US20240159439A1

US20240159439A1 - Variable frequency air source heat pump cold water unit and parallel control method thereof

Info

Publication number: US20240159439A1
Application number: US18/510,780
Authority: US
Inventors: Zhenhua Liao; Long Xiong
Original assignee: Guangdong Carrier Hvac Co Ltd
Current assignee: Guangdong Carrier Hvac Co Ltd
Priority date: 2022-11-16
Filing date: 2023-11-16
Publication date: 2024-05-16
Also published as: EP4372296A1; CN115854608A

Abstract

A control method includes: setting a set water temperature for each subunit, acquiring a total outlet water temperature; according to an operating mode of the unit, judging whether the total outlet water temperature meets a first unloading condition, and if so, controlling the subunit with the lowest operating frequency to perform an unloading action; acquiring the total outlet water temperature in real time, and judging whether the total outlet water temperature meets a second unloading condition, and if so, controlling the subunit with the lowest operating frequency to perform the unloading action; judging whether the total outlet water temperature meets a start-up loading condition in real time, and if so, controlling the first subunit with the shortest accumulative operating time of the subunits performing the unloading action to perform a start-up loading action; and judging whether the total outlet water temperature meets the start-up loading condition every a specified time.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No. CN202211435385.5 filed on Nov. 16, 2022, which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to the technical field of heat pumps, in particular to a variable frequency air source heat pump cold water unit and a parallel control method thereof.

BACKGROUND OF THE INVENTION

In a variable frequency air source heat pump cold water unit, during the operation of the unit, the problem of “shutting down before down conversion” has occurred to single units within the entire parallel system, because the operating frequency of each unit is under uniform control. When the water temperature of some units reaches a set temperature, all units are unloaded directly. As a result, there may be some units that have not reached the set temperature. However, the capacity output of the unit is just in balance with the energy consumption. Even if the unit is turned on with the maximum output, it can only maintain the water temperature without falling or rising. Therefore, there are some units in the parallel system that have been shut down without reaching the set temperature. On the other hand, in order to reach the set temperature, a compressor is caused to be turned on again, which leads to frequent turn-on and shut-down of the compressor, shortens the service life of the compressor, and affects the service life of the entire system.
In addition, in the start-up procedure of the parallel system, a number of units to be turned on is usually determined directly once starting up. Units in a water system cannot directly respond to specific requirements of terminals via water temperature differences. Therefore, some units should have been turned on but were not. This causes a misjudgment on system requirements.

