DEVICE AND METHOD FOR OPERATING A THERMODYNAMIC CYCLE

Abstract

The invention relates to a device for operating a thermodynamic cycle, in particular an ORC process, comprising: a feed pump for conveying liquid working medium to an evaporator by increasing the pressure; the evaporator for evaporating and optionally additionally superheating the working medium by supplying heat; an expansion machine for producing mechanical energy by expanding the evaporated working medium; and at least two condensers connected in parallel between the expansion machine and the feed pump for condensing and optionally subcooling the expanded working medium. The invention further relates to a corresponding method for operating a thermodynamic cycle.

Claims

1. Device for operating a thermodynamic cycle, in particular an ORC process, comprising: a feed pump for conveying liquid working medium to an evaporator by increasing the pressure; the evaporator for preheating, evaporating and optionally additionally superheating the working medium by supplying heat; an expansion machine for producing mechanical energy by expanding the evaporated working medium; and at least two condensers connected in parallel between the expansion machine and the feed pump for deheating, condensing and optionally additionally subcooling the expanded working medium.

2. Device according to claim 1, wherein at least two condensers comprise a liquid-cooled condenser and an air condenser.

3. Device according to claim 2, wherein the liquid-cooled condenser is provided with a pump in a liquid circuit, in particular a heating circuit and/or wherein the air condenser comprises a fan.

4. Device according to claim 3, wherein the fan and/or the pump are controllable, in particular the rotational speed of the fan or the mass flow of liquid conveyed by the pump.

5. Device according to claim 1, wherein each condenser additionally is connected to the feed pump via a siphon, wherein by means of the vertex of the siphon, a minimum filling height of condensed working medium is determined in the condenser.

6. Device according to claim 1, wherein furthermore, between the condensers and the feed pump a pressure-tight container is provided.

7. Device according to claim 1, wherein for each of the parallel connected condensers a back-pressure valve is provided between the respective condenser and the feed pump and/or between the expansion machine and the respective condenser, wherein each back-pressure valve only allows a flowing in the direction of the feed pump.

8. Device according to claim 4, further comprising: a temperature sensor for measuring the temperature of a liquid/heating circuit return flow, and/or a temperature sensor for measuring the temperature of the liquid/heating circuit flow line; and a control device for adjusting a rotational speed of the fan and/or for adjusting a mass flow of liquid conveyed by the pump based on the measured temperature or the measured temperatures, in particular for limiting the return flow temperature to a maximum value and/or for adjusting a desired flow line temperature, in particular a constant flow line temperature.

9. Method for operating a thermodynamic cycle, in particular an ORC process, the method during normal operation comprising the following steps: conveying liquid working medium to an evaporator by increasing the pressure by a feed pump; preheating, evaporating and optionally additionally superheating the working medium by supplying heat in the evaporator; expanding the evaporated working medium in an expansion machine; deheating, condensing and optionally additionally subcooling of the expanded working medium with at least two condensers connected in parallel between the expansion machine and the feed pump.

10. Method according to claim 9, wherein a mass flow of the expanded working medium can be divided in mass flows of the expanded working medium into the respective condensers in a self-regulating manner by means of a pressure equilibrium.

11. Method according to claim 9, wherein the at least two condensers comprise an air condenser with a fan and/or a liquid-cooled condenser in a liquid circuit, and the method comprising the following further step: adjusting a rotational speed of the fan and/or adjusting a mass flow of the liquid conveyed by the pump, wherein by switching off the fan, no condensation of the working medium takes place in the air condenser while the pump is running, or wherein by switching off the pump, no condensation takes place in the liquid-cooled condenser while the fan is running.

12. Method according to claim 9, wherein during a start-up operation carried out before normal operation, the method further comprises: providing a sufficient positive suction head of liquid working medium in front of the feed pump in order to prevent cavitation in the feed pump; starting the thermodynamic cycle with the condenser, in which the lowest condensation pressure is present; and switching on the further condensers in the order of increasing condensation pressure.

