Four-pipe electric drive enhanced flue gas waste heat recovery system and control method thereof

Abstract

The invention discloses a four-pipe electric drive enhanced flue gas waste heat recovery system and a control method thereof, and relates to the field of flue gas waste heat utilization and heating. Circulating pumps and heat meters are arranged on water loops corresponding to the first-stage heat exchanger and the second-stage heat exchanger, and an electric heating pump is arranged on a water loop corresponding to the second-stage heat exchanger; the electric heat pump comprises an evaporator and a condenser. The heat meter monitors loop information on the water loop; the PLC performs frequency conversion control on the circulating pump according to the loop information, and performs gradient utilization control on the flue gas waste heat based on a first-stage heat exchanger and a second-stage heat exchanger in the heat exchangers; wherein the primary heat exchanger directly recycles flue gas waste heat by adopting return heating water; the second-stage heat exchanger recovers waste heat of the low-temperature part of the smoke through an evaporator, hot water with the low temperature is replaced to serve as a heat source of an electric heat pump, and the temperature of heating water is increased through the electric heat pump. The flue gas waste heat can be deeply recycled, and the flue gas waste heat recycling efficiency is improved.

Description

Technical Field

[0001] This application relates to the fields of flue gas waste heat utilization and heating, and in particular to a four-pipe electrically driven enhanced flue gas waste heat recovery system and its control method. Background Technology

[0002] Gas-fired boilers generate a large amount of flue gas during operation, which contains a significant amount of heat, especially latent heat of condensation. Although most boilers implement flue gas waste heat recovery, the recovery is not thorough, and a considerable amount of heat remains wasted in the flue gas.

[0003] Currently, most gas-fired boilers have implemented flue gas waste heat recovery. One type uses traditional finned tube flue gas heat exchangers, where the flue gas temperature is generally still above 60℃. The other type uses high-efficiency plate flue gas heat exchangers, which can lower the flue gas temperature even further, but it is still generally above 40℃, leaving room for energy saving.

[0004] Another type of boiler room uses a combination of flue gas waste heat recovery and heat pump operation, using all the recovered flue gas waste heat as the heat source for the heat pump unit; this system can reduce the flue gas temperature to about 30°C.

[0005] Conventional flue gas waste heat recovery combined with heat pump operation has the following disadvantages: 1) All the recovered waste heat from the flue gas is heated by a heat pump, which results in a large selection of heat pumps and a high investment.

[0006] 2) Because heat pumps are large-scale, they require a large amount of power distribution capacity and a large installation space, making them difficult to implement in existing projects.

[0007] 3) Due to the large fluctuations in boiler load, the boiler load during the severe cold period is generally more than three times that of the early and late cold periods; and the flue gas temperature is even higher during the severe cold period, so the waste heat load of the flue gas during the severe cold period is more than five times that of the early and late cold periods. This makes it difficult to select a heat pump. If the heat pump is selected according to the severe cold period, it is easy for the heat pump to not be able to start during the early and late cold periods due to the low load (heat pump units have minimum operating load requirements); if the heat pump is selected according to the early and late cold periods, it is easy for the heat pump to be insufficient during the severe cold period.

[0008] 4) The coefficient of performance (COP) of the heat pump is not high enough, resulting in poor economic efficiency.

[0009] 5) When the boiler is running at low load, the boiler load changes frequently or even stops, resulting in very little or no heat being extracted from the evaporator side, causing the electric heat pump to shut down. After the electric heat pump stops, it takes more than 10 minutes to restart, which causes three problems: First, during the time interval between the electric heat pump stopping and restarting, the waste heat of the flue gas from the boiler cannot be recovered, resulting in waste of flue gas waste heat; second, the frequent start-up and shutdown of the electric heat pump increases power consumption (the heat pump consumes a lot of power and has low efficiency during the start-up process); third, the frequent start-up and shutdown of the electric heat pump results in a short lifespan of the heat pump and high maintenance costs.

[0010] 6) Since the condenser side of the electric heat pump is filled with heating water, the temperature of the heating water varies greatly during the day, night, and is affected by the weather. The load changes frequently, making it difficult to control the electric heat pump, especially the load control, resulting in low efficiency of the electric heat pump in actual operation.

[0011] Therefore, it is crucial to achieve deep recovery of flue gas waste heat in order to improve the efficiency of flue gas waste heat recovery. Summary of the Invention

[0012] The purpose of this application is to provide a four-pipe electric drive enhanced flue gas waste heat recovery system and its control method, which can deeply recover flue gas waste heat and improve the flue gas waste heat recovery efficiency.

