EP3277933B1

EP3277933B1 - Combined control method of an organic rankine cycle

Info

Publication number: EP3277933B1
Application number: EP16719517.1A
Authority: EP
Inventors: Mario Gaia; Roberto Bini; Roberto Bertanzi; Paolo Belotti
Original assignee: Turboden SpA
Current assignee: Turboden SpA
Priority date: 2015-04-03
Filing date: 2016-03-31
Publication date: 2020-02-26
Anticipated expiration: 2036-03-31
Also published as: WO2016157116A1; EP3277933A1

Description

Technical field

The present invention relates to a combined control method for an organic Rankine cycle (hereafter, also ORC cycle). The control method is said combined for the simultaneous presence of a vapor admission valve in the turbine and a by-pass valve of the same turbine and thus for the adjustment by means of both valves.

Background art

As known, a thermodynamic cycle is a finite sequence of thermodynamic processes (e.g. isothermal, isochoric, isobaric and adiabatic processes) after which a system returns to its initial state. In particular, an ideal Rankine cycle is a thermodynamic cycle comprising two adiabatic processes and two isobars. Its purpose is to transform heat into work. This cycle is generally adopted principally in thermal power plants for the production of electrical energy and uses as working fluid, water, either in liquid form or in the form of steam, with the so-called steam turbine.
Document US2013074497 describes a waste heat recovery system that includes a Rankine cycle device in which working fluid circulates through a pump, a boiler, an expander and then through a condenser, heat exchange occurs in the boiler between the working fluid and intake fluid that is introduced into the internal combustion engine while being cooled, a determination device for determining required cooling load for the intake fluid, a pressure reducing device for reducing evaporation pressure in the Rankine cycle device, and a controller for controlling the pressure reducing device so as to reduce the evaporation pressure below a predetermined evaporation pressure if the required cooling load determined by the determination device exceeds a threshold.
Document US2011115445 describes a power system for delivering a custom level of electrical power to an industrial or commercial facility, comprising a local generator connected to a turbine operating with an organic Rankine cycle, one or more control devices operatively connected to the local generator for regulating active and reactive power generated by the generator, a detector for sensing active voltage induced by said generator, a detector for sensing reactive voltage produced by the generator, and a controller in electrical communication with said one or more control devices and with the active and reactive voltage detectors, wherein the controller directs the one or more control devices to regulate the generator such that the active power and reactive power generated by the generator are sufficient to satisfy active and reactive load conditions, of local industrial or commercial facilities.
More specifically, organic Rankine cycle (ORC) have been designed and realized, such ORC using organic fluids having high molecular mass for different applications, in particular also for the exploitation of thermal sources at low-medium enthalpy. As in other steam cycles, the system for an ORC cycle includes one or more pumps to supply the organic working fluid, one or more heat exchangers to achieve preheating, vaporization and possible overheating stages or heating stage for supercritical working fluid, a steam turbine for the expansion of the fluid, the turbine being mechanically connected to an electric generator or to a working machine, a condenser that returns the organic working fluid in the liquid state and eventually a regenerator to recover the heat downstream of the turbine and upstream of the condenser.
In particular, the supply of the turbine is realized by means of one or more admission lines, controlled by corresponding control valves. At the same time, for installation requirements, it is provided one or more by-pass lines, controlled by corresponding by-pass valves. In the following we will refer to a simplified schematic system that has a single line with a single admission valve and a single line with a single valve by-pass, but what will be said will not lose generality and remain valid also in the case of installations having more admission and / or by-pass lines.
In normal operating conditions of the system, the organic working fluid from the evaporator passes through the turbine with its maximum rate, in order to produce the maximum power. In this case the admission valve is open 100% and the by-pass valve is fully closed.
In some circumstances, the system operator or the grid operator can require a quick adjustment of the supplied power, with a time response of few seconds; in other words, it may be required that the power is reduced in a few seconds to reach a desired value (set point).
If the admission valve AV closes partially by means of a isenthalpic process, the fluid pressure is reduced at the turbine inlet, so as to decrease the power produced. The pressure reduction also involves a reduction of the fluid flow rate through the turbine. Therefore, the flow coming from the evaporator is lower and, consequently, the evaporator pressure begins to grow, due to an imbalance between the flow rate of the liquid entering the evaporator and the flow rate of the steam exiting the evaporator (more steam is produced than needed by the turbine). Therefore, it is understandable that the entire thermodynamic cycle is conditioned by an adjustment made by acting solely on the admission valve. This adjustment creates instability in some controls, for example in the control of the supply pump of the organic working fluid, because of rapid changes in the operating conditions. In cogeneration systems in which the heat discharged to the condenser is used, a variation of the heat supply to the user would also occur.

