EP3167166B1

EP3167166B1 - System and method for recovering waste heat energy

Info

Publication number: EP3167166B1
Application number: EP14846776.4A
Authority: EP
Inventors: Ivan Vladimirovich Nikolin; Viacheslav Vsevolodovich Schuchkin
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens AG
Priority date: 2014-09-08
Filing date: 2014-09-08
Publication date: 2020-11-04
Anticipated expiration: 2034-09-08
Also published as: WO2016039655A1; EP3167166A1; RU2017111353A; RU2017111353A3; RU2673959C2; ES2848307T3

Description

The present invention concerns a system and method for waste heat energy recovery or waste heat energy utilization.
The development of efficient concepts for waste heat recovery or utilization is a major objective of modern industry, not only in view of the associated cost savings but also in view of the environmental benefits. As will be appreciated, many industrial processes including metal manufacture, glass production, and chemical processing, as well as technological processes in compressors, internal combustion engines etc., involve large amounts of heat. While numerous efforts and technologies have been directed to the problem of recovering or utilizing heat which is otherwise lost as waste, to date such concepts effectively utilize only a small fraction of available waste heat energy. An average total efficiency of existing technologies for waste heat utilization would not be expected to exceed a value of about 10%. Thus, about 90% of thermal energy is still wasted to the atmosphere.
A majority of waste heat is contained in the flue or exhaust gases of various industrial processes and the exhaust gases of different drives and engines and the net efficiency values in the discussion below are based on waste heat utilization from flue/exhaust gases. This is as distinct from waste heat utilization in solid bodies, such as solar collectors or some other high temperature solid structures, which may produce significantly higher values of net efficiency.
Waste heat can be utilized by turbine-generator systems which employ thermodynamic methods, such as the Rankine cycle, to convert heat into work. Typically, this method is steam-based with the waste heat being used to raise steam in a boiler to drive a turbine. However, a key short-coming of a steam-based Rankine cycle is its high temperature requirement; that is, it usually requires a relatively high-temperature waste heat stream (e.g. 300°C or higher) or a very large overall heat content. Further, the complexity of boiling water at multiple pressures/temperatures to capture heat energy at multiple temperature levels as the heat source stream cools is costly. In addition, a steam-based Rankine cycle is not a practical option for streams of low flow rate and/or low temperature.
The organic Rankine cycle (ORC) addresses short-comings of a steam-based Rankine cycle by replacing water with a lower-boiling-point fluid, such as a light hydrocarbon like propane or butane, or a HCFC fluid (e.g. R245fa). The boiling heat transfer restrictions remain, however, and new issues such as thermal instability, toxicity or flammability of the fluid arise. Accordingly, widely implementing the organic Rankine cycle (ORC) technology does not permit a utilization of all the waste heat potential due to the limited thermal stability of the organic fluid, which affects thermal efficiency of ORC systems when the waste heat source temperature exceeds 250°-300°C. On average, the total net efficiency of existing ORC units does not exceed a value of about 10%, such that up to 90% of the thermal energy is still wasted to the atmosphere.
Supercritical carbon-dioxide (S-CO₂) waste heat utilization technology has been used to address some of these issues. The supercritical state of CO₂ provides improved thermal coupling with multiple heat sources and allows for development of more effective (up to 20% net efficiency) and very compact units compared with ORC systems for a variety of applications. Such S-CO₂ systems often require very complex system layouts and unique heat transfer equipment, however, which lead to high capital costs and also technical difficulties. Examples of such known systems for waste heat recovery and utilization are described in the patent publications WO2011/119650A2 , WO2012/074905A2 , WO2012/074911A2 , and WO2012/074940A2 .
EP 2 426 435 A2 discloses a cooling installation for low energy consumption, consisting of a primary cooling circuit in which there is an object to be cooled, a compressor, a primary condenser and a primary turbine that drives an electricity generator or is directly coupled to the primary compressor, characterised in that the primary condenser is coupled to a secondary cooling circuit that follows a Rankine cycle or a TFC (Trilateral Flash Cycle system), whereby heat from the primary cooling circuit is utilised in the secondary cooling circuit that contains a secondary turbine that is coupled to an electricity generator, or is directly coupled to the primary compressor, and further contains a secondary condenser and a pressure pump.
US 4 760 705 A , WO 95/24822 A2 and WO 2011/030285 A1 disclose different installations comprising a binary Rankine cycle power plant.
WO 2013/115668 A1 discloses a heat engine and a method for utilizing waste heat.
In view of the above, an object of the present invention is to provide a new and improved system and method for utilizing and/or recovering waste heat energy from a waste heat source, such as flue gases or exhaust gases.
In accordance with the invention, a system for recovering or utilizing waste heat energy as recited in claim 1 and method of recovering or utilizing waste heat energy as recited in claim 6 are provided. Advantageous and/or preferred features of the invention are recited in the dependent claims.
According to one aspect, therefore, the invention provides a system for recovering and/or utilizing waste heat energy from a waste heat source, comprising:

