Solar energy and air source heat pump coupled gas-steam boiler feed water preheating system

Abstract

The invention discloses a solar energy and air source heat pump coupled gas-steam boiler feed water preheating system, and belongs to the technical field of industrial energy conservation. The system comprises a heat pump subsystem capable of switching heat sources, a gas-steam boiler system and an intelligent control system. The heat pump subsystem capable of switching the heat sources comprises a solar heat collector, a heat storage water tank, a variable frequency pump, an evaporator, an air cooling heat exchanger, a compressor, a condenser and an expansion valve. The solar heat collector, the heat storage water tank and the variable frequency pump form a first circulation loop through a pipeline; the evaporator, the air-cooled heat exchanger, the compressor, the condenser and the expansion valve form a second circulation loop through a pipeline; the intelligent control system comprises a solar irradiance sensor and a PLC controller. According to the system, through complementation of solar energy and the air source heat pump, gradient utilization of energy is achieved, all-weather stable preheating of boiler feed water is achieved, natural gas consumption is effectively reduced, carbon dioxide emission is reduced, and the comprehensive energy efficiency of the whole steam supply system is improved.

Description

Technical Field

[0001] This invention belongs to the field of industrial energy conservation technology, specifically relating to an energy-saving feedwater preheating system for gas-fired steam boilers, and in particular, a system that utilizes solar energy and an air source heat pump coupled together to preheat feedwater for gas-fired steam boilers. Background Technology

[0002] Industrial gas-fired steam boilers are core equipment in many production processes, but heating ambient-temperature feedwater to boiling to produce steam requires a significant amount of energy. Although gas-fired boilers themselves have high thermal efficiency, the low temperature of the feedwater entering the boiler means that a large amount of fuel heat is used in the basic heating stage, resulting in energy waste.

[0003] Currently, a common practice to improve boiler efficiency is to use boiler flue gas waste heat recovery devices to preheat boiler feedwater. However, this method has limited preheating temperature, and its energy-saving potential is nearing its limit. Solar energy, as a clean energy source, has been explored (Chinese patent CN103499088A) for boiler feedwater preheating. Typically, solar collectors are used to produce hot water, which is then preheated through a heat exchanger. However, this solution relies entirely on solar energy. At night, on cloudy or rainy days, or in winter, insufficient solar irradiance leads to a sharp decline in preheating efficiency, or even complete failure, resulting in system instability and hindering the continuous and efficient operation of the boiler.

[0004] An air source heat pump is a highly efficient device that can extract heat from the ambient air. However, when used alone to heat cold water to a higher temperature, its coefficient of performance (COP) decreases as the outlet water temperature increases, resulting in poor economic efficiency. Summary of the Invention

[0005] To address the problem of high natural gas consumption and consequently high carbon dioxide emissions in existing industrial boilers, this invention provides a gas-fired steam boiler feedwater preheating system based on the coupling of solar energy and an air source heat pump. This system prioritizes the use of zero-cost solar energy, and activates a high-efficiency air source heat pump when solar energy is insufficient, achieving stable preheating of boiler feedwater around the clock. This effectively reduces natural gas consumption, decreases carbon dioxide emissions, and improves the overall energy efficiency of the entire steam supply system.

[0006] This invention provides a gas-fired steam boiler feedwater preheating system coupled with solar energy and air source heat pump, comprising: a heat pump subsystem with switchable heat sources, a gas-fired steam boiler system, and an intelligent control system;

[0007] The switchable heat source heat pump subsystem includes a solar collector, a hot water storage tank, a variable frequency pump, an evaporator, an air-cooled heat exchanger, a compressor, a condenser, and an expansion valve. The solar collector is a flat-plate solar collector. The solar collector, the hot water storage tank, and the variable frequency pump form a first circulation loop via pipelines. The evaporator and the hot water storage tank are connected via pipelines for heat exchange. The evaporator, the air-cooled heat exchanger, the compressor, the condenser, and the expansion valve form a second circulation loop via pipelines. The air-cooled heat exchanger in this loop is located on a branch and can be selectively connected. The condenser is connected to the preheating water supply tank of the gas-fired steam boiler system and is used to heat the hot water flowing to the gas-fired boiler. The switchable heat source heat pump subsystem can selectively use the evaporator to exchange heat with the hot water storage tank or use the air-cooled heat exchanger to exchange heat with the air.

