A chiller multi-unit load allocation system and method

Abstract

This invention discloses a multi-unit load allocation system and method for chillers, which reliably manages the load distribution of multiple chillers. The system utilizes end-user valve opening sequence values ​​and chiller load distribution values ​​to derive a frequency control algorithm for the chilled water system's delivery pumps and a method for calculating the diversion opening of each chiller's inlet diversion device, as well as a corresponding system architecture. This invention implements a relatively reliable and easy-to-implement control method for multi-unit load distribution in chillers, eliminating the complex tasks of multi-unit communication, control protocol integration, and device controller upgrades.

Description

Technical Field

[0001] The present invention relates to the technical field of refrigeration and air-conditioning chilled water system control, and in particular to a chilled water system variable flow control method and system with constant water supply temperature. Background Art

[0002] Centrifugal and screw chillers have been widely used in campus buildings, large single-unit buildings, and regional energy stations, and are trending towards larger and more centralized installations. The demand for energy conservation and carbon reduction in large chillers and energy plant rooms is increasing.

[0003] The optimal efficiency point for centrifugal and screw chillers is typically between 40% and 60% load. While chiller power configurations must meet design loads, they also face the objective reality of intraday partial load fluctuations during the cooling season due to factors such as weather and insufficient occupancy. Therefore, at partial loads, systems with multiple chillers require load distribution and control between units. This ensures that each unit operates near its optimal efficiency point while meeting the total cooling load, thereby reducing overall power consumption. Summary of the Invention

[0004] To achieve load allocation among multiple chillers, it is necessary to actively control the flow rate entering the chiller. The conventional method of regulating the flow rate of the chiller is to install a regulating valve at the outlet of the chiller to throttle the flow of chilled water through the unit. This method will significantly increase throttling losses and reduce the overall efficiency of the system's fluid transportation.

[0005] Against this backdrop, the present invention aims to achieve more economical and efficient load allocation across multiple chillers (a many-to-many system) by controlling the flow ratios entering each chiller and the operating frequency of the chiller pumps, while ensuring safe operation. The present invention employs the following solutions:

[0006] A method for load allocation of multiple units of a chiller first records the sequence of opening values ​​of each terminal balancing valve within the current strategy period. r (n, j), calculate the delivery frequency of the system chilled water pump at the end of the current strategy period, and adjust the frequency to the calculated value, which is divided into the following 7 steps:

[0007] (1) Calculate the average opening of each balancing valve within the strategic period. The process is as follows:

[0008] ;

[0009] Among them, r(n,j) is the opening value sequence of the jth balancing valve at the end in the current strategy period, and the opening values ​​of each balancing valve total n;

[0010] (2) Calculate the deviation matrix of each balancing valve opening:

[0011] ;

[0012] (3) Calculate the correlation coefficient matrix between each balancing valve:

[0013] ;

[0014] (4) Take the lower half of the correlation coefficient matrix A, that is:

[0015] ;

[0016] (5) Eliminate all valves whose row values ​​in the lower half of the correlation coefficient matrix A are not positive;

[0017] (6) Calculate the total opening value of the remaining m balancing valves at the end of the current strategy period after elimination:

[0018] ;

[0019] (7) Calculate the chilled water pump delivery frequency:

[0020] ;

[0021] Among them, y min and y max are the upper and lower limits of the water pump operating frequency, k is the steepness coefficient of the frequency conversion curve, z is the overall opening value of the balancing valve at the end of the current strategy period, and c is the center coefficient of the frequency conversion curve. The upper and lower limits of the water pump operating frequency need to take into account factors such as the performance of the water pump inverter, the safe operation requirements of the unit, and the maximum resistance of the system. The purpose of using this function is mainly to reversely regulate the system flow through the overall opening z of the balancing valve: a larger opening of the balancing valve indicates that the flow demand at each terminal is larger at this time, and the water pump operating frequency needs to be increased at this time; a smaller opening of the balancing valve indicates that the flow demand at each terminal is smaller at this time, and the water pump operating frequency needs to be reduced at this time. Since valves usually have equal percentage characteristics, this function distribution is used. The steepness coefficient of the frequency conversion curve and the center coefficient of the frequency conversion curve are usually determined by the system characteristics.

[0022] According to the cooling load demand during the current strategic period Load , calculate the load distribution value of each chiller in the current strategy period w(j) The prediction method of the total cooling load can be LSTM, historical average, typical daily average, etc., and the calculation of the load distribution value can be calculated by mixed integer linear programming and other methods. Then calculate and adjust the diversion opening of each diversion device. The inlet of the diversion device is connected to the upstream of the chilled water return main, and the two outlets are the return branch pipe of the chiller inlet and the downstream of the chilled water return main. By adjusting the diversion opening, the proportion of chilled water flowing to the two outlets can be controlled, but the sum of its flow area remains unchanged, so it basically does not affect the overall resistance along the water system. Since the diversion amount of the diversion device is linearly related to its angle, the calculation method of the diversion opening pointing to the chiller is:

[0023] ;

[0024] in Reg (j) is the diversion opening of the diversion device pointing to the outlet of the j-th chiller at the end of the current strategic period, p (j) is the ratio of the load distribution value of the jth chiller at the end of the current strategy period to the total load demand at the current moment, that is:

[0025] ;

[0026] If the overall load demand during the current strategy period Load If it is greater than the value of the previous strategy period, the operating frequency of the chilled water pump is adjusted first. f , then adjust the diversion device diversion opening Reg If the overall load demand during the current strategy period is Load If it is less than the value of the previous strategy period, the diversion device opening is adjusted first. Reg , then adjust the operating frequency of the chilled water pump f .

