Discussion on the Design of Central Air Conditioning Billing System

1. Introduction

With the progress of society, people's requirements for living environment and quality of life are constantly improving. Central air conditioning is also widely used in residential buildings, commercial buildings, high-end office buildings and multi-functional comprehensive buildings. The method of charging by cooling/heating will undoubtedly provide users with a fair, reasonable and scientific charging basis; in addition, it can also establish a direct connection between the user's economic interests and energy consumption, fundamentally enhance the user's energy-saving awareness, and minimize energy waste. Based on this billing method, this article designs and studies the structure, function and management software of household energy meters.

2. Principle of central air conditioning control system

2.1 Energy billing formula

Q = ∫ ₀ ^t Q _m Δhdt

Q----Released cold/heat, KJ

Q _m ---- liquid mass flow rate flowing through the energy meter, Kg/s

Δh----The difference in specific enthalpy between the inlet temperature and the outlet temperature of the heat exchanger, KJ/Kg

t----time, s

2.2 Control system

Two temperature sensors and one flow sensor are used in the central air conditioning control system. The temperature sensors are installed on the water supply pipe and return pipe of the heat exchanger circuit respectively, and the flow sensor is installed on the inlet or outlet pipe of the heat exchange circuit. When the flow sensor sends out a flow signal, the paired temperature sensor gives the inlet and outlet temperature signals. The energy meter automatically and continuously collects the flow and temperature signals, and displays the energy value released by the cooling liquid from the inlet to the outlet after calculation.

3 Energy meter components

3.1 Hardware Structure

3.1.1 Microcontroller

The 89S52 chip is a low-power, high-performance CMOS 8-bit microcontroller manufactured using Atmel's high-density non-volatile memory technology and has 8K in-system programmable Flash memory, providing a highly flexible and ultra-effective solution for this system.

3.1.2 Display Circuit

The DM-162 LCD display module is selected. It is a LCD display that uses 5x7 dot matrix graphics to display characters. It is programmed in the commonly used 2-line 16-character format and can cyclically display the current time, indoor temperature, set temperature, energy consumption, payable fees, IC card data, and alarm information.

3.1.3 Clock and signal acquisition circuit

The clock/calendar circuit can calculate the seconds/minutes/hours/days/dates/weeks/months/years before 2100, and accurately display the time in real time. The number of days per month and leap years can be automatically adjusted; the display can continue even in a power-off state.

The signal acquisition circuit first collects the analog signals transmitted by the temperature sensor and flow sensor through the analog/digital converter, and then converts them into corresponding digital signals. The sampling frequency is 5ms by default, and the parameters can also be set according to actual needs. The temperature sensor uses Pt100 platinum resistance as a temperature sensing element to measure the supply and return water temperature, which is 1.6KΩ at 25℃; the flow sensor uses a mechanical flow meter to measure the refrigerant flow of the loop. The level signal converted by the signal acquisition circuit can provide the original basis for central air conditioning billing.

3.1.4 Billing Circuit

The temperature signal sent by the temperature sensor is amplified and converted into an electrical pulse frequency signal proportional to the measured temperature difference by the analog/digital conversion circuit and then transmitted to the single-chip microcomputer. The single-chip microcomputer calculates the energy consumed by the user through the PID algorithm, and then receives inquiries from the host computer through the communication circuit.

3.1.5 IC card management

The IC card is managed by the management department for payment. It consists of a card writer and a card reader. The card writer is used by the management department to charge and write the card, and the card reader is identified by the single-chip microcomputer and stores the IC card data; the single-chip microcomputer can automatically decrease the stored data according to the calculated Q value; when the stored data is 0, the single-chip microcomputer instructs the electric valve to automatically close, the terminal air conditioner stops working, and displays an alarm signal.

3.1.6 Watchdog Circuit

The watchdog uses the MAX813 chip, which not only has a power-off protection function, but also enables the controller to automatically resume work in the event of a system failure or program runaway, thereby enabling reliable and safe operation.

