Typical Applications of PLC in Water Source Heat Pump Air Conditioning Systems
The HOLLiAS-LECG3 small integrated PLC was applied to the water source heat pump air conditioning control system. The PLC 's I/O point allocation table is given, the composition of the control system and the software design ideas are introduced, and a compressor control method with random start and stop is proposed.
1 Introduction
Water source heat pump air conditioning system is an air conditioning system that uses natural water source as cold and heat source. Its core technology is water source heat pump technology. Water source heat pump technology is a technology that uses the low temperature and low heat energy resources formed by the solar energy and geothermal energy absorbed by shallow water sources on the earth's surface, and uses the heat pump principle to transfer low heat energy to high heat energy through a small amount of high-grade electrical energy input. River water, lake water, groundwater and other shallow water sources on the earth's surface absorb the energy of solar radiation, and the temperature of the water source is very stable. In summer, water source heat pump air conditioning system transfers the heat in the building to the water source. Because the water source temperature is low, it can efficiently remove the heat. In winter, water source heat pump air conditioning system extracts energy from the water source, and according to the heat pump principle, it uses air or water as a refrigerant to raise the temperature and deliver it to the building. Usually, water source heat pump consumes 1kW of energy, and users can get more than 4kW of heat or cooling. Because water source heat pump air conditioning system has the advantages of high efficiency, energy saving and environmental protection, it has been increasingly used in recent years [1][2].
Air conditioning system control is mainly divided into several levels: relay control system, direct digital controller (DDC) system and programmable logic controller (PLC) system. Due to its obvious disadvantages such as high failure rate, system complexity and high power consumption, relay control system has been gradually phased out. Although DDC control system has made great progress in intelligence, its application is limited due to its poor anti-interference ability, difficulty in networking, low information integration and hierarchical and step-by-step structure. On the contrary, PLC control system has been widely used in intelligent buildings due to its significant advantages such as reliable operation, convenient use and maintenance, strong anti-interference ability and suitability for new high-speed network structure. In order to improve the economy, reliability and maintainability of air conditioning system, air conditioning system currently tends to use advanced, practical and reliable PLC for control[3].
This article introduces the successful application of Hollysys' HOLLiAS-LECG3 small integrated PLC in a water source heat pump air conditioning control system, demonstrating that the HOLLiAS-LECG3 small integrated PLC can effectively realize intelligent control of central air conditioning, thereby reducing ineffective energy consumption, improving energy utilization efficiency, and protecting air conditioning equipment.
2. Introduction to the Air Conditioning System
An office building in Beijing uses a water source heat pump central air conditioning system. The total building area is 8,550 m2, and the building height is 20.5 m. The air-conditioned area is approximately 6,840 m2. The basement level contains various equipment rooms and operating rooms, the ground floor contains a staff canteen, lobby, and conference rooms, and the second to sixth floors are commercial office space.
The design requirements for technical parameters such as indoor temperature and relative humidity are shown in Table 1. The design cooling capacity of the water source heat pump central air conditioning system is 860kW, and the heating capacity is 950kW. The main unit of the air conditioning system consists of four compressors, and the water source system consists of a water intake well, a seepage well, and water treatment equipment.
Table 1 Design Requirements for Indoor Technical Parameters
3 Control System Hardware Design
This water source heat pump central air conditioning system primarily controls the start and stop of four compressors based on changes in the inlet and outlet water temperatures of the evaporator and condenser, ensuring the water temperature remains stable within a set range. The four compressors are divided into two groups, A and B, with two compressors in each group. The system's I/O point allocation is shown in Table 2, which includes 6 digital input points, 4 analog input points, 5 digital output points, and 1 analog output point.
Table 2 System I/O Point Allocation Table
According to the input and output requirements, the controller of the water source heat pump central air conditioning system is the HOLLiAS-LECG3 small integrated PLC with independent intellectual property rights from Hollysys. Considering that the system needs a certain number of spare I/O points, the CPU module is selected as LM3107 with 24 switching points, of which 14 are switching inputs and 10 are switching outputs. The analog input module is selected as LM3312 with four channels of RTD input and LM3320 with two channels of analog output. The human-machine interface of the PLC is selected as EView touch screen. The composition of the PLC control system and related equipment is shown in Figure 1. These configurations can fully meet the system requirements [4][5].
Figure 1. Composition of the PLC control system
4 Control System Software Design
The main function of the control system is to automatically start and stop the heat pump, display operating parameters such as temperature, pressure, and flow rate, display the compressor's operating status, record equipment operating time and fault causes, and achieve intelligent control of the water source heat pump central air conditioning system. Based on the main functions of the control system, to increase program readability and reduce program code, the PLC program adopts a program structure of main program calling function blocks and function blocks calling functions. The PLC program consists of one main program, 11 function block subroutines, and one function, and their calling relationship is shown in Figure 2. The program compilation and decoding space occupies 30K.
The program design involves the PLC continuously monitoring operating parameters such as temperature, pressure, and flow rate after power-on. These monitoring functions are primarily implemented in the detection program, fault program, and A/B group fault shutdown program. If all parameters are normal, the start-up function block subroutine runs, starting the compressor. During start-up, temperature is simultaneously checked. If the temperature reaches the set value, the adjustment function block subroutine is entered, stopping the start-up function block subroutine and completing the start-up process. Based on temperature changes, the adjustment function block subroutine controls the compressor's start and stop. Inverter control is achieved by calling the loading and unloading programs.
Among these programs, the adjustment function block subroutine is the most cumbersome to meet the compressor's operating requirements. For example, the compressor's start-up time must be less than 30 seconds, and the compressor's start-up frequency should not exceed 5 times per hour. To balance compressor running time and increase the air conditioner's lifespan, traditional program design uses control methods such as first-in-first-out (FIFO), first-out-first-start (FIFO), and rotating start-up order during startup to coordinate compressor running time. However, if this system uses this method, the problem of one compressor running for too long still exists. Therefore, it was decided to improve the traditional method by using a random start-stop control method instead of the FIFO or FIFO control methods, thus solving the problem of unbalanced compressor running time.
Figure 2 Program call relationship diagram
The human-machine interface uses an EView touchscreen, and the homepage is shown in Figure 3. After entering the password, click the function menu. In the pop-up shortcut window, you can select sub-menus such as parameter query, running time, fault query, running status, parameter setting, adjustment display, and operation interface to perform related operations and displays.
Figure 3. Homepage of the human-computer interface
5. Conclusion
Traditional relay control systems for heat pump control suffer from numerous mechanical contact points, complex wiring, inconvenient parameter adjustments, low operating frequencies of these contacts leading to easy damage and poor reliability. While direct digital controllers (DDCs) can reduce wiring and improve reliability, their inherent limitations—poor anti-interference capabilities, difficulty in networking, low information integration, and hierarchical, distributed structure—make them increasingly unsuitable for complex and ever-changing intelligent control requirements.
Using a PLC to control a heat pump system not only allows for complex logic control through programming, but also significantly simplifies hardware wiring, improves control system reliability, provides a user-friendly interface, offers high information aggregation, and facilitates intelligent control. Therefore, in the field of heat pump air conditioning, the replacement of DDC control systems with PLC control systems is an inevitable trend.
