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

The present invention relates to a system for effectively utilizing thermal energy in an evaporator of a Rankine cycle system when an engine stops. The system includes a temperature control means for controlling 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 control means for manipulating the rotational speed of the expander by changing the 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. This allows for 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. The system can efficiently recover the thermal energy remaining in the evaporator and prevent the temperature inside the engine compartment from increasing.

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