WO2013170861A1

WO2013170861A1 - A flue gas tube for thermoelectric generator

Info

Publication number: WO2013170861A1
Application number: PCT/DK2013/050147
Authority: WO
Inventors: Paw Vestergård MORTENSEN; Ali Asghar Enkeshafi; Lasse Aistrup ROSENDAHL
Original assignee: Aalborg Universitet
Priority date: 2012-05-16
Filing date: 2013-05-15
Publication date: 2013-11-21
Also published as: EP2877714A1

Abstract

The invention relates to a flue gas tube (FGT) (1) for generation of thermoelectric power having thermoelectric elements (8) that are integrated in the tube. The FTG may be used in combined heat and power (CHP) system (13) to produce directly electricity from waste heat from, e.g. a biomass boiler. The CHP system may also be operated in a heating or cooling mode, thus being able to heat or cool water by feeding electricity to the system.

Description

A FLUE GAS TUBE FOR THERMOELECTRIC GENERATOR FIELD OF THE INVENTION

The present invention relates to a flue gas tube (FGT) for thermoelectric generators, which can be employed in a combined heat and power (CHP) system.

BACKGROUND OF THE INVENTION

A flue gas tube, also called flue gas pipe, is a tube, generally a metallic tube allowing for flue gas produced in a combustion chamber to move from the combustion chamber to a chimney so as to be released in the atmosphere.

As flue gases are produced in a combustion chamber at a high temperature, FGTs are surrounded by water so as to reduce the temperature of the flue gases passing through the FGTs. In this way heat is transferred to the water and flue gases are cooled before they reach the chimney and released in the atmosphere. FGTs are probably the most common way for transferring heat from a combustion chamber to a water circuit, in particular for small and medium scale boiler, producing hot water for industrial and domestic use.

In marine application boilers employing FGTs are also used for efficient heat transfer due to their compactness.

FGTs are widely used worldwide and are now recognized as a standard product globally provided in different ranges and sizes for any use.

In general, FGTs have the advantage of allowing for at least partial recover of heat as well as cooling of flue gas so as to allow safe release in the environment.

Hence, an improved FGT would be advantageous, and in particular a more efficient FGT that is able to recover more energy from hot gases would be advantageous.

OBJECT OF THE INVENTION

It is an object of this application to provide a FGT which is able to recover more energy from hot gases.

It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide a FGT that solves the above mentioned problems of the prior art with a FGT that is able to recover more energy from hot gas by converting part of that heat transferred directly into electricity.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a flue gas tube for generation of thermoelectric power.

A FGT for generation of thermoelectric power may be also referred to as a FGT- power generator i.e. a flue gas tube having thermoelectric elements that are integrated inside the FGT.

A FGT for generation of thermoelectric power therefore produces directly electricity thus being able to directly convert heat transfer into electrical energy production.

In some embodiments according to the first aspect of the invention the flue gas tube comprises: a first or inner portion having an internal and external surface, the first or inner portion having thermoelectric elements located on its external surface; a second or outer portion having an internal and external surface, the second or outer portion surrounding the first or inner portion; means for transferring heat located between the thermoelectric elements and the second or outer portion.

In some embodiments the second or outer portion comprises the means for transferring heat. In some further embodiments the means for transferring heat are located and transfer heat between the thermoelectric elements and the internal surface of the second or outer portion.

In general, the thermoelectric elements (TEs) are integrated into the FGT.

Integrated is herein defined as formed into a whole, i.e. the TEs are not simply attached to the external surface of a FGT, but integrated within the tube, between the inner portion and the outer portion of the tube.

Thus, the invention relates to a thermoelectric flue gas tube that may be characterized by the presence of integrated thermoelectric module within, i.e. inside, the tube.

In some embodiments the first or inner portion and the second or outer portion are hermetically sealed, thereby no physical contact between the fluid flowing inside the inner portion and the fluid flowing outside the second or outer portion is present. The first or inner portion and the second or outer portion are hermetically sealed so that a first fluid, e.g. water, flowing in outside the tube, in contact with the external surface of the second or outer portion cannot not get in contact with a second fluid, e.g. flue gas, flowing inside the tube, in contact with the internal surface of the first or inner portion.

Thus, the invention relates to a thermoelectric flue gas tube that may also be characterized by the presence of integrated thermoelectric modules within, i.e. inside, the tube which are hermetically sealed from the flue gas.

The internal surface of the first or inner portion is suitable for fluid flow, such as hot gas, thus it is adapted to resist to corrosive and hot environment generated by the passage of the hot gas, e.g. may be made of corrosion resistant materials. The external surface of the second or outer portion is suitable for flow of fluid such as water, thus it is adapted to resist to the corrosive environment, such as oxidative environment, generated by the contact with water, e.g. may be made of stainless steel or Ni alloy.

