Patent Description:
Heat engine systems are used to provide power in many different applications. One particular application is the powering of water vessels, including submarines, using a nuclear reactor as a heat source to heat a working fluid which is passed through a closed cycle comprising a compressor and turbine to rotate a shaft and from which other machines can be driven (for example electrical generators).

The heat source and main machinery (e.g. compressor and turbine) is housed within the vessel. Such systems have issues with maintaining the desired mass of working fluid in the cycle. For example leaks may lead to a drop off in power and, if the heat source is nuclear in nature, leaks may be a contamination risk to the rest of the vessel. Additionally, maintaining the mass of working fluid in the closed cycle may mean the power output of the system may only be varied within a limited range.

Hence a heat engine system which provides for improved power output control while also preserving the volume of working fluid available for use in the cycle is highly desirable.

<CIT> relates to a power generation system, and more particularly, to a miniature power generation system in a modular form in which both a power generation cycle and an output control device are included in a containment vessel.

According to the present disclosure there is provided an apparatus, system and method as set forth in the appended claims.

There may be provided a heat engine system (<NUM>, <NUM>) comprising a compressor (<NUM>) having an inlet (<NUM>) and an outlet (<NUM>), a heat source (<NUM>) having an inlet (<NUM>) and an outlet (<NUM>), and a turbine (<NUM>) having an inlet (<NUM>) and an outlet (<NUM>). The compressor (<NUM>), heat source (<NUM>) and turbine (<NUM>) may define part of a working fluid flow circuit (<NUM>). The heat engine system may further comprise a housing (<NUM>) which is operable to be sealed to define a reservoir (<NUM>) in which the compressor (<NUM>), heat source (<NUM>), turbine (<NUM>) and working fluid flow circuit (<NUM>) are located. The working fluid flow circuit (<NUM>) may further comprise: a compressor-to-heat-source duct (<NUM>) which extends between the compressor outlet (<NUM>) and the heat source inlet (<NUM>); a heat-source-to-turbine duct (<NUM>) which extends between the heat source outlet (<NUM>) and the turbine inlet (<NUM>); and a turbine-to-compressor duct (<NUM>) which extends between the turbine outlet (<NUM>) and the compressor inlet (<NUM>). A bleed valve (<NUM>) may be provided in flow communication with the compressor outlet (<NUM>), operable to bleed working fluid into the reservoir (<NUM>). An intake valve (<NUM>) may be provided in flow communication with the compressor inlet (<NUM>) operable to allow the passage of working fluid from the reservoir (<NUM>) to the compressor inlet (<NUM>).

The bleed valve (<NUM>) may be provided in the compressor-to-heat-source duct (<NUM>), operable to bleed working fluid passing through the compressor-to-heat-source duct (<NUM>) into the reservoir (<NUM>).

The intake valve (<NUM>) is provided in the turbine-to-compressor duct (<NUM>), operable to allow the passage of working fluid from the reservoir (<NUM>) into the turbine-to-compressor duct (<NUM>) for delivery to the compressor inlet (<NUM>).

The heat engine system (<NUM>) may further comprise a fluid flow junction conduit (<NUM>) which defines a cavity (<NUM>) in flow communication with the bleed valve (<NUM>), the intake valve (<NUM>) and an opening (<NUM>) for fluid communication with the reservoir (<NUM>).

The heat engine system (<NUM>) may further comprise a control system. The control system may be operable to control the opening and closing of the bleed valve (<NUM>). The control system may be operable to control the opening and closing of the intake valve (<NUM>). The control system may be operable to control the bleed valve (<NUM>) and intake valve (<NUM>) independently of one another.

The control system may be operable to control the bleed valve (<NUM>) and intake valve (<NUM>) to open at the same time as one another and/or close at the same time as one another.

The control system may be operable to control the bleed valve (<NUM>) and intake valve (<NUM>) to vary the flow through the bleed valve (<NUM>) and intake valve (<NUM>) relative to one another.

The control system may be operable to control the rate at which the bleed valve (<NUM>) and/or intake valve (<NUM>) open and close.

The control system may be operable to vary the rate at which the bleed valve (<NUM>) and intake valve (<NUM>) open and close relative to one another.

