Heat engine

Abstract

A closed working fluid type heat engine (100) for power generation in the environment of a nuclear fusion power system (200). The heat engine includes a primary heat exchanger (102) connected to a primary heat source (106) and a secondary heat exchanger (108) connected to a secondary heat source (110). The secondary heat source is thermally independent of the primary heat source, and the temperature of the primary heat source is higher than that of the secondary heat source. The heat engine also includes a main turbine (104) and an auxiliary turbine (116). The turbines are arranged in fluid parallel, and a parallel shunt point is arranged between the primary heat exchanger and the secondary heat exchanger. The main turbine receives the working fluid heated by the secondary heat exchanger and the main heat exchanger, and the auxiliary turbine receives the working fluid heated by the secondary heat exchanger. The heat engine also includes a first regenerator (124) configured to thermally connect working fluid flowing out of the main turbine with working fluid flowing into the main heat exchanger. Suitable nuclear fusion power systems and methods of operation thereof are also described.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to thermal energy transfer and power generation systems, and more particularly to thermal energy transfer and power generation systems related to the extraction of energy from nuclear fusion reactors (i.e. power systems as part of nuclear fusion power plants); in particular Tokamak type reactors / power plants. More specifically, the present disclosure relates to thermal energy transfer and power generation systems based on modified Brayton cycles. BACKGROUND

[0002] One of the biggest challenges facing nuclear fusion technology is how to convert the energy produced by nuclear fusion reactors into usable electrical energy. In particular in the case of Tokamak type reactors, closed working fluid heat engines operating according to a (closed) Brayton cycle have been identified as a potentially advantageous power generator.

[0003] One difficulty in the efficient utilisation of heat from nuclear fusion reactors (in particular Tokamak type reactors) in a Brayton cycle / closed cycle gas turbine is that nuclear fusion reactors typically have one or more low grade heat sources (i.e. low temperature heat sources) in addition. While the high grade heat (i.e. high temperature) output from the reactor can be suitably used to power the heat engine, how to deal with the low grade heat produced at the same time is still a problem. One simple answer is to simply say that the heat (and the energy used to produce the heat) is wasted, but this of course has an impact on the efficiency of the power generation.

[0004] Accordingly, it would be desirable to develop an energy transfer system which efficiently utilises all of the heat produced by a nuclear fusion reactor, in particular to integrate the excess low grade heat from the reactor into the energy cycle. SUMMARY

[0005] The invention is defined according to the independent claims. Additional features will be understood from the dependent claims and the description. Any embodiments described but not falling within the scope of the claims are to be interpreted merely as examples to better understand the invention.

[0006] The example embodiments are provided in order to address at least some of the difficulties encountered with current methods of transferring thermal energy from nuclear fusion reactors, whether these difficulties have been specifically mentioned hereinabove or will be understood from the discussion herein.

[0007] Thus, in one aspect of the application, a heat engine for power generation in a nuclear fusion power plant is provided. The heat engine comprises a primary heat exchanger connected to a primary heat source and a secondary heat exchanger connected to a secondary heat source, the heat exchangers being configured to heat a working fluid of the heat engine. The primary heat source and the secondary heat source are thermally independent, the primary heat source having a higher temperature than the secondary heat source, and suitably, the primary heat source can provide high grade heat (e.g. heat from the outer blanket assembly) while the secondary heat source can provide low grade heat (e.g. heat from the inner first wall). The heat engine further comprises a primary turbine and an auxiliary turbine configured in a fluid parallel arrangement. A parallel split point is arranged between the primary heat exchanger and the secondary heat exchanger, such that the primary turbine receives working fluid heated by the secondary heat exchanger and the primary heat exchanger (in series), while the auxiliary turbine receives working fluid heated only by the secondary heat exchanger. The heat engine further comprises a first recuperator configured to thermally connect an outflow of working fluid from the primary turbine with an inflow of working fluid into the primary heat exchanger.

[0008] Advantageously, the auxiliary turbine integrates energy from the second (low grade) heat source into the power generation cycle, thereby maximizing the recovery of heat from the power plant reactor.

