Rankine cycle system

Abstract

A Rankine cycle system includes an evaporator for heating water with thermal energy of exhaust gas of an engine so as to generate steam; a displacement type expander for converting the thermal energy of the steam generated by the evaporator into mechanical energy; a temperature controller for manipulating the amount of water supplied to the evaporator so that the temperature of the steam supplied from the evaporator to the expander coincides with a target temperature; and a pressure controller for manipulating the rotational speed of the expander by changing a load of the expander so that the pressure of the steam supplied from the evaporator to the expander coincides with a target pressure. The temperature controller and the pressure controller control the amount of water supplied to the evaporator and/or the rotational speed of the expander according to at least an internal density of the evaporator.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2005-69365 filed on Mar. 11, 2005 the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Rankine cycle system that includes an evaporator for heating a liquid-phase working medium with thermal energy of the exhaust gas of an engine so as to generate a gas-phase working medium, and a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy.

2. Description of Related Art

Japanese Utility Model Registration Publication No. 2-38162 discloses an arrangement wherein the temperature of steam generated by waste heat from a boiler using exhaust gas of an engine rotating at a constant speed as a heat source is compared with a target temperature. When a water supply signal obtained from this deviation is used-in a feedback control of the amount of water supplied to the waste heat from the boiler, a feedforward signal obtained by correcting with steam pressure a degree of throttle opening signal of the engine is added to the above-mentioned feedback signal, thus compensating for variation in the load of the engine and thereby improving the precision of control.

WO03/031775 discloses an arrangement in which the steam temperature is controlled by manipulating the amount of water supplied to an evaporator of a Rankine cycle system. The steam pressure is controlled by manipulating the rotational speed of a displacement type expander into which steam flows.

The steam temperature and the steam pressure can be controlled by a conventional technique to a degree corresponding to load variation accompanying normal acceleration/deceleration after an engine and a Rankine cycle system are warmed up. However, in the process of operations from starting the engine in a low temperature state to completing warm-up of the Rankine cycle system, there are unstable states involving the effect of phase changes of a working medium within a system in going from water to saturated steam and then to superheated steam, and control of the amount of water supplied until the temperature gradient of the interior of the evaporator becomes stable. Furthermore, when the engine stops, high temperature, high pressure steam remains in the interior of the evaporator, and if the Rankine cycle system is stopped at the same time there is a loss from the viewpoint of the efficiency of energy recovery. In particular, when the engine and the Rankine cycle system are started up, it is important that the steam attains a target temperature and a target pressure as soon as possible so that the Rankine cycle system makes a transition to a high operating efficiency state.

For example, as shown in FIG. 14, even when the engine starts from a low temperature state and the exhaust gas energy increases, if the evaporator exit temperature is very low, the amount of water supplied to the evaporator is maintained at 0 by temperature feedback control (ref. region a). Once the water remaining within the evaporator is heated and becomes superheated steam, even by increasing the amount of water supplied, it becomes impossible to decrease the steam temperature quickly, so that the target temperature might be overshot (ref. region b).

As shown in FIG. 15A, when the expander is rotated simultaneously with an ignition switch being turned ON, since the steam leaks at first and the steam pressure does not increase, it is necessary to rotate as a motor a motor/generator connected to the expander, so that it becomes difficult to recover energy by means of the motor/generator.

Moreover, as shown in FIG. 15B, if an attempt is made to start the expander in a state in which time has elapsed after the ignition switch is turned ON and the steam pressure is high, the expander cannot rotate due to a large static frictional force such as a valve pressing force, and even if the motor/generator generates a maximum torque as a motor in order to rotate the expander (ref. region c), the expander might not be able to rotate. In such a case, there is a possibility that the steam pressure might greatly exceed the target pressure and cause damage to the expander.

SUMMARY OF THE INVENTION

The present invention has been accomplished under the above-mentioned circumstances, and it is an object of an embodiment of the present invention to appropriately control the temperature and pressure of steam generated by an evaporator of a Rankine cycle system even when starting an engine.