[0008] The gas-fired steam boiler system includes a preheating water supply tank and a gas-fired steam boiler; ambient temperature makeup water first flows through the condenser, is heated and then enters the preheating water supply tank, and is then transported to the gas-fired steam boiler by the boiler feed water pump;

[0009] The intelligent control system includes a solar irradiance sensor and a PLC controller. Both the solar irradiance sensor and the PLC controller require additional configuration.

[0010] Furthermore, a variable frequency pump is connected between the solar collector and the hot water storage tank, and the variable frequency pump is used to change the flow rate of the first circulation loop.

[0011] Furthermore, the first circulation loop is equipped with a water replenishment device, which is connected to the hot water storage tank.

[0012] Furthermore, the solar irradiance sensor is connected to the solar collector, and the solar irradiance sensor is used to monitor solar irradiance in real time.

[0013] Furthermore, the solar collector and the variable frequency pump are respectively connected to the PLC controller. The PLC controller is used to receive the input signal from the solar irradiance sensor, process it, and output a control signal to the variable frequency pump to dynamically adjust the operating frequency of the variable frequency pump.

[0014] Furthermore, the heat transfer medium in the first circulation loop is a mixture of water and ethylene glycol, mixed in a volume ratio of 1:1; the heat transfer medium in the second circulation loop is R134a (tetrafluoroethane).

[0015] Furthermore, the evaporator and the air-cooled heat exchanger are connected by pipes; valves are provided on the pipes to control the flow direction of the working fluid.

[0016] Furthermore, the evaporator is connected to the hot water storage tank. The evaporator is used to enable heat exchange between the first circulation loop and the second circulation loop, absorbing solar radiation heat.

[0017] Furthermore, the hot water storage tank is equipped with an internal heat exchanger, and the heat transfer medium in the first circulation loop flows into the internal heat exchanger to heat the water stored in the hot water storage tank.

[0018] Furthermore, the intelligent control system adopts a composite control method combining a main control loop (which controls the flow rate of the first circulation loop by monitoring solar irradiance and changing the frequency of the variable frequency pump) and an auxiliary protection loop (which controls the frequency of the variable frequency pump by setting the outlet temperature threshold of the solar collector and the temperature threshold of the hot water storage tank to prevent the system from overheating). Both the main control loop and the auxiliary protection loop control the frequency of the variable frequency pump, which is implemented by the PLC controller. The control method of the main control loop is as follows:

[0019] S101, Parameter settings: Set the minimum solar irradiance threshold Eth, irradiance variation interval Eb, maximum safe speed of variable frequency pump Nth, and variable frequency pump speed variation interval Nb.

[0020] S102, data acquisition, real-time acquisition of solar irradiance E and variable frequency pump real-time speed N;

[0021] S103, Variable frequency pump start control: When E≥Eth, the variable frequency pump starts running; when E≤Eth, the variable frequency pump stops running.

[0022] S104, variable frequency pump frequency control, irradiance increases Eb, variable frequency pump speed increases Nb;

[0023] The auxiliary protection circuit method is as follows:

[0024] S201, parameter setting, set the maximum outlet temperature T1th of the solar collector and the maximum temperature T2th of the hot water storage tank;

[0025] S202, Data Acquisition, Real-time acquisition of solar collector outlet temperature T1 and hot water storage tank temperature T2;

[0026] S203, temperature protection control: when T1≥T1th, the variable frequency pump speed is forcibly reduced by 50% to prevent system vaporization; when T2≥T2th, the variable frequency pump in the first circulation loop is shut down as a system overheat protection.

