EP3004754B1 - Heat pumps for use of environmentally safe refrigerants - Google Patents

Description

The present invention relates to heat pumps and the use of refrigerant therein.

Previously used in heat pumps refrigerants are either toxic or harmful to the environment, i. they have a high global warming potential. Others are flammable or, at least problematic, at least hazardous to health. Previously known approaches to work with non-toxic environmentally friendly refrigerants failed so far because these tools can not provide adequate performance of the heat pump or can not be used in conventional heat pump assemblies.

The use of a refrigerant in a heat pump is characterized by the so-called temperature lift. The temperature lift is the difference between condensation and evaporation temperature. The temperature lift thus means how much the heat source is raised in the temperature level to be used at the heat sink. In the FIG. 1 To illustrate the problem, the phase boundary line of a suitable environmentally friendly refrigerant is shown, which is characterized by a strongly overhanging dew line. In addition, a heat pump process is shown for a temperature elevation of 50 Kelvin from 75 ° C evaporation temperature to 125 ° C condensation temperature. In order to operate a heat pump with such a refrigerant, the compression endpoint must maintain a minimum distance from the dew line to still be in the gas phase region. For example, if the temperature lift were only 20 Kelvin, the condensation temperature would be only 95 ° C, as in FIG. 3 Thus, the compression end point within the phase boundary line would be in the mixed phase region. This would lead to liquid beats in the compressor and prevent stable operation of the heat pump.
So far, only one approach is known for the use of such new working fluids with these special thermodynamic properties, which is aligned with the transient starting process of a heat pump. In the German application 10 2013 203243.9 a heat pump is described with an internal heat exchanger, which, as in FIG. 2 shown graphically, by supercooling the condensate from state 4 to state 5, the resulting heat transfers to the state 7 and thus overheating the suction gas before compression. The distance from state 4 to state 5 and the distance from state 7 to state 1 is the same enthalpy difference, as can be seen from the pressure-enthalpy diagrams 1 to 4. How out FIG. 3 However, the approach with the internal heat exchanger is not suitable for every temperature lift. At a temperature lift of, for example, 20 Kelvin, the amount of heat which the internal heat exchanger can supply for overheating the suction gas is insufficient, and the compression end point is problematically again within the phase boundary line.
Fluids heretofore used in heat pumps and chillers, such as R134a (1,1,1,2-tetrafluoroethane) have the problem that the compression end point in the two-phase region does not exist at all and can therefore be used with heat pumps and chillers known from the prior art operate.
The US 2010/0192607 A1 describes an air conditioning system and a heat pump with an injection circuit and an automatic control of the injection circuit. The injection circuit is used to cool a portion of a working fluid of the heat pump by means of an expansion valve and then to use in a heat exchanger to cool the working fluid at a location in the cycle of the working fluid before the branch of the injection circuit. document US2010 / 0192607 A1 discloses a heat pump according to the preamble of claim 1.

The EP 2 752 627 A1 describes a refrigerator in which a working fluid of the refrigerator is superheated at the input side of a compressor in a liquid / gas heat exchanger, wherein the overheating takes place by means of a portion of the working fluid provided by a liquid / gas separator disposed at an outlet of a steam condenser becomes.

It is an object of the present invention to provide a heat pump and a method of operation thereof, which allows the use of environmentally friendly working fluids and ensures stable steady state operation, wherein a working fluid can be overheated in a particularly efficient manner before it enters a compressor.

The object is achieved by means of a heat pump according to claim 1 and a method for their operation according to claim 5 and by the inventive use of new working fluids according to claim 4. Embodiments of the invention are the subject of the dependent claims.

The heat pump according to the invention comprises a compressor, a condenser, an internal heat exchanger, an expansion valve, an evaporator and a control device which is designed to bring the temperature of the working fluid at the outlet of the compressor to a predeterminable minimum distance, above the dew point. The minimum temperature distance refers to the working fluid at constant pressure and is in particular at least one Kelvin, preferably at least 5 Kelvin. This has the advantage that environmentally friendly non-toxic safe working media, which are often characterized by very specific thermodynamic properties such as a very low taulin assisting of less than 1000 (kg K ² ) / kJ in the temperature-entropy diagram, can be used and a stationary stable heat pump operation is enabled.

