EP2321589B1 - High temperature heat pump and method for the control thereof - Google Patents

Description

The invention relates to a high-temperature heat pump for heating a fluid, preferably water, to a temperature level up to 150 ° C, which is operated with carbon dioxide as a refrigerant in the trans-critical range. The heat pump is particularly suitable for industrial heat generation and heat recovery as well as for energy storage methods where hot water is used as the storage medium.

Heat pumps operated with carbon dioxide (R744) as refrigerant are becoming increasingly important. Carbon dioxide is a natural refrigerant and is characterized by a much lower global warming potential than conventional refrigerants, such. Hydrofluorocarbons, off. In contrast to the other natural refrigerants, carbon dioxide is not toxic, e.g. Ammonia and propylene ether, nor can it form explosive mixtures with air, e.g. Propane and butane.

The critical point of carbon dioxide is 31.1 ° C and 73.6 bar. Since heat pumps are almost always used to generate higher temperatures, they are inevitably operated in the transcritical range.

In DE 10 2005 044 029 B3 is a heat pump described, the refrigerant circuit is operated with a desuperheater, an evaporator, a compressor and a throttle body in the supercritical range. The heat pump has a control unit for controlling the throttle body. The throttle body is operated in response to a first pressure on the high-pressure side of the refrigerant circuit when a permissible overheating of the refrigerant in the refrigerant circuit is present. If the refrigerant in the refrigerant circuit is outside the allowable superheat, the throttle body is operated in accordance with a first temperature before the compressor.

With WO 2004/057245 A1 is a heat pump is described in which caused by overheating of the refrigerant upstream of the suction side of the compressor, an increase in the temperature of the refrigerant at the outlet of the compressor, without the pressure of the refrigerant is additionally increased on the output side of the compressor. The overheating should be achieved for example with a countercurrent heat exchanger. Thus, the desired heating of water to 60-90 ° C is achieved.

In heat pumps operated with carbon dioxide, so-called internal heat exchangers are frequently used with which heat is transferred from warmer carbon dioxide flowing back from the gas cooler into the evaporator to colder carbon dioxide emerging from the evaporator. With internal heat exchangers, on the one hand, an increase in the enthalpy change ΔH is achieved, on the other hand, however, the density of the refrigerant and, as a result, the refrigerant mass flow are reduced. When using carbon dioxide as the refrigerant, the performance figures of heat pumps can be increased by means of internal heat exchangers, since the advantageous increase in the enthalpy change outweighs the disadvantageous reduction of the density.

EP 1 396 689 A1 discloses a refrigerant circuit consisting of a compressor, a heat exchanger, a refrigerant collector, an expansion valve and an evaporator. To further cool the refrigerant after exiting the heat exchanger and before entering the refrigerant receiver, this flows through an internal heat exchanger. The energy extracted from the refrigerant is returned to the refrigerant after it has left the evaporator and before it re-enters the compressor. In the case of carbon dioxide heat pumps with smaller outputs in the range from 5 to 50 kW, which are used to supply single and multi-family houses with hot water and for heating, a high-pressure regulation is usually installed. The evaporator is followed by a refrigerant collector, which receives from the evaporator leaking liquid carbon dioxide. With such heat pumps, however, no optimal performance figures can be achieved, since the refrigerant injection is not regulated in the evaporator and can not be avoided that liquid carbon dioxide exiting the evaporator. However, with smaller power heat pumps these disadvantages are usually accepted due to the lower manufacturing cost.

A transfer of this principle to heat pumps with larger outputs of 0.5 to 20 MW, as used for example to generate heat in industrial plants but is not acceptable. Compared to heat pumps for the building services sector, such heat pumps have a much higher purchase price. Therefore, the additional costs for an improved control are less significant, while on the other hand an improvement in the coefficient of performance results in a higher absolute energy gain.

Finally, in JP 2002 162123 A a run with carbon dioxide as a high-temperature heat pump with an evaporator, at least one internal heat exchanger, a compressor, three series condensers (gas cooler), a refrigerant collector and a refrigerant injection valve shown, in which the input of the refrigerant collector via a controllable expansion valve and the first inner Heat exchanger with the outlet of the series circuit of the gas cooler and the outlet of the refrigerant collector is connected to the expansion valve. Another internal heat exchanger, which serves to heat the carbon dioxide before entering the compressor by means of hot water, is connected on the refrigerant side between the outlet of the first internal heat exchanger and the inlet of the compressor. With the heat pump, temperatures up to 150 ° C should be achievable.

With this heat pump, which improves the process management, but no control of a fixed process, in particular no control of the volume flow of the hot water allows the pending object of the invention is not achieved.