[0009] In one example, the split point comprises a controllable valve configured to control the flow of working fluid from the secondary heat exchanger to the primary turbine and the auxiliary turbine. Advantageously, the flow to the primary turbine and the auxiliary turbine can be optimized based on operating parameters of the heat engine, e.g. inlet / outlet temperatures, primary coolant used, heat distribution between components; the cooling capacity (i.e. temperature) can also affect the feasibility of this cycle. In one example, a temperature sensor can be connected to the working fluid flowing into the primary heat exchanger, and the valve can be controlled to reduce the flow to the primary heat exchanger when the temperature of the fluid flowing into the primary heat exchanger exceeds a predetermined value (e.g. 500°C).

[0010] Furthermore, the provision of the auxiliary turbine on a parallel working fluid path mitigates uncertainties in operation: if the heat distribution between the reactor components or the outlet temperature (in other words, the difference between the first heat source and the second heat source) does not comply with expectations, then the present system can still integrate all reactor heat into the working fluid by manipulating the split and the specific temperature of the two turbines. Likewise, the present technology also mitigates design uncertainties of STEP reactors and similar reactors: if the heat distribution between the reactor components or the outlet temperature changes during further design of a particular reactor, then the present system can still integrate all reactor heat into the working fluid by manipulating the split of the two turbines.

[0011] In one example, the heat engine further comprises a further recuperator configured to thermally connect an outflow from the primary turbine with an inflow to the secondary turbine. In this way, enthalpy can be removed from the outflow of the primary turbine and transferred to the secondary inlet, rather than being wasted. Thus, the recuperator provides a final heating step for the working fluid flowing to the secondary turbine. Furthermore, the temperature of the low pressure CO2 stream at the recuperator outlet can also be controlled to effectively determine how much heat is added to the secondary turbine stream. In addition to controlling the flow rate of each turbine, this temperature manipulation allows for further optimisation of power generation.

[0012] In one example, the heat engine further comprises a combustor configured to heat the working fluid received by the primary turbine. This arrangement advantageously provides a supplementary heating system to maximise power output during operation (i.e. by accommodating temperature variations of the primary and secondary sources).

[0013] Suitably, in the related aspects of the application, there is provided a nuclear fusion power system comprising the aforementioned heat engine. More particularly, the nuclear fusion power system comprises a reactor comprising a primary heat source and a secondary heat source, the temperature of the secondary heat source being lower than the temperature of the primary heat source during normal operation of the reactor; a primary heat exchanger connected to the primary heat source; and a secondary heat exchanger connected to the secondary heat source (the heat exchangers being configured to heat a working fluid). The system further comprises a primary turbine and a secondary turbine configured in a fluid parallel manner, with a parallel split point arranged between the primary heat exchanger and the secondary heat exchanger, such that the primary turbine receives the working fluid heated by the (series) secondary heat exchanger and the primary heat exchanger, and the secondary turbine receives the working fluid heated only by the secondary heat exchanger.

[0014] In one example, the reactor is a tokamak type reactor, and suitably, the primary heat source comprises heat from at least one of a reactor blanket (e.g. a lithium breeding blanket) and an outer first wall, and the secondary heat source comprises heat from at least one of an inner first wall and an inner shield.

[0015] Some reactors can further comprise a tertiary heat source having a temperature lower than the temperature of the primary heat source (and lower than the temperature of the secondary heat source). Suitably, the system can further comprise a tertiary heat exchanger connected to the tertiary heat source (the heat exchanger being suitably configured to heat the working fluid), wherein the tertiary heat exchanger is arranged in series with the secondary heat exchanger to pre-heat the working fluid prior to the secondary heat exchanger.

[0016] Here, the tertiary heat source can comprise heat from a divertor assembly of a tokamak type reactor.