Invention summary

Aim of the present invention is to provide a combined control method for vapor thermodynamic cycles, in particular for ORC cycles, that is able to avoid the above described drawbacks. The control method will exploit the simultaneous presence of the vapor admission valve in the turbine and the by-pass valve of the same turbine, performing a combined adjustment that uses both valves.
According to the present invention, a combined control method for ORC cycles is described, the method having the characteristics as in the enclosed independent claim 1.
Further embodiments of the invention, preferred and/or particularly advantageous, are described according to the characteristics as in the enclosed dependent claims.
A further embodiment of the present invention is an organic Rankine cycle power plant as defined by appended independent claim 6, comprising at least one feed pump, at least one heat exchanger, an expansion turbine, a condenser and a control unit configured to operate a method according to any of the embodiments described above.
The method, according to any of its aspects, can be performed by means of a computer program as defined by appended independent claim 7, comprising a software to perform all the steps of the above described method, in the form of computer program product as defined by appended independent claim 8, comprising the computer program.
The computer program product is configured to be able to instruct a controller for an Organic Rankine cycle power plant, comprising a control unit, a data carrier and a computer program stored on the data carrier, so that the controller defines the embodiments of the invention in the same way as defined by the method. In this case, when the controller executes the computer program, they are also carried out all of the method steps as described above.

Brief description of the drawings

The invention will be now described by reference to the enclosed drawings, which show some non-limitative embodiments, namely:

Figure 1 schematically represents an ORC plant, for which the control method according to the present invention can be used;
Figure 2 is a graph showing the typical relationship between the flow coefficient of a throttle valve, both admission or by-pass valve, and the opening degree of the valve itself;
Figure 3 is a graph showing the output power, expressed as a percentage of the nominal power, as a function of the opening degree of the admission valve, in a typical installation;
Figure 4 is a further graph showing the relationship between the opening degree of the admission valve and the opening degree of the by-pass valve, to maintain a constant total flow rate;
Figure 5 is a logic diagram which allows to obtain the opening degree of both admission valve and by-pass valve as a function of the power set point;
Figure 6 is a block diagram that allows a system finer adjustment by means of a feedback control.

