EP2499343B1 - Thermodynamic machine and method for the operation thereof - Google Patents

Description

The invention relates to a thermodynamic machine with a circulation system in which a particularly low-boiling working fluid circulates alternately in gas and liquid phase. The machine comprises a heat exchanger, a relaxation machine, a condenser and a liquid pump. The invention further relates to a method for operating such a thermodynamic machine, wherein the working fluid is heated in a circuit, relaxed, condensed and conveyed by pumping the liquid working fluid.

Under such a thermodynamic machine is particularly understood a machine that operates on the thermodynamic Rankine cycle. The Rankine cycle is characterized by pumping the liquid working medium, evaporating the working medium at high pressure, depressurizing the gaseous working fluid to perform mechanical work, and condensing the gaseous working fluid at low pressure. For example, today's conventional steam power plants operate according to the Rankine cycle. Typically, fossil-fired steam power plants produce water vapor at temperatures above 500 ° C at a pressure of over 200 bar. The condensation of the relaxed water vapor takes place at about 25 ° C and a pressure of about 30 mbar.

A working according to the Rankine cycle thermodynamic machine and a method for their operation is for example from the WO 2005/021936 A2 known. The working fluid is water.

If heat sources for the evaporation of the working fluid to be used, which only have a relatively low temperature difference to the heat sink, the achievable with the working fluid water efficiency is no longer sufficient an economic way of working. However, such heat sources can be developed with the help of so-called ORC machines, in which instead of the working fluid water, a low-boiling, in particular organic fluid is used. The term "low-boiling" is understood to mean that such a fluid boils at lower pressures relative to water or has a higher vapor pressure compared to water. An ORC machine operates according to the so-called Organic Rankine Cycle (ORC) cycle, ie essentially with a different, especially organic, low-boiling working fluid from water. As working fluids for an ORC machine, for example, hydrocarbons, aromatic hydrocarbons, fluorinated hydrocarbons, carbon compounds, especially alkanes, fluoroether, fluoroethane or synthesized silicone oils are known.

By means of ORC machines or systems, for example, the heat sources available in geothermal or solar power plants can be used economically to generate electricity. Also, with an ORC engine so far unused waste heat of an internal combustion engine from exhaust air, cooling circuit, exhaust gas, etc. can be used to perform work or to generate electricity.

If the vapor pressure of a liquid, which belongs to a particular temperature, is reached, it evaporates. The undershooting of the vapor pressure can take place in quiescent or in moving liquids. For example, in a flowing liquid due to a sharp deflection or acceleration of the flow locally below the vapor pressure, so that a local evaporation takes place. The locally produced vapor bubbles condense at points of higher pressure and collapse. The whole process is called cavitation.

In a thermodynamic machine of the type mentioned above, cavitation occurring in the liquid phase of the working fluid represents a not inconsiderable problem. Because of the small size of the vapor bubbles, the condensation takes place very quickly. A sudden implosion of the vapor bubbles may form a microbeam. Is this directed to a surrounding wall, local pressure peaks of up to 10,000 bar can be achieved. In addition, due to the high pressures, local temperatures of well over 1000 ° C can be reached, which can lead to melting processes in the wall material. Destruction effects from cavitation can occur within hours.

In addition, in a pump, the occurrence of cavitation undesirably reduces the flow rate of fluid. Since the density of the vapor bubbles is generally clearly different from that of the liquid, even with a small mass fraction of the working fluid, the mass flow which can be conveyed is reduced as steam at a given volume flow. With a strong formation of steam, the mass flow possibly even breaks down. For example, if the work machine is used as a pump in an ORC system, the entire cycle process may possibly come to a standstill. Due to the lack of pump power it comes to the backflow of the liquid working fluid in the condenser, whereby its effect is significantly reduced. As a result, the heat dissipation comes to a standstill. This state of the overall system is difficult to leave. It is necessary to wait until the working fluid undercooled itself by cooling. Next breaks the flow in the evaporator together, so that no heat can be dissipated. If necessary, the working fluid used can then be damaged by exceeding its stability limit.