SUMMARY OF THE INVENTION

In view of the above problems, the present invention provides a variable frequency air source heat pump cold water unit and a parallel control method thereof. The operating frequency of each of the subunits is independently controlled to avoid the problem of “shutting down before down conversion”, greatly improving the energy efficiency of the entire system, and achieving the purpose of energy saving. In addition, in an unloading procedure, whether there is a requirement for start-up loading is judged. This guarantees that the unit can meet the requirement of a desired water temperature under the premise of sufficient capacity, to meet the usage requirement.
The present invention provides a parallel control method of a variable frequency air source heat pump cold water unit, wherein the variable frequency air source heat pump cold water unit includes a plurality of subunits disposed in parallel, the parallel control method including the steps of:
setting a set water temperature of each of subunits, and acquiring a total outlet water temperature of the variable frequency air source heat pump cold water unit;
according to an operating mode of the variable frequency air source heat pump cold water unit, judging whether the total outlet water temperature meets a first unloading condition, and if so, controlling the subunit with the lowest operating frequency to perform an unloading action; the first unloading condition being: in a heating/domestic hot water mode, the total outlet water temperature≥the set water temperature−(a number of subunits turned on−a first preset value); and in a cooling mode, the total outlet water temperature≤the set water temperature+(a number of subunits turned on−a second preset value);
acquiring the total outlet water temperature in real time, and judging whether the total outlet water temperature meets a second unloading condition, and if so, controlling the subunit with the lowest operating frequency to perform the unloading action;
judging whether the total outlet water temperature meets a start-up loading condition in real time, and if so, controlling the first subunit with the shortest accumulative operating time of the subunits performing the unloading action to perform a start-up loading action; and
judging whether the total outlet water temperature meets the start-up loading condition every a specified time, and if so, controlling the subunit to perform the start-up loading action, the subunit performing the start-up loading action being shut down according to conditions in a non-parallel mode.
The present invention judges whether a subunit needs to be unloaded according to whether the subunit meets a first unloading condition, and judges whether a further subunit need to be unloaded according to whether the subunit meets a second unloading condition, that is, adjusts a number of subunits to be unloaded in real time according to a total outlet water temperature obtained in real time and a change condition of the total outlet water temperature, and adjusts the operation of the unit according to the total outlet water temperature, which improves the efficiency of unit operation and achieves the purpose of energy saving. The operating frequency of each of the subunits is independently controlled to avoid the problem of “shutting down before down conversion”, greatly improving the energy efficiency of the entire system, and further achieving the purpose of energy saving. In addition, judging whether there is a requirement for start-up loading can avoid the condition in which there are some subunits in a parallel unit that have been shut down without reaching the set water temperature, guaranteeing that the unit operation is stable and reliable, and guaranteeing that the unit can meet the requirement of a desired water temperature under the premise of sufficient capacity, to meet the usage requirement. The subunit performing a start-up loading action being shut down according to conditions in a non-parallel mode can prevent unloading of the same subunit which leads to frequent turn-on and shut-down of the subunit in a short period of time, thereby improving the service life of the subunit.
In an optional technical solution of the present invention, the start-up loading condition is: in the heating/domestic hot water mode, whether the total outlet water temperature is not higher than the set water temperature, and whether the total outlet water temperature has risen within a first preset time period, and if the total outlet water temperature is not higher than the set water temperature, and the total outlet water temperature has not risen within the first preset time period, the first subunit with the shortest accumulative operating time of the subunits performing the unloading action is controlled to perform the start-up loading action; and
in the cooling mode, whether the total outlet water temperature is not lower than the set water temperature, and whether the total outlet water temperature has fallen within a second preset time period, and if the total outlet water temperature is not lower than the set water temperature, and the total outlet water temperature has not fallen within the second preset time period, the first subunit with the shortest accumulative operating time of the subunits performing the unloading action is controlled to perform the start-up loading action.
According to this technical solution, in an unloading process, whether the total outlet water temperature meets the requirement is judged, and when the requirement is not met, the start-up loading action is performed for the first subunit with the shortest accumulative operating time of the subunits performing the unloading action, which can improve an output capacity of the unit, so that the total outlet water temperature meets the water temperature requirement of a user.
In an optional technical solution of the present invention, the second unloading condition is: in the heating/domestic hot water mode, the total outlet water temperature rises by a first specified temperature; and in the cooling mode, the total outlet water temperature falls by a second specified temperature.