The invention has for its object to provide a powered with carbon dioxide refrigerant heat pump, which allows simultaneous control of refrigerant superheat in the evaporator, the high pressure in the gas cooler system and the heating of the refrigerant upstream of the compressor by internal heat exchanger, whereby high hot water outlet temperatures can be achieved. With the method high performance numbers should be achievable.

This object is achieved by the features of claims 1 and 2. Further advantageous embodiments and uses emerge from the claims 3 to 13.

The starting point is a high-temperature heat pump for heating a fluid to temperatures of up to 150 ° C, which is operated with carbon dioxide as the refrigerant in the transcritical range. It is intended to use preferably water as the fluid, which is heated to temperatures of 100 to 130 ° C. The heat pump consists of an evaporator, at least two internal heat exchangers, a compressor, one or more series-connected gas coolers, a refrigerant collector and at least one refrigerant injection valve.

The inlet of the refrigerant collector is connected via a control valve (and via the first inner heat exchanger) to the (refrigerant side) outlet of the series connection of the gas cooler and the outlet of the refrigerant collector to the refrigerant injection valve. With this arrangement can be adjusted by the control valve, the high pressure in the gas cooler.

The coefficient of performance of the heat pump reaches its maximum value at a certain level of pressure in the gas cooler. If the pressure increases further, the hot water outlet temperature increases, but the coefficient of performance decreases. According to the invention, the control loop is also used to increase the hot water outlet temperature by raising the pressure. In order to keep the resulting reduction in the coefficient of performance as low as possible, the pressure is increased only so far that the required hot water outlet temperature is reached exactly.

From certain values of the pressure, only a small increase in the hot water outlet temperature is achieved by further pressure increase, while the performance figures are still sharply lower. Consequently, it is necessary to set a maximum limit in the control for the value of the high pressure, in which the disadvantage of reducing the coefficient of performance outweighs the advantage of the temperature increase.

According to the invention, in addition to the first inner heat exchanger, which causes a preheating of the carbon dioxide flowing to the compressor by the flowing back from the gas cooler carbon dioxide to the carbon dioxide before To further heat the inlet to the compressor, a second application-related internal heat exchanger, in which the carbon dioxide is heated by the hot water generated by the heat pump used. The second internal heat exchanger is connected on the refrigerant side between the outlet of the first internal heat exchanger and the inlet of the compressor.

Since the gas cooler each heat the water to different temperature levels and for preheating the carbon dioxide, a high temperature level is required, the water-side inlet of the second internal heat exchanger via at least one control valve (3-way valve) connected to the gas cooler water outlet side, the refrigerant side output over the first inner heat exchanger is connected to the refrigerant collector; This gas cooler produces the water with the lowest temperature level.

According to the invention, in the heat pump, the refrigerant superheating is controlled via the inflow of the refrigerant into the evaporator and the high pressure in the gas cooler via the volume flow of carbon dioxide from the gas cooler into the refrigerant collector.

To control the refrigerant superheat, the pressure and the temperature of the carbon dioxide at the outlet of the evaporator is measured, from which the refrigerant superheat is determined and compared in a control unit with a setpoint. If the target value is undershot, the influx of carbon dioxide into the evaporator is throttled by means of a refrigerant injection valve; if it is exceeded, the inflow is correspondingly increased by opening the refrigerant injection valve.

As the refrigerant injection valve of the evaporator either a pressure-controlled thermostatic valve or an electronic valve is used, which is controlled by means of temperature sensors for the evaporation temperature and the refrigerant outlet temperature from the evaporator.

To regulate the high pressure in the gas cooler, the high pressure is measured in one of the refrigerant pipes between the compressor outlet and the refrigerant collector with a pressure sensor arranged there. By means of a regulator, the actual value compared with the setpoint. If no increase in temperature of the hot water is required by an increase in pressure in the gas cooler, the setpoint corresponds to the pressure value at which the heat pump works with maximum coefficient of performance; otherwise it is, according to the necessary increase in temperature, above. When the setpoint is undershot, the flow of carbon dioxide from the gas cooler into the refrigerant collector is throttled by means of the control valve arranged between the gas cooler outlet and the refrigerant collector; if it is exceeded, the flow of carbon dioxide is correspondingly increased.

To further increase the coefficient of performance of the heat pump, in addition to the regulation of the refrigerant superheating and the high pressure, a stepwise regulation of the heat pump to the temperature value of the hot water is provided.

For this purpose, the temperature of the hot water is measured and compared in a controller with a setpoint. If the setpoint is exceeded, more is passed by means of a 3-way valve with actuator and one by-pass, and less if less carbon dioxide is passed by the first and second internal heat exchangers.