The control device is a temperature control device which is designed to increase the temperature of the working fluid at the inlet of the compressor. The temperature control device is designed such that it regulates the pipeline heating via the temperature of the working fluid at the compressor outlet. Depending on which temperature is measured by the temperature control device at the compressor output, the pipeline heating is switched on or off or varies in temperature. The pipeline heating can thus start briefly, for example, in fluctuating heat sources or Wärmesenketemperaturen or be in continuous operation. This has the advantage of compensating for a too low temperature lift. The temperature limit for the temperature lift depends on the refrigerant used, or working fluid. The temperature lift depends on various properties and parameters of the heat pump.

The temperature control device comprises a bypass line with a valve which connects the high-pressure region at the outlet of the compressor to the low-pressure region at the inlet of the compressor such that the working fluid flowing from the internal heat exchanger to the compressor can be overheated by means of the hot gas which can be returned via the bypass line. The temperature control device is in particular designed such that it regulates the passage through the valve of the bypass line via the temperature of the working fluid at the compressor outlet. Also, this embodiment has the advantage in a temperature lift, which would land without additional intervention in the heat pump process with the compression end point in the two-phase region, so to regulate that the heat pump can be operated stable with the working fluid used in a steady state. The bypass valve used may be, for example, a thermostatic or an electronically controlled valve.

In an exemplary embodiment of the heat pump, the control device is a pressure control device which is configured is to lower the pressure of the working fluid at the inlet of the compressor. For this purpose, the pressure control device may in particular comprise an automatic expansion valve, which is arranged as an expansion valve in the heat pump cycle between the internal heat exchanger and the evaporator. An automatic expansion valve is a pure evaporator pressure control valve by means of which it is possible to set the evaporation temperature and therefore the evaporation pressure.

By lowering the pressure in the evaporator, a higher pressure ratio P _ratio can be generated between the pressure side after the compressor and the low pressure side before the compressor. Because the compressor has to implement a higher pressure ratio P _ratio , a higher compressed gas temperature T ₂ is also generated at the compressor outlet. The higher the pressure ratio P _ratio , the higher the temperature T _{2 of} the compressed gas after the compressor. $$\frac{T_2}{{\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">T</figure-callout>}}_1}=P_{\mathit{reason}}{}^{\frac{\kappa -1}{\kappa }}$$

In this case, κ is the isentropic exponent, T ₂ and T _{1 are} the temperatures before and after the compressor, and P _ratio is the pressure ratio of the gas pressures upstream and downstream of the compressor. As an alternative to an increase in the temperature T ₁ , therefore, the pressure upstream of the compressor can also be lowered. Instead of the additional heating power in this case, an additional compressor power for the increased pressure ratio to be implemented is necessary. This embodiment has the advantage of being able to dispense with additional heating elements and temperature control devices and by replacing the expansion valve by the automatic expansion valve to require no additional components in the heat pump for a steady operation.

The use of an automatic expansion valve in the heat pump has the additional advantage of a control option represent for the application that the temperature lift is not below a threshold temperature but well above the limit temperature. If the temperature lift is just too far above it, the pressure gas temperature T ₂ after the compressor would also be very far above the minimum distance to the dew point to be maintained. This may be another problem when, for example, the compressor has an upper service temperature limit. Such an upper temperature limit of use of a compressor may be due, for example, to the thermal stability of the lubricants or to excessive expansion for close fits in the compressor. By the automatic expansion valve, however, the pressure in the evaporator can be increased so far that the working fluid only slightly overheated or even partially evaporated. The then still necessary overheating for the minimum distance from the dew line could be done by means of the internal heat exchanger. The embodiment with the automatic expansion valve at a temperature elevation above the threshold temperature has the added advantage of increasing the overall efficiency of the heat pump due to the pressure increase because decreasing the temperature difference in the evaporator decreases the pressure ratio and demands less compressor power. At the same time, the density of the fluid increases, thus increasing the power density in the compressor. In addition, an increased service life of the compressor can be ensured by the lower pressure gas temperature.

For this purpose, the heat pump preferably comprises a working fluid which, in the temperature-entropy diagram, has a pitch of the dew line below 1000 (kg K ² ) / kJ. The advantage of using such a working fluid is its excellent environmental and safety properties. For example, as such, working fluids from the family of fluoroketones can be used. Particularly advantageous therefrom are the working fluids Novec649 (dodecafluoro-2-methylpentan-3-one) and Novec524 (decafluoro-3-methylbutan-2-one). Novec649 has a dew line slope of 601 (kgK ² ) / kJ, Novec524 has a dewline slope of 630 (kgK ² ) / kJ, and a Another suitable example is R245fa (1,1,1,3,3-pentafluoropropane), which has a slope in the TS diagram of 1653 (kgK ² ) / kJ, the slope being indicated in each case for a saturation temperature of 75 ° C.