[0013] To achieve the above objectives, this application provides the following solution: In a first aspect, this application provides a four-pipe electric drive enhanced flue gas waste heat recovery system, which is applied in a natural gas hot water boiler scenario; the four-pipe electric drive enhanced flue gas waste heat recovery system includes: a control cabinet, a circulating pump, a heat exchanger, and a heat meter; the control cabinet includes a PLC controller; the circulating pump, heat exchanger, and heat meter are all connected to the PLC controller. The heat exchanger includes a primary heat exchanger and a secondary heat exchanger; each of the primary and secondary heat exchangers is equipped with a circulating pump and a heat meter on its corresponding water circuit, and an electric heat pump is installed on the water circuit corresponding to the secondary heat exchanger; the electric heat pump includes an evaporator and a condenser. The heat meter is used to monitor loop information in the water circuit; the loop information includes: temperature, temperature difference, flow rate, thermal power, and heat. The PLC controller is used to perform frequency conversion control of the circulating pump based on loop information, and to perform gradient utilization control of the flue gas waste heat in the natural gas hot water boiler scenario based on the primary and secondary heat exchangers. The primary heat exchanger directly recovers the flue gas waste heat using heating water return water; the secondary heat exchanger recovers the waste heat of the low-temperature part of the flue gas using an evaporator and replaces it with lower-temperature hot water as the heat source for the electric heat pump, which then raises the temperature of the heating water. The low-temperature part of the flue gas is the flue gas after waste heat recovery by the primary heat exchanger; the lower temperature is the temperature below the waste heat temperature of the low-temperature part of the flue gas.

[0014] Secondly, this application provides a four-pipe electric drive enhanced flue gas waste heat recovery control method, wherein the four-pipe electric drive enhanced flue gas waste heat recovery control method is implemented using the aforementioned four-pipe electric drive enhanced flue gas waste heat recovery system; the four-pipe electric drive enhanced flue gas waste heat recovery control method includes: Obtain loop information on the water circuit; the loop information includes: temperature, temperature difference, flow rate, thermal power, and heat. The circulating pump is controlled by frequency conversion based on the loop information, and the waste heat of flue gas in the natural gas hot water boiler scenario is utilized by gradient control based on the primary and secondary heat exchangers. The primary heat exchanger directly recovers the waste heat of flue gas using the return water from the heating system. The secondary heat exchanger recovers the waste heat of the low-temperature part of the flue gas using an evaporator and replaces it with lower-temperature hot water, which serves as the heat source for the electric heat pump, and the electric heat pump raises the temperature of the heating water. The low-temperature part of the flue gas is the flue gas after waste heat recovery by the primary heat exchanger. The lower temperature is the waste heat temperature below the low-temperature part of the flue gas.

[0015] According to the specific embodiments provided in this application, the following technical effects are disclosed: This application provides a four-pipe electric drive enhanced flue gas waste heat recovery system and its control method. Based on a PLC controller, the circulating pump is frequency-controlled according to loop information, and the flue gas waste heat is utilized through gradient control via a primary and secondary heat exchanger. The primary heat exchanger directly recovers flue gas waste heat using heating water return water. The secondary heat exchanger recovers waste heat from the low-temperature portion of the flue gas using an evaporator, displacing it with lower-temperature hot water as a heat source for the electric heat pump, which then raises the temperature of the heating water. This application enhances the synergistic cooperation between the various components of the four-pipe electric drive enhanced flue gas waste heat recovery system. By frequency-controlling the circulating pump, the water flow rate of the primary heat exchanger can be adjusted, and the heat exchange capacity of the primary heat exchanger can be changed, ensuring the electric heat pump operates normally under different loads of the gas boiler and achieving high COP. Furthermore, this application achieves deeper recovery of flue gas waste heat and improves the efficiency of flue gas waste heat recovery through gradient utilization control. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a structural diagram of a device for recovering waste heat from flue gas in related technologies.

[0018] Figure 2 A schematic diagram illustrating the design principle of a four-pipe electrically driven enhanced flue gas waste heat recovery system.

[0019] Figure 3 This is a schematic diagram of the control principle of a four-pipe electrically driven enhanced flue gas waste heat recovery system.

[0020] Figure 4 This diagram illustrates common operating conditions for a four-pipe electrically driven enhanced flue gas waste heat recovery system.

[0021] Figure 5 A flowchart for a four-pipe electric drive-enhanced flue gas waste heat recovery control method. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] Figure 1 In related technologies, the method of using flue gas waste heat recovery in conjunction with heat pump operation uses all the flue gas waste heat as the heat source of the heat pump unit. However, it has the problems of high investment cost, incomplete recovery, and low recovery efficiency.

[0025] The four-pipe system refers to the configuration of two independent circulating water circuits, with a total of four pipes, to achieve independent operation of the hot and cold water circuits without interference between them.

[0026] In one exemplary embodiment, such as Figure 2 and Figure 3 As shown, a four-pipe electrically driven enhanced flue gas waste heat recovery system is provided. This system is applied in natural gas hot water boiler scenarios. The system includes a control cabinet, a circulating pump, a heat exchanger, and a heat meter. The control cabinet contains a PLC controller. The circulating pump, heat exchanger, and heat meter are all connected to the PLC controller.

[0027] The heat exchanger includes a primary heat exchanger and a secondary heat exchanger; each of the primary and secondary heat exchangers is equipped with a circulating pump and a heat meter on its corresponding water circuit, and an electric heat pump is installed on the water circuit corresponding to the secondary heat exchanger; the electric heat pump includes an evaporator and a condenser.

[0028] A heat meter is used to monitor loop information in a water circuit; loop information includes: temperature, temperature difference, flow rate, thermal power, and heat.