If already the entire refrigerant flow passed through the first and second heat exchanger and yet the target temperature of the hot water is not reached, then the volume flow of hot water through the second inner heat exchanger is additionally increased by means of another 3-way valve with actuator.

If the target temperature of the hot water is not reached, although the entire refrigerant flow is already passed through the first and second heat exchanger and also the volume flow of hot water through the second internal heat exchanger has already reached the maximum value, then the pressure in the gas cooler is increased.

If the target temperature of the hot water is not reached, although already the entire refrigerant flow is passed through the first and the second heat exchanger and the volume flow of hot water through the second internal heat exchanger and the pressure in the gas cooler have reached their maximum control limits, then the hot water volume flow is reduced by the gas cooler last.

The hot water volume flow through the gas cooler is adjusted via the speed of the water pump of the water cycle, by means of a throttle valve or via a controllable by a 3-way valve bypass to the water pump, which causes a partial return of the hot water.

Looking at the cycle of carbon dioxide-driven heat pumps in the temperature-entropy diagram (T-s diagram), it can be seen that the cooling of carbon dioxide in the gas cooler takes place along a curved line. The resulting heating of the water takes place in the T-s diagram along straight line sections that run below the curved line of the carbon dioxide. In this case, the area lying between the curved line and the straight line sections represents the power loss occurring in the heat transfer process. Consequently, the more accurately the curved line of the carbon dioxide is followed with a number n of straight line sections, the more effectively the heat transfer becomes. However, n gas coolers are required for this purpose, which each heat water to n different temperature levels. It is therefore in each case to weigh the expenditure on equipment against the achieved thereby increasing the efficiency.

The cycle of a carbon dioxide-powered heat engine runs in the opposite direction, but otherwise is almost identical.

With a heat pump according to the invention with n gas coolers energy can be stored with comparatively high efficiency and removed again by the heated by the gas cooler water with n different temperature levels stored in n separate hot water tanks and to remove the energy of a water-powered with carbon dioxide heat engine with n evaporators is supplied. In order to achieve high efficiencies, the n temperature values of the gas cooler and the evaporator must correlate with respect to the refrigerant circuit.

In industrial manufacturing processes, heat at different temperature levels is often required for the individual process steps. In a further advantageous use of the heat pump according to the invention with n gas coolers, the hot water with n temperature levels is used to supply industrial plants specifically with water to the temperature levels required for the individual steps of the production process.

In addition, the heat pump should be used primarily for the production of hot water with a temperature of 65 ° C. If the demand is met, the heat pump can also support the heating of the building. This is z. B. at a floor heating a flow temperature of 40 ° C is sufficient. In this case, the flow temperature is regulated down in reverse order by means of the control circuits.

Fig. 1:

Anlagenschema einer Wärmepumpe mit zwei Gaskühlern;

Fig. 2:

T-s-Diagramm des Wärmepumpenprozesses einer Wärmepumpe mit zwei Gaskühlern.

The invention will be explained in more detail with reference to an embodiment; show:

Fig. 1:

System diagram of a heat pump with two gas coolers;

Fig. 2:

Ts diagram of the heat pump process of a heat pump with two gas coolers.

In the case of the heat pump operated with carbon dioxide as the refrigerant, which is equipped with two gas coolers 2, 3, heat is transferred from a heat source to the carbon dioxide in the evaporator 6 by means of a liquid or gas stream ( Fig. 1 ). The incoming and outgoing liquid or gas flow is designated in each case by t _WQE and t _WQA .

From the evaporator 6, the carbon dioxide flows to the 3-way valve 18, which is controlled by means of the regulator 16, in which the temperature value of the hot water t _HWA2 (or t _HWA1 ) is compared with a predetermined desired value, and the actuator 17. If the real temperature value of the water is higher than the set value, the 3-way valve 18 is set so that more carbon dioxide through the bypass 9 on the series connection of the first inner heat exchanger 4 and second inner heat exchanger 5 is passed. If the real temperature value is lower, more carbon dioxide is conducted through the internal heat exchangers 4, 5 to the compressor 1.