[0027] Furthermore, under normal circumstances, the above control parameters can be set as follows: the recommended range for the minimum solar irradiance threshold Eth is 30-150 W / m². 2 The recommended range for the irradiance variation interval Eb is 50-300 W / m². 2The recommended range for the maximum safe speed Nth of the variable frequency pump is 85%-100% of the rated speed; the recommended range for the speed change interval Nb of the variable frequency pump is 50-500 r / min; the recommended range for the highest outlet temperature T1th of the solar collector is 75-95°C; the recommended range for the highest temperature T2th of the hot water storage tank is 60-85°C; and the recommended range for the minimum speed Nmin of the variable frequency pump is 15%-30% of the rated speed.

[0028] Preferably, the irradiance variation interval Eb is 100 W / m. 2 The variable frequency pump speed changes at intervals of 200 r / min. The highest outlet temperature of the solar collector, T1th, is 80°C. The highest temperature of the hot water storage tank, T2th, is 80°C. When the variable frequency pump is turned on, it operates at a minimum speed of Nmin = 500 r / min.

[0029] Furthermore, the minimum temperature difference T3th between the solar collector outlet and the hot water storage tank is monitored. When the temperature difference between the solar collector outlet and the hot water storage tank is lower than the minimum temperature difference setting T3th, the system switches from solar mode to air source mode.

[0030] Furthermore, the start-up and shutdown conditions of the variable frequency pump based on solar irradiance are expressed as follows:

[0031]

[0032] In the formula, In variable frequency pump mode. This represents the current solar irradiance. This is the minimum threshold for solar irradiance.

[0033] The formula for calculating the speed of a variable frequency pump based on solar irradiance is as follows:

[0034]

[0035] In the formula, For the variable frequency pump speed, This is the floor function. This represents the current solar irradiance. This represents the minimum threshold for solar irradiance.

[0036] The formula for changing the frequency of the variable frequency pump based on the collector outlet temperature can be expressed as:

[0037]

[0038] In the formula, The variable frequency pump speed is controlled by temperature T1. This represents the current speed of the variable frequency pump.

[0039] The formula for controlling the start and stop of the variable frequency pump based on the temperature of the hot water storage tank can be expressed as:

[0040]

[0041] In the formula, The variable frequency pump is started and stopped under temperature T2 control.

[0042] The final variable frequency pump speed decision is as follows:

[0043]

[0044] In the formula, This refers to the final variable frequency pump speed.

[0045] This invention provides a method for preheating feedwater in a gas-fired steam boiler coupled with solar energy and an air source heat pump, comprising the following steps:

[0046] When the solar irradiance reaches a set threshold, the variable frequency pump in the first circulation loop starts, driving the heat transfer medium to flow. The heat transfer medium absorbs heat through the solar collector and enters the internal heat exchanger of the hot water storage tank. After exchanging heat with the water inside the hot water storage tank, it flows back into the solar collector through the variable frequency pump for further heat exchange. In the second circulation loop, two modes can be switched according to the actual weather conditions: solar mode and air source heat pump mode. In solar mode, the air-cooled heat exchanger is closed, and the heat transfer medium in the second circulation loop only flows through the evaporator. The heat transfer medium first evaporates and absorbs heat in the evaporator, becoming a low-temperature, low-pressure gaseous medium, and then flows into the compressor for compression. The heat pump fluid then transforms into a high-temperature, high-pressure gaseous working fluid, which enters the condenser and exchanges heat with the boiler feedwater flowing through it. After releasing heat, it transforms into a high-temperature, high-pressure liquid working fluid and enters the expansion valve. After passing through the expansion valve, it becomes a low-temperature, low-pressure liquid-gas mixture, which then passes through the evaporator again, completing one heat pump cycle. When solar irradiance is insufficient, such as on cloudy days or at night, the system switches to air-source heat pump mode. The evaporator stops operating, and the heat exchange fluid only flows through the air-cooled heat exchanger. After absorbing heat and evaporating in the air-cooled heat exchanger, the heat exchange fluid passes through the compressor and becomes a high-temperature, high-pressure state. It then enters the condenser, condenses, and releases heat. After passing through the expansion valve, it becomes a low-temperature, low-pressure state and enters the air-cooled heat exchanger again, completing one heat pump cycle.

