Heat Exchanger for Direct Evaporation in Organic Rankine Cycle Systems and Method

Abstract

Systems and methods include heat exchangers using Organic Rankine Cycle (ORC) fluids in power generation systems. The system includes a heat exchanger configured to be mounted inside an exhaust stack that guides hot flue gases and having an inlet and an outlet, the heat exchanger being configured to receive a liquid stream of a first fluid through the inlet and to generate a vapor stream of the first fluid and the heat exchanger is configured to include a double walled pipe, where the first fluid is disposed within an inner wall of the double walled pipe and a second fluid is disposed between the inner wall and an outer wall of the double walled pipe.

Description

TECHNICAL FIELD

The embodiments of the subject matter disclosed herein generally relate to power generation systems and more particularly to Organic Rankine Cycle (ORC) systems.

BACKGROUND

Rankine cycles use a working fluid in a closed cycle to gather heat from a heating source or a hot reservoir by generating a hot gaseous stream that expands through a turbine to generate power. The expanded stream is condensed in a condenser by transferring the heat to a cold reservoir. The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. A system for power generation using a Rankine cycle is shown in FIG. 1. These systems for power generation can be described based on the power generated as primary power generation and secondary power generation systems. Additionally, secondary power generation systems tend to use the waste heat, e.g., hot exhaust gases, from the primary power generation system to improve overall system efficiency. The power generation using the Rankine cycle is traditionally used as a secondary power generation system.

The power generation system 100 includes a heat exchanger 2, or in some cases a boiler, a turbine 4, a condenser 6 and a pump 8. Walking through this closed loop system, beginning with the heat exchanger 2, an external heat source 10, e.g., hot flue gases, heats the heat exchanger 2. This causes the received pressurized liquid medium 12 to turn into a pressurized vapor 14 which flows to the turbine/generator 4. The turbine 4 receives the pressurized vapor stream 14 and can generate power 16 by, for example, rotating a mechanical shaft (not shown) as the pressurized vapor 14 expands inside the turbine 4. The expanded lower pressure vapor stream 18 then enters a condenser 6 which condenses the expanded lower pressure vapor stream 18 into a lower pressure liquid stream 20. The lower pressure liquid stream 20 then enters a pump 8 which both generates the higher pressure liquid stream 12 and keeps the closed loop system flowing. The higher pressure liquid stream 12 then is pumped to the heat exchanger 2 to continue this process.

One working fluid that can be used in a Rankine cycle is an organic working fluid. Such an organic working fluid is referred to as organic rankine cycle (ORC) fluid. ORCs systems have been deployed as retrofits for small-scale and medium-scale gas turbines, to capture waste heat from the hot flue gas stream generated by an engine. These systems may generate up to an additional 20% power on top of the engine's baseline output, i.e., ORC systems are typically used in secondary power generation systems. This ORC working fluid is typically a hydrocarbon with a boiling temperature slightly above the International Organization for Standardization's baseline for atmospheric pressure. Because of the concern that such hydrocarbon fluids can degrade if exposed directly to the high-temperature (approximately 500 degrees Celsius) gas turbine exhaust stream, measures need to be taken to limit the surface temperature of the heat exchanging surfaces in an evaporator which contains the ORC working fluids. A currently used method for limiting the surface temperature of the heat exchanging surfaces in an evaporator which contains the ORC working fluids is to introduce an intermediate thermo-oil loop into the heat exchange system, i.e., to avoid the ORC liquid circulating through the exhaust stack of the gas turbine.

Another potential concern with ORC systems, when exposed directly to hot gasses, is their potential flammability. If a leak were to occur in a system using the ORC fluids, and the ORC fluid were to leak into the hot exhaust gas stream, e.g., the hot flue gas, combustion and/or an explosion could occur which could potentially be of a catastrophic nature to the power generation system and/or power plant. A currently used method for both limiting the surface temperature of the heat exchanging surfaces in an evaporator which contains the ORC working fluids and reducing the risk of explosion is to introduce the intermediate thermo-oil loop into the heat exchange system, which separates the ORC fluid from the exhaust stack as discussed next.

The intermediate thermo-oil loop can be used between the hot flue gas and the vaporizable ORC fluid. In this case, the intermediate thermo-oil loop is used as an intermediate heat exchanger, i.e., heat is transferred from the hot flue gas to the oil, which is in its own closed loop system, and then from the oil to the ORC fluid using a separate heat exchanger. Separating the ORC fluid from direct exposure to the hot flue gas can protect the ORC fluid from degradation and decomposition. Additionally, while the oil used in the intermediate thermo-oil loop is flammable, this oil is generally less flammable than ORC working fluids. However, this thermal oil system takes additional physical space and can represent up to one quarter of the cost of an ORC system.

