GB2508193A

GB2508193A - Heat pump arrangement with an expander for extracting work from heat energy input

Info

Publication number: GB2508193A
Application number: GB1221097.7A
Authority: GB
Inventors: Mark Trebilcock; Sarah Anne Trebilcock
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-11-23
Filing date: 2012-11-23
Publication date: 2014-05-28
Also published as: WO2014080164A3; WO2014080164A2; GB201221097D0

Abstract

A heat pump arrangement 10 has a fluid circuit 12 which includes a heat exchanger 18, 20, an expander 22 downstream of the heat exchanger, a condenser 24 downstream from the expander, a first reservoir 32 downstream of the condenser and which in use can receive condenser fluid, a second reservoir 34 which in use can receive fluid from the first reservoir and convey the fluid to the heat exchanger. The arrangement further includes valve arrangements 36, 38, 40, 44 for controlling conveying of fluid from the first reservoir and the second reservoir, such that the valve arrangement permits selectively filling the second reservoir from the first reservoir, and the valve arrangement further permitting discharging the second reservoir at high pressure to the heat exchanger. In use, the heat exchanger is able to receive heat energy from a source and the condenser is able to reject heat energy to a sink at a lower temperature than the source. Flow of fluid around the circuit may be assisted by gravity, and the arrangement can be an organic Rankine cycle.

Description

HEAT PUMP ARRANGEMENT WITH AN EXPANDER FOR EXTRACTING WORK

FROM HEAT ENERGY INPUT

Field of the Invention

The present invention relates to a heat pump.

Background of the Invention

The Rankine cycle is well known and is often used to convert heat energy from a heat source (often referred to as waste heat) to produce useful work. An organic Rankirie cycle converts heat energy from a low heat energy source. Typically, a Rankine cycle uses water as a working fluid and a source of heat energy above 100°C (degrees centigrade). An organic Rankine cycle is typically used to produce useful work, using an organic working fluid, from a heat source at less than 100°C degrees centigrade. fl

*.....

* The Rarikine cycle operates by generating a high pressure in the working fluid in one region of a closed circuit, which is usually used to drive an expander, such as a steam turbine. The high pressure is generated by a generator or boiler which transfers heat, from the heat source, to the working fluid principally with the intention * of increasing the pressure of the working fluid but in practical systems the heat *s.*.* * energy also causes an increase in enthalpy of the working fluid. Downstream of the expander is a condenser which reduces the pressure of the fluid and transfers heat to a low temperature source. A pump pumps the fluid through the system.

There are continued attempts to improve the Rankine cycle and aspects of it and generally these attempts have been aimed at improving the efficiency of the expander.

An object of the present invention is to provide an improved heat pump.

Summary of Invention

exchanger for receiving heat energy from a first external source for supply to the working fluid; an expander downstream from the heat exchanger for generating work from the supplied heat energy; a condenser downstream from the expander for transferring heat energy from the working fluid to a second external source at a lower temperature than the first external source; and a pressure transfer region downstream of the condenser and upstream of the heat exchanger for conveying fluid around the fluid circuit and comprising: a first reservoir arranged to receive fluid from the condenser; a second reservoir arranged to receive fluid from the first reservoir and to convey it downstream towards the heat exchanger; and a valve arrangement for controlling conveying of fluid from the first and the second reservoirs; wherein the valve arrangement is configured for selectively filling the second reservoir from the first reservoir, isolating the first reservoir from the second reservoir and discharging the second reservoir at high pressure to the heat exchanger.

The valve arrangement may comprise a first valve having an open condition for allowing fluid communication between the first reservoir and the second reservoir and a closed condition for resisting said fluid communication.

The valve arrangement may comprise a second valve having an open condition for conveying fluid from the second reservoir towards the heat exchanger and a closed condition for resisting the conveyance of fluid.

The valve arrangement may comprise a third valve for conveying fluid pressure from a location downstream of the second valve to a location downstream of the first valve between the first reservoir and the second reservoir.

The valve arrangement may comprise a fourth valve for conveying fluid. pressure from a location downstream of the second reservoir and upstream of the second valve to a location between the expander and the condenser.

