Power cycle system

Abstract

A power cycle system for an intermittent heat source such as a fusion reactor comprises a primary heating means 108 thermally coupled to the reactor 102, a primary compressor 106 for driving working f

Description

Field of the Invention

[01] The present disclosure relates generally to power cycle systems for generating electricity in a power plant, and more specifically to a power cycle system for use with a fusion reactor; in particular, a tokamak type reactor / power plant. Yet more specifically, the present disclosure relates to a power cycle system based on a modified Brayton or Rankine cycle. Background

[02] One of the remaining challenges facing nuclear fusion technology is that of converting the energy generated by a fusion reactor into useable electricity. In the context of tokamak type reactors particularly, a closed loop heat engine operating according to a Brayton or Rankine cycle - i.e., a closed cycle gas turbine operating with supercritical, subcritical or transcritical fluid - has been postulated as a potentially advantageous electrical generator.

[03] However, existing power cycle technologies for fusion reactors face a problem of how to handle reactor uncertainty in heat split (quantity and quality), intermittency (pulsed operation), start-up I shut down, trip and dynamics (rapid load ramp rate), and so on.

[04] Current solutions to at least some of these problems utilise a thermal storage whereby heat is stored in a suitable salt composition. However, the thermal storage loses heat overtime and can be inefficient to heat and maintain, particularly as currently available high temperature salt compositions can be highly corrosive. Also, the thermal response to deliver heat to the power cycle is very slow, and so while it can be useful in a startup phase, it is unsuitable for dealing with operational uncertainty or periods of low activity such as dwell phases.

[05] Another approach utilises an external steam Rankine or Brayton engine to smooth power performance. However, thermal response time is limited by the thermal inertia of the associated steam boiler, again making such a system unsuitable for responding to short term operational uncertainty.

[06] Hence it is desirable to develop a power cycle system which may more readily adapt to reactor conditions. Summary

[07] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[08] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current approaches to power cycle systems for in a fusion reactor, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein.

[09] In particular, the example embodiments provide a closed loop power cycle (such as a supercritical, subcritical or transcritical Brayton or Rankine cycle) configured to run a high temperature compressor to generate high grade heat during a period of dwell, maintenance periods, or when the reactor is off or about to start, instead of using auxiliary heat from a thermal storage. Here a dwell period may be an idle time between reactor pulses (if operating in a pulsed fashion) or otherwise idle time when the reactor is not in full operation but also not being shut down. During dwell periods heat may still be added to the system from various heat sources of the reactor (circa 50-100 MWth, although in some cases up to 500 MWth), as well as ancillary systems such as heat generated in primary circulators. There may also be available decay heat which may be utilised as well (circa 1 to 200 MWth). Upgrading the heat by the compressor (or indeed generating ‘new’ heat by the compressor) allows the power cycle system to be maintained in an idle mode - i.e., keeping the system ready to be switched to a full power mode when the reactor is back in normal operation. The present invention allows for a highly power dense solution with rapid ramp ability - while also being compact. In some embodiments, only one piece of equipment is required in addition to previously accepted fusion mode configurations - and minimum changes to the configuration are required.

[10] Accordingly, in one aspect of the invention there is provided a power cycle system for an intermittent heat source. The power cycle comprises a primary heating means thermally coupled to the intermittent heat source, a primary compressor for driving working fluid around the power cycle system, a main turbine, and a secondary compressor. The power cycle comprises a first mode of operation, corresponding to a normal operating time of the intermittent heat source, in which the main turbine receives working fluid output by the primary heating means. The power cycle also comprises a second mode of operation, for example corresponding to a dwell time of the intermittent heat source, in which the main turbine receives working fluid output by the secondary compressor, and working fluid output from the main turbine is divided between a return path to the primary compressor and an input path to the secondary compressor. In the first mode, the secondary compressor may be operated in an idle state or ‘off’ state. In the second mode, the secondary compressor may be operated in a powered or ‘on’ state.

[11] In an example, the secondary compressor may be arranged in parallel flow with the primary compressor (when the system is in the second mode). Relatedly, an input path to the secondary compressor may comprise secondary heating means thermally coupled to the intermittent heat source; in this way heat may be added to the system which can then be upgraded by the secondary compressor. Here, working fluid output from the secondary compressor may be combined with working fluid output from the primary heat exchanger, prior to being input to the main turbine.

[12] In another example, the secondary compressor may be arranged in series flow with the main turbine (when the system is in the second mode). In a related example, the power system may further comprise an auxiliary turbine configured in flow parallel with the main turbine. The auxiliary turbine input may be thermally coupled to the main turbine output to efficiently recover excess heat from the main turbine output. Furthermore, the working fluid output from the auxiliary turbine may be divided between a return path to the primary compressor and the input path to the secondary compressor. Suitably, by being a series flow, the secondary compressor may be arranged so that it receives working fluid output by the primary heat exchanger.

[13] In an example, the primary heating means may comprise a set of heat exchangers corresponding to a set of heat sources comprised by the intermittent heat source. Relatedly, the set of heat sources may comprise a low temperature heat source, a medium temperature heat source, and a high temperature heat source. In some examples there may also be other heat sources that may be coupled to the primary heating means.

[14] In an example, the working fluid may comprise super critical carbon dioxide. Other examples of working fluids include steam (water), helium, nitrogen and air; preferably in their supercritical states.

[15] In an example, the primary turbine and secondary compressor may be coupled by a common drive shaft.

[16] In a related aspect of the present invention, there may be provided another power system for an intermittent heat source. The system comprises a primary heating means thermally coupled to the intermittent heat source and configured to heat a first working fluid, a secondary heating means thermally coupled to the intermittent heat source and configured to heat a second working fluid, a thermal coupling between the first working fluid and the second working fluid, a primary compressor for driving the first working fluid around the power cycle system, a main turbine, and a secondary compressor. The secondary compressor receives second working fluid output by the secondary heating means, and outputs working fluid to the thermal coupling, and the main turbine receives first working fluid output by the primary heating means and output by the thermal coupling. In other words, the secondary compressor is provided on a separate fluid loop but may still be used to provide heated fluid for the main turbine via the thermal coupling between the two loops.