Accordingly, systems and methods for reducing cost and improving the safety of using ORC systems in power generation systems are desirable.

SUMMARY

According to an exemplary embodiment there is a system for power generation using an Organic Rankine Cycle (ORC). The system includes: a heat exchanger configured to be mounted inside an exhaust stack that guides hot flue gases and having an inlet and an outlet, the heat exchanger being configured to receive a liquid stream of a first fluid through the inlet and to generate a vapor stream of the first fluid and the heat exchanger is configured to include a double walled pipe, wherein the first fluid is disposed within an inner wall of the double walled pipe and a second fluid is disposed between the inner wall and an outer wall of the double walled pipe; an expander fluidly connected to the outlet of the heat exchanger and configured to expand the vapor stream of the first fluid, to generate power; a condenser fluidly connected to an outlet of the expander and configured to receive and condense an expanded vapor stream; and a pump fluidly connected to an outlet of the condenser and configured to receive the liquid stream of the first fluid, to pressurize the liquid stream of the first fluid and to circulate the liquid stream of the first fluid to the inlet of the heat exchanger.

According to another exemplary embodiment there is a method for vaporizing an Organic Rankine Cycle (ORC) fluid in a power generation system. The method includes: transferring heat from a flue gas in an exhaust stack through a first wall of a heat exchanger to a heat pipe medium which changes a first phase of the heat pipe medium from a liquid phase to a gaseous phase inside a compartment of the heat exchanger; and vaporizing the ORC fluid when transferring heat from the heat pipe medium in the gaseous phase through a second wall to the ORC fluid which is contained inside the heat exchanger within the exhaust stack which changes a second phase of the heat pipe medium from the gaseous phase to the liquid phase, wherein the second wall is provided inside the first wall.

According to still another exemplary embodiment there is a heat exchanger in an exhaust stack directly exposed to hot flue gases. The heat exchanger includes: a first pipe configured to receive a heat pipe fluid and further includes a second pipe, wherein a volume between the first pipe and the second pipe is hermetically sealed and is divided into compartments which are bound by an inner wall of the first pipe, an outer wall of the second pipe and separating walls between the compartments; the second pipe configured to receive an Organic Rankine Cycle (ORC) fluid; and the separating walls configured to link the first pipe to the second pipe, the heat exchanger configured to receive heat from the hot flue gases and configured to receive a liquid stream of the ORC fluid through an inlet and to generate a vapor stream of the ORC fluid through an outlet.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate exemplary embodiments, wherein:

FIG. 1 depicts a conventional Rankine Cycle;

FIG. 2 illustrates a heat exchanger which uses an organic fluid disposed within an exhaust stack according to exemplary embodiments;

FIG. 3 shows a double walled pipe according to exemplary embodiments;

FIG. 4 illustrates a partial cross section of the double walled pipe of FIG. 3 with compartments according to exemplary embodiments;

FIG. 5 shows a view of the double walled pipe with toroid compartments according to exemplary embodiments;

FIG. 6 is a flowchart for a method for heat exchange according to exemplary embodiments;

FIG. 7 illustrates exhaust paths according to exemplary embodiments; and

FIG. 8 shows a flowchart for a method for vaporizing an ORC fluid according to exemplary embodiments.

DETAILED DESCRIPTION

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

As described in the Background, and shown in FIG. 1, a Rankine cycle can be used in secondary power generation systems to use some of the wasted energy from the hot exhaust gases of the primary power generation systems. A primary system produces the bulk of the energy while also wasting energy. A secondary system can be used to capture a portion of the wasted energy from the primary system. An Organic Rankine Cycle (ORC) can be used in these power generation systems depending upon system temperatures and other specifics of the power generation systems. According to exemplary embodiments, ORCs can be used for small to mid-sized gas turbine power generation systems to capture additional heat/energy from the hot flue gas which may be released directly to the atmosphere.