The control system may be operable to control the bleed valve (<NUM>) and intake valve (<NUM>) so that one of the bleed valve (<NUM>) and intake valve (<NUM>) opens as the other closes such that when one of the bleed valve (<NUM>) and intake valve (<NUM>) is fully open, the other is fully closed; and when one of the bleed valve (<NUM>) and intake valve (<NUM>) is <NUM>% open, the other is <NUM>% open.

The compressor (<NUM>) and turbine (<NUM>) may be rotatable around a common axis (<NUM>) and coupled to rotate together around the common axis (<NUM>).

The heat source (<NUM>) may comprise a nuclear reactor.

There may further be provided a vehicle comprising a heat engine system (<NUM>, <NUM>) according to the present disclosure.

There may also be provided a method of operation of a heat engine system (<NUM>, <NUM>) according to the present disclosure wherein the bleed valve (<NUM>) is controlled to bleed working fluid into the reservoir (<NUM>). The intake valve (<NUM>) may be controlled to allow the passage of working fluid from the reservoir (<NUM>) to the compressor inlet (<NUM>). The bleed valve (<NUM>) and intake valve (<NUM>) may be controlled to open and close independently of one another.

The bleed valve (<NUM>) and intake valve (<NUM>) may be controlled to open at the same time as one another and/or close at the same time as one another.

The bleed valve (<NUM>) and intake valve (<NUM>) may be controlled such that one of the bleed valve (<NUM>) and intake valve (<NUM>) opens as the other closes; and when one of the bleed valve (<NUM>) and intake valve (<NUM>) is fully open, the other is fully closed; and when one of the bleed valve (<NUM>) and intake valve (<NUM>) is <NUM>% open the other is <NUM>% open.

The bleed valve (<NUM>) and intake valve (<NUM>) may be controlled to vary the flow through the bleed valve (<NUM>) and intake valve (<NUM>) relative to one another.

The bleed valve (<NUM>) and/or intake valve (<NUM>) may be controlled to open and close at different rates relative to one another.

Hence there is provided a heat engine system (e.g. heat source, for example a reactor, and gas turbine) housed within a single (i.e. common) compartment (the reservoir <NUM>) with an atmosphere of the working fluid of the working fluid flow circuit <NUM>. In this way leaks from the working fluid flow circuit <NUM> can be recycled back into the working fluid flow circuit <NUM> through the compressor <NUM>, and hence not be lost to the environment outside of the housing <NUM>.

Additionally the system is configured to enable power output control from the system by varying the mass of working fluid in the working fluid flow circuit <NUM> by flowing working fluid into the reservoir <NUM> from the working fluid circuit <NUM> and drawing working fluid from the reservoir into the working fluid circuit <NUM>.

Example of the apparatus, system and method of operation of the present disclosure will now be described by way of example only with reference to the figures, in which:.

The present disclosure relates to a heat engine system <NUM>, <NUM>. The present disclosure also relates a vehicle <NUM> comprising a heat engine system <NUM>, <NUM> according to the present disclosure, and a method of operation of a heat engine system <NUM>, <NUM> according to the present disclosure.

The vehicle <NUM> may be provided as a vessel, which may be provided as a submersible such as a submarine <NUM> (as shown in <FIG>), comprising a heat engine system <NUM>, <NUM> according to the present disclosure housed in the vessel (for example as shown in <FIG>).

<FIG> shows a fist example of a heat engine <NUM> according to the present disclosure. <FIG> shows a second example of a heat engine <NUM> according to the present disclosure.

Both examples may comprise a compressor <NUM> having an inlet <NUM> and an outlet <NUM>. There is further provided a turbine <NUM> having an inlet <NUM> and an outlet <NUM>. The compressor <NUM> and turbine <NUM> are rotatable around a common axis <NUM> and coupled to rotate together around the common axis <NUM>. The compressor <NUM> and turbine <NUM> may form part of a gas turbine <NUM> assembly.

Both examples of heat engine system <NUM>, <NUM> may further comprise a heat source <NUM> having an inlet <NUM> and an outlet <NUM>. The heat source <NUM> may be any appropriate source of heat. The heat source <NUM> may be a nuclear reactor. In alternative examples the heat source <NUM> may be chemically fuelled, for example using coal, diesel or ethanol as the fuel source.