Preferably, the first valve is operable to convey working fluid from the first reservoir for filling the second reservoir when the liquid level in the second reservoir is reduced below a predetermined level, the second valve is operable to convey fluid from the second reservoir to the heat exchanger when the liquid level in the second reservoir is increased above a predetermined level, the third valve is operable to equalise the pressure in the second reservoir with the pressure in the heat exchanger generally prior to conveying working fluid from the second reservoir, and the fourth valve is operable to equalise the pressure in the second reservoir with the pressure upstream of the condenser generally prior to conveying working fluid from the first reservoir to the second reservoir.

In one embodiment, a third reservoir is located downstream of the second reservoir and upstream of the heat exchanger for receiving liquid from the second reservoir at a first pressure and discharging liquid to the heat exchanger at a second higher pressure.

A boiler vessel may be provided for receiving heat energy from an external heat source and transferring the heat energy to liquid being conveyed from the second reservoir to the third reservoir.

Preferably, a second valve arrangement is provided for selectively conveying liquid from the boiler vessel to the third reservoir and from the third reservoir to the heat exchanger.

The second valve arrangement may comprise a fifth valve having an open condition for allowing fluid communication between the boiler vessel and the third reservoir and a closed condition for resisting said fluid communication.

The second valve arrangement may comprise a sixth valve having an open condition for conveying fluid from the third reservoir towards the heat exchanger and a closed condition for resisting the conveyance of fluid.

The second valve arrangement may comprise a seventh valve for conveying fluid pressure from a location downstream of the sixth valve to a location downstream of the fifth valve between the boiler vessel and the third reservoir.

The second valve arrangement may comprise an eighth valve for conveying fluid pressure from a location downstream of the third reservoir and upstream of the sixth valve to a flow path extending between the boiler vessel and the heat exchanger.

Preferably1 in use; expanded working fluid exhausts from the expander, passes to the condenser and subsequently as condensate passes to the first reservoir at a low pressure; when a first predetermined condition is met the second valve opens and drains condensate from the first reservoir to the second reservoir at an intermediate pressure; when a second predetermined condition is met the second valve closes, so as to isolate the first and second reservoirs one from another; when a third predetermined condition is met, the third valve opens and condensate drains from the second reservoir to a third reservoir at a high pressure; when a fourth predetermined condition is met the second valve closes, so as to isolate the second and third reservoirs one from another; when a fifth predetermined condition is met, the fourth valve opens and condensate is returned to the heat exchanger, where it is reheated from an external heat source to repeat the cycle.

In the embodiments illustrated, the fluid circuit is closed; the working fluid is organic and has a boiling temperature at atmosphere of less than 100°C, preferably less than 80°C and more preferably less than 60°C; and the heat pump is configured for use with a low temperature heat source less than 100°C, preferably less than 80°C and more preferably less than 60°C.

Brief Description of the DrawinQs

In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 is an overall schematic of a closed Rankine cycle with a waste heat source, a heat pump, an expander and a condenser; Figure 2 is a modification of the Figure 1 arrangement; Figure 3 is schematic drawing of an alternative embodiment and shows in greater detail: a heat source, heat exchangers, a heat pump, an expander a condenser, as well as valves and intermediate sealed vessels; and Figure 4 is a modification of the Figure 3 arrangement.

Detailed description of an Embodiment of the Invention Referring to Figure 1, a heat pump 10 is shown comprising a fluid circuit 12 shown in bold arrows for a working fluid. The fluid circuit includes a heat exchanger arrangement for receiving heat energy from a first external source 16 for supply to the working fluid. The heat pump 10 is particularly but not exclusively for use in a so-called organic Rankine cycle with a low heat energy source at a temperature of less than 100° C, or depending on application less than 80° C or less than 60° C, such as waste heat or solar heating. In an organic Rankine cycle the working fluid may be for example carbon dioxide, n-pentane, toluene, HC hydrocarbon gasses, HFC hydro flouro carbon gasses (such as R134a) HF hydro-flouro and HFO refrigerants or ammonia (ammonia has a high specific heat capacity and makes a good heat transfer fluid) in place of water as used in higher temperature applications.

