DE102008046620B4 - High-temperature heat pump and method for its regulation - Google Patents

Abstract

High-temperature heat pump for heating a fluid to a temperature level up to 150 ° C, which is operated with carbon dioxide as the transcritical refrigerant, with an evaporator (6), at least two internal heat exchangers (4, 5), a compressor (1), or a plurality of series connected gas coolers (2, 3), a refrigerant collector (7) and a refrigerant injection valve (8), wherein the input of the refrigerant collector (7) via a control valve (15) and via the first internal heat exchanger (4) with the exit of the Series connection of the gas cooler (2, 3) and the output of the refrigerant collector (7) with the expansion valve (8) is connected, and the second internal heat exchanger (5), which serves to heat the carbon dioxide before entering the compressor (1) by means of hot water , refrigerant side, between the output of the first inner heat exchanger (4) and the input of the compressor (1) is connected, wherein the water - side entrance of the second internal heat exchanger (5) via at least one control valve (24) connected to one of the water-side outputs of the gas cooler (2, 3) ...

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. As fluorohydrocarbons, from. In contrast to the other natural refrigerants, carbon dioxide is neither toxic, such as. As ammonia and propylene ether, nor can it form explosive mixtures with air, such as. As 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 power from 0.5 to 20 MW, as z. B. are used 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.

The invention has for its object to provide a powered with carbon dioxide refrigerant heat pump, which allows simultaneous control of the 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, creating 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 3. Further advantageous embodiments and uses emerge from the claims 2 and 4 to 14.

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 internal 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 heat the carbon dioxide before entering the compressor, a second 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. On the water side, its inlet is connected via at least one 3-way valve to one of the water-side outlets of the gas cooler.

Since the gas coolers each heat the water to different temperature levels and no high temperature level is required for preheating the carbon dioxide, the second internal heat exchanger is preferably connected to the gas cooler water outlet side, the refrigerant side output is connected via the first inner heat exchanger 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 controller, the actual value is 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 gas is arranged between the gas cooler outlet and the refrigerant collector Control valve, the flow of carbon dioxide from the gas cooler throttled into the refrigerant collector, at an excess, the current of carbon dioxide is increased accordingly.

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 the entire refrigerant flow is already 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 reduced by the gas cooler.

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.

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

1 : Plant schematic of a heat pump with two gas coolers;

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 a refrigerant, with two gas coolers 2 . 3 equipped, is in the evaporator 6 Transfer heat from a heat source to the carbon dioxide by means of a liquid or gas stream ( 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 , 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 setpoint, and the actuator 17 is controlled. If the real temperature value of the water is higher than the setpoint, the 3-way valve becomes 18 adjusted so that more carbon dioxide through the bypass 9 on the series connection of the first internal heat exchanger 4 and second internal heat exchanger 5 is passed. If the real temperature value is lower, more carbon dioxide will pass through the internal heat exchangers 4 . 5 to the compressor 1 directed.

In the 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 through the first gas cooler 2 and then through the second gas cooler 3 directed. The gas cooler 2 . 3 act as heat exchangers, so in the first gas cooler 2 the carbon dioxide is cooled and at the same time hot water with a temperature t _HWA2 up to 145 ° C and in the gas cooler 3 hot water 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 internal heat exchanger 4 via the control valve 15 into the refrigerant collector 7 , The control valve 15 is from the regulator 13 which is the high pressure in one of the refrigerant pipes between the outlet of the compressor 1 and entering the control valve 15 measures and compares this with the setpoint of the high pressure. The pressure in the gas cooler 2 . 3 is increased in a fall below the setpoint, that by means of the control valve 15 and the actuator 14 the flow of carbon dioxide flowing out of the gas cooler 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 becomes the refrigerant injection valve 8th of the evaporator 6 that's about the electronic control unit 12 is controlled, directed. The control unit 12 uses temperature sensors to measure the evaporation temperature and the refrigerant outlet temperature from the evaporator 6 and determines from this the refrigerant overheating (overheating of the carbon dioxide). This is compared with a setpoint. When the setpoint is undershot, the refrigerant injection valve is used 8th the influx of carbon dioxide to the evaporator 6 throttled and when exceeded by opening the refrigerant injection valve 8th elevated.

With the water pump 11 First, cool water in the second gas cooler 3 promoted. By means of the regulator 19 , the actuator 20 and the bypass above the water pump 22 , which causes a partial return of the water from the pressure to the suction side of the pump, the incoming amount of water is regulated. In the regulator 19 the current temperature _value t _HWA2 (or t _HWA1 ) is measured and compared with the predetermined setpoint. Unless the 3-way valves 18 and 24 already by means of the regulator 16 as well as the control valve 15 by means of the regulator 13 are controlled to the control limit, but the setpoint is not yet reached, by means of the 3-way valve 21 the amount of water through the bypass 22 elevated.

After leaving the second gas cooler 3 becomes part of the water, which now has the temperature t _HWA1 (also called the mean temperature), with the help of the 3-way valve 10 diverted. The remaining water becomes a 3-way valve 24 directed the flow of water into a stream through the first gas cooler 2 and a current through the second internal heat exchanger 5 divides. By means of the 3-way valve 24 and the actuator 23 the flow of water through the second internal heat exchanger 5 increases 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 through the series connection of the two internal heat exchanger 4 . 5 is directed.

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 at the temperature t _HWA1 .

