GB2532263A

GB2532263A - A pumping apparatus

Info

Publication number: GB2532263A
Application number: GB1420224.6A
Authority: GB
Inventors: Miller Jeremy; Oliver David; Islam Nashtara
Original assignee: Spirax Sarco Ltd
Current assignee: Spirax Sarco Ltd
Priority date: 2014-11-13
Filing date: 2014-11-13
Publication date: 2016-05-18
Anticipated expiration: 2034-11-13
Also published as: WO2016075466A1; GB201420224D0; GB2532263B

Abstract

A pumping apparatus 50 for a heat engine 10 e.g. using an Organic Rankine Cycle, uses working fluid extracted from the working circuit. A fraction of the working fluid is extracted in extraction line 60 after the main heat exchanger 14 and is further heated in a pumping heat exchanger 52 before being compressed in a compressor 54. Low pressure liquid is admitted to pumping chambers 62, 64 via non-return valves 78, and the compressed pumping fluid is alternately admitted to pumping chambers 62, 64 to pump the liquid by direct contact.

Description

A PUMPING APPARATUS

The invention relates to a pumping apparatus for a heat engine.

A heat engine is used to convert thermal energy into mechanical energy. An example of a typical heat engine is a Rankine cycle, in which work is generated by the expansion of a working fluid, and in which the working fluid changes phase from liquid to gas and vice versa.

The working circuit is typically a closed system containing a fixed quantity of a working fluid. The working fluid is typically pumped around the working circuit by a mechanical pump. However, traditional mechanical pumps are subject to relatively high mechanical losses.

It is therefore desirable to provide an improved pumping apparatus for a heat engine.

According to a first aspect of the invention there is provided a pumping apparatus for a heat engine, comprising: an extraction line arranged to extract a fraction of working fluid from a working circuit of a heat engine; a compressor for compressing the extracted working fluid to provide a pressurised motive gas; and a pressure-operated pump for pumping the working fluid around the working circuit, wherein the pump is driven by the pressurized motive gas.

The extraction line may be arranged to extract at least 1%, at least 2%, at least 5%, at least 10%, or at least 15% of the working fluid by flow rate (for example, by volume flow rate). The extraction line may be arranged to extract less than 1%, less than 2%, less than 5%, less than 10% or less than 15% of the working fluid by flow rate. The extraction line may be arranged to extract between 1% and 20%, between 2% and 18%, between 5% and 15%, or between 8% and 12% of the working fluid by flow rate.

The extraction line may be configured to extract a fraction of liquid working fluid, the pumping apparatus may further comprise a pumping heat exchanger for vaporising the extracted liquid working fluid, and the compressor may be arranged downstream of the pumping heat exchanger to compress the vaporised extracted working fluid.

The pressure-operated pump may comprise at least first and second vessels arranged to pump liquid working fluid under the action of the motive gas, and each vessel may comprise a liquid inlet and a liquid outlet for receiving and discharging liquid working fluid respectively, a gas inlet for receiving the pressurised motive gas, and a gas outlet for discharging exhaust gas from the vessel.

The pumping apparatus may further comprise a controller configured to selectively open and close valves for the inlets and outlets of each vessel to alternate between operating the first vessel in a filling mode, in which the vessel receives liquid working fluid from the gas inlet and discharges exhaust gas from the gas outlet; and a pumping mode, in which the vessel receives pressurised motive gas from the gas inlet and discharges liquid working fluid under pressure through the liquid outlet. The controller may be configured to operate the second vessel in the filling mode when the first vessel is in the pumping mode, and to operate the second vessel in the pumping mode when the first vessel is in the filling mode. The controller may be configured so that at least one of the vessels operates in the pumping mode at all times.

According to a second aspect of the invention there is provided a heat engine comprising: a working circuit arranged to transfer thermal energy from a heat source to a working fluid flowing around the circuit, and to convert thermal energy to mechanical energy; and a pumping apparatus according to the first aspect of the invention for pumping the working fluid around the working circuit.

The working circuit may comprise, in order with respect to the direction of motion of the working fluid: a main heat exchanger for transferring heat from the heat source to the working fluid; an expander for converting thermal energy in the working fluid to mechanical energy; a condenser for condensing a vapour phase of the working fluid; and the pressure-operated pump of the pumping apparatus.