For a machine operating according to the Rankine cycle, the problem of the occurrence of cavitation is, for example, in EP 1 624 269 A2 described. There is a cavitation in the working fluid water within the condenser and the subsequent pump can be prevented by the fact that the condenser, a specific pressure and temperature control is provided. For this purpose, appropriate pressure and temperature sensors are included. In particular, the water level in the condenser is maintained at a predetermined level. This is supported by a drain valve, which discharges water or non-condensing gases to the outside.

Also in the US 7,131,290 B2 For a Rankine cycle machine, the meaning of a constant water level in the condenser is described. In particular, the effect of a variable water level on the coming into effect cooling surfaces in the capacitor is specified. If, due to the negative pressure conditions prevailing in the condenser, non-condensing gas such as air penetrates into the circulatory system of the working fluid, this accumulates in particular in the condenser. To prevent a resulting loss of cooling performance, suggests the US 7,131,290 B2 a corresponding separation and discharge device before.

From the DE 10 2006 013 190 A1 For example, a complex fluid machine is known which operates according to the Rankine cycle. The fluid machine has a pump for pressurizing and pumping out a liquid-phase working fluid and an expansion device connected in series with the pump for generating a driving force by expanding the working fluid which is heated to become a gas-phase working fluid. It is provided to transfer the heat of the working fluid at an outlet side of the expansion device to the working fluid at an outlet side of the fluid pump.

From the DE 36 41 122 A1 a transportable drive unit for converting heat is known, which is designed as a thermodynamic machine of the type mentioned and operates according to the Rankine cycle.

From the DE 7 225 314 U a steam power plant is known, wherein an organic working fluid is used in the Rankine cycle.

Also from the US 4,291,232 is a thermodynamic machine of the type mentioned above known. As a working fluid, a gas / liquid solution, in particular an ammonia / water solution, circulates. By dissolving the gas in the liquid, the pressure of gas and liquid is lowered. By separating the gas under Termperaturerhöhung the pressure is increased.

Next is from the DE 198 53 206 C1 for a steam power plant, for a cavitation-free operation to divert an extraction steam flow into the steam space above the condensate in a feed water tank so that the pressure there is maintained. From the supply water tank thus charged, the feed water is supplied to the steam generator by means of a feed pump.

The object of the invention is to develop a thermodynamic machine of the type mentioned in that the occurrence of cavitation in the liquid or in the liquid working fluid is avoided as possible. Furthermore, it is an object of the invention to provide a corresponding method for operating such a thermodynamic machine, cavitation in the liquid being avoided as far as possible.

With regard to the machine, the object is achieved according to the invention by the feature combination according to claim 1. Accordingly, it is provided for a thermodynamic machine of the type mentioned that the liquid working fluid in the flow of the liquid pump by adding a non-condensing auxiliary gas, a system pressure-increasing partial pressure is impressed.

The invention is based on the recognition that, especially in the design of an ORC machine, the possibility of an occurrence of cavitation in the liquid phase is underestimated. So it happens that in the overall design, for example, a specified for a pump flow height is not met. Such a flow height caused by the fluid column at the intake there is a necessary pressure increase. Because of the upstream condenser namely the fluid is without regard to the flow height of the pump with the saturation or condensation vapor pressure, assuming that no hypothermia takes place. When the pump is switched on, the saturation vapor pressure can then be exceeded without regard to the flow height due to the resulting suction power. It comes to cavitation.

The flow height for a pump is typically given by the so-called NPSH value. The NPSH value (Net Positive Suction Head) is understood to mean the necessary minimum inlet height above the saturation vapor pressure. In other words, the necessary NPSH value expresses the suction power of the pump. The NPSH value is given in meters. It is typically a few meters for a pump suitable here. Therefore, if the NPSH value is not met for a given pump in advance, it will happen during the Operation to not insignificant cavitation problems. There is an undesirable formation of vapor bubbles.

Disadvantageously, the pump has to be lowered relative to the system level, especially in the design of a small and compact ORC machine, which leads to an undesirable increase in installation space.

Alternatives to avoid cavitation in the liquid phase of the working fluid, such as subcooling the working fluid to lower the vapor pressure, are expensive because of the added expense. Also results in an additional space requirement. In addition, more energy must be applied to heat the supercooled working fluid. To the same extent the use of a backing pump for generating an additional pressure at the suction is not economical. Incidentally, additional space is required by an additional pump.