[17] As above the first working fluid may comprise super critical carbon dioxide. The second working fluid may be the same or different. Different working fluids include steam (water), helium, nitrogen and air; preferably in their supercritical states.

[18] In another aspect of the present invention there is provided a power cycle system for an intermittent heat source comprising a primary heating means thermally coupled to the intermittent heat source, a primary compressor for driving working fluid around the power cycle system, a main turbine, a secondary compressor, and a thermal storage. In a first mode of operation, the main turbine receives working fluid output by the primary heating means, and in a second mode of operation, working fluid output from the main turbine is divided between a return path to the primary compressor and an input path to the secondary compressor, while working fluid output from the secondary compressor is divided between an input path to the main turbine and a path for heating the thermal storage.

[19] In an example, the power cycle may comprise an auxiliary turbine configured in flow parallel with the main turbine. Relatedly, an input to the auxiliary turbine may comprise working fluid heated by the thermal storage.

[20] Other optional features discussed above may also be applied to this aspect of the invention.

[21] Relatedly, in some examples, instead of heated working fluid being diverted to heat the thermal storage, the heat generated may be instead diverted for use in other purposes such as process heat application (eg hydrogen generation) rather than storage.

[22] In a related aspect of the present invention there is provided a power cycle system for an intermittent heat source comprising a primary heating means thermally coupled to the intermittent heat source, a primary compressor for driving working fluid around the power cycle system, a main turbine, a secondary compressor, and a thermal storage. In a first mode of operation, the main turbine receives working fluid output by the primary heating means, and in a second mode of operation, working fluid output from the main turbine is divided between a return path to the primary compressor and an input path to the secondary compressor, and working fluid input to the main turbine comprises a combination working fluid output by the secondary compressor and working fluid heated by the thermal storage.

[23] Other optional features discussed above may also be applied to this aspect of the invention.

[24] In yet another aspect of the present invention, there is provided a fusion power plant comprising a fusion reactor and the aforementioned power cycle system. The power cycle system may be operated in the first mode during a power generation phase of the fusion reactor, and operated in the second mode during a one of a dwell phase, a maintenance period, a reactor off phase, ora start up phase, of the fusion reactor.

[25] In another aspect of present invention, there is provided a solar thermal power plant (i.e., a power plant for thermal based solar energy production) comprising the aforementioned power cycle system. The power cycle system may be operated in a first mode when the sun is providing suitable heating for power generation, and operated in the second mode when the sun is not able to provide heating for power generation (e.g., during the night).

[26] In another aspect of the present invention, there is provided a nuclear fission power plant comprising the aforementioned power cycle system. Suitably, the power cycle system may be operated in the first mode during a power generation phase of the fission reactor (i.e., during peak demand), and operated in the second mode during an idle, or low power output phase, of the fission reactor (i.e., during low demand). Such operation is not typical for a fission reactor, which are usually operated continuously as a base load power plant. The present invention however may allows the fission plant to be utilised instead as a load following power plant.

[27] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word “about”, even if the term does not expressly appear. The term “about”, or substantially, when used herein means +1- 5% of the stated value. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein, and the terms “from” and “to” a pair of values are intended to indicate such values are included in the range. Singular encompasses plural and vice versa. Additionally, although the present invention has been described in terms of “comprising”, the processes, materials, and coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’. Brief Description of the Drawings

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

[29] Fig. 1 shows a schematic of an example power cycle system for an intermittent heat source;

[30] Fig. 2 shows an example of the power cycle of Fig. 1 applied to a fusion reactor;

[31] Fig. 3 shows a schematic of another example power cycle system for an intermittent heat source;

[32] Fig. 4 shows an example of the power cycle of Fig. 3 applied to a fusion reactor;

[33] Fig. 5 shows a schematic of yet another example power cycle system for an intermittent heat source;

[34] Fig. 6 shows an example of the power cycle of Fig. 5 applied to fusion reactor;

[35] Fig. 7 shows a schematic of yet another example power cycle system for an intermittent heat source;

[36] Fig. 8 shows an example of the power cycle of Fig. 7 applied to fusion reactor;

[37] Fig. 9 shows an example power cycle incorporating thermal storage;

[38] Fig. 10 shows the example of Fig. 9 applied to a fusion reactor;

[39] Fig. 11 shows another example power cycle related to the example of Fig. 10.

[40] In each of the Figures, the following key is used: ■ continuous lines indicate working fluid paths which correspond to all modes of operation of the power cycle, including a main turbine path; ■ dash lines indicate working fluid paths corresponding to a compression mode of the power cycle; ■ dash-dot lines indicate working fluid paths corresponding to a reactor mode of the power cycle; ■ dash-dot-dot lines indicate energy flow (usually from the heat source to the power cycle). Detailed Description

[41] At least some of the following example embodiments provide a power cycle system for use in generating electricity from energy provided by an intermittent heat source, such as a fusion reactor - in particular, improved power cycle systems based on a modified Brayton cycle. Other advantages and improvements may also be apparent from the discussed embodiments herein.

[42] Herein, an intermittent heat source may be a heat source that does not provide a constant and reliable energy output, either due to planned dwell periods, reactor variability, maintenance periods, or other operational uncertainties. Examples of intermittent heat sources include fusion reactors, thermal solar arrays / power plants, fission reactors (particularly as a load-following plant), to name but a few. It will of course be appreciated that the list is non-exhaustive and the disclosure is not limited thereto. In some examples the intermittent heat source may be taken to comprise a plurality, or set, of heat sources corresponding to different grades of heat output by the intermittent heat source.

