Energy recovery system and method using an organic rankine cycle with condenser pressure regulation

An energy recovery system and method using an organic rankine cycle is provided for recovering waste heat from an internal combustion engine, which effectively controls condenser pressure to prevent unwanted cavitation within the fluid circulation pump. A coolant system may be provided with a bypass conduit around the condenser and a bypass valve selectively and variably controlling the flow of coolant to the condenser and the bypass. A subcooler may be provided integral with the receiver for immersion in the accumulated fluid or downstream of the receiver to effectively subcool the fluid near the inlet to the fluid pump.

FIELD OF THE INVENTION

The present invention generally relates to energy recovery from the waste heat of a prime mover machine such as an internal combustion engine.

BACKGROUND OF THE INVENTION

It is well known that the thermal efficiency of an internal combustion engine is very low. The energy not extracted as usable mechanical energy is typically expelled as waste heat into the atmosphere by way of the engine's exhaust gas emission, charge air cooling and engine coolant heat rejection.

It is known to employ a relatively simple, closed-loop Organic Rankine Cycle (ORC) system to recapture the engine's waste heat otherwise lost to the surrounding ambient. Such a system typically comprises a circulating pump, pumping a liquid phase organic, working fluid through a boiler wherein the working fluid undergoes a phase change from a liquid to a pressurized, gaseous phase. The boiler receives its heat input from the engine's waste heat streams. The gaseous phase working fluid expands through a turbine wherein mechanical work is extracted from the turbine. A low pressure vapor, typically exiting the turbine, then enters a condenser intended to cool and return the two phase fluid to a saturated liquid phase for recirculation by the circulating pump. A receiver is typically placed between the condenser and the recirculation pump to accumulate and separate the liquid portion of the fluid from any surviving gaseous phase exiting the condenser. The fluid passing through the condenser is typically cooled by a suitable cooling medium directed through the condenser. However, improvements are desirable.

SUMMARY OF THE INVENTION

The present invention achieves various functions and advantages as described herein and includes a system and method of recovering energy from a source of waste heat using an organic fluid, comprising providing a waste heat source, providing a heat exchanger, passing a heat conveying medium from said waste heat source through the heat exchanger, providing a fluid pump to pressurize the organic fluid, and passing the pressurized organic fluid through the heat exchanger. The system and method further include directing the organic fluid from the heat exchanger through an energy conversion device, passing the organic fluid from the turbine through a cooling condenser, directing the organic fluid from the condenser into and through a receiver, returning the organic fluid from the receiver to said pump, providing a condenser coolant fluid flow through the condenser to cool the organic fluid flowing through the condenser, and selectively bypassing coolant flow around the condenser.

The system and method may further include selectively varying the bypassed coolant flow based on at least one of a temperature and a pressure of the organic fluid upstream of the fluid pump, and further may be based on a saturation pressure of the organic fluid near an inlet of the fluid pump. A subcooler may be positioned within the receiver so as to be immersed in the organic fluid accumulated in the receiver. A subcooler may be provided downstream of the receiver and upstream of the fluid pump. A bypass valve may be positioned upstream of the condenser along a coolant flow circuit to selectively bypass coolant flow around the condenser. The method and system may also include measuring an inlet temperature of the organic fluid entering the fluid pump, measuring an inlet pressure of the organic fluid entering the organic fluid pump, determining a saturation pressure corresponding to the measured inlet temperature, comparing said measured inlet pressure to the saturation pressure, and increasing the bypass flow of coolant around the condenser thereby decreasing the flow of coolant through the condenser when the measured inlet pressure of the organic fluid is not greater than the saturation pressure plus a specified delta pressure.

The present invention is also directed to a system of recovering energy from a source of waste heat using an organic fluid, comprising an organic fluid circuit, a heat exchanger arranged along the organic fluid circuit to receive a heat conveying medium and the organic fluid, an energy conversion device positioned to receive organic fluid from the heat exchanger, a cooling condenser positioned to receive the organic fluid from the heat exchanger, a receiver positioned downstream of the cooling condenser to receive the organic fluid, a pump to receive organic fluid from the receiver and direct the organic fluid through the heat exchanger, a coolant circuit to direct coolant through the cooling condenser, and a subcooler positioned along the coolant circuit upstream of the condenser. The subcooler is positioned along the organic fluid circuit downstream of the receiver and upstream of the pump to cool the organic fluid flowing from the receiver prior to entering the pump.

