Air-to-water heat pump system with a defrost unit and method of optimizing air-to-water heat pump operation

Description of the invention

The subject of the invention is an air-to-water heat pump with a defrost unit and a method for optimizing the operation of an air-to-water heat pump, especially the defrosting process.

In air heat pumps, due to the low temperature of the refrigerant in the evaporator, a layer of ice forms on the exchanger. Heat exchanger frosting usually occurs when the outside air temperature is below 7°C. Due to the high humidity of the outside air, water vapor condenses on the cold surface of the evaporator and freezes. The accumulation of the ice layer restricts the air flow and causes the heat exchanger to lose its ability to exchange heat with the outside air. This leads to a significant decrease in the efficiency of the evaporator, and thus a decrease in the efficiency of the entire heat pump system and its COP (Coefficient of Performance). Due to the need to periodically remove the ice layer, the heat pump evaporator cannot run continuously. The number of defrost cycles depends on the actual outdoor air conditions.

Known from the state of the art is a solution in which a medium with a higher thermodynamic potential is directed through the evaporator, the system is equipped with a separate circuit containing a heat source to supply the evaporator. During the defrost process, the system is inactive and the condenser does not supply heat.

Known from the state of the art is a heat pump system that uses reverse operation of the evaporator-condenser to defrost the exchanger. The system is equipped with a 4-way valve installed on the inlet and outlet lines of the compressor. To defrost the evaporator, the flow direction of the refrigerant in the circuit is reversed. Changing the position of the 4-way valve causes the evaporator to become the condenser. In the process of defrosting the evaporator, the refrigerant transfers its heat to the defrosted evaporator, which takes on the function of the condenser. There are interruptions in heat production during the defrost process. The heat pump, during the periods of ice removal from the evaporator surface, operates in the cooling mode, using the energy taken from the serviced system.

A heat pump system is known that uses an additional exchanger for reversible operation to defrost the evaporator, where the circulation of the refrigerant is controlled by means of 2-way valves. The patent description EP 2 516 942 B1 presents a system containing a solution for connecting an additional heat exchanger in series between the condenser and the evaporator. The additional coil operates in defrost mode reversing as an evaporator or as a condenser, depending on the position of the 2-way valves. From the description of patent EP 0 128 108 B1, a solution is known in which the system includes many exchangers connected in parallel to the circuit. The refrigerant flow configuration is changed using 2-way valves. The system includes, in order to defrost the exchanger, at least two reversible external exchangers working alternately as an evaporator or as a condenser, where the circuit of the indoor unit is cut off for the time of defrosting.

There is known a heat pump system that uses serial inclusion of two or more evaporators in the system and their alternating operation to perform the process of defrosting the exchanger. EP 1 598 611 discloses a heat pump system comprising two evaporator rows operating alternately in defrost mode, where the system performs the process using one or two 4-way valves and two expansion valves. EP 2 447 096 A1 discloses a heat pump system equipped for the defrost process with two evaporators operating alternately as a subcooler/evaporator and two expansion valves and a 4-way valve. Patent PL 209 839 B1 discloses a water heat pump comprising, a condenser, a refrigerant, one expansion element and a 4-way valve for alternately reversing refrigerant circulation in defrost mode. Patent application P.430903 discloses a heat pump comprising a lower and an upper heat source and one expansion element, where the lower heat source includes a reversible evaporator and refrigerant working alternately to perform the defrosting process, where the refrigerant circuit is changed by a 4-way valve.

An important problem not addressed in the previous solutions is the lack of drying of the evaporator after the defrosting process, which causes the heat exchange surface of the evaporator to become frosted again in a short time. The purpose of the solution according to the invention is to implement the process of defrosting and drying the heat exchange surface of the evaporator, as well as to improve the energy efficiency of the heat pump system.