Detailed description

Referring now to the drawings and in particular to Figure 1, an Organic Rankine cycle (ORC) power plant 10 is indicated in its entirety. Each embodiment forming part of the invention comprises at least all features of at least one of the appended independent claims. Any embodiment not comprising at least all features of at least one of the appended independent claims does not form part of the invention but is merely helpful for understanding the present invention.
The power plant typically comprises at least a feed pump 1 to supply an organic working fluid, in liquid phase, to at least one heat exchanger 2. In the heat exchanger, which can in turn comprise a preheater, an evaporator and a superheater, the organic working fluid is heated until the change into the vapor phase and its possible overheating. Exiting the heat exchanger the vapor passes through an expansion turbine 3 producing the positive work of the ORC cycle. Such work is a positive mechanical work collected to the turbine shaft which is rigidly connected with an electric machine or other processing machine, for example an electric generator 4 which receives mechanical energy and transforms it into electrical energy. Finally, the organic working fluid passes through a condenser 5 that transforms it in the liquid phase to be sent from the pump 2 again to the heat exchanger. Advantageously, to increase the performance of the cycle, between the expansion turbine 3 and the condenser 5, a heat regenerator unit 6 can be added, i.e. a heat exchanger which exchanges heat between the organic working fluid in liquid phase, which is pumped by the pump 1 to the heat exchanger 2 and the organic working fluid in vapor phase that from the turbine 3 is directed to the condenser 5.
The supply of the organic working fluid in vapor phase to the expansion turbine 3 is realized by means of at least one admission line 7, controlled by a corresponding admission valve AV. Moreover, for installation requirements, at least one by-pass line 8 is provided, said by-pass line directly connecting the heat exchanger 2 with the heat regenerator unit 6 or with the condenser 5 and controlled by a corresponding by-pass valve BV. A control unit manages all control processes of the power plant.
In normal operating conditions of the power plant, the organic working fluid coming from the heat exchangers 2 passes through the expansion turbine 3 with its maximum flow rate, in order to produce the maximum possible power. In this case, the admission valve AV is 100% open and the by-pass valve BV is fully closed.
However, there are circumstances which asks the ORC power plant for a rapid adjustment of the delivered power, with response times of a few seconds; in other words, it may be required that the power is reduced in a few seconds to reach a desired value (set point). According to an embodiment of the present invention, the consequent reduction of the flow rate of the organic working fluid, due to the partial closure of the admission valve AV, can be compensated by suitably opening the by-pass valve BV. In practice, the flow rate of the organic working fluid that passes through the by-pass valve BV must be substantially equal to the difference between the flow rate corresponding to maximum power (with the admission valve AV 100% open and the by-pass valve closed) and the flow rate of the organic working fluid which passes through the admission valve AV, in partialized conditions, as required by the power plant controller. By doing so, the flow rate of the fluid flowing out from the heat exchanger 2 will not be modified, thus avoiding control instabilities suffered by the known applications. Similarly, the thermal power supplied to the condenser and then to users in the event of thermal cogeneration power plant remains almost unaltered.
Even if the fluid flow rate does not change during the adjusting step of the output power, the produced electrical power (and therefore the efficiency) will be reduced since a part of the thermal power extracted from the hot source is directly dissipated by the fluid flow rate which reaches the heat regenerator unit 6 or the condenser 5 through the by-pass line 8. Of course, if the power plant is a cogeneration type, i.e. a power plant for the production of electric and thermal power, the power discharged on the condenser is used for the thermal loads and therefore is not dissipated.
The proposed method is thus based on the fact that the thermodynamic cycle does not change if the total flow rate of the organic working fluid does not change either, nor the control logic of the cycle itself is modified, with respect to standard operating conditions. Therefore, oscillations of the fluid level or disturbances in general will be so avoided, not leading to instability in the control itself or variations of the thermal power discharged to the condenser.
The relationship between the opening degree of the admission valve AV and the by-pass valve BV is rather complex for the following reasons:

unlike the admission valve AV, the whole pressure drop existing between the heat exchanger (evaporator) 2 and the condenser 5 is applied to the by-pass valve BV, and therefore the latter valve usually operates in sonic conditions. It should be noted that the admission valve AV operates in sonic conditions only below a certain opening degree, in other words, when the valve is almost closed (if the evaporation and the condensation pressures are in a ratio greater than the so called "critical" one);
the number and the size of the admission lines (and therefore of the admission valves) may be different from the number and size of the by-pass lines; thus also the corresponding valves may have different characteristics;
the flow coefficient Cv of the butterfly valves does not have a linear trend with the opening degree of the valves themselves (this assumption is valid for both the admission valve and the by-pass valve): in Fig. 2 there is an example of a graph showing the relationship between the flow coefficient Cv of an admission valve (or by-pass valve) and the opening degree %AV, %BV of the valve itself;
the turbine normally operates under sonic conditions, and therefore the adjustment on the admission valve AV should produce a significant pressure drop in order to obtain a not negligible power reduction. As evident from the graph of Fig. 3, which shows the output power, expressed as a percentage of the nominal power, depending on the opening degree of the admission valve AV, the variation of the output power %P as a function of the same opening degree %AV has a strongly non-linear behavior. In other words, almost all power variation (in the example in Fig. 3, from 10% to 90%) is obtained by varying the opening degree %AV of the valve from 20% to 40%. Out of this range, the variation of the opening degree %AV implies negligible changes of the output power;
in case of large diameter valves (for example, greater than or equal to 10"), the fine adjustment is very difficult. In fact, since the organic working fluids are characterized by a high molecular weight, ORC systems often require large volumetric flow rates and consequently large diameter valves.

