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Title:
POWER GENERATION AND ENERGY RECOVERY SYSTEMS AND METHODS
Document Type and Number:
WIPO Patent Application WO/2008/124868
Kind Code:
A1
Abstract:
A power generation system (100) includes a working fluid (6) circulating around a closed working fluid circuit (101) which includes means (5) to superheat the working fluid, energy conversion means (7) adapted to convert internal energy of the superheated working fluid vapour to useful energy and latent heat recovery means, wherein the latent heat recovery means includes means to retain, recover or accumulate latent heat, and the system includes means to return a proportion of the latent heat to the working fluid before or as it enters the working fluid heating means (5). Methods of energy recover and a combined turbine/generator/pump are also described.

Inventors:
DRYSDALE KENNETH WILLIAM PATTERSON (AU)
MOLYNEAUX ALEX K (GB)
HARRIS MARTYN RUTHERFORD (GB)
Application Number:
PCT/AU2008/000506
Publication Date:
October 23, 2008
Filing Date:
April 11, 2008
Export Citation:
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Assignee:
RENEWABLE ENERGY SYSTEMS LTD
DRYSDALE KENNETH WILLIAM PATTERSON (AU)
MOLYNEAUX ALEX K (GB)
HARRIS MARTYN RUTHERFORD (GB)
International Classes:
F01K25/10; F01D13/00; F01D25/16; F16C32/06; F28C3/08
Domestic Patent References:
WO2007071944A12007-06-28
WO1997013961A11997-04-17
WO2004009964A12004-01-29
WO2008022407A12008-02-28
Foreign References:
US20050076639A12005-04-14
EP0790391A21997-08-20
US20060254251A12006-11-16
US5842345A1998-12-01
DE3321739A11983-10-13
US5311741A1994-05-17
US3931371A1976-01-06
US5336451A1994-08-09
US3943374A1976-03-09
US4362020A1982-12-07
US6071091A2000-06-06
US5841285A1998-11-24
GB2046370A1980-11-12
Other References:
PATENT ABSTRACTS OF JAPAN
DATABASE WPI Week 199822, Derwent World Patents Index; Class Q75, AN 1998-243820
Attorney, Agent or Firm:
BALDWINS (Wellesley Street, Auckland 1141, NZ)
Download PDF:
Claims:

CLAIMS:

1. A power generation system including a working fluid circulating around a closed working fluid circuit, the working fluid circuit including; ■ working fluid heating means to superheat the working fluid;

■ an energy conversion means adapted to convert internal energy of the superheated working fluid vapour to useful energy; and • a latent heat recovery means; wherein the latent heat recovery means includes means to retain, recover or accumulate the latent heat.

2. The power generation system of claim 1 wherein the system includes means to return a proportion of the latent heat to the working fluid before it enters the working fluid heating means.

3. The power generation system of claim 1 or 2 wherein the system includes means to return a proportion of the latent heat to the working fluid as it enters the working fluid heating means.

4. The power generation system of claim 1 , 2 or 3 wherein the working fluid completes a Rankine cycle

5. A power generation system including a working fluid circulating around a closed working fluid circuit, the working fluid circuit including a working fluid heating means adapted to receive working fluid though an inlet and to heat the working fluid so that it becomes a superheated vapour, an energy conversion means adapted to convert internal energy of the superheated working fluid vapour to useful energy, an inlet of the energy conversion means in fluid connection with an outlet of the working fluid heating means, condensing means adapted to remove superheat and latent heat from the working fluid, the condensing means having an inlet in fluid communication with an outlet of the energy conversion means, wherein working fluid moves from the condensing means to an inlet of the working fluid heating means through a fluid path which includes means for heating the working fluid with heat removed from the working fluid vapour in the condensing means.

6. The power generation system of claim 5 wherein the working fluid heating means is a boiler means.

7. The power generation system of claim 5 or 6 wherein the working fluid remains liquid between the condensing means and the working fluid heating means.

8. The power generation system of any one of claims 5 to 7 wherein the working fluid circuit includes a subcooling means having an inlet in fluid communication with an outlet of the condensing means, the subcooling means adapted to subcool the working fluid received from the condensing means before the working fluid moves to the working fluid boiler means.

9. The power generation system of any one of claims 5 to 8 wherein the condensing means includes a desuperheater having an inlet in fluid communication with an outlet of the energy conversion means, the desuperheater adapted to remove superheat from working fluid vapour received from the energy conversion means, and a separate condenser having an inlet in fluid communication with an outlet of the desuperheater, the condenser adapted remove latent heat from working fluid vapour received from the energy conversion means.

10. The power generation system of any one of claims 5 to 9 wherein the energy conversion means includes a turbine.

11. The power generation system of any one of claims 5 to 10 wherein the working fluid ϊS R4109 or R134a.

12. The power generation system of claim 5 including a thermally conductive liquid circulating around a closed thermally conductive liquid circuit, wherein the means for heating the liquid working fluid with heat removed from the working fluid in the condensing means includes a thermally conductive liquid circulating around a dosed thermally conductive liquid circuit.

13. The power generation system of claim 12 wherein the thermally conductive liquid receives heat from the working fluid in the condensing means and transfers heat to the working fluid in the working fluid heating means.

14. The power generation system of claim 13 wherein the thermally conductive liquid is further heated between the condensing means and the working fluid heating means.

15. The power generation system of any one of claims 12 to 14 wherein the temperature of the thermally conductive liquid is decreased before the thermally conductive liquid enters the condenser heat exchanger, and wherein the decrease in temperature is achieved by accelerating the fluid, thereby reducing its temperature and pressure.

16, The power generation system of any one of claims 5 to 8 wherein the temperature of the working fluid is decreased before the heat removed from the working fluid vapour in the condensing means is added to it, wherein the decrease in temperature is achieved by accelerating the working fluid, thereby reducing its temperature and pressure.

17, An energy recovery system including the power generation system of any one of the previous claims, wherein the heat used to heat the working fluid is heat rejected from a primary process.

18, A method of preheating a working fluid entering a boiler means of a power generation cycle having a energy conversion means in fluid communication with the boiier means, the method including transferring superheat and latent heat from working fluid exiting the energy conversion means to the working fluid entering the boiler means.

19, The method of claim 18 wherein the working fluid is accelerated prior to receiving the superheat and latent heat, thereby reducing its temperature.