In this embodiment the heat exchanger arrangement comprises a boiler 18 and heat exchanger 20. Fluid is circulated about a circuit 14 including the external heat energy source 16, the boiler and the heat exchanger thereby transferring heat energy to fluid conveyed along the fluid circuit 12. Working fluid enters the heat exchanger arrangement as a liquid at high pressure where it is heated at generally constant pressure by the external heat source to become a dry saturated vapour.

The boiler is intended to elevate pressure, temperature and enthalpy of received working fluid to a saturated vapour condition. However, if the working fluid conveyed from the boiler contains sOme liquid it may damage the expander. Therefore in the illustrated arrangement, a second heat exchanger 18 is provided for ensuring that the working liquid is in a saturated vapour state.

An expander 22 is located on the fluid circuit 12 downstream from the heat exchanger arrangement 18, 20. The expander receives vaporised fluid at high pressure and generates useful work. The expander may be a turbine, or other mechanical mechanism such as a rotary vane compressor. The working fluid loses energy as it passes through the expander as work energy is extracted causing a drop in working fluid pressure to a low pressure. However, the working fluid still retains sufficient heat energy to remain a vapour.

A condenser 24 is located in the fluid circuit 12 downstream from the expander for transferring heat energy from the working fluid to a second external source 26 at a lower temperature than the first external source 16. The condenser may be a heat exchanger for transferring heat energy to a fluid circulated about a cold or ambient temperature fluid circuit 28 including the external source 26. The external source may be a source of cold water and for example may be the sea if the heat energy source is a marine propulsion unit of a ship Working fluid transferred through the condenser is reduced in heat energy and returns to its liquid state.

In known Rankine cycles a pump is located downstream of the condenser for increasing the pressure of the liquid and pumping it round the circuit. The present embodiment does not require a pump and instead uses a pressure transfer region 30 downstream of the condenser and upstream of the heat exchanger for conveying fluid around the fluid circuit.

The pressure transfer region 30 comprises a first reservoir 32 arranged to receive fluid from the condenser 24, a second reservoir 34 arranged to receive fluid from the first reservoir and to convey it downstream towards the heat exchanger arrangement 18, 20; and a valve arrangement for controlling conveying of fluid from the first and the second reservoirs.

The volumetric capacity required in each of the reservoirs is dependent on the capacity of the system for example a larger capacity system will typically require larger capacity reservoirs. The amount of work that can be generated by the system is dependent on the mass flow rate of working fluid through the system, the specific heat capacity of the working fluid and the difference in temperature between the source of heat and the cold, or ambient, temperature sink. For example, in a 10 kW system reservoir 34 may hold a full charge of 2kg and in a 100kw system the reservoir may hold 20kg.

Elevation of the reservoirs is required to harness gravity for supplying working fluid to the heat exchangers. Additionally or alternatively, pressure difference may induce flow and overcome gravity.

The valve arrangement comprises a first valve 36 having an open condition for allowing fluid communication between the first reservoir 32 and the second reservoir 34 and a closed condition for resisting said fluid communication. The first valve is located on the fluid circuit 12 downstream of the first reservoir and upstream of the second reservoir. The first valve may comprise any suitable valve mechanism such as a motorised ball valve.

The valve arrangement also comprises a second valve 38 having an open condition for conveying fluid from the second reservoir 34 towards the heat exchanger arrangement 18, 20 and a closed condition for resisting the conveyance of fluid. The second valve is located on the fluid circuit downstream of the second reservoir and upstream of the heat exchanger arrangement. The second valve may comprise any suitable valve mechanism such as a motorised ball valve.

Additionally, the valve arrangement comprises a third valve 40 for conveying fluid pressure from a location downstream of the second valve 38 to a location downstream of the first valve 36 between the first reservoir 32 and the second reservoir 34. The third valve may comprise any suitable valve mechanism and in the present embodiment the valve comprises a solenoid valve in combination with a flow reducing orifice 42.

Further, the valve arrangement comprises a fourth valve 44 for conveying fluid pressure from a location downstream of the second reservoir 34 and upstream of the second valve 38 to a location between the expander 22 and the condenser 24. The fourth valve may comprise any suitable valve mechanism and in the present embodiment the valve comprises a solenoid valve in combination with a flow reducing orifice 46.