Surprisingly, the invention now recognizes that the problem of the formation of cavitations in a thermodynamic machine can be solved by the use of a noncondensing gas. While hitherto in the machines operating in the Rankine cycle, the non-condensing gas in circulation has been undesirably removed since the efficiency has been lowered, the invention now deliberately introduces it.

The invention recognizes that, in the case of a non-condensing gas in circulation, its partial pressure in the gas phase adds to the condensation pressure. The resulting system pressure, which has been raised in the desired manner, is impressed on the liquid working fluid, in particular in the supply line of the fluid pump. The disadvantages associated with the addition of a non-condensing gas to the circuit, in particular an increase in the backpressure for the expansion machine, are eliminated in the case of a low-boiling working fluid by the advantages of avoiding cavitation. In the case of a low-boiling working fluid, condensation is made with water at higher pressures. Typically, at room temperature be condensed above atmospheric pressure. The partial pressure necessarily generated by the auxiliary gas has less effect on the overall efficiency in the sense of the overall concept and negligible.

In detail, the invention makes it possible to choose the added amount of substance of the auxiliary gas so that the flow height for the pump in the sense of the available space can be reduced accordingly. At the same time, it can be noted that the counterpressure hindering the expansion machine remains at a generally acceptable level.

The invention offers the distinct advantage that a compact thermodynamic machine for the utilization of low-temperature heat sources can be designed. The space is no longer mandatory given by the necessary flow height of the pump. Since, in principle, the non-condensing auxiliary gas can be introduced once during filling of the system, if necessary even no additional structural measures are required. In this respect, the invention offers an extremely cost-effective option for further compaction of a thermodynamic machine. The invention is thus outstanding, to design small mobile machines that are used for example on motor vehicles for the use of engine, coolant or exhaust heat.

In an advantageous embodiment of the resulting by the addition of the auxiliary gas partial pressure is sufficiently large, so that the saturation vapor pressure is not exceeded in the flow during operation of the liquid pump. As will be explained below, under certain simplification assumptions (no additional supercooling of the liquid) this is the case, for example, when the resulting partial pressure is at least equal to the NPSH value of the liquid pump. A flow height of the pump may possibly even be omitted altogether. Under real conditions, the amount of auxiliary gas supplied must be such that the resulting partial pressure exceeds the suction pressure or the converted NPSH value.

The invention is not necessarily limited to a thermodynamic machine operating on the Rankine cycle. By way of example, it is also possible to include a machine which does not comprise any evaporation of the working fluid upstream of the expansion machine but in which a flash evaporation of the working fluid takes place in the expansion machine through a continuously increasing working space. In particular, continuous phase conversions can be made.

In the sense of an ORC machine, mixtures of different working media can also be used as the working fluid so as to achieve an ideal mode of operation of the machine adapted to the given conditions.

With reference to Fig. 2 , Left field, adjusts in a thermodynamic machine of the prior art in the condenser according to the given temperature of the saturation vapor pressure p _{S of} the working fluid. When the pump is turned on to draw off the liquid phase of the working fluid, a suction pressure is generated at the intake manifold according to the given NPSH value. Around this suction pressure p _NPSH , the saturation vapor pressure p _S decreases. As a result, the pump results in an inlet pressure p _E which is less than the saturation vapor pressure p _S. Consequently, the formation of vapor bubbles and thus cavitation.

By an added non-condensing auxiliary gas (right part of the image FIG. 2 ) results in a system pressure at the pump, which is added from the saturation vapor pressure p _S and the partial pressure p _{part of} the auxiliary gas. After switching on the pump, this system pressure is again reduced by the suction pressure p _NPSH specified by the NPSH value. If the partial pressure p _{part of} this noncondensing gas resulting from the introduced auxiliary gas is greater than or at least equal to the suction pressure p _NPSH at the intake manifold of the pump, then the inlet pressure p _E is at least equal to or greater than the saturation vapor pressure p _s . Cavitation is thus prevented.

