Rankine cycle system

Abstract

A Rankine cycle system includes an evaporator for heating water with thermal energy of exhaust gas of an engine for generating steam with a displacement type expander for converting thermal energy into mechanical energy. A temperature controller manipulates 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. A pressure controller manipulates 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 / or the pressure controller continue to control the amount of water supplied to the evaporator and / or the rotational speed of the expander in set ranges at least in a state in which the engine has stopped and the thermal energy of the exhaust gas has disappeared.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2005-69367 filed on Mar. 11, 2005 the entire contents of which are hereby incorporated by reference. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a Rankine cycle system that includes an evaporator for heating a liquid-phase working medium with thermal energy of an 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. [0004] 2. Description of Background Art [0005] Japanese Utility Model Registration Publication No. 2-38162 discloses an arrangement in which 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 temperat...

AI Technical Summary

Benefits of technology

[0009] The present invention has been accomplished under the above-mentioned circumstances, and it is an object of an embodiment thereof to effectively utilize thermal energy remaining in the interior of an evaporator when an engine stops and to allow a Rankine cycle system to make a transition to a stable stopped state.

[0011] With the first feature, in an arrangement in which 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, a control of the amount of liquid-phase working medium supplied to the evaporator and / or control of the rotational speed of the expander are continued so as to be in the set ranges even after the engine has stopped and the thermal energy of the exhaust gas has disappeared. Therefore, it is possible to effectively recover the thermal energy remaining in the interior of the evaporator while making a transition to a stable stopped state by inhibiting a rapid increase in the rotational speed of the expander after the engine stops. Moreover, it is possible to prevent, by converting the thermal energy into mechanical energy, the temperature of the interior of an engine compartment from increasing.

[0013] With the second feature, after the engine has stopped, the temperature control means continues to supply the liquid-phase working medium to the evaporator until the temperature of the gas-phase working medium decreases to the temperature at which the expander does not generate an output. Therefore, it is possible to use the thermal energy remaining in the evaporator efficiently to the very end.

[0015] With the third feature, after the engine has stopped, the pressure control means continues to control the rotational speed of the expander until the pressure of the gas-phase working medium decreases to the pressure at which the expander does not generate an output. Therefore, it is possible to use the thermal energy remaining in the evaporator efficiently to the very end.

[0017] With the fourth feature, when the rotational speed of the expander decreases to the set rotational speed, the pressure control means maintains this set rotational speed; and when the expander attains a state in which no output is generated the pressure control means stops controlling the rotational speed of the expander and allows it to rotate freely in a non-load state. Therefore, it is possible to recover energy by allowing the expander to rotate at a stable rotational speed while inhibiting a rapid increase in the rotational speed of the expander due to the thermal energy remaining in the evaporator, and to allow the Rankine cycle system to make a smooth transition to a stable stopped state while inhibiting a rapid increase in the rotational speed of the expander due to the thermal energy remaining in the evaporator.

Problems solved by technology

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.

If the Rankine cycle system is also stopped at the same time there is a loss from the viewpoint of the efficiency of energy recovery.

Moreover, if the expander is made to freely rotate (freely run) without load by the high temperature, high pressure steam remaining in the interior of the evaporator at the same time as the engine stops, there are problems that the rotational speed of the expander increases rapidly and the high temperature, high pressure steam remaining in the interior of the evaporator causes the temperature of an engine compartment to become high.

In this process, water continues to evaporate in the interior of the evaporator as the pressure decreases, the internal density decreases excessively, and as a result the next time the engine is started the evaporator is heated while empty, leading to a possibility that the steam temperature might overshoot (ref. region b).

If the load of the motor / generator is made 0 at the same time as the engine stops, not only is it impossible to regeneratively recover energy by means of the motor / generator, but also the remaining steam pressure brings the expander in a free operating state and the rotational speed increases excessively, leading to a possibility that the expander might be damaged (ref. region c).

In this process, even if rotation of the expander is stopped at the same time as the engine stops to avoid damage due to excessive rotation, there is still a problem that the energy of the remaining high temperature, high pressure steam cannot be recovered.

Images