SE542633C2 - Device for rapid defrosting without compressor stop of the evaporator in an air-to-water heat pump and for running the heat pump at extremely low evaporator temperatures and at extremely low loads - Google Patents

Abstract

The invention relates to a heat pump comprising a compressor (101), a condenser (102), a refrigerant supply (103), an expansion valve (104), controlled by a control unit (106) and an evaporator (105). The heat pump is provided with a bypass duct (200) which runs past at least the refrigerant supply (103) and the expansion valve (104). A tempered refrigerant judgment (301) is provided, comprising a closed container which is connected at the bottom via an open connecting pipe to the high-pressure side of the refrigerant circuit. The mandrel is arranged so that it is in flow communication with either the outlet (102u) of the condenser (102) or the outlet (101u) of the compressor. A bypass valve (201) is configured to be operated externally between its three alternative positions, fully closed, fully open and reduced open, corresponding to the heat pump's three alternative operating positions, normal operation, defrost operation and power-reduced operation, respectively. A special control unit (203) is arranged to operate the bypass valve (201) between its three alternative positions.

Description

The invention intends to reduce the defrosting times in an Air-Water heat pump by more than 96 percent. In addition, the need to shut off the heat pump compressor during the defrost process is eliminated. Also, it is not required that the air flow through the evaporator 105 contributes to the defrost in addition to preferably maintaining a temperature in excess of 0 ° C so that it does not counteract the defrosting process. The invention also intends to make it possible to extend the permissible operating range of the heat pump to include extremely low evaporator temperatures and / or extremely low output effects without risk of compressor damage. Background to the invention The evaporator 105 in a highly efficient air heat pump usually maintains a temperature substantially below zero degrees and often even below zero. Thus, the air moisture will precipitate on the evaporator 105 and form an increasingly thicker and increasingly heat-insulating ice / frost layer which causes the evaporator's 105COP (coefficient of performance) to degrade more and more, which in turn has a detrimental effect on the heat pump's COP. Therefore, the evaporator 105 must be defrosted regularly so that its capacity is restored. During the defrosting process, the heat pump will not be able to absorb any heat energy from the heat source air), which over time means that the heat pump's average energy capacity degrades to a corresponding extent. Thus, a 6 kW heat pump that, for example, needs to be defrosted during about 15% of the operating time will lose as much as 15% in capacity and thus not perform an average heat output higher than 4.5 kW.

The need for defrosting therefore constitutes a not insignificant disadvantage affecting all types of air source heat pumps.

Provided that the air temperature exceeds zero degrees, defrosting is most easily achieved by switching off the compressor and letting the air flow from the fan thaw the frost on the evaporator. (This simple defrosting is the method traditionally applied to exhaust air heat pumps, where the air flow normally always holds room temperature.) ° An outdoor air heat pump cannot be defrosted via the air flow when it is freezing cold, but then the defrosting energy must be supplied in another way. Heat is then taken from the compressor or from additional electricity, which in turn greatly degrades the COP average. On the other hand, the outdoor air can be regarded as an unlimited energy supply, which is why you can always dimension the outdoor air machine so that the remaining operating time can produce all the heating energy the house needs.

° For an exhaust air heat pump, on the other hand, the exhaust air itself is always warm enough, just over twenty degrees, for the evaporator 105 to be able to defrost. However, such defrosting takes a fairly long time if you want to ensure that all ice has been removed. During the defrosting period, no heat energy can be taken out for the self-heating. At the same time, the exhaust air is usually a limited energy resource that one needs to utilize as far as possible in order to be able to meet the heating need during cold periods. There is no possibility that the additional heat outlet in addition to what effect fl the fate of the exhaust air allows. Similarly, exhaust air energy night is seen as a "fresh product". The energy in the flow that has not been used for the preheating has irrevocably been lost. So, if the heat demand of the house exceeds what the heat pump is able to produce on average over time, you are forced to cover the shortfall by shooting for expensive peak energy, usually frayed cartridges. This in turn becomes devastating for the overall COP preheating system. A VP-COP of, for example, respectable 4.0 is probably enough for the needs of the house. But if the defrosting times were to amount to a not unreasonable 20% of the total operating time, the average COP would only be a mediocre 3.2, while the total consumption for the heating would increase by as much as 25%.