From the Ts diagram of the heat pump process ( 2 ) it can be seen that even with two straight sections that the gas cooler 2 . 3 represent, a comparatively good adaptation to the curved curve representing the cooling of the carbon dioxide is possible when the location of the mean temperature (characterized by the intersection of the two straight line sections) by means of the 3-way valve 10 is regulated 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

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 (14)

High-temperature heat pump for heating a fluid to a temperature level up to 150 ° C, which is operated with carbon dioxide as a refrigerant in the transcritical range, with an evaporator ( 6 ), at least two internal heat exchangers ( 4 . 5 ), a compressor ( 1 ), one or more gas coolers connected in series ( 2 . 3 ), a refrigerant collector ( 7 ) and a refrigerant injection valve ( 8th ), whereby the entrance of the refrigerant collector ( 7 ) via a control valve ( 15 ) as well as the first internal heat exchanger ( 4 ) with the exit of the series connection of the gas cooler ( 2 . 3 ) and the outlet of the refrigerant collector ( 7 ) with the expansion valve ( 8th ), and the second internal heat exchanger ( 5 ), the heating of carbon dioxide before entering the compressor ( 1 ) by means of hot water, refrigerant side between the output of the first inner heat exchanger ( 4 ) and the input of the compressor ( 1 ), wherein the water-side entrance of the second inner heat exchanger ( 5 ) via at least one control valve ( 24 ) with one of the water-side exits of the gas cooler ( 2 . 3 ) connected is. High-temperature heat pump according to claim 1, characterized in that the water-side inlet of the second internal heat exchanger ( 5 ) with the water-side outlet of that gas cooler ( 3 ) whose refrigerant-side outlet is connected to the first internal heat exchanger ( 4 ) connected. Method for controlling the high-temperature heat pump according to claim 1, characterized in that - the refrigerant pressure (p ₀ ) and the refrigerant temperature (t ₀ ) at the outlet of the evaporator ( 6 ), from which the refrigerant overheating is determined and stored in a control unit ( 12 ) is compared with a setpoint value, wherein when the setpoint is undershot by the refrigerant injection valve ( 8th ) of the evaporator ( 6 ) the influx of carbon dioxide to the evaporator ( 6 ) and when exceeded by opening the refrigerant injection valve ( 8th ), - the high pressure (p _KKA ) in one of the refrigerant pipes between the compressor outlet and the inlet to the control valve ( 15 ) is measured, by means of the controller ( 13 ) the actual value with a setpoint, which, if no increase in temperature of the hot water via an increase in pressure in the gas cooler ( 2 . 3 ) is required, the pressure value at which the high-temperature heat pump works with maximum coefficient of performance, and otherwise the necessary temperature increase is correspondingly above this pressure value, is compared, wherein when the setpoint is undershot by means of the between the first inner heat exchanger ( 4 ) and the refrigerant collector ( 7 ) arranged control valve ( 15 ) the flow of carbon dioxide from the gas cooler ( 2 . 3 ) into the refrigerant collector ( 7 ) is throttled and increased if exceeded. A method according to claim 3, characterized in that as a refrigerant injection valve ( 8th ) a pressure-controlled thermostatic valve is used. A method according to claim 3, characterized in that as a refrigerant injection valve ( 8th ) an electronic valve is used, which by means of temperature sensors for the evaporation temperature and the refrigerant outlet temperature from the evaporator ( 6 ) is regulated. A method according to claim 3 to 5, characterized in that the temperature of the hot water measured and in the controller ( 16 ) is compared with a setpoint value, whereby if the setpoint is exceeded more and less if there is less carbon dioxide on the first ( 4 ) and at the second internal heat exchanger ( 5 ) is passed. A method according to claim 6, characterized in that, if at high flow rates of the Hot water by means of the 3-way valve ( 18 ) already the entire refrigerant flow through the first ( 4 ) and second internal heat exchanger ( 5 ) and the setpoint temperature of the hot water is not reached, by means of a 3-way valve ( 24 ) with actuator ( 23 ) the volume flow of the hot water through the second internal heat exchanger ( 5 ) is increased. A method according to claim 7, characterized in that, if the volume flow of the hot water through the second internal heat exchanger ( 5 ) has reached the maximum value and the target temperature of the hot water is not reached, the pressure in the gas cooler ( 2 . 3 ) is increased. Method according to claim 8, characterized 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 through the gas cooler ( 2 . 3 ) is reduced. A method according to claim 9, characterized in that the hot water volume flow through the gas cooler ( 2 . 3 ) via a means of the controller ( 19 ), the actuator ( 20 ) and the 3-way valve ( 21 ) adjustable bypass ( 22 ) for the hot water, which causes a partial return of the hot water is set. A method according to claim 9, characterized in that the hot water volume flow through the gas cooler ( 2 . 3 ) about the speed of a water pump ( 11 ) of the water cycle or by means of a throttle valve. Use of the heat pump according to claim 1 for energy storage, wherein the heat pump has n gas cooler, each supplying n separate hot water tank with water at different temperature levels, and for removing the energy, the water of the n hot water tank each by one of n coolant side connected in series evaporators with Carbon dioxide-powered heat engine is passed, in each case the water temperature values in the n gas coolers of the heat pump and in the also n evaporators of the engine along the flow path in these apparatuses are specifically adapted to the temperature differences required for the heat transport. Use of the heat pump according to claim 1 for energy storage, wherein the heat pump has n gas cooler, each supplying n heat consumers at different temperature levels. Use of the heat pump according to claim 1 for the heating of fluids for technological processes, water heating, heating or any combination of applications, each with different setpoints for hot water flow temperatures through the sequence of engagement of the individual control loops according to the method of claims 3 and 6 to 11.

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