[0017] In related aspects of the invention, a method of operating a closed working fluid heat engine in a nuclear fusion power system is also provided. The power system includes a reactor that generates a primary heat source and a secondary heat source having a temperature lower than the temperature of the primary heat source. The method includes heating a working fluid using a secondary heat exchanger coupled to the secondary heat source; controllably distributing the flow of working fluid heated by the secondary heat exchanger between parallel fluid paths; on one parallel fluid path, additionally heating the working fluid using a primary heat exchanger coupled to the primary heat source, and operating a primary turbine using the working fluid heated by the primary heat exchanger and the secondary heat exchanger; on another parallel path, operating an auxiliary turbine using the working fluid heated by the secondary heat exchanger; and recombining the working fluid flowing out of the primary turbine and the auxiliary turbine into a common flow path coupled to the fluid flowing into the secondary heat exchanger via at least one compressor. BRIEF DESCRIPTION OF DRAWINGS

[0018] For a better understanding of the present disclosure, reference will now be made, by way of example only, to the accompanying drawings in which:

[0019] Figure 1 A schematic flow diagram of a closed Brayton cycle is shown;

[0020] Figure 2 A schematic flow diagram of an example heat engine based on a modified Brayton cycle is shown;

[0021] Figure 3 A schematic flow diagram of another example heat engine based on a modified Brayton cycle is shown;

[0022] Figure 4 A schematic flow diagram of another example heat engine based on a modified Brayton cycle is shown;

[0023] Figure 5 A schematic diagram of an example nuclear fusion power system is shown. DETAILED DESCRIPTION

[0024] At least some of the following exemplary embodiments provide a heat engine for power generation in a nuclear fusion power plant, in particular, an improved heat transfer power generation system based on a modified Brayton cycle. Other advantages and improvements are also readily apparent from the embodiments discussed herein.

[0025] Figure 1A basic schematic flow diagram of a closed Brayton cycle is shown. A working fluid (gas) is compressed by a compressor to increase the temperature and pressure of the working fluid. The high pressure working fluid is further heated by a heat exchanger (ideally isobaric heating) and the heated fluid is then used to turn a turbine to produce useful external work, such as electricity generation. The turbine is an expander, such that the working fluid is reduced in pressure on the turbine outlet side before being passed through another heat exchanger to remove any excess heat. The working fluid is then directed back to the compressor to continue the cycle.

[0026] Figure 2 A schematic flow diagram of a heat engine 100 based on a modified (improved) Brayton cycle is shown. The heat engine is specifically developed for use with a tokamak type reactor, such as the spherical tokamak energy production (STEP) project being implemented by the applicant. The heat engine utilises a working fluid in a closed cycle configuration. The working fluid is preferably a gas, more preferably supercritical carbon dioxide. Suitably, Figure 2 The working fluid network structure shown in Figure 1 can be formed by appropriate arrangement of pipes, inlets, outlets, valves and other fluid connection components familiar to those skilled in the art.

[0027] The heat engine 100 comprises a primary heat exchanger 102 and a primary turbine 104 which produces external work as a result of the (heated) working fluid flowing through the primary turbine 104. The primary heat exchanger 102 is suitably connected to a primary heat source 106 which can be considered to provide high grade temperature. In this way, the working fluid can be suitably heated by the primary heat exchanger 102 using energy from the primary heat source 106. In the case of a tokamak (in particular, STEP), the primary (high grade) heat source 106 can be heat from the blanket modules around the vacuum vessel of the reactor and / or heat from the outer first wall of the vacuum vessel. Here, the primary heat (i.e. high grade temperature) provided to the primary heat exchanger is preferably at least 500 °C (degrees Celsius), more preferably at least 600 °C.

[0028] The heat engine 100 also comprises a secondary heat exchanger 108 which is suitably connected to a secondary heat source 110. In this way, the working fluid can be suitably heated by the secondary heat exchanger 108 using energy from the secondary heat source 110. The secondary heat source 110 can be considered to provide low grade heat. In other words, the secondary heat source 100 provides a lower temperature than the primary heat source 106. In the case of a tokamak (in particular, STEP), the secondary heat source 110 can be heat from the inner first wall and / or inner radiation shield of the vacuum vessel. Here, the secondary heat (i.e. low grade temperature) provided to the secondary heat exchanger is typically substantially equal to or less than 300 °C.