In compressor 1, the pressure of the carbon dioxide is increased from about 45 to about 130 bar. The temperature of the carbon dioxide rises from less than 50 up to 150 ° C. The up to 150 ° C hot carbon dioxide is first passed through the first gas cooler 2 and then through the second gas cooler 3. The gas cooler 2, 3 act as a heat exchanger, so that cooled in the first gas cooler 2, the carbon dioxide and hot water at a temperature t _HWA2 up to 145 ° C and hot gas in the gas cooler 3 with a temperature t _HWA1 of about 70 ° C is generated ,

After leaving the second gas cooler 3, the carbon dioxide passes through the first inner heat exchanger 4 via the control valve 15 in the refrigerant collector 7. The control valve 15 is the regulator 13, the high pressure in one of the refrigerant lines between the outlet of the compressor 1 and the entry into the Control valve 15 measures and compares this with the setpoint of the high pressure, controlled. The pressure in the gas coolers 2, 3 is increased by falling below the setpoint, that by means of the control valve 15 and the actuator 14, the flow of the effluent from the gas cooler carbon dioxide is throttled. If the setpoint is exceeded, the flow of the effluent carbon dioxide is increased accordingly.

From the refrigerant collector 7, the carbon dioxide is directed to the refrigerant injection valve 8 of the evaporator 6, which is controlled by the electronic control unit 12. The control unit 12 measures via temperature sensors, the evaporation temperature and the refrigerant outlet temperature from the evaporator 6 and determines from the refrigerant superheat (overheating of carbon dioxide). This is compared with a setpoint. If the setpoint is undershot, the influx of carbon dioxide to the evaporator 6 is throttled by means of the refrigerant injection valve 8 and increased when it is exceeded by opening the refrigerant injection valve 8.

With the water pump 11, cool water is first conveyed into the second gas cooler 3. By means of the regulator 19, the actuator 20 and over the water pump bypass 22, a partial return of the water from the Pressure on the suction side of the pump, the incoming water quantity is regulated. In the controller 19, the current temperature _value t _HWA2 (or t _HWA1 ) is measured and compared with the predetermined setpoint. If the 3-way valves 18 and 24 are already controlled by means of the regulator 16 and the control valve 15 by means of the controller 13 to the control limit, but the target value is not reached yet, by means of the 3-way valve 21, the amount of water through the Bypass 22 increased.

After leaving the second gas cooler 3, part of the water, which now has the temperature t _HWA1 (also called the mean temperature), is diverted by means of the 3-way valve 10. The remaining water is passed to the 3-way valve 24, which divides the water flow into a flow through the first gas cooler 2 and a flow through the second inner heat exchanger 5. By means of the 3-way valve 24 and the actuator 23, the water flow through the second inner heat exchanger 5 is increased if the current temperature _value t _HWA2 (or t _HWA1 ) is below the setpoint and already by means of the 3-way valve 18 the entire carbon dioxide is passed through the series connection of the two inner heat exchanger 4, 5.

From the internal heat exchanger 5, water exits at a temperature t _HWA3 . Since the temperature t _{HWA3 is} only slightly smaller than the temperature t _HWA1 , this water is added to the water of the temperature T _HWA1 .

From the Ts diagram of the heat pump process ( Fig. 2 ) it can be seen that even with two straight line sections which represent the gas coolers 2, 3, a comparatively good adaptation to the curved curve, which represents the cooling of the carbon dioxide, is possible if the position of the middle temperature (characterized by the intersection of the two Straight sections) is controlled by means of the 3-way valve 10 to a value of about 80 ° C. It can also be seen from the diagram that if only one gas cooler were used, considerable heat transfer losses would occur.

List of reference numbers used

1

compressor

2

first gas cooler

3

second gas cooler

4

internal heat exchanger (refrigerant / refrigerant)

5

internal heat exchanger (refrigerant / hot water)

6

Evaporator

7

Refrigerant collector

8th

Refrigerant injection valve

9

Bypass (internal heat exchanger)

10

3-way valve (setting the middle temperature)

11

water pump

12

Control unit (evaporator injection)

13

Regulator (high pressure)

14

Actuator (high pressure)

15

Control valve (high pressure)

16

Regulator (input compressor)

17

Actuator (inlet temperature compressor)

18

3-way valve (inlet temperature compressor)

19

Regulator (incoming water quantity)

20

Actuator (incoming water quantity)

21

3-way valve (incoming water quantity)

22

Bypass (to the water pump)

23

Actuator (amount of water through internal heat exchanger)

24

3-way valve (amount of water through internal heat exchanger)

Claims (13)