13. Method according to claim 11, wherein during a start-up operation carried out before normal operation, the method further comprises: providing a sufficient positive suction head of liquid working medium in front of the feed pump in order to prevent cavitation in the feed pump; starting the thermodynamic cycle with the condenser, in which the lowest condensation pressure is present; and switching on the further condensers in the order of increasing condensation pressure, wherein the step of starting with a running fan of the air condenser and a switched off pump of the liquid circuit occurs, and wherein the step of switching on the liquid-cooled condenser occurs by switching on or increasing the conveyed mass flow of the pump.

14. Device according to claim 2, wherein each condenser additionally is connected to the feed pump via a siphon, wherein by means of the vertex of the siphon, a minimum filling height of condensed working medium is determined in the condenser.

15. Device according to claim 3, wherein each condenser additionally is connected to the feed pump via a siphon, wherein by means of the vertex of the siphon, a minimum filling height of condensed working medium is determined in the condenser.

16. Device according to claim 4, wherein each condenser additionally is connected to the feed pump via a siphon, wherein by means of the vertex of the siphon, a minimum filling height of condensed working medium is determined in the condenser.

17. Device according to claim 2, wherein furthermore, between the condensers and the feed pump a pressure-tight container is provided.

18. Device according to claim 3, wherein furthermore, between the condensers and the feed pump a pressure-tight container is provided.

19. Device according to claim 4, wherein furthermore, between the condensers and the feed pump a pressure-tight container is provided.

20. Device according to claim 5, wherein furthermore, between the condensers and the feed pump a pressure-tight container is provided.

Description

DRAWINGS

[0027] FIG. 1 schematically shows a first embodiment of the device according to the invention.

[0028] FIG. 2 shows the course of the condensate temperature during the starting process.

[0029] FIG. 3 shows the filling height in the air condenser and the heat condenser.

[0030] FIG. 4 shows the height ratios and filling levels of the condensers.

[0031] FIG. 5 illustrates the change in the heat quantity decoupled in the heat condenser without regulation of the heating water circulation pump and, thus, the change in the flow line temperature in the heating water.

[0032] FIG. 6 illustrates the change in the heat quantity decoupled in the heat condenser with the same flow line temperature in the heating water at the same time, with regulation of the heating water circulation pump.

[0033] FIG. 7 shows further embodiments of the device according to the invention, in particular with a siphon (FIG. 7a) and/or a container (FIG. 7b), and/or with back-pressure valves (FIG. 7c).

[0034] FIG. 8 shows the formation of a natural circulation during heating of the unused condenser 3.

EMBODIMENTS

[0035] When operating an ORC system with two parallel condensers, there are different operating states, for which particular operating parameters are to be ensured, respectively. The operating states to be considered are: start-up, stationary operation, load alternation between heat condenser and air condenser operation, and parallel operation of heat condenser and air condenser.

[0036] The operating parameters to be ensured are: appropriate fluid distribution for the load cases 100% air condenser operation, 100% heat condenser operation and parallel operation, as well as sufficient positive suction head for the feed pump in the different operating modes.

[0037] In the simplest embodiment of the ORC system, the necessary operating parameters can be achieved in all different operating modes via control-technological methods as well as an appropriate arrangement of components and a corresponding filling quantity with working medium. Additional components such as valves, etc. are not required. In the following, the devices and methods are described, by means of which the operating parameters may be kept in the simplest embodiment.

[0038] FIG. 1 shows the standard wiring of the system in a simplified manner. The liquid working medium is preheated, evaporated in the heat exchanger (evaporator) 1 by supplying heat, and subsequently expanded in an expansion machine 2 (e.g. screw expander, turbine). Downstream of the expansion machine, the distribution of the working medium mass flow takes place to the liquid-cooled condenser (heat condenser) 3 and the air condenser 4 (with a fan 7).

[0039] During condensation of the working medium in the heat condenser, heat is emitted to the heating water system, wherein the heating water is circulated via a pump 6. The circuit is closed by a feed pump 5 increasing the pressure of the working medium to the evaporation pressure and conveying it repeatedly into the evaporator 1. In the wiring, the flow of the working medium or the distribution of the working medium is not regulated via valves, but occurs merely thermally driven.