[0029] At least some of the working fluid output from the secondary heat exchanger 108 is input to the primary heat exchanger 102. In this way, the secondary heat exchanger 108 pre-heats the working fluid input to the primary heat exchanger 102, thereby making the heating of the working fluid by the primary heat exchanger 102 more efficient. Suitably, the primary turbine 104 receives working fluid output from the primary heat exchanger 102 that has been heated by the secondary heat exchanger and the primary heat exchanger. In other words, the secondary heat exchanger 108, the primary heat exchanger 102 and the primary turbine 104 are connected in series flow path 112. For example, for STEP, the temperature of the working fluid output after the primary heat exchanger and therefore input to the primary turbine 104 can be above 500°C (i.e. the temperature after the heating stage). The working fluid flow 114 (which can be, for example, in the temperature range of 300 to 500°C) from the primary turbine 104 flows back to the input flow of the secondary heat exchanger 108 along a suitable (recuperation) flow circuit; components of the working fluid recuperation circuit will be discussed below.

[0030] The heat engine 100 also comprises an auxiliary turbine 116 that is arranged in parallel with the primary turbine. More specifically, a fluidic parallel split point 118 in the heat engine 100 is located between the primary heat exchanger 102 and the secondary heat exchanger 108. In this way, the auxiliary turbine 116 is also isolated from the primary heat exchanger 102 (by being arranged on a fluidic path 120 that is separate from the fluidic path 112), such that the auxiliary turbine 116 receives working fluid that has been heated by the secondary heat exchanger 108, rather than working fluid that has been heated by the primary heat exchanger 102. The outflow 122 from the auxiliary turbine merges with the outflow 114 from the primary turbine, returning to the second heat exchanger 108 along the same recuperation circuit.

[0031] The split point 118 can suitably comprise a controllable valve that is configured to control the flow of working fluid from the secondary heat exchanger 108 to the primary turbine 104 (via the primary heat exchanger 102) and the auxiliary turbine 116. In this way, the flow of working fluid along the paths 112 and 120 can be suitably controlled based on the desired operating parameters of the system.

[0032] In one example, the allocation of flow can vary depending on the temperature of the primary heat source 106 and the secondary heat source 110 (e.g. to compensate for lower primary heat source or secondary heat source temperatures).

[0033] In another example, the flow distribution between the two turbines can be based on the energy output of the turbines. That is, by varying the flow to each turbine, the flow distribution can be optimally adjusted to achieve energy production. In particular, computer simulations of the heat engine 100 can be used to calculate the flow for each path 112, 120 that will result in maximum performance (i.e. power production) of the turbines based on real-world input data of the primary and secondary heat sources (as well as other performance parameters of the heat engine 100, such as fluid temperatures, etc.).

[0034] The distribution of flow can also vary according to operating parameters of the heat engine 100 in order to, for example, ensure safe or efficient operation of the heat engine. One example parameter is the temperature of the working fluid, and suitable temperature sensors (not shown) can be positioned at various locations throughout the heat engine 100. In particular, when the temperature of the incoming working fluid exceeds a predetermined value (excessively high inlet temperatures can reduce the efficiency of the primary heat exchanger 102), a sensor arranged to measure the temperature of the fluid flowing into the primary heat exchanger 102 can be used to control the split point valve 116 to reduce the flow into the primary heat exchanger 102.

[0035] As shown, the heat engine 100 can also include one or more regenerators: a regenerator is a heat exchanger arranged to thermally connect a turbine inflow stream path with a turbine outflow stream path, which allows for the recovery of excess heat from the turbine outflow stream (i.e. removal of enthalpy from the turbine outflow stream). In this way, the inflow stream can be heated more efficiently, and heat waste can be reduced.

[0036] The first regenerator 124 can be configured to thermally connect the working fluid outflow stream 114 from the primary turbine with the working fluid inflow stream 126 into the primary heat exchanger 102. Suitably, the split point 116 between the flow paths 112, 120 is arranged before the first regenerator 124 (i.e. between it and the secondary heat exchanger 108). In other words, the first regenerator 124 is part of the flow path 112, and thus in series with the secondary heat exchanger 108 and the primary heat exchanger 102.