Another unfortunate limitation is that the compressor can be seriously damaged if it ends up in such operating conditions that it is exposed on its suction side to excessively low temperatures, pressures and or gas fumes. You are then usually forced to close the decompressor until its acceptable operating conditions have been restored. This in turn, of course, has a detrimental effect on the heat pump's energy production and COP.

Summary of the Invention The present invention consists of additions to the traditional refrigerant circuit of an Air-to-Water heat pump.

More specifically, 1. A controlled bypass duct is included via which a direct connection with low pressure drop between compressor outlet or condenser outlet 102u and evaporator inlet 105i can be completely closed, fully opened or partially opened by means of a manoeuvrable valve, bypass valve 201. Open valve directs bypass line 20 for condensation with accompanying high-power defrost, defrost operation. Closed valve provides normal VP operation, normal operation. Partly open valve provides power-reduced operation in that a very small part fl waste of hot steam is diverted directly to the condenser inlet, where temperature, pressure and gas flow increase in condenser pressure. At the same time, the heat output produced in the condenser is reduced at an unchanged compressor speed. 2. A control unit 203 with associated sensors which analyzes the need for defrosting and / or power reduction and based thereon controls the bypass valve 201. The analysis function of the unit can be made infinitely complex and it can be connected to a variety of different sensors. In its simplest embodiment, the control unit 203 is connected to only one sensor for the evaporator temperature and it bases its analysis entirely on the time that has elapsed since the last defrost and the current evaporator temperature. A refrigerant judgment 301 which, upon defrosting, drives its contents of refrigerant condensate into the heat condenser 102 of the heat pump for decoction undertake of considerable amounts of energy from the hot circulating water flowing through the heat exchanger of the condenser 102.

The open bypass duct according to point 1 conducts the compressor power of approx. 1-2 kW inside the evaporator 105 which is heated. Above all, the evaporator's coldest part heats up, where you can expect the strongest frosting. As a result, the ice quickly melts away from the evaporator metal, any loose frost residues are completely exposed to the exhaust air fate and melt away quickly. In this way, a single defrost that with traditional methods normally takes at least about ten minutes (20% of normal time between two consecutive defrosts) can be cleared in less than two minutes (4% of this time).

The refrigerant judgment 301 according to paragraph 3 above does not add any defrosting function on its own. But when the pressure in the verdict drops sharply due to. If the bypass channel 200 above is opened, the contents of the refrigerant condensate in the dome will be expended in considerable quantities through the hot condenser 102 where large amounts of heat energy are immediately supplied from the condenser 102 heat exchanger (effects of about thirty to forty kW can be expected). it condenses rapidly and relays its approximately thirty to forty kW. These supplied about thirty to forty kW melt away defrost in less than ten seconds (0.33% of normal time between two consecutive defrosts)! The melt water may need an additional ten seconds to leave the evaporator. In total, complete defrosting of the evaporator 105 is achieved in less than twenty seconds (<0.7% of normal time between two consecutive defrosts).

Thanks to the extremely short defrosting time, defrosting can often be allowed here without the heat production decreasing significantly. Then the average ice coating on the evaporator 105 decreases as well as the required time for each individual defrost, which in turn leads to a not insignificant increase in the evaporator temperature and efficiency as well as of the COP and the heat production for the entire heat pump.

