Control method of transcritical carbon dioxide composite heat pump system

Abstract

A control method of a transcritical carbon dioxide composite heat pump system is disclosed, wherein the transcritical carbon dioxide composite heat pump system includes: a CO.sub.2 main circuit compressor, an air-cooling-air-cooling recombiner, a supercooling-evaporation recombiner, an evaporator and a CO.sub.2 auxiliary compressor; wherein the air-cooling-air-cooling recombiner comprises a CO.sub.2 main circuit, a CO.sub.2 auxiliary circuit and a water circuit; the supercooling-evaporation recombiner comprises a CO.sub.2 main circuit supercooling section and a CO.sub.2 auxiliary circuit evaporation section. The present invention includes two working modes according to the return water temperature, so that the unit has a wider application range and meets daily needs. There is only one heat exchanger for refrigerant and water. Compared with the three water and refrigerant heat exchangers in the conventional transcritical CO.sub.2 composite heat pump, the circulating water circuit is a single circuit with one inlet and one outlet.

Claims

1. A control method of a transcritical carbon dioxide composite heat pump system, wherein the transcritical carbon dioxide composite heat pump system comprises: a CO.sub.2 main circuit compressor (1), an air-cooling-air-cooling recombiner (2), a supercooling-evaporation recombiner (3), an evaporator (5) and a CO.sub.2 auxiliary compressor (6); wherein the air-cooling-air-cooling recombiner (2) comprises a CO.sub.2 main circuit (9), a CO.sub.2 auxiliary circuit (10) and a water circuit (11); the supercooling-evaporation recombiner (3) comprises a CO.sub.2 main circuit supercooling section (12) and a CO.sub.2 auxiliary circuit evaporation section (13); the transcritical carbon dioxide composite heat pump system comprises a main circuit and an auxiliary circuit; for the main circuit: an outlet of the CO.sub.2 main circuit compressor (1) is connected to an inlet of the CO.sub.2 main circuit (9) of the air-cooling-air-cooling recombiner (2); an outlet of the CO.sub.2 main circuit (9) of the air-cooling-air-cooling recombiner (2) is connected to an inlet of the CO.sub.2 main circuit supercooling section (12) of the supercooling-evaporation recombiner (3); an outlet of the CO.sub.2 main circuit supercooling section (12) of the subcooling-evaporation recombiner (3) is connected to an inlet of the evaporator (5); and an outlet of the evaporator (5) is connected to an inlet of the CO.sub.2 main circuit compressor (1); for the auxiliary circuit: an outlet of the CO.sub.2 auxiliary compressor (6) is connected to an inlet of the CO.sub.2 auxiliary circuit (10) of the air-cooling-air-cooling recombiner (2); an outlet of the CO.sub.2 auxiliary circuit (10) of the air-cooling-air-cooling recombiner (2) is connected to an inlet of the CO.sub.2 auxiliary circuit evaporation section (13) of the supercooling-evaporation recombiner (3); an outlet of the CO.sub.2 auxiliary circuit evaporation section (13) of the supercooling-evaporation recombiner (3) is connected to an inlet of the CO.sub.2 auxiliary compressor (6); a CO.sub.2 main circuit expansion valve (4) is arranged between the supercooling-evaporation recombiner (3) and the evaporator (5) on the main circuit; and a CO.sub.2 auxiliary expansion valve (7) is arranged between the air-cooling-air-cooling recombiner (2) and the supercooling-evaporation recombiner (3) on the auxiliary circuit; the transcritical carbon dioxide composite heat pump system operates in a cyclic heating mode, and the control method comprises steps of: starting the CO.sub.2 main circuit compressor (1) and turning on the main circuit; starting the CO.sub.2 auxiliary compressor (6) and turning on the auxiliary circuit, and keeping a fan (8) at a working state; for the main circuit: compressing a CO.sub.2 working fluid from a state