a first heat engine having a first working fluid, such as carbon-dioxide (CO₂), the first heat engine configured and arranged to transfer heat from the waste heat source to the first working fluid, and
a second heat engine, especially an organic Rankine cycle (ORC) heat engine, having a second working fluid, the second heat engine configured and arranged for transfer of heat from the first working fluid to the second working fluid, especially for cooling the first working fluid after expansion thereof in a turbine of the first heat engine.

In this way, the inventors have developed a way of combining the advantages of both the ORC and S-CO₂ approaches, namely: the simplicity of an ORC system that allows implementation of conventional heat transfer equipment and the high efficiency of S-CO₂ technology. Thus, the present invention may provide a new system of combined supercritical carbon-dioxide (S-CO₂) (first cycle) and ORC (second cycle) waste heat recovery or utilization. The layout of the system may include a single waste heat exchanger and two turbines with respective coolers (heat sinks) and respective pumps/compressors for the first (S-CO₂) cycle and the second (ORC) cycle. The invention thus provides a layout which allows high net efficiency (e.g. of about 20%) of waste heat utilization from flue gases with a single waste heat exchanger while avoiding complex known S-CO₂ system layouts that require expensive recuperators for supercritical carbon-dioxide.
The first heat engine typically defines a first thermodynamic circuit for circulation of the first working fluid. The first heat engine includes: a first heat exchanger arranged in the first circuit and configured to transfer heat from the waste heat source to the first working fluid, and a second heat exchanger in the first circuit for cooling the first working fluid after an expansion thereof in a turbine of the first heat engine. Thus, after the turbine of the first heat engine converts heat energy of the first working fluid into work via thermal expansion, the second heat exchanger is configured and arranged to transfer residual heat from the first working fluid to the second working fluid in the second heat engine. In other words, in the combined cycle of the system, only the first heat engine obtains heat energy directly from the waste heat exchanger. Heat energy transferred to the second heat engine is non-utilized heat from the first circuit. The transfer of heat energy transfer between the two heat engines of this combined cycle system is thus performed in a "first cycle cooler/second cycle heater" device which interfaces or joins the two circuits of the combined cycle. A joint "first (S-CO₂) cycle cooler/second (ORC) cycle heater" contributes to heat transfer equipment costs optimization. Desirably, the "first (S-CO₂) cycle cooler/second (ORC) cycle heater" may be represented by a one-piece recuperator in the case where the second (ORC) cycle is operating at supercritical parameters or it may comprise two parts (e.g. preheater and evaporator) for a subcritical second (ORC) cycle.
The second heat engine includes a second turbine for expansion of the second working fluid at a position in the second circuit following or downstream of the second heat exchanger. This second turbine converts the heat energy of the second working fluid into work. The second heat engine includes a by-pass conduit or path in the second circuit for by-passing the second turbine. Preferably, the second heat engine also includes a third heat exchanger for cooling the second working fluid after expansion thereof in the second turbine.
In a particularly preferred embodiment of the invention, the second heat engine includes a fourth heat exchanger which is configured as a recuperator for recuperating heat energy from the second working fluid after an expansion thereof in the second turbine. In this regard, the fourth heat exchanger is to arranged to transfer heat to the second working fluid at a position in the second circuit adjacent to and/or upstream of the second heat exchanger.
In a preferred embodiment, the first heat engine includes a fifth heat exchanger for further cooling the first working fluid after expansion thereof in the first turbine, wherein the fifth heat exchanger is arranged or located following or downstream of the second heat exchanger.
In a particular embodiment, therefore, the invention provides a waste heat utilization system, comprising:

a supercritical carbon-dioxide heat engine having carbon dioxide (CO₂) as working fluid, the first heat engine having a first heat exchanger for transferring heat from a waste heat source to the carbon-dioxide and a second heat exchanger for cooling the carbon-dioxide after expansion thereof in a first turbine; and
an organic Rankine cycle (ORC) heat engine having a second working fluid, wherein the second heat exchanger is configured and arranged to transfer heat from the carbon-dioxide (CO₂) working fluid to the second working fluid.

According to another aspect, the present invention provides a method of recovering and/or utilizing waste heat energy from a waste heat source, comprising:

providing a first heat engine having a first working fluid, especially carbon-dioxide, and defining a circuit for circulation of the first working,
providing a second heat engine, especially an organic Rankine cycle (ORC) heat engine, having a second working fluid and defining a second circuit for circulation of the second working fluid,
transferring heat from the waste heat source to the first working fluid in the first heat engine, and
transferring heat from the first working fluid in the first heat engine to the second working fluid in the second heat engine, especially to cool the first working fluid after its expansion in a turbine of the first heat engine.

As noted above, the step of transferring heat from the first working fluid in the first heat engine to the second working fluid in the second heat engine will typically comprise both cooling the first working fluid (i.e. after thermal expansion thereof in a turbine of the first heat engine) and heating the second working fluid in the second heat engine. The method may additionally comprise the step of further cooling the first working fluid after the step of transferring heat from the first working fluid to the second working fluid.
The second heat engine includes a second turbine for expansion of the second working fluid at a position in the second circuit following or downstream of the second heat exchanger. The method of the invention preferably further includes cooling the second working fluid in the second heat engine after thermally expanding same in a second turbine.
The method includes directing the second working fluid to by-pass the second turbine of the second heat engine, e.g. via a by-pass conduit or path in the second circuit. In this way, the secondary ORC cycle can be "turned off" using an ORC turbine bypass valve. Thus, the second circuit may operate as a simple cooler of the primary S-CO₂ cycle, thereby providing flexible power management to meet consumer requirements and respond to variations in the waste heat source and environmental parameters.
In a particularly preferred embodiment, the method further comprises recuperating heat from the second working fluid after expansion thereof in the second turbine, especially by heat transfer to the second working fluid at a position in the second circuit adjacent to and/or upstream of the second heat exchanger.
For a more complete understanding of the invention and the advantages thereof, exemplary embodiments of the invention are explained in more detail in the following description with reference to the accompanying drawing figures, in which like reference characters designate like parts and in which:

Fig. 1: is a schematic illustration of a waste heat recovery system according to one preferred embodiment;
Fig. 2: is a schematic illustration of a waste heat recovery system according to another preferred embodiment;
Fig. 3: is a flow diagram which schematically illustrates a method according to a preferred embodiment.