Defrost function: 1. Defrost is started immediately and without the compressor having to be stopped when the control unit 203 opens the bypass valve 201 completely. Then in the bypass line 202 a strong refrigerant fl occurs in vapor phase from condenser to evaporator inlet which partly reduces the pressure difference between them to a minimum (condenser pressure drops sharply and evaporator pressure rises sharply), partly a very high power transmits the evaporator. Likewise, the pressure difference across the compressor is reduced to the corresponding minimum, which means that it ends up at idle and thus only consumer bypass. 2. The strong pressure increase in the evaporator 105 gives rise to a strong condensation with concomitant strong heating and especially then in the coldest parts of the evaporator 105. The heat energy flows out through the evaporator walls and melts away existing frost quickly. The melting takes place very quickly in the coldest parts, where the frosting is thickest. 3. In the hot condenser 102 the pressure drops correspondingly sharply, which gives rise to a lightning-fast decoction of the condensate present there, expands strongly and flows further through the open bypass channel 200 to the cold evaporator 105 where it condenses and delivers seat energy content just as lightning fast. 4. Where appropriate, the rapidly lowering pressure in the condenser 102 also causes new condensate to flow into the condenser 102 from the pressurized and hot refrigerant, a condensate de fate passing the condenser 102 and where the condenser evaporates rapidly during heavy energy uptake from the circulating water, flows further through the bypass channel 200. 105, where the steam is condensed again and emits the heat energy taken up during the passage through the condenser 102.

. When the defrost is removed from the evaporator 105, its temperature rises rapidly. The control unit 203 then closes the bypass valve 201, whereby the heat pump immediately switches to ordinary operation with full power and then in a very short time, only a few seconds, returns the utilized defrosting energy to the recipient. 6. The time required for a complete defrost cycle as above can be estimated to be significantly less than one minute.

Power reduction function: 1. The control unit 203 monitors any need for power reduction and / or increase in temperature, pressure or gas fl in the compressor inlet. 2. Should such a need exist, the control unit opens the bypass valve 201 as much fitting as required. Thereby a subset of the compressor 101's gas gas gas is diverted and supplied to the evaporator inlet 105i. Then gas ökar fate, pressure and temperature increase in the evaporator 105 and in the compressor inlet. At the same time, the amount of gas entering the condenser 102 decreases, resulting in reduced condensation, reduced power output to the circulating water and reduced pressure. All in all, in this way the compressor pressure ratio is reduced, the most important parameter to avoid compressor damage, as well as increasing gas fl fate and thus the lubrication of the compressor. 3. In certain circumstances, it may be economically advantageous to rationalize away the speed control of the compressor and replace it with a single-pass-driven power reduction.

The new invention is defined in claim 1.

Preferred embodiments are defined in the dependent claims.

Brief description of the drawings Fig. 1 shows the principle design of a traditional heat pump with its threaded refrigerant circuit and control equipment.

Figs. 2 - 6 show how the traditional heat pump according to f1g. 1 has been supplemented with the invention in different configurations and different status: 1. A mandatory coarse bypass channel that drives the defrost. The duct itself includes the bypass line 202 and a bypass valve 201 which is operated by an associated own control unit 203. 2. A refrigerant judgment 301.

Fig. 2 shows the heat pump according to Fig. 1 with supplied complete function for bypass from compressor outlet 101u to evaporator inlet. In the figure, the bypass valve 201 is completely closed.

Fig. 3 shows a heat pump with bypass according to Fig. 2, but where the bypass line 202 is connected to the condenser outlet 102u instead of as in Fig. 2 to the compressor outlet.

Fig. 4 shows the heat pump according to Fig. 2 supplemented with a return water temperature refrigerant 301 connected on the opposite side of the condenser 102 relative to the connection of the bypass line 202.

Fig. 5 shows in analogy with Fig. 4 the heat pump according to Fig. 3 supplemented with a return water temperature refrigerant 301 connected on the opposite side of the condenser 102 relative to the connection of the bypass line 202.