According to this technical solution, in different modes, once the total outlet water temperature rises or falls by a specified temperature, the subunit with the lowest operating frequency is unloaded, which reduces energy waste while improving energy efficiency, achieving the purpose of energy saving.
In an optional technical solution of the present invention, the method also includes: judging whether the total outlet water temperature meets the maximum water temperature/minimum water temperature limiting conditions, and if not, performing a “shut-down for temperature-reaching” process.
According to this technical solution, the water temperature of the unit can be controlled within a reasonable range, to improve the comfort of use and the reliability of the unit operation.
In an optional technical solution of the present invention, the method also includes: judging whether a subunit meets “shut-down for power-off”/“shut-down for temperature-reaching” conditions, and if so, sequentially unloading one subunit every a specified time according to the accumulative operating time of the subunit.
In an optional technical solution of the present invention, the method also includes a start-up control logic: according to the operating mode of the variable frequency air source heat pump cold water unit, judging whether the total outlet water temperature meets a first start-up condition, and if so, turning on the subunit in a shut-down state with the shortest accumulative operating time; and acquiring the total outlet water temperature every a first specified time, and judging whether the difference between the total outlet water temperatures of adjacent time intervals meets a second start-up condition every a second specified time, and if so, turning on the subunit in the shut-down state with the shortest accumulative operating time; and if not, not turning on the subunit.
According to this technical solution, after start-up requirements are met, the total outlet water temperature is acquired in real time, and whether the temperature difference between the total outlet water temperatures meets the second start-up condition is judged, so that the number of subunits to be turned on can be adjusted in real time according to the requirements of the unit, to improve accuracy in the number of subunits to be turned on. Further, whether to start up is judged every a specified time and the rise/fall ranges of the total outlet water temperature is determined before starting up, whereby whether the unit can quickly meet the set water temperature requirement is judged, so as to judge whether to continue to turn on the subunit. This avoids misjudging the requirement and avoids frequent turn-on and shut-down of the subunit, guaranteeing the service life of the compressor in the subunit and reducing energy waste.
In an optional technical solution of the present invention, the first start-up condition is: in the heating/domestic hot water mode, the total outlet water temperature≤the difference between the set water temperature and a third preset value; and in the cooling mode, the total outlet water temperature≥the sum of the set water temperature and a fourth preset value; and
the second start-up condition is: in the heating/domestic hot water mode, the total outlet water temperature≤the difference between the set water temperature and the third preset value, and the difference between the total outlet water temperatures≤a first temperature threshold; and in the cooling mode, the total outlet water temperature≥the sum of the set water temperature and the fourth preset value, and the difference between the total outlet water temperatures≥a second temperature threshold.
According to this technical solution, the start-up loading action is performed when: in the heating/domestic hot water mode, the total outlet water temperature is lower than the set water temperature, and the rise range of the total outlet water temperature is small; and in the cooling mode, the total outlet water temperature is higher than the set temperature, and the fall range of the total outlet water temperature is small. This can improve the output capacity of the unit, so that the total outlet water temperature and the rise/fall rates of the total outlet water temperature can meet the requirements of a user.
In an optional technical solution of the present invention, the method also includes: acquiring the total outlet water temperature every 5 min, and judging whether the difference between the total outlet water temperatures meets the second start-up condition every 30 min, and if so, turning on the subunit in a shut-down state with the shortest accumulative operating time; and if not, not turning on the subunit.
According to this technical solution, frequent turn-on and shut-down of the subunit can be avoided, to improve the reliability of the unit operation.
In an optional technical solution of the present invention, if the difference between the total outlet water temperatures or the total outlet water temperature does not meet any of the start-up conditions for a specified time, and no unloading action of the subunit is detected during the specified time, one subunit with the shortest accumulative operating time is forcibly turned on.
According to this technical solution, under the above conditions, forcibly turning on one subunit with the shortest accumulative operating time can improve the output capacity of the unit, so that the total outlet water temperature can meet the water temperature requirement of the user.
The present invention further provides a variable frequency air source heat pump cold water unit, which performs the aforementioned parallel control method of the variable frequency air source heat pump cold water unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a shut-down control logic of a parallel control method of a variable frequency air source heat pump cold water unit in an embodiment of the present invention.