point a through the CO.sub.2 main circuit compressor (1) to reach a state point b, and moving the CO.sub.2 working fluid into the CO.sub.2 main circuit (9) of the air-cooling-air-cooling recombiner (2) for heating circulating water in the water circuit (11); self-cooling the CO.sub.2 working fluid to an appropriate temperature to reach a state point c, then passing the CO.sub.2 working fluid through the CO.sub.2 main circuit supercooling section (12) of the supercooling-evaporation recombiner (3) to exchange heat with the CO.sub.2 auxiliary circuit evaporation section (13), so as to further cool down to reach a state point d; then moving the CO.sub.2 working fluid into the CO.sub.2 main circuit expansion valve (4) to be expanded, in such a manner that an expanded low-pressure working fluid reaches a state point e; moving the expanded low-pressure working fluid into the evaporator (5) for evaporation and absorbing heat, so as to return to the state point a, and finally returning to the inlet of the CO.sub.2 main circuit compressor (1); for the auxiliary circuit: compressing the CO.sub.2 working fluid from a state point f through the CO.sub.2 auxiliary compressor (6) to reach a state point g, and moving the CO.sub.2 working fluid into the CO.sub.2 auxiliary circuit (10) of the air-cooling-air-cooling recombiner (2) for heating the circulating water in the water circuit (11); self-cooling the CO.sub.2 working fluid to reach a state point h, then moving the CO.sub.2 working fluid into the CO.sub.2 auxiliary circuit expansion valve (7) to be expanded to reach a state point i; moving the expanded low-pressure working fluid into the CO.sub.2 auxiliary circuit evaporation section (13) of the supercooling-evaporation recombiner (3) for exchanging heat with and further cooling the CO.sub.2 main circuit supercooling section (12); self-evaporating the expanded low-pressure working fluid to absorb heat and reach the state point f, and finally returning to the inlet of the CO.sub.2 auxiliary compressor (6); wherein an auxiliary circuit control method comprises steps of: collecting an ambient temperature t.sub.ambient, a CO.sub.2 auxiliary circuit outlet temperature t.sub.g,out auxiliary of the air-cooling-air-cooling recombiner (2) set by a user, and a water circuit outlet temperature t.sub.return of the air-cooling-air-cooling recombiner (2) set by the user; calculating an exhaust pressure P.sub.co.sub.2.sub.,auxiliary of the CO.sub.2 auxiliary compressor (6) by a formula (I), and adjusting an opening degree of the CO.sub.2 auxiliary expansion valve (7) to achieve a preset pressure; and calculating a motor frequency f.sub.compress 6 of the CO.sub.2 auxiliary compressor (6) by a formula (II), in such a manner that a compressor frequency changes with a working condition;
P.sub.co.sub.2.sub.,auxiliary=0.0036t.sub.g,out auxiliary.sup.2−0.02t.sub.g,out auxiliary−0.035t.sub.ambient+0.067t.sub.return+7.38  (I);
f.sub.compress 6=50−0.005t.sub.g,out auxiliary.sup.2+0.17t.sub.g,out auxiliary+0.65t.sub.ambient+0.13t.sub.return  (II); wherein a main circuit control method comprises steps of: collecting the ambient temperature t.sub.ambient, a CO.sub.2 main circuit outlet temperature t.sub.g,out main of the air-cooling-air-cooling recombiner (2) set by the user, and the water circuit outlet temperature t.sub.return of the air-cooling-air-cooling recombiner (2) set by the user; then calculating an optimal discharge pressure P.sub.CO2,main of the CO.sub.2 main circuit compressor (1) by a formula (III), and adjusting an opening degree of the CO.sub.2 main circuit expansion valve (4) to achieve a preset pressure; $\begin{array}{cc}{P}_{\mathrm{CO}2,\mathrm{main}}=\frac{50}{f_{\mathrm{compressor}1}}\left(0.00171t_{g,\mathrm{out}\mathrm{main}}^2-0.03t_{g,\mathrm{out}\mathrm{main}}-0.018t_{\mathrm{ambient}}-0.03\sqrt{t_{\mathrm{return}}}+7.38\right).& \left(\mathrm{III}\right)\end{array}$