In order to achieve the above-mentioned object, according to a first feature of the invention, there is provided a Rankine cycle system including an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium. A displacement type expander is provided for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy. A temperature control means is provided for manipulating the amount of liquid-phase working medium supplied to the evaporator so that the temperature of the gas-phase working medium supplied from the evaporator to the expander coincides with a target temperature. Pressure control means are provided for manipulating the rotational speed of the expander by changing a load of the expander so that the pressure of the gas-phase working medium supplied from the evaporator to the expander coincides with a target pressure. The temperature control means and the pressure control means controls the amount of liquid-phase working medium supplied to the evaporator and/or the rotational speed of the expander according to at least an internal density of the evaporator.

With the first feature, when the temperature control means manipulates the amount of liquid-phase working medium supplied to the evaporator in order to make the temperature of the gas-phase working medium coincide with the target temperature, and the pressure control means manipulates the rotational speed by changing the load of the expander in order to make the pressure of the gas-phase working medium coincide with the target pressure, the amount of liquid-phase working medium supplied and/or the rotational speed of the expander are controlled according to at least the internal density of the evaporator. Therefore, it is possible to set a liquid-phase working medium supply amount or a rotational speed of the expander according to the internal density of the evaporator, and quickly converge the temperature of the gas-phase working medium on the target temperature.

According to a second feature of an embodiment of the present invention, in addition to the first feature, when the engine is started, the temperature control means and the pressure control means control the amount of liquid-phase working medium supplied to the evaporator and/or the rotational speed of the expander according to at least the internal density of the evaporator.

With the second feature, since the amount of liquid-phase working medium supplied or the rotational speed of the expander is controlled according to the internal density of the evaporator when starting the engine, it is possible to quickly converge the steam temperature or the steam pressure on the target temperature or the target pressure without overshooting.

According to a third feature of an embodiment of the present invention, in addition to the second feature, the temperature control means increases the amount of liquid-phase working medium supplied when the internal density of the evaporator is lower than a set value, and decreases the amount of liquid-phase working medium supplied or makes it zero when the internal density of the evaporator is higher than the set value.

With the third feature, the amount of liquid-phase working medium supplied is increased, if the internal density of the evaporator is lower than the set value when starting the engine. Therefore, it is possible to increase the internal density of the evaporator and quickly converge the temperature of the gas-phase working medium on the target temperature. Furthermore, the amount of liquid-phase working medium supplied is decreased or made 0, if the internal density of the evaporator is higher than the set value when starting the engine. Therefore, it is possible to decrease the internal density of the evaporator and quickly converge the temperature of the gas-phase working medium on the target temperature.

According to a fourth feature of an embodiment of the present invention, in addition to the second or third feature, the pressure control means controls the rotational speed of the expander so that the expander stops or rotates at a very low rotational speed that is close to stopping when the internal density of the evaporator is lower than a set value, and controls the rotational speed of the expander so that the expander is rotated in advance when the internal density of the evaporator is higher than the set value.

With the fourth feature, the rotational speed of the expander is controlled so that it stops or rotates at a very low rotational speed that is close to stopping, if the internal density of the evaporator is lower than the set value when starting the engine. Therefore, it is possible to brake the expander to thus prevent spontaneous rotation, thereby quickly raising the pressure of the gas-phase working medium to start the expander. Furthermore, the rotational speed of the expander is controlled so that it is rotated in advance, if the internal density of the evaporator is higher than the set value when starting the engine. Therefore, liquid-phase working medium that has built up in the interior of the evaporator can be discharged efficiently.

The above-mentioned object, other objects, characteristics, and advantages of the present invention will become apparent from an explanation of a preferred embodiment that will be described in detail below by reference to the attached drawings.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a diagram showing the overall arrangement of a Rankine cycle system;

FIG. 2 is a layout diagram of the Rankine cycle system;

FIG. 3 is a control block diagram of temperature control means;

FIG. 4 is a detail of part A in FIG. 3;

FIG. 5 is a control block diagram of pressure control means;

FIG. 6 is a detail of part B in FIG. 5;

FIG. 7 is a diagram for explaining a method for estimating the internal density of an evaporator;

FIG. 8 is a graph showing the relationship between optimum steam temperature and maximum efficiency of an evaporator and an expander

