FIELD OF THE INVENTION

The present invention relates to a modified Organic Rankine Cycle (ORC) process. The process according to the present invention reduces or prevents ORC pump cavitation caused by vapor bubbles in the ORC working medium entering the ORC pump.

BACKGROUND OF THE INVENTION

Natural gas is much used as fuel and is supplied in gaseous form but is often stored as liquid natural gas, LNG. LNG needs to be stored very cold.

The exhaust gas from natural gas fuelled engines is clean. Heat recovery from gas engine exhaust gas is suitable since a heat exchanger in the exhaust gas flow will remain clean. High temperature exhaust and cooling water in machinery is used for power generation by expander machine. An example of power cycle with expander machine used for this purpose is Organic Rankine Cycle (ORC).

ORC uses low boiling point working medium in order to utilize heat sources with relatively low temperatures.

An ORC is a thermodynamic process where a working medium is circulated in a closed loop. Liquid working medium is pumped up to a certain pressure. Then, it is heated. The heat source is often waste heat from combustion machinery.

Part of the energy taken up by the working medium during heating is transferred to an expansion machine and is utilized as shaft power from the expansion machine.

The working medium flow out from the expansion machine is condensed in a condenser by a cooling medium. The condenser cooling medium is normally cooling water from sea or aircooler. The condenser is transferring working medium vapor into liquid condition and the working medium out from the condenser is in saturated condition. The condensed working medium is flowing from the condenser and in return to the pump. In the working medium flow from the condenser to the pump there is a risk for vapor bubbles in the liquid since the liquid is in saturated condition in the condenser and has a low boiling point.

The risk for cavitation in a pump is related to vapor bubbles transported in the liquid flow into a pump. With increasing pressure in the pump the vapor bubbles are collapsing in the pump and may damage the pump. For a pump used in an ORC cycle sufficient net positive suction head (NPSH) is important in order to avoid bubbles into the pump. NPSH gives a value for how close the liquid into the pump is from its boiling point (bubble formation) and NPSH should be as high as possible to avoid pump cavitation. The pump manufacturers specify a minimum NPSH in order to avoid cavitation and some pumps are constructed for

lower NPSH than other pumps.

The lower the pump is placed in relation to the condenser in an ORC the higher NPSH. The more sub-cooling of liquid into the pump the higher NPSH. Where there is limited space available and the ORC condenser is placed close to same height as the ORC pump, e.g. on board a ship, it is necessary with a pump with low NPSH requirement. Means may also be required to avoid that bubbles from the condenser are led into the pump.

US201 1/0048012 describes an energy recovery system and method where the ORC working medium is condensed and sub-cooled in a single flow by a cooling medium. US201 1/0048012 is thereby a temperature control system for sub-cooling of working medium in order to reduce the risk for pump cavitation.

WO 201 1/057724 describes that a non-condensable gas is supplied above the liquid downstream of an ORC working medium condenser. By introducing a partial pressure from a non-condensable gas during condensation of the working medium the working medium liquid flowing to the pump will be sub-cooled since there is no non-condensable gas in the liquid flowing to the pump. WO 201 1/057724 is thereby a pressure control system to reduce the risk for pump cavitation.

US 2012/0042656 describes that the heat in a standard, single flow ORC cycle is utilized to vaporize LNG and it is also utilizing the limited cooling effect of the LNG in the ORC working medium.

The object of the present invention is to provide an alternative solution for reducing or preventing pump cavitation in an Organic Rankine Cycle (ORC) process.

SUMMARY OF THE INVENTION

The present invention relates to an Organic Rankine Cycle (ORC) process, which is modified for reducing or preventing cavitation of the pump in the ORC cycle. In this process, the flow from the ORC expansion machine is split into two flows, of which one flows through a condenser and the other flows through a cooler and is cooled to a temperature which preferably is significantly below its condensing temperature by heat exchanging with LNG. The two flows are re-connected in a mixing device so that vapor bubbles in the flow from the condenser are separated from the liquid and mixed with the cold flow from the LNG heat exchanger and thereby condensing the bubbles by selective cooling and thereby reducing or preventing cavitation of the ORC pump. The process is preferably utilizing waste heat from LNG fuelled machinery.

The present invention provides a modified Organic Rankine Cycle (ORC) process, for reducing or preventing pump cavitation, wherein an ORC working medium flow is pressurized in a pump, heated in a heat exchanger by the use of a waste heat flow, and expanded in an expansion machine, characterized in that

the working medium flow from the expansion machine is split in a splitting device into two parallel working medium flows, the working medium flow is condensed in a condenser by the use of a cooling medium flow, the working medium flow is cooled in a heat exchanger to a temperature lower than the temperature of the working medium flow from the condenser by heat exchanging with an LNG flow flowing from an LNG fuel storage tank through the heat exchanger and to a LNG fuelled machinery, and the condensed working medium flow from the condenser and the cooled working medium flow from the heat exchanger are combined in a mixing device positioned upstream of the pump.

In an embodiment of process of the present invention, the condensed working medium flow from the condenser is introduced below the cooled working medium flow from the heat exchanger in the mixing device.

In a further embodiment of process of the present invention, the working medium flow is cooled in the heat exchanger to about the temperature of the LNG flow.

In a further embodiment of process of the present invention, the waste heat flow comes from the LNG fuelled machinery.

In a further embodiment of process of the present invention, the cooling medium flow for the condenser is cooling water from sea or air cooler.