As described in the generic Rankine Cycle of FIG. 1, heat can be introduced into the cycle through a heat exchanger 2 or some similar process, e.g., an evaporator or boiler. Previous ORC systems have used an intermediate thermo-oil loop system to transfer heat from the hot flue gases to the ORC working fluid. In these cases the ORC system is outside of the path of the hot flue gases and located outside of the stack. According to exemplary embodiments, a more direct approach for heat exchange can be used which removes the need for thermo-oil loop and locates the heat exchanger for the ORC system in the exhaust stack in contact with the hot flue gases, e.g., temperatures between 350 degrees Celsius and 600 degrees Celsius, which can be found in electrical power generation systems. However, according to other exemplary embodiments other temperatures and temperature ranges may be used.

According to exemplary embodiments, heat exchanging coils 202 can wind in a serpentine manner through a stack 204, or equivalent waste heat exhaust structure, as shown in FIG. 2. Initially the pressurized liquid ORC fluid 12 enters from an inlet side into the heat exchanger 200. This working fluid may enter into the cooler side of the heat exchanger 200 and travel through the heat exchanging coils 202 and exit from the heat exchanger 200 from the hotter side, e.g., closer to the heat source, as a pressurized vapor ORC fluid 14 at the outlet side. In this view, the coils shown closer to arrow 206 are closer to the heat source (not shown). Arrow 206 represents the direction of travel for the hot flue gases exiting the stack, and “g” indicates the gravity in this exemplary Figure (however other heat exchanger configurations can be in different orientations with respect to gravity). Heat sources for ORCs include, but are not limited to, exhaust gases from combustion systems (power plants or industrial processes), hot liquid or gaseous streams from industrial processes, geothermal and solar thermal sources.

According to exemplary embodiments, a double walled pipe (which can also be considered as a pipe within a pipe) can be used as the heat exchanging coils 202 in the heat exchanger 200 of the ORC system to protect the ORC working fluid from decomposition and degradation. ORC fluid can degrade and/or decompose at localized temperatures of 300° C. or possibly average temperatures of 240° C. in larger volumes of the ORC fluid. This operating range is generally applicable to whichever hydrocarbon is used as the ORC fluid except for aromatic hydrocarbons, e.g., thiophene, which may be able to operate at higher temperatures.

An exemplary diagram illustrating this concept is shown in FIG. 3. The heat exchanging coil 202 of FIG. 2 can include a double walled pipe 300 with an outer wall 302 and an inner wall 306. A heat pipe fluid can be placed in the outer section 304 between the two walls and the ORC working medium is located in the inner section 308, i.e., the volume constrained by the inner wall 306. This exemplary arrangement can allow a high temperature flue gas 206, e.g., in the range of 350-600 degrees Celsius, to transfer heat to the heat pipe fluid in outer section 304 through the outer wall 302. The heat pipe fluid then transfers heat to the ORC fluid in the inner section 308 through inner wall 306. According to exemplary embodiments, this heat exchange between the heat pipe fluid to the ORC fluid can be performed such that the temperature used, and potential temperature fluctuations, can be controlled such that the temperature of the ORC fluid stays below a degradation temperature while still allowing the ORC fluid to vaporize before leaving the heat exchanger 200. In support of this, the heat pipe fluid selected needs to be, at the desired pressure, able to use the heat energy from the hot flue gases to change its phase from a liquid to a vapor. The heat pipe fluid vapor then circulates toward the inner wall 306, where the heat pipe fluid vapor is cooled, e.g., releases heat energy, and condenses into a heat pipe fluid liquid and circulates back toward the outer wall 302. In this manner the temperature of the heat pipe fluid remains relatively constant as the heat pipe fluid is capable to absorb large amounts of heat from the hot flue gases without increasing its temperature, due to the liquid to gas phase change.

In order to control this exemplary heat exchange, various factors can be modified to achieve this effect. These factors can include, but are not limited to, exhaust gas temperature, pipe dimensions, heat exchanger size, stack size, heat pipe fluid, ORC fluid, internal pipe construction and pressure(s). For example, if the exhaust gas temperature is 200 degrees Celsius versus 500 degrees Celsius, different combinations of the above mentioned factors may be used to achieve the desired cost effective heat exchanger 200. More details regarding these various factors are described in the exemplary embodiments below.

According to exemplary embodiments, the heat pipe fluid may be hermetically sealed within section 304. The heat pipe fluid may be selected from various mediums which have some, or possibly all, of the following characteristics: being less flammable than the ORC fluid, capable to undergo a phase change to transfer heat at the desired temperature/pressure ratio, and be capable of self circulating within section 304. Examples of a heat pipe fluid may include water, sodium, thermal oil and silicon-based thermal oil. Additionally, according to exemplary embodiments, the ORC fluid may be a hydrocarbon, such as, pentane, propane, cyclohexane, cyclopentane and butane or a fluorohydrocarbon such as R-245fa, a ketone such as acetone or an aromatic such as toluene or thiophene.

