A carbon dioxide cascade heating system and its control method

Abstract

The present invention discloses a carbon dioxide cascade heating system and a control method thereof, comprising a low-temperature circuit, a high-temperature circuit, and a heating circuit, wherein both the low-temperature compressor and the high-temperature compressor are variable-frequency compressors, and the water pump is a variable-frequency water pump. Because the carbon dioxide cascade heating system of the present invention utilizes a variable-frequency compressor, a variable-frequency water pump, and a variable-frequency fan, it is highly efficient, energy-saving, and environmentally friendly. The control method employed can reasonably adjust its operating parameters based on the heat load demand at the user end and ensure operation at the most efficient state, thereby achieving the effects of saving energy and improving heating comfort, thereby overcoming the shortcomings of existing carbon dioxide cascade heating systems.

Description

Technical Field

[0001] The invention relates to a carbon dioxide cascade heating system and a control method thereof. Background Art

[0002] As refrigerants, both carbon dioxide and R1234yf possess favorable environmental properties, including an ODP value of zero and a low global warming potential (GWP). The new refrigerant R1234yf has already gained widespread use in automotive air conditioning as a replacement for the high-warming refrigerant R134a. Carbon dioxide, as a natural working fluid, also exhibits favorable physical properties at low temperatures. Therefore, a combined carbon dioxide cascade heating system offers superior performance compared to traditional cascade heat pumps while also taking into account environmental considerations. Current carbon dioxide cascade heating systems cannot adjust the unit's load based on changes in ambient and indoor temperatures, tailored to human comfort, nor can they adjust the load at the high and low temperature stages to ensure optimal operation under these operating conditions. Summary of the Invention

[0003] The object of the present invention is to provide a carbon dioxide cascade heating system and method.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is:

[0005] A carbon dioxide cascade heating system includes a low-temperature stage circuit, a high-temperature stage circuit and a heating circuit, wherein the low-temperature stage circuit includes an evaporator, a gas-liquid separator, a low-temperature stage compressor, an intermediate heat exchanger condenser tube and a low-temperature stage expansion valve sequentially arranged in series in the loop, the high-temperature stage circuit includes an intermediate heat exchanger evaporator tube, a high-temperature stage compressor, a condenser condenser tube and a high-temperature stage expansion valve sequentially arranged in series in the loop, the heating circuit includes a condenser evaporator tube, a user-end heat exchange component and a water pump sequentially arranged in series in the loop, the intermediate heat exchanger condenser tube and the intermediate heat exchanger evaporator tube are arranged together for heat exchange with each other, the condenser condenser tube and the condenser evaporator tube are arranged together for heat exchange with each other, the low-temperature stage compressor and the high-temperature stage compressor are both variable-frequency compressors, and the water pump is a variable-frequency water pump.

[0006] Preferably, it also includes an ambient temperature sensor for detecting the external ambient temperature, an indoor temperature sensor for detecting the indoor temperature, and a refrigerant inlet temperature sensor and a refrigerant outlet temperature sensor respectively arranged at the inlet and outlet ends of the condenser tube of the condenser.

[0007] Preferably, the high-temperature stage circuit also includes an internal heat exchanger evaporator tube arranged between the intermediate heat exchanger condenser tube and the high-temperature stage compressor and an internal heat exchanger condenser tube arranged between the condenser evaporator tube and the high-temperature stage expansion valve, and the internal heat exchanger evaporator tube and the internal heat exchanger condenser tube are arranged together for heat exchange with each other.

[0008] Preferably, a defrost pipeline is connected to the inlet end of the evaporator and the inlet end of the condenser tube of the intermediate heat exchanger, and a solenoid valve is provided in the defrost pipeline.

[0009] Preferably, the user-end heat exchange assembly includes a plurality of user-end heat exchange units arranged in parallel, and the user-end heat exchange unit includes a flow regulating valve and a user-end heat exchanger arranged in series.

[0010] Preferably, the low-temperature stage circuit further includes a fan for driving outdoor air to exchange heat with the evaporator, and the fan is a variable frequency fan.