FIG. 9 is a diagram showing a map in which a target steam pressure is looked up from steam energy and steam temperature;

FIG. 10 is a time chart for explaining control in a case in which the internal density of the evaporator is normal when an ignition switch is turned ON;

FIG. 11 is a time chart for explaining control in a case in which the interior of the evaporator is empty when the ignition switch is turned ON

FIG. 12 is a time chart for explaining control in a case in which the interior of the evaporator is full of water when the ignition switch is turned ON;

FIG. 13 is a time chart for explaining control when the ignition switch is turned OFF;

FIG. 14 is a time chart for explaining conventional temperature control of an evaporator; and

FIGS. 15A and 15B are time charts for explaining conventional pressure control of the evaporator.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 shows the overall arrangement of a Rankine cycle system R to which the present invention is applied. The Rankine cycle system R recovers thermal energy of exhaust gas of an engine E and converts it into mechanical energy. The Rankine cycle system R includes an evaporator 11, an expander 12, a condenser 13, and a water supply pump 14. The evaporator 11 heats water with the exhaust gas discharged by the engine E so as to generate high temperature, high pressure steam. The expander 12 is operated by the high temperature, high pressure steam generated by the evaporator 11 so as to generate mechanical energy. The condenser 13 cools decreased temperature, decreased pressure steam that has completed work in the expander 12 so as to turn it back into water. The water supply pump 14 pressurizes water discharged from the condenser 13, and supplies it to the evaporator 11 again.

As shown in FIG. 2, an open/close valve 15 for cutting off the supply of water is disposed between the evaporator 11 and the water supply pump 14, and an open/close valve 16 for cutting off the supply of steam is disposed between the evaporator 11 and the expander 12. Furthermore, a motor/generator 17 is connected to the expander 12, and the rotational speed of the expander 12 is controlled by regulating a load of the motor/generator 17. A Rankine controller Cr controls, based on a signal such as ON/OFF of an ignition switch, a fuel injection quantity Ti, or an engine rotational speed Ne, the rotational speed of a motor 18 for driving the water supply pump 14, the load of the motor/generator 17, and opening/closing of the two open/close valves 15 and 16.

FIG. 3 shows the arrangement of temperature control means 21 included in the Rankine controller Cr. The temperature control means 21 includes feedforward water supply amount calculation means 22, feedback water supply amount calculation means 23, water supply amount control changeover means 24, and rotational speed calculation means 25. The feedforward water supply amount calculation means 22 calculates a feedforward water supply amount for the evaporator 11 from the engine rotational speed Ne, the fuel injection quantity Ti, and the exhaust gas temperature of the engine E. The feedback water supply amount calculation means 23 calculates a feedback water supply amount by multiplying a deviation of the steam temperature at the exit of the evaporator 11 from a target steam temperature at the entrance of the expander 12 by a predetermined gain. The water supply amount control changeover means 24 changes the control of the water supply amount for the evaporator 11 according to the internal density of the evaporator 11 when the ignition switch of the engine E is turned ON or the internal energy of the evaporator 11 when the ignition switch is turned OFF. The rotational speed calculation means 25 calculates a target rotational speed for the water supply pump 14 from a target water supply amount outputted by the water supply amount control changeover means 24 and a steam pressure at the exit of the evaporator 11, and controls the rotational speed of the motor 18 for driving the water supply pump 14 so that the rotational speed coincides with the target rotational speed.

The target steam temperature is determined as follows: as shown in FIG. 8, the efficiency of the evaporator 11 and the efficiency of the expander 12 of the Rankine cycle system change according to the steam temperature. When the steam temperature increases, the efficiency of the evaporator decreases and the efficiency of the expander increases, whereas when the steam temperature decreases, the efficiency of the evaporator increases and the efficiency of the expander decreases. Therefore, there is an optimum steam temperature (a target temperature) at which the overall efficiency of the two becomes a maximum.

The internal density of the evaporator 11 is obtained as follows: as shown in FIG. 9, a flow rate Qin of water supplied from the water supply pump 14 to the evaporator 11 and a flow rate Qout of steam supplied from the evaporator 11 to the expander 12 are measured using a flowmeter and an internal density p of steam in the interior of the evaporator 11 is calculated from ρ=∫{Qin(t)−Qout(t)}dt/V.