Fig. 6 shows a simplified embodiment of the heat pump according to Fig. 4, co-rationalized return water tempering of the refrigerant judgment 301 which is instead tempered via the exhaust air flow. Fig. 7 shows the corresponding simplified embodiment of the heat pump according to Figs. Fig. 8 shows the principle of a heat pump without refrigerant judgment in defrosting operation. As an example, the heat pump according to Fig. 2 has been chosen here. The bypass valve 201 is kept maximally open.

Fig. 9 shows the principle of a heat pump with refrigerant judgment in defrost operation. As an example, the heat pump according to Fig. 6 has been selected here. The bypass valve 201 is kept open to the maximum.

Fig. 10 shows the principle of a heat pump without refrigerant judgment in power-reduced operation. As an example, the heat pump according to Fig. 2 has been chosen here. The bypass valve 201 is kept open for a limited time and then lets a small subset of hot gas through the evaporator inlet directly.

Fig. 1 1 shows the principle of a heat pump with refrigerant judgment in defrosting operation. As an example, the heat pump according to Fig. 6 has been selected here. The bypass valve 201 is kept open for a limited time and then lets a small subset of the hot gas flow directly to the evaporator inlet.

Detailed Description The invention relates to the modification of a conventional refrigerant circuit, Fig. 1, in a single heat pump. The modification is defined above in the descriptions of Figures 2-6.

The usual refrigerant circuit consists of a closed loop with successive components: a compressor 101, a condenser 102, a refrigerant supply 103 (sometimes referred to as a "receiver"), an expansion valve 104 and an evaporator 105. The flow through the refrigerant circuit is controlled by a dedicated machine control 106 .

The modification includes: 1. A shortcut (bypass duct 200) for the hot gas from the compressor 101, bypass refrigerant supply 103 and expansion valve 104, directly to the inlet of circuit evaporator 105. In an alternative route, the bypass duct 200 may also be passed by condenser 102. The capacity of bypass duct 200 may be shifted between completely blocked, partially open and fully wide open with free passage and only insignificant pressure drop for all gas fl fate from the compressor, preferably a maximum of 100kPa.

The capacity control of the bypass channel 200 takes place via an operable bypass valve 201, the position of which can be switched between fully closed, partially open and fully open, respectively. A control unit 203 which operates the bypass valve 201, based on the signals from connected external sensors. A refrigerant 301 which is kept tempered by either the water circulation through the heat pump (Figures 4 and 5) or by the exhaust air through the heat pump (Figures 6 and 7) and which during defrosting continuously supplies the condenser 102 with condensate for evaporation.

In Fig. 4, for example, the heat pump according to Fig. 2 is shown supplemented with a single refrigerant judgment 301 tempered by return water 122 and connected to the refrigerant circuit. For best effect, according to the invention, the connection 30 of the dome 301 is always located on the opposite side of the condenser 102 relative to the connection of the bypass line 202. In this case thus at the condenser outlet.

The refrigerant mandrel 301 consists of a closed container, connected at the bottom via an open connecting pipe to the high-pressure side of the refrigerant circuit. (The connection pipe is thus connected to the refrigerant circuit somewhere between the compressor outlet 101u and the refrigerant supply 103. In addition, the best effect is obtained if the connection is located on the opposite side of the condenser 102 relative to the connection of the bypass duct 200.) Through the connection between the 301 and refrigerant circuit, refrigerant flows freely. the circuit is immediately eliminated. Due to the fact that the connection is located at the bottom of the judgment, the fate of the judgment will always consist of any refrigerant condensate that has accumulated, which has accumulated at the bottom of the judgment. The kan fate, on the other hand, can be constituted by both steam and condensate, depending entirely on the current media phase connection to the refrigerant circuit.

As a result of the design, tempering and connection of the judgment 301, during normal operation the refrigerant condensate and sub-defrost operation will very quickly drive all its condensate content out into the refrigerant circuit.

Explanation of this as follows: As previously mentioned, the media pressure inside the judgment 301 is the same as the refrigerant circuit outside.