DETAILED DESCRIPTION OF THE INVENTION

Applicants have recognized that during large transient heat inputs from the waste heat or abrupt changes in the temperature of the coolant flowing through the condenser, a rapid condenser pressure decrease may occur causing the fluid in the receiver to boil. As a result, the circulation pump, in the ORC, may undesirably experience cavitation. Applicant has recognized that measures can be taken to assure that sufficient fluid pressure is maintained thereby preventing pump cavitation.

In particular,FIG. 1presents a schematic of a closed loop Organic Rankine Cycle (ORC) system10in accordance with an exemplary embodiment of the present invention which addresses the aforementioned issue. The ORC system10includes a circulating pump12for circulating a liquid phase organic fluid, such as R-245fa, or any other suitable refrigerant, through an organic fluid circuit including conduits22,24,26, and28. A heat exchanger or boiler13, positioned downstream of pump12, receives a high temperature heat conveying medium20, such as high temperature exhaust gas, from a waste heat source Q, such as an internal combustion engine, and transfers the waste heat to the organic fluid causing the organic fluid to change from a liquid phase fluid to a high pressure gaseous phase.

The gaseous phase fluid flows from boiler13through conduit24to an energy conversion device such as turbine14. The gaseous fluid expands through turbine14creating mechanical work W at the turbine shaft. An expanded, low pressure vapor generally exits turbine14through passage26and is directed through a condenser15wherein the vapor returns to its liquid phase by the cooling effect of the coolant flowing through condenser15. The resulting re-liquefied or condensed fluid exits condenser15and is conveyed through a conduit28to a receiver16for accumulating a sufficient supply of organic fluid for supplying pump12and for recirculation through the system10. However, the present embodiment also includes a subcooler18positioned along conduit28downstream of receiver16and upstream of pump12. The re-liquefied fluid within conduit28is thus further cooled below the fluid's saturation temperature by flowing through subcooler18prior to entering the intake port of re-circulation pump12.

ORC system10further includes a separate closed loop condenser coolant system50whereby a suitable coolant is circulated through coolant system50including a coolant circuit including conduits52and54. Coolant system50includes subcooler18and a coolant pump58, positioned along conduit52, to circulate the coolant through subcooler18, wherein excess heat is removed from the re-liquefied fluid passing through conduit28prior to entering the intake port of pump12thereby reducing the temperature of the organic fluid.

During normal operation, the coolant passing through conduit52flows from subcooler18through condenser15thereby causing condensation of the two-phase organic fluid passing through condenser15by extracting heat from the two-phase fluid. The heated coolant exiting condenser15through conduit54is then passed through radiator60where the coolant is re-cooled to a desired working temperature by, for example, air flow, for recycling through coolant system50by coolant pump58.

Coolant system50of ORC system10also includes a bypass valve55positioned along conduit52to control the coolant flow to condenser15and a bypass conduit56. Bypass valve55is connected to conduit56which functions as a bypass passage directing flow around, i.e. in parallel with, the condenser15by connecting conduit52to conduit54. Bypass valve55is preferably adjustable to selectively vary the quantity of the coolant flow through condenser15and thus vary the quantity of coolant flow through bypass conduit56as desired. For example, bypass valve55may be a variable position three-way valve capable of completely blocking flow to condenser15while permitting bypass flow, completely blocking flow to the bypass conduit56while allowing flow to the condenser, or allowing a portion of coolant flow through the condenser and a portion of coolant flow through bypass conduit56simultaneously. Bypass valve55preferably is capable of modulating or variably controlling the quantity of coolant flow through the condenser15and bypass conduit56based on operating conditions to ensure appropriate condenser pressure to prevent boiling of the working organic fluid and thus prevent cavitation at pump12through operation at various operating conditions.