2. The control method, as recited in claim 1, wherein in the air-cooling-air-cooling recombiner (2), the CO.sub.2 working fluid of the main circuit is cooled and releases heat in the CO.sub.2 main circuit (9), and the CO.sub.2 working fluid of the auxiliary circuit is cooled and releases heat in the CO.sub.2 auxiliary circuit (10); the circulating water absorbs heat to reach a preset temperature; in the supercooling-evaporation recombiner (3), the CO.sub.2 working fluid of the main circuit is further cooled and releases heat in the CO.sub.2 main circuit supercooling section (12), and the CO.sub.2 working fluid of the auxiliary circuit is evaporated and absorbs heat in the CO.sub.2 auxiliary circuit evaporation section (13); the CO.sub.2 auxiliary circuit evaporation section (13) exchanges heat with the CO.sub.2 main circuit supercooling section (12) to maintain heat balance.

3. The control method, as recited in claim 1, wherein the air-cooling-air-cooling recombiner (2) comprises three inner pipes and one outer pipe, wherein two of the inner pipes serve as the CO.sub.2 main circuit (9), and the other one of the inner pipes serves as the CO.sub.2 auxiliary circuit (10); a circuit between the outer pipe and the three inner pipes is the water circuit (11); the three inner pipes are arranged in an equilateral triangle form with identical pipe spacings D.sub.L and identical diameters D.sub.2; the outer pipe has a diameter D.sub.1; first ends of the two CO.sub.2 main circuits (9) are combined into one pipe connected to an exhaust port of the CO.sub.2 main circuit compressor (1) on an external side of the air-cooling-air-cooling recombiner (2), and second ends of the two CO.sub.2 main circuits (9) are combined into one pipe connected to the CO.sub.2 main circuit supercooling section (12) on the external side of the air-cooling-air-cooling recombiner (2); a relationship between the pipe spacings D.sub.L of the three inner pipes, the diameters D.sub.2 of the three inner pipes, and the diameter D.sub.1 of the outer pipe is:
D.sub.L=1.7D.sub.2  (IV)
D.sub.1/D.sub.2=3.7  (V).

4. The control method, as recited in claim 1, wherein the fan (8) is installed on the evaporator (5).

5. The control method, as recited in claim 1, wherein the CO.sub.2 auxiliary compressor (6) is an inverter compressor.

6. The control method, as recited in claim 1, wherein the transcritical carbon dioxide composite heat pump system operates in the cyclic heating mode when a water circuit outlet temperature of the air-cooling-air-cooling recombiner (2) set by the user is greater than or equal to 30° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a structural view of a transcritical carbon dioxide composite heat pump system of the present invention;

(2) FIG. 2 is a structural view of the transcritical carbon dioxide composite heat pump system in a direct heating according to the present invention;

(3) FIG. 3 is a schematic diagram of a cycle of the transcritical carbon dioxide composite heat pump system in the direct heating mode according to the present invention;

(4) FIG. 4 is a structural view of the transcritical carbon dioxide composite heat pump system in a cyclic heating mode according to the present invention;

(5) FIG. 5 is a schematic diagram of a cycle of the transcritical carbon dioxide composite heat pump system in the cyclic heating mode according to the present invention;

(6) FIG. 6 is a sketch view of internal piping arrangement of an air-cooling-air-cooling recombiner of the transcritical carbon dioxide composite heat pump system according to the present invention.

(7) Element reference: 1—CO.sub.2 main circuit compressor; 2—air-cooling-air-cooling recombiner; 3—supercooling-evaporation recombiner; 4—CO.sub.2 main circuit expansion valve; 5—evaporator; 6—CO.sub.2 auxiliary compressor; 7—CO.sub.2 auxiliary expansion valve; 8—fan; 9—CO.sub.2 main circuit; 10—CO.sub.2 auxiliary circuit; 11—water circuit; 12—CO.sub.2 main circuit supercooling section; 13—CO.sub.2 auxiliary circuit evaporation section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(8) Referring to the drawings, the present invention will be further illustrated bellow.