When the machine is running in normal operation or in power-reduced operation, the media pressure is determined by the conditions in the condenser and corresponds to the condensing temperature of the medium at the prevailing condenser temperature. Because the dome 301 is tempered to a single temperature which during normal operation is always below the condenser temperature (at outside air temperature of the dome 301 this means that the condenser temperature should not be below the exhaust air temperature (which probably happens very rarely)), the pressure inside the dome significantly exceeds the refrigerant pressure temperature during normal operation. current temperature, so that all refrigerant vapor in the judgment 301 will condense and allow the inflow of additional refrigerant. Thus, after a short period in normal operation or power-reduced operation, the judgment 301 will be completely filled with refrigerant condensate.

On the other hand, when the machine goes into defrosting operation and the bypass duct 200 is wide open, the medium pressure becomes the same as the prevailing very low condensing pressures in the cold evaporator. A small subset of the tempered condensate contents of the mandrel 301 will then evaporate rapidly and drive all other condensate out into the refrigerant circuit, further through the hot condenser for rapid heat absorption, evaporation, expansion and flow through the wide open bypass channel 200 into the cold evaporator. hot steam to quickly give off its heat energy and condense. This energy transfer can be expected to be greatest in the coldest parts of the evaporator, ie. departments that are most frosted. The defrost will thus quickly thaw from within, detach from the evaporator material and be blown away by the exhaust air, which may require further thawing and drainage.

As can be seen from the above, the total energy content of the refrigerant vapor from the cathondensate should be at least as large as the energy required to thaw the frost on the evaporator. The 301 "minimum permissible" volume of the judgment is thus determined by the amount of condensate volume that must be accommodated in it. This volume is in turn determined by the amount of ice that needs to be melted, the heat of melting the ice per liter, and the heat of formation of the current refrigerant per liter of condensate. For the melting heat, the melting heat of the ice per liter of zero-degree melting water can be set at 335 kJ.

With the addition of the heating energy of the ice to a melting temperature from the original minus minus fifteen degrees, it can be expected that 400 kJ per liter of melt water will be required. For the steam generation heat, for example, the commonly occurring refrigerant R410a has a specific steam generation heat of approximately 70 kJ per liter of condensate. For R410a, therefore, 5.7 liters of condensate are required to thaw frost corresponding to 1 liter of melt water. For various reasons, mandock should avoid condensate volumes in excess of 6 deciliters. This limits the amount of melt water that can be thawed up to about 1 dl per defrost occasion. This in turn sets the maximum limit for how long it should be allowed to build up defrost on the condenser, which in turn is the time that elapses between two consecutive defrosts.

The invention will now be described below with reference to Figures 7 and 7, respectively. (Corresponding reasoning applies to Figures 6 and 9, respectively, but is not reported here.) Figure 7 shows the heat pump according to Figure 6 in normal operation. The bypass valve 201 is kept completely closed and the function of the machine completely corresponds to that of a traditional machine according to Fig. 1.

In this mode of operation, energy is supplied in the condenser 102 from the condensation of the hot gas (= refrigerant vapor) from the compressor 101, energy which is transferred to the circulating water via via the condenser heat exchanger. The temperature in the condenser 102 thus exceeds the flow temperature of the circulation circuit as well as much more of the return temperature. The pressure in the condenser 102 in turn exceeds the refrigerant condensing pressure at the prevailing temperature. Typical values in this operating mode are the temperature difference across the condenser heat exchanger, approx. 3 ° C and the transfer power to the circulating water, approx. 6 kW.

