An air-water heat pump arrangement and a method for defrosting of an air-water heat pump arrangement

Abstract

The invention is an air-water heat pump arrangement (200) for heating water of a heating system. The air-water pump arrangement has a heating cycle and a defrosting cycle. The arrangement comprises a main circuit (206) comprising a main circuit evaporator (208), a main circuit compressor (209), a main circuit condenser (210), and a main circuit expansion valve (211 ). The main circuit further comprises a first bypass pipe (218), a second bypass pipe (219), a first three-way valve (220), and a second three-way valve (221 ). The bypass pipes and the three-way valve are positioned in such a way that refrigerant flows through the main circuit compressor same direction both in the heating cycle and the defrosting cycle.

Description

[0001] An air-water heat pump arrangement and a method for defrosting of an airwater heat pump arrangement

[0002] The invention relates to an air-water heat pump arrangement for heating water of a heating system, and the air-water heat pump arrangement comprises a main circuit, and the main circuit comprises a main circuit evaporator, a main circuit compressor, a main circuit condenser, a main circuit expansion valve, and a main circuit pipe system for transferring a refrigerant, and the air-water heat pump arrangement comprises a heating cycle and a defrosting cycle, and in the heating cycle the refrigerant is configured to flow from the main circuit evaporator, through the main circuit compressor, the main circuit condenser and main circuit expansion valve back to the main circuit evaporator. The invention also relates to a method for defrosting an airwater heat pump arrangement.

[0003] BACKGROUND

[0004] Air-water heat pumps (or air source heat pumps) are commonly used for to provide cooling and heating in buildings. These heat pumps absorb heat from the outside air and then transfer it via central heating system to provide hot water heating. An outdoor unit (an evaporator) takes in heat from the ambient air and transfers it to a refrigerant. The heated refrigerant is used for warming water in a heating system of a building.

[0005] Generally, a refrigerant-circulating cycle, when an air-water heat pump is used for heating (a heating cycle), comprises a closed loop formed by a compressor adapted to compress a refrigerant to high temperature and high pressure, a condenser adapted to condense the high temperature and high pressure refrigerant discharged from the compressor to a liquid phase, and expansion valves adapted to expand the liquid-phase refrigerant discharged from the condenser to a low pressure by means of a throttling action. Further, the refrigerant-circulating cycle comprises an evaporator adapted to evaporate the throttled refrigerant to a gaseous phase by means of the heat absorption of the outdoor air (or similar). The air-water heat pump can be used for cooling when the refrigerant-circulating cycle is reversely operated.

[0006] The evaporator or an air source for the evaporator is placed outside the building, where it can access the outside air. The evaporator comprises a system, for example a fan or similar, to draw in air from outside and pass it over an evaporator coil containing the refrigerant. The refrigerant absorbs heat from the air and evaporates, turning into a gas. The temperature of the refrigerant must be below outside (air) temperature. The gas is then compressed by the compressor, which raises its temperature even further. The heated gaseous refrigerant is then passed through the condenser, where it releases heat to the water in the heating system. As the gas cools down, it condenses back into a liquid. The liquid refrigerant is then passed through the expansion valve, which reduces its pressure and temperature, and the refrigerant returns to the evaporator to start the cycle again.

[0007] Figure 1 shows a simplified example of a conventional air-water heat pump 100. The air-water heat pump comprises an evaporator 101 , a compressor 101 , a condenser 102 and an expansion valve 104. Said components are connected in a loop (or a circuit) by a pipe system 105 that is configured to contain a refrigerant. Arrows are added in Figure 1 for indicating direction of the refrigerant flow when the airwater heat pump is used for a heating cycle. In this embodiment, the evaporator is placed outside B of a building A. The building comprises a heating system (not marked in Figure 1 ). The condenser is configured to transfer heat from the refrigerant to water in the heating system. It must be noted that the air-water heat pump can be configured in many ways. For example, the heat pump components are all outside the building and heated water is transferred to the heating system.

[0008] It must be noted that even refrigerators and freezers use corresponding type of technologies as air-water heat pumps for buildings, they are quite different. Air-water heat pumps can be used for cooling as well as heating. This means that the process works in reversible or dual-mode operation, i.e. the heat pump actively runs in reverse mode. Because cooling loads differ from heating loads, proper sizing of equipment is important. Also, amount of refrigerant is significantly bigger in the air-water heat pumps than refrigerators. Typical household refrigerators use below 100 g of refrigerant. In commercial kitchen equipment, such as refrigerator rooms, amounts are somewhat larger, but less than 500g, and with current refrigerants they are generally less than 150g. Small residential systems (5-10 kW heating capacity) air-water heat pumps use 2 to 3.5 kg of refrigerant, medium systems (10-20 kW) 3.5 to 6 kg, and larger systems (20+ kW) 6 kg and above. Industrial sized air-water heat pumps use several tens of kilograms. It is clear that that event the amount of the refrigerant causes effects in the air-water heat pumps that are not evident or even occur in small scale applications. These effects are, for example, inertia, resonance, friction and sounds.

[0009] The efficiency of the air-water heat pumps can be measured by the coefficient of performance (COP). A COP of 4 means the heat pump produces 4 units of heat energy for every 1 unit of electricity it consumes. Within temperature ranges of -3 °C (27 °F) to 10 °C (50 °F), the COP for many machines is fairly stable. In mild weather with an outside temperature of 10 °C (50 °F), the COP of efficient air source heat pumps ranges from 4 to 6. However, on a cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. The heat pump's performance is limited by the Carnot cycle and will approach 1 .0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach -18 °C (0 °F). Some refrigerants may allow a COP of greater than 2 even down to -20 °C, pushing the break-even figure downward to -30 °C (-22 °F). It is clear that the efficiency of the air-water heat pumps is very important for regions where outside temperatures can be low (i.e. the temperature difference between outside and inside is large). Even small improvements in the efficiency can produce significant effects.

[0010] Conventional air-water heat pumps lose their capacity as the external temperatures fall below -10 °C (14 °F). Some refrigerants may allow operating in temperatures as low as -30 °C (-22 °F), although they may not be as efficient in cooling during the summer season. Usually, if conventional air-water heat pumps are used in colder climates, the system needs an auxiliary source of heat to supplement the heat pump in the event of extremely cold temperatures or when it is simply too cold for the heat pump to work at all.