[0047] The beneficial effects of this invention are:

[0048] (1) It realizes the cascade utilization of energy, giving priority to the use of zero-cost solar energy, and transferring its heat to the higher temperature feed water through a heat pump; even when solar energy is insufficient, the heat pump can operate at a high COP, and the energy saving effect is very significant compared to directly using gas to heat cold water;

[0049] (2) By complementing solar energy and air source heat pump, the instability of a single solar energy system is overcome, and the boiler feedwater is preheated around the clock without interruption, ensuring the continuity of industrial production.

[0050] (3) The introduction of a control strategy (i.e., main control loop) that uses solar irradiance to control the frequency of the variable frequency pump overcomes the inherent lag of traditional temperature difference control, enabling the system to respond immediately to weather changes, maximize solar energy collection, and minimize the power consumption of the circulating pump.

[0051] (4) By finely adjusting the flow rate, the solar collector can operate in a more stable temperature range, reducing the impact on system components caused by frequent pump start-up and shutdown or drastic temperature fluctuations, which helps to extend the equipment life. Attached Figure Description

[0052] Figure 1 A schematic diagram of the structure of a gas-fired steam boiler feedwater preheating system coupled with solar energy and an air source heat pump.

[0053] In the diagram: 1 is a solar collector, 2 is a hot water storage tank, 3 is a variable frequency pump, 4 is a solar irradiance sensor, 5 is a PLC controller, 6-1 is the first gate valve, 6-2 is the second gate valve, 6-3 is the third gate valve, 6-4 is the fourth gate valve, 7 is an expansion valve, 8 is a condenser, 9 is a compressor, 10 is an air-cooled heat exchanger, 11 is an evaporator, 12-1 is the first water pump, 12-2 is the second water pump, 12-3 is the third water pump, 13 is a preheating water supply tank, 14 is a gas-fired steam boiler, 15-1 is the first water supply valve, and 15-2 is the second water supply valve. Detailed Implementation

[0054] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but is not limited to the following embodiments.

[0055] Example 1:

[0056] like Figure 1 As shown, a gas-fired steam boiler feedwater preheating system coupled with solar and air source heat pumps includes a heat pump subsystem with switchable heat sources, a gas-fired steam boiler system, and an intelligent control system. The heat pump subsystem with switchable heat sources includes a solar collector 1, a hot water storage tank 2, a variable frequency pump 3, a first gate valve 6-1, a second gate valve 6-2, a third gate valve 6-3, a fourth gate valve 6-4, an expansion valve 7, a condenser 8, a compressor 9, an air-cooled heat exchanger 10, an evaporator 11, a first water pump 12-1, a second water pump 12-2, and a first water supply valve 15-1. The gas-fired steam boiler system includes a preheating water supply tank 13, a third water pump 12-3, a gas-fired steam boiler 14, and a second water supply valve 15-2. The intelligent control system includes a solar irradiance sensor 4 and a PLC controller 5.

[0057] The solar collector 1 is a flat-plate solar collector; the solar collector 1, the variable frequency pump 3, and the hot water storage tank 2 are connected in sequence to form a first circulation loop; the evaporator 11, the expansion valve 7, the condenser 8, the compressor 9, the air-cooled heat exchanger 10, the first gate valve 6-1, the second gate valve 6-2, the third gate valve 6-3, and the fourth gate valve 6-4 are connected to form a second circulation loop, in which the air-cooled heat exchanger is set on a branch and can be selectively connected; the hot water storage tank 2 is connected to the evaporator 11 through a pipe, and a first water pump 12-1 is installed on the connecting pipe; the solar irradiance sensor 4 is connected to the solar collector 1 and the PLC controller 5 to receive data input from the solar collector 1 and provide real-time data to the PLC controller 5; the PLC controller 5 receives the input signal from the solar irradiance sensor 4 and provides an output signal to the variable frequency pump 3 to control the flow rate of the first circulation loop;

[0058] The condenser 8 is connected to the preheating water tank 13 of the gas-fired steam boiler system. The condenser 8 is used to heat the hot water flowing to the gas-fired boiler. The heat pump subsystem with switchable heat source can selectively use the evaporator 11 to exchange heat with the hot water storage tank 2 or use the air-cooled heat exchanger 10 to exchange heat with air. In the gas-fired steam boiler system, the ambient temperature makeup water first flows through the condenser 8, is heated, enters the preheating water tank 13, and is then transported to the gas-fired steam boiler 14 by the boiler feed water pump (i.e., the third water pump 12-3).

