A HEAT EXCHANGER SYSTEM AND METHOD FOR RECOVERING ELECTRIC POWER FROM A HEATED FLUID

TECHNICAL FIELD

The present invention relates to a heat exchanger system for recovering electric power from a heated fluid by use of thermoelectric elements.

BACKGROUND

Thermoelectic generator technology is based on the well-known concept that a temperature differential may be converted into electricity and vice versa. The Seebeck effect describes the conversion of a temperature differential directly into electricity. A thermoelectric generator is usually made from two different sheets of plates of a semi conducting material such as a semi conducting material. The sheets are electrical separated, but thermal connected at two end points. The construction of thermoelectric generators or elements is well known.

Thermoelectric generators or elements hold great promises for widespread use due to their solid state structure, silent operation, high reliability and long service life. Thermoelectric generators can produce electricity from many different heat sources, and this could be used in different energy conversion processes in order to increase overall efficiency, reduce pollutant emissions and lower costs.

Most of today's solar hot water (SHW) systems use plate collectors or vacuum tubes to absorb solar radiation and heat up water, which may contain antifreeze, or other fluids. In order to heat up water for household in a hot water tank, most systems use a coil made of copper, aluminum or steel. The water in the coil is connected to the plate collectors via a solar loop system, and the temperature of the coils increases above the temperature of the water in the tank, thereby operating as a heat exchanger to heat up water for household. In today's society, there is a great demand for electricity and heated water. Thus, there is a need for a system which can combine the generation of electric power and heating of a fluid, where the fluid can be household water. SUMMARY

It is an object of the present invention to provide a solution for a heat exchanger system, which combines the generation of electric power and heating of a fluid. The foregoing and other objects are achieved by the features of the independent claim. Further implementation forms are apparent from the dependent claims, the description and figures.

According to the present invention there is provided a heat exchanger system for recovering electric power from a heated fluid, said system comprising:

a high temperature fluid passage part for conducting a first fluid, said fluid passage part having an inner heat absorbing surface for contacting the first fluid, and an outer heat releasing surface; and

one or more thermoelectric elements or generators with a heat receiving side being thermally connected to the outer heat releasing surface of the fluid passage part, and a heat releasing or cooling side opposite the heat receiving side.

The heat exchanger system may further comprise:

a low temperature fluid container having a wall part with an inner surface encompassing the heat releasing or cooling sides of the thermoelectric elements and thereby the high temperature fluid passage part, whereby the heat releasing or cooling sides of the thermoelectric elements are arranged for thermally connecting to a second fluid contained within the wall part of the fluid container. Thus, according to the present invention there is also provided a heat exchanger system for recovering electric power from a heated fluid, said system comprising: a high temperature fluid passage part for conducting a first fluid, said fluid passage part having an inner heat absorbing surface for contacting the first fluid, and an outer heat releasing surface; one or more thermoelectric elements or generators with a heat receiving side being thermally connected to the outer heat releasing surface of the fluid passage part, and a heat releasing or cooling side opposite the heat receiving side; and

a low temperature fluid container having a wall part with an inner encompassing the heat releasing or cooling sides of the thermoelectric elements and thereby the high temperature fluid passage part, whereby the heat releasing or cooling sides of the thermoelectric elements are arranged for thermally connecting to a second fluid contained within the wall part of the fluid container. Thus, when the first fluid is conducted through the high temperature fluid passage part, the second fluid is contained in the fluid container, and the first fluid is heated to a higher temperature than the second fluid, then electric power is generated by the thermoelectric elements and heat is absorbed by the second fluid. The present invention covers one or more embodiments, wherein the system of the invention comprises a plurality of the thermoelectric elements or generators, where the thermoelectric elements or generators are electrical connected to each other by serial or parallel connection. It is preferred that at least two thermoelectric elements are connected to the outer heat releasing surface of the fluid passage part. It is also preferred that the outer heat releasing surface of the fluid passage part has one or more, such as at least two or three, plane surface parts. It is preferred that each or part of these plane surface parts is thermally connected to the heat receiving side of one or more thermoelectric elements. The outer heat releasing surface of the fluid passage part may have four plane surface parts, and each or part of the four plane surface parts may be thermally connected to the heat receiving side of one or more thermoelectric elements.