[0037] The second regenerator 128 can be configured to thermally connect the outflow 114 from the main turbine 104 to the inflow 130 into the auxiliary turbine 116. Suitably, the second regenerator is in series flow with the auxiliary turbine 116 (along the path 120). Such a regenerator can significantly increase the temperature of the inflow to the auxiliary turbine (e.g., up to 450°C) to a temperature higher than that achievable by the secondary heat source 110 alone providing heat to the working fluid. In other words, the second regenerator 128 is provided to maximize the amount of heat entering the auxiliary turbine 116, thereby improving its performance. The second regenerator 128 also provides a useful auxiliary heat rejection device for the outflow 114, since it is undesirable to recover heat to the inflow 126 via the first regenerator 124 in the event that the temperature of the primary heat source is too high.

[0038] The third regenerator 132 may be configured to thermally connect an outflow 134 of the working fluid from the combination of the main turbine 104 and the auxiliary turbine 116 to an inflow 135 of the secondary heat exchanger 108 .

[0039] Some reactors may include more than two low-grade heat sources, so the heat engine 100 may also include a tertiary heat exchanger 136 suitably connected to a tertiary heat source 138. The temperature of the tertiary heat source 138 is suitably lower than the temperature of the primary heat source 106, and preferably also lower than the temperature of the secondary heat source 110. In the case of a tokamak (particularly a STEP), the tertiary heat source 138 may be heat from the reactor divertor assembly.

[0040] A tertiary heat exchanger 136 is arranged in series with the second heat exchanger 108 (optimally via the third regenerator 132 ) to preheat the working fluid prior to the secondary heat exchanger 108 .

[0041] The combined outflow / outflow 134 from the two turbines has passed through one or more of the above-mentioned regenerators (not all of which need be present, although preferably at least one regenerator is used from a turbine similar to Figure 1 The heat is then fed back to the secondary heat exchanger 110 (or, if present, the tertiary heat exchanger 136) through the (main) compressor 140 to repeat the cycle. That is, the outflows from the main turbine 104 and the auxiliary turbine 116 are reconnected to a common flow path 134, which is connected to the inflow 135 of the secondary heat exchanger 108 via at least one compressor, similar to a conventional Brayton cycle.

[0042] The present disclosure also optionally allows for further modification of the Brayton cycle to incorporate an oxy-combustion direct cycle. Here, the heat engine 100 includes a burner 142 that burns natural gas and oxy-fuel. The burner 142 provides additional, non-fusion-based heating to the working fluid inflow to the main turbine 104. By providing a condenser 144, the recovery loop can be suitably adapted to accommodate the oxy-combustion element, extracting excess H2O and CO2 (introduced by combustion) via appropriate fluid paths 147, 148.

[0043] Furthermore, a fourth regenerator 150 may be arranged to thermally connect the combined turbine effluent stream 134 with the influent stream of the secondary heat exchanger 108 or the tertiary heat exchanger 136 , as appropriate.

[0044] Figure 3 Shown Figure 2 An alternative arrangement of the heat engine 100 is provided in which the system 100 includes a bypass 152 to the secondary regenerator 128. Specifically, one or both of the outflow 114 from the main turbine 104 or the inflow 130 to the auxiliary turbine 116 may be provided with a bypass 152. The bypass 152 is particularly advantageous in situations where unexpected high and low-grade heat may be encountered, thereby preventing the inflow 130 to the auxiliary turbine 116 from becoming overheated by preventing heat transfer from the main turbine outflow 114. Suitably, a temperature sensor may be connected to the secondary heat source 110, or preferably to the working fluid path 120 (i.e., the inflow to the second regenerator 128), and data from that path may be used to determine that the system would benefit from isolating the second regenerator 128, and to control the bypass accordingly.

[0045] Figure 4 Another embodiment of a heat engine 100 is shown, comprising a set of auxiliary turbines 116, here 116a and 116b. Each turbine 116a, 116b in the set of turbines 116 can preferably be configured to operate at a different optimal fluid temperature, with each auxiliary turbine 116 preferably arranged in parallel. Here, a flow splitting point 154 is provided between the first auxiliary turbine 116a and the second auxiliary turbine 116b, such that the inflow of the first auxiliary turbine 114a is thermally coupled to the main turbine outflow 116, while the second auxiliary turbine receives only flow from the secondary heat exchanger 108.