In the refrigerant judgment 301, which is tempered by the circulation return and is in direct communication with the condenser 102, the normal pressure prevails during normal operation as in the refrigerant circuit and the condenser 102, but significantly lower temperature, well below the condensing temperature in the condenser 301 at prevailing pressure. is therefore condensed and the volume reduction due to the transition of the steam to condensate is compensated by a corresponding inflow of refrigerant, in liquid and / or gas phase, from the refrigerant circuit. Any fl liquefied gas is condensed and new refrigerant flows in, etc. Finally, the dome301 is completely filled with refrigerant condensate. Transition to defrost operation occurs when the bypass valve 201 between the system high pressure side and its low pressure side is opened. Due to the pressure drop that occurs on the high-pressure side, a strong decoction of the existing refrigerant condensate occurs in the hot condenser 102. Large amounts of evaporated refrigerant, causing large amounts of energy, are formed in the condenser 102 and flow out via condenser inlet and bypass line 202 to the evaporator 105 where it immediately condenses and emits its energy. See further below under Fig. 8.

Fig. 8 shows the heat pump according to Fig. 7 in defrost operation. The bypass valve 201 is kept open to the maximum. And an almost total pressure equalization prevails between evaporator 105 and condenser 102. Thus the condenser pressure will be much lower than in normal operation and will very much fall below the evaporation pressure in condenser 102 and in the communicating refrigerant 301. All possible refrigerant condensate in condenser 102 will thus rapidly evaporate during condensation. drops to a new equilibrium temperature, significantly lower than in normal operation.

The pressure also drops in the refrigerant dome 301 and causes the refrigerant in it, heated by the circulation return, to evaporate and expel condensate from the domain bottom via the refrigerant supply 103 and up into the condenser 102 where it supplies the continued decoction process which takes place there. Thus, the refrigerant is quickly emptied of its condensate content and is instead filled with refrigerant vapor.

The temperature drop in the condenser 102 causes a large temperature deficit in the refrigerant relative to the circulating water. The temperature deficit in turn in turn causes very strong heat loss through the condenser heat exchanger from the circulating water and into the refrigerant, where it drives the evaporation process further.

(Typical value for the heat exchanger capacity of the condenser 102 is 2 kW per degree temperature difference. At the new equilibrium temperature, one can expect a temperature difference over the exchanger of fifteen to twenty degrees, which in turn gives a transfer power of about thirty to forty kW. out of the condenser outlet, carrying its approximately thirty to forty kW heat output, and further through the bypass line 202 the evaporator inlet.

The ordinary refrigerant flow from the expansion valve 104 to the evaporator inlet will cease almost completely as the driving pressure difference between condenser and evaporator 105 disappears.

At the same time, against a very low back pressure, the compressor 101 pumps around all the siting flow through the evaporator 105, where the supplied compressor power (in principle only the passing power of the compressor) contributes a little more to the energy supply.

In the evaporator 105, the pressure equalization will correspondingly give a pressure which is much higher than during normal operation and which is far above the condensing pressure at the prevailing evaporator temperature. Thereby the condensed shield medium vapor in the evaporator 105, existing as well as the inflowing via pass line 202 and emits its about thirty to forty kW inside the evaporator 105, the internal temperature of which increases so rapidly that the emitted heat effect can flow further out through the evaporator heat exchanger near the temperature exchanger. the melting point of the water rises and causes the frost to begin to melt quickly. The evaporator 105 also has a heat exchanger capacity of about 2 kW per degree temperature difference, so the released heat output about thirty to forty kW brings an equilibrium temperature about fifteen to twenty degrees above the zero degrees prevailing in the melt water on the outside of the evaporator 105. These about thirty to forty to forty. (30-40) kJ / s) converts every second an amount of ice of (30-40) / 400 kg = 75 - 80 g to ten-degree melt water that drains away quickly. In just under one and a half seconds, almost a hg of ice, corresponding to one dl of melt water, an associated respectable amount, has thus been removed from the evaporator 105.

In addition, during defrosting, a quantity of refrigerant condensate 301 1 will accumulate in the evaporator 105 and in any connected condensate depots. During the transition to normal operation, this accumulated condensate will evaporate rapidly and return the utilized defrost heat to the circulating water in less than one minute via the compressor.