The extraction line of the pumping apparatus may be arranged to extract working fluid from between the main heat exchanger of the working circuit and the expander. The main heat exchanger may be configured so that the working fluid is liquid when exiting the main heat exchanger. The working fluid may have a dryness of 0% when exiting the main heat exchanger. The expander may be configured so that at least part of the working fluid exiting the expander is liquid, for example, sub-cooled liquid, 0% dry saturated liquid, or two-phase flow comprising a liquid phase and a gas phase. The expander may be a two-phase expander.

The heat engine may further comprise a phase separator disposed between the expander and the condenser of the working circuit for separating the working fluid into liquid and gas streams, and the working circuit may be arranged so that the liquid stream bypasses the condenser. The phase separator may be in fluid communication with the pressure-operated pump so that the phase separator provides liquid working fluid to the pressure-operated pump and receives exhaust gas from the pressure-operated pump.

The working fluid may have a boiling point lower than the water-steam boiling point, so that the heat engine is an Organic Rankine Cycle. It will be appreciated that the boiling point of the working fluid may be less than the water-steam boiling point under the same pressure conditions.

According to a third aspect of the invention there is provided a method of operating a heat engine comprising: a working circuit configured to transfer thermal energy to a working fluid and to convert thermal energy to mechanical energy; and a pumping apparatus for pumping the working fluid around the working circuit; the method comprising: extracting a fraction of the working fluid from the working circuit; compressing the extracted working fluid to provide a pressurised motive gas; and providing the pressurised motive gas to a pressure-operated pump of the pumping apparatus to pump the working fluid around the working circuit.

The fraction of working fluid may be extracted from a portion of the working circuit in which the working fluid is liquid; and the method may further comprise vaporising the extracted liquid working fluid prior to compressing the vaporised extracted working fluid.

The pressure-operated pump may be a positive displacement pump.

The method may comprise the steps of operating a pressure operated pump in accordance with the first aspect of the invention and/or operating a heat engine in accordance with the second aspect of the invention.

The invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 schematically shows a heat engine according to an embodiment of the invention.

Figure 1 shows a heat engine 10 for converting thermal energy from a heat source to mechanical energy. In this embodiment, the heat source is a condensate flow 100 from a steam system. The heat engine 10 comprises a working circuit 12 having, in order with respect to the transport of the working fluid around the circuit 12, a main heat exchanger 14, an expander 16, a phase separator 18, a condenser 20 and a pressure-operated pump 56. A working fluid flows around the working circuit 12.

The heat engine 10 also comprises a pumping apparatus 50 for pumping the working fluid around the working circuit. The pumping apparatus 50 comprises a pumping heat exchanger 52 (or secondary heat exchanger), a compressor 54 and the pressure-operated pump 56 which forms part of the working circuit 12.

A heat source side of the main heat exchanger 14 is arranged to receive the condensate flow 100 so that working fluid in a heat sink side of the main heat exchanger 14 is heated. The main heat exchanger 14 is fluidically coupled to the expander 16 by a fluid line so that the heated working fluid flows to the expander 16 and is expanded to a lower pressure.

The output of the expander 16 is fluidically coupled to the phase separator 18 by a fluid line. The phase separator 18 is configured to separate liquid and gas phases of the expanded working fluid into separate streams. In this embodiment, the phase separator 18 is a simple gravity-based separation vessel as is known in the art. In alternative embodiments, the phase separator may be a centrifugal separator such as a hydrocyclone. The phase separator 18 comprises a surge tank or buffer tank that can store both liquid working fluid and gaseous working fluid, so that either one may be drawn upon when the working circuit is temporarily operating in a non-equilibrium state (i.e. owing to a change in the rate of heat input at the main heat exchanger 14).

The phase separator 18 is fluidically coupled to the condenser 20 by a fluid line so that the gas phase stream of the expanded working fluid is condensed downstream of the phase separator. The condenser 20 comprises a heat source side which receives the gas phase stream of the working fluid, and a heat sink side which receives a cooling fluid, such as cool liquid water.