According to the invention, a working fluid is used in a heat pump, which has a slope in the tau line in the temperature-entropy diagram of less than 1000 (kg K ² ) / kJ.

In the method according to the invention for operating a heat pump, the temperature of a working fluid after compression is brought to a predeterminable minimum distance, in particular of one Kelvin, above the dew point.

Figur 1

zeigt ein logarithmisches Druck-Enthalpie-Diagramm eines neuen Arbeitsmediums und einen damit gefahrenen Wärmepumpenprozess mit 50 Kelvin Temperaturlift.

Figur 2

zeigt den Wärmeübertrag durch den internen Wärmeübertrager in einem logarithmischen Druck-EnthalpieDiagramm.

Figur 3

zeigt ein logarithmisches Druck-Enthalpie-Diagramm des Arbeitsmediums wie in Figur 1 mit einem Wärmepumpenprozess mit 20 Kelvin Temperaturlift.

Figur 4

zeigt ein logarithmisches Druck-Enthalpie-Diagramm des Arbeitsmediums wie in Figur 1 mit einem Wärmepumpenprozess mit 60 Kelvin Temperaturlift.

Figur 5

zeigt ein Fließbild einer Wärmepumpe mit Rohrleitungsheizung,

Figur 6

ein Fließbild einer Wärmepumpe mit Heißgas-Bypass und

Figur 7

zeigt ein Fließbild einer Wärmepumpe mit automatischem Expansionsventil.

Embodiments of the present invention will be described by way of example with reference to FIGS FIGS. 1 to 7 the attached drawing:

FIG. 1

shows a logarithmic pressure-enthalpy diagram of a new working fluid and a heat pump process with 50 Kelvin temperature lift.

FIG. 2

shows the heat transfer through the internal heat exchanger in a logarithmic pressure-enthalpy diagram.

FIG. 3

shows a logarithmic pressure-enthalpy diagram of the working medium as in FIG. 1 with a heat pump process with 20 Kelvin temperature lift.

FIG. 4

shows a logarithmic pressure-enthalpy diagram of the working medium as in FIG. 1 with a heat pump process with 60 Kelvin temperature lift.

FIG. 5

shows a flow diagram of a heat pump with pipeline heating,

FIG. 6

a flow diagram of a heat pump with hot gas bypass and

FIG. 7

shows a flow diagram of a heat pump with automatic expansion valve.

The FIGS. 1 to 4 show pressure-enthalpy diagrams in which the pressure p is plotted on a logarithmic scale. In the diagrams 1, 3 and 4, the isotherms IT and dotted the isentropes IE are shown in dashed lines. The temperatures for the isotherms IT in degrees Celsius, the entropy values for the isentropes IE in kJ / (kg · K) are given. The consistently drawn curve is in each case the phase boundary line PG of a new working medium, for example the fluid Novec649. This has a critical point at 169 ° C. The dew line would be inclined by 601 (kgK ² ) / kJ in the temperature-entropy diagram. Another suitable example of a working medium is Novec524 with a critical point at 148 ° C.

In the FIG. 1 In addition, a heat pump process WP is shown in dashed lines. Beginning from the state point 1, a compression leads to the state point 2 or 3, which coincide in purely theoretical considerations and will be referred to below only as the state point 2. By means of a condensation process, the state point 4 is reached. From the state point 4 to the state point 5 there is a subcooling. From the state point 5 to the state point 6, one arrives via an expansion process and from the state point 6 to the state point 7 via an evaporation process. The path from state point 7 back to the starting point 1 is an overheating of the working medium. The heat pump process WP shown has an evaporation temperature at 75 ° C and a condensation temperature at 125 ° C, so a temperature of 50 Kelvin. The subcooling from 4 to 5 or the superheating from 7 to 1, as in FIG. 2 clarified, coupled by an internal heat exchanger IHX. This uses the heat generated during the supercooling and transfers it to the Condition 7. At constant pressure, supercooling will reduce enthalpy by the same amount as overheating. The distance of state 2 from the tau line TL in the heat pump process WP, ie the temperature difference of state 2 to its dew point at the same pressure is 10 Kelvin. This minimum distance is sufficient to ensure a stable operation of the heat pump 10 without endangering the compressor 11 by liquid hammer. In order to reliably place the compression end point, ie state 2, outside the mixed phase region l + g, ie outside the phase boundary line PG, a minimum distance should be maintained, which must be set for each system of working fluid and heat pump 10 depending on possible fluctuation parameters. In particular, however, a minimum distance of one Kelvin, advantageously a minimum distance of 5 Kelvin should be maintained.

