Re-understanding and development trend of building automation system in intelligent building?

The following Zhong Da Consulting will introduce the automatic control system and development trend of intelligent buildings for your reference.

The building equipment monitoring system provides a good working environment for building users and convenient management means for building managers through comprehensive computer monitoring and management of various electromechanical facilities in the building, adopting decentralized control and centralized management technology, thus reducing building energy consumption, prolonging the service life of equipment, improving labor productivity and reducing labor management costs.

Although the current building automation system has developed to a certain extent, both hardware and software are advanced and reliable, but there is still a lot of work to be done to achieve the expected goal. Based on years of work experience, the author provides some new understandings and views here.

1. Understanding of investment and benefit based on building automation system

Since 2000, the initial investment in building automation system of intelligent buildings has dropped significantly, from 2000 yuan/point controlled by BA to less than 1000 yuan/point, which is still in a downward trend, while the real estate price has been rising continuously, with an average increase of 3 times so far, which is still at a high level. It can be seen that the initial investment of building automation system in intelligent building accounts for less and less of the whole building investment, and the average life span of modern high-rise buildings is above 100 years. Although the building automation equipment will continue to lag behind, the whole wiring framework exists and it will be very convenient to upgrade.

Building automation system of intelligent building is an important means of energy saving, and we must realize its urgency. First, strengthen the propaganda of building energy conservation, so that everyone can realize that energy conservation is not only a matter of money and environment, but also a prerequisite for the sustained and stable development of the country. At present, the energy-saving effect of building automation system in intelligent building is not ideal for many reasons. First, some managers think that the building automation system is useless because of misunderstanding, and it is better to open it manually by several people. This has entered a misunderstanding. In fact, it is impossible to achieve energy-saving, efficient and safe operation of building equipment by relying on manual control. Therefore, we must change our understanding, strengthen the training and construction of the property team, entrust a professional company to keep the equipment after the warranty period expires, improve the service life of the equipment and give full play to the role of the system. Otherwise, due to the low level of equipment management and high operating cost, the investment is wasted.

Energy-saving of building automatic control system must start from details and form habits. When it comes to energy-saving design, electrical designers usually think of choosing suitable transformers, frequency converters, soft starters and energy-saving ballasts for lamps. The coil system adopts three-speed fan and electric valve, which are common design contents in building automatic control system. In addition, the energy-saving design is also reflected in the details of adapting to local conditions, such as the design of energy-saving switch in hotel rooms. When people leave, they turn on the power supply, and when people leave, they cut off the power supply to achieve the purpose of energy saving.

2. Intelligent building automation system puts forward higher requirements for building mechanical and electrical equipment.

The building automation system of intelligent building needs the cooperation of various professions, and the energy saving of building automation is not only a matter of a certain profession, but also needs the cooperation of intelligence, building structure, HVAC, electric power, water supply and drainage and other professions, and puts forward some methods with energy saving potential. As an independent subsystem of the building, lighting system includes public lighting in public areas (such as parking lot, lobby, corridor, restaurant lighting, indoor lighting (office, conference room, etc. ), outdoor lighting (decorative lighting, road lighting, floodlighting) and so on. The energy consumption of building lighting accounts for about 20% of the whole building, and the lighting system has the following characteristics:

First, in the process of power transmission, it is necessary to ensure the working voltage of the terminal equipment with higher voltage transmission. With the decrease of equipment load, the output voltage will increase. Therefore, the building automatic control system is used for dynamic voltage regulation control to achieve the purpose of saving electricity.

Second, the demand for lighting function is different at different time periods every day, especially the public areas can be grouped, controlled in sections or directly turned off in combination with outdoor lighting sensors, such as the control of underground garage lights.

Thirdly, with the development of new lighting sources, new energy-saving lighting lamps are constantly emerging, replacing the traditional switch control of fluorescent lamps, energy-saving lamps, incandescent lamps, metal halide lamps and other lamps. Fully combining the intelligent control of digital technology and computer technology, the soft start mode is adopted to control the surge voltage and surge voltage of the power grid, protect the filament from thermal shock and prolong the service life of the lamp. Intelligent lighting system can usually prolong the life of lamps by 2-4 times, which not only saves a lot of lamps, but also greatly reduces the workload of replacing lamps and effectively reduces the operating cost of lighting system, which is of special significance to areas where lamps are widely used and difficult to install. In addition, the intelligent lighting system has potential value return, which makes the whole system work in the most comfortable state, thus ensuring people's physical and mental health and improving work efficiency.