20. A method of energy recovery including repeating the following steps in a continuous process: i. transferring heat from a heat source to a thermally conductive liquid with a first heat exchange means; ii. moving the thermally conductive liquid to a second heat exchange means and transferring heat from the thermally conductive liquid to a first working fluid with the second heat exchange means;

iii, using energy conversion means to convert a portion of the energy added to the first working fluid to useful energy then moving the working fluid to a third heat exchange means; ^ iv. cooling the thermally conductive liquid from the second heat exchange means and then moving the thermally conductive liquid to the third heat exchange means; v. transferring heat from the first working fluid to the thermally conductive liquid with the third heat exchange means; vi. moving the thermally conductive liquid back tα the first heat exchange means.

21. The method of claim 20 including the step of adding additional heat to the first working fluid between step ii. and step iii. 22. The method of claim 20 or 21 wherein the energy conversion means includes a turbine.

23. A method of increasing a pressure of a superheated gaseous phase substance, the method including the steps of: i, containing the gaseous phase substance in a chamber at an initial pressure; ii. adding a quantity of the substance into the chamber in a liquid phase, wherein the pressure of the liquid phase substance is substantially equal to the pressure of the gaseous phase substance in the chamber; whereby iii. the liquid phase substance is heated by the superheated gaseous substance such that at least a portion of the liquid phase substance boils or evaporates.

24. The method of claim 23 wherein the temperature of the liquid phase substance entering the chamber is slightly below the boiling temperature of the substance at the initial pressure.

25. The method of claim 23 or 24 wherein the chamber has an inlet and an outlet for the gaseous phase substance.

26. The method σf claim 23, 24 or 25 wherein the inlet is provided with a one-way valve.

27. The method of any one of claims 23 to 26 inciudiπg providing the chamberwith a substantially continuous flow of the gaseous phase substance.

28. The method of any one of claims 23 to 27 including providing the chamber with a substantially continuous flow of the liquid phase substan.ee.

29. The method of any one of claims 23 to 28 wherein the ratio of the mass flow rate of the gaseous phase substance to the mass flow rate of the liquid phase substance is approximately 10:1.

30. A power generation system substantially as herein described with reference to any one of the embodiments of the present invention and as shown in the figures.

31. An energy recovery system substantially as herein described with reference to any one of the embodiments of the present invention and as shown in the figures,

32. A method of preheating a working fluid entering a boiler means substantially as herein described with reference to any one of the embodiments of the present invention and as shown in Figures 1 to 6.

33. A method of increasing a pressure of a superheated gaseous phase substance substantially as herein described with reference to Figures 5 and 6.

34. A combined turbine/geπerator/purnp substantially as herein described with reference to any one of the embodiments and as shown in the Figures 7 to 11, 18 and 19 or Figures 12, 18 and 19.

35. A composite bearing substantially as herein described with reference to Figures 18 and 19.

Description:

POWER GENERATION AND ENERGY RECOVERY SYSTEMS AND METHODS

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for creating useful energy or power from low temperature heat sources, and has particular application to generation or recovery of power from waste heat. BACKGROUND TO THE INVENTION

Vapour compression heat pump systems are used in refrigeration and air conditioning systems around the world to keep the atmosphere in a designated area below an ambient temperature, or to cool fluids. Heat is taken from the target fluid or atmosphere and is rejected to a heat sink, usually atmospheric air or a water source. Energy is consumed during this process, usually in the form of electricity to drive one or more refrigerant compressors. It is the heat extraction which is the desired result of such systems, with the rejection of heat to the heat sink being a by-product,

Most such systems reject heat at a relatively low temperature, which is not suitable for powering traditional power generation apparatus, and so there is no attempt to utilise the rejected heat.

Many power generation systems of the prior art operate on a Rankine cycle. The Rankine cycle may be summarised as follows:

■ A working fluid, for example water, is heated in a boiler (also referred to herein as an evaporator) until it becomes a high temperature, high pressure superheated vapour. The temperature of the working fluid may be 400°C or greater at this stage, if traditional working fluids such as water are used.

• The working fluid enters an energy conversion device, typically a turbogenerator (referred to hereinafter as a turbine), which converts a proportion of the energy in the working fluid to electrical energy. The temperature and pressure of the working fluid are lowered as a result of this work.

■ The working fluid enters a condenser, where superheat and latent heat in the working fluid are rejected. The working fluid leaves the condenser as a liquid.

■ The liquid is pumped back to the boiler and the cycle begins again.

It would be desirable to create a system which could convert this waste heat, as well as waste heat from other relatively low temperature sources, to useful energy.

The term "fluid" is used, to describe a fluid in any state, including liquid, gas or saturated vapour, except where the context clearly requires otherwise.

The term "vapour" is used to describe saturated and/or superheated vapour, except where the context clearly requires otherwise.

Where a system of the present invention is powered by heat rejected from a primary process and creates electrical or other power which is used to help power that primary process, the system may be considered to be operating in an "energy recovery" mode. Where the system does not contribute to the running of the primary process, or uses heat from an alternative energy source such as solar or geothermal energy, the system may be considered to be running in a "power generation" mode. Nevertheless, in both cases the systems in accordance with the present invention will be described as "power generation systems", in reference to the fact that electrical or other power is produced by the energy conversion means. The primary process may be any operation which requires the generation of heat. Examples of primary processes include heating of boilers for industrial heating or processing, or refrigeration or air conditioning systems.

OBJECT OF THE INVENTION

It is an object of a preferred embodiment of the invention to provide a power generation system and/or an energy recovery system which will overcome or ameliorate problems with such systems at present.

It is an alternative object to provide a method of preheating a working fluid entering a boiler means of a power generation cycle which will overcome or ameliorate problems With such methods at present.

It is a further alternative object to provide a method of increasing a pressure of a superheated gaseous phase substance Which will overcome or ameliorate problems with such methods at present

The above objects should be read disjunctively with the further alternative object of the invention of at least providing a useful choice.

Other objects of the present invention may become apparent from the following description, which is given by way of example only.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a power generation system including a working fluid circulating around a closed working fluid circuit, the working fluid circuit including;

■ working fluid heating means to superheat the working fluid;

■ an energy conversion means adapted to convert interna! energy of the superheated working fluid vapour to useful energy; and

- a latent heat recovery means; wherein h the latent heat recovery means includes means to retain, recover or accumulate latent heat.

Preferably the system includes means to return a proportion of the latent heat to the working fluid before it enters the working fluid heating means.

Preferably the system includes means to return a proportion of the latent heat to the working fluid as it enters the working fluid heating means.

Preferably the working fluid completes a Rankine cycle.