A first flow path 48 is provided including the third valve 40 and a second flow path 50 is provided including the fourth valve 44.

The valve arrangement is configured for selectively preventing fluid being conveyed from the first reservoir 32 to the second reservoir 34 and allowing the second reservoir to convey fluid downstream towards the heat exchanger arrangement 18, 20 and subsequently to allow fluid in the first reservoir to be conveyed to the second reservoir. The valve arrangement prevents the conveyance of fluid from the first reservoir to the second reservoir for increasing the pressure of fluid in the first reservoir. In this regard, liquid working fluid flows from the condenser into the first reservoir 32 generally continuously throughout the cycle. The fluid charged into the first reservoir is at low pressure. The first reservoir is principally provided for periodic charging of the second reservoir. The second reservoir is charged from the first reservoir and then connected to the high pressure region of the cycle for increasing the pressure in the second reservoir. At the selected time the contents of the second reservoir are released and their pressure together with gravity is sufficient to drive the heat exchanger arrangement and circulate fluid around the circuit 12.

The use of heat pump 10 will now be described in more detail. In a first stage in the cycle the fluid level in the second reservoir 34 is reducing. At this stage, the first valve 36 is closed, the second valve 38 is open, the third valve 40 is energised (i.e. open if the solenoid valve is a normally closed valve) and the fourth valve 44 is de-energised (i.e. closed if the solenoid valve is a normally closed valve). When the level in the second reservoir reaches a predetermined low level limit, a float switch (not shown) energises a fill relay which causes the second valve 38 to start to close.

When the valve is almost or substantially closed a cam switch de-energises the third valve 40 and energises the fourth valve 44.

At this stage, the first valve 36 is closed, the second valve 38 is closed, the third valve 40 is de-energised and the fourth valve 44 is energised.

When the fourth valve 44 is eriergised the pressure in the second reservoir 34 will reduce until it is generally the same as the pressure at the inlet of the condenser 24.

A differential pressure switch opens first valve 36. At this stage, first valve 38 is open, second valve 38 is closed, third valve 40 is de-energised and fourth valve 44 is energised. With the first valve 36 open and the fourth valve 44 energised fluid in the first reservoir 32 can drain into the second reservoir 34.

As the fluid level in the second reservoir 34 increases the float switch rises, and once a predetermined level is reached, a drain relay causes closing of the first valve 36.

When the first valve is almost closed a cam switch enables the fourth valve 44 to de-energise and the third valve 40 to energise. At this stage the pressure in the second reservoir equalises with the pressure in the heat exchanger arrangement 181 20. A differential pressure switch senses when the pressure has equalised and causes the second valve 38 to open to allow the content of the second reservoir to drain into the heat exchanger arrangement 18, 20 with the assistance of gravity. Since the third valve 40 is open gas locking is avoided. The cycle then repeats.

A modification of the Figure 1 embodiment is shown in Figure 2 comprising a regenerator vessel 51 and a further reservoir 52. In this modification the regenerator vessel is arranged to receive or exchange heat energy exhausted from the expander 22 for pre-heating pressurised working fluid conveyed downstream from the second reservoir 34. The reservoir 52 receives the working fluid from the regenerator vessel 51 prior to it being conveyed to the boiler 20 to ensure a substantially constant or regular feed to the boiler, although in some modifications the reservoir 52 may be omitted. The provision of the regenerator improves the efficiency of the cycle.

A second embodiment of the invention is shown in Figure 3 which is a two-stage modification of the first embodiment and the same reference numerals will be used for similar components.

Referring to Figure 3 in greater detail, the second embodiment 58 comprises a second stage 60 of the pressure transfer region comprising a similar pressure step-up arrangement as in the first embodiment. The second stage 60 is located downstream of second valve 38 and upstream of boiler 20. The operation of the first stage 30 of the pressure transfer region is the same as described above and will not be repeated. More than two pressure transfer regions may be provided as required.