The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate particular embodiments of the invention as defined in the claims and together with the description serve to explain the principles of the invention. Other embodiments of the invention and many of the attendant advantages of the invention will be readily appreciated as they become better understood with reference to the following detailed description.
It will be appreciated that common and/or well understood elements that may be useful or necessary in a commercially feasible embodiment are not necessarily depicted in order to facilitate a more abstracted view of the embodiments. The elements of the drawings are not necessarily illustrated to scale relative to each other. It will further be appreciated that certain actions and/or steps in an embodiment of a method may be described or depicted in a particular order of occurrences while those skilled in the art will understand that such a specificity with respect to sequence may not actually be required. It will also be understood that the terms and expressions used in the present specification have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study, except where specific meanings have otherwise been set forth herein.
With reference to Fig. 1 of the drawings, one embodiment of a system 1 for recovering and/or utilizing waste heat energy from a waste heat source S according to the present invention is shown schematically. The heat source S may be a waste heat stream, such as gas turbine exhaust, process stream exhaust, or other combustion product exhaust stream, including furnace or boiler exhaust streams. The thermodynamic system 1 may be configured to transform the waste heat into electricity for a range of different applications including but not limited to bottom cycling in gas turbines, diesel engine generator sets, industrial waste heat recovery (e.g. in manufacturing plants, refineries, compression stations), and hybrid alternatives to internal combustion engines. In other exemplary embodiments, the heat source S could derive thermal energy from renewable sources of thermal energy such as, but not limited to, solar thermal and geothermal sources.
The system 1 comprises a first heat engine H1 and a second heat engine H2, each of which defines a respective first and second thermodynamic cycle or circuit A, B for an associated first and second working fluid F1, F2. Each of the first and second heat engines H1, H2 is used to convert thermal energy to work via thermal expansion of the respective first and second working fluid F1, F2. In particular, the thermodynamic system 1 comprises a first heat engine H1 having a working fluid circuit A in thermal communication with a waste heat source S via a first heat exchanger E1. While it will be appreciated that any number of heat exchanger devices may be utilized in conjunction with one or more waste heat sources, in this exemplary embodiment the first heat exchanger E1 is a single waste heat exchanger. In other exemplary embodiments, the first heat exchanger E1 may include multiple stages of a combined waste heat exchanger. While the waste heat source S may be a fluid stream of a high temperature source itself, in other exemplary embodiments the waste heat source S may be a thermal fluid in contact with the high temperature source. The thermal fluid may thus deliver the thermal energy to the waste heat exchanger E1 to transfer the energy to the working fluid F1 in the first circuit A.
As illustrated, the first or waste heat exchanger E1 serves as a high temperature, or relatively higher temperature, heat exchanger adapted to receive a flow or stream of the waste heat source S. In exemplary embodiments of the disclosure, the initial temperature of the waste heat source S entering the system 1 may range from about 200°C to greater than about 700°C. In the particular embodiment shown, the stream of the waste heat source S may have a temperature of about 500°C or higher. In this regard, however, operative temperatures and pressures and flow rates are given by way of example and are not in any way to be considered as limiting the scope of the disclosure.
The working fluid F1 circulated in the first circuit A of the first heat engine H1 is carbon dioxide (CO₂). Carbon dioxide as a working fluid for power generating cycles has many advantages. It is a neutral working fluid that is non-toxic, non-flammable, low cost, readily available, and has no need of recycling. Due in part to its relative high working pressure, a CO₂ system can be built that is much more compact than systems using other working fluids. The high density and volumetric heat capacity of CO₂ with respect to other working fluids makes it more "energy dense" meaning that the size of all system components can be considerably reduced without losing performance. It should be noted that the term "carbon dioxide" as used herein is not intended to be limited to CO₂ of any particular type, purity, or grade. In an exemplary embodiment, for example, an industrial grade CO₂ may be used without departing from the scope of this disclosure.