FIG. 2 is a schematic flow diagram of a start-up control logic of a parallel control method of a variable frequency air source heat pump cold water unit in an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The technical solutions in the embodiments of the present invention are clearly and fully described below with reference to the accompanying drawings in the embodiments of the present invention. It is clear that the described embodiments are merely some embodiments of the present invention rather than all the embodiments of the present invention. All other embodiments, obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative efforts, shall fall within the protection scope of the present invention.
As shown in FIG. 1 , the present invention provides a parallel control method of a variable frequency air source heat pump cold water unit. The variable frequency air source heat pump cold water unit includes a plurality of subunits disposed in parallel. The parallel control method includes the following steps.
A set water temperature TS of each of the subunits is set, and a total outlet water temperature T1 of the variable frequency air source heat pump cold water unit is acquired.
According to an operating mode of the variable frequency air source heat pump cold water unit, whether the total outlet water temperature T1 meets a first unloading condition is judged, and if so, the subunit with the lowest operating frequency is controlled to perform an unloading action; as shown in a dashed block in FIG. 1 , the first unloading condition is: in a heating/domestic hot water mode, the total outlet water temperature T1≥the set water temperature TS−(a number of subunits turned on−a first preset value); and in a cooling mode, the total outlet water temperature T1≤the set water temperature TS+(a number of subunits turned on−a second preset value).
The total outlet water temperature T1 is acquired in real time, and whether the total outlet water temperature T1 meets a second unloading condition is judged, and if so, the subunit with the lowest operating frequency is controlled to perform the unloading action; as shown in the dashed block in FIG. 1 , the second unloading condition is: in the heating/domestic hot water mode, the total outlet water temperature T1 rises by a first specified temperature a° C., that is, whether a temperature difference between the total outlet water temperatures T1′−T1 within two adjacent temperature acquisition time intervals is not less than a° C. is judged, where T1′ is the total outlet water temperature acquired this time, and T1 is the total outlet water temperature acquired last time; and in the cooling mode: the total outlet water temperature T1 falls by a second specified temperature a′, that is, whether the temperature difference between the total outlet water temperatures T1′−T1 within two adjacent temperature acquisition time intervals is not more than −a° C. is judged.
Whether the total outlet water temperature T1 meets a start-up loading condition is judged in real time, and if so, the first subunit with the shortest accumulative operating time of the subunits performing the unloading action is controlled to perform a start-up loading action; and whether the total outlet water temperature T1 meets the start-up loading condition is judged every a specified time, and if so, the subunit is controlled to perform the start-up loading action, the subunit performing the start-up loading action being shut down according to conditions in a non-parallel mode.
The present invention judges whether a subunit needs to be unloaded according to whether the subunit meets a first unloading condition, and judges whether a further subunit need to be unloaded according to whether the subunit meets a second unloading condition, that is, adjusts a number of subunits to be unloaded in real time according to a total outlet water temperature obtained in real time and a change condition of the total outlet water temperature, and adjusts the operation of the unit according to the total outlet water temperature, which improves the efficiency of unit operation and achieves the purpose of energy saving. In addition, the comparison of the total outlet water temperature with a set water temperature takes a number of subunits that are turned on in consideration. For example, in a heating/domestic hot water mode, when there is a small number of subunits that are turned on, the total outlet water temperature may be more than the set water temperature before entering the unloading procedure; and when there is a large number of subunits that are turned on, the total outlet water temperature may be lower than the set water temperature before entering the unloading procedure. The influence of a number of subunits that are turned on upon a change speed of the total outlet water temperature is fully considered, improving the accuracy of unloading. The operating frequency of each of the subunits is independently controlled to avoid the problem of “shutting down before down conversion”, greatly improving the energy efficiency of the entire system, and further achieving the purpose of energy saving. In addition, judging whether there is a requirement for start-up loading can avoid the condition in which there are some subunits in a parallel unit that have been shut down without reaching the set water temperature, guaranteeing that the unit operation is stable and reliable, and guaranteeing that the unit can meet the requirement of a desired water temperature under the premise of sufficient capacity, to meet the usage requirement. The subunit performing a start-up loading action being shut down according to conditions in a non-parallel mode can prevent unloading of the same subunit which leads to frequent turn-on and shut-down of the subunit in a short period of time, thereby improving the service life of the subunit. It should be noted that the set water temperature of the present invention is the set water temperature of each of the subunits, and the set water temperatures of the subunits may be the same or different.