According to a second aspect of the present invention there is provided a power generation system including a working fluid circulating around a closed working fluid circuit, the working fluid circuit including a working fluid heating means adapted to receive working fluid though an inlet and to heat the working fluid so that it becomes a superheated vapour, an energy conversion means adapted to convert internal energy of the superheated working fluid vapour to useful energy, an inlet of the energy conversion

nαeans in fluid connection with an outlet of the working fluid heating means, condensing means adapted to remove superheat and latent heat from the working fluid, the condensing means having an inlet in fluid communication with an outlet of the energy conversion means, wherein working fluid moves from the condensing means to an inlet of the working fluid heating means through a fluid path which includes means for heating the working fluid with heat removed from the working fluid vapour in the condensing means.

Preferably, the working fluid heating means is a boiler means.

Preferably, the working fluid remains liquid between the condensing means and the working fluid heating means.

Preferably, the working fluid circuit includes a subcooling means having an inlet in fluid communication with an outlet of the condensing means, the subcooling means adapted to subcool the working fluid received from the condensing means before the working fluid moves to the working fluid boiler means.

Preferably the condensing means includes a desuperheater having an inlet in fluid communication with an outlet of the energy conversion means, the desuperheater adapted to remove superheat from working fluid vapour received from the energy conversion means, and a separate condenser having an inlet in fluid communication with an outlet of the desuperheater, the condenser adapted remove latent heat from working fluid vapour received from the energy conversion means,

Preferably the energy conversion means includes a turbine,

Preferably the working fluid is R410a or R 134a,

Preferably the power generation system includes a thermally conductive liquid circulating around a closed thermally conductive liquid circuit, wherein the means for heating the [iquid working fluid with heat removed from the working fluid in the condensing means includes a thermally conductive liquid circulating around a dosed thermally conductive liquid circuit.

Preferably the thermally conductive liquid receives heat from the working fluid in the condensing means and transfers heat to the working fluid in the working fluid heating means.

Preferably the thermally conductive liquid is further heated between the condensing means and the working fluid heating means.

Preferably the temperature of the thermally conductive liquid is decreased before the themπaliy conductive liquid enters the condenser heat exchanger, and wherein the decrease in temperature is achieved by accelerating the fluid, thereby reducing its temperature and pressure.

Preferably the temperature of the working fluid is decreased before the heat removed from the working fluid vapour in the condensing means is added to it, wherein the decrease in temperature is achieved by accelerating the working fluid, thereby reducing its temperature and pressure.

Preferably the heat used to heat the working fluid is heat rejected from a primary process.

According to a third aspect of the present invention there is provided a method of preheating a working fluid entering a boiler means of a power generation cycle having a energy conversion means in fluid communication with the boiler means, the method including transferring superheat and latent heat from working fluid exiting the energy conversion means to the working fluid entering the boiler means.

Preferably the working fluid is accelerated prior to receiving the superheat and latent heat, thereby reducing its temperature.

According to a fourth aspect of the present invention there is provided a method of energy recovery including repeating the following steps in a continuous process: i. transferring heat from a heat source to a thermally conductive liquid with a first heat exchange means; ii. moving the thermally conductive liquid to a second heat exchange means and transferring heat from the thermally conductive liquid to a first working fluid with the second heat exchange means;

iii. using energy conversion means to convert a portion of the energy added to the first working fluid to useful energy then moving the working fluid to a third heat exchange means; iv. cooling the thermally conductive liquid from the second heat exchange means and then moving the thermally conductive liquid to the third heat exchange means; v. transferring heat from the first working fluid to the thermally conductive liquid with the third heat exchange means; vi. moving the thermally conductive liquid back to the first heat exchange means.

Preferably the method includes the step of adding additional heat to the first working fluid between step ii and step iii.

Preferably the energy conversion means includes a turbine.

According to a fifth aspect of the present invention there is provided a method of increasing a pressure of a superheated gaseous phase substance, the method including the steps of; i, containing the gaseous phase substance in a chamber at an initial pressure; ii. adding a quantity of the substance into the chamber in a liquid phase, wherein the pressure of the liquid phase substance is substantially equal to the pressure of the gaseous phase substance in the chamber; whereby iii. the liquid phase substance is heated by the superheated gaseous substance such that at least a portion of the liquid phase substance boils or evaporates.

Preferably the temperature of the liquid phase substance entering the chamber is slightly below the boiling temperature of the substance at the initial pressure.

Preferably the chamber has an inlet and an outlet for the gaseous phase substance.

Preferably the inlet is provided with a one-way valve.

Preferably the method includes providing the chamber with a substantially continuous flow of the gaseous phase substance.

Preferably the method includes providing the chamber with a substantially continuous flow of the liquid phase substance.

Preferably the ratio of the mass flow rate of the gaseous phase substance to the mass flaw rate of the liquid phase substance is between substantially 10:1 and 5:1.

According to a- further aspect of the present invention there is provided a power generation system substantially as herein described with reference to any one of the embodiments of the present invention and as shown in the figures.

According to a still further aspect .of the present invention there is provided an energy recovery system substantially as herein described with reference to any one of the embodiments of the present invention and as shown in the figures.

According to a still further aspect of the present invention there is provided a method of preheating a working fluid entering a boiler means substantially as herein described with reference to any one of the embodiments of the present invention and as shown in Figures 1-6.

According to a still further aspect of the present invention there is provided a method of increasing a pressure of a superheated gaseous phase substance substantially as herein described with reference to Figures 5 and 6.

According to a stiil further aspect of the present invention there is provided a combined turbine/generator/pump substantially as herein described with reference to any one of the embodiments and as shown in the Figures 7 to 11, 18 and 19 or Figures 12, 18 and 19.

According to a stilf further aspect the invention comprises a composite bearing substantially as herein described with reference to Figures 18 and 19.

In a still further aspect the present invention consists in any new feature or combination of features disclosed herein.

Further aspects of the invention, which should be considered in all its novel aspects, will become apparent from the following description given by way of example of possible embodiments of the invention.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 Is a schematic diagram of a preferred embodiment of a power generation/energy recovery system of the present invention.

Figure 2 Is a schematic diagram of a second preferred embodiment of a power generation/energy recovery system of the present invention.

Figure 3 Is a schematic diagram of a third preferred embodiment of a power generation/energy recovery system of the present invention.

Figure 4 Is a schematic diagram of a fourth preferred embodiment of a power generation/energy recovery system of the present invention.

Figure 5 Is a schematic diagram of a fifth preferred embodiment of a power generation/energy recovery system of the present invention.

Figure © Is a schematic diagram of a sixth preferred embodiment of a power generation/energy recovery system of the present invention.