1. A high-temperature heat pump for heating a fluid to a temperature level up to 150°C, said pump being operated in the transcritical range with carbon dioxide as coolant, comprising an evaporator (6), at least one inner heat exchanger (4), a compressor (1), one or more gas coolers (2, 3) connected in series, a coolant collector (7), and a coolant injection valve (8), wherein the inlet of the coolant collector (7) is connected via a control valve (15) and via the first inner heat exchanger (4) to the exit of the series connection of the gas coolers (2, 3), and the outlet of the coolant collector (7) is connected to the relief valve (8), and a further application-based inner heat exchanger (5), which is used to heat the carbon dioxide by means of hot water prior to entry into the compressor (1), is connected on the coolant side between the outlet of the first inner heat exchanger (4) and the inlet of the compressor (1), characterised in that the water-side inlet of the second inner heat exchanger (5) is connected via at least one control valve (24) to the water-side outlet of the gas cooler (3), of which the coolant-side outlet is connected to the first inner heat exchanger (4).
2. A method for controlling the high-temperature heat pump according to claim 1, characterised in that

   - the coolant pressure (po) and the coolant temperature (to) are measured at the exit of the evaporator (6), coolant overheating is determined on this basis, and this is compared in a control unit (12) with a target value, wherein, if the temperature falls below the target value, the inflow of carbon dioxide to the evaporator (6) is throttled by the coolant injection valve (8) of the evaporator (6), and, if the temperature exceeds the target value, the inflow of carbon dioxide to the evaporator (6) is increased by opening the coolant injection valve (8),

   - the high pressure (p_KKA) in one of the coolant pipelines between the compressor exit and the entry into the control valve (15) is measured and the actual value is compared by means of the controller (13) with a target value, which, if no temperature increase of the hot water via a pressure increase in the gas coolers (2, 3) is necessary, corresponds to the pressure value at which the high-temperature heat pump operates with maximum coefficient of performance, and otherwise the necessary temperature increase lies above this pressure value accordingly, wherein, if the temperature falls below the target value, the flow of the carbon dioxide from the gas coolers (2, 3) into the coolant collector (7) is throttled by means of the control valve (15) arranged between the inner heat exchanger (4) and the coolant collector (7) and is increased if the temperature exceeds the target value.
3. The method according to Claim 2, characterised in that a pressure-controlled thermostatic valve is used as coolant injection valve (8).
4. The method according to Claim 2, characterised in that an electronic valve is used as coolant injection valve (8) and is controlled by means of temperature sensors for the evaporation temperature and the coolant exit temperature from the evaporator (6).
5. The method according to Claim 2 to 4, characterised in that the temperature of the hot water is measured and is compared with a target value in the controller (16), wherein, if the temperature exceeds the target value, more carbon dioxide is passed by the first (4) and the second inner heat exchanger (5), and, if the temperature falls below the target value, less carbon dioxide is passed by the first (4) and the second inner heat exchanger (5).
6. The method according to Claim 5, characterised in that, if at high volume flow rates of the hot water by means of the 3-way valve (18) the total coolant flow is already conveyed through the first (4) and second inner heat exchanger (5) and the target temperature of the hot water is not reached, the volume flow rate of the hot water through the second inner heat exchanger (5) is increased by means of a 3-way valve (24) with actuating drive (23).
7. The method according to Claim 6, characterised in that, if the volume flow rate of the hot water through the second inner heat exchanger (5) reaches the maximum value and the target temperature of the hot water is not reached, the pressure in the gas coolers (2, 3) is increased.
8. The method according to Claim 7, characterised in that, if the pressures in the gas coolers (2, 3) have reached their maximum values and the target temperature of the hot water is not reached, the hot water volume flow rate through the gas coolers (2, 3) is reduced.
9. The method according to Claim 8, characterised in that the hot water volume flow rate through the gas coolers (2, 3) is adjusted via a bypass (22) for the hot water, which bypass can be controlled by means of the controller (19), the actuating drive (20) and the 3-way valve (21) and brings about a partial return flow of the hot water.
10. The method according to Claim 8, characterised in that the hot water volume flow rate through the gas coolers (2, 3) is adjusted via the rotational speed of a water pump (11) of the water circuit or by means of a throttle valve.
11. Use of the heat pump according to Claim 1 for storing energy, wherein the heat pump has n gas coolers, which each supply n separate hot water stores with water at a different temperature level, and, for removal of the energy, the water of the n hot water stores is conveyed in each case through one of n evaporators, connected in series on the coolant side, of a heat engine operated with carbon dioxide, wherein the water temperature values in the n gas coolers of the heat pump and in the likewise n evaporators of the heat engine are in each case adapted selectively along the flow path into these apparatuses to the temperature differences necessary for the heat transport.
12. Use of the heat pump according to Claim 1 for storing energy, wherein the heat pump has n gas coolers, which each supply n heat consumers at different temperature levels.
13. Use of the heat pump according to Claim 1 for the heating of fluids for technological processes, water heating, heating, or any combinations of the applications each with different target values for hot water inflow temperatures by the sequence of the intervention of the individual control loops in accordance with methods according to Claims 3 and 6 to 11.

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