Despite these difficulties, it is possible to get the curves that represent:

the function between the opening degree %AV of the admission valve and the power output %P, as the one shown in Fig. 3;
the behavior of the opening degree %BV of the by-pass valve as a function of the opening degree %AV of the admission valve AV, as in the graph of Fig. 4, in order to maintain unchanged the total flow rate of the organic working fluid.

The adjustment curves of the admission valve AV and the by-pass valve BV are theoretically calculated on the basis of the flow coefficients of the same valves and according to the specific application. Alternatively, such adjustment curves can be evaluated experimentally by searching for each opening degree of the admission valve AV the corresponding opening degree of the by-pass valve BV which maintains the same total flow rate.
By means of these curves it is possible to control the position of the valves even without recurring to sophisticated control systems with feedback. Fig. 5 shows a logic diagram that allows to apply the proposed method, according to one aspect of the present invention. In detail, this method allows to obtain the opening degree of both admission and by-pass valves as a function of the power set point. The set point of the desired power S500, corresponding to the electrical energy request, sets S510 the opening degree %AV of the admission valve and the latter sets S520 the opening degree %BV of the by-pass valve, in order to ensure that the total flow rate of the organic working fluid remains unchanged. Therefore, in the presence of a predetermined electric power request, this method allows to establish S530 both the corresponding values of the opening degree %AV, %BV of the admission valve AV and the bypass valve BV.
It is clear that, if the boundary conditions of the ORC power plant significantly change (for example, the pressure in the condenser 5) compared to nominal conditions, it is possible to define a family of curves (%AV as a function of %P and %BV as a function %AV), whose individual curves will be used for each characteristic condition, with different power and pressure levels.
A practical example will further clarify the essence of the procedure. Initially, the expansion turbine 3 operates at rated power with the admission valve AV fully open and the by-pass valve BV fully closed. The supervisor of the power plant requires a power equal to 10% of the rated value, and then, from the Fig. 3 graph, the admission valve AV should be choked with an opening degree %AV equal to 20%. With this partialized value of the admission valve and using the graph in Fig. 4, it results that the by-pass valve BV must be opened with an opening degree %BV of about 30%. These opening degree values of the two valves are the final values, corresponding to the required power setting, in other words they are the target values. However, it must be considered that the admission valve AV must vary its opening degree of about 80% (100% - 20%), while the by-pass valve BV will vary its opening degree of only 30%. Furthermore, during the adjustment the two valves cannot move independently, otherwise there would be times when the invariance of the total flow rate would not be respected. In other words, the function between %BV and %AV shown in Fig. 4 has always to be respected in each instant of the transient. Transient which is not temporally negligible, since both valves AV, BV are normally moved by pneumatic actuators which have a relatively low speed (when compared with the speed of equivalent hydraulic actuators) typically comprised between 25 and 10 percentage points of the opening degree per second (the larger the valve, less quickly it can be adjusted).
In the above example, assuming a maximum speed equal to 10%/s (the same for both valves) the by-pass valve BV may reach the target in 3 seconds, while the admission valve AV in 8 seconds; then the position of the by-pass valve BV must be continuously modulated (slowed down) until the admission valve AV has reached the required position, respecting the constancy of the flow rate, as in Fig. 4.
Therefore, the slowest valve and/or the one that must change more the opening degree establishes the overall speed with which both valves will arrives to the final position, passing from intermediate positions that realize the constancy of the flow rate.
According to a different aspect, the present methodology can be implemented also using for a finer adjustment a controller of proportional-integral-derivative PID type, as shown in the block diagram of Fig. 6.
According to this scheme, having fixed S600 the value of the power set point at the generic time t, said set point value is compared S610 with the current value of the power. From the obtained difference, the PID controller determines S620 the theoretical value of the opening degree %AV. Whereas the valve speed is relatively low (especially for large valves), the opening degree required at time t + 1 is corrected S630 by taking into account said speed limit. Finally, according to the effective opening degree %AV, the transmitted power is measured S640, and this is the value of the control feedback.
In order to respect the constancy of the flow rate, the bypass valve BV assumes instant by instant the opening degree value calculated in S650 as a function of the relationship S520 that links the opening degree %BV with the opening degree %AV. For the same reason explained with regard to the admission valve, the opening degree required at time t + 1 is corrected S660 by taking into account the speed limit of the valve movement. More in particular, the positions of the AV and BV valves are modulated so as to ensure that the various opening degrees %AV, %BV of the valves (which have been obtained during movements of the same valves) realize the substantial constancy of the total flow rate. This means that one of the two valves will have to "wait" for the other in order to ensure the above mentioned principle.
The logics described in Fig. 5 and 6 can be used individually or in combination. For example, the admission valve AV and by-pass valve BV are moved toward the positions calculated using the logic of Fig. 5 and, once reached the calculated position, their final positions will be corrected using the PID controller, according to the logic of Fig. 6.
The proposed methodology, according to one or more of the above described aspects, defines a relatively simple mode for controlling the power delivered by the ORC power plant, without modifying the thermodynamic cycle.
Other than the embodiments of the invention, as above disclosed, it is to be understood that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims.