Fluid lines extending downstream from the condenser 20 and phase separator 18 join so that condensed (liquid) working fluid from the condenser 20 joins with the liquid working fluid from the phase separator 18.

The fluid line carrying the liquid working fluid from the condenser 20 and phase separator 18 is coupled to the pressure operated pump 56, which is arranged to pump the liquid working fluid around the working circuit 12, as will be described in detail below.

The pumping apparatus 50 comprises an extraction valve 58 disposed in the fluid line extending between the main heat exchanger 14 and the expander 16. An extraction line 60 is coupled to the extraction valve 58 so that a fraction of the working fluid can be extracted from the working circuit 12 into the extraction line 60.

The extraction line 60 is fluidically coupled to the pumping heat exchanger 52, which is configured to receive the condensate flow 100 on a heat source side and the extracted working fluid on a heat sink side. The pumping heat exchanger 52 is arranged to vaporise the working fluid by evaporation to provide a flow of vaporised extracted fluid.

The pumping heat exchanger 52 is coupled to the compressor 54 by a fluid line so that the vaporised extracted fluid flow is received at the compressor from the pumping heat exchanger 52, and compressed so as to provide a pressurised motive gas (i.e. at higher pressure relative to the working fluid between the main heat exchanger 14 and the expander 16).

There may be a buffer tank between the compressor 54 and the pressure-operated pump 56, so that there is a ready supply of pressurised motive gas, which may be drawn upon, for example, if a controller for the heat engine initiates an increase in the flow rate of the working fluid.

The pressure-operated pump 56 is arranged to receive the pressurised motive gas and to drive the liquid working fluid received therein from the phase separator 18 and condenser 20 around the working circuit 12. The pressure-operated pump 56 comprises first and second pumping vessels 62, 64, each having a gas inlet 66, gas outlet 68, liquid inlet 70 and liquid outlet 72 provided with respective control valves 74, 76, 78, 80.

The pressure-operated pump 56 has a controller (not shown) configured to control the valves for the respective inlets and outlets so that each of the first and second pumping vessels 62, 64 can be operated in either a filling mode or a pumping mode. In the filling mode, the respective vessel is arranged to receive and store liquid working fluid from the phase separator 18 and condenser 20, and so the valves 76, 78 for the gas outlet 68 and the liquid inlet 70 respectively are open, whereas the valves 74, 80 for the gas inlet 66 and the liquid outlet 72 respectively are closed. In the pumping mode, the respective vessel is arranged to pump out liquid working fluid stored therein under the action of the pressurised motive gas received from the compressor 54, and so the valves 74, 80 for the gas inlet 66 and the liquid outlet 72 respectively are open, whereas the valves 76, 78 for the gas outlet 68 and the liquid inlet 70 respectively are closed.

In order to constantly pump working fluid around the working circuit, the controller is configured so that during operation of the heat engine at least one (in this embodiment, only one) of the first and second pumping vessels 62, 64 is in the pumping mode at any one time (during operation), whereas the other vessel 62, 64 operates in the filling mode in. The first and second vessels 62, 64 are therefore configured to be alternately operated in the pumping and filling modes respectively.

During filling, gas is displaced from the respective vessel 62, 64 as liquid working fluid is received therein. The gas is exhausted via the gas outlet 68, which is in fluid communication with the phase separator 18 by a fluid line. Accordingly, the gas received at the phase separator 18 from the vessels 62, 64 mixes with the gas stream of working fluid from the expander 16, and is subsequently condensed in the condenser 20, before flowing back to the pressure-operated pump 56 as liquid working fluid, thereby rejoining the working circuit 12. In other embodiments, the gas may be exhausted from the gas outlet 68 directly to the condenser 20.