(9) Referring to FIG. 1, a transcritical carbon dioxide composite heat pump system of the present invention comprises: a CO.sub.2 main circuit compressor 1, an air-cooling-air-cooling recombiner 2, a supercooling-evaporation recombiner 3, an evaporator 5 and a CO.sub.2 auxiliary compressor 6.

(10) The air-cooling-air-cooling recombiner 2 comprises a CO.sub.2 main circuit 9, a CO.sub.2 auxiliary circuit 10 and a water circuit 11; the supercooling-evaporation recombiner 3 comprises a CO.sub.2 main circuit supercooling section 12 and a CO.sub.2 auxiliary circuit evaporation section 13.

(11) The transcritical carbon dioxide composite heat pump system comprises a main circuit and an auxiliary circuit;

(12) for the main circuit: an outlet of the CO.sub.2 main circuit compressor 1 is connected to an inlet of the CO.sub.2 main circuit 9 of the air-cooling-air-cooling recombiner 2; an outlet of the CO.sub.2 main circuit 9 of the air-cooling-air-cooling recombiner 2 is connected to an inlet of the CO.sub.2 main circuit supercooling section 12 of the supercooling-evaporation recombiner 3; an outlet of the CO.sub.2 main circuit supercooling section 12 of the subcooling-evaporation recombiner 3 is connected to an inlet of the evaporator 5; and an outlet of the evaporator 5 is connected to an inlet of the CO.sub.2 main circuit compressor 1;

(13) for the auxiliary circuit: an outlet of the CO.sub.2 auxiliary compressor 6 is connected to an inlet of the CO.sub.2 auxiliary circuit 10 of the air-cooling-air-cooling recombiner 2; an outlet of the CO.sub.2 auxiliary circuit 10 of the air-cooling-air-cooling recombiner 2 is connected to an inlet of the CO.sub.2 auxiliary circuit evaporation section 13 of the supercooling-evaporation recombiner 3; an outlet of the CO.sub.2 auxiliary circuit evaporation section 13 of the supercooling-evaporation recombiner 3 is connected to an inlet of the CO.sub.2 auxiliary compressor 6.

(14) A CO.sub.2 main circuit expansion valve 4 is arranged between the supercooling-evaporation recombiner 3 and the evaporator 5 on the main circuit; and a CO.sub.2 auxiliary expansion valve 7 is arranged between the air-cooling-air-cooling recombiner 2 and the supercooling-evaporation recombiner 3 on the auxiliary circuit.

(15) A fan 8 is installed on the evaporator 5. By changing the speed of the fan, the proper heat transfer coefficient can be adjusted.

(16) The CO.sub.2 auxiliary compressor 6 is an inverter compressor.

(17) Referring to FIG. 6, the air-cooling-air-cooling recombiner 2 comprises three inner pipes and one outer pipe, wherein two of the inner pipes serve as the CO.sub.2 main circuit 9, and the other one of the inner pipes serves as the CO.sub.2 auxiliary circuit 10; a circuit between the outer pipe and the three inner pipes is the water circuit 11; the three inner pipes are arranged in an equilateral triangle form with identical pipe spacings D.sub.L and identical diameters D.sub.2; the outer pipe has a diameter D.sub.1; first ends of the two CO.sub.2 main circuits 9 are combined into one pipe connected to an exhaust port of the CO.sub.2 main circuit compressor 1 on an external side of the air-cooling-air-cooling recombiner 2, and second ends of the two CO.sub.2 main circuits 9 are combined into one pipe connected to the CO.sub.2 main circuit supercooling section 12 on the external side of the air-cooling-air-cooling recombiner 2.