[0027] A multi-unit load allocation system for a chiller is applied to a multi-unit load allocation method for a chiller, comprising a plurality of chilled water pumps (chilled water pump groups) arranged in parallel, and a plurality of chillers arranged in parallel. The chilled water return main pipe is connected from the chilled water pump group outlet to each chilled water return branch pipe, and the chilled water return branch pipe is connected to the inlet of each chiller. Except for the farthest chiller, a diversion device is provided at the bifurcation between each chilled water return branch pipe and the main pipe. The chilled water supply branch pipe of the chiller is connected to the outlet of each chiller, and after confluence, it is led to the end user. After the chilled water is heated by heat exchange and cooling at the end user, it is connected to the inlet of the chilled water pump group through the chilled water return main pipe.

[0028] The present invention provides a multi-unit load allocation system and method for a chiller, which realizes the control problem of load distribution of multiple chillers in a relatively reliable manner, including obtaining a frequency control algorithm for the delivery pump of the chilled water system and a method for calculating the diversion opening of each chiller inlet diversion device, as well as a corresponding system architecture by using the end-user valve opening sequence value and the chiller load distribution value. The present invention realizes a relatively reliable and easy-to-use control method suitable for load distribution control of multiple units of a chiller, eliminating the relatively complex tasks of multi-unit communication, control protocol docking, equipment controller upgrade, etc. of the chiller.

[0029] The beneficial effects of the present invention are:

[0030] For systems with a many-to-many relationship between chilled water pump groups and chillers, this method ensures mutual redundancy between the pump groups, thereby avoiding reliability issues associated with a one-to-one relationship between chilled water pumps and chillers. The method of the present invention is suitable for the efficient transformation of such multi-unit cooling systems, enabling flow control of each chiller in a many-to-many relationship between pumps and chillers.

[0031] The method of the present invention actively reduces or increases the load power of the chiller by controlling the flow rate of each chiller, thereby adjusting the distribution ratio of the cooling load among the chillers, so that each chiller operates at the power value specified by the optimal scheduling method without changing the set temperature difference during the flow rate change, thereby realizing the distribution of the cooling load and improving the overall energy efficiency of the system.

[0032] The method of the present invention is highly practical and realizes a relatively reliable and easy control method suitable for load distribution and regulation of multiple units of a chiller, eliminating the relatively complicated work of multi-unit communication, control protocol docking, and equipment controller upgrade of the chiller. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a simplified diagram of the multi-unit load allocation system of the chiller of the present invention;

[0034] Figure 2 This is a frequency conversion curve diagram of the chilled water pump delivery frequency of the present invention;

[0035] In the figure: 1. Chilled water pump; 2. Diverter device; 3. Chiller; 4. Chilled water return main pipe; 5. Chilled water return branch pipe; 6. Terminal balancing valve; 7. Each end user. DETAILED DESCRIPTION

[0036] Embodiments of the present invention will be described below with reference to the accompanying drawings.

[0037] The chilled water system in a building's cooling room consists of three parallel chilled water pumps (55kW motors), three centrifugal chillers, and associated piping. The rated cooling power and cooling COP of the three centrifugal chillers are shown in Table 1:

[0038] Table 1

[0039] ;

[0040] Under normal operating conditions, the chilled water supply temperature is 7°C and the return water temperature is 12°C.

[0041] There are 5 balancing valves on the user side, and 5 opening values ​​are recorded during the current strategy period. The opening value sequence is as follows:

[0042] ;

[0043] Calculate the average of the opening values:

[0044] ;

[0045] Calculate the deviation matrix of each balancing valve opening:

[0046] ;

[0047] Compute the correlation coefficient matrix:

[0048] ;

[0049] The lower half of the correlation coefficient matrix is:

[0050] ;

[0051] The fifth row is not positive, so it is discarded.

[0052] Calculate the total opening value of the remaining 4 balancing valves at the end of the current strategy period:

[0053] ;

[0054] Based on the performance of the water pump inverter in the system, the safe operation requirements of the unit, the maximum resistance of the system, etc., the experiment shows that the minimum frequency is 25Hz and the maximum frequency is 50Hz. The steepness coefficient of the frequency conversion curve is 10, and the center coefficient of the frequency conversion curve is 0.4. Substituting it into the calculated delivery frequency of the chilled water pump, we get f=46.75. After rounding the frequency to one place, we get 46.8Hz. Figure 2 As shown;

[0055] Using the prediction algorithm and the historical load data of the dispatching system, the cooling load in the current strategic period is 2900kW. Using the mixed integer linear programming algorithm, the optimal cooling load of chillers 1 and 3 under this cooling load is 900kW, and the optimal cooling load of chiller 2 is 1100kW.