As in the Figures 3 and 4 2, the temperature lift of the heat pump process WP changes whether the exchanged heat quantity Q _IHX through the internal heat exchangers IHX for overheating the suction gas before the compressor 11 is sufficient to place the compression end point 2 in the gas phase region g.

In the FIG. 3 is, for example, again a heat pump process WP with the working medium Novec649 as in the FIG. 1 which, however, has a condensation temperature of only 95 ° C. This temperature lift of 20 Kelvin is thus below the limit for this system. The internal heat exchanger IHX would operate in this example with a power of 0.64 kW.

The in FIG. 4 shown heat pump process WP has a very high temperature lift of 60 Kelvin up to a condensation temperature of 135 ° C. In this heat pump process WP, the internal heat exchanger IHX, for example, operates with a power of 5.9 kW. In this case, the compression end point 2 is very far away from the tau line TL, the temperature lift thus clearly exceeds the limit value of the temperature lift for this system of heat pump 10 and work equipment.

The example _values for the transferred heat output Q _IHX through the internal heat exchanger IHX refer to a capacitor output of 10 kW. In these examples, therefore, not enough heat can be transferred at a small temperature lift of 20 Kelvin to maintain a minimum distance of, for example, 5 Kelvin for this system. At a temperature _lift of 60 Kelvin, however, the transferred heat Q _{IHX of} the internal heat exchanger IHX is sufficient for the minimum distance. The temperature lift of 60 Kelvin is therefore above the limit temperature lift for this system. For the system of heat pump 10 with Novec649 and 10 kW capacitor capacity described here by way of example at an evaporation temperature of 70 ° C., the limit temperature lift is 37 Kelvin. If, for example, Novec524 were used as the working fluid with otherwise identical parameters, the limit temperature lift would be 31 Kelvin.

Accordingly, a limit temperature lift can be determined correspondingly for each heat pump working fluid system, above which an internal heat exchanger IHX can maintain the necessary heat for maintaining the minimum distance of the compression end point 2 from the tau line TL. If the temperature lift is below the limit temperature lift, work must be done with a system as described in this application to ensure the compression end point 2 at the minimum distance to the tau line TL. Only in this way can a stable stationary operation be realized with low taulin control fluids in heat pumps 10.

The FIGS. 5 to 7 show embodiments of heat pump 10 with different control options for the use of new work equipment. Thus, heat pump processes WP with too low a temperature lift below the limit temperature lift can nevertheless be operated stably stable. The starting point is in each case an evaporation temperature at 70 ° C and a Condensation temperature at 100 ° C, so a temperature of 30 Kelvin, which would be in both cases for the working fluid Novec649 as well as for Novec524 below the temperature limit lift. The capacitor power, for example, is 10 kW. In the Figures 5 and 6 Two alternative temperature controls are shown. In these cases, the heat pump 10 is operated with a conventional expansion valve 14, which may be, for example, a thermostatic or an electronically controlled expansion valve 14. This expansion valve 14 regulates the flow of the working fluid and the superheat after the evaporator 15. Between the internal heat exchanger 13 and the compressor 11, a piping heater 20 is then arranged around the pipe section between the internal heat exchanger 13 and the compressor 11 around. By means of this pipe heater 20, the working medium flowing therein can be heated. How much the pipe heater 20, the working fluid in the state 1 is heated over the temperature T ₂ at state 2, that is regulated at the output of the compressor 11. For this purpose, the temperature T _{2 is} measured there and, via an adjustment to a minimum distance of the temperature T _1, the heating is switched on or off or its heating power is lowered or increased.

In the FIG. 6 shown temperature control device 30 includes a hot gas bypass 31, the compressed gas from the pressure side 2 of the compressor 11 back to the suction side 1 of the compressor 11 and thus further heated by the hot pressurized gas, the suction gas. The increase in the temperature T _{1 of} the suction gas is limited by a bypass valve 31, which in turn is controlled by the temperature T ₂ in state 2. The valve 31 may be a thermostatically or electronically controlled valve 31. The additional power required for this temperature control 30 is, for example, 0.58 kW, which is an additional compressor output in an isentropic pressure and temperature increase.