(18) In order to ensure proper cooling temperature of the CO.sub.2 when the return water temperature is changed, thereby ensuring high system performance, the present invention sets two operating modes:

(19) Direct heating mode (return water temperature is 30° C. or lower): referring to FIGS. 2 and 3, starting the CO.sub.2 main circuit compressor 1 and turning on the main circuit; shutting down the CO.sub.2 auxiliary compressor 6 and turning off the auxiliary circuit, and keeping the fan 8 at a working state; for the main circuit: compressing a CO.sub.2 working fluid from a state point a through the CO.sub.2 main circuit compressor 1 to reach a state point b, and moving the CO.sub.2 working fluid into the CO.sub.2 main circuit 9 of the air-cooling-air-cooling recombiner 2 for heating circulating water in the water circuit 11; self-cooling the CO.sub.2 working fluid to an appropriate temperature to reach a state point c, then passing the CO.sub.2 working fluid through the CO.sub.2 main circuit supercooling section 12 of the supercooling-evaporation recombiner 3 and maintaining the state point c; then moving the CO.sub.2 working fluid into the CO.sub.2 main circuit expansion valve 4 to be expanded, in such a manner that an expanded low-pressure working fluid reaches a state point d; moving the expanded low-pressure working fluid into the evaporator 5 for evaporation and absorbing heat, so as to return to the state point a, and finally returning to the inlet of the CO.sub.2 main circuit compressor 1.

(20) Cyclic heating mode (return water temperature is above 30° C.): referring to FIGS. 4 and 5, starting the CO.sub.2 main circuit compressor 1 and turning on the main circuit; starting the CO.sub.2 auxiliary compressor 6 and turning on the auxiliary circuit, and keeping a fan 8 at a working state; for the main circuit: compressing a CO.sub.2 working fluid from a state point a through the CO.sub.2 main circuit compressor 1 to reach a state point b, and moving the CO.sub.2 working fluid into the CO.sub.2 main circuit 9 of the air-cooling-air-cooling recombiner 2 for heating circulating water in the water circuit 11; self-cooling the CO.sub.2 working fluid to an appropriate temperature to reach a state point c, then passing the CO.sub.2 working fluid through the CO.sub.2 main circuit supercooling section 12 of the supercooling-evaporation recombiner 3 to exchange heat with the CO.sub.2 auxiliary circuit evaporation section 13, so as to further cool down to reach a state point d; then moving the CO.sub.2 working fluid into the CO.sub.2 main circuit expansion valve 4 to be expanded, in such a manner that an expanded low-pressure working fluid reaches a state point e; moving the expanded low-pressure working fluid into the evaporator 5 for evaporation and absorbing heat, so as to return to the state point a, and finally returning to the inlet of the CO.sub.2 main circuit compressor 1;

(21) for the auxiliary circuit: compressing the CO.sub.2 working fluid from a state point f through the CO.sub.2 auxiliary compressor 6 to reach a state point g, and moving the CO.sub.2 working fluid into the CO.sub.2 auxiliary circuit 10 of the air-cooling-air-cooling recombiner 2 for heating the circulating water in the water circuit 11; self-cooling the CO.sub.2 working fluid to reach a state point h, then moving the CO.sub.2 working fluid into the CO.sub.2 auxiliary circuit expansion valve 7 to be expanded to reach a state point i; moving the expanded low-pressure working fluid into the CO.sub.2 auxiliary circuit evaporation section 13 of the supercooling-evaporation recombiner 3 for exchanging heat with and further cooling the CO.sub.2 main circuit supercooling section 12; self-evaporating the expanded low-pressure working fluid to absorb heat and reach the state point f, and finally returning to the inlet of the CO.sub.2 auxiliary compressor 6.

(22) In the air-cooling-air-cooling recombiner 2, the CO.sub.2 working fluid of the main circuit is cooled and releases heat in the CO.sub.2 main circuit 9, and the CO.sub.2 working fluid of the auxiliary circuit is cooled and releases heat in the CO.sub.2 auxiliary circuit 10; the circulating water absorbs heat to reach a preset temperature; in the supercooling-evaporation recombiner 3, the CO.sub.2 working fluid of the main circuit is further cooled and releases heat in the CO.sub.2 main circuit supercooling section 12, and the CO.sub.2 working fluid of the auxiliary circuit is evaporated and absorbs heat in the CO.sub.2 auxiliary circuit evaporation section 13; the CO.sub.2 auxiliary circuit evaporation section 13 exchanges heat with the CO.sub.2 main circuit supercooling section 12 to maintain heat balance.