The air-water heat pump system according to the invention includes a heat source unit and a heat source unit connected in a thermodynamic cycle, where the heat source unit is supplied with external air, and the heat source has at least two alternately operating evaporators forming a heat source from a fan mounted axially to the unit in its upper part, equipped with a defrost unit. This assembly is characterized by the fact that at least two alternately operating evaporators are connected to a valve assembly consisting of two pairs of automatic three-way valves, respectively for each evaporator of the lower heat source assembly equipped with a shutter. Evaporators are connected in series in a closed system with a condenser. The discharge port of the first evaporator is connected through the port of the second three-way valve to the suction side of the compressor, connected on the discharge side to the condenser, the discharge port of which is connected through the by-pass of the third three-way valve to the inlet of the second evaporator, wherein the outlet of the second evaporator is connected via a by-pass of the fourth three-way valve to the expansion valve and through the port of the first three-way valve is connected to the inlet of the first evaporator. The discharge port of the second evaporator is connected through the by-pass of the second three-way valve with the suction side of the compressor, connected on the discharge side with the condenser, the discharge port of which is connected through the port of the third three-way valve with the inlet of the first evaporator, where the outlet of the first evaporator is connected via the port the fourth three-way valve with the expansion valve and through the by-pass of the first three-way valve connects to the inlet of the second evaporator. Said shutter is formed on the brine unit to shield the frosted evaporator going into defrost mode.

Preferably, said valve assembly is provided with actuators connected to the controller for automatically reversing the circulation of the refrigerant and changing the function of the evaporators.

Preferably, the shutter is connected to a controller to automatically direct the shutter to the evaporator defrost shield position.

Preferably, the shutter is automatically directed to the defrosted evaporator's cover position in a rotary motion or in a reciprocating motion in the horizontal or vertical direction, or by closing the single or multi-blade dampers.

Preferably, the evaporators forming the lower heat source are finned or segmented bent exchangers shaped as an open or closed polygon, including a regular or irregular polygon, or as a plane, or as a circle, semicircle or other irregular shape.

Preferably, the shutter is in the form of concentric semicircles mounted on both sides of the evaporator, coaxially with respect to the fan of the heat source unit.

Preferably, the shutter is in the form of a movable element shaped as a jacket or a damper, adapted to change its position by sliding means, mounted to the frame structure on both sides of the evaporator, the shutter having a shape corresponding to the shape of the evaporator.

Preferably, the shutter consists of an inner and an outer part which are thermally insulated.

Preferably, both parts of the shutter are equipped with a brush element located along its height, preventing the infiltration of outside air into the space between the defrosting evaporator and the shutter, while maintaining free movement of the shutter.

Preferably, the heat pump circuit heats the water or glycol solution in the upper source.

The method of optimizing the operation of the air-to-water heat pump, which uses alternating operation of at least two evaporators, is characterized by the fact that the defrosting process of the second evaporator is carried out by directing the refrigerant from the first evaporator through the port of the second three-way valve to the compressor, then the refrigerant is directed to the condenser, then the condensed refrigerant is directed through the by-pass of the third three-way valve to the evaporator, then the subcooled refrigerant is directed through the by-pass of the fourth three-way valve to the expansion valve, then the depressurized refrigerant is directed through the port of the first three-way valve to the first evaporator, at the same time the shutter is directed to the second evaporator shield position. The defrost process of the first evaporator is carried out by directing the refrigerant from the second evaporator through the by-pass of the second three-way valve to the compressor, then the refrigerant is directed to the condenser, then the condensed refrigerant is directed through the port of the third three-way valve to the first evaporator, then the subcooled refrigerant is directed through the port of the fourth three-way valve to the expansion valve, then the expanded medium is directed through the by-pass of the first three-way valve to the second evaporator, at the same time the shutter is directed to the position of shielding the first evaporator to limit the heat exchange between the external air and the shielded first evaporator. The fan operating continuously forces the flow of external air flowing around the bare evaporator. The process of defrosting one of the mentioned evaporators is carried out until the end of the exchanger defrosting cycle, where the controller, by switching the three-way valves operation mode, generates a new change in the direction of the refrigerant circulation, and thus a change in the function of the evaporators and a change in the position of the shutter to protect the defrosted evaporator.

Thanks to the solution according to the invention, it is possible to perform the defrosting process of the air heat pump evaporator, while maintaining high energy efficiency of the system. The reduction of electricity consumption for the defrosting process results from the mutual interaction of system elements and optimization of its operation. The solution according to the invention enables the defrosting process to be carried out with complete drying of the heat exchange surface of the evaporator. The use of a diaphragm ensures the process of sublimation or ice melting, heating of the condensate and its subsequent evaporation. Thanks to the solution according to the invention, the number of defrost cycles is reduced and the time between them is extended. The non-dried surface of the heat exchanger becomes frosted again in a shorter time due to the presence of crystallization nuclei in the form of moisture droplets and impurities suspended in it. The solution according to the invention ensures drying of the heat exchange surface, which, especially in conditions of sub-zero temperatures, is extremely important for extending the time of the exchanger's activity.