During operation, if the pressure in condenser15decreases, for example, because of transients or changes in engine load or coolant temperature, bypass valve55is programmed to close-off or block, all or a portion of the coolant flow to condenser15and direct all or an increased portion of coolant through conduit56around condenser15directly to radiator60. Thus the pressure within condenser15may be controlled, thereby preventing boiling within receiver16caused by an accompanying pressure drop. It should be noted that such transients may include, for example, the engine of waste heat source Q changing from a high load to a low load condition thus rapidly decreasing the heat input to the ORC system causing less heat to be rejected in the condenser resulting in a pressure decrease. Also, a coolant temperature decrease, causing a sudden condenser pressure drop, may be initiated by a sudden decrease in the temperature of the, for example, air flow through radiator60.

FIG. 2presents a schematic of an alternate embodiment of the waste heat recovery system illustrated inFIG. 1. The primary difference between theFIG. 1embodiment and that ofFIG. 2is that receiver16and subcooler18, of theFIG. 1embodiment, has been replaced by an integrated receiver/subcooler30wherein a coolant subcooler coil32is integral to the receiver34. Thus subcooler32is immersed in the organic fluid accumulating in receiver34. The functioning of all components remains the same as the embodiment ofFIG. 1.

Turning now toFIG. 3, a simplified flow chart is illustrated for controlling the flow of condenser coolant through conduit52, condenser15, bypass valve55and bypass loop conduit56. During normal, steady state, operating conditions bypass valve55is in a first position permitting all of the condenser coolant to flow through condenser15while blocking flow through bypass conduit56. In steps102and104, a control system monitors and detects or measures the inlet pressure102(P.sub.in) and inlet temperature, respectively, of the organic fluid at the inlet to pump12using appropriate sensors71, an electronic controller70, and an appropriate signal connection72between the sensors and the electronic controller70. In step106, using the inlet pressure and temperature of the organic fluid at or near the inlet of pump12, electronic controller70determines the corresponding saturation pressure P.sub.sat using an appropriate known look-up table, such as a fluid saturation table, for the particular organic fluid used in the system10. The measured pump inlet pressure P.sub.in is compared in step108to the fluid saturation pressure P.sub.sat plus a predetermined cavitation margin AP appropriate for the given system. The net inlet pressure requirement (or cavitation margin) is the excess pressure above the fluids saturation pressure for the given inlet temperature. Each pump has it's own unique net inlet pressure requirement to prevent the pump from cavitating based on the pump style and geometry. If the inlet pressure to the pump is not at or above the net inlet pressure requirement, it will cavitate and may cause pump damage or loss of the ability to pump fluid. If P.sub.in is greater than P.sub.sat plus .DELTA.P, then in step110, the flow rate of coolant through the condenser is increased thereby providing increased cooling of the organic fluid in the condenser while decreasing coolant flow through bypass conduit56. However, if P.sub.in is less than P.sub.sat plus .DELTA.P, then in step112, controller70controls bypass valve55toward a second position to increase the valve opening to conduit56to provide more bypass flow around condenser15while reducing the valve opening to conduit52to decrease the flow rate of coolant to condenser15. Coolant flow through the condenser is slowly increased, or decreased, as dictated by the subcooling requirement. That is, electronic controller70determines and applies the margin, compares the pressures, and generates and sends a control signal via control connector74to bypass valve55to selectively and variably adjust the position of bypass valve55to variably control the flow of coolant through condenser15and bypass conduit56to achieve the desired effect.

Thus by variable operation of bypass valve55, the system50bypasses coolant flow around condenser15as needed as dictated by working fluid subcooling level. The system may also include a subcooler, either integrated in the receiver or positioned downstream of the receiver, to subcool the working fluid prior to the working fluid entering the circulation pump intake port to assist in cooling the working fluid to a temperature sufficiently below the working fluid's boiling temperature for a given system pressure thereby maintaining the fluid in a liquid state. As a result, the pressure within the condenser, and thus the receiver, may be controlled, i.e., maintained at a sufficiently elevated level, to prevent unwanted boiling within receiver16and cavitation at pump12.

While we have described above the principles of our invention in connection with a specific embodiment, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of our invention as set forth in the accompanying claims.