(23) Auxiliary circuit control: when the return water temperature is high (greater than or equal to 30° C.), the exhaust pressure of the CO.sub.2 auxiliary circuit is higher, the heat generation is larger, and the operation effect is better. From the perspective of the control principle, a fitting formula of the CO.sub.2 auxiliary circuit exhaust pressure is proposed for controlling the exhaust pressure of the compressor 6, so as to ensure the high performance of the entire system. An auxiliary circuit control method comprises steps of: with given t.sub.g,out auxiliary (a CO.sub.2 auxiliary circuit outlet temperature of the air-cooling-air-cooling recombiner 2), t.sub.ambient (an ambient temperature), and t.sub.return (a water circuit outlet temperature of the air-cooling-air-cooling recombiner 2), calculating P.sub.co.sub.2.sub.,auxiliary (an exhaust pressure of the CO.sub.2 auxiliary compressor 6) by a formula, and adjusting an opening degree of the CO.sub.2 auxiliary expansion valve 7 to achieve a preset pressure, so as to ensure the high performance of the entire system. The present invention proposes an adaptive frequency control formula for the CO.sub.2 auxiliary compressor 6 when the working conditions change. The temperature at d in the cycle needs to be kept at a suitable value. In order to balance the power consumption of the CO.sub.2 auxiliary compressor 6, the compressor frequency can be varied with the working conditions to keep the system operating efficiently, and the following fitting formula is proposed. f.sub.compress 6 (a motor frequency of the CO.sub.2 auxiliary compressor 6) can be calculated by the formula, in such a manner that a compressor frequency changes with a working condition to ensure the high performance of the entire system.
P.sub.co.sub.2.sub.,auxiliary=0.0036t.sub.g,out auxiliary.sup.2−0.02t.sub.g,out auxiliary−0.035t.sub.ambient+0.067t.sub.return+7.38  (I);
f.sub.compress 6=50−0.005t.sub.g,out auxiliary.sup.2+0.17t.sub.g,out auxiliary+0.65t.sub.ambient+0.13t.sub.return  (II);

(24) A main circuit control method comprises steps of: with given t.sub.g,out main (a CO.sub.2 main circuit outlet temperature of the air-cooling-air-cooling recombiner 2), t.sub.ambient (an ambient temperature), and t.sub.return (a water circuit outlet temperature of the air-cooling-air-cooling recombiner 2), calculating P.sub.CO2,main (an optimal discharge pressure of the CO.sub.2 main circuit compressor 1) by a formula, and adjusting an opening degree of the CO.sub.2 main circuit expansion valve 4 to achieve a preset pressure, so to ensure the high performance of the entire system.

(25) $\begin{array}{cc}{P}_{\mathrm{CO}2,\mathrm{main}}=\frac{50}{f_{\mathrm{compressor}1}}\left(0.00171t_{g,\mathrm{out}\mathrm{main}}^2-0.03t_{g,\mathrm{out}\mathrm{main}}-0.018t_{\mathrm{ambient}}-0.03\sqrt{t_{\mathrm{return}}}+7.38\right).& \left(\mathrm{III}\right)\end{array}$

(26) In order to ensure sufficient heat exchange between CO.sub.2 and water, the present invention creatively proposes the optimal structural parameters of the air-cooling-air-cooling recombiner 2: when selecting the air-cooling-air-cooling recombiner 2, D.sub.L: a center distance between the inner tubes can be calculated based on the parameter D.sub.1: a diameter of the outer tube and a formula, so as to select the air-cooling-air-cooling recombiner 2 that is most suitable for the system.
D.sub.L=1.7D.sub.2  (IV)
D.sub.1/D.sub.2=3.7  (V).