The compressor <NUM>, heat source <NUM> and turbine <NUM> define part of a working fluid flow circuit <NUM> through which, in operation, a working fluid (for example nitrogen) is passed. The working fluid flow circuit <NUM> may define a flow path to direct a working fluid to pass through the compressor <NUM>, through the heat source <NUM> and through the turbine <NUM>, returning to the compressor <NUM>. The working fluid flow circuit <NUM> may define a flow path to control the flow of working fluid into, through and out of the compressor <NUM>, then into, through and out of the heat source <NUM>; and then into, through and out of the turbine <NUM> to return to the compressor <NUM> to restart the cycle.

The working fluid flow circuit <NUM> may further comprise a free spinning turbine <NUM>, through which working fluid is passed. The free spinning turbine <NUM> may be used to drive a power off take. As shown in relation to the first example of heat engine system <NUM> in <FIG>, the working fluid flow circuit <NUM> may further comprise a first bypass circuit <NUM>, which provides a bypass flow route past the free spinning turbine <NUM>. The first bypass <NUM> may comprise a first bypass flow control valve <NUM>.

The working fluid flow circuit <NUM> may further comprise a heat exchanger <NUM>, through which working fluid is passed. The heat exchanger <NUM> may be operable to remove heat from the working fluid. As shown in relation to the second example of heat engine system <NUM> in <FIG>, the working fluid flow circuit <NUM> may further comprise a second bypass circuit <NUM>, which provides a bypass flow route past the heat exchanger <NUM>. The second bypass <NUM> may comprise a second bypass flow control valve <NUM>.

As shown in relation to the first example of heat engine system <NUM> in <FIG>, the working fluid flow circuit <NUM> may further comprise a third bypass circuit <NUM>, which provides a bypass flow route from the compressor <NUM> to a region downstream of the free spinning turbine <NUM>. The third bypass circuit <NUM> may comprise a third bypass flow control valve <NUM>. The third bypass circuit <NUM> may additionally or alternatively comprise a filter <NUM> configured for the filtration of particulates and/or chemicals. The third bypass flow control valve <NUM> may be provided in series or in parallel with the filter <NUM>.

The working fluid flow circuit <NUM> may further comprise a fourth bypass circuit <NUM>, which provides a bypass flow route between the heat source inlet <NUM> and heat source outlet <NUM>. The third bypass <NUM> may comprise a third flow control valve <NUM>.

The heat engine system may further comprise a housing <NUM> which is operable to be sealed to define a reservoir <NUM> in which the compressor <NUM>, heat source <NUM>, turbine <NUM> and working fluid flow circuit <NUM> are located. That is to say, the housing <NUM> delimits (e.g. determine the limits or boundaries of) the reservoir <NUM> in which the compressor <NUM>, heat source <NUM>, turbine <NUM> and working fluid flow circuit <NUM> are located. The housing <NUM> may be a discrete (i.e. dedicated) structure, or may comprise a combination of walls which also form parts of the surrounding structure. For example, in the case of a vessel, the walls defining the housing <NUM> may comprise bulkheads and regions of the hull.

The housing <NUM> is sealable and configured to contain the working fluid (for example nitrogen). That is to say, the housing <NUM> may be configured to prevent fluid exchange across the boundary defined by the housing <NUM>. Put another way, the housing <NUM> may be configured to prevent fluid loss from the reservoir <NUM> (i.e. from inside the boundary defined by the housing <NUM> to outside of the boundary defined by the housing <NUM>) and configured to prevent fluid entry into the reservoir <NUM> (i.e. from outside the boundary defined by the housing <NUM> to inside the boundary defined by the housing <NUM>).

As shown in <FIG>, <FIG>, the working fluid flow circuit <NUM> may comprise a compressor-to-heat-source duct <NUM> which extends between the compressor outlet <NUM> and the heat source inlet <NUM> for the passage of working fluid between the compressor outlet <NUM> and heat source inlet <NUM>. The working fluid flow circuit <NUM> may comprise a heat-source-to-turbine duct <NUM> which extends between the heat source outlet <NUM> and the turbine inlet <NUM> for the passage of working fluid between heat source outlet <NUM> and turbine inlet <NUM>. The working fluid flow circuit <NUM> may comprise a turbine-to-compressor duct <NUM> extends between the turbine outlet <NUM> and the compressor inlet <NUM> for the passage of working fluid between turbine outlet <NUM> and the compressor inlet <NUM>.

A bleed valve <NUM> may be provided in flow communication with the compressor outlet <NUM>, operable to bleed working fluid into the reservoir <NUM>. An intake valve <NUM> may be provided in flow communication with the compressor inlet <NUM> operable to allow the passage of working fluid from the reservoir <NUM> to the compressor inlet <NUM>.