[0011] In addition to the dropping efficiency, the cold temperatures outside cause also other problems. The surface temperature of the exterior heat exchanger, i.e. the evaporator, during the heating operation in winter seasons is lower than a dew-point temperature of outdoor air. In such situations frost and ice is generated on the surface of the evaporator. Problems arise when frost is form on the evaporator heat exchanger coils or other parts of the evaporator. The frost reduces the heat transfer capability of the heat exchanger which leads to lower evaporation temperature and lower energy efficiency of the system. The frost should be removed from the system in several intervals during the operation. This causes several technical problems for heat pump operation, interrupt the thermal comfort, and jeopardizes the reliability of these systems. This lowers the evaporator’s ability to transfer heat and thus efficiency of the whole heat pump system. Thus, many systems have a defrost mode.

[0012] In some defrost modes, the hot refrigerant leaving the compressor is bypassed directly to the evaporator. The bypass can occur by reducing the removal of heat in the condenser or can be a bypass of some refrigerant around the condenser. This hot gas bypass defrosting operation has been presented in conventional practices. For example, patent publication US8006506 discloses a that kind of method using four-way valve. However, these bypass methods cause pauses and irregularities for functioning of the heat pump.

[0013] In some defrost modes, the heat pump systems have built-in electric heating elements installed on or near the evaporator coils. When defrosting is required, the system activates these heating elements. They generate heat to melt the frost or ice on the coils. Electric resistance defrosting is typically used as a backup or supplemental defrosting method. There are also methods where waste heat from the system (particularly heat generated by the compressor) is used for defrosting.

[0014] The air-water heat pump requires the evaporator containing air moving mechanical components such as fans which produce noise. Modem devices offer schedules for silent mode operation with reduced fan speed. This, of course, will reduce the maximum heating power but can be applied at mild outdoor temperatures without efficiency loss. Acoustic enclosures are another approach to reduce the noise. In insulated buildings, operation can be paused at night without significant temperature loss. Only at low temperatures, frost protection forces operation after a few hours. Another feature of air-water heat pumps evaporators is their need to stop the fan from time to time for a period of several minutes in order to get rid of frost that accumulates in the outdoor unit in the heating mode. After that, the heat pump starts to work again. This part of the work cycle results in two sudden changes of the noise made by the fan. The acoustic effect of such disruption is especially powerful in quiet environments where background night-time noise may be as low as 0 to 10dBA. Usually, noise reduction is traditionally implemented by improving casing of the evaporator or noise making parts of the evaporator. That kind of approach is presented in patent publication FR3142536.

[0015] For reversing the flow of the refrigerants in the air-water heat pumps four-way valves are commonly used at least since the 1950s. For example, patent publications US6698452 and US7152416 discloses such a system. The technology is regularly recommended. For example, Danfoss tells (https: / w w.danfoss.co / en / se ice- and-support / case-stories / dcs / components-for-heat-pumps-part-5-four-way-revers- ing-valves / ) that four-way reversing valves are used to completely reverse the cycle of one-to-one heat pump systems. Such valves may be used to facilitate using the system for both heating and cooling, or to provide an effective and energetically optimized defrosting method. Also, they state that four-way valves are components that offer an extremely efficient way of defrosting air-to-air and air-to-water heat pumps. Complete reversal of the cycle makes it possible to also use the system for cooling. The design is not at all complicated and follows from the dimensions of the suction line, with no need to worry about minimum pressure drops.

[0016] However, it is well known that the refrigerant flow reversing causes sounds in addition that was described earlier. This is tried to prevent using structures that isolate sound producing parts. For example, patent publications US11971192 and US10337775 disclose that kind of methods. However, they are only hiding the problem. Sound waves in the pipes of the air-water heat pumps cause pressure variations and resonances that could eventually damage the system.

[0017] It would be beneficial if an air-water heat pump efficiency can be improved. Also, it would be beneficial if the air-water heat pump could be effectively defrosted without detectable variations in the heating. Furthermore, noise reduction and control is important.

[0018] BRIEF DESCRIPTION

[0019] The object of the invention is a solution that can significantly reduce the disadvantages and drawbacks of the prior art. In particular, the object of the invention is a solution where an arrangement is provided that significantly improves efficiency and time allows fast defrosting process for an air-water heat pump and at the same time reduces sounds the refrigerant flow reversing causes.

[0020] The objects of the invention are attained with an arrangement that is characterised by what is stated in the independent patent claims. Some advantageous embodiments of the invention are disclosed in the dependent claims.

[0021] The invention is an air-water heat pump arrangement for heating water of a heating system. The air-water pump arrangement has a heating cycle and a defrosting cycle. The arrangement comprises a main circuit comprising a main circuit evaporator, a main circuit compressor, a main circuit condenser, and a main circuit expansion valve. The main circuit further comprises a first bypass pipe, a second bypass pipe, a first three-way valve, and a second three-way valve. The bypass pipes and the three-way valve are positioned in such a way that refrigerant flows through the main circuit compressor same direction both in the heating cycle and the defrosting cycle.

[0022] In one embodiment of the invention is an air-water heat pump arrangement for heating water of a heating system for a building. The air-water heat pump arrangement comprises a main circuit. The main circuit comprises a main circuit evaporator, a main circuit compressor, a main circuit condenser, a main circuit expansion valve, and a main circuit pipe system for transferring a refrigerant. The air-water heat pump arrangement comprises a heating cycle and a defrosting cycle. In the heating cycle the refrigerant is configured to flow from the main circuit evaporator, through the main circuit compressor, the main circuit condenser and main circuit expansion valve back to the main circuit evaporator. In one advantageous embodiment of the invention, the main circuit further comprises a first bypass pipe, a second bypass pipe, a first three-way valve, and a second three-way valve. The first three-way valve is on the main circuit pipe system between the main circuit evaporator and the main circuit compressor, and the second three-way valve is on the main circuit pipe system between the main circuit compressor and the main circuit condenser. One end of the first bypass pipe is connected to the main circuit pipe system between the main circuit evaporator and the first three-way valve, and another end of the first bypass pipe is connected to the second three-way valve, and one end of the second bypass pipe is connected to the first three-way valve and another end of the second bypass pipe is connected to the main circuit pipe system between the second three- way valve and the main circuit condenser. In the defrosting cycle the first three-way valve is in a position that allows the refrigerant flow from the second bypass pipe towards the main circuit compressor, and the second three-way valve is in a position that allows the refrigerant flow from the main circuit compressor to the first bypass pipe and towards the main circuit evaporator.

