CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage of International Application No. PCT/FR2019/052315, filed Oct. 1, 2019, which claims priority to French Patent Application No. 1859135, filed Oct. 2, 2018, the disclosures of which are herein incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to cycles for converting heat into mechanical energy, in particular machines using the Organic Rankine Cycle, or ORC.

BACKGROUND

An ORC cycle typically uses a working fluid with a lower boiling point than water at ambient pressure. The fluid is often an organic refrigerant, such as a hydrocarbon gas (ethane, propane, butane, propylene, etc.). A machine using an ORC cycle generally comprises four components:

An evaporator or generator heated by a heat source, which vaporizes liquid fluid at high pressure.

An expansion device, often a turbine, powered by the high-pressure steam produced by the evaporator. This expansion device produces mechanical energy that can be converted into electricity.

A condenser cooled by a heat sink, which collects the steam discharged at low pressure by the turbine and liquefies it.

A pump to feed the evaporator at high pressure from the condenser's low pressure liquid fluid.

Regardless of the refrigerant used, the temperature of the heat source at the evaporator is in practice rarely below 100° C., since the machine would then not be economically viable. This excludes many waste heat recovery applications, usually having temperatures well below 100° C.

U.S. Pat. No. 5,685,152 discloses a machine based on an ORC cycle that does not use a pump, which would allow a better exploitation of waste heat sources. The pump is replaced by a transfer tank connected between the condenser and the evaporator through respective valves. The tank operates in four steps. In the first step, the tank is opened to the condenser to receive liquid fluid at low pressure by gravity. In a second step, the tank is closed and heated by the heat source. The fluid in the tank vaporizes, at least partially, and its pressure increases. In a third step, when the pressure in the tank is close to that of the evaporator, the tank is opened to the evaporator. The pressures in the tank and the evaporator equalize, while the liquid remaining in the tank is transferred to the evaporator by gravity. In a fourth step, the tank is closed and cooled by the heat sink. The vapor in the enclosure liquefies and the pressure drops.

In principle, such a machine can supply steam to a turbine relatively continuously, but it is difficult to alternately heat and cool the transfer tank fast enough to obtain a usable flow rate.

SUMMARY

A machine for converting heat into mechanical energy is generally provided, comprising an expansion device producing mechanical energy from a flow of vapor of a fluid; an evaporator heated by a heat source to a high temperature and configured to supply vapor to the expansion device; a condenser cooled by a heat sink to a low temperature and configured to condense the vapor discharged from the expansion device; a liquid circuit connecting a liquid phase of the condenser to a liquid phase of the evaporator; a vapor circuit connecting a vapor phase of the evaporator to a vapor phase of the condenser; and valves configured to, during a first, active stroke, close the liquid and vapor circuits, and, during a second, inactive stroke, open the liquid and vapor circuits.

The machine may further comprise a buffer vapor tank cooled by the heat sink to the low temperature with a corresponding saturating steam pressure; and a valve configured to connect the buffer tank to the condenser during the active stroke and close the buffer tank during the inactive stroke.

The liquid and vapor circuits may be configured to perform transfers passively, respectively by pressure equalization in the vapor circuit and by gravity in the liquid circuit.

The liquid circuit may be configured to perform transfers by level equalization.

The machine may further comprise a first transfer stage inserted in the liquid and vapor circuits, and heated from the heat source to a first intermediate temperature between the high and low temperatures; low-pressure-side valves on the liquid and vapor circuits between the first transfer stage and the condenser, configured to close during the active stroke and open during the inactive stroke; and high-pressure-side valves on the liquid and vapor circuits between the first transfer stage and the evaporator, configured to open during the active stroke and close during the inactive stroke.

The machine may further comprise a second transfer stage inserted in the liquid and vapor circuits between the evaporator and the high-pressure-side valves of the first transfer stage, and heated from the heat source to a second intermediate temperature between the high temperature and the first intermediate temperature; and high-pressure-side valves on the liquid and vapor circuits between the second transfer stage and the evaporator, configured to close during the active stroke and open during the inactive stroke.

The expansion device may be a positive displacement device, comprising a cylinder; a piston sliding in the cylinder and defining two variable volumes in the cylinder, a first of the two variable volumes being connected to the evaporator; a discharge valve configured to connect the second of the two variable volumes to the condenser during the active stroke; and a check valve configured to connect the second variable volume to the evaporator during the inactive stroke.

The machine may comprise a valve between the expansion device and the condenser, configured to open during the active stroke and close during the inactive stroke.