In a preferred embodiment of the present invention, as shown in the dashed block in FIG. 1 , the start-up loading condition is: in the heating/domestic hot water mode, whether the total outlet water temperature T1 is not higher than the set water temperature TS (T1≤TS), and whether the total outlet water temperature T1 has risen within a first preset time period, and if the total outlet water temperature T1 is not higher than the set water temperature TS (T1≤TS), and the total outlet water temperature T1 has not risen within the first preset time period, the first subunit with the shortest accumulative operating time of the subunits performing the unloading action is controlled to perform the start-up loading action; and
in the cooling mode, whether the total outlet water temperature T1 is not lower than the set water temperature TS, and whether the total outlet water temperature T1 has fallen within a second preset time period, and if the total outlet water temperature T1 is not lower than the set water temperature TS, and the total outlet water temperature T1 has not fallen within the second preset time period, the first subunit with the shortest accumulative operating time of the subunits performing the unloading action is controlled to perform the start-up loading action.
According to different operating modes of the system, whether the total outlet water temperature meets the water temperature requirements for cooling or heating/domestic hot water is judged, respectively, which improves the accuracy of control of the total outlet water temperature. In an unloading process, whether the total outlet water temperature T1 meets the water temperature requirement is judged, and when the water temperature requirement is not met, the start-up loading action is performed for the first subunit with the shortest accumulative operating time of the subunits performing the unloading action, which can improve an output capacity of the unit, so that the total outlet water temperature T1 meets the water temperature requirement of a user.
In a preferred embodiment of the present invention, the method also includes: judging whether each of the subunits meets a single-subunit shut-down condition, and if so, controlling the subunit to be shut down. In particular, the single-subunit shut-down condition is that the capacity requirement equals to 0 or over-temperature protection occurs. If the unit meets any of the shut-down conditions, a shut-down action will be performed.
In a preferred embodiment of the present invention, the method also includes: judging whether the total outlet water temperature T1 meets the maximum water temperature/minimum water temperature limiting conditions, and if not, performing a “shut-down for temperature-reaching” process. With the above method, the water temperature of the unit can be controlled within a reasonable range, to improve the comfort of use and the reliability of the unit operation.
In a preferred embodiment of the present invention, the method also includes: judging whether the subunit meets “shut-down for power-off”/“shut-down for temperature-reaching” conditions, and if so, sequentially unloading one subunit every a specified time according to the accumulative operating time of the subunit. In particular, if the subunit meets the “shut-down for power-off”/“shut-down for temperature-reaching” conditions, one subunit can be unloaded every 10 s according to the accumulative operating time of the subunit.
As shown in FIG. 2 , in a preferred embodiment of the present invention, the method also includes a start-up control logic: according to the operating mode of the variable frequency air source heat pump cold water unit, judging whether the total outlet water temperature T1 meets a first start-up condition, and if so, turning on the subunit in a shut-down state with the shortest accumulative operating time; acquiring the total outlet water temperature T1 every a first specified time, and judging whether the difference between the total outlet water temperatures T1 of adjacent time intervals meets a second start-up condition every a second specified time, and if so, turning on the subunit in a shut-down state with the shortest accumulative operating time; and if not, not turning on the subunit.