[0023] In one embodiment of the air-water heat pump arrangement, the amount of the refrigerant in the main circuit is 2 kg or more. This means that the invention is applicable small air-water heat pumps (5-10 kW heating capacity) and upwards.

[0024] In one embodiment of the air-water heat pump arrangement, the amount of the refrigerant in the main circuit is 3,5 kg or more. This means that the invention is applicable medium air-water heat pumps (10-20 kW heating capacity) and upwards. The inventors have found out that there is significant noise increase when the amount of the refrigerant exceeds this.

[0025] In one embodiment of the air-water heat pump arrangement, the first three-way valve and the second three-way valve are motorized valves. The motorized valves produce calmer flow changes than electrical (solenoid) valves, i.e. electrical valves are practically on-off valves. For noise reduction it is better to avoid these kind (on- off) valves.

[0026] In one embodiment of the air-water heat pump arrangement, diameters of holes of the first three-way valve and the second three-way valve are same size as diameters of the pipes that are adjacent to the three-way valves, i.e. the cross-sections through the three-way valves and surrounding pipes are constant. This minimizes the flow resistance through the opened three-way valve that improves efficiency.

[0027] In one embodiment of the air-water heat pump arrangement, the three-way valves are arranged reverse flow of the refrigerant gradually, and when the three-way valves are closed, the flow of the refrigerant in vicinity of the three-way valve is stopped or speed of the refrigerant flow is 5% or less of the normal speed of the refrigerant. This means that when the three-way valve opens, there is no significant pressure differences in the refrigerant in the pipes on both sides of the valve. This reduces noises and prevents possible shock waves in the pipes.

[0028] In one embodiment of the air-water heat pump arrangement, time at which the three- way valves change the flow direction (i.e. travel time of the valve from one extreme position to another) is 90 seconds or more.

[0029] In one embodiment of the air-water heat pump arrangement, time at which the three- way valves change the flow direction is 120 seconds or more. In one embodiment of the air-water heat pump arrangement, the amount of the refrigerant in the main circuit is 4 kg or more. The inventors have found out that gradual change of reversal of the refrigerant (i.e. gradual shifting of the three-way valve from one flow position to another flow position) improves significantly noise reductions and lessens pressure changes in the pipes and the heat pump components. These are quite evident effects especially when the amount of the refrigerant increases.

[0030] In one embodiment of the air-water heat pump arrangement, the pipes of the main circuit are arranged in such a way that flow length of the refrigerant between the three-way valves and the main circuit evaporator is less than flow length of the refrigerant between the three-way valves and the main circuit expansion valve. This feature improves the defrosting cycle.

[0031] In one embodiment, the flow length of the refrigerant between the three-way valves and the main circuit evaporator is less than 50% of the flow length of the refrigerant between the three-way valves and the main circuit expansion valve.

[0032] In one embodiment of the air-water heat pump arrangement, the pipes of the main circuit are arranged in such a way that the three-way valves are directly connected to the main circuit evaporator. This feature allows constructions that has been found improving efficiency of the air-water heat pump and especially the main circuit compressor. In one embodiment of the air-water heat pump arrangement, the main circuit compressor is between the three-way valves. This feature allows compact component placement that improves efficiency.

[0033] In one embodiment of the air-water heat pump arrangement, the three-way valves are directly connected to the main circuit compressor.

[0034] In one embodiment of the air-water heat pump arrangement, in the heating cycle the first three-way valve and the second three-way valve are in a position where the refrigerant does not flow through the first bypass pipe and the second bypass pipe, i.e. the first three-way valve is in a position that allows flow from the main circuit evaporator towards the main circuit compressor but not to the second bypass pipe, and the second three-way valve is in a position that allows flow from the main circuit compressor to the main circuit condenser but not to the first bypass pipe.

[0035] In one embodiment of the air-water heat pump arrangement, the second three-way valve is dimensioned for higher temperatures than the first three-way valve.

[0036] In one embodiment of the air-water heat pump arrangement, the second three-way valve is dimensioned for higher pressures than the first three-way valve.

[0037] In one embodiment of the air-water heat pump arrangement, the air-water heat pump arrangement further comprises a second circuit comprising a second circuit evaporator, a second circuit compressor, a second circuit condenser, a second circuit expansion valve, and a second circuit pipe system for transferring a second refrigerant, and the main circuit and the second circuit are connected in such a way that between the main circuit condenser and the main circuit expansion valve the main circuit pipe system is configured to go through the second circuit evaporator, and the second circuit evaporator is configured to receive heat from the refrigerant thus cooling the refrigerant before entering the main circuit expansion valve.

[0038] In one embodiment of the air-water heat pump arrangement, the refrigerant and the second refrigerant are same substance.

[0039] In one embodiment of the air-water heat pump arrangement, the heating system is configured to transfer water through both the main circuit condenser and the second circuit condenser. In one embodiment of the air-water heat pump arrangement, the heating system comprise parallel pipes for the main circuit condenser and the second circuit condenser.

[0040] In one embodiment of the air-water heat pump arrangement, maximum power of the second circuit compressor is less than maximum power of the main circuit compressor.

[0041] In one to be turned off when the capacity requirement of the air-water heat pump arrangement falls below a predetermined limit value.

[0042] In one embodiment of the air-water heat pump arrangement, if the defrosting cycle is longer than 30 seconds, the flow of the second refrigerant in the second circuit pipe system is stopped or significantly slowed.

[0043] In one embodiment of the air-water heat pump arrangement, if the defrosting cycle is longer than 1 minute, the second circuit is turned off.

[0044] In one embodiment of the air-water heat pump arrangement, the second circuit comprises only a heating cycle.

[0045] One embodiment of the invention is a method defrosting an air-water heat pump arrangement. In one advantageous embodiment of the method, an arrangement disclosed previously is used. The method comprises steps where in the defrosting cycle the first three-way valve is in a position that allows the refrigerant flow from the second bypass pipe towards the main circuit compressor and the second three- way valve is in a position that allows the refrigerant flow from the main circuit compressor to the first bypass pipe and towards the main circuit evaporator, and the refrigerant warmed by the main circuit compressor defrost the main circuit evaporator.