Air conditioning system is another subsystem, which is divided into fresh air unit, air conditioning unit, VAV variable air volume and other air handling terminal equipment, and the other part is cold source system, whose power consumption generally accounts for about 70% of the whole building. How to operate and manage air-conditioning related equipment is more important. Using advanced building automation system can not only save electricity, prolong equipment life, but also reduce the number of equipment managers while meeting the comfort of conference rooms. Now analyze and elaborate one by one.

(1) The main function of the fresh air system is to deliver fresh air with a certain temperature according to the building area, so as to ensure the indoor air quality. The automatic control design of fresh air system generally determines a supply air temperature point, and if there is a high humidity requirement, a humidity point needs to be added. If the air conditioning pipeline adopts two controls, supplying hot water in winter and cold water in summer, choose electric regulating valve. For buildings with high temperature and humidity requirements, air conditioning pipes are generally equipped with four pipes, and cold and hot water pipes are installed separately, each with an electric control valve. In order to prevent the condenser from freezing and cracking in winter, an anti-freezing switch and a fresh air electric valve are installed. When the outdoor temperature is low, the air conditioner does not directly use the fresh air door or start the hot water circulating pump, and the hot water runs at a small flow rate.

When the air conditioner is running, the control target parameters are supply air temperature and supply air humidity. Compared with the set value, DDC calculates the deviation value and outputs a signal to control the opening of the cold (hot) electric two-way valve. The building control system of the fresh air unit temperature set point can be automatically adjusted according to the outdoor temperature, which is 30-32℃ in winter and 18-20℃ in summer.

(2) The improvement of temperature and humidity environment in public areas (lobby, dining room) is mainly realized by air treatment of air conditioning units, and its process flow chart is more complicated than that of new fans, and an air return pipe is added.

The equipment for automatic control configuration is generally cold and hot water valves, humidification valves, new return air valves, supply air temperature and return air temperature, and some need to install temperature sensors in public areas. The air supply volume can be adjusted by frequency conversion. The determination of target control parameters is generally based on the return air temperature, but it is not necessarily appropriate. For example, if the return air outlet is close to the door, the sampled data will be very inaccurate. It is also possible to arrange multiple temperature sensors in a common area and then take the average value. As for the control strategy, it is more complicated than the supply air temperature control of the new fan, and its own characteristics are pure lag and slow response. If the conventional PID control is adopted, the controlled valve will open and close frequently and the temperature will fluctuate greatly. Therefore, unconventional control strategies, such as dead zone mode, are adopted. When the target parameter enters the deviation range DT 1 (generally 0.5 degrees) recognized by the set value, the cold (hot) water valve will remain unchanged. Variable PID control mode means that the control effect is strong when the target parameters and set values are large, and weak when the target parameters and set values are small. You can also use fuzzy control method according to temperature deviation and rising or falling speed. When the temperature is within the positive deviation DT2( 1 degree) from the set value, the water valve will open slowly during the descending process; when the temperature is within the negative deviation DT2, the water valve will close slowly during the ascending and descending process. In short, for the control of target parameters, we should fully consider the measured temperature value, set value and temperature deviation value in unit time. Only by "seeing more and moving less" can we really control the target parameters. In order to save energy, according to the outdoor temperature, the set temperature can be reduced by 1 degree in winter and increased by 1 degree in summer. At the same time, we should make full use of the interlocking function of the new return air valve, fully open the fresh air valve in the transitional season, and close it to the minimum in midsummer or severe winter, which can save energy 12%.

Building automation system not only has rich control functions, but also has powerful management functions and humanized interface design. When the filter screen is blocked and the fan fails, it will give an audible and visual alarm to remind managers to clean and maintain, count the running time of the equipment and maintain the equipment regularly. In the process of use, make full use of the functions of the building control system and the characteristics of the air conditioning unit of the fresh air unit, and start/stop the operation according to the time program, which is embodied in:

Intermittent operation: make the equipment start and stop reasonably intermittently, but it will not affect the environmental comfort and process requirements.