With the above method, after start-up requirements are met, the total outlet water temperature T1 is acquired in real time, and whether the temperature difference between the total outlet water temperatures T1 meets the second start-up condition is judged, the number of subunits to be turned on can be adjusted in real time according to the requirements of the unit, to improve accuracy in the number of subunits to be turned on. In addition, turning on the subunit with the shortest accumulated operating time is beneficial for prolonging the overall service life of the variable frequency air source heat pump cold water unit and reducing the cost of use. Further, whether to turn on is judged every a specified time and the rise/fall ranges of the total outlet water temperature is determined before turning on, whereby whether the unit can quickly meet the set water temperature requirement is judged, so as to judge whether to continue to turn on the subunit. This avoids misjudging the requirement and avoids frequent turn-on and shut-down of the subunit, guaranteeing the service life of the compressor in the subunit and reducing energy waste.
In a preferred embodiment of the present invention, the first start-up condition is: in the heating/domestic hot water mode, the total outlet water temperature T1≤the difference between the set water temperature TS and a third preset value (such as T1≤TS−2); and in the cooling mode, the total outlet water temperature T1≥the sum of the set water temperature TS and a fourth preset value (such as T1≥TS+6). The second start-up condition is: in the heating/domestic hot water mode, the total outlet water temperature T1≤the difference between the set water temperature TS and the third preset value, and the difference between the total outlet water temperatures T1≤a first temperature threshold (such as ΔT1 (ΔT1=T1 at 10 min−T1 at 5 min)≤5° C.); and in the cooling mode, the total outlet water temperature T1≥the sum of the set water temperature TS and the fourth preset value, and the difference between the total outlet water temperatures T1≥a second temperature threshold (ΔT1 (ΔT1=T1 at 10 min−T1 at 5 min)≥−5° C.).
With the above method, the start-up loading action is performed when: in the heating/domestic hot water mode, the total outlet water temperature T1 is lower than the set water temperature TS, and the rise range of the total outlet water temperature T1 is less than the third preset value; and in the cooling mode, the total outlet water temperature T1 is higher than the set temperature, and the fall range of the total outlet water temperature T1 is less than the fourth preset value. This can improve the output capacity of the unit, so that the total outlet water temperature T1 and the rise/fall ranges of the total outlet water temperature T1 can meet the requirements of a user.
In a preferred embodiment of the present invention, the method also includes: acquiring the total outlet water temperature T1 every 5 min, and judging whether the difference between the total outlet water temperatures T1 meets the second start-up condition every 30 min, and if so, turning on the subunit in a shut-down state with the shortest accumulative operating time; and if not, not turning on the subunit that is shut down. With the above method, by obtaining the total outlet water temperature T1 in real time, the outlet water temperature of the unit can be known in time, so as to avoid the water temperature being too high or too low, affecting the operation reliability of the unit or the comfort of the user. In addition, by prolonging the time interval to judge whether the second start-up condition is met, frequent turn-on and shut-down of the subunit can be avoided, improving the reliability of the unit operation.
In a preferred embodiment of the present invention, if the difference between the total outlet water temperatures T1 or the total outlet water temperature T1 does not meet any of the start-up conditions for a specified time, and no unloading action of the subunit is detected during the specified time, one subunit with the shortest accumulative operating time is forcibly turned on. Under the above conditions, forcibly turning on one subunit with the shortest accumulative operating time can improve the output capacity of the unit, so that the total outlet water temperature T1 can meet the water temperature requirement of the user.
In a preferred embodiment of the present invention, during the specific operation, the capacity requirement of each of the subunits follows the capacity requirement mode of the single subunit. For example, the operating frequency of the subunits is increased or decreased at different rates based on different temperature difference sections according to the temperature difference between the set water temperature TS and the total outlet water temperature T1.
In a preferred embodiment of the present invention, the method also includes: before entering the unloading control logic and the start-up control logic, judging whether the variable frequency air source heat pump cold water unit is a parallel unit, and in the start-up control logic, judging whether to start up.
The present invention further provides a variable frequency air source heat pump cold water unit, which performs the aforementioned parallel control method of the variable frequency air source heat pump cold water unit.
The above merely gives preferred embodiments of the present invention, and is not intended to limit the present invention. Any modification, equivalent substitution, and improvement made within the spirit and principle of the present invention shall fall in the protection scope of the present invention.