The bleed valve <NUM> may be provided in the compressor-to-heat-source duct <NUM>, operable to bleed working fluid passing through the compressor-to-heat-source duct <NUM> into the reservoir <NUM>, the bleed valve <NUM> having, and/or in fluid communication with, a vent <NUM> which opens into the reservoir <NUM>. The intake valve <NUM> may be provided in the turbine-to-compressor duct <NUM>, operable to allow the passage of working fluid from the reservoir <NUM> into the turbine-to-compressor duct <NUM> for delivery to the compressor inlet <NUM>, the intake valve <NUM> having an intake aperture <NUM> which opens into the reservoir <NUM>.

As shown in <FIG>, in relation to the second example of a heat engine <NUM>, the heat engine system <NUM> may further comprise a fluid flow junction conduit <NUM> which defines a cavity (e.g. volume and/or chamber) <NUM> in flow communication with the bleed valve <NUM>, the intake valve <NUM> and an opening <NUM> for fluid communication with the reservoir <NUM>. The opening <NUM> may be at the end of a pipe <NUM>, and fluid flow may be from the fluid flow junction conduit <NUM> into the reservoir <NUM> or from the reservoir <NUM> into the fluid flow junction conduit <NUM>.

The fluid flow junction conduit <NUM> may comprise a filter <NUM>. The filter <NUM> may be operable to prevent the passage of particulates and/or chemicals from and/or into the working fluid circuit <NUM>.

The heat engine system <NUM>, <NUM> may further comprise a control system (i.e. a controller) (not shown) operable to control the opening and closing of the bleed valve <NUM>. The control system (not shown) may be operable to control the opening and closing of the intake valve <NUM>. The control system may be operable to control the bleed valve <NUM> and intake valve <NUM> independently of one another.

The control system (not shown) may be operable to control the bleed valve <NUM> and intake valve <NUM> to open at the same time as one another. The control system (not shown) may be operable to control the bleed valve <NUM> and intake valve <NUM> to close at the same time as one another.

The control system (not shown) may be operable to control the bleed valve <NUM> and intake valve <NUM> to vary the flow through the bleed valve <NUM> and intake valve <NUM> relative to one another. That is to say, the control system may be operable to control the bleed valve <NUM> and intake valve <NUM> to vary the flow through the bleed valve <NUM> relative to the flow throw the intake valve <NUM> and to vary the flow through the intake valve <NUM> relative to the flow throw the bleed valve <NUM>.

The control system may be operable to control the rate at which the bleed valve <NUM> and/or intake valve <NUM> open and close.

The control system may be operable to vary the rate at which the bleed valve <NUM> and intake valve <NUM> open and close relative to one another.

The control system may be operable to control the bleed valve <NUM> and intake valve <NUM> so that one of the bleed valve <NUM> and intake valve <NUM> opens as the other closes such that as the flow rate through one of the bleed valve <NUM> and intake valve <NUM> increases the flow rate through the other of the bleed valve <NUM> and intake valve <NUM> decreases. In one example, when one of the bleed valve <NUM> and intake valve <NUM> is fully open, the other is fully closed, and when one of the bleed valve <NUM> and intake valve <NUM> is <NUM>% open the other is <NUM>% open.

In operation, the bleed valve <NUM> and intake valve <NUM> of the heat engine systems <NUM>, <NUM> of the present disclosure are controlled to open and close, example under the control of the control system (not shown). The relative timing of when one of the bleed valve <NUM> and intake valve <NUM> is closed or open, or is closing or opening, relative the other bleed valve <NUM> and intake valve <NUM> may be controlled according to the required power output of (i.e. power demand on) the heat engine systems <NUM>, <NUM>. That is to say, the relative timing of when one of the bleed valve <NUM> and intake valve <NUM> is closed or open, or is closing or opening, relative the other bleed valve <NUM> and intake valve <NUM> may be controlled to govern the mass of working fluid passing through the flow circuit <NUM>.

The basic operation of the systems <NUM>, <NUM> is described below in table <NUM>.

Hence the bleed valve <NUM> may be controlled to open to bleed a proportion of the working fluid flowing through the flow circuit <NUM> into the reservoir <NUM>, and the intake valve <NUM> is controlled to open to allow the passage of working fluid from the reservoir <NUM> to the compressor inlet <NUM> into the flow circuit <NUM>.