[0059] Both the solar irradiance sensor and the PLC controller require additional configuration.

[0060] Furthermore, a variable frequency pump 3 is connected between the solar collector 1 and the hot water storage tank 2, and the variable frequency pump 3 is used to change the flow rate of the first circulation loop.

[0061] Furthermore, the solar irradiance sensor 4 is used to monitor solar irradiance in real time.

[0062] Furthermore, the solar collector 1 and the variable frequency pump 3 are respectively connected to the PLC controller 5. The PLC controller 5 is used to receive the input signal from the solar irradiance sensor 4, process it, and output a control signal to the variable frequency pump to dynamically adjust the operating frequency of the variable frequency pump.

[0063] In this example, a first water supply valve 15-1 is installed on the first circulation loop, and the first water supply valve 15-1 is connected to the hot water storage tank 2; the first water supply valve 15-1 can replenish the heat transfer medium in the first circulation loop; further, the heat transfer medium in the first circulation loop is a mixture of water and ethylene glycol, mixed in a volume ratio of 1:1; the heat transfer medium in the second circulation loop is R134a (tetrafluoroethane).

[0064] Furthermore, the evaporator 11 is connected to the air-cooled heat exchanger 10 via a pipe; valves are provided on the pipes to control the flow direction of the working fluid; the evaporator 11 is connected to the hot water storage tank 2, and the evaporator is used to enable the first circulation loop and the second circulation loop to exchange heat and absorb solar radiation heat.

[0065] Furthermore, the hot water storage tank is equipped with an internal heat exchanger, and the heat transfer medium in the first circulation loop flows into the internal heat exchanger to heat the water stored in the hot water storage tank.

[0066] In the above example, the ambient temperature makeup water first flows through the condenser 8 to be heated, then enters the preheated water supply tank 13 for temporary storage, and finally is pressurized by the boiler to the third water pump 12-3 and delivered to the gas-fired steam boiler 14.

[0067] To achieve maximum efficiency, this embodiment employs a composite control strategy with solar irradiance as the primary control signal and temperature as an auxiliary monitoring and protection factor. This includes a composite control method with a main control loop and an auxiliary protection loop. The main control loop controls the flow rate of the first circulation loop by monitoring solar irradiance and changing the frequency of the frequency converter. The main control loop control method is as follows:

[0068] S101, Parameter settings: Set the minimum solar irradiance threshold Eth, irradiance variation interval Eb, maximum safe speed of variable frequency pump Nth, and variable frequency pump speed variation interval Nb.

[0069] S102, data acquisition, real-time acquisition of solar irradiance E and variable frequency pump real-time speed N;

[0070] S103, Variable frequency pump start control: When E≥Eth, the variable frequency pump starts running; when E≤Eth, the variable frequency pump stops running.

[0071] S104, variable frequency pump frequency control, irradiance increases Eb, variable frequency pump speed increases Nb;

[0072] The auxiliary protection circuit controls the frequency of the variable frequency pump by setting the outlet temperature threshold of the solar collector and the temperature threshold of the hot water storage tank to prevent the system from overheating. The auxiliary protection circuit method is as follows:

[0073] S201, Parameter settings: Set the maximum outlet temperature T1th of the solar collector, the maximum temperature T2th of the hot water storage tank, and the minimum temperature difference T3th between the outlet of the solar collector and the hot water storage tank.

[0074] S202, Data Acquisition, Real-time acquisition of solar collector outlet temperature T1 and hot water storage tank temperature T2;

[0075] S203, temperature protection control: when T1≥T1th, the variable frequency pump speed is forcibly reduced by 50% to prevent system vaporization; when T2≥T2th, the variable frequency pump in the first circulation loop is shut down as a system overheat protection.