According to exemplary embodiments, various implementations can be used to implement the double walled pipe in the heat exchanger portion of the ORC system one of which is shown in FIGS. 4 and 5. FIG. 4 shows a partial cross section of the double walled pipe 300 between the outer wall 302 and the inner wall 306. FIG. 5 shows a view of the double walled pipe 300 with a plurality of toroid shaped compartments 406. According to exemplary embodiments, the outer section 304 (extending the length of the outer pipe) located between the outer wall 302 and the inner wall 306 can be further compartmentalized with a plurality of compartments 406. These compartments 406 contain the heat pipe fluid, e.g., water or sodium, in liquid and gaseous phases. The heat pipe fluid can be under a higher pressure than the ORC working fluid in the inner area 308. Pressures used in both the piping section containing the ORC fluid and the piping section containing the heat pipe fluid, which can be at different pressures, can be established to set the desired boiling point of the respective fluid. Additionally, spacers 404 can be used to assist in creating the compartments 406 as well as providing structural support for the heat exchanging coil 202. These compartments 406 may be, for example, toroids in shape, i.e., the spacers 404 can be circular spacer between the inner and outer pipe.

The piping used in the heat exchanger can be of varying sizes and shapes to promote the desired heat exchange and to allow for/assist in the desired self circulation of the heat pipe fluid, however the diameter of the outer wall 302 is larger than the diameter of the inner wall 306. For example, in some exemplary embodiments, the diameter of the inner most pipe may be in the range of about 12.77 mm-25.4 mm, the diameter of the outer pipe may be in the range of 25.4 mm-50.8 mm. The spacers 404 which connect/support the inner wall 306 to the outer wall 302 may have a length in the range of 5 mm-25.4 mm with a potential separation of up to 152.4 mm between each spacer. However, depending upon the heat exchanger design and use environment, other dimensions can be used. Additionally, according to exemplary embodiments, a capillary structure, e.g., a wire mesh capillary structure, can cover (or partially cover) the spacers 404 and other heat exchanging surfaces to enable the return flow of the liquid heat pipe fluid. The capillary tubes are configured to increase the heat pipe fluid in its liquid phase brought into contact with the heated surface. According to exemplary embodiments, compartment 406 is bounded by spacers 404, inner wall 306 and outer wall 302. Additionally, some of the spacers 404 may be configured to allow fluid communication between selected adjacent compartments.

Heat exchange from the hot exhaust gas to the ORC fluid can be realized through a series of exemplary steps as shown in the flowchart of FIG. 6. Initially convection from the hot exhaust gas to the heat exchanger outer wall 302 occurs in step 602. Then a phase change, e.g., vaporization, of the heat pipe fluid occurs on the inner surface of the outer wall 302 in step 604. The vaporized heat pipe fluid flows toward the inner wall 306 in step 606. Condensation of the heat pipe fluid occurs on the outer surface of the inner wall 306 in step 608. On the ORC fluid side, convection (if preheating or superheating) or a phase change (if boiling) of the ORC fluid occurs on or near the inner wall 306 in step 610. Continuous back stream of the liquid heat pipe fluid towards the inner surface of the outer wall 302 then occurs in compartment 404 (in part via capillaries on the heat exchanging surfaces and the spacers 404) in step 612. According to an alternative exemplary embodiment, evaporation and condensation of the heat pipe fluid does not need to occur, instead buoyancy-driven self circulation can occur without the driving mechanisms of evaporation and condensation, as thermal differences within the compartment can drive self circulation which still results in the desired heat exchange occurring with the ORC fluid.

According to other exemplary embodiments, the double walled pipe configuration can improve the safety in power generation systems. In one exemplary embodiment, the heat pipe fluid in the outer section is at a higher pressure than the ORC fluid in the inner section. In this case, if a leak occurred between the inner and outer sections of the double walled pipe the ORC fluid would not get into the exhaust stack to become a fire hazard. Sensors can be used to monitor a pressure of the heat pipe fluid such that if a leak were to occur it could be detected and allow the system to be shutdown. Similarly, if a leak were to occur that allowed the heat pipe fluid to enter the exhaust stack, the pressure loss could be detected and again allow for a shutdown of the system. Additionally, heat pipe fluids can be chosen which are inflammable or significantly less flammable than the ORC fluid.