[0046] In one embodiment of the method, the air-water heat pump arrangement further comprises a second circuit comprising a second circuit evaporator, a second circuit compressor, a second circuit condenser, a second circuit expansion valve, and a second circuit pipe system for transferring a second refrigerant. The main circuit and the second circuit are connected in such a way that between the main circuit condenser and the main circuit expansion valve the main circuit pipe system is configured to go through the second circuit evaporator, and the second circuit evaporator is configured to receive heat from the refrigerant thus cooling the refrigerant before entering the main circuit expansion valve. The method further comprises step where, the refrigerant cooled by the defrosting main circuit evaporator and the main circuit expansion valve is warmed by the second refrigerant in the second circuit evaporator before entering the main circuit condenser.

[0047] In one embodiment, the method comprises a step where at least partly gaseous the second refrigerant is at least partly liquified in the second circuit condenser, and in this process water in the heating system is heated and the second refrigerant leaving the second circuit condenser is cooled. The method further comprises steps where at least partly liquid the second refrigerant is at least partly vaporized in the second circuit expansion valve, and the processes in the second circuit are dimensioned in such a way that the second refrigerant in the second circuit evaporator is colder than the refrigerant in the main circuit after the main circuit condenser, and where in the second circuit evaporator the second refrigerant cools the refrigerant.

[0048] In one embodiment of the method the method comprises a step where the refrigerant cooled by the defrosting main circuit evaporator and the main circuit expansion valve is warmed by the second refrigerant in the second circuit evaporator before entering the main circuit condenser.

[0049] It is an advantage of the invention that it provides an air-water heat pump arrangement that significantly improves the efficiency of the defrosting cycle. Furthermore, the invention decreases the noises of the heat pump, and especially noises caused by the main circuit evaporator, which is outside or connected to the outside air, and thus main source of noises especially in the defrosting cycles. Also, sounds caused by the refrigerant turbulences during flow reversing process are significantly reduced.

[0050] In some embodiments, when a second circuit is used, the efficiency of the air-water heat pump arrangement is improved and at the same time also defrosting cycles are also more efficient especially in short defrosting cycles (shorter that one minute).

[0051] One advantage of the invention is that it saves energy, i.e. the operating costs of the invention are significantly less than the operating costs of the known heat pump arrangements.

[0052] DESCRIPTIONS OF THE FIGURES In the following, the invention is described in detail. The description refers to the accompanying drawings, in which

[0053] Figure 1 shows a simplified example of a conventional air-water heat pump,

[0054] Figure 2a shows an example of a main circuit of an air-water heat pump in a heating cycle in accordance with an embodiment,

[0055] Figure 2b shows the main circuit of Figure 2a in a defrosting cycle,

[0056] Figure 3 shows a simplified a second example of an air-water heat pump arrangement in accordance with an embodiment comprising a main circuit and a second circuit, and

[0057] Figure 4 shows a third example of an air-water heat pump arrangement in accordance with an embodiment comprising a main circuit and a second circuit.

[0058] DETAILED DESCRIPTION

[0059] The embodiments in the following description are given as examples only and someone skilled in the art can carry out the invention also in some other way than was described in the description, the invention being defined by the appended claims. Though the description may refer to a certain embodiment or embodiments in several places, this does not mean that the reference would be directed towards only one described embodiment or that the described characteristic would be usable only in one described embodiment. The individual characteristics of two or more embodiments may be combined and new embodiments of the invention may thus be provided, within the scope of the invention as defined by the appended claims.

[0060] Figures 2a and 2b show an example of an air-water heat pump arrangement 200 according to an embodiment. Figure 2a shows the air-water heat pump arrangement in a heating cycle and Figure 2b the air-water heat pump arrangement in a defrosting cycle. Arrows in Figures indicate flowing directions of a refrigerant.

[0061] The heat pump arrangement 200 in Figure 2a and 2b comprises a main circuit 206. The arrangement in Figure 2a is for heating water of a heating system, i.e. the airwater heat pump is configured to work in a heating cycle (i.e. the heat pump comprises a heating cycle). In the heating cycle, the air-water heat pump arrangement gathers heat from air and transmits it to water of the heating system. The heating system is for heating a heating destination, which is a building, by circulating heated water and bring water back for heating. The heating system comprises water transferring arrangements comprising water pipes.

[0062] The air-water heat pump arrangement 200 comprises a main circuit 206. The main circuit is a closed loop and it transfers a refrigerant. In some embodiments, a second circuit is used for improving the efficiency of the air-water heat pump arrangement. This is explained in Figure 3.

[0063] The main circuit 206 comprises a main circuit evaporator 208, a main circuit compressor 209, a main circuit condenser 210, a main circuit expansion valve 211 , and a main circuit pipe system 212 for transferring the refrigerant. In the heating cycle the refrigerant is configured to flow from the main circuit evaporator, through the main circuit compressor, the main circuit condenser and main circuit expansion valve back to the main circuit evaporator. One of the water pipes of the heating system goes through the main circuit condenser. The main circuit evaporator is positioned outside of the building.

[0064] In the main circuit condenser 210 the gaseous or partly gaseous refrigerant is at least partly condensed, i.e. liquified. This process warms the water of the heating system. The decrease in the temperature of the refrigerant from the condensing temperature to the vaporization temperature occurs in the flow through the main expansion valve 21 1 , whereupon part of the liquid evaporates, cooling the refrigerant. The amount of evaporated (gasified) refrigerant is usually 25 - 40%. It is all the more significant the greater the difference between the condensing and vaporizing temperatures.

[0065] Primary function of the main circuit evaporator 208 is to absorb heat from the surrounding environment and transfer it forward to the air-water heat pump 200 for further processing. The main circuit evaporator comprises coils or similar means for absorbing heat. The refrigerant enters the main circuit evaporator as a low-pressure, low-temperature gas or gas-liquid mixture. As the warm air from the surrounding environment comes into contact with the main circuit evaporator's coils, heat is transferred from the air to the refrigerant. The absorbed heat causes the refrigerant to evaporate, transforming it into a low-pressure vapor. The vaporized refrigerant is then drawn into the main circuit compressor 209, which increases its pressure and temperature. The heat pump system transfers the compressed and heated refrigerant to the main circuit condenser 210, where it releases heat to the heating system. After releasing heat, the refrigerant returns to the main circuit evaporator as a low-pressure, low-temperature gas to repeat the cycle through the main expansion valve 211. The main expansion valve ensures that the refrigerant undergoes a phase change from a high-pressure liquid (or gas liquid mixture) to a low-pressure, low-temperature mixture of liquid and vapor before entering the main circuit evaporator. For receiving heat in the main circuit evaporator, the temperature of the refrigerant must be lower than the temperature of the outside air.