[0011] A method for controlling a carbon dioxide cascade heating system, using the above-mentioned carbon dioxide cascade heating system, comprises the following steps:

[0012] 1) Obtain the comfortable temperature T of room i by calculating the room information s,i , obtain the outdoor temperature T through the ambient temperature sensor a , obtain the indoor temperature T of room i through the indoor temperature sensor t,i ;

[0013] 2) Through T s,i 、T a and T t,i Calculate the required temperature T of the outlet of the coolant in the condenser out = (T t,1 , T t,2 ...T t,n , T s,1 , T s,2 ...T s,n ), and calculate the heat load Q required for room i i =φ i (T t,i , T s,i , T a );

[0014] 3) Based on the current temperature T of the i-th room t,i and comfort temperature T s,i The difference and heat load Q i Calculate the appropriate opening EXP of the i-th flow control valve i = EXP i ( , Qi ), and through EXP i Adjust the flow control valve according to the value.

[0015] 4) Based on the heat load Q required for room i i And the heat leakage loss Q during the heating process of the user-side heat exchange unit s The heating capacity provided by the heating unit is calculated as Q = According to the outlet temperature T of the coolant in the condenser out The heating capacity provided by the heating unit is Q;

[0016] 5) Based on the obtained heating capacity Q and water outlet temperature T out According to the formula, the appropriate operating frequency V1 of the high temperature compressor is adjusted. g (Q, T out ) and the appropriate operating frequency V3 of the water pump = h (Q, T out );

[0017] 6) According to the target intermediate temperature T m With the ambient temperature T a , adjust the appropriate operating frequency of the low temperature compressor V2 according to the formula = m (T m , T a );

[0018] 7) The high-temperature compressor is adjusted according to the value of V1, the low-temperature compressor is adjusted according to the value of V2, and the water pump is adjusted according to the value of V3.

[0019] Preferably, the control method for the fan operation is:

[0020] When T a ≤T a,1 When the variable frequency fan is V f,1 The operating frequency is:

[0021] When T a,1 <T a ≤T a,2 The variable frequency fan is V f,2 The operating frequency is:

[0022] When T a >T a,2 The variable frequency fan is V f,3 The operating frequency is:

[0023] Where V f,1 >V f,2 >V f,3 .

[0024] Preferably, in step 3), when When , the i-th flow control valve is no longer adjusted.

[0025] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0026] Since the carbon dioxide cascade heating system of the present invention adopts a variable frequency compressor, a variable frequency water pump and a variable frequency fan, it has the characteristics of high efficiency, energy saving and environmental protection. The control method it adopts can reasonably adjust its operating parameters according to the size of the user's end heat load demand and ensure that it operates in the most efficient state, thereby achieving the effect of saving energy and improving heating comfort, and improving the shortcomings of the existing carbon dioxide cascade heating system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Attachment Figure 1 This is a system principle diagram of the present invention.

[0028] Attachment Figure 2 It is a control flow chart of the present invention.

[0029] In the above figures: 1. Evaporator; 2. Gas-liquid separator; 3. Low-temperature compressor; 4. Intermediate heat exchanger; 5. Low-temperature expansion valve; 6. Variable frequency fan; 7. High-temperature compressor; 8. Condenser; 9. High-temperature expansion valve; 10. Water pump; 11. Flow control valve; 12. User-end heat exchanger; 13. Internal heat exchanger; 14. Solenoid valve. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to the embodiments shown in the accompanying drawings:

[0031] See attached Figure 1As shown, a carbon dioxide cascade heating system comprises a low-temperature circuit, a high-temperature circuit, and a heating circuit. The low-temperature circuit includes an evaporator 1, a gas-liquid separator 2, a low-temperature compressor 3, an intermediate heat exchanger condenser tube, and a low-temperature expansion valve 5, arranged in series within the loop. The evaporator 1 is also provided with a variable-frequency fan 6. In this embodiment, carbon dioxide is used as the medium in the low-temperature circuit. The high-temperature circuit includes an intermediate heat exchanger evaporator tube, a high-temperature compressor 7, a condenser tube, and a high-temperature expansion valve 9, arranged in series within the loop. In this embodiment, R1234yf is used as the medium in the high-temperature circuit. The heating circuit includes a condenser evaporator tube, a user-side heat exchange assembly, and a water pump 10, arranged in series within the loop. The intermediate heat exchanger condenser tube and the intermediate heat exchanger evaporator tube are arranged together for heat exchange to form the intermediate heat exchanger 4, and the condenser tube and the condenser evaporator tube are arranged together for heat exchange to form the condenser 8. In this embodiment, both the low-temperature compressor 3 and the high-temperature compressor 7 are variable-frequency compressors, and the water pump 10 is a variable-frequency water pump 10. Furthermore, the user-end heat exchange assembly includes a plurality of user-end heat exchange units arranged in parallel, and the user-end heat exchange unit includes a flow regulating valve 11 and a user-end heat exchanger 12 arranged in series.