In FIG. 7 Finally, an alternative embodiment for temperature control 30 is shown, namely a control over the suction gas pressure: By using an automatic expansion valve 40, so a pure evaporator pressure control valve, it is possible to adjust the evaporation pressure and thus the evaporation temperature. By a reduction in pressure in the evaporator 15, the pressure ratio that the compressor 11 has to implement increases and thus the pressure gas temperature T ₂ in state 2. For the example with the temperature elevation of 30 Kelvin from 70 ° C to 100 ° C, the pressure of 1.96 bar are lowered to 1.35 bar so as to maintain the minimum distance of 5 Kelvin. For this purpose, for example, an additional compressor power at isentropic pressure and temperature increase by the compressor 11 of 0.45 kW is necessary.

It is possible with the regulation possibility through an automatic expansion valve, as in FIG. 7 shown, also another problem that can occur in the new working media to solve: if the temperature lift is very far above the limit temperature. Too high a distance of the compression end point 2 to the tau line T2 can be problematic because the compressor 11 may have an upper temperature limit. By the automatic expansion valve 40, however, it is possible to increase the pressure in the evaporator 15 so far that the fluid during the evaporation process only slightly overheated or even partially evaporated. The then possibly still necessary overheating for the minimum distance would again be possible via the internal heat exchanger 13. Thus, it is possible with this pressure control to cause an increase in pressure, which increases the overall efficiency of the heat pump 10, as by means of the temperature reduction at the state points 1 and 2, the pressure ratio P _ratio decreases, accordingly, a lower compressor power is necessary, at the same time the density of the fluid increases , which causes a higher power density in the compressor 11. In addition, due to the lower pressure gas temperature T _2, an increased service life of the compressor 11 can be assumed.

decreases, accordingly, a lower compressor power is necessary, at the same time the density of the fluid increases, causing a higher power density in the compressor 11. In addition, due to the lower pressure gas temperature T _2, an increased service life of the compressor 11 can be assumed.

Claims (5)

1. Heat pump (10) having a compressor (11), a condenser (12), an internal heat exchanger (13), an expansion valve (14), an evaporator (15) and a control device (21, 30), wherein a primary side of the internal heat exchanger (13), having the evaporator (15) and the compressor (11), and a secondary side of the internal heat exchanger (13), having the condenser (12) and the expansion valve (14), are fluidically connected, wherein the control device (21, 30) is designed to bring the temperature of the working fluid at the outlet of the compressor (11) to a predefinable minimum temperature difference with respect to the dew point;
   wherein the control device is a temperature control device (21, 30) which is designed to raise the temperature of the working fluid at the inlet to the compressor (11); characterized in that
   the temperature control device (30) comprises a bypass line with a valve (31), which connects the high-pressure region (2) at the outlet of the compressor (11) with the low-pressure region (1) at the inlet to the compressor (11) such that the working fluid flowing from the internal heat exchanger (13) to the compressor (11) can be superheated by means of the hot gas which can be recirculated via the bypass line (31).
2. Heat pump (10) according to Claim 1, wherein the control device (21, 30, 40') is designed to bring the temperature of the working fluid at the outlet of the compressor (11) to a predefinable minimum temperature difference of at least 1 kelvin above the dew point.
3. Heat pump (10) according to either of the preceding claims, having a working fluid which, in the temperature-entropy diagram, has a gradient of the dew line (TL) of less than 1000 (kgK²)/kJ.
4. Use of a working fluid in a heat pump (10) according to one of the preceding claims, wherein the working fluid has, in the temperature-entropy diagram, a gradient of the dew line (TL) of less than 1000 (kgK²)/kJ.
5. Method for operating a heat pump (10), having the following steps:

   providing a condenser (12), an internal heat exchanger (13), an expansion valve (14), an evaporator (15) and a control device (21, 30), wherein working fluid is conveyed from the evaporator (15) through a primary side of the internal heat exchanger (13) to the compressor (11), wherein working fluid is conveyed from the expansion valve (14) through a secondary side of the internal heat exchanger (13) to the condenser (12),
   wherein the temperature of a working fluid after compression is brought to a predefinable minimum temperature difference, in particular 1 kelvin, above the dew point;

   wherein the temperature of the working fluid at the inlet of the compressor (11) is raised; and

   superheating is effected using a bypass line with a valve (31) that connects the high-pressure region (2) at the outlet of a compressor (11) to a low-pressure region (1) at the inlet to the compressor (11).

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