The present invention also covers embodiments, where the outer heat releasing surface of the fluid passage part has one or more curved surface parts, including embodiments where the heat releasing surface of the fluid passage part is tubular. It is preferred that each or part of these curved surface parts is thermally connected to the heat receiving side of one or more thermoelectric elements. Here, at least part of the thermoelectric elements may be curve shaped in order to fit the curve of a wall part. In order to obtain an effective heat transport between the first fluid and the thermoelectric elements or generators, it is preferred that the inner heat absorbing surface of the high temperature fluid passage part is at least partly formed as one or more heat sinks having fins extending into the inside of the fluid passage part.

The present invention also covers one or more embodiments, wherein one or more heat sinks are thermally secured to the heat releasing or cooling side of one or more of the thermoelectric elements. The one or more heat sinks may be encompassed by the outer wall of the low temperature fluid container.

It is preferred that one or more heat sinks are thermally secured to the heat releasing or cooling side of one or more thermoelectric elements, which thermoelectric elements are thermally connected to a surface part of the outer heat releasing surface of the fluid passage part. It is also preferred that for each surface part of the outer heat releasing surface of the fluid passage part, which surface part is thermally connected to one or more thermoelectric elements, one or more heat sinks are thermally secured to the heat releasing or cooling side of the thermoelectric elements.

The heat sinks being thermally secured to the cooling side of the thermoelectric elements may have fins extending outwards from the heat releasing or cooling side of the thermoelectric elements. It is within one or more embodiments of the invention comprises that the system comprises a plurality of thermoelectric elements, which thermoelectric elements are electrical connected to each other by serial or parallel connection.

It is preferred that one or more embodiments of the system of the invention comprises an electrical power outlet being electrical connected to the one or more thermoelectric elements.

The high temperature fluid passage part may be sealingly arranged within the low temperature fluid container, whereby the first fluid of the high temperature passage part cannot be mixed with the second fluid of the low temperature fluid container. It is preferred that the low temperature fluid container has a fluid inlet and a fluid outlet for inlet and outlet of the second fluid, or a combined fluid inlet/outlet for inlet and outlet of the second fluid.

The present invention also covers one or more embodiments, wherein several of said high temperature fluid passages part with thermoelectric elements are provided and encompassed by the wall part of the low temperature fluid container.

According to one or more embodiments of the invention, the system may further comprise a fluid heating system for heating a fluid and being connected to the high temperature fluid passage part(s) for passage of the heated fluid through the high temperature fluid passage part.

According to one or more embodiments of the invention, the system may further comprise a solar collector loop for containing a fluid to be heated by solar energy, wherein the solar collector loop is connected to the high temperature fluid passage part(s) for passage of the heated fluid from the solar collector loop through the high temperature fluid passage part.

According to the present invention there is also provided a method for recovering electric power from a heated first fluid and heating a second fluid having a lower temperature than the first fluid. The method may be performed by use of a heat exchanger system according to any one of the embodiments of the system of the invention comprising a low temperature container for holding the second fluid, said method comprising:

filling the second fluid into the low temperature container to thereby provide a thermal connection between the heat releasing or cooling sides of the thermoelectric elements and the second fluid;

heating the first fluid to a temperature being higher than the temperature of the second fluid; and

conducting the first fluid through the high temperature fluid passage.

It should be understood that it is within embodiments of the system and method of the present invention that both the first fluid and the second fluid are liquids. The system and method of the present invention also cover embodiments for which the first fluid is a gas and the second fluid is a liquid, or the first fluid is a liquid and the second fluid is a gas. Furthermore, the system and method of the present invention also cover embodiments for which the first fluid is a gas and the second fluid is a gas.

It should be understood that it is also within the inventive principles and embodiments of the present invention that the thermoelectric elements or generators being thermally connected to the outer heat releasing surface of the fluid passage part could also be turned 180°, with the cooling side being thermally connected to the outer heat releasing surface of the fluid passage part, and the heat receiving side being opposite the cooling side. For this to operate, the second fluid has to be heated to a higher temperature than the first fluid, and the fluid container containing the second fluid will be a high temperature container, while the fluid passage part for conducting the first fluid will be a low temperature fluid passage. Here, a fluid heating system may be connected to fluid inlet and outlets of the container for holding the second fluid. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram of heat exchanger system according to an embodiment of the invention; Fig. 2 shows a cut-through view of a high temperature fluid passage part with thermoelectric elements and heat sinks according to an embodiment of the invention;