The carbon dioxide (CO₂)first working fluid F1 circulated in the first circuit A of the first heat engine H1 is delivered in a pressurized state to the waste heat exchanger E1 via a first pump or compressor P1 which is arranged in the first circuit A. As noted above, the CO₂ first working fluid F1 is heated in the waste heat exchanger E1 by thermal contact with the waste heat source S. The compressed and heated CO₂ then undergoes an expansion in a first turbine T1 which converts a portion of the thermal energy in the working fluid F1 drawn from the waste heat source S into mechanical work. A first generator G1 operably coupled to the first turbine T1 may then transform that mechanical work into electrical energy.
Downstream of the first turbine T1 in the circuit A of the first heat engine H1, a second heat exchanger E2 is provided for cooling the CO₂ working fluid F1 after its expansion in the turbine T1. And this second heat exchanger E2 provides an interface with the second heat engine H2. In this regard, the second heat engine H2 defines a second thermodynamic cycle or circuit B, specifically an organic Rankine cycle (ORC), in which a second working fluid F2, such as a light hydrocarbon (e.g. propane or butane, or a HCFC fluid) circulates. Thus, the second heat exchanger E2 is effectively arranged in both the first circuit A and the second circuit B and is adapted or configured to transfer heat energy remaining in the carbon dioxide working fluid F1 after its expansion in turbine T1 to the second working fluid F2 of the second heat engine H2. Although the CO₂ working fluid F1 may only have a temperature in the range of 70°C to 250°C, and preferably 100°C to 200°C, on entry to the second heat exchanger E2, the relatively low boiling temperature of the second working fluid F2 in the ORC heat engine H2 still enables that heat energy to be recovered very effectively. After cooling in the second heat exchanger E2, the first working fluid F1 completes the first circuit A by returning to the first pump or compressor P1 to again be pressurized downstream of the waste heat exchanger E1.
The ORC working fluid F2 circulated in the second circuit B of the second heat engine H2 is delivered in a pressurized state to the second heat exchanger E2 via a second pump or compressor P2 arranged in the second circuit B. As already noted above, the second working fluid F2 is heated by thermal contact with the first working fluid F1 in the second heat exchanger E2. The compressed and heated ORC working fluid F2 then undergoes an expansion in a second turbine T2 which converts a portion of the thermal energy in the second working fluid F2 into work. As in the first heat engine H1, a second generator G2 which is operably coupled to the second turbine T2 may transform that work into electrical energy. Finally, a third heat exchanger E3 is provided as a cooler in the second (ORC) circuit B for cooling the ORC working fluid F2. To this end, the third heat exchanger E3 may be fluidly connected with a heat sink S2, optionally including a forced convection device (e.g. fan) or a cooling tower arrangement. In this way, the second working fluid F2 is cooled prior to returning in the ORC circuit B to be pressurized again via the second pump or compressor P2 downstream of the second heat exchanger E2.
It will be noted that the second circuit B includes a by-pass path or conduit BP with by-pass valve V arranged in parallel with the path through the second turbine T2. This by-pass path BP enables the turbine T2 of the ORC heat engine H2 to be by-passed and effectively deactivated via the by-pass valve V. By doing so, the second circuit B may operate as a simple cooling circuit form the primary S-CO₂ circuit A, thereby allowing flexible power management to meet consumer requirements and to respond to variation in the waste heat source S and/or to environmental parameters.
The preferred embodiment illustrated in Fig. 1 thus provides a simple layout of a combined S-CO₂ and ORC system 1 which allows for flexible operation during waste heat recovery and utilization with high efficiency. The heat engine layout in the system 1 of Fig. 1 has at least two continuously arranged non-regenerative cycles A, B with single waste heat exchanger E1 and internal heat transfer to a secondary ORC cycle in the cooler of the primary S-CO₂ cycle having high net efficiency of the combined system (e.g. up to 20%) comparable with much more complex S-CO₂ prior art systems. The internal heat transfer to the secondary ORC circuit B in the cooler E2 of the primary S-CO₂ circuit A and the single flow streams F1, F2 continuously flowing through all elements of each circuit A, B results in the use of less heat transfer equipment due to the absence of internal recuperators. It will also be seen that the layout of system 1 avoids the need for any internal flow split-up points and thus simplifies mass-flow management and control.
With reference to Fig. 2 of the drawings, another preferred embodiment of a waste heat recovery and utilization system 1 according to the invention is illustrated schematically. With this alternative embodiment, the general arrangement of the first and second heat engines H1, H2 in the system 1 remains substantially unchanged, but the second heat engine H2 itself differs from the embodiment in Fig. 1 in that it includes a fourth heat exchanger E4 which is configured as a recuperator for recuperating heat energy from the second working fluid F2 after an expansion thereof in the second turbine T2. In this regard, the fourth heat exchanger E4 forms a thermal interface in the second circuit B between the second fluid F2 as it exits the second turbine T2 and the second fluid F2 before it enters the second heat exchanger E2. Thus, the arrangement uses residual heat in the second fluid F2 after its expansion in the turbine to pre-heat the second fluid F2 at a later or downstream position in the second circuit B adjacent to the second heat exchanger E2. Thus, it is possible to increase an internal thermal efficiency of the second ORC heat engine H2 by adding the recuperator E4 to the system layout.
