The invention concerns a fired heat exchanger with a thermoelectric generator, designated in particular for central heating and/or domestic water installations.

The market today expects new technical solutions and devices able to serve a growing number of functions, while at the same time getting more and more universal. Moreover, the entire industry is taking intense effort to develop new methods of generating electric energy. The majority of the micro-generating devices currently available in the market are based on principle of conversion of some form of kinetic energy to electric energy. Known are solutions employing different kinds of reciprocating engines, Stirling engines, turbines, etc. The solutions are characterised by

substantial limitations resulting from the high level of their complexity which entails high production cost, as well as by low reliability and high costs related to ensure the required maintenance.

Known are thermoelectric devices employing the Seebeck effect, which enable generation of electric energy. The devices contain

thermoelectric technical means where the temperature difference between specific areas enables generation of electric energy.

The current state of the art in the area of thermoelectric coatings implies that no solution has been found yet which would provide instruction on how to apply thermoelectric composites in a comprehensive manner and use them in devices. There are, however, some scientific publications which discuss specific methods of manufacturing the semiconductor components only, intended for thermocouples.

It should also be mentioned here that attempts are made to lay layers of bismuth telluride under the thermal spray method. Because of its high imprecision, however, the method proves poorly effective, and the produced systems of poor performance. Because of the open structure of the semiconductor junctions of the 'p' and 'n' type, the systems cannot be used safely.

Currently, the market is saturated only as concerns prefabricated

thermocouples. Their fairly substantial limitations stem from their absolute lack of formability and their standardized sizes.

Recently, however, a new application group has been developed for the electricity generating thermocouples: they are used to generate electricity in the so-called hiking power generators. The devices employ heat energy to enable charging small receivers. Because of the size of the target market, however, and the technical solutions used one can hardly speak about industrial scale here. The heretofore experimental attempts at using prefabricated thermocouple modules do not stand any true chance of commercialization because of their production and size-related limitations. Another factor which effectively hampers the development of this specific industry branch is the high unit cost of thermocouples.

Known from GB 2 451 521 A is a portable water heater having a combustion chamber, where the heater burns fuel to ensure the source of heating. The combustion chamber is fitted with a fuelled burner and blower supplying air to the burner. The blower is fed with power from a

thermoelectric generator which contains a Peltier-Seebeck device having one side hot from the heat produced by the burner and one side cold, cooled by the air supplied by the blower. The fuel is delivered to the burner gravitationally from a petrol can. The thermoelectric generator contains a set of semiconductor modules made of bismuth telluride (BI-TE) connected in series, where the modules produce electric voltage whenever a

temperature difference occurs throughout the stack. The water heater does - J - not need to be energy fed and can be used in field kitchens, on a boat, etc.

The purpose of the invention is to improve the energy efficiency of heat exchangers, and in particular to provide a fired heat exchanger intended especially for central heating and/or domestic water installations, which would at the same time generate electric energy used for supplying power to additional electronic accessories or external devices, or fed back to the power grid. In addition, the purpose of the invention is to solve the problems discussed in the first paragraphs hereof by providing technical means which would overcome low efficiency of thermoelectric devices and reduce their production costs.

The purpose has been achieved by developing a heat exchanger fitted with thermoelectric technical means which employ the heat produced in the fuel burning process to generate electric energy.

A fired heat exchanger intended for central heating and/or domestic water installations, having a thermoelectric generator, the functioning of which is based on the Seebeck effect, and fitted with an external jacket, combustion chamber, and flame pipes fixed in the sieve bottoms, according to the invention is characterised in that the thermoelectric generator takes the form of a thin thermoelectric coating applied under the PVD (Physical Vapour Deposition) technology to these elements of the heat exchanger which are in thermal contact with combustion gases, where the coating contains a thermoelectric layer having two semiconductor sub-layers 'p' and 'n' which do not contact each other and are interconnected in series with thin-layered conducting elements fitted with connection ends to evacuate the generated electric energy, and where the coating is electrically insulated on both sides with layers of electrical insulator based on inorganic oxides. Preferably, the thickness of each semiconductor sub-layer 'p' and 'n' ranges from Ι μιη to ΙΟμιτι.