The machine is kept by the control unit 203 in defrost mode for as long as, but only for as long as, any frost remains on the evaporator 105. Due to the very efficient defrost, the total time in this position will be extremely short, no more than a few seconds, almost negligible in comparison with the total time for the corresponding traditional defrost. Similarly, energy consumption and energy loss due to the defrost to be extremely much lower than they would have been with traditional exhaust-driven defrost.

The control unit 203 puts the machine into defrost mode at regular intervals, selected to keep the accumulated amount of frost on the evaporator 105 at an optimal level. By optimal level is meant here the level that over an entire heating / defrosting cycle gives the highest average heat output and the highest average COP, in optimal balance between them. The control unit 203 also monitors the defrosting process and closes the bypass valve 201 as soon as all defrosting has been removed from the evaporator 105. Transition to normal operation takes place when the bypass valve 201 is closed. Thus, the commercial condensate 301 1 on the suction side of the compressor 101 is to be evaporated, transferred by the compressor to hot gas with high pressure and pressed further into the condenser 102 where it emits its energy during condensation. The condensate that has accumulated on the suction side during defrosting will, as long as some of it remains, contribute to an orderly but short-lived increase in the refrigerant fate and the condenser effect, which in centrifuge helps to quickly return the used defrosting energy to the circulating water.

Figures 10 and 11 show the heat pump according to Fig. 2 and Fig. 6, respectively, in power-reduced operation. The bypass valve is then controlled by the control unit 203 to release a suitable proportion of the hot gas fl of the compressor (101) directly to the evaporator inlet. Thereby, gas fl fate and power flow into the condenser (102) will decrease as well as pressure and temperature in the condenser (102). At the same time, the medium påverk fate is influenced into the evaporator inlet, here a mixture of hot bypass gas and cold refrigerant from the expansion valve 104, to obtain the intended design in terms of temperature, pressure and fl extent of fate.

Numbered components in the drawings, figures 1-1 1 101 Compressor 101u Compressor outlet102 Condenser 102u Condenser outlet103 Refrigerant storage104 Expansion valve105 Evaporator 105i Evaporator inlet 6 Ordinary machine control1 122 Return fl fate 200 Bypass bevel valve1 Bypass valve122 Control bar Bypass valve

Claims (5)

(visible changes): A heat pump comprising a compressor (101), a condenser (102), a refrigerant supply (103), an expansion valve (104), controlled by a control unit (106), an evaporator (105), and a bypass duct (200) extending past the expansion valve ( 104), and wherein the bypass duct contains a bypass line (202) and a bypass valve (20 1), characterized by a tempered refrigerant (301), comprising a closed container which is connected at the bottom via an open connecting pipe to the high pressure side of the refrigerant circuit, and arranged so as to stand in fl fatal connection either to the outlet (102u) of the co-condenser (102) or to the outlet (101u) of the compressor, and in that the bypass channel (200) runs from the compressor outlet (101u) to the evaporator inlet (105i) or that the bypass channel (200) runs from the condenser outlet (102u) to , and in that the refrigerant dome (301) is connected on the opposite side of the condenser (102) relative to the connection of the bypass duct (200). Heat pump according to the preceding claim, wherein the bypass valve (201) is configured to be operated externally between its three alternative positions, fully closed, fully open or reduced open, corresponding to the heat pump's three alternative operating positions, normal operation, defrost operation and power reduction operation. A heat pump according to any one of the preceding claims, wherein a special control unit (203) operates the bypass valve (201) between its three alternative positions. Heat pump according to one of the preceding claims, wherein the bypass duct (200) is dimensioned so that, with the bypass valve (201) in the fully open position, it has a flow capacity so large that the entire compressor can be released through a limited pressure drop, maximum 100 kPa. A heat pump according to claim 3, wherein the bypass valve (201) of the control unit (203) is operated to transmit a limited leakage of the hot gas evaporator inlet (105i) sufficient to raise the temperature, pressure and gas flow of the evaporator and compressor inlet to non-hazardous values. precompressor damage, and reduce the heat output produced to the intended level.