[43] In some examples herein the intermittent heat source is assumed to be a tokamak type (fusion) reactor, for example the Spherical Tokamak for Energy Production ‘STEP’ project. Tokamak type reactors comprise a variety of heat sources which provide different grades of temperature corresponding to different components of the reactor.

[44] In the case of a tokamak type reactor, a primary heat source supplying a primary heat, which may be regarded as high-grade temperature, may be heat from a blanket module surrounding the vacuum vessel of the reactor, and / or from an outboard first wall of the vacuum vessel. Here the primary heat may be at least 500°C (degrees centigrade), and possibly at least 600°C. Heat from a primary heat source may also be termed a high temperature heat herein.

[45] Heat from a secondary heat source supplying a secondary heat (low grade), may be heat from an inboard first wall of the vacuum vessel and / or inboard radiation shield. Here the secondary heat may be about 300°C or lower, and in general will be always lowerthan the primary heat. Heat from a secondary heat source may also be termed a medium heat source herein.

[46] A third heat source supplying tertiary heat (low grade), which may be heat from heat from a divertor component of the reactor, and maybe lower than the second heat. Here the tertiary heat may be above 150°C, and in general will be always lower than the primary and secondary heats. Heat from a tertiary heat source may also be termed a low temperature heat source herein.

[47] Heat lower than 150°C may be regarded as waste heat from parts of the plant such as heating and current drive, (potentially from) cryogenic plant etc. In some examples such heat may also be utilised as a fourth (or further) heat source. In the following, a fourth source may correspond to a decay heat - that is, heat resulting from radioactive decay of activated structural material of the reactor.

[48] It will however be appreciated that the present techniques may be applied to any suitable intermittent heat source and are not necessarily limited to fusion reactors, nor specifically tokamak reactors. The present disclosure may be readily applied to any fusion reactor where startup phases, shutdown phases, dwell phases, reactor intermittence, and so on, can be expected, and any reactor where heat grade and / or temperature output levels vary.

[49] In the present embodiments, all of the described power cycles are based on a closed Brayton (or Rankine) cycle. As a brief introduction, in such cycles a working fluid (liquid or gas) is compressed by a compressor to increase the temperature and pressure of the working fluid. The high-pressure working fluid is heated (ideally isobarically) by a heat exchanger, and then the heated fluid is used to turn a turbine to produce useful external work - e.g., generating electricity. The turbine is an expander such that the working fluid reduces temperature and pressure on the outlet side of the turbine before being passed through a cooler to remove any excess heat. The working fluid is then directed back into the compressor for the cycle to continue.

[50] In the present examples the working fluid is preferably a gas, for example di-nitrogen, argon, helium, air, water, or carbon dioxide; preferably in their supercritical states. In particular, the example embodiments have been developed with supercritical carbon dioxide in mind as the working fluid. In some examples the working fluid may be a binary or tertiary mixture of (supercritical) carbon dioxide with other organic based working fluids such as Hydrofluorocarbons (e.g., CeFe, C4F8), C2H3N, TiCL4, NO2, SO2, SiCk, WCb, WFe, UFe; such mixtures facilitate condensing of the working fluid even at higher ambient temperature (~50°C), thereby reducing the pumping power. In other examples the working fluid may be purely organic, such as iso butane (R600a). iso pentane (R601a), toluene, and so on, as will be appreciated by those familiar with organic rankine cycles.

[51] The working fluid may be suitably transported by various fluidically coupling means including a network of piping, inlets, outlets, valves, and other suitable components as would be familiar to those in the art. Hybrid power cycle

[52] Figure 1 shows an example power cycle 100 modified for use with an intermittent heat source 102. As mentioned above, the power cycle 100 is based on a modified Brayton (or Rankine) cycle. Suitably the power cycle 100 comprises a main turbine 104 and a primary compressor 106 (shown here as part of a pump apparatus) which drive the working fluid around the power cycle 100.

[53] The power cycle 100 also comprises heating means 108, such as a heat exchanger, for receiving heat from the intermittent heat source 102 and heating the working fluid, and cooling means 110. Here (and in other examples), the heating means 108 may be considered a means of indirectly heating the working fluid. In the present example, the heat exchanger 108 may be considered as a primary heat exchanger, or primary heat exchanging means.

[54] Suitably, working fluid which has been heated by the intermittent heat source 102 via heating means 108 is used to drive the main turbine 104. Put another way, the main turbine 104 is arranged, or disposed, in the system 100 to receive working fluid heated by the intermittent heat source 102, as shown by flow path 112. The main turbine 104 may be suitably coupled to an electrical generator (not shown) to produce electricity.

[55] The power cycle system 100 also comprises a secondary compressor 114, here shown arranged in parallel flow with the fluid loop comprising the main turbine 104 and primary compressor 106. That is, there is a flow path which includes the main turbine 104 and primary compressor 106, and a flow path that includes the main turbine 104 and the secondary compressor 114.

[56] Suitably, the secondary compressor 114 is arranged to receive working fluid controllably split off from the output line 116 from the main turbine 104 at a junction 118. Put another way, working fluid output from the main turbine 104 is divided between a return path 120 heading towards the primary compressor 106, and an input path 122 for the secondary compressor 114. The secondary compressor 114 is configured to heat the received working fluid (i.e., adding heat into the system), thereby ‘upgrading’ the heat.

[57] Working fluid output from the secondary compressor 114 - i.e., path 124 - is configured to rejoin the main fluid loop of the power cycle; i.e., to combine with input path 112 to the main turbine 104. In other words, the heated working fluid output from the secondary compressor 114 is input to the main turbine. In this way the upgraded heat from the secondary compressor 114 can be used to run the main turbine 104. This is particularly beneficial when components need to be kept in a ready state, or idle mode, during a corresponding idle state of the intermittent heat source 102.