[0076] In the above example, the irradiance variation interval Eb is set to 100 W / m. 2 The variable frequency pump speed variation interval is 200 r / min, the maximum speed of the variable frequency pump is 2950 r / min, and the minimum threshold for solar irradiance is 50 W / m². 2 ;

[0077] In the above example, the highest outlet temperature T1th of the solar collector is 80°C, and the highest temperature T2th of the hot water storage tank is 80°C.

[0078] Both the main control circuit and the auxiliary protection circuit control the frequency of the variable frequency pump, and the frequency control of the variable frequency pump is implemented by the PLC controller.

[0079] Furthermore, the minimum temperature difference T3th between the solar collector outlet and the hot water storage tank is monitored. When the temperature difference between the solar collector outlet and the hot water storage tank is lower than the minimum temperature difference setting T3th, the system switches from solar mode to air source mode.

[0080] The start / stop conditions of the variable frequency pump based on solar irradiance are expressed as follows:

[0081]

[0082] In the formula, In variable frequency pump mode. This represents the current solar irradiance. This is the minimum threshold for solar irradiance.

[0083] The formula for calculating the speed of a variable frequency pump based on solar irradiance is as follows:

[0084]

[0085] In the formula, For the variable frequency pump speed, This is the floor function. This represents the current solar irradiance. This represents the minimum threshold for solar irradiance.

[0086] The formula for changing the frequency of the variable frequency pump based on the collector outlet temperature can be expressed as:

[0087]

[0088] In the formula, The variable frequency pump speed is controlled by temperature T1. This represents the current speed of the variable frequency pump.

[0089] The formula for controlling the start and stop of the variable frequency pump based on the temperature of the hot water storage tank can be expressed as:

[0090]

[0091] In the formula, The variable frequency pump is started and stopped under temperature T2 control.

[0092] The final variable frequency pump speed decision is as follows:

[0093]

[0094] In the formula, This refers to the final variable frequency pump speed.

[0095] In this example, there are two operating modes depending on the actual weather: solar mode and air source mode. The gas-fired steam boiler feedwater preheating method coupled with solar and air source heat pumps provided in this embodiment includes the following steps:

[0096] When solar radiation is sufficient and reaches a set threshold, in the first circulation loop, the variable frequency pump 3 starts, driving the heat transfer medium to flow. The heat transfer medium flows through the solar collector 1 to absorb heat, enters the hot water storage tank 2, and after heat exchange in the internal heat exchanger of the hot water storage tank, it flows back into the solar collector 1 through the variable frequency pump 3 for heat exchange again. In the second circulation loop (two modes can be switched according to actual weather conditions), in solar mode, the first gate valve 6-1 and the third gate valve 6-3 are closed, the air-cooled heat exchanger 10 is closed, and the second valve 6-2 and the fourth valve 6-4 are open. The heat transfer medium only flows through the evaporator 11. The heat transfer medium first evaporates and absorbs heat in the evaporator 11, becoming a low-temperature, low-pressure gaseous medium, and then flows through the compressor 9, becoming a high-temperature, high-pressure gaseous medium, and then enters the... The heat pump fluid enters the condenser 8 and exchanges heat with the boiler feedwater flowing through it. After releasing heat, it becomes a high-temperature, high-pressure liquid working fluid and enters the expansion valve 7. After passing through the expansion valve 7, it becomes a low-temperature, low-pressure liquid-gas mixture and flows through the evaporator 11 again, completing one heat pump cycle. When solar radiation is insufficient, such as on cloudy days or at night, the system switches to air source heat pump mode. The evaporator stops operating, and the first gate valve 6-1 and the third gate valve 6-3 in the second circulation loop are in the open state, while the second gate valve 6-2 and the fourth gate valve 6-4 are in the closed state. The heat exchange working fluid only flows through the air-cooled heat exchanger. After absorbing heat and evaporating in the air-cooled heat exchanger 10, the heat exchange working fluid becomes a high-temperature, high-pressure state after passing through the compressor 9 and enters the condenser 8 to condense and release heat. After passing through the expansion valve 7, it becomes a low-temperature, low-pressure state and then enters the air-cooled heat exchanger 10, completing one heat pump cycle.