FIG. 5 shows the arrangement of pressure control means 26 included in the Rankine controller Cr. The pressure control means 26 includes feedforward rotational speed calculation means 27, feedback rotational speed calculation means 28, rotational speed control changeover means 29, and PI feedback term calculation means 30. The feedforward rotational speed calculation means 27 calculates a feedforward rotational speed based on a target pressure of steam supplied to the expander 12, a commanded water supply amount, and a steam temperature at the entrance of the expander 12. The feedback rotational speed calculation means 28 calculates a feedback rotational speed by multiplying a deviation of the steam pressure at the entrance of the expander 12 from the target pressure for steam at the entrance of the expander 12 by a predetermined gain.

The target pressure is set by applying the energy (flow rate) and temperature of steam supplied from the evaporator 11 to the expander 12 to the map of FIG. 9. This target pressure corresponds to a steam pressure at which the expander 12 is operated at maximum efficiency.

The rotational speed control changeover means 29 controls the entrance steam pressure of the expander 12 by changing, based on an ON/OFF signal of the ignition switch, a positive torque (a torque in a direction that assists rotation of the expander 12) or a negative torque (a torque in a direction that inhibits rotation of the expander 12) generated by the motor/generator 17.

The PI feedback term calculation means 30 calculates a target torque for the motor/generator 17 from a deviation of the rotational speed of the motor/generator 17 (that is, the rotational speed of the expander 12) from a target rotational speed outputted by the rotational speed control changeover means 29. The rotational speed of the expander 12 is feedback-controlled at the target rotational speed by generating the above target torque in the motor/generator 17.

Functions of the temperature control means 21 and the pressure control means 26 when the ignition switch is turned ON are now explained.

As shown in FIG. 4, FIG. 6, and FIG. 10, in the case where the internal density of the evaporator 11 is normal when the ignition switch is turned ON, a smaller amount of water is supplied than when there is normal temperature control (ref. region d) so that the interior of the evaporator 11 does not become empty simultaneously with an increase in the exhaust gas energy, and when the steam temperature becomes close to the target temperature, the operation shifts to water supplied by normal feedback control (ref. region e). Until the steam pressure attains a starting pressure for the expander 12, a torque in the direction opposite to the rotational direction of the expander 12 is generated in the motor/generator 17 (ref. region f), thereby braking the expander 12 so that it does not rotate spontaneously. When the steam pressure attains the starting pressure (ref. region g), a torque in the rotational direction of the expander 12 is generated in the motor/generator 17 for a moment (ref. region h) to thus start rotation of the expander 12 at the lowest rotational speed that allows stable rotation (ref. region i), thereby smoothly starting the expander 12.

As shown in FIG. 4, FIG. 6 and FIG. 11, in the case where the interior of the evaporator 11 is empty when the ignition switch is turned ON, the amount of water supplied to the evaporator 11 is temporarily increased simultaneously with an increase in the exhaust gas energy (ref. region i), thus preventing any response lag in the steam temperature. In this process, the amount of water supplied is not an amount that would make the evaporator 11 full of water, but is somewhat larger than when normal in order to make an easy transition to a stable control state, and the amount of water supplied is decreased accompanying an increase in the internal density of the evaporator 11. Torque control of the motor/generator 17 is carried out in the same manner as for the above-mentioned case where the internal density of the evaporator 11 is normal, and starting rotation of the expander 12 at the lowest rotational speed allowing stable rotation enables a smooth start.

As shown in FIG. 4, FIG. 6 and FIG. 12, in the case where the interior of the evaporator 11 is full of water when the ignition switch is turned ON, even if the exhaust gas energy increases, water supply to the evaporator 11 is maintained in a suspended state (ref. region k), and water supply is started after the internal density of the evaporator 11 has become appropriate. When the steam temperature becomes close to the target temperature, the operation shifts to normal feedback temperature control. The motor/generator 17 generates a positive torque to rotate the expander 12 at a low speed (ref. region m) before the steam pressure starts rising, thereby discharging water in a passage that is downstream of the evaporator 11, particularly in a portion between the evaporator 11 and the expander 12, and that is not heated by exhaust gas.