[0029] The PLC controller is used to perform frequency conversion control of the circulating pump based on loop information, and to perform gradient utilization control of flue gas waste heat in the natural gas hot water boiler scenario based on the primary and secondary heat exchangers. Specifically, the primary heat exchanger directly recovers flue gas waste heat using heating water return water; the secondary heat exchanger uses an evaporator to recover waste heat from the low-temperature portion of the flue gas and displaces it into lower-temperature hot water, which serves as the heat source for the electric heat pump, and the electric heat pump then raises the temperature of the heating water; the low-temperature portion of the flue gas is the flue gas after waste heat recovery by the primary heat exchanger; the lower temperature refers to the waste heat temperature below the low-temperature portion of the flue gas.

[0030] The primary and secondary heat exchangers use heat exchangers with different terminal differences; the terminal differences are determined based on the flue gas outlet temperature and the heat exchanger inlet water temperature.

[0031] A buffer water tank is installed between the secondary heat exchanger and the evaporator; the buffer water tank is used to buffer changes in the waste heat of the flue gas; the water volume in the buffer water tank corresponds to the circulation flow of the secondary heat exchanger and the evaporator for a set time period. The buffer water tank is replenished with heating water return water, and the opening and closing is controlled by a float valve.

[0032] When the electric heat pump and the boiler in the natural gas hot water boiler scenario are not one-to-one, the four-pipe electric drive enhanced flue gas waste heat recovery system also includes: electric valves and temperature sensors; the temperature sensors are used to monitor the outlet water temperature of the heat exchanger.

[0033] In one embodiment, the control cabinet further includes a touch screen, a frequency converter, and a smart meter; the touch screen, frequency converter, and smart meter are all connected to the PLC controller.

[0034] Specifically, the design scheme of this application is as follows: This application utilizes the waste heat of flue gas in stages: in the high-temperature section of the flue gas, a primary heat exchanger with a 2°C temperature difference is used, and the waste heat of the flue gas is directly recovered through the return water of the heating system. This can reduce the temperature of the flue gas to a relatively low level, that is, only 2°C higher than the temperature of the return water of the heating system. This part of the heat exchange efficiency is extremely high, and only a very small amount of electricity is needed to exchange a large amount of heat.

[0035] After heat exchange through the primary heat exchanger, the flue gas temperature is already quite low and cannot be directly used for heat exchange with the heating return water. However, it can be used in conjunction with an electric heat pump. Considering the economic efficiency of the heat exchanger, a product with a terminal temperature difference of 5℃ is selected for the secondary heat exchanger. The secondary heat exchanger is used to further recover the waste heat from the flue gas. The lower-temperature water produced by the evaporator recovers the waste heat from the low-temperature part of the flue gas, displacing it into lower-temperature hot water, which serves as the heat source for the heat pump unit (electric heat pump). Then, the heating water temperature is increased through the heat pump (electric heat pump).

[0036] A buffer tank is installed between the secondary heat exchanger and the evaporator of the electric heat pump. The water volume in the tank is the circulation flow rate of the secondary heat exchanger and evaporator for 10 minutes (e.g., if the circulation flow rate of the secondary heat exchanger and evaporator is 60 m³ / s). 3 / h, then the water volume in the tank is 10m³. 3 This is used to buffer changes in the waste heat of flue gas and ensure that the secondary heat exchanger can still recover the waste heat of flue gas during the 10-minute interval between the shutdown and restart of the electric heat pump, so as not to waste the waste heat of flue gas.

[0037] The buffer tank is replenished with water using the return water from the heating system, which is controlled by a float valve.

[0038] This application is configured with an intelligent control cabinet, which includes a PLC controller, a touch screen, a frequency converter, a smart meter, etc. Figure 2 and Figure 3 All three water pumps (circulating pumps) are frequency converters with minimum frequency limits. Heat meters are installed on each water circuit to monitor temperature, temperature difference, flow rate, thermal power, and heat output, and data is transmitted to the PLC controller in the intelligent control cabinet via communication.

[0039] The flue gas circuit is equipped with a temperature sensor to monitor the flue gas temperature; when the boiler and electric heat pump are not one-to-one, an electric valve and a heat exchanger outlet water temperature sensor are also required.

[0040] The electric heat pump and three water pumps (circulating pumps) are each equipped with a smart meter to monitor power consumption, electricity usage, etc., and transmit data with the PLC controller in the intelligent control cabinet via communication. The PLC controller in the intelligent control cabinet transmits data with the electric heat pump via communication, controls the heat pump temperature and load, and monitors the internal operating data of the heat pump.

[0041] The four-pipe electrically driven enhanced flue gas waste heat recovery system of this application requires an evaporator-side outlet water temperature of 20℃ during operation. Since the secondary heat exchanger has a 5℃ terminal temperature difference, the final exhaust gas temperature is 25℃. The specific approach for selecting the appropriate system is as follows: First, according to Table 1, which shows the ratio of waste heat from flue gas to the lower heating value of fuel gas at different flue gas temperatures, the waste heat from flue gas below 25℃ is very small, less than 2.7%. Extracting heat from flue gas below 25℃ is very costly; therefore, 25℃ was chosen as the final exhaust temperature. Second, considering the economic efficiency of the heat exchanger, a 5℃ differential heat exchanger was selected for the secondary stage. However, to reduce the flue gas temperature to 25℃, water at 20℃ is required. Third, to improve the COP of the electric heat pump, the evaporator-side water temperature cannot be too low. Taking all these factors into account, the final evaporator-side outlet water temperature was determined to be 20℃, and the final exhaust temperature to be 25℃.