[58] Suitably, the system 100 may be configured to provide control over the amount of working fluid divided between the return path 120 and the secondary compressor input path 122. Thus, the system 100 may be operated in different modes depending on the operating condition of the intermittent heat source (e.g., reactor conditions). Flow control may be effected by suitable means such as valves 126 arranged on one or both of the paths 120,122 (here shown only on secondary compressor input 122).

[59] Here a first mode (also termed reactor mode) may correspond to a normal operating state, or time, of the intermittent heat source 102. That is, a time during which the intermittent heat source 102 is active in producing heat for energy production. In the first mode, the secondary compressor 114 may be operated in an idle state, or ‘off’state, in which the secondary compressor 114 is operated with low power draw. That is, in the first mode, the secondary compressor 114 may be substantially inactive, so that it is not upgrading heat for the input 112 to the main turbine 104. Put another way, in the first mode, the main turbine 104 may be operate using heat only provided by the intermittent heat source. Suitably, in the first mode a flow of working fluid along input path 122 to the secondary compressor 114 may be minimised or completely stopped. That is, it will be appreciated that there may be some times in which it is desirable for some flow to be maintained in order to preserve pressure dynamics of the system 100 as a whole and to maintain a ‘hot condition’ of the secondary compressor 114, and there may be times in which it is desirable to completely isolate the secondary compressor 114.

[60] A second mode (also termed compression mode) may correspond to a period of inactivity of the intermittent heat source 102 (such as a dwell period between reactor pulses). That is, a time during which the intermittent heat source 102 is not active in producing heat for energy generation, or is in a low heat state, idle state, etc. Suitably, in the second mode the secondary compressor 114 may be operated in a powered or ‘on’ state, so that it is active in upgrading heat for the system 100 so as to maintain an idle state of the main turbine.

[61] Optionally, the system 100 may also comprise a secondary heat exchanger, or heating means, 128 configured to receive heat from the intermittent heat source 102. In particular, the secondary heat exchanger 128 may be arranged to receive low grade heat produced by the intermittent heat source 102 when it is in an idle (or low power) state (and the system 100 is operated in the second mode). For example, the heat received by heat exchanger 128 may be a decay heat from components activated by operation of the intermittent heat source. Suitably this additional heat may be efficiently utilised in maintaining operation of the system even during downtime of the intermittent heat source.

[62] Figure 2 shows the example power cycle 100 of Fig. 1 being utilised with a nuclear fusion reactor 102 (i.e., an intermittent heat source). Reference numerals are shared with Fig. 1 to show like components and so obviate a need for detailed repeat description thereof.

[63] Here, the primary heating means 108 comprises a set of heat exchangers configured to heat working fluid at various points in the power cycle 100. A first heat exchanger 108a in the set is thermally coupled to a first heat 102H of the reactor 102 (i.e., a high temperature, high grade heat), a second heat exchanger 108b in the set is thermally coupled to a second heat 102M of the reactor 102 (i.e., mid temperature, low grade heat), and a third heat exchanger 108c in the set is thermally coupled to a third heat 102L of the reactor (i.e., low temperature, low grade heat).

[64] Each of the heat exchangers 108a-c may themselves be part of a subset of heat exchangers. In this example, the second heat exchange means 108b thermally coupled to the second heat 102M comprises a pair of such heat exchangers 108b-1, 108b-2. Likewise, the third heat exchange means 108c comprises two heat exchangers 108c-1,108c-2.

[65] The thermal coupling between the reactor and set of heat exchangers 118 may be direct or indirect. For example, as shown the reactor 12 may heat a set of heating loops 130a-c corresponding to each member of the set of heat exchangers 108a-c. Each indirect heating loop 130 may utilise the same working fluid as the rest of the power cycle system 100 (e.g., super critical carbon dioxide) or may utilise a different working fluid. The working fluid may also be different between different heating loops 130, and may be in different phases (e.g., liquid vs gas).

[66] The power cycle 100 comprises a common flow path (solid lines) through which working fluid flows during all modes of operation of the power cycle 100. Suitably, the common flow path includes the main turbine 104, cooling means 110 (in the form of indirect cooling comprising a loop with e.g., cooling towers 132, and the pump / primary compressor 106.

[67] The common flow loop may also include an auxiliary turbine 134 which is configured in flow parallel with the main turbine. That is, the auxiliary turbine 134 is not in a direct flow path with the first heat exchanger 108a, so that the auxiliary turbine 134 does not receive working fluid heated by the first heat exchanger 108a. Suitably the auxiliary turbine 134 may form part of the same electrical generator as the main turbine 104 (and may be e.g., on the same shaft) or may be coupled to a separate electrical generator.

[68] The common flow loop may also include one or more recuperators; recuperators are heat exchangers arranged to thermally couple turbine inflow paths with turbine outflow paths, which allows excess heat to be recovered from the turbine outflows (i.e., to remove enthalpy from a turbine exit stream). In this way the inflow heating may be made more efficient, and less heat wasted. In particular, a first recuperator 136, also termed a high temperature recuperator, may be configured to thermally couple working fluid outflow with working fluid inflow to the first heat exchanger 108a.

[69] The power cycle 100 also comprises a reactor mode flow path (dash-dot lines); that is, a working fluid loop corresponding to the first mode / configuration. In this mode a set of first valves (e.g., valve 138) coupled to the common flow path are controlled to be open (e.g., by an operator or automatic control unit). For example, valves 138 may control fluid flow to / from the first heat exchanger 108a. Other valves may control fluid flow to / from other components, such as to the heat exchangers 108b,c of the other indirect heating loops 130. Put another way, the first set of valves may be used to define a first controllable flow path for the working fluid. In this first configuration, a corresponding set of second valves coupled to the common flow path are controlled to be closed (or in practice, to allow for a predetermined minimum flow rate). This second set of valves typically provide a bypass path for the set of heat exchangers 108 - for example valve 140 - and also include the valve 126 which controls fluid flow to the second compressor 114. Suitably, in the first / reactor mode, the working fluid which is used to operate the main turbine 104 has been heated by a combination of each of the heat exchangers in the set of primary heat exchangers 108a-c.