[0046] Appropriately, in Figures 2-4 In each example, the main turbine 104 and the auxiliary turbine 116 are connected to suitable generators to generate electricity to be distributed over suitable power infrastructure 156 .

[0047] Figure 5 An example nuclear fusion power system 200 (ie, for producing electricity) incorporating an example heat engine 100 is shown.

[0048] Here, the nuclear fusion power system includes a reactor 201 that includes a primary heat source 206 and a secondary heat source 210. As noted above, the primary heat source 206 can be considered a high-grade heat source, while the secondary heat source can be considered a low-grade heat source, and thus the primary heat source is at a higher temperature than the secondary heat source (at least during normal operation of the reactor).

[0049] In a preferred example, the reactor 201 is a tokamak. Suitably, the primary heat source 206 can include heat from at least one of the reactor blanket 203 and the outer first wall 205 (i.e., components facing outward from the reactor 201), while the secondary heat source includes heat from at least one of the inner first wall 207 and the inner (radiation) shield 209 (i.e., components facing inward from the reactor 201, near the central cylindrical “z” axis of the tokamak). Suitably, the reactor 201 can also include a tertiary heat source 238, which can include heat from the divertor 211.

[0050] Suitably, the respective heat sources 206, 210, 238 can be connected to respective heat exchangers (primary heat exchanger, secondary heat exchanger, tertiary heat exchanger) of the aforementioned heat engine 100, in order to transfer heat energy from the reactor 201 to the turbine of the heat engine, from which electricity is generated.

[0051] In summary, exemplary embodiments of an improved heat energy transfer system for a nuclear fusion power plant have been described.

[0052] The described exemplary embodiments enable more efficient utilization of the heat generated by a reactor in a controllable manner that is applicable to different reactors. In particular, the exemplary embodiments enable high efficiency for nuclear fusion reactors with a large proportion of low-grade heat.

[0053] The example apparatus can be industrially produced. Industrial applicability of the example embodiments will be apparent from discussion herein. Moreover, the described exemplary embodiments facilitate manufacturing and are easy to use.

[0054] While preferred embodiments of the application have been shown and described, it will be understood by those skilled in the art that modifications can be made to the application without departing from the scope of the application as defined in the claims and as described above.

[0055] Please note that all documents and literature related to the present application that are filed with or prior to the present specification are hereby incorporated by reference into the present specification, and all contents of all such documents and literature are incorporated herein by reference.

[0056] All of the features disclosed in this specification and / or all of the steps of any methods or processes described herein can be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[0057] Unless expressly stated otherwise, each feature disclosed in this specification can be replaced by an alternative feature having the same, equivalent or similar effect. Thus, unless expressly stated otherwise, each feature disclosed is merely an example of a generic series of equivalent or similar features.

[0058] The present invention is not restricted to the details of the foregoing embodiment. The present invention extends to any novel one, or any novel combination, of the features disclosed in this specification, or to any novel one, or any novel combination, of the steps of any methods or processes so disclosed.

Claims

1. A heat engine for generating electricity in a nuclear fusion power system, comprising: a primary heat exchanger connected to a primary heat source; a secondary heat exchanger connected to a secondary heat source, the secondary heat source being thermally independent of the primary heat source, and the primary heat source having a higher temperature than the secondary heat source; wherein the primary heat exchanger and the secondary heat exchanger are configured to heat a working fluid; a primary turbine and an auxiliary turbine configured in a fluidic parallel, wherein a parallel split point is arranged between the primary heat exchanger and the secondary heat exchanger, and wherein the primary turbine receives working fluid heated by both the secondary heat exchanger and the primary heat exchanger, and the auxiliary turbine receives working fluid heated only by the secondary heat exchanger; and a first recuperator configured to thermally connect working fluid exiting the primary turbine with working fluid entering the primary heat exchanger.

2. The heat engine of claim 1, wherein, The split point comprises a controllable valve configured to control the flow of working fluid from the secondary heat exchanger to the primary turbine and the auxiliary turbine.

3. The heat engine of claim 2, further comprising a temperature sensor coupled to the working fluid flowing into the primary heat exchanger, and wherein, The valve is controlled to reduce the flow into the primary heat exchanger when the temperature of the working fluid entering the primary heat exchanger exceeds a predetermined value.