When both the bleed valve <NUM> and intake valve <NUM> are open the net flow of working fluid into the flow circuit <NUM> from the reservoir <NUM> may be positive (i.e. to increase the amount of working fluid in the working fluid flow circuit <NUM>) or negative (i.e. to decrease the amount of working fluid in the working fluid flow circuit <NUM>. If both the bleed valve <NUM> and intake valve <NUM> are closed then there is no controlled flow between the flow circuit <NUM> and the reservoir <NUM>. That is to say, if both the bleed valve <NUM> and intake valve <NUM> are closed, with the exception of any leaks from the flow circuit <NUM>, there is no flow from the flow circuit <NUM> into the reservoir <NUM>.

The bleed valve <NUM> and intake valve <NUM> may be controlled to open and close independently of one another. Hence the bleed valve <NUM> and intake valve <NUM> may be at least partially open (i.e. configured to allow flow of the working fluid) at the same time or at different times).

The bleed valve <NUM> and intake valve <NUM> may be controlled to open at the same time as one another and/or close at the same time as one another.

Alternatively or additionally, the bleed valve <NUM> and intake valve <NUM> may be controlled such that as one of the bleed valve <NUM> and intake valve <NUM> opens, to allow flow of the working fluid therethrough, the other closes, such that as the flow rate through one of the bleed valve <NUM> and intake valve <NUM> increases the flow rate through the other of the bleed valve <NUM> and intake valve <NUM> decreases.

Alternatively or additionally the bleed valve <NUM> and intake valve <NUM> may be controlled such that when one of the bleed valve <NUM> and intake valve <NUM> is fully open, the other is fully closed.

Alternatively or additionally the bleed valve <NUM> and intake valve <NUM> may be controlled such that when one of the bleed valve <NUM> and intake valve <NUM> is <NUM>% open the other is <NUM>% open.

The bleed valve <NUM> and intake valve <NUM> may be controlled to vary the flow through the bleed valve <NUM> and intake valve <NUM> relative to one another.

The bleed valve <NUM> and/or intake valve <NUM> may be controlled to open and close at different rates relative to one another.

The bleed valve <NUM> and/or intake valve <NUM> may have the same flow area capacity, or may have different flow are capacities. That is to say, when fully open, the bleed valve <NUM> (and vent <NUM>) and intake valve <NUM> (and intake aperture <NUM>) may have the same flow area such that the maximum flow rate through both is the same. That is to say, when fully open, the bleed valve <NUM> and vent <NUM> may allow the same maximum flow rate as the intake valve <NUM> and intake aperture <NUM>.

During operation, the reservoir <NUM> delimited by the housing <NUM> provides a reserve of working fluid (for example nitrogen gas) at the same pressure as the turbine inlet <NUM>. The pressure at in the reservoir <NUM> may be controlled to be atmospheric temperature. The pressure in the working fluid flow circuit <NUM> is greater than atmospheric pressure. This ensures leaks from the pressurised working fluid flow circuit <NUM> are eliminated as a concern as leaked working fluid can be inducted into the compressor <NUM> at the compressor inlet <NUM> whilst keeping the pressures in the reservoir <NUM> low.

The use of the housing <NUM> as described as a pressure vessel such that working fluid is vented from the high pressure working fluid flow circuit <NUM> into the reservoir <NUM>, enables the reduction of mass flow through the turbine <NUM> to therefore reduce output power levels whilst maintaining constant pressure ratios. This enabled turbine efficiencies close to design at part load to be achieved.

Maintaining the pressure in the reservoir <NUM> at about atmospheric pressure also obviates the need for the housing <NUM> to a heavy duty (and hence expensive, heavy and large) pressure vessel. Hence the housing <NUM> may be a relatively light structure.

As the flow rate through the bleed valve <NUM> and intake valve <NUM> are changed the balance of working fluid lost from the reservoir <NUM> versus gas working fluid gained in the reservoir <NUM> is changed resulting in either a net loss of gas or net gain of working fluid to the working fluid flow circuit <NUM> from the reservoir <NUM> until the system achieves a steady state.

The relatively large reservoir <NUM> volume required to house the reactor, compressor, turbine and other equipment means that working fluid can be vented into, or withdrawn from, the reservoir small pressure change in the reservoir <NUM>.