Claims

Combined control method of an Organic Rankine Cycle (ORC) power plant (10), wherein
- said plant comprises at least a feed pump (1), a heat exchanger (2), an expansion turbine (3) and a condenser (5), being the heat exchanger (2) and the expansion turbine (3) in fluid dynamic connection by means of at least one admission line (7) which is provide with an admission valve (AV) and being the heat exchanger (2) and the condenser (5) in fluid connection by means of at least one by-pass line (8) which is provided with a by-pass valve (BV), said Organic Rankine Cycle (ORC) power plant (10) furthermore comprising a control unit and a means for receiving a set point value of a required power,

- operating said Organic Rankine Cycle power plant (10) includes a step of feeding an organic working fluid, a step of heating and/or vaporizing the same working fluid, an expansion phase and a step of condensing the same working fluid,

- said combined control method comprising that said control unit regulates the power supplied from the Organic Rankine Cycle (ORC) power plant by determining an opening degree (%AV) of the admission valve (AV) as a function of the received set point value of the required power and determining an opening degree of the by-pass valve (%BV) as a function of the opening degree (%AV) of the admission valve (AV), so that the total flow rate of the organic working fluid remains substantially constant during changing of the power supply output.
Method according to claim 1, characterized in that said Organic Rankine Cycle (ORC) power system (10) further comprises means for obtaining the current value of power and in that said control unit is an integral-proportional- derivative controller (PID) that determines the value of the opening degree (%AV) of the admission valve (AV) as a function of the difference between said received set-point value of the power and the obtained current value of the power.
Method according to claim 1 or 2, characterized in that the opening degree (%AV) value of the admission valve (AV) is also a function of a predetermined limit speed of the admission valve (AV) movement .
Method according to claim 1 or 2, characterized in that the opening degree (%BV) value of the by-pass valve (BV) is also a function of a predetermined limit speed of the by-pass valve (BV) movement.
Method according to claims 3 and 4, characterized in that the activation speed of the valves (AV) and (BV) are modulated so as to ensure that in each instant of time of the adjusting period the opening degrees (%AV, %BV) of the valves allow to realize the substantial constancy of the total flow rate.
Organic Rankine cycle power plant, comprising at least the apparatus features specified for the Organic Rankine Cycle power plant (10) in one of the preceding method claims 1-5, wherein the control unit is configured to operate a method according to said one of the preceding claims.
A computer program comprising a software suitable for implementing the method according to one of claim 1 to 5 when run on a computer means comprised in a control unit of an Organic Rankine Cycle power plant (10) comprising at least the apparatus features specified for the Organic Rankine Cycle power plant (10) in said one of claims 1 to 5.
A computer program product on which is stored the computer program according to claim 7.