[0097] This system fully utilizes solar and air energy by coupling solar energy with an air source heat pump, effectively reducing natural gas consumption. The system is equipped with two heat sources, solar energy and an air source heat pump, and the system operation mode is reasonably selected according to different environmental conditions, realizing stable preheating of the gas-fired steam boiler around the clock.

[0098] The various embodiments of the present invention have been described in detail above. Finally, it should be noted that the above embodiments of the present invention are merely examples for illustrating the present invention and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations and modifications can be made based on the above description, and it is impossible to exhaustively list all the implementation methods here. Any obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A feedwater preheating system for a gas-fired steam boiler coupled with solar energy and an air source heat pump, characterized in that, This includes a heat pump subsystem with switchable heat sources, a gas-fired steam boiler system, and an intelligent control system; The switchable heat source heat pump subsystem includes a solar collector, a hot water storage tank, a variable frequency pump, an evaporator, an air-cooled heat exchanger, a compressor, a condenser, and an expansion valve. The solar collector is a flat-plate solar collector. The solar collector, the hot water storage tank, and the variable frequency pump form a first circulation loop via pipelines. The evaporator and the hot water storage tank are connected via pipelines for heat exchange. The evaporator, the air-cooled heat exchanger, the compressor, the condenser, and the expansion valve form a second circulation loop via pipelines. The air-cooled heat exchanger in this loop is located on a branch and can be selectively connected. The condenser is connected to the preheating water supply tank of the gas-fired steam boiler system and is used to heat the hot water flowing to the gas-fired boiler. The switchable heat source heat pump subsystem can selectively use the evaporator to exchange heat with the hot water storage tank or use the air-cooled heat exchanger to exchange heat with the air. The gas-fired steam boiler system includes a preheating water supply tank and a gas-fired steam boiler; ambient temperature makeup water first flows through the condenser, is heated and then enters the preheating water supply tank, and is then transported to the gas-fired steam boiler by the boiler feed water pump; The intelligent control system includes a solar irradiance sensor and a PLC controller.

2. The gas-fired steam boiler feedwater preheating system coupled with solar energy and air source heat pump as described in claim 1, characterized in that, A variable frequency pump is connected between the solar collector and the hot water storage tank. The variable frequency pump is used to change the flow rate of the first circulation loop. The first circulation loop is equipped with a water replenishment device, which is connected to the hot water storage tank.

3. The gas-fired steam boiler feedwater preheating system coupled with solar and air source heat pumps according to claim 1, characterized in that, The solar irradiance sensor is connected to the solar collector and is used to monitor solar irradiance in real time. The solar collector and the variable frequency pump are respectively connected to the PLC controller. The PLC controller is used to receive the input signal from the solar irradiance sensor, process it, and output a control signal to the variable frequency pump to dynamically adjust the operating frequency of the variable frequency pump.

4. The gas-fired steam boiler feedwater preheating system coupled with solar energy and air source heat pump according to claim 1, characterized in that, The heat transfer medium in the first circulation loop is a mixture of water and ethylene glycol in a volume ratio of 1:1; the heat transfer medium in the second circulation loop is R134a.

5. The gas-fired steam boiler feedwater preheating system coupled with solar and air source heat pumps according to claim 1, characterized in that, The evaporator is connected to the hot water storage tank. The evaporator is used to enable heat exchange between the first circulation loop and the second circulation loop, absorbing solar radiation heat.

6. The gas-fired steam boiler feedwater preheating system coupled with solar and air source heat pumps according to claim 1, characterized in that, The hot water storage tank is equipped with an internal heat exchanger, and the heat transfer medium in the first circulation loop flows into the internal heat exchanger to heat the water stored in the hot water storage tank.

7. The gas-fired steam boiler feedwater preheating system coupled with solar and air source heat pumps according to claim 1, characterized in that, The intelligent control system adopts a composite control method that combines a main control loop and an auxiliary protection loop. Both the main control loop and the auxiliary protection loop control the frequency of the variable frequency pump, and the frequency control of the variable frequency pump is implemented by the PLC controller. The main control loop controls the flow rate of the first circulation loop by monitoring the solar irradiance and changing the frequency of the variable frequency pump. The auxiliary protection loop controls the frequency of the variable frequency pump by setting the outlet temperature threshold of the solar collector and the temperature threshold of the hot water storage tank to prevent the system from overheating.