Figure 7 Is a diagrammatic vertical cross section through a combined turbo/generator/pump suitable for use with the present invention,

Figure 8 Shows the rotor of the combined turbine/generator/pump.

Figure 9 Shows a longitudinal cross section of the rotor of Figure 8.

Figure 10 Is a perspective view of the rotor of Figure 8.

Figure 11 Is a transverse cross section through the generator section of the combined turbine/generator/pump shown in Figure 7.

Figure 12 Is a transverse cross section through an alternative configuration of generator section.

Figure 13 Is a partial cross section of a hydrodynamic bearing of the prior art.

Figure 14 Is a partial cross section of the journal of an alternative embodiment of the bearing shown in Figure 13, showing the grooves in the interna! surface.

Figure 15 Is a partial cross section of a hydrostatic bearing of the prior art.

Figure 16 is a partial cross section of the journal of the bearing shown in Figure 15.

Figure 17 Is a partial cross section of an alternative embodiment of a journal for a hydrostatic bearing.

Figure 18 Is a partial cross section of a hybrid bearing of the present invention.

Figure 19 Is a partial cross section of the hybrid bearing of Figure 18.

BEST MODES FOR PERFORMING THE INVENTION

Referring first to Figure 1 , a power generation system according to one preferred embodiment of the invention is generally referenced by arrow 100.

The system 100 has a first heat exchanger 1, which receives heat from a first heating fluid 2, As is described further below, the heating fluid may be hot water, a refrigerant vapour, or any other fluid which can transfer heat from a heat source 3. The first heat exchanger 1 transfers heat from the first heating fluid to a first thermally conductive fluid 4, which circulates around a closed circuit. The thermally conductive fluid is preferably a suitable liquid, such as thermal oil, but is more preferably water based, with the addition of appropriate corrosion inhibiters and boiiing/freezing point modifiers. A preferred thermally conductive liquid is a mixture of 65% water and 35% ethylene glycol additive such as Dowcal 10™, manufactured by the Dow Chemical Company.

The heated thermally conductive liquid 4 travels to a second heat exchanger 5 where it exchanges heat with a first working fluid 6, which also circulates around a closed circuit 101.

In a preferred embodiment the first working fluid preferably has a relatively low evaporating temperature. In a particularly preferred embodiment the first working fluid Is

a blend of 50% refrigerant R32 and 50% R125, the mixture being more commonly known to those skilled in the art as R41 Oa. This means that the working fluid can be vaporised at a temperature of, for example, 55 9 C 1 at 35 bar pressure. The working fluid is heated until it becomes a superheated vapour within the second heat exchanger 5 at a temperature of around 65 e C and pressure of 35 bar.

After heating by the second heat exchanger 5, the first working fluid S moves to an energy conversion means such as a turbine 7, which converts a portion of the internal energy of the working fluid to useful energy such as electrical potential. The electrical energy may be used for any suitable purpose, for example to partially power the various pumping means provided for the system, or it may be sold to an electricity distributor. In some embodiments other energy conversion means such as one or more Seebeck effect type thermoelectric generators may alternatively or additionally be used. In still further embodiments more than one turbine may be used, either in parallel or series as required, or a combination of turbines and thermoelectric generators.

The first working fluid then moves to a third heat exchanger 8, where heat is transferred from the first working fluid 6 to the first thermally conductive fluid 4, thereby at least partially condensing the first working fluid θ, The first working fluid is then pumped by a pumping means 9 back to the second heat exchanger 5. A receiver (not shown) may also be provided upstream of the pump if required. In a particularly preferred embodiment, the turbine 7 and pump 9 may be combined into a single unit, as described further below with reference to Figures 7 to 14.

Prior to entry to the third heat exchanger 8, the first thermally conductive fluid passes through a cooling means 10, such as a liquid to air cooler. A pump 11 is provided to circulate the first thermally conductive liquid around the circuit.

The first thermally conductive liquid enters the third heat exchanger 8 at a sufficiently low temperature to remove the superheat and latent heat from the first working fluid, thereby causing a required amount of condensation of the first working fluid. If required, the cooling means 10 may include a heat pump in order to ensure that sufficient heat is removed from the thermally conductive liquid before it enters the third heat exchanger 8.

Those skilled in the art will appreciate that the first working fluid essentially follows a Raπkiπe cycle as it travels around the first working fluid circuit 101. A non-return valve

(not shown) may be provided as required, for example between the third heat exchanger and the first heat exchanger and between the second heat exchanger and the pump. Those skilled in the art will appreciate suitable locations for the non-return valve.

Referring next to Figure 2, an alternative embodiment of the present invention is generally referenced 200, with similar numerals referring to similar features as in Figure 1.

In this embodiment the first heat exchanger 1 receives heat from a vapour compression refrigeration or heat pump cycle or system 201. In this embodiment the first heating fluid is a working fluid 12 of the heat pump cycle 201. In a preferred embodiment the first heat exchanger 1 acts as a condenser of the heat pump cycle 201.

The cooling means for the thermally conductive liquid 4 preferably includes a fourth heat exchanger 1Oa 1 which moves heat from the thermally conductive liquid 4 to the second working fluid 12, before the thermally conductive liquid 4 moves to the third heat exchanger 8. The fourth heat exchanger 10a is preferably in parallel with an evaporator 13 of the heat pump 12, so that both the fourth heat exchanger 10a and evaporator 13 receive second working fluid from an expansion means, such as a capillary tube or a thermostatic expansion valve (TX valve) 14, of the heat pump 201. The second working fluid 12 flows from the evaporator 13 and fourth heat exchanger 10a back to the first heat exchanger 1 via a pumping means (not shown).

If required, a fifth heat exchanger 15 may be provided to add further heat from a second heating fluid (not shown) to the ' first working fluid 6 between the second heat exchanger 5 and the turbine 7, in order to ensure that the working fluid enters the turbine 7 at a required temperature.

Referring next to Figure 3, another alternative embodiment of the present invention is generally referenced 300, with similar numerals referring to similar features as in Figures 1 and 2.

In this embodiment a heating fluid 2a, which has been heated by a heat source (not shown), enters the fifth heat exchanger 15. The heating fluid leaves the fifth heat exchanger 15 and then enters the first heat exchanger 1. In this way the first working fluid 6 is heated to the maximum possible temperature before it enters the turbine 7, This embodiment may be particularly useful for recovering heat from industrial boilers or the

like. In this embodiment the heating fluid 2a may also be a thermally conductive liquid such as Dowcai 10.