[0042] Table 1. Ratio of Waste Heat from Flue Gas to Lower Heating Value of Combustion Gas at Different Flue Gas Temperatures The solution mentioned in this application is very versatile and can be applied to many natural gas hot water boiler scenarios, especially in scenarios with low electricity prices and high gas prices. Common operating conditions include... Figure 4 As shown.

[0043] In one exemplary embodiment, a four-pipe electrically driven enhanced flue gas waste heat recovery control method is provided. The four-pipe electrically driven enhanced flue gas waste heat recovery control method is implemented using the aforementioned four-pipe electrically driven enhanced flue gas waste heat recovery system.

[0044] like Figure 5 As shown, the four-pipe electrically driven enhanced flue gas waste heat recovery control method includes: Step 100: Obtain loop information for the water circuit. Loop information includes: temperature, temperature difference, flow rate, thermal power, and heat output.

[0045] Step 200: The circulating pump is controlled by frequency conversion based on the loop information, and the waste heat from the flue gas in the natural gas hot water boiler scenario is utilized using gradient control based on the primary and secondary heat exchangers. Specifically, the primary heat exchanger directly recovers waste heat from the flue gas using heating water return water; the secondary heat exchanger recovers waste heat from the low-temperature portion of the flue gas using an evaporator and replaces it with lower-temperature hot water, which serves as the heat source for the electric heat pump, and the electric heat pump then raises the temperature of the heating water; the low-temperature portion of the flue gas is the flue gas after waste heat recovery by the primary heat exchanger; the lower temperature refers to the waste heat temperature below the low-temperature portion of the flue gas.

[0046] Specifically, the circulating pump is frequency-controlled based on loop information, and the waste heat from the flue gas in the natural gas hot water boiler scenario is utilized through gradient control based on the primary and secondary heat exchangers. This includes: Based on the flow rate in the loop information corresponding to the first-stage heat exchanger, the upper and lower limits of the frequency of the circulating pump corresponding to the first-stage heat exchanger are determined, and the PID control method is used to control the flow rate based on the terminal difference of the first-stage heat exchanger.

[0047] When the frequency reduction condition is met, the circulating pump corresponding to the primary heat exchanger is controlled to start operating at a reduced frequency. When the frequency reduction condition is no longer met, the operating frequency is kept unchanged. The frequency reduction condition includes: the outlet water temperature on the evaporator side corresponding to the evaporator is lower than the preset temperature, and the heat power of the secondary heat exchanger is lower than the set ratio of the rated heat output power of the electric heat pump.

[0048] Based on the flow rates of the secondary heat exchanger and the evaporator side of the electric heat pump, the upper and lower limits of the frequency of the circulating pump corresponding to the secondary heat exchanger are determined, and the PID control method is adopted to control the flow rate based on the terminal difference of the secondary heat exchanger; the evaporator side of the electric heat pump is the side of the electric heat pump closest to the evaporator.

[0049] The frequency adjustment range of the electric heat pump is determined based on the flow rate on the condenser side, and the temperature difference between the inlet and outlet water on the condenser side is determined using a PID control method. The inlet water temperature on the condenser side is the same as the heating return water temperature. The outlet water temperature on the condenser side is automatically matched according to the heat power. The condenser side is the side of the electric heat pump closest to the condenser.

[0050] The gradient utilization control includes two control modes: cooling mode and heating mode. The heating mode controls the condenser side outlet water temperature, i.e., the hot water outlet water temperature. The cooling mode controls the evaporator side outlet water temperature, i.e., the cold water outlet water temperature.

[0051] In heating mode, controlling the condensate outlet water temperature specifically includes: Determine the initial set value of the condenser side outlet water temperature; the condenser side is the side of the water circuit corresponding to the secondary heat exchanger that is close to the condenser; the initial set value of the condenser side outlet water temperature is determined based on the condenser side inlet water temperature and the preset temperature difference on the condenser side.

[0052] The evaporator side outlet water temperature is acquired in real time at preset time intervals; the evaporator side is the side of the water circuit corresponding to the secondary heat exchanger that is close to the evaporator.

[0053] The initial setting value of the condenser outlet temperature is adjusted according to the evaporator outlet temperature; wherein, when the evaporator outlet temperature is within the preset threshold range, the initial setting value of the condenser outlet temperature is not adjusted; when the evaporator outlet temperature is less than the lower limit of the preset threshold range, the initial setting value of the condenser outlet temperature gradually decreases; when the evaporator outlet temperature is greater than the upper limit of the preset threshold range, the initial setting value of the condenser outlet temperature gradually increases.

[0054] In practical applications, the specific operating procedures for cooling and heating modes are as follows: Cooling mode: 1) The system starts and stops based on the flue gas inlet temperature of the first-stage heat exchanger. When the flue gas inlet temperature of the first-stage heat exchanger is higher than the start-up temperature setpoint, the system determines that the gas boiler is running and starts the system. The equipment start-up sequence is: electric valve (if any) on the water circuit corresponding to the first-stage heat exchanger - circulating pump on the water circuit corresponding to the first-stage heat exchanger - circulating pump corresponding to the electric heat pump - electric valve (if any) on the water circuit corresponding to the second-stage heat exchanger - circulating pump on the water circuit corresponding to the second-stage heat exchanger - heat pump unit (electric heat pump).