[0032] In addition, it also includes an ambient temperature sensor for detecting the external ambient temperature, an indoor temperature sensor for detecting the indoor temperature, and a refrigerant inlet temperature sensor and a refrigerant outlet temperature sensor respectively arranged at the inlet and outlet ends of the condenser tube of the condenser.

[0033] In this embodiment, the high-temperature stage circuit also includes an internal heat exchanger evaporator tube arranged between the intermediate heat exchanger condenser tube and the high-temperature stage compressor 7 and an internal heat exchanger condenser tube arranged between the condenser evaporator tube and the high-temperature stage expansion valve 9. The internal heat exchanger evaporator tube and the internal heat exchanger condenser tube are arranged together for heat exchange to form an internal heat exchanger 13.

[0034] A defrost pipeline is connected between the inlet end of the evaporator 1 and the inlet end of the condenser pipe of the intermediate heat exchanger. A solenoid valve 14 is provided in the defrost pipeline. When defrosting is required, the solenoid valve 14 opens, and the carbon dioxide medium bypasses the intermediate heat exchanger 4 and returns to the evaporator 1.

[0035] The working mode of the carbon dioxide cascade heating system of this embodiment is as follows:

[0036] In the heating mode, the solenoid valve 14 is closed, the variable frequency fan 6, the low temperature stage compressor 3 and the high temperature stage compressor 7 are turned on, and the variable frequency water pump 10 is turned on.

[0037] In the low-temperature stage circuit, the carbon dioxide medium enters the intermediate heat exchanger condenser tube driven by the low-temperature stage compressor 3 and is cooled by the R1234yf medium in the intermediate heat exchanger evaporator tube. It then passes through the low-temperature stage expansion valve 5 and enters the evaporator 1 to absorb heat from the air. It then enters the gas-liquid separator 2 and returns to the low-temperature stage compressor 3.

[0038] In the high-temperature stage circuit, the R1234yf medium enters the condenser tube of the condenser driven by the high-temperature stage compressor 7 and transfers heat to the refrigerant in the condenser evaporator tube. It then enters the internal heat exchanger 13 and is further cooled by the low-temperature R1234yf. After passing through the high-temperature stage expansion valve 9, it enters the intermediate heat exchanger condenser tube to absorb heat from the carbon dioxide medium in the intermediate heat exchanger evaporator tube. It then enters the internal heat exchanger 13 for further superheating and returns to the high-temperature stage variable frequency compressor.

[0039] In the heating circuit, the refrigerant enters the condenser evaporator tube driven by the variable frequency water pump 10 to absorb heat from the R1234yf medium in the condenser tube. It is then divided into n paths, each of which is regulated by a flow control valve 11 to adjust the flow of the refrigerant. The refrigerant releases heat in the heat exchanger to heat the room and then re-enters the variable frequency water pump 10.

[0040] In order to ensure that the variable frequency fan 6, low temperature compressor 3, high temperature compressor 7 and variable frequency water pump 10 in the carbon dioxide cascade heating system can be properly adjusted according to the different ambient temperatures and the heat load of each room, and to ensure that the unit operates at the optimal state point, the control method includes the following steps:

[0041] 1) Obtain the comfortable temperature T of room i by calculating the room information s,i , obtain the outdoor temperature T through the ambient temperature sensor a , obtain the indoor temperature T of room i through the indoor temperature sensor t,i ;

[0042] 2) Through T s,i 、T a and T t,i Calculate the required temperature of the refrigerant outlet in condenser 8:

[0043] T out =(T t,1 , T t,2 ...T t,n , T s,1 , T s,2 ...T s,n )= ,

[0044] And calculate the required heating load for room i:

[0045] Q i =φi (T t,i , T s,i , T a ) =A i [K1(T a -T s,i )+K2(T t,i -T s,i )]; where K1 and K2 are characteristic coefficients, A i is the characteristic heating area of room i.