The fourth heat exchanger 10a may be omitted from this embodiment, although in some embodiments a heat pump may be required to sufficiently cool the thermally conductive fluid 4 entering the third heat exchanger 8.

Figure 4 shows anther embodiment of the invention, generally referenced 400.

The power generation system 400 includes a closed working fluid circuit 401 which is provided with a suitable working fluid. In the embodiment described below the working fluid is R410a, but those skilled in the art will be familiar with other working fluids which will be suitable for alternative embodiments of the invention.

A working fluid boiler means 20 receives liquid working vapour and heats the working fluid until it is a superheated vapour. In one embodiment the working fluid may leave the boiler means at around 65°C and 35 bar. The input of the boiler contains a πon return valve V. The output of the. boiler 20 contains a valve P with a preset upper pressure of 35 Bar which maintains the pressure of the boiler output at 35 Bar prior to entering the energy conversion means.

The superheated vapour travels to an energy conversion means, for example a turbine 21 , where a portion of the internal energy in the working fluid vapour is converted into useful energy, for example in the form of electricity, or rotational energy of the turbine output shaft. The working fluid leaves the turbine 21, for example at a temperature of around 33°C and pressure of 20 bar, and moves to a condensing means, generally referenced 22. The condensing means 22 transfers the superheat and latent heat from the working fluid, leaving it in a substantially liquid state, or as a slightly subcooled liquid, for example at a temperature of around 3O 0 C and a pressure of around 18 bar. The heat is transferred to a stream of working fluid liquid, as is described. further below.

The condensing means 22 may be a single heat exchanger unit, but it is preferred that the condensing- means includes separate desuperheater 23 and condenser 24 heat exchangers. By separating the desuperheatϊng and condensing functions the performance of each heat exchanger may be improved, and "pinch points" avoided. In the embodiment shown the working fluid leaves the desuperheater 23 and enters the

Gondeπser 24 at a temperature of around 30 α C and pressure of 18 bar. In the example shown in Figure 4 the working fluid leaves the condensing means 22 as a liquid at a temperature of around 30° C and pressure of 18 bar.

The liquid working fluid leaves the condensing means 22 and passes through a subcooler 25 which removes any residual latent heat remaining in the working fluid, and subcools the liquid. In a preferred embodiment the subcooler 25 includes a heat pump 25A and the heat from the working fluid is rejected to a suitable cooling medium by the heat pump 25λ However, in other embodiments an alternative heat sink such as a cooling tower or source of suitably cool water may be used to subcool the working fluid, depending on the required temperature of the subcooled working fluid. In the embodiment shown the working fluid may be cooled by the subcooler to a temperature of around 20 D C and pressure of 18 bar.

A control means {not shown) controls the heat removed from the working fluid by the

■ subcooler in order to obtain a required temperature- of the working fluid leaving the subcooler.

The subcooled working fluid moves from the subcooler 25 to a receiver 26, A pump 27 is provided to pump the subcooled working fluid from the receiver 26 to the secondary side of the condenser means 22 via a nozzle N which Increases the speed of the fluid and reduces its temperature, The working fluid moves to the secondary side of the condenser 22 where it receives heat from the working fluid vapour exiting the turbine 21, and flashes to a vapour. In one embodiment the turbine 21, receiver 26 and pump 27 may all be provided as a single unit, for example a turbopump as described below. If this type of turbopump is used then an auxiliary electric pump (not shown) may be required for startup of the system. Alternatively the turbopump generator may be adapted for use as an electrical motor during the startup phase.

The preheated working fluid liquid moves to a diffuser D which reduces its speed and increases its pressure to around that immediately upstream of the nozzle N, The working fluid then returns to the working fluid boiler means 20 via a non return valve V.

The pump 27 pumps the working fluid to the boiler 20, where the pressure pulse from the pump forces open the non return valve V at the entrance to the boiler 20 and allows an increment of working fluid to enter. The working fluid undergoes auto compression by heating the working fluid vapour in a closed environment to raise it's pressure from 15 bar to 35 bar. The boiler also acts as a heat store with a capacity large enough to ensure a

sufficient flow of heat energy to operate the turbine 21. The working fluid then moves to the turbine 21 through a preset valve P 1 which only allows the working fluid to flow from the boiler 20 when its pressure has been raised to that required by the turbine 21.

The boiler means 20 may heat the working fluid with heat derived from a dedicated heat source, for example combustion of a fuel, but in a more preferred embodiment the boiler means 20 is a heat exchanger which heats the working fluid with waste heat from a separate process (not shown). In one embodiment the heat may originate from the flue of a central heating furnace. The heat may be transferred from the separate process to the boiler means 20 by a suitable fluid, for example Dowcal 10. In this way the present invention may generate useful power from heat which would otherwise be rejected to the environment, in another embodiment the working fluid may be selected so that heat rejected from an air conditioning system may be used to heat the working fluid in the boiler means 20, The amount of heat introduced into the system may be controlled by a control means (not shown). Heat balance in the system can be maintained by adjusting the heat input to the boiler in conjunction with the heat rejected through the subcooler.

To ensure that the turbine 21 is not exposed to liquid working fluid the control system may monitor the temperature of the working fluid leaving the boiler 20 and may actuate a valve (not shown) which bypasses the working fluid around the turbine 21 if the temperature of the working fluid is too low. The turbine 21 may be bypassed during the startup phase, or if the heat supply to the boiler 20 is interrupted for any reason for a sufficiently long period outside that normally introduced by the dynamic operation of the various valves in the system.

Those skilled in the art will appreciate that once the system described above has warmed up and is operating in a steady state, the energy rejected to the atmosphere is limited to that removed from the working fluid by the subcooler 25. The energy removed from the working fluid vapour by the condensing means 22 is returned to the liquid working fluid once it has passed through the pump 27. In this way the efficiency of the system is increased in comparison to systems In which all of the superheat and latent heat removed from the working fluid vapour is rejected to the atmosphere.

Figure 5 shows yet another embodiment of invention, generally referenced 500, with similar numbers indicating similar features as in the previous drawings. The power generation system shown in Figure 5 is similar to that shown in Figure 4 in that the

superheat and latent heat removed from the working fluid in the condenser 22 is used to preheat the liquid working fluid before it enters the boiler 20. The working fluid used in this case is (1,1,1 ,2-tetrafluoroethane), otherwise known as Ri34a. Other working fluids such as R401a may be used dependent upon the temperature and pressure ranges required. For example, in an air conditioning or chiller system R41 Oa would be preferable because of its high pressure and low temperature characteristics.