When any one of the above-mentioned three types of control when starting the engine E is completed, normal water supply control for the evaporator 11 is started based on a value obtained by adding the feedforward water supply amount and the feedback water supply amount, and normal rotational speed control is started based on a value obtained by adding the feedforward rotational speed and the feedback rotational speed.

Functions of the temperature control means 21 and the pressure control means 26 when the ignition switch of the engine E is turned OFF are now explained by reference to FIG. 4, FIG. 6, and FIG. 13.

In the case where there is a lot of thermal energy remaining in the interior of the evaporator 11 when the ignition switch of the engine E is turned OFF, if the Rankine cycle system R is stopped immediately, the thermal energy is wasted. Therefore, when the ignition switch is turned OFF, water supply to the evaporator 11 is not stopped immediately and additional water supply is carried out, thus continuing the generation of steam (ref region n). The amount of water supplied in this process is decreased in response to a decrease in the internal energy of the evaporator 11. When the steam temperature attains a temperature at which the expander 12 does not generate an output (for example, the saturated steam temperature), the water supply is suspended.

As a result, the steam pressure is maintained at the target pressure for a predetermined period of time after the ignition switch is turned OFF, the expander 12 is rotated efficiently, and energy can be recovered. When the steam pressure decreases, the expander 12 is rotated at the lowest rotational speed allowing stable rotation, thus further recovering energy (ref. region o). When the regenerative torque of the motor/generator 17 becomes 0, rotation of the expander 12 is stopped, and recovery of energy is completed (ref. region p).

In this way, by continuously supplying water and operating the expander 12 for the predetermined period of time after the ignition switch is turned OFF, not only can the thermal energy remaining in the evaporator 11 be recovered without waste, but also the Rankine cycle system R can be shifted to a stable stopped state while preventing over-rotation of the expander 12 by slowly decreasing the steam pressure. In addition, it is possible to prevent the temperature of the interior of the engine compartment from increasing due to thermal energy remaining in the evaporator 11.

Although one embodiment of the present invention has been described above, the present invention can be modified in a variety of ways as long as the modifications do not depart from the spirit and scope of the present invention.

For example, in the embodiment the amount of water supplied to the evaporator 11 is controlled based on the rotational speed of the water supply pump 14, but it may be controlled by the degree of opening of the open/close valve 15 shown in FIG. 2.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A Rankine cycle system comprising: an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; temperature control means for manipulating the amount of liquid-phase working medium supplied to the evaporator so that the temperature of the gas-phase working medium supplied from the evaporator to the expander coincides with a target temperature; and pressure control means for manipulating the rotational speed of the expander by changing a load of the expander so that the pressure of the gas-phase working medium supplied from the evaporator to the expander coincides with a target pressure, wherein the temperature control means and the pressure control means controls the amount of liquid-phase working medium supplied to the evaporator and/or the rotational speed of the expander according to at least an internal density of the evaporator.

2. The Rankine cycle system according to claim 1 wherein, when the engine is started, the temperature control means and the pressure control means control the amount of liquid-phase working medium supplied to the evaporator and/or the rotational speed of the expander according to at least the internal density of the evaporator.

3. The Rankine cycle system according to claim 2, wherein the temperature control means increases the amount of liquid-phase working medium supplied when the internal density of the evaporator is lower than a set value, and decreases the amount of liquid-phase working medium supplied or makes it zero when the internal density of the evaporator is higher than the set value.

4. The Rankine cycle system according to claim 2, wherein the pressure control means controls the rotational speed of the expander so that the expander stops or rotates at a very low rotational speed that is close to stopping when the internal density of the evaporator is lower than a set value, and controls the rotational speed of the expander so that the expander is rotated in advance when the internal density of the evaporator is higher than the set value.

5. The Rankine cycle system according to claim 3, wherein the pressure control means controls the rotational speed of the expander so that the expander stops or rotates at a very low rotational speed that is close to stopping when the internal density of the evaporator is lower than a set value, and controls the rotational speed of the expander so that the expander is rotated in advance when the internal density of the evaporator is higher than the set value.