[70] Conversely, the power cycle 100 also comprises a compression mode of operation (dashed lines); that is, a working fluid loop corresponding to the aforementioned second mode / configuration. In this mode, the second set of valves are controlled to be opened and the secondary compressor 114 controlled to be powered. The first set of valves are controlled to be substantially closed (including allowing for a predetermined minimum flow rate, or ‘trickle’ rate, which avoids the heat exchangers being completely isolated and so potentially cooling). Put another way, the second set of valves may be used to define a second controllable flow path for the working fluid.

[71] Suitably, in the second mode, as per Fig. 1, some working fluid output from the main turbine 104 is fed as input to the secondary compressor 114 so as to heat the working fluid for operating the turbine (despite heat not being provided to the system 100 from the reactor 102). In this example, a fourth heat exchanger 128 (corresponding to the second heat exchanger in Fig. 1) is coupled to the reactor 102 to receive decay heat 102D.

[72] Optionally, there may also be provided an electric heater (not shown) for further upgrading the heat of the working fluid when in the second mode. In some arrangements, the electric heater may be arranged on the input to the main turbine 104, or otherwise somewhere on the input path 112 to the main turbine 104. In other arrangements, the electric heater may be arranged on the input to the secondary compressor 114.

[73] In some examples, there may also be provided a heat sink (not shown) that removes some high temperature heat prior to the inlet to the main turbine 104. That is, on the input path 112 after the working fluid output 124 from the secondary compressor 114 is recombined with the bulk flow. Heat absorbed by the heat sink may be used for process heating applications or diverted to a thermal storage 154 (see also Figs 9 &10). In this way, the system 100 may be essentially utilised as a heat pump that uses the existing power cycle CO2 turbine, rather than having a dedicated turbine which is the go-to option today for pumped thermal energy storage application. In particular, in this example the heat exchanger 128 may be coupled to a low temperature heat source, and then the heat pump upgrades the low temperature heat source by compression using the secondary compressor 114 to deliver a high temperature heat and expands the high-pressure cooled fluid via the main turbine 104. Once started, this closed-loop can run in steady state without needing to run the other components of CO2 cycle. In this way, there is provided an intensified process integrating heat pump and power cycle by using the same turbine 104. Thus this example is particularly beneficial in non-fusion contexts, where there is an intermittent process heat (low grade heat source) and a requirement to produce electricity. So, the low temperature intermittent process heat is upgraded in one mode and stored in thermal storage (taking cheap electricity from the grid at during high renewable energy penetration to the grid)- charging process, and the stored thermal energy is used to produce electricity when the electricity market price is high - discharging process.

[74] In some configurations, the main turbine 104 and secondary compressor 114 may be provided on the same shaft (using appropriate gearing / clutches to accommodate the compressor 114 being in a low power state in the first system mode). Similarly, the main turbine 104, secondary compressor 114, and auxiliary turbine 134 may each be provided on the same shaft. In other configurations the main turbine 104 and secondary compressor 114 may be hydraulically coupled; optionally they may also be coupled to the auxiliary turbine 134. In some examples the compressor may be driven by an independent motor or set of motors.

[75] It will of course be appreciated that it will require energy input to the system in order to run the secondary compressor 114 to add heat to maintain operation of the main turbine. In one example, assuming minimal decay heat (and adding heat by the secondary compressor 114 only) an energy requirement of the secondary compressor 114 may be about 155 MW, while other components of the system (e.g., pump) may equate to about 7 MW. An output power achievable from the main turbine when running in the second system mode may be about 133 MW. Suitably, the present example may require an energy input of about 30 MW. In another example, the secondary compressor 114 input power may be in the range 250MWto 500MW, whils the main turbine output may be about 250MW. In yet other examples, particularly where the compressor and turbine share a shaft, the compressor duty may be less than 250 MW.

[76] Unless otherwise stated, components described above with reference to Figures 1 and 2 may be taken to generally serve the same or similar function, being configured in the same or similar way, as in other embodiments described herein.

[77] Figure 3 shows another example power cycle 100 for use with an intermittent heat source 102. Suitably, reference numerals are shared with preceding figures to indicate like components and reduce a need for detailed repeat description thereof.

[78] In this example, the secondary compressor 114 is provided on a supplementary heating loop 140. That is, the input 122 to the secondary compressor 114 does not receive working fluid output from the main turbine 104. Rather, the loop 140 is self contained. The supplementary heating loop 140 may utilise the same sort of working fluid as the remainder of the power cycle, or may utilise a different working fluid. In any case, the supplementary heating loop 140 may be considered to comprise a second working fluid, while the working fluid used to drive the main turbine may be considered a first working fluid.

[79] In this example, primary heat from the intermittent heat source 102 is provided to the first working fluid by the primary heat exchanger 108, as before. Similarly, low grade heat (such as decay heat) from the intermittent heat source 102 is used to heat working fluid on the input path 122 to the secondary compressor 114 via secondary heat exchanger 128, however here the working fluid is the second working fluid.

[80] Suitably, the system 100 also comprises a thermal coupling 142, such as a third heat exchanger, by which heat output from the compressor 114 (path 124) is transferred to the input line 112 to the main turbine 104. Put another way, the heated second working fluid output from the secondary compressor 114 is thermally coupled to the first working fluid which is input to the main turbine. Otherwise, the advantages of this embodiment are the same as previous: an ability to maintain operation of the main turbine during e.g., dwell periods.