For a desired pressure difference Δp between the system pressure and the saturation vapor pressure to be applied by the auxiliary gas, this is advantageously at least p _NPSH , the necessary substance amount x _{i of} the auxiliary gas is calculated after $${\mathrm{x}}_{\mathrm{i}}=\frac{\mathrm{\Delta }p}{\mathrm{\Delta }p+p_s}\mathrm{,}$$

For a real system, the amount of substance x _{i of} the auxiliary gas is then dimensioned such that sufficient auxiliary gas is present even under unfavorable conditions, that is to say with reduced condensation temperatures and thus reduced saturation vapor pressures. It should also be noted that part of the auxiliary gas goes into solution and thus is no longer available for generating a pressure difference. Also, different operating phases of the machine (partial load, full load) can be taken into account in the dimensioning of the supplied amount of material of the auxiliary gas.

In a preferred embodiment of the machine according to the aforementioned embodiments, the height can be correspondingly reduced by the fact that the actual flow height of the liquid pump with respect to a necessary flow height, which takes into account the NPSH value and optionally a supercooling of the liquid working fluid is reduced. By an additional subcooling of the liquid, the necessary flow height will decrease due to the reduced vapor pressure. The possible, further reduction of the actual flow height is given by the partial pressure of the introduced auxiliary gas. In this case, to maintain certain reserves even a low flow height can be maintained despite appropriate supply of the auxiliary gas. A reduction of the flow height is compensated insofar by a corresponding amount of substance of the auxiliary gas.

The introduction point for the auxiliary gas can in principle be provided at any point in the circulation system of the machine. The introduction point can be designed here for a single introduction or for a repeated introduction of the auxiliary gas. In a preferred embodiment is a Einbringstelle provided for the auxiliary gas between the expansion machine and the liquid pump. Thus, the auxiliary gas is available directly at the required point in the circulation. The auxiliary gas is introduced into the liquid phase on the cold side of the cyclic process. In particular, the auxiliary gas can also be easily removed there, since it can be collected in the condenser. For this example, the machine can be "cold run", whereby the auxiliary gas flows slowly into the condenser. For example, a compressor may be used to add the auxiliary gas. Alternatively, a pressure bottle can be connected. An addition of the auxiliary gas on the hot side of the cycle is associated with additional expense.

The non-condensing auxiliary gas is such a gas which does not condense under the conditions prevailing or prevailing in the cycle of the thermodynamic machine. For example, noble gases or nitrogen are suitable as such an auxiliary gas. Also suitable organic gases come into question.

The non-condensing auxiliary gas will move to some extent with the working fluid in the cycle of the thermodynamic machine. Usually, in machines operating according to the Rankine cycle with the working fluid, water is provided for the condenser, so-called tube bundle heat exchangers. The tubes are flowed through by a cooling liquid inside.

The gaseous working fluid flows along the outside of the tubes, condenses on their surface and drips off as condensate or liquid phase.

However, disadvantageously, the non-condensing auxiliary gas optionally accumulates in such a condenser depending on its orientation. In this case, the auxiliary gas remains as an insulating layer around the tubes, thereby reducing the efficiency of the condenser. The non-condensing auxiliary gas can only be reduced by a withdrawal against the flow direction of the condensate or by diffusion.

In order to avoid this disadvantage when adding a non-condensing auxiliary gas, the condenser is advantageously configured to entrain the auxiliary gas in the flow direction of the condensate or of the liquid working fluid. Such a capacitor is designed, for example, as an air condenser or by means of plate heat exchange elements. In an air condenser, the gaseous working fluid flows through the interior of pipes, which are flowed around outside, for example, by air, but also by another coolant. In this case, the auxiliary gas is at least partially pushed by subsequent gaseous working fluid through the tubes in the flow direction. This also applies to capacitors which are formed by means of plate heat exchange elements. Again, the gaseous working fluid flows through the interstices of the plate heat exchange elements and will take part of the auxiliary gas from the condenser. The given for a tube bundle heat exchanger undesirable effect of forming an insulating layer is thereby reduced.

More preferably, a sensor for detecting the auxiliary gas concentration is arranged in the reservoir. By means of such a sensor, which will be arranged in the gas space above the collected liquid of the working fluid, for example, located in the circulatory system substance amount of the auxiliary gas can be measured and when falling below or exceeding a predetermined limit, a warning signal can be output. According to the warning signal then a certain amount of substance of the auxiliary gas can be supplied or withdrawn.