1. Start-Up

[0040] For the operation of an ORC system with two condensers, it is important to ensure a reliable system start. In order to ensure a start without cavitation at the feed pump, it is required to increase the pressure in front of the feed pump monotonously, moreover, at the beginning of the starting process, a minimum net positive suction head NPSH.sub.r has to be ensured

[0041] In case of a switched off, cold system, a low condensation pressure with low condensation temperature occurs. Even in case of a warm heat condenser, the condensation pressure due to the heat emission to the environment via the air condenser, will assume the saturation pressure at ambient temperature. During the starting process, the condensation pressure now increases, whereby also the condensation temperature increases. If the pressure is now dropped in front of the pump, heated working medium with low pressure would be present. Thus, the present subcooling of the working medium decreases, which may lead to cavitation in the pump. Consequently, it has to be ensured that sufficient subcooling always prevails during the starting process. This may be achieved by two ways. On the one hand, via the filling height and the fluid distribution in the condensers, a subcooling must be ensured, which allows pressure fluctuation without the risk of cavitation. On the other hand, it can be ensured via the regulation that during the starting process, the condensation pressure increases monotonously. This may be achieved due to the fact that the system is started in the air condenser operation. Thus, the system starts its operation under low pressure. Subsequently, the system smoothly turns into the heat condenser operation. If the temperature of the heat condenser is higher than the ambient temperature (which almost always applies), the condensation pressure will slowly increase monotonously.

TABLE-US-00001 TABLE 1 (starting process): Position Heating Air Condensation working Phase condenser condenser pressure medium 1. warm, cold, since low (saturation in the air System since on cooled due at ambient air condenser switched temperature to ambient temperature) off due to air heating water 2. warm cold, increases still in the air System growing condenser, start warmer since the heat (beginning) condenser temperature is still higher than the condensation temperature, which is defined by the air condenser 3. warm warm further Depending on System increased the state of start equilibrium, (progressed) divided in the air condenser and the heat condenser 4. warm warmer high working System than the medium mainly start heat in the heat (completed) condenser condenser

[0042] Table 1 shows the sequence of the starting process. In phase 1, the system is switched off. The condensate temperature and, thus, the condensation pressure are low (see FIG. 2). The condensate temperature T.sub.kond is equal to the temperature of the ambient air T.sub.L.

[0043] In phase two, the system is started, the condensation pressure slowly increases. Fluid begins to move into the heat condenser (see FIG. 3). The filling height L.sub.HK increases in the heat condenser. The condensate temperature increases to the temperature T.sub.VL of the flow line in the heating system. From the condensate temperature, which allows a condensation in the heat condenser (phase 3), the condensation in the heat condenser takes place substantially. The filling height LHC in the air condenser is reduced in this phase. The condensate temperature approaches the temperature T.sub.RL of the return flow in the heating system. In phase 4, the start is completed and a mere heat condenser operation is active.

2. Stationary Operation

[0044] In the stationary operation, the working medium will always flow into the colder condenser, since a lower pressure prevails there. Due to the self-regulating system, the colder condenser is the one, in which the condensation shall take place. In the air condenser operation, the air condenser is flowed through with cold outside air, while the heat condenser in the stationary state assumes the temperature of the exhaust steam. This results in a lower pressure in the air condenser and the fluid (working medium) flows to the condensation through the air condenser. The condensation heat is emitted to the ambient air. In the heat condenser operation, the heat condenser is flowed through with the return flow of the heating water. This is colder than the exhaust steam temperature. Since the air condenser, when the fans are switched off, assumes a temperature (due to heat loss, only) near the temperature of the exhaust steam, the condensation takes place in the colder heat condenser.

[0045] 100% heat condenser or 100% air condenser operation:

[0046] The 100% operation conditions are achieved by switching off or reducing the performance of the fans or the heating water circulation pump, respectively, so that in one of the condensers, no heat can be emitted. Since the condensers on the working medium side are not separated by valves, a small part of the exhaust steam always flows through the non-required condenser and is cooled by natural convection or heat conduction.