The proposed solution allows for continuous operation of the heat pump in outdoor conditions with the formation of an ice layer on the surface of the heat exchanger. The alternating operation of the lower heat source exchangers, provided by the automatically controlled set of valves and shutters according to the invention, guarantees uniform operation of the heat pump system and constant supply of heat energy to the upper source. The high stability and reliability of the system allows to keep the output temperature parameters required by the receiving system. No additional electrical power is required to defrost the evaporator. The defrosting unit in the solution according to the invention allows for recovery of energy from the exchanger defrosting process to subcool the liquid medium.

The effect of the above-mentioned set of technical features of the invention is a high year-round balance of thermal power given off by the condenser, with simultaneous lower consumption of electricity, and thus high energy efficiency of the system.

The proposed invention includes an optimized brine defrost unit with a simplified design. In the solution according to the invention, the use of a shutter reduces the number of fans, and therefore the heat source system includes one common fan for servicing two heat exchangers. In addition, no additional expansion valves or heating circuits are used to implement the defrost process according to the invention. According to one of the preferred embodiments, the cylindrical shape of the brine system according to the invention makes it possible to scale its size by increasing the diameter of the cylinder made of the evaporator assembly and the fan, allowing for proportional adjustment of the system components to obtain the desired efficiency while maintaining the optimization of its construction.

The subject of the invention is presented in an embodiment in the drawing, in which Fig. 1 shows a schematic diagram of the heat pump system for the configuration in which the evaporator 2 is defrosted and the evaporator 1 is active; Fig. 2 is a schematic plan view of the heat pump system according to embodiment 2; Fig. 3 shows a simplified section through the upper part of the heat pump system according to embodiment 3, Fig. 4 a simplified side view of the heat pump according to embodiment 3 is shown; Fig. 5 shows a temperature-entropy diagram of a refrigeration circuit implemented by a heat pump for a preferred embodiment of the system and method in accordance with example 4; in fig. 6 the course of changes in the temperature of the liquid medium in the defrosting process, the temperature of the heat exchange surface and the amount of exchanged heat, carried out by the heat pump, depending on the temperature and time, for the preferred variant of the system and method according to example 4, is presented; Fig. 7 shows a diagram of the heating power of the device over 1 hour for the alternating system without a shutter, the reverse system and the system with a shutter according to the invention, respectively.

Example 1

The air-water heat pump system according to the invention includes a lower heat source unit and an upper heat source unit connected in a thermodynamic cycle, where the lower heat source unit is supplied with external air, and the lower heat source has at least two alternating evaporators 1, 2 forming the lower a heat source with a fan 10 mounted axially in relation to the brine unit in its upper part, provided with a defrost unit. Alternately operating evaporators 1, 2 are connected to a valve assembly consisting of two pairs of automatic three-way valves 3, 4 and 5, 6 for each evaporator 1 and 2 of the lower heat source unit equipped with a shutter 11, respectively. Evaporators 1, 2 are connected in series in closed system with condenser 9. The outlet port of the first evaporator 1 is connected through the port of the second three-way valve 4 with the suction side of the compressor 8, connected on the discharge side with the condenser 9, whose outlet port is connected through the by-pass of the third three-way valve 5 with the inlet of the second evaporator 2, where the outlet from the second evaporator 2 is connected through the by-pass of the fourth three-way valve 6 to the expansion valve 7 and through the port of the first three-way valve 3 it connects to the inlet of the first evaporator 1. The outlet port of the second evaporator 2 is connected through the by-pass of the second three-way valve 4 with the suction side of the compressor 8, connected on the discharge side with the condenser 9, whose outlet port is connected through the port of the third three-way valve 5 with the inlet of the first evaporator 1, where the outlet from the first evaporator 1 is connected through the port of the fourth 3-way valve 6 with the expansion valve 7 and through the by-pass of the first 3-way valve 3 connects to the inlet of the second evaporator 2. The diaphragm 11 is formed on the heat source unit to cover the frosted evaporator 1 or 2 going into defrost mode. The valve assembly is equipped with actuators connected to the controller 12 to automatically reverse the refrigerant circuit and change the function of evaporators 1 and 2. Diffuser 11 is connected to the controller 12 to automatically move the damper to the defrosting position of evaporator 1 or 2. Evaporators 1 and 2 forming the lower heat source are bent, ribbed or segment heat exchangers. The shutter 11 consists of an inner and an outer part which are thermally insulated. The heat pump circuit heats the water or glycol solution in the upper source.