Claims

1. A parallel control method of a variable frequency air source heat pump cold water unit, wherein the variable frequency air source heat pump cold water unit comprises a plurality of subunits disposed in parallel, the parallel control method comprising the steps of:

setting a set water temperature of each of the subunits, and acquiring a total outlet water temperature of the variable frequency air source heat pump cold water unit;

according to an operating mode of the variable frequency air source heat pump cold water unit, judging whether the total outlet water temperature meets a first unloading condition, and if so, controlling the subunit with the lowest operating frequency to perform an unloading action; the first unloading condition being: in a heating/domestic hot water mode, the total outlet water temperature≥the set water temperature−(a number of subunits turned on−a first preset value); and in a cooling mode, the total outlet water temperature≤the set water temperature+(a number of subunits turned on−a second preset value);

acquiring the total outlet water temperature in real time, and judging whether the total outlet water temperature meets a second unloading condition, and if so, controlling the subunit with the lowest operating frequency to perform the unloading action;

judging whether the total outlet water temperature meets a start-up loading condition in real time, and if so, controlling the first subunit with the shortest accumulative operating time of the subunits performing the unloading action to perform a start-up loading action; and

judging whether the total outlet water temperature meets the start-up loading condition every a specified time, and if so, controlling the subunit to perform the start-up loading action, the subunit performing the start-up loading action being shut down according to conditions in a non-parallel mode.

2. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 1, wherein the start-up loading condition is: in the heating/domestic hot water mode, whether the total outlet water temperature is not higher than the set water temperature, and whether the total outlet water temperature has risen within a first preset time period, and if the total outlet water temperature is not higher than the set water temperature, and the total outlet water temperature has not risen within the first preset time period, the first subunit with the shortest accumulative operating time of the subunits performing the unloading action is controlled to perform the start-up loading action; and

in the cooling mode, whether the total outlet water temperature is not lower than the set water temperature, and whether the total outlet water temperature has fallen within a second preset time period, and if the total outlet water temperature is not lower than the set water temperature, and the total outlet water temperature has not fallen within the second preset time period, the first subunit with the shortest accumulative operating time of the subunits performing the unloading action is controlled to perform the start-up loading action.

3. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 1, wherein the second unloading condition is: in the heating/domestic hot water mode, the total outlet water temperature rises by a first specified temperature; and in the cooling mode, the total outlet water temperature falls by a second specified temperature.

4. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 1, further comprising: judging whether the total outlet water temperature meets the maximum water temperature/minimum water temperature limiting conditions, and if not, performing a “shut-down for temperature-reaching” process.

5. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 1, further comprising: judging whether the subunit meets “shut-down for power-off”/“shut-down for temperature-reaching” conditions, and if so, sequentially unloading one subunit every a specified time according to the accumulative operating time of the subunit.

6. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 1, further comprising a start-up control logic:

according to the operating mode of the variable frequency air source heat pump cold water unit, judging whether the total outlet water temperature meets a first start-up condition, and if so, turning on the subunit in a shut-down state with the shortest accumulative operating time; and

acquiring the total outlet water temperature every a first specified time, and judging whether the difference between the total outlet water temperatures of adjacent time intervals meets a second start-up condition every a second specified time, and if so, turning on the subunit in the shut-down state with the shortest accumulative operating time; and if not, not turning on the subunit.

7. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 6, wherein the first start-up condition is: in the heating/domestic hot water mode, the total outlet water temperature≤the difference between the set water temperature and a third preset value; and in the cooling mode, the total outlet water temperature≥the sum of the set water temperature and a fourth preset value; and

the second start-up condition is: in the heating/domestic hot water mode, the total outlet water temperature≤the difference between the set water temperature and the third preset value, and the difference between the total outlet water temperatures≤a first temperature threshold; and in the cooling mode, the total outlet water temperature≥the sum of the set water temperature and the fourth preset value, and the difference between the total outlet water temperatures≥a second temperature threshold.

8. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 6, further comprising: acquiring the total outlet water temperature every 5 min, and judging whether the difference between the total outlet water temperatures meets the second start-up condition every 30 min, and if so, turning on the subunit in the shut-down state with the shortest accumulative operating time; and if not, not turning on the subunit.

9. The parallel control method of the variable frequency air source heat pump cold water unit according to claim 6, wherein if the difference between the total outlet water temperatures or the total outlet water temperature does not meet any of the start-up conditions for a specified time, and no unloading action of the subunit is detected during the specified time, one subunit with the shortest accumulative operating time is forcibly turned on.

10. A variable frequency air source heat pump cold water unit, which performs the parallel control method of the variable frequency air source heat pump cold water unit according to claim 1.