As described above, various exemplary configurations for the double walled pipe in the heat exchanger can be used. According to other exemplary embodiments, this double walled pipe can be used in various heat exchanger designs, such as, for example, shell, tube and plate heat exchangers. Additionally, multiple double walled pipes may be used in parallel configurations.

According to another exemplary embodiment, in lower temperature applications, an ORC system can be placed in the path of the exhaust gases without the use of a double walled pipe. In this case care is to be taken to avoid allowing the ORC fluid to leak into the path of the exhaust gases. However, it is envisioned that low level leakage may occur which is difficult to detect. This low level leakage (very low rate leakage of the ORC fluid) can be of a concern when the system is not in operation for extended periods of time. When the system is not in operation for extended periods of time, if a low level leak occurs allowing the ORC fluid to leak into the flue system, a buildup of the ORC fluid in the general area of the heat exchanger may occur since no flue gases are ventilating out past the heat exchanger. When this occurs, it is possible for enough ORC fluid to accumulate such that when the system is turned back on, the hot flue gases come in contact with the leaked ORC fluid, and combust or explode. According to exemplary embodiments, ventilation systems can be put in place to reduce and/or remove this risk as shown in FIG. 7.

Under normal operations, the hot exhaust gas follows a path from the heat source through over the heat exchanger coils 702 and out an exhaust stack 714 as shown by directional arrow 704. According to an exemplary embodiment, in this case, a baffle 706 is placed in a closed position A such that the only flow path available is the flow path shown by directional arrow 704. However, there are times when the ORC system will not be operational. In this case, the exhaust follows the path designated by directional arrow 708, and flows directly out to atmosphere. To make this occur, the baffle 706 is place in an open position B such that the only flow path available is the flow path shown by directional arrow 708. As described above, if the power generation system is shut down for an extended period of time, and a leak of ORC fluids has occurred that was too small to be detected while the unit was operating, a flammable concentration of ORC fluid vapor may accumulate slowly over time within the exhaust stack.

According to exemplary embodiments, to prevent this, baffle 710 is opened which allows air to enter the stack at this point and flush the area around the heat exchanger coils 702 such that no appreciable amount of combustible ORC fluids can stay in the area. The path of the flushing air is shown by the directional arrow path 712. Note that during normal operations baffle 710 is in a closed position C. When flushing, various circulating methods can be used to introduce this air into the stack, e.g., fans. Also, the controls for opening and closing the baffles 706 and 710 may be interlinked or not as desired. According to one exemplary embodiment, the heat exchanger coils 702 may be of the double walled pipe design described above.

Utilizing the above-described exemplary systems according to exemplary embodiments, a method for vaporizing an ORC fluid is shown in the flowchart of FIG. 8. Initially a method for vaporizing an ORC fluid in a power generation system includes: transferring heat from a flue gas in an exhaust stack through a first wall of a heat exchanger to a heat pipe medium in step 802; changing a first phase of said heat pipe medium from a liquid phase to a gaseous phase inside a compartment of said heat exchanger in step 804; transferring heat from said heat pipe medium in said gaseous phase through a second wall to said ORC fluid which is contained inside said heat exchanger within said exhaust stack in step 806; changing a second phase of said heat pipe medium from said gaseous phase to said liquid phase in step 808; and vaporizing said ORC fluid in step 810.

The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other example are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements within the literal languages of the claims.

Claims

1. A system for power generation using an Organic Rankine Cycle (ORC), the system comprising:

a heat exchanger configured to be mounted inside an exhaust stack that guides hot flue gases and having an inlet and an outlet, said heat exchanger being configured to receive a liquid stream of a first fluid through said inlet and to generate a vapor stream of said first fluid and said heat exchanger is configured to include a double walled pipe, wherein said first fluid is disposed within an inner wall of said double walled pipe and a second fluid is disposed between said inner wall and an outer wall of said double walled pipe;

an expander fluidly connected to said outlet of said heat exchanger and configured to expand said vapor stream of said first fluid, to generate power;

a condenser fluidly connected to an outlet of said expander and configured to receive and condense an expanded vapor stream; and

a pump fluidly connected to an outlet of said condenser and configured to receive said liquid stream of said first fluid, to pressurize said liquid stream of said first fluid and to circulate said liquid stream of said first fluid to said inlet of said heat exchanger.