[81] Figure 4 shows the example power cycle 100 of Fig. 3 being utilised with a nuclear fusion reactor 102 (i.e., an intermittent heat source). Reference numerals are shared with preceding figures to indicate like components and reduce a need for detailed repeat description thereof.

[82] In this example, the workings of the common flow loop and first mode flow loop are substantially the same as described with reference to Fig. 2. Likewise, the workings of the second mode flow loop are similar, with some alterations to account for the supplementary heating loop 140. Mainly, instead of the valve 126 controlling a split flow from the main turbine output, the valve 126 controls a flow rate through the supplementary heating loop 140. Thus, in the first mode, minimal (or no) additional heat is provided to the first working fluid by the third heat exchanger 142, whereas in the second mode, heat is provided to the input path 112 to the main turbine 104 by the third heat exchanger 142.

[83] Figure 5 shows another example power cycle 100 for use with an intermittent heat source 102. Once again reference numerals are shared with preceding figures to indicate like components and reduce a need for detailed repeat description thereof.

[84] In this example the secondary compressor 114 is arranged in series with the main turbine 104. In the first mode, the secondary compressor 114 is bypassed (path 144), and in the second mode the bypass 144 is shutoff (or substantially reduced to a predetermined minimum) to enforce the working fluid flow through the secondary compressor 114.

[85] Furthermore, this example includes the auxiliary turbine 134, which as before is arranged on a parallel flow loop to the loop comprising the main turbine 104. Suitably, the primary heating means 108 in this example comprises a set of heat exchangers comprising at least two heat exchangers 108a,b: a first heat exchanger 108a and a second heat exchanger 108b. The second heat exchanger 108b providing heat for operating the auxiliary turbine 134, and providing heat to be combined with heat from the first heat exchanger 108a for operating the main turbine 104. Moreover, working fluid output by the auxiliary turbine 134 may be controllably split between a return path 146 to the primary compressor 106, and a path 148 which combines with the input flow 122 to the secondary compressor 114; that is, the path 148 recombines with the bulk (common loop) flow before the secondary compressor 114, when the system is operating in the second mode).

[86] As in Figs 1 &2, working fluid output by the main turbine 104 is controllably split between a return path 120 to the primary compressor 106 and the input path 122 to the secondary compressor 114. Here, however, the split from the main turbine towards the secondary compressor 114 recombines with the bulk (common loop) flow before the secondary compressor 114 rather than after (to achieve the series flow). Further in contrast to previous examples, here the input path 122 to the secondary compressor 114 which issplit off from the main turbine outflow comprises the primary heating means 108 (specifically, the first heating means 108a). In this example, decay heat which was previously added to the input path 120 by the secondary heating means 128, is instead coupled to the working fluid by the primary heating means 108. Put another way, the input path 122 to the secondary compressor comprises at least one of the primary heating means 108 arranged to receive heat from the intermittent heat source.

[87] Figure 6 shows the example power cycle 100 of Fig. 5 being utilised with a nuclear fusion reactor 102 (i.e., an intermittent heat source). Reference numerals are shared with preceding figures to indicate like components and reduce a need for detailed repeat description thereof.

[88] In this example, the thermal coupling between the low temperature heat source 102L and the third heat exchange means 108c has been simplified to a single indirect heating loop 130c. As in Fig. 5, decay heat 120D may heat the working fluid via the same heating means as for the high grade heat 120H.

[89] Also, each of the common, reactor (first) mode, and compression (second) mode flow paths have been reworked, particularly with respect to the auxiliary turbine 134. That is, while the auxiliary turbine 134 is still configured in flow parallel with the main turbine 104, the flow paths now allow for an input path 150 to the auxiliary turbine 134 to recover heat from the main turbine outflow 116 via the recuperator 136. Put another way, when the system is in the second mode, some outflow from the main turbine is looped back on the input path 122 to the secondary compressor 114, while remaining working fluid on the return path 120 to the primary compressor 106 may be thermally coupled to the input path 150 to the auxiliary turbine 134. When the system is in the first mode, the recuperator 136 serves to recover some heat from the return path 120 back into the input 112 to the main turbine 104. As in previous examples, the main turbine 104, secondary compressor 114, and optionally auxiliary turbine 134, may all be arranged on a common shaft.

[90] Thermal coupling to the auxiliary turbine 134 in this way allows for heat remaining in the main turbine 104 outlet stream 116, 120 to be utilised efficiently. The configuration has two main advantages vs previous examples: a higher temperature can be achieved on the inlet of the main turbine 104 - which is more in line with what it would be in operations, while the auxiliary turbine 134 may be more efficiently utilised to increase power generation. A downside however is added complexity, and that switching between modes by rearranging the flow path is slower, although it is otherwise notable that the present example and that of Figs 1 &2 perform virtually the same in terms of thermodynamic efficiencies.

[91] In this example, assuming a common shaft, shaft power out from the main and auxiliary turbines 104, 134 may be about 261 MW, while the power into the secondary compressor 114 and other components of the system may be about 360 MW, such that an input power required to run the system 100 in idle mode may be about 100 MW. It will be appreciated that other example powers in / out may also apply, depending on the exact turbine and compressor arrangement.

[92] Figure 7 shows a further modification of the example power system according to Fig. 5. Here working fluid output from the secondary compressor 114 is split between the path 124 which is then input to the main turbine 104, and a path 152 which combines with an input to the auxiliary turbine.

[93] Figure 8 shows the same modification as in Fig. 7 as applied to the system 100 shown in Fig. 6. Pumped Thermal Energy Storage

[94] The following now focusses on variations of the above techniques for achieving a pumped thermal storage solution for an intermittent heat source.

[95] In the following, low grade heat added at the inlet of the secondary compressor is upgraded by the secondary compressor and the upgraded heat is removed prior to the expansion in the turbine (essentially a heat pump that is using the same power cycle turbine, instead of a dedicated heat pump expansion valve / expansion turbine).