As described above, the specified thermodynamic machine is particularly suitable for a mobile system in a motor vehicle, wherein the heat exchanger is thermally coupled to a waste heat source of the vehicle. Such a waste heat source constitutes, for example, the coolant, other equipment such as e.g. Oil, the engine block itself or the exhaust gas.

The expansion machine coupled to generate electricity with a corresponding generator is preferably designed as a positive displacement machine. Such a displacement machine is for example a screw or piston expansion machine or a scroll expansion machine. Also, a vane machine can be used.

The object directed to a method according to the invention is achieved by the feature combination according to claim 9. Accordingly, it is provided for a method for operating a thermodynamic machine that the fluid pressure in a pump flow by adding a non-condensing auxiliary gas, a system pressure-increasing partial pressure is impressed.

Further preferred embodiments can be taken from the subclaims directed to a method. The advantages mentioned for the machine can be correspondingly transferred accordingly.

Fig. 1

schematisch eine ORC-Maschine mit einem im Pumpenvorlauf aufgeprägten Partialdruck eines Hilfsgases und

Fig. 2

eine schematische Darstellung verschiedener Druckverhältnisse.

Embodiments of the invention will be explained in more detail with reference to a drawing. Showing:

Fig. 1

schematically an ORC machine with an imprinted in the pump flow partial pressure of an auxiliary gas and

Fig. 2

a schematic representation of various pressure conditions.

In Fig. 1 is schematically shown an ORC machine 1, as it is particularly suitable as a mobile system for utilizing the waste heat of internal combustion engines. In this case, the ORC machine 1 comprises, in a circulation system 2 as a heat exchanger 3, an evaporator, an expansion machine 5, a condenser 6 and a liquid pump 8. The illustrated ORC machine 1 operates according to the Rankine cycle, wherein the expansion machine 5 Work to drive a generator 9 is performed. The generator 9 is designed in particular for feeding in the recovered current into the vehicle's on-board electrical system or connected thereto. As the working fluid 10, a hydrocarbon is used, which has a significantly higher vapor pressure compared to water. The working fluid 10 is in a closed circuit.

The conveyed via the liquid pump 8 liquid working fluid 10 is evaporated in the evaporator 3 at a high pressure. At the expansion machine 5, which is designed as a positive displacement machine, the gaseous working fluid 10 relaxes while performing the work. The expanded gaseous working fluid 10 is condensed in the condenser 6 at low pressure. The saturation vapor pressure in the condenser 6 is about 1.2 bar. The condensate or the liquid working fluid 10 is collected in a storage tank 11 before it is again pumped by the pump 8 for evaporation.

For cooling the condenser 6, a waste heat removal 14 is provided. For example, this may be circulating air of a motor vehicle, wherein the heat of condensation of the working fluid of the circulating air is supplied for heating the passenger compartment, for example. The condenser 6 is designed as an air condenser in which the working fluid 10 to be cooled flows in the interior of flow-around tubes.

For the evaporation of the funded by the pump 8 working fluid 10, the heat is supplied to the evaporator 3 via a waste heat supply 16. For this purpose, the evaporator 3 is supplied via a suitable heat exchange heat from the exhaust gas of the vehicle engine. Alternatively, heat can be supplied from the cooling circuit of the internal combustion engine. Also, the waste heat of the internal combustion engine and the exhaust gas generated can be supplied to the evaporator 3 in total via a corresponding third medium.

Between the expansion machine 5 and the liquid pump 8, a supply point 18 for introducing a non-condensing auxiliary gas 20 into the circuit of the ORC machine 1 is provided on the condenser 6. Via a corresponding valve, one or more times a specific amount of substance x _{i of} the auxiliary gas 20 can be introduced into the circulation of the ORC machine. The amount of substance x _i is dimensioned so that in the flow of the pump 8, the partial pressure of the auxiliary gas 20 and the saturation vapor pressure of the working fluid 10 (resulting from the condensation in the condenser 6) added to a system pressure such that after switching on the pump, the saturation vapor pressure of Working fluid is not fallen below. It is thereby also prevented from falling below the saturation vapor pressure at deflections of the flowing working fluid in the liquid phase. In particular, the amount of substance x _{i is} dimensioned such that the resulting partial pressure of the auxiliary gas is greater than the suction pressure corresponding to the NPSH value of the pump. In this respect, cavitation is prevented in the flow and in particular at the suction nozzle of the liquid pump. Since the saturation vapor pressure of the working fluid 10 does not fall below during operation, there are no vapor bubbles formed there.