A controller for the heat engine (not shown) is provided for controlling the operation of the heat engine. In particular, the controller may monitor the temperature of at least the condensate flow 100, and the temperature, pressure and dryness fraction of the working fluid at various locations around the working circuit. The controller may control the operation of various parts of the system to ensure it continues to operate as desired. For example, the controller may increase the flow rate of the pressure-operated pump 56 in response to an increase in the temperature or mass flow rate of the condensate flow 100. The controller for the heat engine is linked to the controller for the pressure operated pump 56. The controller may also be required to adjust the fraction of working fluid extracted by the extraction valve 58, and/or or the compression ratio of the compressor 54, in order to adjust the flow rate of the working fluid. The controller may also monitor the levels of liquid phase and gas phase working fluid stored in the various buffer tanks, and adjust various parameters of the system accordingly. For example, if more liquid working fluid is required, the controller may increase the flow rate of the cooling liquid 102 in the condenser, or reduce the mass flow rate of the condensate flow 100, so as to reduce the temperature of the working fluid in the working circuit as a whole. The controller may also be configured to control the mass flow rate of the condensate flow 100. For example, this may be reduced if the controller determines that the working fluid is being evaporated in the main heat exchanger 14.

An example method of operating the heat engine will now be described. In this example, the heat engine is configured so that the working fluid remains in the liquid phase between the main heat exchanger 14 and the expander 16. Consequently, the expander 16 is a two-phase expander, such as a screw-type expander, configured so that part of the flow of working fluid exiting the expander 16 is a multi-phase flow. This type of heat engine is often called a tri-lateral flash cycle.

In this example embodiment, the working fluid is tetrafluoroethane (otherwise known as 1,1,1,2-Tetrafluoroethane or R-134a, or under a number of product names such as Genetron 134a), which is an inert gas refrigerant.

The heat source is a condensate flow 100 of liquid water from a steam system having a temperature of 85°C which is received in the heat source side of the main heat exchanger 14 at a flow rate of 7.2kg/s. The cooling fluid is liquid water at 15°C, received in the heat sink side of the condenser 20 at a flow rate of 68.8kg/s.

Upstream of the main heat exchanger 14, the working fluid is in the liquid phase (which may be sub-cooled liquid or 0% dryness fraction liquid at saturation temperature) at 25.73bar pressure and 23.2°C. As the working fluid flows through the main heat exchanger 14, its temperature increases to 76.5°C whilst the pressure reduces to 24.45bar. The working fluid remains in the liquid phase. Correspondingly, the temperature of the condensate flow 100 reduces from 85°C to 28.9°C as it flows through the main heat exchanger 14, and to a drain 101. In other embodiments, the working fluid may be heated so that it is two-phase after the main heat exchanger 14, such as 20% dryness fraction saturated fluid (80% saturated liquid, 20% saturated vapour).

A main portion of the working fluid flows along the fluid line between the main heat exchanger 14 and the two-phase expander 16 (approximately 90% by mass), whereas the extraction valve 58 extracts a minor portion (approximately 10% by mass) of the working fluid from the fluid line and diverts the extracted fluid along the extraction line towards to the pumping heat exchanger 52. This extracted portion is no longer considered to be part of the working circuit, as it does not flow through the expander 16, and so does not generate mechanical work/energy. The pumping heat exchanger 52 receives the condensate flow 100 in a heat source side of the heat exchanger and receives the extracted working fluid in a heat sink side, so that heat is transferred from the condensate flow 100 to the extracted working fluid to vaporise the extracted working fluid by evaporation. Consequently, the extracted fluid exits the pumping heat exchanger as a gas (100% dryness fraction).

The vaporised extracted fluid flows via a fluid line to the compressor 54, which operates to provide a pressurised motive gas at a pressure of 26.44bar. The pressurised motive gas flows to the pumping vessels 62, 64 in the manner described above to drive the working fluid around the working circuit 12.

Referring back to the main working circuit 12, the main portion of the working fluid continues to flow along the working circuit 12 from the main heat exchanger 14 to the two-phase expander 16. As the fluid flows through the two-phase expander 16, the pressure reduces to 6.77bar and the temperature reduces to 25.6°C. The expander 16 extracts approximately 120kW of mechanical power as the working fluid expands. The working fluid exiting the expander comprises both liquid and gas phases (approximately 42% dryness fraction), and enters the phase separator 18 for separation into separate liquid and gas streams.

The gas phase of the working fluid flows through the heat source side of the condenser 20, where it condenses and cools slightly. Correspondingly, the temperature of the cooling water increases (in this embodiment, from 15°C to 20.6°C).