Figs. 3a and 3b are cut-through and side views, respectively, of a high temperature fluid passage part with thermoelectric elements according to an embodiment of the invention;

Fig. 4 is a cut-through view of a low temperature fluid container encompassing a high temperature fluid passage part with thermoelectric elements and heat sinks according to an embodiment of the invention;

Figs. 5a and 5b are cut-through views of alternative embodiments of high temperature fluid passage parts according to embodiments of the inventions; and

Figs. 6a and 6b are perspective and exploded views, respectively, of a high temperature fluid passage part with thermoelectric elements and heat sinks according to an embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

Fig. 1 is a block diagram of heat exchanger system 100 according to an embodiment of the invention. The system 100 can be used to recover or generate electric power from a heated fluid. The system 100 comprises a heat exchanger 101 with an inlet part 102 and an outlet part 103 for conducting a first fluid through the heat exchanger 101. The heat exchanger 101 , which will be described in details in the following, holds thermoelectric elements 203, 303 for generating electric power when exposed to a temperature difference, and an electrical power outlet 107 is provided and connected to the thermoelectric elements 203, 303.

The heat exchanger 101 is fully encompassed by a low temperature fluid container 104, which has a fluid inlet 105 and a fluid outlet 106 for inlet and outlet of a second fluid into the container 104. The heat exchanger 101 with inlet 102 and outlet 103 is sealingly arranged within the low temperature fluid container 104, whereby when the first fluid is within the heat exchanger 101 , it cannot be mixed with the second fluid within the low temperature fluid container 104. A fluid heating system 108 may be provided for heating the first fluid, which can be conducted through the heat exchanger 101 via an outlet pipe 1 10 connected to the inlet part 102, a pipe 11 1 connected to the outlet part 103, and further connected to a pump 109, which is connected to the heating system 108 through a pipe 1 12. According to an embodiment of the invention the fluid heating system may comprise plate collectors to be heated by the sun, and the fluid heating system 108 including the pipes 1 10, 1 11 , 1 12 and pump 109 may be a solar collector loop arranged for conducting the first fluid being heated by the sun through the heat exchanger 101.

It is noted that a system comprising the heat exchanger 101 and the encompassing low temperature fluid container 104 is also within an embodiment of the present invention, and such a system can be used in connection with other fluid heating system than the system described in with Fig. 1.

Today, most solar collector systems have an efficiency of above 70% and some are up to 85%. This means that the solar collector also can increase the temperature of a fluid to a very high temperature, such as 180°C and up to 250°C. Vacuum tubes are able to reach 300°C. Water cannot normally be used in this kind of solar collector systems, and for water systems operating at normal pressure, the temperature should not increase above 120°C. But other fluids can be used for temperatures up to or above 200°C under normal pressure with regulation of pumps and valves. By regulation with pump and valves, it is possible to control the fluid temperature and keep it below 200°C, even at peak periods. Some common types of thermoelectric elements or generators, such as BiTe elements, work well at 200°C and are able to convert heat to electric power at this temperature with an efficiency from 4-6% and up to 10% under optimum conditions. Due to the generally low conversion efficiency around 5% of a thermoelectric element or generator, most of the heat, 95%, from the hot side of a thermoelectric element have to pass through the element and to be removed from the opposite and cold side of the element. From the cold side of the element the heat can be removed by use of air or water or another kind of fluid. Fig. 2 shows a cut-through view of a heat exchanger 101 according to an embodiment of the invention. The heat exchanger 101 has an inner high temperature fluid passage 201 , defined by wall parts 202 having an inner heat absorbing surface for contacting a fluid within the fluid passage 201 , and an outer heat releasing surface. A number of thermoelectric elements or generators 203 are provided with a heat receiving side being thermally connected to the outer heat releasing surface of the wall parts 202, and a heat releasing or cooling side opposite the heat receiving side. Thermally conductive wall parts 204 are arranged in thermally connection with the cooling side of the thermoelectric elements 203. The thermally conductive wall parts 204 operate as heat sinks, and it is preferred that they are provided with cooling fins 205. It is also preferred that the inner surface of the wall parts 202 are provided with cooling or heat absorbing fins 206, to thereby increase absorption of heat from the fluid in the fluid passage 201. The heat exchanger 101 illustrated in Fig. 2 is formed as a rectangle with four wall parts 202, where the outer heat releasing surface of each wall part 202 has a plane surface, and wherein each of the four plane surfaces is thermally connected to the heat receiving side of one or more thermoelectric elements 203. Also four corresponding wall parts or heat sinks 204, 205 are connected to the cooling side of the elements 203. The number of elements 203 arranged on plane surface depends on the dimensions of the thermoelectric elements 203 and the wall parts 202. The heat exchanger 101 may have a lengthwise orientation, and several thermoelectric elements 203 may be arranged below each other on each plane surface of the wall parts 202. The thermoelectric elements 203 are electrical connected to each other, either by a serial connection or a parallel connection or a combination of both serial and parallel connections. Electrical conductors for connecting the elements 203 may be arranged in the spare room 207 between the wall parts 202 and the wall parts 204. The wall parts 202 defining the fluid passage 201 are connected together in a fluid or water tight connection, and also the walls parts 204 are connected together in a fluid or water tight connection, to thereby leave the thermoelectric elements 203 in a dry space without fluid or water. The heat exchanger 101 illustrated in Fig. 2 is formed as a rectangle with four wall parts 202, but is also within embodiments of the invention that the heat exchanger 101 has other forms where the wall parts 202 may have one, two or three plane wall parts, such as a triangular shape. The present invention also covers embodiments, where the heat exchanger 101 has one or more curved wall parts 202, or where the wall parts 202 are tubular. When having curved or tubular wall parts for the heat exchanger 101 it is preferred that at least some of the thermoelectric elements 203 are curve shaped in order to fit the curve of a wall part 202.