However, a different technique is used to ensure that the working fluid entering the secondary side of the condenser heat exchanger is sufficiently cool to ensure that the working fluid entering the primary side of the condenser heat exchanger is fully condensed.

In system shown in Figure 5, the working fluid circuit 501 is provided with a converging nozzle 30 upstream of the secondary side of the condenser heat exchanger 22. The converging nozzle 30 accelerates the liquid working fluid, thereby reducing its temperature, and reducing its pressure to a point at which the liquid working fluid is about to enter a saturated vapour state. In the embodiment shown the pressure is reduced from 3.25MPa to 0.9MPa and the temperature from 40 0 C to 35°C. This high speed, low temperature liquid working fluid receives the heat rejected from the condensing vapour in the condenser heat exchanger 22 and is heated to around 40°C. The heating causes the working fluid to change state to a vapour,

The preheated vapour moves to a boiler heat exchanger 20 where it receives heat from a heat source, preferably via heat transfer fluid. The working fluid vapour, still moving at high speed, then enters a mixing chamber 32 after passing through an optional diffuser (not shown) if required, If the flow is supersonic then the diffuser will reduce the velocity to a subsonic value prior to entering a non-return valve, as is explained further below. The working fluid circuit 501 is provided with a bypass 33 which allows a portion of the liquid working fluid leaving the pump 27 to bypass the converging nozzle 30, condenser heat exchanger 22 and boiler 20. The liquid working fluid in the bypass 33 enters the mixing chamber 32 through a converging nozzle 34, which accelerates the fluid and reduces its pressure to match that of the working fluid vapour entering the chamber 32 from the boiler 20.

The liquid woridng fluid from the bypass 33 merges with the vapour in the mixing chamber 32, expands, and increases the pressure of the working fluid. By combining the liquid and the vapour in the mixing chamber 32 the pressure of the working fluid in the chamber 32 is increased. Noπ return valves (not shown) are provided in both the main and bypass lines to prevent backflow of the working fluid from the mixing chamber 32.

In some embodiments the working fluid may leave the mixing chamber 32 as a vapour. However, in other embodiments the working fluid may leave the chamber 32 just below the vapour temperature. In these embodiments an optional additional heat source heat exchanger 2OA may be required downstream of the mixing chamber 32 to return the working fluid to a vapour state.

The. pressurised working fluid moves from the mixing chamber 32 to the turbine 21. In a preferred embodiment the flow of working fluid through the bypass 33 is controlled with an electronic flow control valve 35. A control means 36 monitors the pressure in the conduit between the mixing chamber 32 and the turbine 21 and controls the electronic flow control valve 35 to ensure that the pressure of the working fluid entering the turbine 21 is maintained within a selected range. Typically the ratio of the mass flow rate of vapour working fluid entering the chamber to liquid entering the chamber is around 10:1 , but could be as low as 5:1 depending on the output pressure required and the volume of the mixing chamber.

If necessary the main working fluid circuit 501 may be provided with a further means of heat rejection, for example a working fluid to air heat exchanger 37, to reduce the temperature of the working fluid entering the receiver 26 from the condenser 22. However, in some embodiments this heat exchanger may not be necessary.

The turbine 21 and/or the main circuit 501 is preferably provided with bypass means (not shown) which bypass the working fluid around the turbine if the temperature of the working fluid is not sufficiently high to ensure that all the working fluid is in vapour form, for example during startup.

Referring next to Figure 6, a further embodiment of the power generation system of the present invention is generally referenced by arrow 600,

In this embodiment heat is transferred from the heat source to a first intermediate heat transfer circuit 601 via a first heat exchanger 40. The intermediate heat transfer fluid in

the intermediate heat transfer circuit 601 is selected to boil or vaporise at a temperature below that of the heat provided by the heat source. In a preferred embodiment the intermediate heat transfer fluid is R134A. In some embodiments (not shown), heat may be transferred directly into the first heat exchanger 40 from the heat source, without the use of a heat source heat transfer circuit.

The R134A vapour leaves the first heat exchanger 40 at a temperature of around 10O'C and travels to a second heat exchanger 41 where heat is exchanged with a working fluid in a main turbine driving circuit 602. in a preferred embodiment the working fluid is heated to around 95 D C at a pressure of around 3.25MPa.

The intermediate heat transfer fluid leaving the second heat exchanger 41 moves through a heat rejection device, such as a radiator 42 or similar, and then through a receiver 43 and pump 44. As with the system shown in Figure 5, the majority of the fluid moves through a converging nozzle 30 and is accelerated before entering the secondary side of a condensing heat exchanger 45 and receiving heat rejected by the condensing working fluid of the main turbine driving circuit 602. The preheated intermediate heat transfer fluid then moves to the mixing chamber 32, via a further heat input heat exchanger (not shown) if required, and a diffuser (not shown) to reduce the velocity of the working fluid vapour to a subsonic value, if the velocity of the working fluid is supersonic. As with the system shown in Figure 5, a portion of the heat transfer fluid bypasses the converging nozzle 30 and condenser heat exchanger 45 and enters the mixing chamber 32 through a converging nozzle 34. The flow through the bypass 33 is controlled by a control system (not shown) and electronic control valve 35.

The turbine driving circuit 602 operates on a Rankiπe cycle, with the working fluid being boiled or evaporated in the second heat exchanger 41, then travelling to a turbine 21. The working fluid leaves the turbine 21 at around 44°C and at a pressure of around 1.34MPa and flows to a condenser heat exchanger 46 which condenses the working fluid and transfers the superheat and latent heat from the working fluid to the intermediate heat transfer fluid. From the condenser heat exchanger 45 the liquid working fluid flows to a receiver 26, and is then pumped back to the second heat exchanger 41 by a pump 27. In the embodiment shown the working fluid is R134A.

As with the embodiment illustrated in Figure 5, the turbine and/or the turbine driving circuit is preferably provided with bypass means (not shown) which bypass the working fluid

around the turbine nozzle tf the temperature of the working fluid is not sufficiently high to ensure that all the working fluid is in vapour form.