It will also be noted that the embodiment of the system 1 in drawing Fig. 2 differs from Fig. 1 in that the first heat engine H1 includes another (fifth) heat exchanger E5 fluidly connected to a heat sink S1 for further cooling the first working fluid F1 after expansion in the first turbine T1. The additional heat exchanger or cooler E5 is desirable in the first (S-CO₂) circuit A in this case to provide the required temperature at the inlet of the first pump/compressor P1 in order to keep a reasonable value of compression work. While the embodiment of the system 1 in Fig. 2 is more complicated than the embodiment of Fig. 1 and requires more heat transfer equipment, it has a higher overall efficiency, offers more flexibility in control, and can be particularly suitable for some applications.
Finally, referring now to Fig. 3 of the drawings, a flow diagram is shown that schematically illustrates the steps in a method of recovering and/or utilizing waste heat according to the preferred embodiments of the invention described above with respect to Figs. 1 and 2. Specifically, the first box I of Fig. 3 represents the step of providing a supercritical CO₂ first heat engine H1 having a carbon-dioxide (CO₂)as the first working fluid F1. The second box II then represents the step of providing an organic Rankine cycle (ORC) second heat engine H2 having a light hydrocarbon as second working fluid F2. The third box III represents the step of transferring heat from a waste heat source S to the first working fluid F1 (e.g. supercritical CO₂) in the first heat engine H1. Then, the final box IV in Fig. 3 of the drawings represents the step of transferring heat from the first working fluid F1 in the first heat engine H1 to the second working fluid F2 in the second heat engine H2 to cool the CO₂ working fluid F1 after expansion in a turbine T1 of the first heat engine H1.
Although some specific embodiments of the invention are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternative and/or equivalent implementations exist. In this regard, for example, it will be noted that the working fluid in the first circuit A and/or the second circuit B may be a binary, ternary, or other working fluid blend. The working fluid blend or combination can be selected for the unique attributes possessed by the fluid combination within the heat recovery system, as described herein. In the first circuit A for example, one such fluid combination includes a liquid absorbent and CO₂ mixture enabling the combined fluid to be pumped in a liquid state to high pressure with less energy input than required to compress CO₂. In another exemplary embodiment, the working fluid may be a combination of CO₂ or supercritical carbon dioxide (S-CO₂) and one or more other miscible fluids or chemical compounds. In other exemplary embodiments, the working fluid may be a combination of CO₂ and propane, or CO₂ and ammonia, without departing from the scope of the disclosure.
Use of the term "working fluid" is not intended to limit the state or phase of matter that the working fluid is in. In other words, the working fluid may be in a liquid phase, a gas phase, a supercritical phase, a subcritical state, or any other phase or state at any one or more points within the fluid cycle. The working fluid may be in a supercritical state over certain portions of a circuit or cycle (the "high pressure side"), and in a subcritical state over other portions of the circuit or cycle (the "low pressure side"). In other exemplary embodiments, an entire circuit or cycle may be operated and controlled such that the working fluid is in a supercritical or subcritical state during circulation of the entire circuit.
It is also to be appreciated that the exemplary embodiment or exemplary embodiments illustrated and described herein are as examples only and are not intended to limit the scope, the applicability, or the configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient explanation or road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of the elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents. Generally, this application is intended to cover any such adaptations or variations of the specific embodiments of the present invention as claimed in the claims.
It will also be appreciated that in this document the terms "comprise", "comprising", "include", "including", "contain", "containing", "have", "having", and any variations thereof, are intended to be understood in an inclusive (i.e. non-exclusive) sense, such that the process, method, device, apparatus or system described herein may include other elements, features, parts or steps not expressly listed or inherent to such process, method, article, or apparatus. Furthermore, the terms "a" and "an" used herein are intended to be understood as meaning one or more unless explicitly stated otherwise. Moreover, the terms "first", "second", "third", etc. are used merely as labels, and are not intended to impose numerical requirements on or to establish a certain ranking of importance of their objects.