[0046] 3) Based on the current temperature T of the i-th room t,i and comfort temperature T s,i The difference and heat load Q i Calculate the appropriate opening of the i-th flow control valve 11:

[0047] EXP i = EXP i ( , Q i ) = , the coefficient a is obtained by fitting the experimental data and its value is 8≤a≤10.

[0048] And through EXP i Adjust the flow control valve 11 only when When , the i-th flow control valve 11 is no longer adjusted.

[0049] 4) Based on the heat load Q required for room i i And the heat leakage loss Q during the heating process of the user-side heat exchange unit s The heating capacity provided by the heating unit is calculated as Q = ; According to the required temperature T of the outlet of the coolant in the condenser 8 out The heating capacity provided by the heating unit is Q;

[0050] 5) Based on the obtained heating capacity Q and water outlet temperature T out The appropriate operating frequency of the high-temperature stage compressor 7 is calculated according to the formula:

[0051] V1= g (Q, T out )=50+5* +3* ,

[0052] And the appropriate operating frequency of the water pump 10:

[0053] V3= h (Q, T out )=1250* ,

[0054] The high temperature stage compressor 7 is regulated through V1, and the water pump 10 is regulated through V3.

[0055] 6) According to the intermediate temperature T of the intermediate heat exchanger 4 m and ambient temperature T a , adjust the appropriate operating frequency of the low-temperature compressor 3 according to the formula:

[0056] V2=m (T m , T a ) =45.27-0.65*T a +0.254*T m -0.003*T a 2 +0.0017*T m 2 +0.0019*T a *T m ,

[0057] The low temperature stage compressor 3 is adjusted by V2 to ensure that the condensing temperature of the low temperature stage reaches the target intermediate temperature T m .

[0058] In addition, the control method also includes the adjustment method of the fan:

[0059] When T a ≤T a,1 When the frequency conversion fan 6 is V f,1 The operating frequency is:

[0060] When T a,1 <T a ≤T a,2 , variable frequency fan 6V f,2 =c* T a +b runs at the operating frequency;

[0061] When T a >T a,2 , variable frequency fan 6V f,3 The operating frequency is:

[0062] Where V f,1 >V f,2 >V f,3 The coefficients c and b in the formula are obtained by fitting experimental data. In this embodiment, c=-4 / 9, b=146 / 3.

[0063] The formulas involved in the above control methods can all be obtained by fitting experimental data.

[0064] The carbon dioxide cascade heating system and its control method can reasonably adjust the operating frequencies of the fan and compressor according to the size of the operating heating load, thereby providing a fast heating rate and saving energy. Moreover, the system can operate in an optimal state under different working conditions, with high system efficiency, thereby achieving the purpose of saving energy.

[0065] Because the CO2 cascade heating system of this embodiment utilizes a variable-frequency compressor, a variable-frequency water pump, and a variable-frequency fan, it is highly efficient, energy-efficient, and environmentally friendly. Its control method appropriately adjusts the operating frequencies of the water pump 10, the low-temperature compressor 3, and the high-temperature compressor 7, as well as the opening of the flow control valve 11, based on the user's heat load demand, to ensure the system operates at peak efficiency, thereby saving energy and improving heating comfort. This improves the shortcomings of existing CO2 cascade heating systems.