Referring next to Figures 7 to 10, a combined turbine/generator/pump is generally referenced by arrow 700, hereinafter referred to as a turbopump 700. The turbopump 700 includes a turbine section 701 , a generator section 702 and a pump section 703, A rotor 50 is provided which includes a turbine wheel 51 , a permanent magnet rotor 52 and a pump rotor or impeller 53, all mounted in line on a common shaft 54 and supported by bearings 55. in one embodiment the turbine section 701 is supplied with 5.2 kg/s of R134A refrigerant vapour at-28 bar pressure, and drives the generator 702 and pump 703 at a shaft speed of 18000 rev/min. The generator 702 produces 90 kW 3-phase maximum electrical output power This output is passed to an uncontrolled diode rectifier bridge (not shown) which produces DC power, which in turn is passed to an inverter (not shown) that produces 3-phase 50 Hz electrical power for injection into the 3-phase mains system. The pump 703 produces the required flow of refrigerant in the liquid state around the system and raises its pressure from 9 bar to 28 bar.

Figures 8 to 10 show the generator rotor 52 which holds 36 segmental pieces of ceramic rare-earth neodymium magnet 56 (best seen in Figure 11) arranged in three pancakes or layers of 12 magnet pieces each. The magnet pieces 56 are held securely in place against centrifugal force by a framework of aluminium cradles 57 which fit closely to the magnets 56 and surround them on all sides. Figures 8 and 10 show the assembled framework of cradles 57 with the actual magnets omitted. Figure 11 shows a transverse cross-section of the generator 702, showing the magnet pieces 56 and part of the surrounding aluminium framework 57. The inner surfaces of the magnet pieces 56 fit to a ferromagnetic steel hub 58 that provides a yoke or return magnetic circuit for the magnetic flux of the generator. In some embodiments it may be constructioπally convenient to allow radial gaps (not shown) between the magnets 56 and yoke 58 of about 1 mm, which are filled with an aluminium web forming part of said aluminium framework. In the complete rotor assembly, all interfaces between magnets 56, aluminium framework 57 and rotor hub 58 are preferably filled with an epoxy adhesive which bonds all parts together and greatly increases strength and rigidity of the structure.

In the structure shown in Figure 11, the polarity of the outer faces 59 of the magnets 56 alternates in a circumferential direction: N, S, N, S, N, S, N, S, N, S, N, S, producing a 12- pole heteropolar configuration. The three pancakes or layers of magnets 56 have their

polarities axially aligned, so that the polarity of magnet faces in the axial direction is alternately N, N 1 N and S 1 S, S 1 The overall axial length of the magnet structure and of the stator core is approximately 80 mm in a preferred embodiment.

The stator 60 of the generator comprises a core of laminated electrical sheet steel, each lamination of thickness suitable for operation at 1.8 kHz, preferably circa 0.2 mm or less, stamped with 36 slots 61 as shown in Figure 11. A 3-phase electrical winding {not shown) is inserted into the slots to form a 3-phase 12-pole star-connected configuration, two coil sides per slot, full-pitch coils. Sheet insulation (not shown) is provided between the slot walls and the coil sides, and between phases in the end-windings. The entire winding is trickle-impregnated with varnish after completion. The conductor of the coil is formed from a number of parallel stranded enamelled round copper wires, and the number of turns ' of conductor per coil might conveniently be 2 or 3, with all colls per phase in series, depending upon the voltage required to suit the DC input requirements of said inverter. Choice of insulation material, varnish, and wire enamel must be made with due regard to withstanding attack by the specified refrigerant vapour, in accordance with data available from manufacturers, as will be well understood by those skilled in the art.

The generator as described above is capable of 65 kW output at 18000 rev/min with a high-flux-density grade of ceramic neodymium-iron-boroπ magnet However at 90 kW output the voltage regulation is excessive, that is to say, the fall in voltage from no-load to full-load is an undesirably large fraction of no-load voltage and the operating point is close to an unstable complete collapse of output voltage. This situation may be completely remedied by inserting series metallised film capacitors in each of the three output lines from the generator, such as is described in international publication No,WO03/103111 in the name of Bowman Power Systems Ltd. With 2 turns per coil a suitable capacitance value is 50 micro-farad, with a current rating of say 120 A at 1.8 kHz. This provides very moderate voltage regulation and 90 kW is comfortably achieved.

In an alternative preferred embodiment, the generator design comprises a four-pole structure, as shown diagrammatically in Fig 12. The four ceramic rare-earth rotor magnets 62 are of bread-loaf profile, that is, having a bottom end 63, two sides 64 orthogonal to the bottom end 63, and an arcuate top 65. The magnets θ2 are retained against a ferromagnetic rotor hub 66 by a pre-stressed carbon-fibre sleeve 67. The 3-phase stator winding is contained in a 24-slot stator 68. As compared to the above 12-pole arrangement, the 4-pole design offers the advantages of lower working frequency (and

therefore lower power loss in the core), easier rotor assembly, and higher magnetic flux due to the greater volume of magnet material accommodated on the rotor.

Referring next to Figure 7, the generator is preferably cooled by external and internal cooling circuits. In the external cooling circuit liquid refrigerant flows through a cooling jacket which surrounds and has intimate thermal contact with the outer diameter of the stator core either by direct contact with the stator core material or with a separate aluminium (or other high thermal conductivity material) cooling jacket shrunk onto the stator laminations (or bonded with high thermal conductivity adhesive). The principal cooling effect is by pool boiling of the liquid into vapour at the jacket/core interface; this allows efficient cooling of the stator at a constant low temperature. The internal cooling circuit comprises a controlled flow of refrigerant vapour in a sub-cooled condition created by expanding high pressure liquid-phase refrigerant, with the Joule-Thomson effect creating the cooling. The flow passes into the end-winding void at one end of the machine, along the physical gap between stator and rotor, and out of the end-winding void at the other end. This flow must be sufficient to remove without excessive temperature rise a substantial fraction of the considerable heat that is generated within the body of the machine, in the stator core and stator windings, by fluid friction in the gap, and by induced eddy currents in the rotor structure. However ihe flow must also not be too high or very considerable extra fluid loss will result. In a preferred embodiment it has been found that a suitable flow rate is circa 0.2 kg/s.

The refrigerant pump 703 is mounted on the turbine-generator shaft because by so doing the efficiency of the pump may be made much higher than if a separate iower-speed proprietary motor-pump unit were employed. However, this has the disadvantage that there is zero refrigerant flow at standstill and thus the turbine 701 is incapable of starting . the system. Therefore, at starting, an electrical variable-speed drive unit (not shown) is connected to the generator stator winding and the generator is driven as a motor in sensoriess PM mode, this terminology being familiar to those skilled in the art. At somewhere around half-speed, the variable-speed drive unit is disconnected and the alternator becomes available for generation purposes. At this point the turbine drive power is then sufficient to accelerate the shaft to full-speed.