The two streams of liquid working fluid from the phase separator 18 and condenser 20 combine upstream of the pressure-operated pump 56 at a pressure of 6.43bar and a temperature of 21.9°C.

The pressure-operated pump 56 receives high temperature pressurised motive gas at a pressure of 26.44bar, which is selectively alternately distributed by the controller to the pump vessels 62, 64 to pressurise and pump the liquid working fluid received therein from the phase separator 18 and condenser 20. The temperature of the working fluid also increases to 23.2°C as it is pressurised and owing to contact with the relatively high temperature pressurised motive gas.

The pressurised liquid working fluid leaves the pressure-operated pump 56 at a pressure of 25.73bar and a temperature of 23.2°C, and flows to the main heat exchanger 14.

The thermal cycle of the working circuit 12 repeats as outlined above. The thermal cycle is controlled by the controller for the heat engine 10. For example, the controller may regulate the mass flow rates of the condensate flow 100, the cooling fluid 102 and the working fluid as discussed above, dependent on the temperature, pressure and dryness fraction of the working fluid at different parts of the heat engine.

The applicant has calculated that the extraction of a fraction of the working fluid (approximately 10%), and its subsequent vaporisation and compression for use as a motive gas in the pressure-operated pump 56 results in significant efficiency savings when compared to the use of a mechanical pump for the liquid working fluid.

The applicant has calculated that above described system results in a power output of approximately 120kW from the two-phase expander 16, and requires a power input of approximately 7kW to drive the compressor 54 of the pump apparatus 50.

By way of comparison, the applicant has calculated that a modified system using a conventional mechanical pump for pumping the liquid working fluid around the working circuit 12 would result in a power output of approximately 127kW (marginally higher than above, since 100% of the working fluid is used in the working circuit), but requires a significantly higher power input of approximately 78kW(based on pump manufacturer data).

Accordingly, it can be seen that extracting, vaporising and compressing a portion of the working fluid to drive the working fluid around the working circuit using a pressure-operated pump results in a significantly higher overall power output (i.e. taking into account the power input required). Since working circuits are closed systems, the use of a pressure-operated pump has not previously been considered as these require the provision of a high pressure motive gas. However, the above described embodiments generate a high pressure motive gas from the working fluid itself, which eventually condenses and rejoins the main working fluid. Accordingly, there is no net addition or reduction of working fluid in the working circuit over time (i.e. it remains a closed system).

These efficiency savings are in part a result of the relatively low power required to compress the vaporised working fluid, when compared with the relatively high power required to pump the dense liquid fluid. In a conventional heat engine having a two-phase expander, the only part of the heat engine in which the working fluid is (even partly) vaporised is in the low pressure region downstream of the expander. However, owing to the low pressure of this fluid, it would not be feasible to compress this portion of the fluid for use in a pressure-operated pump.

In an alternative embodiment, the working fluid between the main heat exchanger and the expander may be two-phase.

In a further alternative embodiment, the heat engine may be configured so that the working fluid vaporises by evaporation as it passes through the main heat exchanger 14, for example, an Organic Rankine Cycle (ORC) heat engine. In such embodiments, the pump apparatus 50 would extract the vaporised working fluid from between the main heat exchanger 14 and the expander 16, and there would be no need for a pumping heat exchanger 52 for vaporising the extracted working fluid. However, it may be desirable to provide a pumping heat exchanger 52, for example, to eliminate any wetness from the working fluid by heating (i.e. so that the working fluid has a dryness fraction of 100%).

In an ORC heat engine, the expander 16 would be a conventional expander configured to operate on dry vapour, such as a turbine. Consequently, the working fluid exiting the expander would also be dry, and there would be no requirement for a phase separator between the expander and the condenser. Similarly, gas from the pumping vessels would be exhausted directly to the condenser.

An example of a suitable working fluid for ORC is pentafluoropropane (also known as HFC-245fa, or 1,1,1,3,3,-Pentafluoropropane).