[0066] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A method for controlling a carbon dioxide cascade heating system, characterized in that: The carbon dioxide cascade heating system includes a low-temperature stage circuit, a high-temperature stage circuit and a heating circuit. The low-temperature stage circuit includes an evaporator, a gas-liquid separator, a low-temperature stage compressor, an intermediate heat exchanger condenser tube and a low-temperature stage expansion valve sequentially arranged in series in the loop. The high-temperature stage circuit includes an intermediate heat exchanger evaporator tube, a high-temperature stage compressor, a condenser condenser tube and a high-temperature stage expansion valve sequentially arranged in series in the loop. The heating circuit includes a condenser evaporator tube, a user-end heat exchange component and a water pump sequentially arranged in series in the loop. The intermediate heat exchanger condenser tube and the intermediate heat exchanger evaporator tube are arranged together for heat exchange to form an intermediate heat exchanger. The condenser condenser tube and the condenser evaporator tube are arranged together for heat exchange to form a condenser. The low-temperature stage compressor and the high-temperature stage compressor are both variable-frequency compressors, and the water pump is a variable-frequency water pump. The carbon dioxide cascade heating system further includes an ambient temperature sensor for detecting the external ambient temperature, an indoor temperature sensor for detecting the indoor temperature, and a brine inlet temperature sensor and a brine outlet temperature sensor respectively provided at the inlet and outlet ends of the condenser tube of the condenser; The low-temperature circuit uses carbon dioxide as the medium, and the high-temperature circuit uses R1234yf as the medium; The low-temperature circuit also includes a fan for driving outdoor air to exchange heat with the evaporator, and the fan is a variable frequency fan; The control method comprises the following steps: 1) Obtain the comfortable temperature Ts,i of the i-th room through room information calculation, and obtain the ambient temperature T through the ambient temperature sensor a , obtain the indoor temperature Tt,i of the i-th room through the indoor temperature sensor; 2) Through T s,i 、T a and T t,i Calculate the required temperature of the outlet of the coolant in the condenser , and calculate the required heating load for room i: Q i =φ i (T t,i , T s,i , T a )=A i [K1(T a - T s,i )+ K2(T t,i - T s,i )], where K1 and K2 are characteristic coefficients, A i is the characteristic heating area of room i; 3) Based on the current temperature T of the i-th room t,i and comfort temperature T s,i The difference ΔT i and heat load Q i Calculate the appropriate opening EXP of the i-th flow control valve i =EXP i (ΔT i , Q i ) = (ΔT i ×Q i ) / a×100%, the coefficient a is obtained by fitting the experimental data, and the value is 8≤a≤10, and according to EXP i The flow control valve is adjusted numerically. When |ΔT i When |≤1℃, the i-th flow control valve will no longer be adjusted; 4) According to the heat load Q required by room i i And the heat leakage loss Q during the heating process of the user-side heat exchange unit s Calculate the heating capacity provided by the heating unit as , according to the required temperature T of the outlet of the coolant in the condenser out The heating capacity provided by the heating unit is Q; 5) Based on the obtained heating capacity Q and water outlet temperature T out The appropriate operating frequency of the high temperature compressor is calculated according to the formula: V1=g(Q,T out )=50+5×Q / Q0+3×T out / T out,0 , And the appropriate operating frequency of the water pump V3 = h(Q, T out )=1250×Q / [ Q0×(T out -40)]; 6) According to the intermediate temperature T of the intermediate heat exchanger m With the ambient temperature T a , calculate the appropriate operating frequency of the low-temperature compressor according to the formula: V2=m(T m ,T a )=45.27-0.65×T a +0.254×T m -0.003×T a 2 +0.0017×T m 2 +0.0019×T a ×T m ; 7) Adjust the high-temperature compressor according to the value of V1, adjust the low-temperature compressor according to the value of V2, and adjust the water pump according to the value of V3; The control method for the operation of the variable frequency fan is: When T a ≤T a,1 When the variable frequency fan is V f,1 The operating frequency is: When T a,1 <T a ≤T a,2 The variable frequency fan is V f,2 =c×T a +b operating frequency operation, coefficients c and b are obtained by fitting the experimental data; When T a >T a,2 The variable frequency fan is V f,3 The operating frequency is: Where V f,1 >V f,2 >V f,3 .

2. The control method of the carbon dioxide cascade heating system according to claim 1, characterized in that: The high-temperature stage circuit also includes an internal heat exchanger evaporator tube arranged between the intermediate heat exchanger condenser tube and the high-temperature stage compressor and an internal heat exchanger condenser tube arranged between the condenser evaporator tube and the high-temperature stage expansion valve. The internal heat exchanger evaporator tube and the internal heat exchanger condenser tube are arranged together for heat exchange.

3. The control method of the carbon dioxide cascade heating system according to claim 1, characterized in that: The inlet end of the evaporator and the inlet end of the condenser tube of the intermediate heat exchanger are connected with a defrost pipeline, and a solenoid valve is provided in the defrost pipeline.

4. The control method of the carbon dioxide cascade heating system according to claim 1, characterized in that: The user-end heat exchange assembly includes a plurality of user-end heat exchange units arranged in parallel, and the user-end heat exchange unit includes a flow regulating valve and a user-end heat exchanger arranged in series.

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