The bearings 55 shown in Figures 8 to 10 are ball bearings, ideally with ceramic balls, lubricated with oil or liquid refrigerant; this latter choice allows oil free system operation.

An improvement to the ball bearings is the use of hybrid bearings lubricated with liquid or vapour phase (gas) refrigerant These hybrid bearings are a combination of aspects of hydrodynamic (self acting) bearings with aspects of hydrostatic (externally pressurised) bearings.

The principle features of plain hydrodynamic bearings are shown in Figure 13. Vapour or liquid in the clearance formed between the rotating shaft 70 and the stationary journal bush 71 creates a pressure due to the rotation of the shaft. Without rotation there is no pressure generation and load capability. An improvement to the plain hydrodynamic bearing has grooves 72 as shown in Figure 14. These grooves 72 improve the stability and reliability of hydrodynamic bearings and are essential for vapour lubricated bearings. These grooves 72 have depths typically twice the clearance between the shaft and journal and have various patterns, herringbone and logarithmic spirals are typical. Other geometries are found in ihe bearing clearance including lobes or steps, but they offer no significant advantages over spiral grooves.

The principle features of hydrostatic bearings are shown in Figures 15 and 16. Vapour or liquid is injected under pressure through small holes 73 directly into the clearance formed between the rotating shaft 70 and the stationary journal bush 71. The pressure generated inside the clearance by the injected fluid supports the loads and provides stability. The size of the injection holes is critical to the successful operation of the bearing. An alternative construction is shown in Figure 17 where the fluid is injected through small holes (or tubes) 71 into pockets 74. These latter pockets 74 spread the pressure providing more load capability. Hydrostatic bearings can work without shaft rotation.

The principle features of the hybrid bearing are shown in Figures 1B aπd 19. Features of both the hydrostatic and hydrodynamic bearings are included to allow injection of fluid (lubricant, either vapour of liquid) into pockets 74 so that load capacity is achieved at stationary conditions (zero rotational speed) but is augmented by the hydrodynamic action at significant rotational speeds. This means that at low speeds (zero speed at start up and shut down) there is no contact between the surfaces that would result for purely hydrodynamic bearings, but at high rotational speeds the bearings are essentially hydrodynamic arid hence there is no need for the pressurised fluid supply - thus reducing pumping losses, There are also improvements in bearing stability and load capacity with the addition of the spiral groove hydrodynamic effects at high rotational speeds. The form

aπd size of the pockets 74 can vary with individual applications to reduce losses. They may be circular, rectangular or take the form of an array of small grooves.

The result is a bearing system that has no wear, nearly infinite life and is completely oil fr^. All the above bearings may be made for radial use Qournal bearings) or axial use (thrust bearings).

In applications where low losses are important the vapour phase (gas) can be used as the lubricant and in applications where higher loads are required the liquid phase can be used as the lubricant. In some applications both vapour and liquid phase refrigerant bearings could be considered in order to cope with different loading and/or operating conditions.

An additional feature in liquid injected hybrid bearings is that by allowing the liquid refrigerant to vaporise during its passage from the injection holes 73 through the bearing the power loss in the bearing is considerably reduced compared to a bearing where pure liquid injection is used. In addition the heat generated in the bearing is removed by the latent heat of vaporisation - this keeps the bearing cool and improves reliability,

During the starting process it may be convenient to restrict the turbine power to substantially less than the rated value, in order to slow the rate of acceleration up to full- speed, which will otherwise occur in typically less than a second. This restriction of power is conveniently achieved by reducing the flow of the hot-source fluid that vaporises the refrigerant in an evaporator or boiler before it is passed to the turbine. The heat input into the system may be increased to its full value when the turbine-generator has settled to full-speed. After the electrical output power has been produced by the generator at high frequency, rectified, and converted by the inverter to alternating current at mains frequency and voltage, the most convenient way of utilising it is to feed into the local mains supply. This must be done in accordance with G59 standards that guarantee the operating mode is acceptable to the supply utility, using an inverter specifically designed for this mode of operation, all this being familiar to those skilled in the art. With such an inverter, when the generator reaches rated speed, effective electrical connection to the mains supply is very quickly enabled and power begins to be generated into the supply, so that further acceleration of the shaft is prevented. Shaft speed is thereafter maintained by means of a local controller that senses speed and communicates with the inverter, which in response causes power generation into the mains to be continuously finely adjusted in such a way as to enable speed to stay always close to the rated value.

Shut-down of the system in normal working circumstances may be conveniently achieved by shutting off the hot-source fluid flow so that the heat input into the system is decreased or completely stops. The stored energy in the thermodynamic system may then be allowed to gradually fall as energy circulates around the system and is dissipated or utilised in various ways. This may take several seconds. When the remaining energy flow is insufficient to maintain the generator at full speed, it is allowed to decelerate and the inverter automatically ceases to generate into the mains when the generator can no longer supply sufficient output power and/or voltage to sustain normal inverter operation. A switch then disconnects the inverter from the mains, and the system slows to standstill over a further period of seconds.

Emergency shut-down may be achieved by shutting off the said hot-source fluid flow and additionally disconnecting the system from the mains while at the same time automatically switching a brake resistor across the DC busbar of the inverter. The resistor is so designed that it can absorb substantial energy over a period of seconds and its initial rate of energy dissipation while the generator is at full speed is matched to the rated power of the system. The system is thus very quickly disconnected from the mains and the shaft immediately begins to decelerate, slowing to standstill over a period of seconds. Emergency shut-down may be invoked if a variety of mal-functions occur in the system, particularly if for whatever reason an over-speed event occurs, which if not checked will threaten the mechanical integrity of spinning components and raise generator output voltage to a level injurious to the inverter.

A further measure may be introduced for emergency shut-down, which is to provide a quick-acting valve that opens rapidly to divert a substantial fraction of the refrigerant vapour away from the turbine into a by-pass circuit. The bypass circuit should contain a pressure-reducing unit, such that the high pressure of the vapour entering the by-pass circuit may be reduced in a controlled manner to the low pressure appropriate to the turbine output. A first further alternative (or in addition) is to provide a by-pass valve around the refrigerant pump, the effect of which upon the whole system, for shut-down purposes, is similar to that of the turbine by-pass. A second further alternative (or in addition) is to by-pass the pump outlet flow directly into the turbine outlet flow. The liquid will vaporise due to the low pressure, and thus increase the pressure at this point, thereby shutting down the turbine flow.

Where in the foregoing description, reference has been made to specific components or integers of the invention having known equivalents, then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described byway of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the spirit or scope of the appended claims.