The method of optimizing the operation of the air-to-water heat pump, which uses alternating operation of at least two evaporators and the process of defrosting the second evaporator 2, is carried out by directing the refrigerant from the first evaporator 1 through the passage port of the second three-way valve 4 to the compressor 8, then the refrigerant is directed to the condenser 9, then the condensed medium is directed through the by-pass of the third three-way valve 5 to the second evaporator 2, then the subcooled medium is directed through the by-pass of the fourth three-way valve 6 to the expansion valve 7, then the depressurized medium is directed through the port of the first three-way valve 3 to the first evaporator 1, at the same time the shutter 11 is directed to the position of protecting the second evaporator 2. The defrost process of the first evaporator 1 is carried out by directing the refrigerant from the second evaporator 2 through the by-pass of the second 3-way valve 4 to the compressor 8, then the refrigerant is directed to the condenser 9, then the condensed refrigerant is directed through the port of the third shunt 5 to the first evaporator 1, then the subcooled refrigerant is directed through the port of the fourth three-way valve 6 to the expansion valve 7, then the expanded medium is directed through the by-pass of the first three-way valve 3 to the second evaporator 2, at the same time the shutter 11 is directed to the position of covering the first evaporator 1 to limit the heat exchange between the external air and the covered first evaporator 1. The fan 10 working continuously forces the flow outside air flowing over bare evaporator 1 or 2. The process of defrosting the evaporator 1 or 2 is carried out until the end of the defrosting cycle of the exchanger, where, through the controller 12, by switching the operation mode of the three-way valves, a new change in the direction of the medium circulation is generated, and thus a change in the function of the evaporators 1, 2 and the change of the position of the shutter 11 to defrost evaporator 1 or 2.

Example 2

The heat pump system and the method of optimizing the operation of the air-water heat pump as in example 1, with the following:

The alternating evaporators 1, 2 are shaped as semi-circles forming a lower heat source with a cylindrical shape of the unit with a fan 10 mounted axially in relation to the lower heat source unit in its upper part. The shutter 11 is in the form of concentric semi-circles mounted on both sides of the evaporator coaxially with the fan 10 of the brine unit. The parts of the damper 11 are equipped with a brush element 13 located along its height, preventing the infiltration of outside air into the space between the defrosting evaporator and the damper, while maintaining the free movement of the damper 11. , (fig. 2) Example 3

The heat pump system and the method of optimizing the operation of the air-water heat pump as in example 1, with the following:

The alternating evaporators 1, 2 are shaped as closed polygons forming a bottom heat source of a cuboidal shape with a fan 10 mounted axially in relation to the bottom heat source unit in its upper part. Evaporator 1 is placed in the upper part of the brine unit and evaporator 2 in its lower part. The shutter 11 is in the form of a movable element in the form of a shell, adapted to change its position by sliding means, mounted to the frame structure 14 on both sides of the evaporator 1, 2, the shutter 11 having a shape corresponding to the shape of the evaporator 1, 2. the position of the defrosted evaporator 1 or 2 cover is performed in a reciprocating motion in the vertical direction, (fig. 3 and fig. 4)

Example 4

The heat pump system and the method of optimizing the operation of the air-water heat pump as in example 1, with the following:

The heat pump circuit uses the natural refrigerant propane R290, assuming that evaporator 2 is defrosting and evaporator 1 is active. The compressor 8 pumps the working medium (process 1-2 in fig. 5) in the amount of 300 kg/h, which in the condenser 9 being the upper heat source gives power to the heating system equal to 37.9 kW (process 2-3 in fig. 5). The condensed working medium at a temperature of 47°C is directed to the evaporator 2, where it is subcooled to a temperature of 5°C (process 3-4), which increases the specific cooling capacity by 115 kJ/kg, while bringing the heating power of 9.5 kW to defrosted evaporator 2, which ensures sublimation of the ice layer. After subcooling, the refrigerant is throttled in the expansion valve (process 4-5) and then directed to the evaporator where it evaporates by taking heat from the environment (process 5-1). In a preferred embodiment, a liquid refrigerant at a temperature of 47°C is directed to the evaporator 2, which is covered by a diaphragm 11. Due to the lack of heat exchange with the environment, which is achieved thanks to the use of the said diaphragm 11, the heat of subcooling the medium is gradually transferred to the thawing evaporator 2, and then to the melting of the ice layer. At a low external temperature of 0°C and a high relative air humidity of 100%, a layer of frost of 17.2 kg is deposited on the surface of one of the evaporators 1, 2 within 1 hour. In a preferred embodiment, said evaporator 1,2 of the system according to the invention has a mass of the finned part equal to 48 kg, of which copper constitutes 22 kg and aluminum 26 kg. According to the calculations above, and due to the fact that the specific heat of copper is 0.38 kJ/(kg*K) and that of aluminum is 0.9 kJ/(kg/*K), the amount of heat needed to heat the heat exchanger to end of defrost, equal to 5°C, is 290 kJ. At the same time, the amount of heat needed to melt the entire layer of frost, taking into account the heat of fusion at the level of 333 kJ/kg, will be 5700 kJ. The mass of melted frost, equal to 57%, flows from the surface of the heat exchanger and flows out of the system. The remaining 43% mass of melted frost in the form of a 100 μm thick layer of moisture remains on the surface of the evaporator 1,2. The partial pressure of saturated water vapor at 0°C, i.e. the frost melting temperature, is 612 Pa. Moisture exchange takes place according to the partial pressure difference between the layer of water wetting the heat exchange surface and the surrounding air. Effective drying of the heat exchange surface requires the partial pressure of water vapor on the surface of the heat exchanger to be higher than the water vapor pressure in the ambient air. Accordingly, the surface of the evaporator 2 is heated to a temperature of at least 5°C, at which the water vapor pressure on the surface of said exchanger is 873 Pa. The resulting partial pressure difference of at least 260 Pa allows the surface of the evaporator 2 to be dried by evaporating the moisture remaining on its surface.

The process of changing the temperature of the liquid medium in the process of defrosting the evaporator 2, according to Fig. 6, is as follows: The liquid working medium with a temperature of 47°C flowing from the condenser 9 heats the heat exchanger, transferring the amount of heat equal to 130 kJ in 115 s, which causes heating exchanger from -4°C to 0°C, which allows to start the process of melting the frost layer, while reducing the temperature of the working medium to 42°C. The process of melting the frost layer ends after 780 s after the working medium transfers 5700 kJ of heat, which causes the temperature of the medium to be lowered to 34°C. Part of the melted frost flows down as condensate, while the remainder, equal to 57%, wets the heat exchanger surfaces. This layer is evaporated due to the supply of another 18070 kJ of heat in 2470 s, which at the same time reduces the temperature of the liquid medium to 10°C. The defrost cycle ends after the refrigerant delivers another 160 kJ of heat in 230 seconds, which results in heating the heat exchange surface to 5°C, ensuring drying of the heat exchanger surface and preventing secondary deposition of moisture. The total defrost time is 3600 s. Fig. 5 shows the beneficial effect of the invention in terms of increasing the heat pump's heat output. The heat flux transferred by the device to the heating system in relation to a unit of time is equal to the area under the graph marked with diagonal hatching, i.e. the area designated by points 2-3-a-5-bd. This field is significantly larger than the field marked with a vertical hatch, determined by points 2-3-a-3*-cd, which is a graphical representation of the heating power of heat pumps without defrosting for subcooling of the liquid medium, i.e. heat pumps with a defrost system known from the state of the art. It should be emphasized that the increase in the heating power of the heat pump according to the invention achieved by using the shutter does not increase the work required for the circulation, described as the surface area determined by points 1-2-3-a.

Fig. 7 illustrates another advantageous effect of the invention. As shown in the diagram, the heat pump according to the invention transmits a constant heating power to the receiving system. Unlike the reverse system and the system without a cover, in which there is a periodic reduction or even a negative heat flux transferred to the heating system.

The analysis of the technical parameters of the exemplary variant 4 shows that the air-to-water heat pump system according to the invention with the defrost unit enables energy-efficient implementation of the defrost process. The shutter according to the invention enhances the defrosting effect, reducing the amount of energy and time required for the process. The use of the diaphragm according to the invention allows for absolute drying of the defrosted exchanger by maintaining a higher temperature of the exchanger surface, and thus ensures a longer time of its operation in the active mode.

Reference list first evaporator second evaporator first 3-way valve 2nd 3-way valve third 3-way valve fourth 3-way valve expansion valve compressor condenser fan shutter driver brush element frame construction