By use of a heat exchanger system 100 as illustrated in Fig. 1 having a heat exchanger 101 as illustrated in Fig. 2, where the heat exchanger 101 is fully surrounded or encompassed by the low temperature fluid container 104, then electric power may be recovered or generated and a second fluid within the container 104 may be heated. This requires that a first fluid is heated to a temperature above or well above the temperature of the second fluid, and that the first fluid is conducted through the high temperature fluid passage 201 of the heat exchanger 101. Thus, the present invention also covers a method for recovering electric power from a heated first fluid and heating a second fluid having a lower temperature than the first fluid, where the second fluid is filled into the low temperature container 104 to thereby provide a thermal connection between the heat releasing or cooling sides of the thermoelectric elements 203 and the second fluid. This thermal connection between the cooling side of the elements 203 and the second fluid may be obtained via the thermally conducting heat sinks 204, 205. The first fluid is heated to a temperature being higher than the temperature of the second fluid, and the first fluid is conducted through the high temperature fluid passage 201 , in which heat is absorbed by the inner absorbing surface 206 of the wall parts 202 and conducted via the outer heat releasing surface of the wall parts 202 to the heat receiving side of the thermoelectric elements 203. Since the temperature of the second fluid is lower than the temperature of the heated first fluid, then a temperature difference is provided across the thermoelectric elements 203, and electric power is generated to be output at the electrical power output 107. At the same time, the second fluid act as a coolant of the elements 203 via the heat sinks 204, 205, and heat is transferred to the second fluid, which is thereby heated to a higher temperature. The first fluid may be heated by use of the fluid heating system 108, such as a solar collector system.

The present invention also covers embodiments in which several of the heat exchangers 101 are provided within the low temperature fluid container 104 and encompassed by the wall part of the low temperature fluid container 104. By using several heat exchangers 101 , more heat can be delivered to the second fluid via a higher number of thermoelectric elements 203, resulting in a higher electrical power output and an increase in the temperature of the second fluid.

Figs. 3a and 3b are cut-through and side views, respectively, of an alternative heat exchanger 300 according to an embodiment of the invention. The heat exchanger 300 is similar to the exchanger 101 of fig. 2, but without the wall parts or heat sinks 204, 205 covering the thermoelectric elements 203. The heat exchanger 300 has an inner high temperature fluid passage 301 , defined by wall parts 302 having an inner heat absorbing surface for contacting a fluid within the fluid passage 301 , and an outer heat releasing surface being thermally connected to a number of thermoelectric elements or generators 303. The side view of Fig. 3b also shows fluid inlet 307 and fluid outlet 308 for the exchanger 300. The heat exchanger 300 has a lengthwise orientation with six thermoelectric elements 303 be arranged below each other on each plane surface of the wall parts 302

Fig. 4 is a cut-through view of a low temperature fluid container 104 encompassing a heat exchanger 101 according to an embodiment of the invention. The heat exchanger is similar to the exchanger 101 of Fig. 2, and holds a high temperature fluid passage part with thermoelectric elements and heat sinks.