8. The gas-fired steam boiler feedwater preheating system coupled with solar and air source heat pumps according to claim 7, characterized in that, The main control loop control method is as follows: S101, Parameter settings: Set the minimum solar irradiance threshold Eth, irradiance variation interval Eb, maximum safe speed of variable frequency pump Nth, and variable frequency pump speed variation interval Nb. S102, data acquisition, real-time acquisition of solar irradiance E and variable frequency pump real-time speed N; S103, Variable frequency pump start control: When E≥Eth, the variable frequency pump starts running; when E≤Eth, the variable frequency pump stops running. S104, variable frequency pump frequency control, irradiance increases Eb, variable frequency pump speed increases Nb; The auxiliary protection circuit control method is as follows: S201, parameter setting, set the maximum outlet temperature T1th of the solar collector and the maximum temperature T2th of the hot water storage tank; S202, Data Acquisition, Real-time acquisition of solar collector outlet temperature T1 and hot water storage tank temperature T2; S203, temperature protection control: when T1≥T1th, the variable frequency pump speed is forcibly reduced by 50% to prevent system vaporization; when T2≥T2th, the variable frequency pump in the first circulation loop is shut down as a system overheat protection.

9. The gas-fired steam boiler feedwater preheating system coupled with solar energy and air source heat pump according to claim 8, characterized in that, The minimum threshold for solar irradiance, Eth, ranges from 30 to 150 W / m². 2 The irradiance variation interval Eb ranges from 50 to 300 W / m. 2 The maximum safe speed Nth of the variable frequency pump ranges from 85% to 100% of the rated speed; the speed change interval Nb of the variable frequency pump ranges from 50 to 500 r / min; the highest outlet temperature T1th of the solar collector ranges from 75 to 95°C; the highest temperature T2th of the hot water storage tank ranges from 60 to 85°C; and the minimum speed Nmin of the variable frequency pump ranges from 15% to 30% of the rated speed.

10. A method for preheating feedwater in a gas-fired steam boiler coupled with solar energy and an air source heat pump, comprising the gas-fired steam boiler feedwater preheating system coupled with solar energy and an air source heat pump as described in any one of claims 1 to 9, characterized in that, Includes the following steps: When the solar irradiance reaches a set threshold, the variable frequency pump in the first circulation loop starts, driving the heat transfer medium to flow. The heat transfer medium absorbs heat through the solar collector and enters the internal heat exchanger of the hot water storage tank. After exchanging heat with the water inside the tank, it flows back into the solar collector through the variable frequency pump for further heat exchange. In the second circulation loop, two modes can be switched according to the actual weather conditions: solar mode and air source heat pump mode. In solar mode, the air-cooled heat exchanger is closed, and the heat transfer medium in the second circulation loop only flows through the evaporator. The heat transfer medium first evaporates and absorbs heat in the evaporator, becoming a low-temperature, low-pressure gaseous medium, and then flows into the compressor. After compression, the working fluid becomes a high-temperature, high-pressure gaseous state, which then enters the condenser and exchanges heat with the boiler feedwater flowing through it. After releasing heat, it becomes a high-temperature, high-pressure liquid working fluid and enters the expansion valve. After passing through the expansion valve, it becomes a low-temperature, low-pressure liquid-gas mixture, which then passes through the evaporator again, completing one heat pump cycle. When solar irradiance is insufficient, the system switches to air-source heat pump mode, the evaporator stops operating, and the heat exchange working fluid only flows through the air-cooled heat exchanger. After absorbing heat and evaporating in the air-cooled heat exchanger, the working fluid passes through the compressor and becomes a high-temperature, high-pressure state. It then enters the condenser, condenses, and releases heat, and then passes through the expansion valve to become a low-temperature, low-pressure state before entering the air-cooled heat exchanger again, completing one heat pump cycle.

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