In a further embodiment of process of the present invention, the LNG flow is vaporized by heat exchanging with the OCR working medium flow in the heat exchanger and supplied as fuel to the LNG fuelled machinery with a temperature higher than the ORC working medium condensing temperature.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 schematically illustrates the modified Organic Rankine Cycle process according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The modified Organic Rankine Cycle process according to the present invention is illustrated in figure 1 .

An ORC working medium flow 6 in the modified Organic Rankine Cycle is pressurized in a pump 4. The pressurized working medium flow 6 from the pump 4 is heated in a heat exchanger 3 by the use of a waste heat flow 7, and the heated working medium flow 6 is then expanded in an expansion machine 5.

The working medium flow 6 from the expansion machine 5 is split in a splitting device 13 into two parallel working medium flows 6a, 6b. Then, the working medium flow 6a from the splitting device 13 is condensed in a condenser 2 by the use of a cooling medium flow 8, and the working medium flow 6b from the splitting device 13 is cooled in a heat exchanger 9 to a temperature lower than the temperature of the working medium flow 6a from the condenser 2 by heat exchanging with an LNG flow 10 flowing from an LNG fuel storage tank

2 through the heat exchanger 9 and to a LNG fuelled machinery 1 .

The condensed working medium flow 6a from the condenser 2 and the cooled working medium flow 6b from the heat exchanger 9 are combined in a mixing device 1 1 which is positioned upstream of the pump 4, so that the cooled working medium flow 6b condenses vapor bubbles separated from the liquid in the working medium flow 6a.

The condensed working medium flow 6a from the condenser 2 can be introduced below the cooled working medium flow 6b from the heat exchanger 9 in the mixing device 1 1 .

The working medium flow 6b can be cooled in the heat exchanger 9 to about the temperature of the LNG flow 10.

The LNG flow 10 can be vaporized by heat exchanging with the OCR working medium flow 6b in the heat exchanger 9 and supplied as fuel to the LNG fuelled machinery 1 with a temperature higher than the ORC working medium condensing temperature.

The present modified Organic Rankine Cycle (ORC) process may take waste heat from LNG fuelled engine, gas turbine or other LNG fuelled machinery 1. Preferably the waste heat flow 7 comes from the LNG fuelled machinery 1 . The waste heat may come from warm exhaust and cooling water.

The ORC working medium is pressurized by pump 4 in liquid condition, heated in a heat exchanger 3 with waste heat flow 7, preferably from the LNG fuelled machinery 1 , and sent through an expansion machine 5. In general, the flow rate of the ORC working medium that is utilizing the waste heat from the LNG machinery is much larger than the flow rate of the LNG fuel to the machinery.

After expansion the circulating ORC working medium is split in two flows 6a, 6b. Flow 6a is sent through condenser 2 where the flow is condensed by cooling medium 8. The cooling medium in the cooling medium flow 8 can be cooling water from sea or air cooler.

Flow 6b is sent through the heat exchanger 9 where it is cooled by LNG that is supplied as fuel from the storage tank 12 to the LNG machinery 1. The working medium flow 6b out from the heat exchanger 9 will be cooled and may be cooled close to LNG temperature.

The flows 6a and 6b are re-connected into one flow in the mixing device 1 1 . The mixing device 1 1 is located upstream of and preferably close to pump 4. The mixing device flow outlet leading to the pump 4 is located at the bottom part of the mixing device 1 1 , the flow 6b is introduced in the top part of the mixing device 1 1 and the flow 6a is introduced between the bottom and top of the mixing device 1 1. The mixing device 1 1 has a flow area that is larger than the flow area of the flow from 1 1 to the pump, and the mixing device thereby has a lower flow velocity than the velocity of the flow from the mixing device 1 1 to the pump 4. The flow area of the mixing device 1 1 is so large and the flow velocity is so low that vapor bubbles in the liquid flow 6a are floating up and separated from the liquid. The separated bubbles are cooled and condensed by mixing with the colder liquid flow 6b and thereby preventing bubbles to be transported into the pump. The present invention is a modified ORC cycle to reduce or prevent ORC pump cavitation by preventing vapor bubbles to flow from downstream condenser to the pump. Bubbles are separated from flow 6a from the condenser 2 and condensed by mixing with the cold flow 6b and thereby are bubbles prevented to flow into pump 2. The cold flow 6b is mixed into the flow to the pump 4 and is thereby cooling the flow to the pump 4.

The condensed working medium flow 6a from the condenser 2 and the cooled working medium flow 6b from the heat exchanger 9 are combined in a mixing device 1 1 positioned upstream of the pump 4 so that vapor bubbles are separated from the liquid in the working medium flow 6a and that the cooled working medium flow 6b is mixed with and condenses the vapor bubbles that have been separated from the liquid in the working medium flow 6a thereby reducing or preventing ORC pump cavitation caused by vapor bubbles.

In the present invention the ORC working medium is split into two flows

downstream of the expander and is condensed by two different cooling media. One of the flows is strongly cooled with LNG as cooling medium. The flows are reconnected downstream of the condensers and the cooled flow is utilized to condense bubbles that are separated from the other, warmer flow. The method for reducing the risk for pump cavitation is an arrangement of cooling needs and heating needs in a way that enables selective cooling of the flow to the pump. The cold branch flow of the ORC working medium is mixed with the other branch flow of the ORC working medium in a location where bubbles are collected.

The present invention is not a separate control system.

In the present invention the ORC working medium is split into two flows so that one of the flows is vaporizing LNG and heating up the vapor from LNG. By arranging the ORC cycle into two flow rates the heat exchanging with LNG makes it possible to cool one of the ORC medium flows to temperatures close to LNG temperature. The very cold ORC working medium flow is utilized for selective cooling and condensing of bubbles in addition to cooling of the flow to the pump.