[0055] 2) When the flue gas inlet temperature of the first-stage heat exchanger is lower than the stop temperature set value, the system determines that the gas boiler is in standby mode, the circulating pump on the corresponding water circuit of the first-stage heat exchanger stops, and the electric valve on the corresponding water circuit of the first-stage heat exchanger closes (if any); when the flue gas inlet temperature of the first-stage heat exchanger is lower than the stop temperature set value for more than 5 minutes, the system determines that the gas boiler is shut down, and the electric heat pump section begins to shut down. The shutdown sequence is: heat pump unit (electric heat pump) - circulating pump on the corresponding water circuit of the second-stage heat exchanger - electric valve on the corresponding water circuit of the second-stage heat exchanger (if any) - heat pump circulating pump (circulating pump corresponding to the electric heat pump).

[0056] 3) The evaporator-side outlet water temperature is set to 20℃, with an adjustment range of ±2℃, i.e., an adjustment range of 18-22℃. The built-in program of the electric heat pump automatically adjusts the heat pump load rate.

[0057] 4) The condensate inlet water temperature is the same as the heating return water temperature, and the outlet water temperature is automatically matched according to the heating power.

[0058] 5) The primary heat exchange circulation pump (the circulation pump on the water circuit corresponding to the primary heat exchanger) sets its upper and lower frequency limits based on the maximum and minimum flow rates of the primary heat exchanger. Regardless of the control strategy used, the frequency adjustment range remains between these limits. Under normal circumstances, the primary heat exchange circulation pump uses PID control, based on the terminal temperature difference of the primary heat exchanger (terminal temperature difference = flue gas outlet temperature of the flue gas heat exchanger - inlet water temperature of the flue gas heat exchanger), with a setpoint of 2℃.

[0059] However, when the evaporator outlet water temperature is below 18℃ and the heat power of the secondary heat exchanger (monitored by the heat meter) is less than 35% of the rated heat output power of the electric heat pump (which can be set according to different models), the circulating pump on the corresponding water circuit of the primary heat exchanger starts to operate at a reduced frequency. The frequency reduction rate is adjustable. When the frequency reduction does not meet the frequency reduction conditions (evaporator outlet water temperature is below 18℃ and the heat power of the secondary heat exchanger is less than 35% of the rated heat output power of the electric heat pump), the operating frequency remains unchanged. PID control is restored when the evaporator outlet water temperature is greater than or equal to 20℃, or the heat power of the secondary heat exchanger is greater than or equal to 40% of the rated heat output power of the electric heat pump.

[0060] The load adjustment range of a typical electric heat pump is 30%-100%, but this varies depending on the model (scroll, screw, centrifugal, etc.).

[0061] 6) The frequency lower limit of the circulating pump in the water circuit corresponding to the secondary heat exchanger is set according to the higher of the minimum flow requirements of the secondary heat exchanger and the electric heat pump evaporator side, and the frequency upper limit is set according to the higher of the maximum flow requirements of the two sides. Regardless of the control strategy used, the frequency adjustment range is between the upper and lower limits. The circulating pump in the water circuit corresponding to the secondary heat exchanger uses PID control, controlled according to the terminal temperature difference of the secondary heat exchanger (terminal temperature difference = flue gas outlet temperature of flue gas heat exchanger - inlet water temperature of flue gas heat exchanger), with a set value of 5℃.

[0062] 7) The circulating pump on the water circuit at the electric heat pump has its frequency upper and lower limits set according to the minimum and maximum flow rates on the condenser side of the electric heat pump; regardless of the control strategy used, the frequency adjustment range is between the upper and lower limits. The circulating pump of the electric heat pump adopts PID control, and the set value is generally 5-7℃ based on the inlet and outlet water temperature difference designed on the condenser side of the electric heat pump.

[0063] Heating mode: 1) The system starts and stops based on the flue gas inlet temperature of the first-stage heat exchanger. When the flue gas inlet temperature of the first-stage heat exchanger is higher than the start-up temperature setpoint, the system determines that the gas boiler is running and starts the system. The equipment start-up sequence is: electric valve (if any) on the water circuit corresponding to the first-stage heat exchanger - circulating pump on the water circuit corresponding to the first-stage heat exchanger - circulating pump at the electric heat pump - electric valve (if any) on the water circuit corresponding to the second-stage heat exchanger - circulating pump on the water circuit corresponding to the second-stage heat exchanger - heat pump unit (electric heat pump).

[0064] 2) When the flue gas inlet temperature of the primary heat exchanger is lower than the stop temperature setpoint, the system determines that the gas boiler is in standby mode, the circulating pump on the water circuit corresponding to the primary heat exchanger stops, and the electric valve on the water circuit corresponding to the primary heat exchanger closes (if any); when the flue gas inlet temperature of the primary heat exchanger is lower than the stop temperature setpoint for more than 5 minutes, the system determines that the gas boiler is shut down, and the electric heat pump section begins to shut down. The shutdown sequence is: heat pump unit (electric heat pump) - circulating pump on the water circuit corresponding to the secondary heat exchanger - electric valve on the water circuit corresponding to the secondary heat exchanger (if any) - circulating pump corresponding to the electric heat pump.