The flow height 21 (shown schematically here) is clearly lowered compared to the NPSH value of the liquid pump 8 to only a few tens of centimeters. In the storage tank 11, a sensor 22 for measuring the concentration of the auxiliary gas 20 is arranged.

LIST OF REFERENCE NUMBERS

1

ORC machine

2

Circulatory system

3

Heat exchanger

5

expansion machine

6

capacitor

8th

liquid pump

9

generator

10

working fluid

11

storage container

14

heat dissipation

16

heat supply

18

concrete placement

20

auxiliary gas

21

forward height

22

sensor

Claims (14)

1. A thermodynamic machine (1) with a cyclic system (2), in which a particularly low-boiling working fluid (10) circulates alternately in a gaseous phase and liquid phase, with a heat exchanger (3), with an expansion machine (5), with a condenser (6), and with a liquid pump (8),
   characterized in that
   a partial pressure, which increases the system pressure, is applied to the liquid working fluid (10) in the head of the liquid pump (8) by the addition of a non-condensing auxiliary gas (20).
2. The thermodynamic machine (1) as claimed in claim 1,
   characterized in that
   the partial pressure which results by the addition of the auxiliary gas (20) is sufficiently high so that the saturation vapor pressure is not fallen short of in the head during operation of the liquid pump (8).
3. The thermodynamic machine (1) as claimed in claim 1 or 2,
   characterized in that
   the actual head height (21) of the liquid pump (8) is reduced compared with a necessary head height which takes into consideration the NPSH value and possibly a subcooling of the liquid working fluid (10).
4. The thermodynamic machine (1) as claimed in one of the preceding claims,
   characterized in that
   a point of introduction (18) for the auxiliary gas (20) is provided between the expansion machine (5) and the liquid pump (8).
5. The thermodynamic machine (1) as claimed in one of the preceding claims,
   characterized in that
   the condenser (6) is designed for entrainment of the auxiliary gas (20) in the flow direction of the working fluid (10), especially as an air condenser or by means of plate-type heat exchange elements.
6. The thermodynamic machine (1) as claimed in one of the preceding claims,
   characterized in that
   the expansion machine (5) is a positive displacement machine.
7. The thermodynamic machine (1) as claimed in one of the preceding claims,
   characterized in that
   a sensor (22) for detecting the auxiliary gas concentration is arranged in a header tank (11) of the liquid working fluid (10).
8. The use of a thermodynamic machine (1), as claimed in one of the preceding claims, as a mobile plant for a motor vehicle, wherein the heat exchanger (3) is thermically connected to a waste heat source (16) of the motor vehicle.
9. A method for the operation of a thermodynamic machine (1), wherein in a cyclic system (2) an especially low-boiling working fluid (10) circulates alternately in a gas phase and a liquid phase, and wherein the working fluid (10) is heated, expanded, condensed, and delivered by pumping of the liquid,
   characterized in that
   a partial pressure, which increases the system pressure, is applied to the liquid working fluid (10) in a pump head by the addition of a non-condensing auxiliary gas (20).
10. The method as claimed in claim 9,
    characterized in that
    the auxiliary gas (20) is introduced in such volume that the resulting partial pressure is sufficiently high so as not to fall short of the saturation vapor pressure in the pump head during delivery of the liquid working fluid (10).
11. The method as claimed in claim 9 or 10,
    characterized in that
    the auxiliary gas (20) is added to the expanded, gaseous working fluid (10).
12. The method as claimed in one of claims 9 to 11,
    characterized in that
    the auxiliary gas (20) is further transported, principally in the flow direction, during the condensing of the working fluid (10).
13. The method as claimed in one of claims 9 to 12,
    characterized in that
    the working fluid (10) is expanded in a positive displacement machine.
14. The method as claimed in one of claims 9 to 13,
    characterized in that
    waste heat (16) of a motor vehicle is used for heating and/or evaporating the working fluid (10).

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