Optimal start-up: according to the user or production process, turn on the air conditioning equipment in advance, and adopt large air volume and low temperature in summer; In winter, the air intake is large and the temperature is high; After the room temperature is stable, run under low air volume.

Optimal shutdown: stop the air conditioning equipment in advance according to the off-duty situation or production process.

(3) Variable air volume system (VAV) is a new type of air conditioning system, which is more and more used in the air conditioning of intelligent buildings. When the indoor ambient temperature changes, the same effect can be achieved by changing the air supply temperature and air supply volume. The central air conditioning system with variable air volume system can save energy by 40%, and the system only uses the maximum air volume when the cooling and heating load reaches the peak, so the energy consumption can be greatly reduced.

VAV system generally consists of air conditioning unit with variable frequency regulating motor and terminal device of VAV adjustable air valve. The monitoring content includes controlling the start and stop of the fan and monitoring the running state of the fan. According to the indoor temperature, the size of the new air return door and the opening of the water valve are automatically adjusted to control the temperature and keep the room temperature stable. All aspects of the air conditioning system with VAV devices need coordinated control, and its contents are mainly reflected in the following aspects:

(1) Due to the different loads in each room, the air volume sent into each room changes, and the air volume of the air conditioning unit will also change accordingly, so the fan should be controlled by adjusting the frequency conversion.

(2) When adjusting the fan speed, the control mode of constant static pressure or variable static pressure can be adopted to keep the pressure in each chamber stable and ensure the normal operation of the device.

(3) For the VAV system, signals such as air volume, temperature and air valve position in each room need to be detected, and the set value of air supply temperature can only be given after comprehensive analysis and processing.

(4) At the same time of air supply adjustment, the new and return air valves should be adjusted to make each room have enough fresh air to ensure the air quality of the room.

(4) Cold source system is the core part of HVAC system, and how to coordinate management is very important, which has a great impact on energy consumption. Generally, it is embodied in two adjustment methods: quantity adjustment and quality adjustment. Quantity adjustment is to adjust the number of chilled water pumps according to the change of load, or to adjust the water quantity through the frequency conversion of pumps, and then control the number of water chillers according to the total load of the cold source system (the product of the temperature difference between supply and return water and the total flow). Quality adjustment is to adjust the outlet temperature of chilled water. Generally, under the condition of low load, the outlet temperature of the water chiller is appropriately increased by several degrees to realize the optimal start-stop time control of the unit, so that the equipment can run alternately and optimize the running time of the equipment.

Taking a building as an example, the refrigeration station system has four refrigeration units, five chilled water pumps (one standby), five cooling water pumps (1 standby), four cooling towers and an expansion water tank. The building automation system completes the control requirements of refrigeration unit through the direct digital controller DDC installed in the refrigeration machine room: realizes the linkage control of water chillers, chilled water pumps, cooling water pumps, cooling tower fans, cooling tower inlet valves and related equipment. Detect the temperature, pressure and flow of chilled water supply and return water, and the temperature and pressure of cooling water supply and return water, and calculate the cooling load of air conditioning system.

Realize linkage control:

Open the chilled water valve → open the chilled water circulating pump (delay 1 min) → open the cooling water valve → open the cooling water circulating pump → open the cooling tower fan → open the cold water main engine.

Linkage stop sequence:

Cold water main organ → (delay 5 minutes) cooling tower fan is turned off → cooling water circulating pump is turned off (delay 20 minutes) → cooling water valve is turned off → chilled water circulating pump is turned off → chilled water valve is turned off.

In addition to the above strict interlocking, in order to protect the running refrigerator, once the freezing water pump has an accident, the water pump stops running, the refrigerator must stop running, and the cooling water pump must also stop running.

Building automatic control system not only completes the above basic functions, but also mainly realizes the optimal control of freezer. The central station collects the data of the site DDC for overall calculation, continuously analyzes and determines the actual load of the building in each time period, determines the number of chillers running, and sends instructions to the site DDC through communication. At present, we focus on solving the following problems.

① How to determine the initial number of chillers?

In the initial stage of system operation, the initial number of water chillers operated by the system is set manually. The system automatically records the climatic conditions and hourly cooling load on all operating days, and starts to execute load forecasting and optimization control software. After the monitoring system has accumulated to a certain extent, it will analyze the previous climatic conditions, load conditions and empirical data of system operation data, and draw certain rules to gradually realize the optimization of chiller system operation control. So as to determine the initial number of water chillers.