The outer portion is surrounded by cooling water providing the temperature gradient necessary to the TEs for the production of electricity.

Thermoelectric elements may be referred herein as P&N legs or thermoelectric modules.

In some embodiments the thermoelectric elements have a first side facing said external surface of the first or inner portion and wherein the thermoelectric elements have a second side facing the internal surface of the second or outer portion.

In some embodiments the second side of the thermoelectric elements is opposite to the first side of the thermoelectric elements.

Opposite is defined as located on the opposite side of the TE.

Thus, the outer portion is surrounded by cooling water providing the temperature gradient between the first or hot side of the TEs and the second or cold side of the TEs, necessary for the production of electricity from the thermoelectric module. In some further embodiments the thermoelectric elements are distributed around the external surface of the first or inner portion.

The distribution may a continuous distribution, i.e. without empty space between arrays of thermoelectric modules. In some other embodiments the TEs may be distributed with empty spaces, i.e. so as to include areas of the external surface of the inner portion where TEs are not present. The inner portion design may be driven by the applicability of standard TE modules.

In some embodiments the first side of said thermoelectric elements follows the outline of the external surface of the first or inner portion.

In some embodiments the first or inner portion has a polygon cross section, such as a square cross section.

As polygon cross section is herein define a portion having a cross section with three or more straight sides. The advantage of having a polygon cross section of the inner portion is that standard thermoelectric modules, generally having straight surfaces, can be easily integrated into the FGT. Thus, the cross section of the inner portion of the tube is adapted to allow for use of standard thermoelectric P&N legs.

In general, the first or inner portion for flue gas has a cross section with sides having straight, i.e. not curved, surfaces.

However, in some embodiments the use of curved thermoelectric modules may allow for the use of FGTs having inner portion with a circular cross section.

Thus, in some embodiments the first or inner portion has a circular cross section.

In some further embodiments the FGT is coaxial tube comprising an inner and outer portion according to the other embodiments of the invention, thus the first and second portion may have the same center.

The means for transferring heat may be in contact with the second side of the TEs allowing for transferral of heat from the second side to the second or outer portion of the FGT. The outer portion is than surrounded by cooling water providing, through the means for transferring heat, the temperature gradient necessary to the TEs for production of electricity.

The means for transferring heat may be positioned onto the TEs.

The means for transferring heat may also be located between the TEs and the internal surface of the second or outer portion.

In some embodiments the internal surface of the second or outer portion comprises the means for transferring heat. In some further embodiments said means for transferring heat may be integrated into the outer portion of the FGT.

In some further embodiments the means for transferring heat are metal capes.

In some embodiments the metal capes may by produced by extrusion. For example, the means for transferring heat may have a desired cross-sectional profile, produced by extrusion so as to be integrated into the FGT. For example, metal capes used as heat transfer may be aluminium capes.

A great advantage of the FGTs according to the invention is that FGTs may be produced in separate parts and connected to each other at a later stage.

According to the invention, the inner portion of the FGT ends before the end part of the FGT, so that in its end part the FGT correspond to the outer portion. The end parts of the FGT can be joined, e.g. by welding or soldering, producing FGTs of desired length. Furthermore, this allows for easier insertion of FGTs in already existing boilers, buy changing the existing FGT where TEs are not present.

Thus, in some embodiments at least one of the end parts of the FGT according to the invention is characterized by the absence of the inner portion.

The invention stems also from the idea of replacing existent tube in boilers, such as biomass boilers. These boilers have walls having holes with a cross section of the dimensions of the tube to be used. In that, tubes having profiles comprising different cross sections cannot be used for replacing tubes already in the boilers. By integrating the TE elements inside the FGT, a single tube having a continuous profile and having the same cross section along the all length of the tube is achieved. This allows for substitution of current FGTs or cooling tubes in standard boilers.

Thus in some embodiments the external diameter of the FGT for generation of thermoelectric power is substantially the same along its entire length.

In some further embodiments the external diameter of the FGT for generation of thermoelectric power is substantially the same at any cross section along its entire length.

External diameter is defined as the diameter at the external surface of the second or outer portion.

In some other embodiments the ends part of the FGT may be complementary, such as that can be at least partially contained in one other. This is the case when one end is larger than the other end. In this case two or more FGTs may be joined, e.g. by press fitting the smaller end of a first FGT into the larger end of a second FGT, and further sealed, e.g. by soldering or welding.

Thus, the above described object and several other objects are intended to be obtained in a second aspect of the invention by providing a method for producing a CHP system by removing cooling tubes in existing boilers and introducing FGT for generation of thermoelectric power. In some embodiments according to the second aspect of the invention, the method comprises introducing FGT for generation of thermoelectric power according to the first aspect of the invention. The above described object and several other objects are intended to be obtained in a third aspect of the invention by providing a combined heat and power (CHP) system comprising flue gas tube for generation of thermoelectric power.