[96] This philosophy can be realised with different fluid connections to / from different thermal storage configuration e.g., single tank thermal storage (sensible heat, or phase change materials-PCM) or two tank thermal storage (molten salt).

[97] The concept itself is agnostic to the type of thermal storage technology and the fluid connections between the system and the thermal storage system, as they would slightly be different depending on the type of thermal storage system.

[98] For example, hot CO2 may be sent to the storage system when the storage medium is solid (e.g., PCM or solid sensible heat storage medium) and the cold return CO2 returns to the cycle at the either downstream the main turbine, with an expansion valve, or sent to auxiliary turbine (see below). During discharge operation, the cold CO2 downstream of the primary heat exchanger is sent to the storage system and the hot CO2 connects to the turbine inlet.

[99] For two tank molten salt storage systems, the liquid molten salt would be pumped, from the cold tank, to receive the heat from the high temperature compressor outlet, via a heat exchanger (heat sink in Fig 9), and send back to the hot tank.

[100] During discharge, the hot molten salt would be pumped to a heat exchanger that is in parallel to the primary heat exchanger (similarto the bypass line 144), such that the stored energy can be sent to the turbine.

[101] With reference now to Figure 9, there is shown an example power cycle 100 for an intermittent heat source 102 that implements the above concept. These embodiments suitably build upon the understanding that has been provided by the previously described examples.

[102] As with previous examples, here the power cycle 100 comprises at least a main turbine 104, a primary compressor 106, a secondary compressor 114 (and a cooler 110). In these examples the system 100 further comprises a thermal energy storage 154.

[103] As previously, heat from the intermittent heat source (fusion reactor) 102 is used to heat working fluid for driving the main turbine 104 by primary heating means 108. That is, the main turbine 104 is configured to receive working fluid which has been heated by the intermittent heat source 102 by one or more primary heating means 108.

[104] Working fluid output from the main turbine 104 is split between a return path 120 to the primary compressor, and input path 122 to the secondary compressor 114. In this example the secondary compressor 114 is arranged in a parallel flow loop analogous to as shown in Figs. 1 and 2. However, it will be appreciated that the secondary compressor 114 may instead be arranged in a series configuration as per Figs. 5 and 6, and that the suitable working fluid loops may be readily rearranged accordingly. In some cases it may also be possible to utilise the compression loop 140 discussed in relation to Figs. 3 and 4.

[105] Here, the system 100 also comprises a thermal storage 154. The thermal storage may be constructed and configured in any manner that will be familiar to those in the art. The thermal storage 154 may be used to collect heat from the system 100 so as to store it for later use (i.e., for conversion into useable electric power). For example, in the case of a fusion reactor 102, the thermal storage 154 may be used to store heat before a pulse or during a pulse. Later, on pulse ramp up or ramp down, or during dwell, the thermal storage 154 may be configured to release its heat back into the system 100 so that the heat may be used to reduce demand (and reliance) on the grid for maintaining an idle state of the power system 100, or in some examples may be able to provide enough useful heat for electrical generation (thereby smoothing over demand).

[106] Suitably, in this example the second (compression) operational mode (dashed lines) may comprise two sub modes: a heat pump mode (long-dash-short-dash lines) and a heat engine mode (dash-dash-dot lines).

[107] In the heat pump mode, thermal energy may be diverted to a heat sink 174, which may then be used for storage (e.g., in a thermal storage) or ancillary industrial processes. In the heat engine mode, thermal energy 176 may be input into the system 100 for driving the main turbine 104 (and / or the secondary compressor 114). The heat energy 176 may be energy which has been previously taken by the heat sink 174 (i.e., from thermal storage) or thermal energy from another source.

[108] Figures 10 &11 take the above concept and apply it to a fusion context. That is, there is shown two examples of a power cycle 100 for an intermittent heat source 102, particularly where that heat source is a fusion reactor. Fig. 10 builds upon the power cycle 100 first introduced in Fig. 2, while Fig. 11 builds upon the power cycle first introduced in Fig. 6.

[109] In these examples, in the heat pump mode, suitable control means, such as valve 156, are opened in order to allow working fluid to heat thermal storage 154 (i.e., heat sink 174 above). That is, working fluid output from the secondary compressor 114 - path 124 - may be split between a main stream 158 which is used to run the main turbine 104 (as previously) and a side stream 160 which is used to heat the thermal storage. In the example shown, the side stream 160 is taken before recombination of the compressor 114 output with the bulk flow to the main turbine (i.e., on the path 124). In another example (not shown) the side stream 160 may be taken after the recombination (i.e., on the input path 112). After heat has been absorbed from working fluid, a high pressure cooled stream 162 may be coupled to the input to the auxiliary turbine 134; thereby making efficient use of any left over energy in the working fluid.

[110] The heat pump mode is particularly suitable for operation during dwell and other non-fusion operating periods. That is, when there is less of a site wide demand for power or when power from the grid is cheap (for running the compressor), the compressor heat output may be partially diverted to supply thermal power to the thermal storage 154.

[111] In the example of Fig. 10, the system 100 is further configured to combine working fluid output from the auxiliary turbine 134 with the main return path 120 from the main turbine 104 before the first recuperator 136, such that any remaining residual useful heat may be coupled back into the input path 112 to the main turbine.

[112] In the example of Fig. 11, the system 100 is further configured to split off some working fluid from the return path 146 for the auxiliary turbine 134 back to the primary compressor 106 into a path 170. Here the working fluid is thermally coupled to the main turbine input 112 fluid path by a second recuperator 172, thereby likewise recovering remaining residual heat.