[0066] The main circuit 206 further comprises a first bypass pipe 218, a second bypass pipe 219, a first three-way valve 220, and a second three-way valve 221 .

[0067] Three-way valves are valves having three ports or connections. The three-way valves comprise a position where one port is closed and other two are open. This allows a T-junction structure where routes can be changed. The first three-way valve

[0068] 220 is on the main circuit pipe system 112 between the main circuit evaporator 208 and the main circuit compressor 209. The second three-way valve 221 is on the main circuit pipe system between the main circuit compressor 209 and the main circuit condenser 210. The main circuit compressor is between the first three-way valve 220 and the second three-way valve 221. In this embodiment, motorized three-way valves are used. They allow gradual route change through the three-way valve. Commonly used valves are electrical valves (solenoid valves) that are practically on-off valves, i.e. the refrigerant in the pipes near the valve experience route change as instantaneous. In this embodiment, the routes (i.e. holes) through the three-way valves are same size as diameters of the pipes that are adjacent to the three-way valves. This means that the cross-sections through the three-way valves and surrounding pipes are constant.

[0069] In this embodiment, the first three-way valve 220 and the second three-way valve

[0070] 221 and pipes of the main circuit are arranged in such a way that the three-way valves are directly connected to the main circuit evaporator 208. The main circuit compressor 209 is between the three-way valves, and the valves are directly connected to the main circuit compressor. By ‘directly’ it is meant here that there is no air-water heat pump arrangement 200 components between them. These features improve the flowing properties of the refrigerant between the main circuit compressor and the main circuit evaporator. Thus, they enhance the defrosting process. In addition, this structure has been found to dampen sounds when reversing flows. In this embodiment, the pipes of the main circuit 206 are arranged in such a way that flow length of the refrigerant between the three-way valves 220, 221 and the main circuit evaporator 208 is less than flow length of the refrigerant between the three-way valves and the main circuit expansion valve 211. In some embodiments, the flow length of the refrigerant between the three-way valves and the main circuit evaporator is less than 50% of the flow length of the refrigerant between the three- way valves and the main circuit expansion valve.

[0071] The bypass pipes are connected in the main circuit pipe system 212 in such a way that they bypass the main circuit compressor 209. The first bypass pipe 218 is connected in such a way that one end of the first bypass pipe is connected to the main circuit pipe system between the main circuit evaporator 208 and the first three-way valve 220 and another end of the first bypass pipe is connected to the second three- way valve 221. The second bypass pipe 209 is connected in such a way that one end of the second bypass pipe is connected to the first three-way valve 220 and another end of the second bypass pipe is connected to the main circuit pipe system between the second three-way valve 221 and the main circuit condenser 210.

[0072] Figure 2a shows the heating cycle of the air-water heat pump arrangement 200. The processes of the heating cycle are similar as was described before. The first three- way valve 220 and the second three-way valve 221 are set in position where the refrigerant flows from the main circuit evaporator 208 through the main circuit compressor 209 to the main circuit condenser 210. After the main circuit expansion valve the refrigerant goes through the main circuit evaporator 208 where the refrigerant absorbs heat from the outside air. If the second circuit is used, the refrigerant is cooled when it goes through the second circuit evaporator 213 towards the main circuit expansion valve 211 .

[0073] In Figure 2b the defrosting cycle is initiated. The main circuit compressor 209 is between the first three-way valve 220 and the second three-way valve 221. The main circuit compressor works similarly as in the heating cycle (power it is working may be changed). The first three-way valve 220 is set in a position where the main circuit compressor is connected to the second bypass pipe 219. The second three- way valve 221 is changed in a position (from the heating cycle position) where the refrigerant is pushed to the first bypass pipe 218 and towards the main circuit evaporator 208 instead of the main circuit condenser 210. The refrigerant goes through the main circuit evaporator and then through the main circuit expansion valve 211. After the main circuit expansion valve the refrigerant goes through second circuit evaporator 213 and then enters the main circuit condenser 210. Because the second three-way valve 221 is in a position where the main circuit compressor and the second bypass pipe are connected (i.e. the second three-way valve connects the main circuit compressor and the first bypass pipe 218), the refrigerant must flow through the second bypass pipe 219 and the first three-way valve towards the main circuit compressor. This structure allows a situation where the direction of the flow of the refrigerant is same in the heating cycle and the defrosting cycle through the main circuit compressor 209.

[0074] The main circuit compressor 209 heats the refrigerant when compressing it. The heated refrigerant warms the main circuit evaporator 208 and thus defrosting the coils and other parts of the main circuit evaporator. The cooled refrigerant goes through the main circuit expansion valve 211. If a second circuit is present, the temperature of the refrigerant is lower than the second refrigerant in a second circuit evaporator (between the main circuit expansion valve and the main circuit compressor), the refrigerant does not give heat to the second refrigerant but instead is warmed by the second refrigerant. This improves the efficiency of the defrosting cycle. The refrigerant goes through the main circuit condenser 210 and back through the main circuit compressor 209. The defrosting cycle is used until the main circuit evaporator 408 is defrosted or some predetermined time.

[0075] Because of the three-way valve structure and heating of the refrigerant, the defrosting cycle is efficient. Thus, the length of the defrosting cycle is shorter than in the conventional defrost methods. Also, the noise caused by the defrosting is reduced. The inventors have found out that using the described three-way valve structure produces a calmer pressure equalization compared to conventional systems, thus further reducing noises caused by the refrigerant flow changes. This also reduces and even eliminates in some embodiments pressure shocks and thus increases the life of the piping and components. Pressure losses are reduced, too. Also, using two three-way valves eliminates harmful heat transfer when the high-temperature hot gas leaving the compressor and the cool suction gas coming to the compressor circulates through their own valves, i.e. , direction of the flow in the main circuit compressor 209 does not change in the heating and the defrosting cycles.