Claims

A system (1) for recovering and/or utilizing waste heat energy from a waste heat source (S), comprising:
a first heat engine (H1) having a first working fluid (F1), and defining a first circuit (A) for circulation of the first working fluid (F1), wherein the first heat engine (H1) is configured and arranged for transfer of heat from the waste heat source (S) to the first working fluid (F1), and

a second heat engine (H2), especially an organic Rankine cycle (ORC) heat engine, having a second working fluid (F2) and defining a second circuit (B) for circulation of the second working fluid (F2),

wherein the second heat engine (H2) is configured and arranged for transfer of heat from the first working fluid (F1) to the second working fluid (F2), especially to cool the first working fluid (F1) after expansion thereof in a turbine (T1) of the first heat engine (H1),wherein the first heat engine (H1) comprises a turbine (t1) and a first heat exchanger (E1) in the first circuit (A) to transfer heat from the waste heat source (S) to the first working fluid (F1) and a second heat exchanger (E2) in the first circuit (A) for cooling the first working fluid (F1) after an expansion thereof in the turbine (T1) of the first heat engine (H1), wherein the second heat exchanger (E2) is also in the second circuit (B) to transfer heat from the first working fluid (F1) to the second working fluid (F2) of the second heat engine (H2), wherein the second heat engine (H2) includes a second turbine (T2) for expansion of the second working fluid (F2) at a position in the second circuit (B) downstream of the second heat exchanger (E2), wherein the second heat engine (H2) includes a by-pass circuit (BP) for by-passing the second turbine (T2), the by-pass circuit including a valve (V) for regulating operation thereof, characterized in that the first working fluid is carbon-dioxide.
A system (1) according to claim 1, wherein the second heat engine (H2) includes a third heat exchanger (E3) for cooling the second working fluid (F2) after expansion thereof in the second turbine (T2).
A system (1) according to claim 2, wherein the second heat engine (H2) includes a fourth heat exchanger (E4) which forms a recuperator in the second circuit (B) to recuperate heat from the second working fluid (F2) after expansion thereof in the second turbine (T2).
A system (1) according to any of claims 1 to 3, wherein the first heat engine (H1) has a fifth heat exchanger (E5) for further cooling of the first working fluid (F1) after expansion thereof in the first turbine (T1), wherein the fifth heat exchanger (E5) is arranged in the first circuit (A) following or downstream of the second heat exchanger (E2).
A system (1), according to claim 1, wherein the second working fluid (F2) is a light hydrocarbon, preferably propane or butane, or a HCFC fluid.
A method of recovering waste heat from a waste heat source (S), comprising:
providing a first heat engine (H1) having a first working fluid (F1), wherein the first working fluid is carbon-dioxide (CO₂),

providing a second heat engine (H2), especially an organic Rankine cycle (ORC) heat engine, having a second working fluid (F2);

transferring heat from the waste heat source (S) to the first working fluid (F1) in the first heat engine (H1), and
transferring heat from the first working fluid (F1) in the first heat engine (H1) to the second working fluid (F2) in the second heat engine (H2), especially to cool the first working fluid (F1) after expansion in a turbine (T1) of the first heat engine (H1), and further comprising optionally by-passing a second turbine (T2) of the second heat engine (H2) via a by-pass circuit.
A method according to claim 6, wherein the step of transferring heat from the first working fluid (F1) in the first heat engine (H1) to the second working fluid (F2) in the second heat engine (H2) comprises both cooling the first working fluid (F1) after expansion thereof in a turbine (T1) of the first heat engine (H1) and heating the second working fluid (F2) in the second heat engine (H2).
A method according to claim 6 or claim 7, wherein the second heat engine (H2) includes a second turbine (T2) for expansion of the second working fluid (F2) at a position downstream of the second heat exchanger (E2).
A method according to any of claims 6 to 8, further comprising cooling the second working fluid (F2) in the second heat engine (H2) after expansion thereof in a second turbine (T2).
A method according to any of claims 6 to 9, further comprising recuperating heat from the second working fluid (F2) after expansion thereof in a second turbine (T2), especially by heat transfer to the second working fluid (F2) at a position adjacent to and/or upstream of the second heat exchanger (E2).
A method according to any of claims 6 to 10, comprising further cooling the first working fluid (F1) after the step of transferring heat from the first working fluid (F1) to the second working fluid (F2).