6. The Rankine cycle system according to claim 1, wherein the internal density of the evaporator is based on a flow rate Qin of water supplied and a flow rate out Qout according to the following formula:

7. The Rankine cycle system according to claim 1, wherein the pressure control means includes a feedforward rotational speed calculation means, feedback rotational speed calculation means, rotational speed control changeover means and feedback term calculation means for manipulating the rotational speed of the expander.

8. The Rankine cycle system according to claim 7, wherein the feedforward rotational speed calculation means calculates a feedforward rotational speed based on a target pressure of steam supplied to the expander, a commanded water supply amount and a steam temperature at an entrance of the expander.

9. The Rankine cycle system according to claim 7, wherein the feedback rotational speed calculation means calculates a feedback rotational speed by multiplying a deviation of steam pressure at an entrance of the expander from the target pressure for steam at the entrance of the expander by a predetermined gain.

10. The Rankine cycle system according to claim 1, wherein the rotational speed changeover means controls an entrance steam pressure of the expander by changing, based on an ON/OFF signal of an ignition switch, a positive torque or a negative torque generated by a motor/generator.

11. A Rankine cycle system comprising: an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; a temperature controller to manipulate the amount of liquid-phase working medium supplied to the evaporator so that the temperature of the gas-phase working medium supplied from the evaporator to the expander coincides with a target temperature; and a pressure controller to manipulate the rotational speed of the expander by changing a load of the expander so that the pressure of the gas-phase working medium supplied from the evaporator to the expander coincides with a target pressure, wherein the temperature controller and the pressure controller control the amount of liquid-phase working medium supplied to the evaporator and/or the rotational speed of the expander according to at least an internal density of the evaporator.

12. The Rankine cycle system according to claim 11 wherein, when the engine is started, the temperature controller and the pressure controller control the amount of liquid-phase working medium supplied to the evaporator and/or the rotational speed of the expander according to at least the internal density of the evaporator.

13. The Rankine cycle system according to claim 12, wherein the temperature controller increases the amount of liquid-phase working medium supplied when the internal density of the evaporator is lower than a set value, and decreases the amount of liquid-phase working medium supplied or makes it zero when the internal density of the evaporator is higher than the set value.

14. The Rankine cycle system according to claim 12, wherein the pressure controller controls the rotational speed of the expander so that the expander stops or rotates at a very low rotational speed that is close to stopping when the internal density of the evaporator is lower than a set value, and controls the rotational speed of the expander so that the expander is rotated in advance when the internal density of the evaporator is higher than the set value.

15. The Rankine cycle system according to claim 13, wherein the pressure controller controls the rotational speed of the expander so that the expander stops or rotates at a very low rotational speed that is close to stopping when the internal density of the evaporator is lower than a set value, and controls the rotational speed of the expander so that the expander is rotated in advance when the internal density of the evaporator is higher than the set value.

16. The Rankine cycle system according to claim 11, wherein the internal density of the evaporator is based on a flow rate Qin of water supplied and a flow rate out Qout according to the following formula:

17. The Rankine cycle system according to claim 11, wherein the pressure controller includes a feedforward rotational speed calculation means, feedback rotational speed calculation means, rotational speed control changeover means and feedback term calculation means for manipulating the rotational speed of the expander.

18. The Rankine cycle system according to claim 17, wherein the feedforward rotational speed calculation means calculates a feedforward rotational speed based on a target pressure of steam supplied to the expander, a commanded water supply amount and a steam temperature at an entrance of the expander.

19. The Rankine cycle system according to claim 17, wherein the feedback rotational speed calculation means calculates a feedback rotational speed by multiplying a deviation of steam pressure at an entrance of the expander from the. target pressure for steam at the entrance of the expander by a predetermined gain.

20. The Rankine cycle system according to claim 11, wherein the rotational speed changeover means controls an entrance steam pressure of the expander by changing, based on an ON/OFF signal of an ignition switch, a positive torque or a negative torque generated by a motor/generator.