[0076] Because in this embodiment, the first three-way valve 220 and the second three- way valve 221 are configured to be gradually change from one route to another route through the valve (i.e. change valve from one extreme position to another), pressure differences around the three-way valve are significantly reduced. This is against common practise, where defrosting processes are aimed to be as fast as possible. The inventors have found that optimal changing times (reversing times) are related to the amount of the refrigerant in the main circuit 206. When the amount of the refrigerant increases, also the valve reversing time increases. Small or medium sized air-water heat pumps have reversing time 60 seconds or more, and bigger arrangements have reversing time 90 second or more. Industrial air-water heat pumps may contain refrigerant 10 kg or more and they may have reversing time as high as 150 seconds. The reversing of the three-way valves is done in such a way that when the three-way valve is closed, the flow of the refrigerant in vicinity of the three-way valve is stopped or speed of the refrigerant flow is reduces to 5% or less of the normal speed of the refrigerant. By the normal speed here is meant the speed of the refrigerant through the three-way valve before the reversing process (changing the ports of the three-way valve) was started.

[0077] In some embodiments, the first three-way valve 220 is rated for lower temperatures than the second three-way valve 221. In other words, the second three-way valve, which receives the heated refrigerant from main circuit compressor 209 is graded for higher temperatures than the first three-way valve. This means that cheaper products can be used for the first three-way valves than the second three-way valves.

[0078] In some embodiments, the second three-way valve 221 is dimensioned for higher pressures than the first three-way valve 220.

[0079] In some embodiments, when the second circuit is present, if the defrosting cycle is longer than 30 seconds, the flow of the second refrigerant in the second circuit pipe system is stopped or significantly slowed. This is because the second refrigerant is cooled by the refrigerant, and there is no benefit after that. In some embodiments, if the defrosting cycle is longer than 1 minute, the second circuit is turned off. In this case, there are no benefits from the second circuit and energy can be saved.

[0080] Figure 3 shows a second example of an air-water heat pump arrangement 300 according to an embodiment. The heating cycle is similar described in Figure 1. The air-water heat pump arrangement comprises a main circuit 306 and a second circuit 307. The main circuit is a closed loop and it transfers a refrigerant. The second circuit is another closed loop and it transfers a second refrigerant. In some embodiments, both refrigerants are same substance. In some embodiments, different refrigerants are used. The refrigerants are typically fluids with a low boiling point, such as R-410A. Figure 3 is for explaining the second circuit 307. For sake of simplicity, the three- way valves and the bypass pipes are left out.

[0081] The main circuit 306 comprises a main circuit evaporator 308, a main circuit compressor 309, a main circuit condenser 310, a main circuit expansion valve 311 , and a main circuit pipe system 312 for transferring the refrigerant. The main circuit further comprises the bypass pipes the three-way valves (not in Figure 3).

[0082] The second circuit 307 comprises a second circuit evaporator 313, a second circuit compressor 314, a second circuit condenser 315, a second circuit expansion valve 316, and a second circuit pipe system 317 for transferring the second refrigerant. The functioning and processes of the second circuit are practically similar as the processes in the main circuit 306. The second circuit evaporator absorbs heat from the main circuit’s refrigerant. The second circuit evaporator is positioned in such way that the main circuit pipe system 312 goes through the second circuit evaporator. This place is for the second circuit evaporator is between the main circuit condenser 310 and the main circuit expansion valve 311 .

[0083] In some embodiments, the main circuit pipe system 312 comprises shapes, such as curves and such, inside the second circuit evaporator 313 for improving heat absorption to the second refrigerant. The absorbed heat causes the second refrigerant to evaporate at least partly, transforming it into a low-pressure vapor. In this process, temperature of the refrigerant falls. The refrigerant entering the main expansion valve 311 is therefore precooled. This means that the suction power of the main circuit compressor 309 is less needed when the second circuit compressor 314 of the second circuit partly replaces its work. The second circuit works with a higher efficiency than the main circuit because the heat source (the refrigerant) of the second circuit is at a higher temperature level than the heat source of the main circuit (the outside air). This improves the overall efficiency of the processes of the airwater heat pump 300 and thus saves energy.

[0084] Also, because the heat source (the refrigerant) of the second circuit 307 is at a higher temperature level than the heat source of the main circuit 306 (the outside air) can be dimensioned in such a way that maximum power of the second circuit compressor 314 is less than maximum power of the main circuit compressor 309 without decreasing the overall efficiency of the air-water heat pump 300. This means that in some embodiments the structure of the second circuit is smaller than the structure of the main circuit, i.e. the components of the second circuit are less efficient than the components of the main circuit. This also decreases noises the airwater heat pump produces.

[0085] The heating system is configured to transfer water through both the main circuit condenser 310 and the second circuit condenser 315 for heating water of the heating system. The heating system comprise parallel pipes for the main circuit condenser and the second circuit condenser. This means that both condensers receive cooled water.

[0086] In some embodiments, the second circuit 307 is configured to be turned off when the capacity requirement of the air-water heat pump arrangement 300 falls below a predetermined limit value. This saves energy. It must be noted that because the second circuit receives heat from the heated refrigerant of the main circuit 306, the second circuit does not work (heat the second refrigerant) if there is no temperature difference between the refrigerant and the second refrigerant or the refrigerant is colder than the second refrigerant. This means that the second circuit cannot work alone for heating the water of the heating system.

[0087] Figure 4 shows a third example of an air-water heat pump arrangement 400 according to an embodiment. The arrangement is for heating water of a heating system 422, when working in a heating cycle. The heating cycle is described previously. The air-water heat pump arrangement comprises a main circuit 406 and a second circuit 407. The main circuit is a closed loop and it transfers a refrigerant. The second circuit is another closed loop and it transfers a second refrigerant.

[0088] The main circuit 406 comprises a main circuit evaporator 408, a main circuit compressor 409, a main circuit condenser 410, a main circuit expansion valve 411 , and a main circuit pipe system 412 for transferring the refrigerant. The main circuit further comprises a first bypass pipe 418, a second bypass pipe 419, a first three-way valve 420, and a second three way-valve 421 .

[0089] The second circuit 407 comprises a second circuit evaporator 413, a second circuit compressor 414, a second circuit condenser 415, a second circuit expansion valve 416, and a second circuit pipe system 417 for transferring the second refrigerant.

[0090] The first three-way valve 420 is on the main circuit pipe system 412 between the main circuit evaporator 408 and the main circuit compressor 409. The second three- way valve 421 is on the main circuit pipe system between the main circuit compressor 409 and the main circuit condenser 310. The main circuit compressor is between the first three-way valve 420 and the second three-way valve 421 . The bypass pipes are connected in the main circuit pipe system 412 similarly as was described with Figure 2.