The second stage 60 comprises an additional boiler vessel 62 for receiving relatively high pressure liquid from the second reservoir 34. The boiler vessel is arranged to receive heat energy from the heat energy source 16 or possibly an additional or different heat energy source. The heat energy elevates the temperature, pressure and enthalpy of the working liquid towards that of the heat exchanger arrangement 18, 20 and that in the second reservoir. Accordingly, the second stage 60 steps up the temperature in addition to the pressure between the condenser 24 and the boiler 20. The cold or ambient temperature heat sink condenses the vapour exhaust from the expander to form a liquid. However, once condensed it is desirable to increase the temperature (and pressure of the liquid) prior to it entering the boiler. Therefore, the boiler vessel provides heating of the working fluid from a cold or ambient temperature at which it is conveyed from the condenser to a temperature at which it is exhausted from the heat exchangers 18, 20 on the hot side of the system.

In a currently preferred arrangement, the boiler vessel 62 elevates the temperature of the working fluid to between the temperature of the hot source 16 and cold source 26. For example if the hot source is at 80°C and the cold source is at 25°C, the effective temperature in boiler vessel 62 may mid way between those temperatures at 52.5°C, although other temperatures can be used. In order to elevate the temperature of the working fluid in this way, the boiler vessel may exchange heat with a different heat source from source 16 or the heat exchanger arrangement itself may be configured to transfer less heat to the working fluid passing through vessel 16.

A flow path 64 may be provided for conveying heated liquid directly to the heat exchanger 18 to provide a pressure relief line. A third reservoir 66 is arranged to receive fluid from the boiler vessel 62 and to convey it downstream towards the heat exchanger arrangement 18, 20. A second valve arrangement selectively conveys fluid from the boiler vessel and the third reservoir.

A non-return valve 61 is located upstream of the boiler vessel 62 to ensure fluid is not conveyed upstream from the boiler vessel to a lower pressure region. Similarly a non-return valve 63 may be located in duct 64 for resisting the flow of fluid from the heat exchanger 18 to the boiler vessel 62 when the pressure between the boiler and the heat exchanger is higher than the pressure in the boiler vessel.

The second valve arrangement comprises a fifth valve 68 having an open condition for allowing fluid communication between the boiler vessel 62 and the third reservoir 66 and a closed condition for resisting said fluid communication. The fifth valve is located on the fluid circuit 12 downstream of the boiler vessel and upstream of the third reservoir. The fifth valve may comprise any suitable valve mechanism such as a motorised ball valve.

The valve arrangement also comprises a sixth valve 70 having an open condition for conveying fluid from the third reservoir 66 towards the heat exchanger arrangement 18, 20 and a closed condition for resisting the conveyance of fluid. The sixth valve is located on the fluid circuit downstream of the third reservoir and upstream of the heat exchanger arrangement. The sixth valve may comprise any suitable valve mechanism such as a motorised ball valve.

Additionally, the valve arrangement comprises a seventh valve 72 for conveying fluid pressure from a location downstream of the sixth valve 70 to a location downstream of the fifth valve 68 between the boiler vessel and the third reservoir. The seventh valve may comprise any suitable valve mechanism and in the present embodiment the valve comprises a solenoid valve in combination with a flow reducing orifice 74.

Further, the second valve arrangement comprises an eighth valve 76 for conveying fluid pressure from a location downstream of the third reservoir 66 and upstream of the sixth valve 70 to the flow path 64 and to the heat exchanger 18. The fourth valve may comprise any suitable valve mechanism and in the present embodiment the valve comprises a solenoid valve in combination with a flow reducing orifice 78.

Flows paths 80, 82 include respectively seventh and eighth valves 72, 76 for conveying fluid as required. The second stage 60 of the pressure transfer region is arranged such that the third reservoir 66 is selectively charged from the boiler vessel 62 at a relatively high pressure (i.e. that pressure between low pressure and high pressure discharged from the second reservoir 34). The liquid pressure in the -reservoir is then increased by connection to the high pressure region in the boiler 20 and then subsequently the high pressure liquid is discharged to the boiler 20. The second valve arrangement is functionally equivalent to the first valve arrangement and the fifth, sixth, seventh and eighth valves 68, 70, 72, 76 are operable in the same way as respective first, second, third and fourth valves 36, 38, 40, 44.

Therefore as operation of the first to fourth valves has already been described operation of the fifth to eighth valves need not be described again.