[0065] 3) The target temperature for the evaporator-side outlet water is 20℃, with an adjustment range of ±2℃, i.e., an adjustment range of 18-22℃. If the evaporator-side outlet water temperature is lower than 15℃ (adjustable), the electric heat pump will shut down for protection.

[0066] 4) The initial setting value of the condenser side outlet water temperature is the condenser side inlet water temperature + condenser side design temperature difference ÷ 2; the control system (i.e., PLC controller) reads the condenser side inlet water temperature every 5 minutes, and adjusts the condenser side outlet water temperature setting value by the same temperature difference based on the 5-minute interval of the condenser side inlet water temperature change.

[0067] Meanwhile, the control system (i.e., the PLC controller) reads the evaporator-side outlet water temperature every 5 minutes. When 18℃≤evaporator-side outlet water temperature≤22℃, the condenser-side outlet water temperature setpoint is not adjusted. When the evaporator-side outlet water temperature<18℃, the condenser-side outlet water temperature setpoint gradually decreases, and the larger the value of "18℃ - evaporator-side outlet water temperature", the faster the decrease, and vice versa. When the evaporator-side outlet water temperature>22℃, the condenser-side outlet water temperature setpoint gradually increases, and the larger the value of "evaporator-side outlet water temperature -22℃", the faster the increase, and vice versa.

[0068] 5) The circulating pump on the water circuit corresponding to the primary heat exchanger sets its upper and lower frequency limits based on the maximum and minimum flow rates of the primary heat exchanger. Regardless of the control strategy used, the frequency adjustment range remains between these limits. Under normal circumstances, the circulating pump on the water circuit corresponding to the primary heat exchanger uses PID control, controlled based on the terminal temperature difference of the primary heat exchanger (terminal temperature difference = flue gas outlet temperature of the flue gas heat exchanger - inlet water temperature of the flue gas heat exchanger), with a setpoint of 2℃.

[0069] However, when the evaporator outlet water temperature is below 18℃ and the heat power of the secondary heat exchanger (monitored by the heat meter) is less than 35% of the rated heat output power of the electric heat pump (which can be set according to different models), the circulating pump on the water circuit corresponding to the primary heat exchanger starts to operate at a reduced frequency. The frequency reduction rate is adjustable. When the frequency reduction does not meet the frequency reduction conditions (evaporator outlet water temperature is below 18℃ and the heat power of the secondary heat exchanger is less than 35% of the rated heat output power of the electric heat pump), the operating frequency remains unchanged. PID control is restored when the evaporator outlet water temperature is greater than or equal to 20℃, or the heat power of the secondary heat exchanger is greater than or equal to 40% of the rated heat output power of the electric heat pump.

[0070] The load adjustment range of a typical electric heat pump is 30%-100%, but this varies depending on the model (scroll, screw, centrifugal, etc.).

[0071] 6) The frequency lower limit of the circulating pump in the water circuit corresponding to the secondary heat exchanger is set according to the higher of the minimum flow requirements of the secondary heat exchanger and the electric heat pump evaporator side, and the frequency upper limit is set according to the higher of the maximum flow requirements of the two sides. Regardless of the control strategy used, the frequency adjustment range is between the upper and lower limits. The circulating pump in the water circuit corresponding to the secondary heat exchanger uses PID control, controlled according to the terminal temperature difference of the secondary heat exchanger (terminal temperature difference = flue gas outlet temperature of flue gas heat exchanger - inlet water temperature of flue gas heat exchanger), with a set value of 5℃.

[0072] 7) The frequency upper and lower limits of the circulating pump corresponding to the electric heat pump are set according to the minimum and maximum flow rates on the condenser side of the electric heat pump; regardless of the control strategy used, the frequency adjustment range is between the upper and lower limits. The circulating pump of the electric heat pump adopts PID control, and the set value is generally 5-7℃ based on the inlet and outlet water temperature difference designed on the condenser side of the electric heat pump.

[0073] Energy efficiency calculation: 1) Calories: The heating capacity of the flue gas waste heat recovery system = heat from the primary heat exchanger + heating capacity of the electric heat pump.

[0074] 2) Power consumption: The power consumption of an electric heat pump system = the power consumption of the electric heat pump unit + the power consumption of the circulating pump corresponding to the secondary heat exchanger + the power consumption of the circulating pump corresponding to the electric heat pump.

[0075] The power consumption of the flue gas waste heat recovery system = the power consumption of the electric heat pump unit + the power consumption of the circulating pump corresponding to the primary heat exchanger + the power consumption of the circulating pump corresponding to the secondary heat exchanger + the power consumption of the circulating pump corresponding to the electric heat pump.

[0076] 3) COP of electric heat pump units: The COP of an electric heat pump unit is calculated as follows: heating capacity of the electric heat pump unit ÷ power consumption of the electric heat pump unit. Data for different time periods can be calculated as needed.