Claims

CLAIMS: 1. A pumping apparatus for a heat engine, comprising: an extraction line arranged to extract a fraction of working fluid from a working circuit of a heat engine; a compressor for compressing the extracted working fluid to provide a pressurised motive gas; and a pressure-operated pump for pumping the working fluid around the working circuit, wherein the pump is driven by the pressurized motive gas.
2. A pumping apparatus according to claim 1, wherein: the extraction line is configured to extract a fraction of liquid working fluid; wherein the pumping apparatus further comprises a pumping heat exchanger for vaporising the extracted liquid working fluid, and wherein the compressor is arranged downstream of the pumping heat exchanger to compress the vaporised extracted working fluid.
3. A pumping apparatus according to claim 1 or 2, wherein the pressure-operated pump comprises at least first and second vessels arranged to pump liquid working fluid under the action of the motive gas, each vessel comprising a liquid inlet and a liquid outlet for receiving and discharging liquid working fluid respectively, a gas inlet for receiving the pressurised motive gas, and a gas outlet for discharging exhaust gas from the vessel.
4. A pumping apparatus according to claim 3, further comprising a controller configured to selectively open and close valves for the inlets and outlets of each vessel to alternate between operating the first vessel in a filling mode, in which the vessel receives liquid working fluid from the gas inlet and discharges exhaust gas from the gas outlet; and a pumping mode, in which the vessel receives pressurised motive gas from the gas inlet and discharges liquid working fluid under pressure through the liquid outlet.
5. A pumping apparatus according to claim 4, wherein the controller is configured to operate the second vessel in the filling mode when the first vessel is in the pumping mode, and to operate the second vessel in the pumping mode when the first vessel is in the filling mode.
6. A pumping apparatus according to claim 4 or 5, wherein the controller is configured so that at least one of the vessels operates in the pumping mode at all times.
7. A heat engine comprising: a working circuit arranged to transfer thermal energy from a heat source to a working fluid flowing around the circuit, and to convert thermal energy to mechanical energy; and a pumping apparatus according to any preceding claim for pumping the working fluid around the working circuit.
8. A heat engine according to claim 7, wherein the working circuit comprises, in order with respect to the direction of motion of the working fluid: a main heat exchanger for transferring heat from the heat source to the working fluid; an expander for converting thermal energy in the working fluid to mechanical energy; a condenser for condensing a vapour phase of the working fluid; and the pressure-operated pump of the pumping apparatus.
9. A heat engine according to claim 8, wherein the extraction line of the pumping apparatus is arranged to extract working fluid from between the main heat exchanger of the working circuit and the expander.
10. A heat engine according to claim 8 or 9, wherein the main heat exchanger is configured so that the working fluid is liquid when exiting the main heat exchanger.
11. A heat engine according to any of claims 7 to 10, wherein the expander is configured so that at least part of the working fluid exiting the expander is liquid.
12. A heat engine according to any of claims 7 to 11, wherein the expander is a two-phase expander.
13. A heat engine according to any of claims 7 to 12, further comprising a phase separator disposed between the expander and the condenser of the working circuit for separating the working fluid into liquid and gas streams, wherein the working circuit is arranged so that the liquid stream bypasses the condenser.
14. A heat engine according to claim 13, wherein the phase separator is in fluid communication with the pressure-operated pump so that the phase separator provides liquid working fluid to the pressure-operated pump and receives exhaust gas from the pressure-operated pump.
15. A heat engine according to any of claims 7 to 14, wherein the working fluid has a boiling point lower than the water-steam boiling point, so that the heat engine is an Organic Rankine Cycle.
16. A method of operating a heat engine comprising: a working circuit configured to transfer thermal energy to a working fluid and to convert thermal energy to mechanical energy; and a pumping apparatus for pumping the working fluid around the working circuit; the method comprising: extracting a fraction of the working fluid from the working circuit; compressing the extracted working fluid to provide a pressurised motive gas; and providing the pressurised motive gas to a pressure-operated pump of the pumping apparatus to pump the working fluid around the working circuit.
17. A method according to claim 16, wherein: the fraction of working fluid is extracted from a portion of the working circuit in which the working fluid is liquid; and wherein the method further comprises vaporising the extracted liquid working fluid prior to compressing the vaporised extracted working fluid.
18. A pumping apparatus or a heat engine substantially as described herein with reference to the drawing.
19. A method of operating a heat engine substantially as described herein with reference to the drawing.