Figs. 5a and 5b are cut-through views of alternative embodiments of high temperature fluid passage parts according to embodiments of the inventions. The fluid passage part 501 of Fig. 5a is defined by wall parts 502 and holds heating fins 503 on each wall part 502, with the fins 503 having different length. The fluid passage part 504 of Fig. 5b is defined by wall parts 505 and holds heating fins 506 on two of wall parts 505, with the fins 505 having equal length.

Figs. 6a and 6b are perspective and exploded views, respectively, of a heat exchanger 600 with a high temperature fluid passage part with thermoelectric elements and heat sinks according to an embodiment of the invention. The heat exchanger 600 is similar to the exchanger 101 of Fig. 2. Turning to Fig. 6b, 601 shows the high temperature fluid passage part, defined by wall parts having an inner heat absorbing surface for contacting a fluid within the fluid passage, and an outer heat releasing surface with a number of thermoelectric elements or generators provided with a heat receiving side being thermally connected to the outer heat releasing surface of the wall parts. The fluid passage part 601 is shown out of scale when compared to the other components of Fig. 6b. Thermally conductive wall parts 602a,b,c,d with heat fins are shown, which wall part are used to be in thermally connection with the cooling side of the thermoelectric elements of the fluid passage part 601. Fig. 6b also shows a fluid passage bottom part 603 and a fluid passage top part for closing the bottom and top, respectively, of the fluid passage part 601.

In principle the wall parts 202 defining the inner high temperature fluid passage 201 and the wall parts 204 defining the heat sinks on the cooling side of the thermoelectric elements can be made of any material that can absorb and transport heat. It may be a material based on metals, polymers or ceramics having good heat conductive properties. When formed by a metal, the metal may for example be aluminum or copper. The thermoelectric elements or generators 203 may be attached to the wall parts 202 and 204 by use of heat conductive and heat resistive glue.

The thermoelectric elements or generators 203 may be commercially available thermoelectric generators with dimensions such as 40x40 mm ² . The heat exchanger system 100 of the present invention may contain at least one heat source 108 for heating the first fluid, but it may also comprise several heat sources. Different heat sources may be used, such as stoker boilers, heat plants, or aggregates for surplus heat harvesting from industry. The heat source 108 for heating the first fluid may be designed for heating a liquid, but may also be designed for heating a gas. Thus, the present invention covers embodiments where the first fluid is a liquid and other embodiments where the first fluid is a gas. It is preferred that the second fluid is a liquid, but the present invention also cover embodiments for which the second fluid is a gas. The electrical power output 107 of the heat exchanger system 100 may be connected to an electrical inverter or converter, which can be connected to a power grid or build up as a stand-alone system. The system 100 may be provided with one or more controllers for optimizing the effect of hot water production and/or optimal electrical power generation.

It should be understood that it is also within the inventive principles and embodiments of the present invention that the thermoelectric elements or generators of the heat exchanger 101 , 300 could also be turned 180°, with the cooling side being thermally connected to the outer heat releasing surface of the fluid passage part, and the heat receiving side being opposite the cooling side. For this to operate, the second fluid has to be heated to a higher temperature than the first fluid, and the fluid container containing the second fluid will be a high temperature container, while the fluid passage part for conducting the first fluid will be a low temperature fluid passage. Here, the fluid heating system 108 may be connected to fluid inlet and outlets of the container for holding the second fluid.

Although the present invention has been described with reference to specific features and embodiments thereof, it is evident that various modifications and embodiments can be made thereto without departing from the spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded as an illustration of the invention as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present invention. The term "comprising" as used in the appended claims does not exclude other elements or steps. The term "a" or "an" as used in the appended claims does not exclude a plurality.

Any method described herein and in the claims may be supplemented by any features of the apparatuses described herein and in the claims in terms of method features.