In some embodiments according to the third aspect of the invention the CHP system comprises one or more flue gas tube according to the first aspect of the invention.

In some further embodiments a CHP system further comprises capillary tube holding wires adapted to collect the electricity produced by the one or more flue gas tube.

The capillary tubes are adapted to keep the wires, for collecting the electricity produced, into a hot water environment allowing the connection between the thermoelectric elements and the outside pol terminal box. Capillary tubes may be made of copper or stainless steel and soldered to the FGTs. The capillary tubes holds wires such as insulated wires, such as glass fibres insulated wires which can be easily bent and have the further advantage that are not inflammable.

In some other embodiments the CHP system, according to third aspect of the invention, is or comprise a water boiler.

The CHP system may further comprise a control unit adapted to switch between a first operating mode being an electricity generating mode and a second mode being a heating or cooling mode.

The first operating mode may be also considered as the normal mode of use of the CHP system, i.e. producing electricity from recovery of waste heat.

However, the presence of the control unit allows for switching between the normal mode and a second mode. The second mode may be a heating mode where the CHP system may produce heat upon being fed electricity from the grid.

This is a great advantage either when the electricity produced by the TEs is not sufficient to self-power the CHP system or when there is a need for heating the water flowing outside and in contact with the external surface of the second or outer portion of the FGT. In this way electricity, e.g. from the grid, can be used to heat water so that even in case of failure of the burner, hot water can be produced.

The second mode may also be a cooling mode where the CHP system may produce cold water upon feeding electricity from the grid to the system. Thus the high reversibility of the system can also be used when cooling of water flowing outside and in contact with the external surface of the second or outer portion of the FGT is required. In this case by inverting the polarity and by feeding electricity from the grid to the system, the water in contact with the external surface of the second or outer portion of the FGT is cooled, thus providing cold water for different purposes, e.g. air conditioning.

For example, a CHP system based on a biomass boiler may produce power up to 500 W. However, in case of lack of biomass by feeding electricity from the grid to the CHP system water heating is achievable, e.g. with a COP around 2.0, i.e. by feeding 100 W, 200W of heat is achieved.

In case of need of cold water, e.g. for green-house or farm use, by inverting polarity and by feeding electricity from the grid to the system, water cooling is achievable, e.g. with a COP around 0.45. FGTs according to the invention may also find application with other system comprising one or more flue gas tube according to the invention, such as engine exhaust, e.g. ship engine exhaust or chimney installation.

The first, second and other aspects and embodiments of the present invention may each be combined with any of the other aspects and embodiments. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The FGT for generation of thermoelectric power according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 shows a schematic view of a cross section of the FGT according to an embodiment of the invention.

Figure 2 shows a schematic view of the FGT according to an embodiment of the invention.

Figure 3 shows a schematic view of a CHP system having arrays of FGTs according to an embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

The FGT 1, in figure 1 is characterized by an inner portion 2 having an internal surface 4 and an external surface 5 and an outer portion 3 having an internal surface 6 and an external surface 7.

When in operation, the first or inner portion 2 is adapted to be heated by a flue gas flow, the flue gas flowing in close contact with the internal surface 4 of the first or inner portion 2.

The second or outer portion 3 is adapted to be cooled by a flow of fluid, such as water in contact with the external surface 7 of the second or outer portion 3. TE elements, such as thermoelectric modules 8, are located on the external surface 5 of the inner portion 2. The presence of means for heat transfer, such as metal capes 9, allows for heat transfer between the thermoelectric modules 8 and the second or outer portion 3.

The cross section of the FGT is shown as a square cross section. However this is not limiting as polygon cross section having sides with straight or curved surfaces may be employed.

Figure 2 shows a schematic view of the FGT according to an embodiment of the invention, i.e. a cross section of the FGT along its length.

Thermoelectric modules 8 are located onto the external surface 5 on the inner portion 2. It can be notices that the inner portion 2 is not present along the all length of FGT 1. The end parts 10 and 11 of the FGT correspond to the outer portion 3. This allows for hermetical sealing by plate 12 so that a first fluid, e.g. water, flowing outside the FGT 1, in contact with the external surface 7 of the second or outer portion 3 cannot not get in contact with a second fluid, e.g. flue gas, flowing inside the FGT, in contact with the internal surface 4 of the first or inner portion 2. The plate 12 also allows for hermetical sealing of the

thermoelectric modules 8, so as to avoid contact between any of the fluids and the thermoelectric modules 8.

The structure of the FGT including end parts 10 and 11, where the inner portion 2 is not present, allows for connecting one of more FGTs, e.g. by welding or soldering producing FGTs of desired length.