Preferably, the width of each semiconductor sub-layer 'p' and 'n' ranges from 0.1mm to 2mm.

Preferably, the thin-layered conducting elements are made of copper and preferably their thickness falls within the range from Ιμιη to 5μιη.

Preferably, the electrical insulator layers are produced based on A1 ₂ 0 ₃ or Si0 ₂ or MgO

Preferably, the cold side of the semiconductor sub-layers 'p' and 'n' of the thermoelectric layer is cooled with water supplied to the heat exchanger. Preferably, the thermoelectric layer is applied to the outer side surface of the combustion chamber and/or to the outer side surface of the flame pipes and/or to the bottom surface of the upper sieve bottom, and/or to the top surface of the lower sieve bottom and/or to at least one side of at least one sieve baffle through which the flame pipes run, and/or to the inner surface of the external jacket.

Preferably, the semiconductor sub-layers 'p' and 'n' of the thermoelectric layer are formed into alternating rings.

The invention enables additional production of electric energy obtained using the thermoelectric coating embedded in the exchanger. Besides heat energy, the heat exchanger, with its embedded thermoelectric coating, generates electric energy in accordance with the Seebeck effect. The advantage of the solution according to the invention lies in e.g. the absence of any movable elements, thanks to which the heat exchanger maintenance will be reduced. Moreover, thanks to the absence of any movable elements, the electric energy generation process is totally sound-free and vibration- free, which will allow installation of the device in higher number of locations, while at the same time minimizing arduousness of the device to the people living and working close to it; it will also ensure scalability of use, ranging from micro-applications to high powers, as well as scalability of production in automated production technology easy to be copied at a large scale. In addition, thanks to taking advantage of the thermoelectric effect, it is possible to generate electricity directly from heat energy, bypassing the conversion to kinetic energy, which will translate to high reliability and lower maintenance costs.

The electric energy produced can find different applications, e.g. in:

- supplying power to electrical sub-assemblies of a complete device, such as control elements or pumping systems (improving energy efficiency)

- building autonomous units independent of external power supply

- delivering energy back to the local power network (reducing the

household demand for electric energy)

- delivering energy back to the power grid (micro sources, prosumer, citizen energy)

These and other characteristics of the invention will be clear from, the following description of a preferential form of embodiment, given as a non- restrictive example, with reference to the attached drawings wherein:

Fig. 1 shows the combustion chamber with flame pipes in perspective view; Fig. 2 illustrates the external jacket of the heat exchanger;

Fig. 3 shows the heat exchanger in vertical section;

Fig. 4 depicts a fragment of the vertical section of the combustion chamber; Fig. 5 shows a fragment of the heat exchanger as in Fig. 6;

Fig. 6 presents the heat exchanger in perspective view with the inside of the combustion chamber shown;

Fig. 7 illustrates the thermoelectric coating in cross-section.

A fired heat exchanger intended for central heating and/or domestic water installations has a combustion chamber 6 on top, flame pipes 7 fixed in the sieve bottoms: upper sieve bottom 8a and lower sieve bottom 8b, as well as transverse baffles 9, where the entire heat exchanger is encased in an external jacket 10 fitted with a stub pipe 11 which supplies cold water and a stub pipe 12 heated evacuating water. Applied to the outer side surface of the combustion chamber 6 under the PVD technology by e.g. evaporation, laser ablation, magnetron sputtering, filtered cathodic arc deposition, or electron beam PVD, is coating 13 which contains a thermoelectric layer 1 having two semi-conductor sub-layers 'p' and 'n' which do not contact each other, the thickness di of which ranges from Ιμιη to ΙΟμιη, preferably amounting to 5 μηι, where the sub-layers are formed into alternating rings 'p' and 'n' of the width s falling within the range from 0.1mm to 2mm, preferably amounting to 1mm, and where the rings are interconnected in series with thin-layered conducting elements 2a, 2b made of copper, the thickness d ₂ of which ranges from 1 to 5μιη, preferably amounting to 3μι ^η , and where the conducting elements are fitted with end connections 4, 5 to evacuate the generated electric energy. The thermoelectric layer 1 is electrically insulated on both sides with layers 3a, 3b of electrical insulator based on inorganic oxides, particularly A1 ₂ 0 ₃ or Si0 ₂ or MgO. The cold side of the semiconductor layers 'p' and 'n' of the thermoelectric layer 1 is cooled with water supplied to the heat exchanger.