2. The system of claim 1, wherein said second fluid is selected from a group comprising water, sodium, thermal oil and a silicon-based thermal oil.

3. The system of claim 1, further comprising:

a plurality of compartments having a volume between said inner wall and said outer wall, some of said plurality of compartments being adjacent and insulated from each other; and

spacers configured to link said inner wall to said outer wall, wherein each compartment within said plurality of compartments is bounded by said inner wall, said outer wall and one or more of said spacers.

4. The system of claim 3, wherein said spacers are configured to allow fluid communication between selected compartments.

5. The system of claim 3, wherein said second fluid self circulates due to a heat flow within said compartments as said hot flue gasses boil said second fluid next to said outer wall and said first fluid condenses said second fluid next to said inner wall.

6. The system of claim 3, wherein said double walled pipe includes a first pipe that forms said inner wall and has an inner diameter in the range of 12.77-25.4 mm and a second pipe that forms said outer wall and has an inner diameter in the range of 25.4-50.8 mm such that the inner diameter of said second pipe is always greater than the inner diameter of said first pipe, further wherein a distance between said inner wall and said outer wall is between 12.7 mm and 25.4 mm.

7. The system of claim 1, wherein said first fluid is an ORC fluid and selected from a group comprising pentane, propane, cyclohexane, cyclopentane, butane, a fluorohydrocarbon, a ketone, an aromatic, and a combination thereof.

8. The system of claim 1, further comprising:

said exhaust stack; and

a first baffle provided in said exhaust stack and configured to redirect said hot flue gasses in said exhaust stack such that said hot flue gasses exhaust bypass said heat exchanger when said heat exchanger is in a non-operational condition.

9. The system of claim 8, further comprising:

a second baffle in said exhaust stack and configured to selectively open to allow air into said exhaust stack to flush gas away from said heat exchanger and into atmosphere when said first baffle is in a closed position.

10. A method for vaporizing an Organic Rankine Cycle (ORC) fluid in a power generation system, the method comprising:

transferring heat from a flue gas in an exhaust stack through a first wall of a heat exchanger to a heat pipe medium which changes a first phase of said heat pipe medium from a liquid phase to a gaseous phase inside a compartment of said heat exchanger; and

vaporizing said ORC fluid when transferring heat from said heat pipe medium in said gaseous phase through a second wall to said ORC fluid which is contained inside said heat exchanger within said exhaust stack which changes a second phase of said heat pipe medium from said gaseous phase to said liquid phase, wherein said second wall is provided inside said first wall.

11. The method of claim 10, wherein said heat pipe fluid is selected from a group comprising water, sodium, thermal oil and a silicon-based thermal oil.

12. The method of claim 10, wherein said ORC fluid is selected from a group comprising pentane, propane, cyclohexane, cyclopentane, butane, a fluorohydrocarbon, a ketone, an aromatic, and a combination thereof.

13. The method of claim 10, wherein a temperature of said flue gas is in a temperature range of 350-600 degrees Celsius.

14. The method of claim 10, wherein an average temperature of said ORC fluid is at an average temperature less than or equal to 240 degrees Celsius.

15. The method of claim 10, wherein said heat pipe fluid is self circulating in said compartment.

16. The method of claim 10, wherein said heat pipe fluid is under a higher pressure than said ORC fluid.

17. The method of claim 10, wherein said heat pipe fluid is in a hermetically sealed volume.

18. A heat exchanger in an exhaust stack directly exposed to hot flue gases, said heat exchanger comprising:

a first pipe configured to receive a heat pipe fluid and further includes a second pipe, wherein a volume between said first pipe and said second pipe is hermetically sealed and is divided into compartments which are bound by an inner wall of said first pipe, an outer wall of said second pipe and separating walls between said compartments;

said second pipe configured to receive an Organic Rankine Cycle (ORC) fluid; and

said separating walls configured to link said first pipe to said second pipe, said heat exchanger configured to receive heat from said hot flue gases and configured to receive a liquid stream of said ORC fluid through an inlet and to generate a vapor stream of said ORC fluid through an outlet.

19. The heat exchanger of claim 18, wherein said distance between said inner wall of said first pipe and said outer wall of said second pipe is between 12.7 mm and 25.4 mm.

20. The heat exchanger of claim 18, wherein said first and second pipes enter and exit said exhaust stack multiple times.