The use of nitrogen as a working fluid ensures that a large power change can be accomplished by simply venting the working fluid flow circuit <NUM> into the reservoir <NUM>.

In examples in which both the bleed valve <NUM> and intake valve <NUM> are opened at the same time (thereby enabling flow therethrough) introduces proportional power control in response to the valve opening/closure position of bleed valve <NUM> and intake valve <NUM>. This proportional power control simplifies the turbine power control system as only valve position needs to be controlled rather than valve position and valve opening period. This allows simple enabling of droop control required for stable splitting of load between multiple generators.

For stable power control when supplying an electrical grid, especially where there are multiple generators, the inclusion of droop is critical. Droop in a traditional turbine is controlled by a governor and is a proportional response of turbine power to an error from a reference speed as indicated by equation [<NUM>] <MAT>.

As described above, the opening of the bleed valve <NUM> and intake valve <NUM> may be coupled. In such a mode of operation the mass of working fluid in the working fluid flow circuit <NUM> becomes a function of the rate of bleed-off from the working fluid flow circuit <NUM> vs re-admission to the working fluid flow circuit <NUM>. Hence power level becomes proportional to the combined open and closure position of the bleed valve <NUM> and intake valve <NUM> may. Droop control is therefore simple to implement through placing the valve position relative to the speed error of the turbine to a reference speed. With this approach part load efficiency can be maintained as a significant proportion of the power reduction is through lowering of the mass within the working fluid flow circuit <NUM> where flow bypassing the turbine contribute a small proportion of the power change increment. Additionally, the flow bypassing the turbine ensures increased damping during power control, and can be tuned through selecting valve opening and closing profile to achieve a desired level of system stability.

The configuration of the heat engine system <NUM>, <NUM> of the present disclosure places the heat source <NUM> (e.g. reactor) and other main machinery within a single compartment. Placing all the heat engine components including the working fluid flow circuit <NUM> flow paths within a single reservoir <NUM> allows the reservoir <NUM> to be filled with the same fluid as the working fluid flow circuit <NUM>. The net result is that leaks from the working fluid flow circuit <NUM> enter the reactor reservoir <NUM> where the pressure is approximately atmospheric with limited pressure driver for further leaks to the environment.

This design approach largely removes the driving mechanism for transport of unwanted features of the heat source (e.g. chemicals or radiation) outside of the plant boundary defined by the housing <NUM>. More importantly with the compressor inlet also operating at the same pressure as the reservoir <NUM>, any gas leaks from the pressurised section of the working fluid flow circuit <NUM> can effectively be drawn back into the working fluid flow circuit <NUM> at the compressor inlet.

Such a system, with a closed cycle gas cooled reactor, offers further advantages relating to power control.

Additionally because of the large volume of gas in the reservoir <NUM> the pressures in the reservoir <NUM> do not vary significantly following pressurisation and depressurisation of the working fluid flow circuit <NUM>. This allows a simple valve to act as bleed into the reservoir <NUM> without the need to include compressors to store the removed working fluid flow circuit <NUM> gas in high pressure cylinders. As a result rapid power changes can be accomplished using inventory control, maintaining design point efficiencies at part load removing the need for further complexity in the plant control design.

Claim 1:
A heat engine system (<NUM>, <NUM>) comprising:
a compressor (<NUM>) having an inlet (<NUM>) and an outlet (<NUM>);
a heat source (<NUM>) having an inlet (<NUM>) and an outlet (<NUM>); and
a turbine (<NUM>) having an inlet (<NUM>) and an outlet (<NUM>);
wherein the compressor, heat source and turbine define part of a working fluid flow circuit (<NUM>);
the heat engine system further comprising a housing (<NUM>) which is operable to be sealed to define a reservoir (<NUM>) in which the compressor, heat source, turbine and working fluid flow circuit are located;
wherein the working fluid flow circuit further comprises:
a compressor-to-heat-source duct (<NUM>) which extends between the compressor outlet and the heat source inlet;
a heat-source-to-turbine duct (<NUM>) which extends between the heat source outlet and the turbine inlet; and
a turbine-to-compressor duct (<NUM>) which extends between the turbine outlet and the compressor inlet;
wherein a bleed valve (<NUM>) is provided in flow communication with the compressor outlet, operable to bleed working fluid into the reservoir; and
an intake valve (<NUM>) is provided in flow communication with the compressor inlet operable to allow the passage of working fluid from the reservoir to the compressor inlet.