② How to control the increase or decrease of the number of water chillers?

First, increase the control mode of water chillers.

When the system load increases, the monitoring system detects that the pressure difference between the two ends of water supply and backwater decreases and the amount of cold water increases. At this point, the unit can be adjusted according to its own load. When the system load of this water chiller rises to 95% of its current percentage FLA (which can be adjusted according to the actual situation), it means that the full-load operation of a single unit and the full-load operation of the water pump are not enough to meet the system load value, and the outlet temperature of chilled water will not be stabilized at the set value of the outlet temperature, so that the electric valve of the second unit will be opened immediately.

The following calculation of k is rather abrupt. It is necessary to make clear what kind of rules it is in order to judge whether it is open or not.

Where: △ t = chwt-chwt.stp

Effluent temperature of CHWT chilled water

CHWT。 STP chilled water outlet temperature setting (7℃)

That is, the outlet temperature of chilled water is set at 7℃. When △ t is greater than or equal to 0.3℃ (which can be adjusted according to the actual situation), and the current percentage of water chillers is greater than or equal to 95%, the second unit (with the shortest running time) will start automatically.

Second, the automatic lightening strategy.

Assuming that two units are running, when the system load decreases, the pressure difference between the two ends of the supply and return water increases, that is, the load reflected on the units decreases accordingly. When the total load of two units is only or even less than that of one unit (assuming Fla

3. The latest development trend of intelligent building equipment automation system.

At first, the purpose of introducing automatic control into buildings was to solve some specific practical problems: temperature control and equipment start-up. With the increasing number of intelligent devices in buildings, the limitations of automatic control systems are constantly exposed. First, the traditional building system is still a relatively closed system, which is manifested in the communication protocol. The manufacturers are still fragmented and incompatible with each other, and the system devices cannot be seamlessly connected. Therefore, for a closed system, there will be a lot of work to be done to integrate all the equipment in the building on a system platform, which is not only high in cost, but also poor in performance; Second, because the system is closed, the design, supply, installation, debugging and upgrading can only be monopolized by the manufacturers, and the owners can only passively accept it, so the initial investment can not be guaranteed; Third, with the rapid development of computers in today's world, the renewal cycle of products is getting shorter and shorter, and new products of building automation are bound to emerge one after another. For a closed system, product renewal will be resisted and monopolized by manufacturers, which will hinder the development of technology. In fact, it is impossible to track the development of advanced technology at low cost.

Therefore, the adoption of open and standard communication protocols is the development trend of building automation system, which needs the joint implementation of various manufacturers to completely change the status quo. A truly open system must adopt standard communication protocols, which must be mainstream and accepted by various manufacturers. At present, LonWorks technology protocol introduced by Echelon Company of the United States is widely used in building automation system, and twisted-pair connection is generally adopted. The communication speed is 76.8K, the communication distance can reach 2500mm from manual bus and the free topology can reach 500 m.. LonTalk is the only point-to-point communication, and its use provides a solution for fully realizing openness and interoperability, making the automation system of the whole building more modern and efficient. Field equipment (such as valves, sensors, etc.) can also be transmitted through the network, which can reduce the amount of pipelines and construction on site. For the current non-standard equipment, a transition period can be defined. As long as the relevant protocols of the equipment can be provided, the building control center station itself has interface plug-in functions such as Modbus, BACnet, DDE and OPC. Through programming interface software, related equipment (such as boilers and freezers) can be connected to the building automation system. With the popularity of the Internet, the operation of the whole building equipment can be viewed in the office or at home through password management, and can also be viewed anywhere with Internet access.

Due to the adoption of advanced and open building automation system, which conforms to the latest international trend, the product selection is more diversified. In terms of system maintenance and upgrade, there are many kinds of equipment choices, including DDC controllers, routers and other products from various manufacturers, which can effectively control operating costs, protect existing investments and play a greater role. At the same time, pay more attention to the details of cognition, design and engineering construction, and adopt the method of comprehensive management to give full play to the function of building automation, which is beneficial to the management and energy saving of the whole building and truly benefits mankind, and the prospect will be broader and broader.

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