[0091] In Figure 4 flow directions of the refrigerant and the second refrigerant in the heating cycle are marked with arrows. Also, flow directions of water in the heating system 422 through the main circuit condenser 410 and the second circuit condenser 415 are marked similarly. Cooled water is heated in both condensers and after heating the heated waters are combined for heating, for example, a building.

[0092] In the heating cycle, the main circuit 406 works conventionally, i.e. the refrigerant flows through the main circuit in following order: the main circuit evaporator 408, the main circuit compressor 409, the main circuit condenser 410, the main circuit expansion valve 411 and back to the main circuit evaporator. The first three-way valve 420 and the second three-way valve 421 are in positions that allow that kind of flow. Between the main circuit condenser and the main circuit expansion valve is the second circuit evaporator 413. The main circuit pipe system 412 is configured to go through the second circuit evaporator. It must be noted that the main circuit pipe system and the second circuit pipe system 417 are separate systems, i.e. the refrigerant and the second refrigerant do not mix.

[0093] In the second circuit 407, at least partly gaseous the second refrigerant is at least partly liquified in the second circuit condenser 415, and in this process water in the heating system 422 is heated and the second refrigerant leaving the second circuit condenser is cooled compared to the temperature of the second refrigerant entering the second circuit condenser. At least partly liquid the second refrigerant is at least partly vaporized in the second circuit expansion valve 416. In this process the second refrigerant is cooled. The processes in the second circuit are dimensioned in such a way that the second refrigerant in the second circuit evaporator 413 is colder than the refrigerant going through the second circuit evaporator. In the second circuit evaporator the second refrigerant cools the refrigerant. Also, the second refrigerant is heated in this process, and after the second circuit compressor 414 has compressed the heated second refrigerant, the second refrigerant releases heat to the heating system in the second circuit condenser 415. The second circuit evaporator therefore cools the refrigerant before the main circuit expansion valve 411 thus improving the efficiency of the air-water heat pump arrangement 400.

[0094] The air-water heat pump arrangement 400 comprises a defrosting cycle where the main circuit evaporator 413 is defrosted in a situation where frost is generated on the surfaces of the main circuit evaporator. It must be noted that due the structure of the air-water heat pump arrangement according to the invention the second circuit evaporator 413 do not gather frost or ice and therefore does not need defrosting. In the defrosting cycle the first three-way valve 420 and the second three-way valve 421 are set in position where the refrigerant starts to flow in opposing direction through the components of the main circuit 406 except the main circuit compressor 409. The warm refrigerant enters the main circuit evaporator and warms it thus executing defrosting. This is explained in more detail in Figures 2a and 2b.

[0095] Some advantageous embodiments of the method and apparatus according to the invention have been described above. The invention is however not limited to the embodiments described above, but the invention can be carried out numerous ways within the scope of the claims.

[0096] Reference numbers

[0097] Air-water heat pump arrangement 100; 200; 300; 400 evaporator 101 compressor 102 condenser 103 expansion valve 104 pipe system 105 main circuit 206; 306; 406 second circuit 307; 407 main circuit evaporator 208; 308; 408 main circuit compressor 209; 309; 409 main circuit condenser 210; 310; 410 main circuit expansion valve 211 ; 311 ; 411 main circuit pipe system 212; 312; 412 second circuit evaporator 313; 413 second circuit compressor 314; 414 second circuit condenser 315; 415 second circuit expansion valve 316; 316 second circuit pipe system 317; 417 first bypass pipe 218; 418 second bypass pipe 219; 419 first three-way valve 220; 420 second three-way valve 221 ; 421 heating system 422

Claims

Patent claims1. An air-water heat pump arrangement (200; 300; 400) for heating water of a heating system (422) for a building, and the air-water heat pump arrangement comprises a main circuit (206; 306; 406), and the main circuit comprises a main circuit evaporator (208; 308; 408), a main circuit compressor (209; 309; 409), a main circuit condenser (210; 310; 410), a main circuit expansion valve (211 ; 311 ; 411 ), and a main circuit pipe system (212; 312; 412) for transferring a refrigerant, and the airwater heat pump arrangement comprises a heating cycle and a defrosting cycle, and in the heating cycle the refrigerant is configured to flow from the main circuit evaporator, through the main circuit compressor, the main circuit condenser and main circuit expansion valve back to the main circuit evaporator, characterised in that the main circuit (206; 306; 406) further comprises a first bypass pipe (218; 418), a second bypass pipe (219; 419), a first three-way valve (220; 420), and a second three-way valve (221 ; 421 ), and the first three-way valve is on the main circuit pipe system between the main circuit evaporator (208; 308; 408) and the main circuit compressor (209; 309; 409), and the second three-way valve is on the main circuit pipe system between the main circuit compressor and the main circuit condenser, and one end of the first bypass pipe is connected to the main circuit pipe system between the main circuit evaporator and the first three-way valve and another end of the first bypass pipe is connected to the second three-way valve, and one end of the second bypass pipe is connected to the first three-way valve and another end of the second bypass pipe is connected to the main circuit pipe system between the second three-way valve and the main circuit condenser (210; 310; 410), and in the defrosting cycle the first three-way valve (220; 420) is in a position that allows the refrigerant flow from the second bypass pipe (219; 419) towards the main circuit compressor (209; 309; 409), and the second three-way valve (221 ; 421 ) is in a position that allows the refrigerant flow from the main circuit compressor to the first bypass pipe (218; 418) and towards the main circuit evaporator (208; 308; 408).

2. The air-water heat pump arrangement (200; 300; 400) according to claim 1 , characterised in that the amount of the refrigerant in the main circuit (206; 306; 406) is 2 kg or more.

3. The air-water heat pump arrangement (200; 300; 400) according to claim 1 , characterised in that the amount of the refrigerant in the main circuit (206; 306; 406) is 3,5 kg or more.

4. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 1 to 3, characterised in that the first three-way valve (220; 420) and the second three-way valve (221 ; 421 ) are motorized valves.

5. The air-water heat pump arrangement (200; 300; 400) according to claim 4, characterised in that diameters of holes of the first three-way valve (220; 420) and the second three-way valve (221 ; 421 ) are same size as diameters of the pipes that are adjacent to the three-way valves.

6. The air-water heat pump arrangement (200; 300; 400) according to claim 4 or 5, characterised in that the three-way valves (220, 221 ; 420, 421 ) are arranged reverse flow of the refrigerant gradually, and when the three-way valves are closed, the flow of the refrigerant in vicinity of the three-way valve is stopped or speed of the refrigerant flow is 5% or less of the normal speed of the refrigerant.

7. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 4 to 6, characterised in that time at which the three-way valves (220, 221 ; 420, 421 ) change the flow direction is 90 seconds or more.

8. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 4 to 7, characterised in that time at which the three-way valves (220, 221 ; 420, 421 ) change the flow direction is 120 seconds or more.

9. The air-water heat pump arrangement (200; 300; 400) according to claim 8, characterised in that the amount of the refrigerant in the main circuit (206; 306; 406) is 4 kg or more.

10. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 1 to 9, characterised in that the pipes of the main circuit (206; 306; 406) arearranged in such a way that flow length of the refrigerant between the three-way valves (220, 221 ; 420, 421 ) and the main circuit evaporator (208; 308; 408) is less than flow length of the refrigerant between the three-way valves and the main circuit expansion valve (211 ; 311 ; 411 ).11 . The air-water heat pump arrangement (200; 300; 400) according to claim 10, characterised in that the flow length of the refrigerant between the three-way valves (220, 221 ; 420, 421 ) and the main circuit evaporator (208; 308; 408) is less than 50% of the flow length of the refrigerant between the three-way valves and the main circuit expansion valve (211 ; 311 ; 411 ).

12. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 1 to 11 , characterised in that the pipes of the main circuit (206; 306; 406) are arranged in such a way that the three-way valves (220, 221 ; 420, 421 ) are directly connected to the main circuit evaporator (208; 308; 408).

13. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 1 to 12, characterised in that the main circuit compressor (209; 309; 409) is between the three-way valves (220, 221 ; 420, 421 ).

14. The air-water heat pump arrangement (200; 300; 400) according to claim 13, characterised in that in the three-way valves (220, 221 ; 420, 421 ) are directly connected to the main circuit compressor (209; 309; 409).

15. The air-water heat pump arrangement (200; 300; 400) according to any of claims 1 to 14, characterised in that in the heating cycle the first three-way valve (220; 420) and the second three-way valve (221 ; 421 ) are in a position where the refrigerant does not flow through the first bypass pipe (218; 418) and the second bypass pipe (219; 419), i.e. the first three-way valve is in a position that allows flow from the main circuit evaporator (208; 308; 408) towards the main circuit compressor (209; 309; 409) but not to the second bypass pipe, and the second three-way valve is in a position that allows flow from the main circuit compressor to the main circuit condenser (210; 310; 410) but not to the first bypass pipe.

16. The air-water heat pump arrangement (200; 300; 400) according to any of claims 1 to 15, characterised in that the second three-way valve (221 ; 421 ) is dimensioned for higher temperatures than the first three-way valve (220; 420).

17. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 1 to 16, characterised in that the second three-way valve (221 ; 421 ) is dimensioned for higher pressures than the first three-way valve (220; 420).

18. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 1 to 17, characterised in that the air-water heat pump arrangement further comprises a second circuit (307; 407) comprising a second circuit evaporator (313; 413), a second circuit compressor (314; 414), a second circuit condenser (315, 415), a second circuit expansion valve (316; 416), and a second circuit pipe system (317; 417) for transferring a second refrigerant, and the main circuit and the second circuit are connected in such a way that between the main circuit condenser and the main circuit expansion valve the main circuit pipe system is configured to go through the second circuit evaporator, and the second circuit evaporator is configured to receive heat from the refrigerant thus cooling the refrigerant before entering the main circuit expansion valve.

19. The air-water heat pump arrangement (200; 300; 400) according to claim 18, characterised in that the refrigerant and the second refrigerant are same substance.

20. The air-water heat pump arrangement (200; 300; 400) according to claim 18 or 19, characterised in that the heating system (422) is configured to transfer water through both the main circuit condenser (210; 310; 410) and the second circuit condenser (315, 415).21 . The air-water heat pump arrangement (200; 300; 400) according to any one of claims 18 to 20, characterised in that the heating system (422) comprise parallel pipes for the main circuit condenser (210; 310; 410) and the second circuit condenser (315, 315).

22. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 18 to 21 , characterised in that maximum power of the second circuit compressor (314; 414) is less than maximum power of the main circuit compressor (209; 309; 409).

23. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 18 to 22, characterised in that the second circuit (307; 407) is configured to be turned off when the capacity requirement of the air-water heat pump arrangement falls below a predetermined limit value.

24. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 18 to 23, characterised in that if the defrosting cycle is longer than 30 seconds, the flow of the second refrigerant in the second circuit pipe system (317; 417) is stopped or significantly slowed.

25. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 18 to 24, characterised in that if the defrosting cycle is longer than 1 minute, the second circuit (307; 407) is turned off.

26. The air-water heat pump arrangement (200; 300; 400) according to any one of claims 18 to 25, characterised in that the second circuit (307; 407) comprises only a heating cycle.

27. A method for defrosting an air-water heat pump arrangement (200; 300; 400), characterised in that the arrangement disclosed in claim 1 is used, and the method comprises steps where- in the defrosting cycle the first three-way valve (220; 420) is in a position that allows the refrigerant flow from the second bypass pipe (219; 419) towards the main circuit compressor (209; 309; 409) and the second three-way valve (221 ; 421 ) is in a position that allows the refrigerant flow from the main circuit compressor to the first bypass pipe (218; 418) and towards the main circuit evaporator (208; 308; 408), and- the refrigerant warmed by the main circuit compressor (209; 309; 409) defrost the main circuit evaporator (208; 308; 408).

28. The method according claim 27, characterised in that the air-water heat pump arrangement (200; 300; 400) further comprises a second circuit (307; 407) comprising a second circuit evaporator (313; 413), a second circuit compressor (314; 414), a second circuit condenser (315; 415), a second circuit expansion valve (316; 416), and a second circuit pipe system (317; 417) for transferring a second refrigerant, and the main circuit and the second circuit are connected in such a way that between the main circuit condenser and the main circuit expansion valve the main circuit pipe system is configured to go through the second circuit evaporator, and the second circuit evaporator is configured to receive heat from the refrigerant thus cooling the refrigerant before entering the main circuit expansion valve the method further comprises step where, the refrigerant cooled by the defrosting main circuit evaporator (208; 308; 408) and the main circuit expansion valve (211 ; 311 ; 41 1 ) is warmed by the second refrigerant in the second circuit evaporator (313; 413) before entering the main circuit condenser (210; 310; 410).

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