[0077] 4) COP of electric heat pump system: The COP of an electric heat pump system is calculated as follows: (Heating capacity of the electric heat pump unit ÷ (Power consumption of the electric heat pump unit + Power consumption of the circulating pump corresponding to the secondary heat exchanger + Power consumption of the circulating pump corresponding to the electric heat pump)). Data for different time periods can be calculated as needed.

[0078] 5) COP of waste heat recovery system: The COP of the waste heat recovery system is calculated as follows: (Heating capacity of the electric heat pump unit + Waste heat recovery amount of the primary heat exchanger) ÷ (Power consumption of the electric heat pump unit + Power consumption of the circulating pump corresponding to the secondary heat exchanger + Power consumption of the circulating pump corresponding to the electric heat pump + Power consumption of the circulating pump corresponding to the primary heat exchanger). Data for different time periods can be calculated as needed.

[0079] 6) Cost savings from flue gas waste heat recovery systems Cost savings from flue gas waste heat recovery system = heating capacity of flue gas waste heat recovery system × heating unit price of gas boiler - electricity consumption of flue gas waste heat recovery system × electricity price.

[0080] The benefits of this application are: 1) Cascaded utilization of flue gas reduces investment and operating costs and improves system economy.

[0081] 2) Significantly reduce the number of electric heat pump units required, thus lowering investment; the number of electric heat pump units required can be reduced to about 30% of the number of units required for conventional heat pump systems; at the same time, it also significantly reduces operating costs, installation space requirements, and electrical capacity requirements.

[0082] 3) The frequency of the primary heat exchange circulation pump (the circulation pump on the water circuit corresponding to the primary heat exchanger) can be adjusted by frequency conversion, thereby adjusting the water flow rate of the primary heat exchanger, changing the heat exchange capacity of the primary heat exchanger, and adjusting the outlet flue gas temperature of the primary heat exchanger to ensure that the electric heat pump can operate normally under different loads of the gas boiler. For example, when the gas boiler is under low load, the frequency of the water pump (circulation pump) corresponding to the primary heat exchanger can be reduced to decrease the heat exchange capacity of the primary heat exchanger, increase the outlet flue gas temperature of the primary heat exchanger, and transfer the heat to the secondary heat exchanger to ensure that the electric heat pump maintains at least the minimum load operation.

[0083] 4) The addition of a buffer water tank avoids the waste of flue gas heat during the interval between the start and stop of the electric heat pump, reduces the number of times the electric heat pump stops, reduces the maintenance cost of the electric heat pump, and increases the life of the electric heat pump.

[0084] 5) Due to the small temperature difference between the evaporator and condenser sides, a high COP can be achieved by adjusting the design of the electric heat pump.

[0085] 6) Implement in-depth control of the electric heat pump unit and match the load with the secondary heat exchanger in real time to ensure efficient operation of the electric heat pump.

[0086] 7) A sophisticated automatic control system enables unattended operation.

[0087] 8) Comprehensive energy consumption statistics and energy efficiency analysis.

[0088] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0089] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A four-pipe electrically driven enhanced flue gas waste heat recovery system, characterized in that, The four-pipe electric drive enhanced flue gas waste heat recovery system is applied in the scenario of natural gas hot water boiler; the four-pipe electric drive enhanced flue gas waste heat recovery system includes: control cabinet, circulating pump, heat exchanger and heat meter; the control cabinet includes PLC controller; the circulating pump, heat exchanger and heat meter are all connected to the PLC controller. The heat exchanger includes a primary heat exchanger and a secondary heat exchanger; each of the primary and secondary heat exchangers is equipped with a circulating pump and a heat meter on its corresponding water circuit, and an electric heat pump is installed on the water circuit corresponding to the secondary heat exchanger; the electric heat pump includes an evaporator and a condenser. The heat meter is used to monitor loop information in the water circuit; the loop information includes: temperature, temperature difference, flow rate, thermal power, and heat. The PLC controller is used to perform frequency conversion control of the circulating pump based on loop information, and to perform gradient utilization control of the flue gas waste heat in the natural gas hot water boiler scenario based on the primary and secondary heat exchangers. The primary heat exchanger directly recovers the flue gas waste heat using heating water return water; the secondary heat exchanger recovers the waste heat of the low-temperature part of the flue gas using an evaporator and replaces it with lower-temperature hot water as the heat source for the electric heat pump, which then raises the temperature of the heating water. The low-temperature part of the flue gas is the flue gas after waste heat recovery by the primary heat exchanger; the lower temperature is the temperature below the waste heat temperature of the low-temperature part of the flue gas.

2. The four-pipe electrically driven enhanced flue gas waste heat recovery system according to claim 1, characterized in that, The primary heat exchanger and the secondary heat exchanger are heat exchangers with different terminal differences; The terminal temperature difference is determined based on the flue gas outlet temperature and the heat exchanger inlet water temperature.

3. The four-pipe electrically driven enhanced flue gas waste heat recovery system according to claim 1, characterized in that, A buffer water tank is provided between the secondary heat exchanger and the evaporator; The buffer tank is used to buffer changes in the waste heat of the flue gas; the water volume in the buffer tank corresponds to the circulation flow rate of the secondary heat exchanger and evaporator for a set duration.