[0047] The sufficient positive suction head of the working medium in front of the feed pump is adjusted by the filling height and the geodetic height of the liquid column above the pump. The geometric relationships between the heat condenser and the air condenser are thereby selected such that at the same filling quantity and operation of respectively one condenser, as much working medium is present in the condenser as required to achieve a sufficient subcooling. In the following section, the positive suction head required in the parallel operation of both condensers, is described in more detail.

Self-Stabilizing Method:

[0048] The method described here is self-stabilizing. This means that the condenser with the higher heat emission, always has the highest filling level, as well. This is due to the fluidic distribution of the fluids. There is always a state of equilibrium, in which there are no pressure differences between the two condensers. The total head p.sub.ges to be considered for this, is composed of the prevailing condensation pressure p.sub.kond and the geodetic pressure Δp.sub.geod, which is adjusted via the filling level Δ.sub.h, respectively.

Δp.sub.geod=ρ.Math.g.Math.Δl

p.sub.ges=p.sub.kond+Δp.sub.geod

[0049] If it is exemplarily assumed that in condenser b more heat is emitted than in condenser a, then, in view of the process parameters, the following table is valid (see FIG. 4 for illustration):

TABLE-US-00002 TABLE 2 (process parameter in FIG. 4): Position Parameter 1 V_dot, p a = b 2 V_dot a < b 3 p_kond a > b 4 p_geod. a < b 5 V_dot, p a = b h a < b Q_dot a < b

[0050] In the table, the process parameters V_dot identify the volume flow, p_kond the condensation pressure, p_geod the geodetic pressure, h the filling height, and Y_dot the heat flow. The positions 1 to 5 correspond for the respective condenser a or b to: after the expansion machine and before the division of the entire mass flow V_dot, p (Position 1), after the division and before the entry into the condenser (Position 2), in the condenser (Position 3), after the condenser and before combining the partial mass flows (Position 4) after combining and before the feed pump (Position 5). The comparison relates to the respective process parameters with regard to the two condensers a and b.

[0051] The higher volume flow in the direction of condenser b results in higher pressure losses than in condenser a (path 1 to 3a/b). Due to the higher pressure loss, the condensation pressure in condenser b must be smaller than in condenser a. Since both condensers are connected to one another, a pressure balance occurs via the geodetic pressure. This causes the filling level in the condenser b to increase to such an extent that there is no pressure difference between the condensers at point 5. Via the higher filling height, it is ensured that in the condenser, in which more heat is emitted, thus, in which also the larger part of the exhaust steam is condensed, a sufficient subcooling of the working medium flow is achieved and therefore, as well, a sufficient positive suction head before the pump is ensured.

[0052] In order to ensure a stable operation, the filling quantity of the system must be chosen such that none of the two condenser drains. Ideally, the filling quantity and the structural height of the condensers to one another interact such that for the moment, no or only minimal fluid is present in the respective unused condenser (100% heat condenser or 100% air condenser). This reduces heat losses and helps saving fluid.

3. Load Alternation Between Heat Condenser and Air Condenser

[0053] By the self-regulating principle, the load alternation is achieved due to fact that by adjusting the rotational speed of the fan and/or the mass flow conveyed by the pump, a sliding regulation of the condensation parts in the air condenser or in the liquid-cooled condenser occurs, in particular by switching off fans or heating pumps, respectively. This results in an increase of the pressure in the unused condenser and the condensation takes place in the other condenser, in which a lower pressure prevails.

4. Parallel Operation Between Heat Condenser and Air Condenser

[0054] If the full heat output is not required in the heating system, only a part of the heat emitted by the ORC system can be condensed into the heating system. The other part will then be emitted via the air condenser. Both condensers are operated in parallel. The parallel operation is achieved, for example, by operating the fans of the air condensers in part load. Thereby, regulation parameter may be a maximum temperature of the heating circuit return flow. In case of a too high heat input by the ORC into the heating circuit, the temperature of the return flow in the combined heat and power station (CHP) may increase. If this exceeds a certain maximum value, the emergency cooler is switched on in order to emit the heat surplus from the system. In order to avoid this, the ORC system must reduce the inputted thermal power at an early stage.