Briefly however in use, expanded working fluid exhausts from the expander, passes to the condenser and subsequently as condensate passes to the first reservoir at a low pressure; when a first predetermined condition is met the second valve opens and drains condensate from the first reservoir to the second reservoir at an intermediate pressure; when a second predetermined condiUon is met the second valve closes, so as to isolate the first and second reservoirs one from another; when a third predetermined condition is met, the third valve opens and condensate drains from the second reservoir to a third reservoir at a high pressure; when a fourth predetermined condition is met the second valve closes, so as to isolate the second and third reservoirs one from another; when a fifth predetermined condition is met, the fourth valve opens and condensate is returned to the heat exchanger, where it is reheated from an external heat source to repeat the cycle.

A modification of the two-stage embodiment of Figure 3 is shown in Figure 4 and comprises a regenerator 84 for receiving or exchanging heat energy with fluid exhausted from the expander 22. The regenerator is located to pre-heat fluid conveyed downstream from the reservoir 34. An additional reservoir 86 is located upstream of the boiler 20 and provides a generally regular or constant flow of fluid to the boiler.

In the Figure 4 arrangement a pressure regulating valve is located in duct 64 to ensure that the vessel 62 is not exposed to excess pressure from the fluid being conveyed between the boiler 20 and the heat exchanger 18.

The illustrated heat pumps can be used in any of the following applications which are given by way of example only.

* Waste heat from Marine propulsion units such as diesel engines in combination with a cold heat source of the sea; * Waste heat from refineries; * Sugar mills and other industrial processes that generate low grade waste heat; * Supermarkets and food process factories using large amounts of vapour compression cycle refrigeration can also harvest low grade waste heat from the high temperature discharge gas, by acting as a de-super heater before using the existing condensers; * Power stations the higher quality low grade waste heat being returned to the ambient condensers can be used to further generate more electricity from the cycle; * Large scale district heating and cooling plants generating low grade waste heat available; and * Solar heating arrangements, particularly in warm climates.

The invention has been described by way of several embodiments, with modifications and alternatives, but having read and understood this description, further embodiments and modifications will be apparent to those skilled in the art.

All such embodiments and modifications are intended to fall within the scope of the present invention as defined in the accompanying claims.

Claims

Claims 1. A heat pump comprising a fluid circuit for a working fluid, the fluid circuit including: a heat exchanger for receiving heat energy from a first external source for supply to the working fluid; an expander downstream from the heat exchanger for generating work from the supplied heat energy; a condenser downstream from the expander for transferring heat energy from the working fluid to a second external source at a lower temperature than the first external source; and a pressure transfer region downstream of the condenser and upstream of the heat exchanger for conveying fluid around the fluid circuit and comprising: a first reservoir arranged to receive fluid from the condenser; a second reservoir arranged to receive fluid from the first reservoir and to convey it downstream towards the heat exchanger; and S...a valve arrangement for controlling conveying of fluid from the first and the second reservoirs; wherein the valve arrangement is configured for selectively filling the second reservoir from the first reservoir, isolating the first reservoir from the second reservoir and discharging the second reservoir to the heat exchanger. S...S