4. The four-pipe electrically driven enhanced flue gas waste heat recovery system according to claim 3, characterized in that, The buffer tank is replenished with heating water return water, and the opening and closing is controlled by a float valve.

5. The four-pipe electrically driven enhanced flue gas waste heat recovery system according to claim 1, characterized in that, When the electric heat pump and the boiler in the natural gas hot water boiler scenario are not one-to-one, the four-pipe electric drive enhanced flue gas waste heat recovery system also includes: an electric valve and a temperature sensor; The temperature sensor is used to monitor the outlet water temperature of the heat exchanger.

6. The four-pipe electrically driven enhanced flue gas waste heat recovery system according to claim 1, characterized in that, The control cabinet also includes: a touch screen, a frequency converter, and a smart meter; The touch screen, the frequency converter, and the smart meter are all connected to the PLC controller.

7. A four-pipe electrically driven enhanced flue gas waste heat recovery control method, characterized in that, The four-pipe electric drive enhanced flue gas waste heat recovery control method is implemented using the four-pipe electric drive enhanced flue gas waste heat recovery system described in any one of claims 1-6; The four-pipe electric drive enhanced flue gas waste heat recovery control method includes: Obtain loop information on the water circuit; the loop information includes: temperature, temperature difference, flow rate, thermal power, and heat. The circulating pump is controlled by frequency conversion based on the loop information, and the waste heat of flue gas in the natural gas hot water boiler scenario is utilized by gradient control based on the primary and secondary heat exchangers. The primary heat exchanger directly recovers the waste heat of flue gas using the return water from the heating system. The secondary heat exchanger recovers the waste heat of the low-temperature part of the flue gas using an evaporator and replaces it with lower-temperature hot water, which serves as the heat source for the electric heat pump, and the electric heat pump raises the temperature of the heating water. The low-temperature part of the flue gas is the flue gas after waste heat recovery by the primary heat exchanger. The lower temperature is the waste heat temperature below the low-temperature part of the flue gas.

8. The four-pipe electrically driven enhanced flue gas waste heat recovery control method according to claim 7, characterized in that, The circulating pump is controlled by frequency conversion based on loop information, and the waste heat from the flue gas in the natural gas hot water boiler scenario is utilized by gradient control based on the primary and secondary heat exchangers in the heat exchanger. Specifically, this includes: Based on the flow rate in the loop information corresponding to the first-stage heat exchanger, the upper and lower limits of the frequency of the circulating pump corresponding to the first-stage heat exchanger are determined, and a PID control method is adopted to control the flow rate based on the terminal difference of the first-stage heat exchanger; the terminal difference is determined based on the flue gas outlet temperature and the heat exchanger inlet water temperature. When the frequency reduction condition is met, the circulating pump corresponding to the primary heat exchanger is controlled to start frequency reduction operation. When the frequency reduction condition is no longer met, the operating frequency is kept unchanged. The frequency reduction condition includes: the outlet water temperature on the evaporation side of the evaporator is lower than the preset temperature, and the heat power of the secondary heat exchanger is lower than the set ratio of the rated heat output power of the electric heat pump. Based on the flow rates of the secondary heat exchanger and the evaporator side of the electric heat pump, the upper and lower limits of the frequency of the circulating pump corresponding to the secondary heat exchanger are determined, and a PID control method is adopted to control the flow rate based on the terminal difference of the secondary heat exchanger; the evaporator side of the electric heat pump is the side of the electric heat pump closest to the evaporator. The frequency adjustment range of the electric heat pump is determined based on the flow rate on the condenser side, and the temperature difference between the inlet and outlet water on the condenser side is determined using a PID control method. The inlet water temperature on the condenser side is the same as the heating return water temperature. The outlet water temperature on the condenser side is automatically matched according to the heat power. The condenser side is the side of the electric heat pump closest to the condenser.

9. The four-pipe electrically driven enhanced flue gas waste heat recovery control method according to claim 7, characterized in that, The control modes corresponding to the gradient utilization control include cooling mode and heating mode; The heating mode is used to control the outlet water temperature on the condenser side; the cooling mode is used to control the outlet water temperature on the evaporator side.

10. The four-pipe electrically driven enhanced flue gas waste heat recovery control method according to claim 9, characterized in that, In heating mode, controlling the condensate outlet water temperature specifically includes: Determine the initial set value of the condenser side outlet water temperature; the condenser side is the side of the water circuit corresponding to the secondary heat exchanger that is closer to the condenser; the initial set value of the condenser side outlet water temperature is determined based on the condenser side inlet water temperature and the preset temperature difference on the condenser side. The evaporator side outlet water temperature is acquired in real time according to a preset time interval; the evaporator side is the side of the water circuit corresponding to the secondary heat exchanger that is close to the evaporator. The initial setting value of the condenser outlet temperature is adjusted according to the evaporator outlet temperature; wherein, when the evaporator outlet temperature is within the preset threshold range, the initial setting value of the condenser outlet temperature is not adjusted; when the evaporator outlet temperature is less than the lower limit of the preset threshold range, the initial setting value of the condenser outlet temperature gradually decreases; when the evaporator outlet temperature is greater than the upper limit of the preset threshold range, the initial setting value of the condenser outlet temperature gradually increases.

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