[0055] The desired flow line temperature may be another regulation parameter for the heating system. Due to a reduction of the fan rotational speed, less heat is emitted in the air condenser. Thereby, the condensation pressure increases from p.sub.1 to p.sub.2 and a part of the exhaust steam flows into the heat condenser and there rises the heat emission into the heating system. In case of an identical water volume flow (unregulated operation of the heating water circulation pump), the outlet temperature (=flow line temperature T.sub.VL) of the heating water increases from T.sub.VL,1 to T.sub.VL,2 (see FIG. 5). Therefore, the system can react to a changing customer's heat demand and input more heat into the heating system, if this is required. However, equally, an excessively large heat input is also prevented. If the heat customer does not consumes the heat, the return flow temperature (coming from the heating system) increases and, thus, also the flow line temperature. If a limit temperature is reached here, the system counteracts this and outputs more heat via the air condenser, by increasing the fan rotational speed again.

[0056] Additionally or alternatively to this, the heating water circulation pump may also be regulated, which allows a constant flow line temperature T.sub.VL in the heating system (see FIG. 6). Thus, heating systems may be supplied, which require a constant flow line temperature with different heat requirements (for example, for temperature-sensitive processes, or for hygienization, etc). By means of a regulation of the heating water circulation pump, the performance of the pump may be adjusted to the actual heat requirement and, thus, the efficiency of the system may be increased.

[0057] The sufficient positive suction head by means of corresponding subcooling of the fluid (working medium) is ensured by the self-regulating principle described under point 2. By means of a sufficient filling quantity with working medium it must be ensured that when dividing the working medium to both condensers, also a sufficient subcooling is present.

[0058] The simple ORC system with two condensers may be improved by various variations of the wiring so that the required operating parameters can be observed more securely (see FIG. 7).

1. Mounting a siphon (FIG. 7a)

[0059] By means of a siphon 8 in the condensate line, a minimum filling height can be determined in the condenser 3, 4, since the liquid level in the condenser must always be as high as the height of the siphon. Thereby, also the defined minimum subcooling is ensured.

2. Container (FIG. 7b)

[0060] A container 9 between the condensers 3, 4 and the feed pump 5 ensures that liquid working medium always flows to the pump. If operation conditions occur, in which one of the condensers drains and, thus, gaseous working medium flows in the direction of the pump, this is deposited in the container. Gas bubbles, as well, flowing along with the liquid working medium, which could cause (partial) cavitation on the feed pump, are also deposited in the container. If the container is not completely filled, and a liquid level is set, the working medium in the container aspires to a saturated state. This results in two possible cases: If the working medium is colder than the environment, it evaporates and a state of equilibrium between the liquid phase and the vaporous state occurs. However, if the working medium in the container is warmer than the ambient temperature, the heat is emitted to the environment and a condensation takes place in the container. This leads to the fact that the liquid level increases until the container is completely filled up. By impressing an additional partial pressure in the container, for example, by means of a non-condensing gas (see e.g. patent DE 10 2009 053 390 B3 on cavitation prevention), a sufficient subcooling is generated.

3. Back-Pressure Valves (FIG. 7c)

[0061] In certain cases, an undesirable natural circulation may occur between heat condenser 3 and air condenser 4 (see FIG. 8). If the unused condenser 3 is nevertheless heated, e.g. flowed through with hot heating water, evaporation occurs therein. The thereby falling filling level would unbalance the pressure equilibrium of the condensation pressure and the geodetic pressure due to different filling heights. In order to maintain this equilibrium, additional condensed working medium flows from the condenser 1. By means of installing back-pressure valves 10, either in the exhaust or the condensate line, this phenomenon is avoided.

[0062] The illustrated embodiments are only exemplary and the full scope of the present invention is defined by the claims.