3.1.7 Power supply circuit

The power circuit in this system uses dual independent power supplies, one for digital power supply and the other for analog power supply. It includes an AC transformer (220V/12V), a bridge rectifier circuit, a filter capacitor, and an integrated voltage regulator block, providing the system with +12V and +5V DC voltages.

3.2 Software Design

3.2.1 Main program flow chart

3.2.2 Main module functions

⑴ Temperature control: The temperature parameters are set by the central management machine, and the control signal is fed back to the proportional integral three-way electric valve through the collected flow and temperature sensor data to automatically adjust the air conditioning refrigerant flow through the terminal air conditioner, so as to achieve the purpose of automatic control of room temperature. The specific contents include temperature adjustment, fan output control, temperature zero drift adjustment, temperature control hysteresis band parameter setting, etc.; the adjustment range is 18℃~30℃, with PID adjustment, and the accuracy is 1℃.

⑵ Output control: Compare the sampling result with the temperature setting value (above 4°C for cooling and below 4°C for heating) and take corresponding actions to control the conduction and closing of the corresponding thyristor, thereby realizing the control of temperature, wind speed and water valve.

⑶ Billing function: Billing is the main task of this system. The billing formula of this system is as follows:

F=A _Q Q+F ₁

Where F is the total fee that the user should pay to the property department.

A _Q ——Cooling/heating cost coefficient

Q ——Released cold/heat, KJ

F1 _- Additional Fees

According to this formula, the user's cooling/heating consumption can be calculated, and the household machine displays the accumulated value and reports the data when the host computer calls. The host computer displays, prints or transfers the fees payable by each user according to the requirements of the property management department.

4 Central Management Agency

The management machine is a PC, and the management software can remotely monitor the energy meter, complete remote centralized control of the room temperature, collect and save daily billing data details, and can perform statistical analysis and print detailed lists and reports to complete centralized management of the room thermostat.

4.1 Management Machine

Operating system: Win9X/2000/XP;

Programming language and database: Delphi7.0+Access;

Database connection: ADO;

Communication method: PC RS232 communication interface;

4.2 Contents sent by the management machine to the controller

Air conditioning start and stop command;

Temperature and fan size setpoints;

Time setting value;

Billing data collection command;

Controller status acquisition command;

4.3 Content sent by the controller to the management machine

Controller working status (air conditioning start/stop, fan status);

Billing data;

4.4 Functional Description

⑴Data collection: Read the billing data of each controller through the serial port automatically (manually if necessary) every day and save the details, and store the collected original terminal data;

⑵Remote control: The management machine can remotely open, close, pause, adjust the temperature, set the rate, etc. of the controller at a specific address;

⑶ Monitoring and fault diagnosis: The management machine monitors the operating status of each controller (temperature setting, startup time, etc.), analyzes the data sent by each controller, and promptly understands the system operating status and problems in operation (such as address conflicts), prompting operators to pay attention;

(4) User management: including management of operating users and management of system users (inquiry, account opening, account cancellation, etc.);

⑸Query: query and retrieve the user's billing list (based on user name, controller name or address, cost, etc.);

⑹ Statistics and reports: daily/weekly/monthly billing lists, and expense statistics reports by time period (day/month/year);

⑺Print: Print user daily/weekly/monthly billing list, monthly billing details, and statistical reports;

5 Conclusion

The innovation of this system lies in the composition of the network, which is realized by using a local area network to access the main network through a communication machine, which can meet the requirements of the actual system: access 1600 measurement points through the RS485 network connection. This system has undergone a series of simulation experiments and can run safely, flexibly and accurately, with certain convenience, rationality and practicality.

References:

[1] He Yaodong, He Qing. Central Air Conditioning[M]. Metallurgical Industry Press, 1998.4.

[2] He Limin, Selected Microcontroller Application Technology 6[M], Beijing University of Aeronautics and Astronautics Press, 1998.8.

[3] Jiang Xiaoxia, Intelligent guest room control system based on At89S52 single chip microcomputer, Microcomputer Information, 2005, 10-2, 78-80