[113] In the heat engine mode, the valves 156 may be closed to prevent further heating of the thermal storage 154, while a corresponding set of control means, such as valve 164 (previously closed during the heat pump mode), may be opened. In the heat engine mode, heated working fluid from the thermal storage 154 - path 166 - is combined with working fluid heated by the secondary compressor 114. Put another way, working fluid input to the main turbine 104 comprises a combination working fluid heated by the secondary compressor 114 and working fluid heated by the thermal storage 154. A side stream of working fluid 168 is taken before the main turbine to complete the fluid loop with the thermal storage. In another example, in this mode the side stream 168 may be taken from upstream of (i.e, before) the primary heat exchanger 108a. This arrangement may have benefits if there is ever a need to discharge heat from the thermal storage 154 when the system is operating in the first mode, so that cold(er) working fluid may be directed towards the thermal storage 154.

[114] The heat engine mode may be particularly suitable for operation when there is a need to supplement the heat / mass flow to the main turbine 104, thereby ensuring a higher power output (and a total net positive power output from the cycle) when it is needed for tokamak start up or when power prices are more expensive on the grid.

[115] In other words, the addition of the thermal storage 154, and the ability to heat it (heat pump compression mode) or utilise its heat efficiently (heat engine compression mode) provides greater opportunity to maximise grid balancing and also offers benefit of providing grid ancillary services such as frequency response by modulating the secondary compressor load. Also, the pumped thermal energy store allows for useful energy to be stored on site for later use without need for other energy storage solutions such as batteries. For example, by increasing the size of the thermal store and relying on the thermal cycle for full supply of electrical power (by converting stored thermal energy) during periods where the grid may not be able to provide all of it. Relying on this rather than batteries ensures the ability to decouple power from energy in the energy storage system.

[116] Also, although the above example has described the use of the energy within the thermal storage 150 with the power cycle 100, it will also be appreciated that the stored thermal energy could also be used elsewhere other than with the power cycle 100 (provided there are suitable thermal couplings); for example, in other industrial processes which may require heat.

[117] In summary, exemplary embodiments of an improved power cycle system, in particular one suited for a nuclear fusion power plant, have been described. The described exemplary embodiments enable the power cycle to better meet operational demand of the fusion reactor and power plant by maintaining operation of the main turbine (and other system components) during reactor inactivity such as dwell periods between pulses, and to additionally store energy for efficient use during ramp up / down phases.

[118] The example apparatus may be manufactured industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[119] Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims and as described above.

[120] All of the features disclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[121] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[122] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, orto any novel one, or any novel combination, of the steps of any method or process so disclosed.

[123] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1. A power cycle system for an intermittent heat source, comprising:a primary heating means thermally coupled to the intermittent heat source,a primary compressor for driving working fluid around the power cycle system,a main turbine, anda secondary compressor, wherein:in a first mode of operation, the main turbine receives working fluid output by the primary heating means, andin a second mode of operation, the main turbine receives working fluid output by the secondary compressor, and working fluid output from the main turbine is divided between a return path to the primary compressor and an input path to the secondary compressor.

2. The power cycle system of claim 1, wherein the secondary compressor is arranged in parallel flow with the primary compressor when the system is in the second mode.

3. The power cycle of claim 2, wherein the input path to the secondary compressor comprises secondary heating means.4, The power cycle of claim 3, wherein the secondary heating means are thermally coupled to the intermittent heat source.

5. The power cycle of claim 3, wherein the secondary heating means receive heat from a source other than the intermittent heat source.

6. The power cycle of any preceding claim, wherein working fluid output form the secondary compressor is combined with working fluid output from the primary heat exchanger.

7. The power cycle system of claim 1, wherein the secondary compressor is arranged in series flow with the main turbine when the system is in the second mode.

8. The power cycle system of claim 7, further comprising an auxiliary turbine configured in flow parallel with the main turbine.

9. The power cycle of claim 8, wherein working fluid output from the auxiliary turbine is divided between a return path to the primary compressor and the input path to the secondary compressor.

10. The power cycle of any of claims 7to 9wherein the secondary compressor receives working fluid output by the primary heat exchanger.

11. The power cycle system of any preceding claim, wherein in the first mode the secondary compressor is operated in an idle state or ‘off’ state, and in the second mode the secondary compressor is operated in a powered or ‘on’ state.

12. The power cycle system of any preceding claim, wherein the first mode corresponds to a normal operating period of the intermittent heat source, and the second mode corresponds to a dwell time of the intermittent heat source.

13. The power cycle of any preceding claim, wherein the primary heating means comprises a set of heat exchangers corresponding to a set of heat sources comprised by the intermittent heat source.

14. The power cycle of claim 13, wherein the set of heat sources comprises one or more low temperature heat sources, one or more medium temperature heat sources, and one or more high temperature heat sources.

15. The power cycle of any preceding claim, wherein the working fluid comprises super critical carbon dioxide.

16. The power cycle of any preceding claim, wherein the primary turbine and secondary compressor are coupled by a common drive shaft.

17. A power cycle system for an intermittent heat source, comprising:a primary heating means thermally coupled to the intermittent heat source and configured to heat a first working fluid,a secondary heating means thermally coupled to the intermittent heat source and configured to heat a second working fluid,a thermal coupling between the first working fluid and the second working fluida primary compressor for driving the first working fluid around the power cycle system,a main turbine, anda secondary compressor, wherein:the secondary compressor receives second working fluid output by the secondary heating means, and outputs working fluid to the thermal coupling, andthe main turbine receives first working fluid output by the primary heating means and output by the thermal coupling.

18. A fusion power plant comprising a fusion reactor and the power cycle system of any preceding claim.

19. The fusion power plant of claim 18, wherein the power cycle system is operated in the first mode during a power generation phase of the fusion reactor.

20. The fusion power plant of claim 18 or 19, wherein the power cycle system is operated in the second mode during one of a dwell phase, a maintenance period, a reactor off phase, or a start up phase, of the fusion reactor.

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