can understand the current and past operating status and all parameters of the entire ice storage cooling system by observing the various information displayed on the PC, and can manage equipment and execute printing tasks using the mouse.
2.2.2 WinCC 5.1 Software Platform
WINCC software can be used for all operator control and monitoring tasks in the field of automation. It can clearly display events occurring in process control, show the current status and record data sequentially. The recorded data can be displayed in full or in a simplified format, and can be edited continuously or as required. It can also output printed reports and trend charts.
WINCC can report critical situations at their initial stages during control processes, issuing signals that can be displayed on the screen or conveyed audibly. It supports troubleshooting using online help and operation guides. A WINCC workstation can be dedicated to process control to ensure that critical process information is not obscured. Software-assisted operation strategies ensure unauthorized process access and provide error-free operation in industrial environments.
WinCC is a top-tier, object-oriented 32-bit application software that runs on PCs under Microsoft Windows 98 or Windows NT 4.0 operating systems. Through the OLE and ODBC window standard mechanisms, it enters the Windows world as an ideal communication partner, thus WinCC can be easily integrated into the company's data processing systems.
3. Control of ice storage system
3.1 Control objective, scope, and main controlled equipment
The purpose of the cold storage control system is to adjust the operating modes of the ice storage system under various application conditions by controlling the refrigeration unit, ice storage device, plate heat exchanger, system water pump, cooling tower, and system pipeline regulating valves, so as to provide a stable water supply temperature to the terminal under the most economical conditions. At the same time, it aims to improve the system's automation level, increase management efficiency, and reduce management labor intensity.
The control scope includes the parameter status display, equipment status and control of the entire ice storage system. The main control equipment includes: dual-mode main unit, electric valve, cooling tower, cooling water pump, ice storage device, primary ethylene glycol pump, plate heat exchanger, secondary ethylene glycol pump, etc.
3.2 Control Functions
The control functions include those required for the stable and economical operation of the entire ice storage system.
3.2.1 Operating Condition Switching Function
Based on the season and machine operating conditions, the automatic control system has the following operating condition switching functions:
a) Dual-mode main unit ice making and cooling mode; b) Dual-mode main unit ice making mode; c) Main unit and ice storage unit combined cooling mode; d) Ice melting and cooling mode; e) Main unit cooling mode.
3.2.2 Start-up, shutdown, display, and fault alarm functions for operating conditions
The control system, based on a pre-programmed time sequence and in conjunction with load forecasting software, controls the number of refrigeration units and peripheral equipment to start and stop, and monitors the operating status and parameters of each device, such as:
- Start-up, shutdown, status, and fault alarms for the chiller unit; - Operating parameters of the chiller unit; - Water shortage protection for the chiller unit; - Remote measurement and display of supply/return water temperature and pressure for the chiller unit; - Start-up, shutdown, status, and fault alarms for the chilled water pump; - Start-up, shutdown, status, and fault alarms for the ethylene glycol pump; - Start-up, shutdown, status, and fault alarms for the cooling water pump; - Differential pressure measurement and display of the differential pressure bypass pipe; - Start-up, shutdown, status, and fault alarms for the cooling tower fan;
- Cooling tower supply/return water temperature control and display; - Supply/return water temperature and differential pressure remote sensing control and display; - Plate heat exchanger inlet and outlet temperature control and display; - Ice storage device inlet and outlet temperature remote sensing control and display; - Chilled water return flow control and display; - Electric valve on/off, regulation, and valve position control and display; - Outdoor temperature and humidity remote sensing control and display; - Ice storage capacity measurement and display; - Terminal cooling load control.
3.2.3 Data recording and printing functions
The control system records year-round trends for necessary monitoring points. It can also record the year-round load data (including daily maximum load and total daily load) and equipment operating time in tables and charts for user access. All monitoring points and calculated data can be automatically printed at scheduled times.
3.2.4 Manual/Automatic Conversion Function
The control system is equipped with a flexible manual/automatic switching function.
3.2.5 Optimize control functions
The system automatically selects between prioritizing the main cooling unit or ice melting based on data such as outdoor temperature, weather forecast, weather trends, and historical records. While meeting the terminal load requirements, it utilizes all stored cooling capacity daily, minimizing the operation of the main cooling unit. This fully leverages the advantages of the ice storage cooling system and saves on operating costs.
3.2.6 Fully automatic operation function
The system can operate independently of a host computer, automatically performing ice making and control system operation, switching of working conditions, automatic diagnosis of system faults, and sending alarms remotely according to a schedule. The touchscreen displays the system's operating status, process flow, parameters of each node, operating records, alarm records, etc.
3.2.7 Holiday Setting Function
The system can operate automatically according to a schedule, and can also be pre-set for holidays to control the amount and time of ice storage, so that the system can stop supplying cooling to places that do not need air conditioning during holidays.
3.2.8 Lower-level machine operation functions
The lower-level machine operation functions are as follows:
a) Human-machine interaction. Operators can interact with the machine via a touch panel. The interface is fully Chinese and includes functions such as prompts, help, parameter settings, key settings, fault query, and history.
b) System Settings. This includes setting the operation password, running settings, running schedule settings, overflow handling, automatic/manual/test selection, holiday settings, and system parameter settings (including temperature and pressure at each node, flow rate of each medium, ice storage capacity, ice making rate, ice melting rate, valve opening, and terminal load, etc.).
c) Fault records, operation records, historical records, etc.
3.3 Remote Monitoring
The control system connects to the expert system via telephone lines or broadband networks to monitor system operation, modify parameters, and collect data, enabling continuous system improvement and software upgrades to provide users with better service. Remote monitoring allows users to remotely monitor the chiller plant via the PSTN (Public Switched Transmission Network). It also enables remote commissioning, real-time remote monitoring, and online maintenance, significantly reducing the workload of engineering personnel and lowering project costs.
3.4 System Extended Control
The control system features a user-friendly interface; both the PLC and touchscreen are expandable, with customizable content and parameters. It integrates the BAS with the ice storage and cooling system via a 485 communication interface or protocol, saving investment and facilitating management. Centralized system control reduces the footprint of the power cabinets and ensures uniformity in model, style, and size. System expansion control is as follows:
a) Automatic control of sewage pumps; b) Ventilation and exhaust control; c) Pressure stabilization control of live water pumps; d) Timed operation, detection, and alarm of waterproof pumps; e) Pressure stabilization control of sprinkler pumps; f) Automatic control of building outline lighting at night; g) Low-voltage metering, switch status detection, and alarm.
4 Conclusion
The widespread application of PLCs in ice storage air conditioning systems has ensured the safe operation and effective energy saving of the systems, while also providing new ideas for the selection of controllers for building equipment control systems. It is believed that in the near future, more and more PLC systems will mature in the application of ice storage air conditioning systems and will also play a significant role in building equipment control systems.
References
[1] Siemens. Industrial Communication, 1999.
[2] Siemens. WinCC Configuration Manual, Volumes 1, 2, and 3, September 1999.
[3] Siemens. WinCC Communication Manual, Volumes 1 & 2, September 1999.
[4] Siemens. SIMATIC Step 7 V5.1 Programming User Manual, 1999.