[0056] ;

[0057] According to the load distribution value of each chiller, calculate the diversion opening of each diversion device:

[0058] Diversion opening of No. 1 diversion device:

[0059] ;

[0060] Diversion opening of No. 2 diversion device:

[0061] ;

[0062] In the previous strategy period, the cooling load was 3500kW. The total load demand of the current strategy period is 2900kW, which is less than the value of the previous strategy period. f , Reg (1), Reg (2) After that, first adjust the diversion opening of the diversion device, and then adjust the operating frequency of the water pump.

[0063] Compared with existing technologies:

[0064] Compared with the same system without the present invention, if the three chillers are turned on, the cooling load will be passively shared evenly, that is, w (1)=966.6kW, w (2)=966.6kW, w (3)=966.6kW, if two chillers are turned on, then w (1) = 1450kW, w (2) = 1450kW, w (3) = 0kW. At the same time, the pump delivery frequency is 46.8Hz, and the power is further reduced compared to the non-frequency reduction. The final comparison results are shown in Table 2:

[0065] Table 2

[0066] ;

[0067] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for load allocation of multiple units of a chiller, based on a load allocation system of multiple units of a chiller, characterized by: The system comprises a plurality of chilled water pumps (1), a plurality of chilled water units (3), a chilled water return main pipe (4), a chilled water return branch pipe (5) and a diversion device (2); the plurality of chilled water pumps (1) are arranged in parallel, and the outlet of each chilled water pump is connected to the chilled water return main pipe (4); the plurality of chilled water units (3) are arranged in parallel, and the chilled water return main pipe (4) is connected to each chilled water return branch pipe (5); except for the farthest chilled water unit (3), a diversion device (2) is provided at the bifurcation between each chilled water return main pipe (4) and the chilled water return branch pipe (5); the chilled water return branch pipe (5) is connected to the inlet of each chilled water unit (3); the outlet of the chilled water unit (3) is connected to the chilled water supply branch pipe; after converging, the chilled water is led to the end user through the end balancing valve; after the chilled water is heated by heat exchange and cooling at the end user, the chilled water is connected to the inlet of the chilled water pump (1) through the chilled water return main pipe; The method comprises the following steps: S1. Obtain the sequence r(n,j) of the opening values ​​of each terminal balancing valve within the current strategy period; S2. Calculate and adjust the system chilled water pump delivery frequency based on the terminal balancing valve opening value sequence r(n,j) within the current strategic period; In S2, the method for calculating the chilled water pump delivery frequency is as follows: 2.1) Calculate the average opening of each balancing valve within the strategic period. The process is as follows: Among them, r(n,j) is the opening value sequence of the jth balancing valve at the end in the current strategy period, and the opening values ​​of each balancing valve total n; 2.2) Calculate the deviation matrix of each balancing valve opening: 2.3) Calculate the correlation coefficient matrix between each balancing valve: 2.4) Take the lower half of the correlation coefficient matrix A, that is: 2.5) Eliminate all valves whose row values ​​in the lower half of the correlation coefficient matrix A are not positive; 2.6) Calculate the total opening value of the remaining m balancing valves at the end of the current strategy period after elimination: 2.7) Calculate the chilled water pump delivery frequency: Among them, y min and y max are the upper and lower limits of the pump operating frequency, k is the steepness coefficient of the frequency conversion curve, z is the overall opening value of the balancing valve at the end of the current strategy period, and c is the center coefficient of the frequency conversion curve; S3. Calculate the load distribution value w(j) of each chiller during the current strategy period according to the cooling load demand Load during the current strategy period; S4. Calculate and adjust the diversion opening of each diversion device according to the load distribution value w(j) of each chiller within the current strategy period; In S4, the method for calculating the diversion opening of each level of diversion device is as follows: Reg(1)=p(1) Where Reg(j) is the diversion opening of the diversion device pointing to the outlet of the j-th chiller at the end of the current strategy period, and p(j) is the proportion of the load distribution value of the j-th chiller at the end of the current strategy period to the total load demand at the current moment, that is: Where w(j) is the load distribution value of the j-th chiller at the end of the current strategy period, and Load is the predicted total cooling load of the current strategy period.

2. A method for load allocation of multiple chillers according to claim 1, characterized in that: If the overall load demand Load in the current strategy period is greater than the value in the previous strategy period, the operating frequency f of the chilled water pump is adjusted first, and then the diversion device diversion opening Reg is adjusted; If the overall load demand Load in the current strategy period is less than the value in the previous strategy period, the diversion device diversion opening Reg is adjusted first, and then the chilled water pump operating frequency f is adjusted.

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