*5**** *
2. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a first valve having an open condition for allowing fluid communication between the first reservoir and the second reservoir and a closed condition for resisting said fluid communication.
3. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a second valve having an open condition for conveying fluid from the second reservoir towards the heat exchanger and a closed condition for resisting the conveyance of fluid.
4. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a third valve for conveying fluid pressure from a location downstream of the second valve to a location downstream of the first valve between the first reservoir and the second reservoir.
5. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a fourth valve for conveying fluid pressure from a location downstream of the second reservoir and upstream of the second valve to a location between the expander and the condenser.
6. A heat pump as claimed in claims 2 to 5, wherein the first valve is operable to convey working fluid from the first reservoir for filling the second reservoir when the liquid level in the second reservoir is reduced below a predetermined level, the second valve is operable to convey fluid from the second reservoir to the heat exchanger when the liquid level in the second reservoir is increased above a predetermined level, the third valve is operable to equalise the pressure in the second reservoir with the pressure in the heat exchanger generally prior to conveying working fluid from the second reservoir, and the fourth valve is operable to equalise the pressure in the second reservoir with the pressure upstream of the condenser generally prior to conveying working fluid from the first reservoir to the second reservoir.
7. A heat pump as claimed in any one of the preceding claims, comprising a third reservoir located downstream of the second reservoir and upstream of the heat exchanger for receiving liquid from the second reservoir at a first pressure and discharging liquid to the heat exchanger at a second higher pressure.
8. A heat pump as claim in claim 7, comprising a boiler vessel for receiving heat energy from an external heat source and transferring the heat energy to liquid being conveyed from the second reservoir to the third reservoir.
9. A heat pump as claimed in claim 7 or 8, comprising a second valve arrangement for selectively conveying liquid from the boiler vessel to the third reservoir and from the third reservoir to the heat exchanger.
10. A heat pump as claimed in claim 9, wherein the second valve arrangement comprises a fifth valve having an open condition for allowing fluid communication between the boiler vessel and the third reservoir and a closed condition for resisting said fluid communication.
11. A heat pump as claimed in claim 9 or 10, when dependent on claim 3, wherein the second valve arrangement comprises a sixth valve having an open condition for conveying fluid from the third reservoir towards the heat exchanger and a closed condition for resisting the conveyance of fluid.
12. A heat pump as claimed in claim 11, wherein the second valve arrangement comprises a seventh valve for conveying fluid pressure from a location downstream of the sixth valve to a location downstream of the fifth valve between the boiler vessel and the third reservoir.
13. A heat pump as claimed in claim 11 or 12, wherein the second valve S...*r" arrangement comprises an eighth valve for conveying fluid pressure from a location downstream of the third reservoir and upstream of the sixth valve to a flow path extending between the boiler vessel and the heat exchanger.
14. A heat pump as claimed in any of the preceding claims, wherein, in use; 5.5: . expanded working fluid exhausts from the expander, passes to the condenser and * subsequently as condensate passes to the first reservoir at a low pressure; when a first predetermined condition is met the second valve opens and drains condensate from the first reservoir to the second reservoir at an intermediate pressure; when a second predetermined condition is met the second valve closes, so as to isolate the first and second reservoirs one from another; when a third predetermined condition is met, the third valve opens and condensate drains from the second reservoir to a third reservoir at a high pressure; when a fourth predetermined condition is met the second valve closes, so as to isolate the second and third reservoirs one from another; when a fifth predetermined condition is met, the fourth valve opens and condensate is returned to the heat exchanger, where it is reheated from an external heat source to repeat the cycle.
15. A heat pump as claimed in any one of the preceding claims, wherein the fluid circuit is closed.
16. A heat pump as claimed in any one of the preceding claims, wherein the working fluid is organic and has a boiling temperature at atmosphere of less than 100°C, preferably less than 80°C and more preferably less than 60°C.
17. A heat pump as claimed in any one of the preceding claims, configured for use with a low temperature heat source less than 100°C, preferably less than 80°C and more preferably less than 60°C.
18. A heat pump as claimed in any one of the preceding claims, arranged when in use such that the elevation of the reservoirs imparts gravitational force to the working fluid when in a liquid state for increasing the pressure of the working fluid prior to entering the heat exchanger. * * * * . * * *.** **** * .Amendments to the claims have been filed as follows Claims 1. A heat pump comprising a fluid circuit for a working fluid, the fluid circuit including: a heat exchanger for receiving heat energy from a first external source for supply to the working fluid; an expander downstream from the heat exchanger for generating work from the supplied heat energy; a condenser downstream from the expander for transferring heat energy from the working fluid to a second external source at a lower temperature than the first external source; and a pressure transfer region downstream of the condenser and upstream of the heat exchanger for conveying fluid around the fluid circuit and comprising: a first reservoir arranged to receive fluid from the condenser; a second reservoir arranged to receive fluid from the first reservoir and to convey it downstream towards the heat exchanger; and S...a valve arrangement for controlling conveying of fluid from the first and the second reservoirs; wherein the valve arrangement is configured for selectively filling the second reservoir from the first reservoir, isolating the first reservoir from the second reservoir and discharging the second reservoir to the heat exchanger. S...S*5**** * 2. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a first valve having an open condition for allowing fluid communication between the first reservoir and the second reservoir and a closed condition for resisting said fluid communication.3. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a second valve having an open condition for conveying fluid from the second reservoir towards the heat exchanger and a closed condition for resisting the conveyance of fluid.4. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a third valve for conveying fluid pressure from a location downstream of the second valve to a location downstream of the first valve between the first reservoir and the second reservoir.5. A heat pump as claimed in any of the preceding claims, wherein the valve arrangement comprises a fourth valve for conveying fluid pressure from a location downstream of the second reservoir and upstream of the second valve to a location between the expander and the condenser.6. A heat pump as claimed in claims 2 to 5, wherein the first valve is operable to convey working fluid from the first reservoir for filling the second reservoir when the liquid level in the second reservoir is reduced below a predetermined level, the second valve is operable to convey fluid from the second reservoir to the heat exchanger when the liquid level in the second reservoir is increased above a predetermined level, the third valve is operable to equalise the pressure in the second reservoir with the pressure in the heat exchanger generally prior to conveying working fluid from the second reservoir, and the fourth valve is operable to equalise the pressure in the second reservoir with the pressure upstream of the condenser generally prior to conveying working fluid from the first reservoir to the second reservoir.7. A heat pump as claimed in any one of the preceding claims, comprising a third reservoir located downstream of the second reservoir and upstream of the heat exchanger for receiving liquid from the second reservoir at a first pressure and discharging liquid to the heat exchanger at a second higher pressure.8. A heat pump as claim in claim 7, comprising a boiler vessel for receiving heat energy from an external heat source and transferring the heat energy to liquid being conveyed from the second reservoir to the third reservoir.9. A heat pump as claimed in claim 7 or 8, comprising a second valve arrangement for selectively conveying liquid from the boiler vessel to the third reservoir and from the third reservoir to the heat exchanger.10. A heat pump as claimed in claim 9, wherein the second valve arrangement comprises a fifth valve having an open condition for allowing fluid communication between the boiler vessel and the third reservoir and a closed condition for resisting said fluid communication.11. A heat pump as claimed in claim 9 or 10, when dependent on claim 3, wherein the second valve arrangement comprises a sixth valve having an open condition for conveying fluid from the third reservoir towards the heat exchanger and a closed condition for resisting the conveyance of fluid.12. A heat pump as claimed in claim 11, wherein the second valve arrangement comprises a seventh valve for conveying fluid pressure from a location downstream of the sixth valve to a location downstream of the fifth valve between the boiler vessel and the third reservoir.13. A heat pump as claimed in claim 11 or 12, wherein the second valve S...*r" arrangement comprises an eighth valve for conveying fluid pressure from a location downstream of the third reservoir and upstream of the sixth valve to a flow path extending between the boiler vessel and the heat exchanger.14. A heat pump as claimed in any of the preceding claims, wherein, in use; 5.5: . expanded working fluid exhausts from the expander, passes to the condenser and * subsequently as condensate passes to the first reservoir at a low pressure; when a first predetermined condition is met the second valve opens and drains condensate from the first reservoir to the second reservoir at an intermediate pressure; when a second predetermined condition is met the second valve closes, so as to isolate the first and second reservoirs one from another; when a third predetermined condition is met, the third valve opens and condensate drains from the second reservoir to a third reservoir at a high pressure; when a fourth predetermined condition is met the second valve closes, so as to isolate the second and third reservoirs one from another; when a fifth predetermined condition is met, the fourth valve opens and condensate is returned to the heat exchanger, where it is reheated from an external heat source to repeat the cycle.15. A heat pump as claimed in any one of the preceding claims, wherein the fluid circuit is closed.16. A heat pump as claimed in any one of the preceding claims, wherein the working fluid is organic and has a boiling temperature at atmosphere of less than 100°C, preferably less than 80°C and more preferably less than 60°C.17. A heat pump as claimed in any one of the preceding claims, configured for use with a low temperature heat source less than 100°C, preferably less than 80°C and more preferably less than 60°C.18. A heat pump as claimed in any one of the preceding claims, arranged when in use such that the elevation of the reservoirs imparts gravitational force to the working fluid when in a liquid state for increasing the pressure of the working fluid prior to entering the heat exchanger. * * * * . * * *.** **** * .