4. The heat engine of claim 3, wherein, The split point is arranged between the first recuperator and the secondary heat exchanger.

5. The heat engine of any one of the preceding claims, further comprising a second recuperator configured to thermally connect fluid exiting the primary turbine with fluid entering the auxiliary turbine.

6. The heat engine of claim 5, further comprising a bypass of the second recuperator, the bypass being on at least one of the fluid exiting the primary turbine and the fluid entering the auxiliary turbine.

7. The heat engine of any one of the preceding claims, further comprising a third recuperator configured to thermally connect working fluid exiting the primary turbine and the auxiliary turbine with working fluid entering the secondary heat exchanger.

8. The heat engine of any one of the preceding claims, further comprising a tertiary heat exchanger configured to heat a working fluid, the tertiary heat exchanger being connected to a tertiary heat source having a lower temperature than the primary heat source.

9. The heat engine of claim 8, wherein, The tertiary heat exchanger is arranged in series with the secondary heat exchanger to preheat the working fluid before the secondary heat exchanger.

10. Heat engine according to claim 8 or 9, when it is also dependent on claim 7, wherein, The third recuperator is arranged between the second heat exchanger and the tertiary heat exchanger.

11. The heat engine of any one of the preceding claims, wherein, The auxiliary turbine is a first of a group of auxiliary turbines.

12. The heat engine of claim 11, wherein, Each turbine in the group of auxiliary turbines is configured to operate at a different fluid temperature.

13. The heat engine of any one of the preceding claims, further comprising a combustor configured to heat the working fluid received by the primary turbine.

14. A nuclear fusion power system, comprising: a reactor, the reactor including a primary heat source and a secondary heat source, the secondary heat source being thermally independent of the primary heat source, and the primary heat source having a higher temperature than the secondary heat source during normal operation; a primary heat exchanger connected to the primary heat source and a secondary heat exchanger connected to the secondary heat source, the primary and secondary heat exchangers configured to heat a working fluid; a primary turbine and an auxiliary turbine configured in a fluid parallel arrangement, wherein a parallel split point is disposed between the primary and secondary heat exchangers, and wherein the primary turbine receives working fluid heated by both the secondary heat exchanger and the primary heat exchanger, and the auxiliary turbine receives working fluid heated only by the secondary heat exchanger; and a first recuperator configured to thermally connect working fluid exiting the primary turbine with working fluid entering the primary heat exchanger.

15. The nuclear fusion power system of claim 14, wherein, the primary heat source includes heat from at least one of a reactor blanket and an outer first wall.

16. The nuclear fusion power system of claim 14 or 15, wherein, the secondary heat source includes heat from at least one of an inner first wall and an inner shield.

17. The nuclear fusion power system of any one of claims 14-16, wherein, the reactor further includes a tertiary heat source having a lower temperature than the primary heat source, and the system further includes a tertiary heat exchanger connected to the tertiary heat source, the tertiary heat exchanger configured to heat a working fluid, and wherein the tertiary heat exchanger is disposed in series with the secondary heat exchanger to preheat working fluid prior to the secondary heat exchanger.

18. The nuclear fusion power system of claim 17, wherein, the tertiary heat source includes heat from a divertor.

19. A method of operating a closed working fluid heat engine in a nuclear fusion power system, the power system including a reactor producing a primary heat source and a secondary heat source, the secondary heat source being thermally independent of the primary heat source, the primary heat source having a higher temperature than the secondary heat source, the method comprising: heating a working fluid using a secondary heat exchanger connected to the secondary heat source; controllably distributing working fluid heated by the secondary heat exchanger between parallel fluid paths; on one parallel fluid path, additionally heating working fluid using a first recuperator connected to working fluid exiting a primary turbine and a primary heat exchanger connected to the primary heat source, and operating the primary turbine using working fluid heated by the primary heat exchanger, the secondary heat exchanger, and the first recuperator; on another parallel path, operating an auxiliary turbine using working fluid heated by the secondary heat exchanger; recombining working fluid exiting the primary and auxiliary turbines into a common flow path, the common flow path being connected to fluid entering the secondary heat exchanger via at least one compressor.

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