Furthermore, this allows for easier insertion of FGTs in already existing boilers, exchanging the current FGTs with FGTs generating thermoelectric power.

The tube can be applied in any heat exchangers, such as biomass boilers, so as to employ part of the heat exchanged between the hot flue gas and the cooling water to produce electricity.

Figure 3 shows a schematic view of a CHP system 13 having arrays of FGTs inserted into a water boiler 14, according to an embodiment of the invention. From a furnace or a burner (not shown) hot gases are produced entering the water boiler through arrows 15 into correspondent FGTs 16. FGTs 16 are shown simplified as normal pipes, however they are the FGTs for generating of thermoelectric power according to the invention, such as the one shown in figures 1 and 2. Water enters the boiler through an inlet (not shown) following arrow 17 and exits the boiler, after being heated through heat exchange with the FGTs 16, through an outlet (not shown) following arrow 18. The gases cooled by heat exchange with the water exit the FGTs 16 and the boiler 14 following arrows 19 and reach the atmosphere via a chimney stack (not shown). A terminal box 20 may be located externally to the boiler and used to collect the electricity produced by the TE elements integrated into the FGTs 16 through wires contained into capillary tubes 21.

The terminal box 20 may be connected to the grid through a grid-tie inverter (not shown) so as to stream overproduction of electricity to the grid or draw electricity from the grid so as to operate the CHP system in a heating or cooling mode. The terminal box 20 may act as a control unit, or a further control unit may be added so as to switch between a first operating mode being an electricity generating mode and a second mode being a heating or cooling mode. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A flue gas tube (FGT) (1) for generation of thermoelectric power, said flue gas tube comprising :

- a first or inner portion (2) having an internal (4) and external (5) surface , said first or inner portion (2) having thermoelectric elements (8) located on said external surface (5);

a second or outer portion (3) having an internal (6) and external (7) surface, said second or outer portion (3) surrounding said first or inner portion (2);

means for transferring heat (9) between said thermoelectric elements 88) and said second or outer portion (3).

2. A flue gas tube for generation of thermoelectric power according to claim 1 wherein said thermoelectric elements (8) have a first side facing said external surface (5) of said first or inner portion and wherein said thermoelectric elements have a second side facing said internal surface (6) of said second or outer portion.

3. A flue gas tube for generation of thermoelectric power according to claim 2 wherein said second side of said thermoelectric elements (8) is opposite to the first side of said thermoelectric elements.

4. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein said thermoelectric elements (8) are distributed around said external surface (5) of said first or inner portion.

5. A flue gas tube for generation of thermoelectric power according to claims 2-4 wherein said first side of said thermoelectric elements (8) follows the outline of said external surface (5) of said first or inner portion.

6. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein said first or inner portion (2) has a circular cross section.

7. A flue gas tube for generation of thermoelectric power according to any of the claims 2-5 wherein said a first or inner portion (2) has a polygon cross section.

8. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein said first or inner portion (2) is adapted to be heated by a flue gas flow, said flue gas flowing in close contact with said internal surface (4) of said first or inner portion (2).

9. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein said second or outer portion (3) is adapted to be cooled by a flow of fluid, such as water in contact with said external surface (7) of said second or outer portion.

10. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein said first or inner portion (2) and said second or outer portion (3) are hermetically sealed, thereby no physical contact between the fluid flowing inside the first or inner portion (2) and the fluid flowing outside the second or outer portion (3) is present.

11. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein said internal surface (6) of said second or outer portion (3) comprises said means for transferring heat (9).

12. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein said means for transferring heat (9) are metal capes.

13. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein at least one of the end parts (10, 11) of said FGT for generation of thermoelectric power is characterized by the absence of said first or inner portion (2).

14. A flue gas tube for generation of thermoelectric power according to any of the preceding claims wherein the external diameter of said FGT is substantially the same along its entire length.

15. A combined heat and power (CHP) system (13) comprising one or more flue gas tubes for generation of thermoelectric power.

16. A CHP system (13) comprising one or more flue gas tubes (1) according to any of the preceding claims 1-14.

17. A CHP system according to claims 15-16, further comprising capillary tube (21) holding wires adapted to collect the electricity produced by said one or more flue gas tube (16).

18. A CHP system according to claims 15-17 wherein said CHP system is or comprises a water boiler (14).

19. A CHP system according to claims 15-18 further comprising a control unit (20) adapted to switch between a first operating mode being an electricity generating mode and a second mode being a heating or cooling mode.

20. An engine exhaust manifolds comprising one or more flue gas tube according to any of the claims 1-14.

21. A ship engine exhaust pipe comprising one or more flue gas tube according to any of the claims 1-14.

22. A chimney installation comprising one or more flue gas tube according to any of the claims 1-14.