First, the process of applying the electrical insulator layer 3a should be performed in the technological chamber, thanks to which the thermoelectric layer will be electrically independent of the base. The insulating layer must be homogenous and continuous in structure. It will make it highly resistant to avalanche breakdown. Then, appropriately located thin-layered conducting elements 2a should be made, which will form the basis and serve as electrical connection between the semiconductor sub-layers. There are many materials which can serve the function. Copper seems a good choice because of the ease of deposition and good conductivity. The most important components of the thermoelectric coating 1 are two

semiconductor layers of the 'p' and 'n' types which can be made of the following material groups: lead telluride; tin selenide; telluride, antimony, and bismuth compounds; inorganic occlusion compounds; skutterudite; semi-Heusler compounds; silicon compounds; and germanium compounds. Thanks to the interconnection between two different semiconductor layers 'ρ' and 'n' achieved using a conducting layer it will be possible to achieve the flow of current whenever the composite is exposed to temperature difference. The material should be selected according to the criteria of e.g. expected performance, the ensuing thermoelectric efficiency (ZT), and the anticipated range of temperatures to which the thermoelectric coating 13 will be exposed during operation. In the next step in the process of producing a thermoelectric layer, the appropriately located thin-layered conducting elements 2 b should be produced to close the electrical circuit of the thermoelectric layer 1. The last step consists in the producing of the second layer 3 b of the electrical insulator.

The cold side of the semiconductor sub-layers 'p' and 'n' of the thermoelectric layer 1 is cooled with water flowing inside the external jacket 10. The thickness of the coating 13 is 50 μηι or less.

In the second exemplary embodiment of the invention the coating 13 having a thermoelectric layer 1 is applied to the outer side surface of the flame pipes 7 of the heat exchanger described in example one.

In the third exemplary embodiment of the invention, the coating 13 having a thermoelectric layer 1 is applied to the bottom surface of the upper sieve bottom 8a and to the top surface of the lower sieve bottom 8b of the heat exchanger described in example one.

In the fourth exemplary embodiment of the invention, the coating 13 having a thermoelectric layer 1 is applied to both sides of the sieve baffles 9 through which the flame pipes 7 of the heat exchanger described in example one run.

In the fifth exemplary embodiment of the invention, the coating 13 having a thermoelectric layer 1 is applied to the inner surface of the external jacket 10 of the heat exchanger described in example one. In the heat exchanger according to the invention fuel is burned in the combustion chamber 6. The thus-produced heat is received by the heated agent, i.e. water which flows through the heat exchanger. The hot side of the thermoelectric layer 1 having semiconductor layers 'p' and 'n' interconnected in series stays in thermal contact with hot combustion gases generated in the process of fuel burning in the combustion chamber 6, whereas the cold side of the same layer stays in thermal contact with the heated agent, i.e. water which flows through the heat exchanger.

In accordance with Seebeck's theory, the difference in temperature resulting therefrom triggers orderly movement of charges in the semiconductor elements 'p' and 'n' contained in the thermoelectric layer 1.

Due to the series connection between the elements, a difference of potential occurs between the outermost connection points, i.e. ends 4, 5.

The thus-produced energy can be used to supply power to electronic accessories or external devices, or delivered back to the power grid.