The present invention relates to a combined hydroelectric- thermoelectric power plant in which the thermoelectric power plant utilizes waste heat generated in the hydroelectric power plant to generate electricity.

Electricity production based on renewable resources produces waste heat, which lowers the efficiency of conversion of primary energy into electrical energy and the electrical power output. For applications where the primary energy source is limited by natural factors, such as precipitation in the drainage area in the case of a hydroelectric power plant, the power output can only be increased by increasing the overall conversion efficiency. In hydroelectric power plants energy losses originate from fluid-dynamic dissipation in turbines and generators, friction in bearings, vibrations, electrical resistances in generators and conductors etc.

Minimizing such losses has been the subject of decades of engineering work and most machine components have reached a very high level of maturity. Few options for potential improvements remain that can be economically exploited.

Instead of reducing waste heat production, waste heat can be reused to generate additional electricity, which further increases the overall conversion efficiency. In hydroelectric power plants, significant sources of waste heat are fluid bearings of shafts of water turbines and electrical generators. For hydroelectric power plants with several hundred megawatts installed power the thermal losses in such bearings can amount to several hundred kilowatts. The waste heat is carried away from the bearings by the lubricant oil, which is forced by pumps to constantly flow through the bearings. In order to keep the lubricant oil at a controlled temperature it needs to be cooled. This is typically done in heat exchangers in which the excess heat is transported to water that is cooler that the oil. Such water is abundant in hydroelectric power plants. The water source typically provides water that is under pressure, or moving, or both, which may allow to operate the cooling water system without pumps .

Recovery of waste heat by thermoelectric generators is known for car engines (DE-A-102008008370) , having a cooling system with split closed-circuit circulation. Reusing waste heat in heat engines coupled to electrical generators in order to produce electricity is uneconomical unless the waste-heat-carrying fluid is sufficiently hot. The aforementioned lubricant oil has typically a temperature of 50-70°C. This is at the lowest limit admissible for Organic Rankine Cycle heat engines, which would therefore yield very low power output and hence high costs of the produced electricity. The integration of Organic Rankine Cycle heat engines and coupled electrical generators in existing hydroelectric power plants is a complex task. Furthermore, a failure of the heat engine could compromise the operation safety of the hydroelectric power plant, since the excess heat produced in fluid bearings may no longer be efficiently disposed . A thermoelectric converter for heat-exchanger according to US-A-5959240 is characterized by a plurality of thermoelectric elements, made of P-type and N-type semiconductors, provided to a heat conduction wall of a heat exchanger for recovering waste heat of an incinerator and so on as to perform a power generation. An insulator is positioned at the hot side of the thermoelectric power generator is faced to a hot fluid and a second insulator poisoned at the cold side of the thermoelectric power generator is faced to a cold fluid, so that a temperature difference over 200° can be obtained.

The present invention relates to a combined hydroelectric- thermoelectric power plant, which comprises a hydroelectric power plant and at least one thermoelectric generator where said hydroelectric power plant is thermally coupled to at least one of said thermoelectric generators. The object of the invention is solved by features of claim 1. Further specification is disclosed by depending claims.

Especially it relates to a combined hydroelectric- thermoelectric power plant in which a liquid lubricant heated by dissipative losses in fluid bearings of the shafts of water turbines or electrical generators in said hydroelectric power plant is used as a heat source for one or more of said thermoelectric power plants and the water that propels said turbines or generators (or passing at the side of the power plant or provided by another reservoir) is used as a heat sink for one or more of said thermoelectric power plants.

The present invention improves the efficiency of hydroelectric power plants by reusing low grade waste heat generated in fluid bearings of water turbines and electrical generators without the disadvantages of the aforementioned approach using heat engines. The present invention aims at increasing the efficiency of hydroelectric power plants by reusing waste heat for electricity production in a thermoelectric power plant, which is thermally coupled to the hydroelectric power plant. Thermoelectric generators have been previously proposed as a means to partially transform waste heat carried by a hot fluid (gas, liquid, smoke) into electrical power. Thermoelectric generators convert heat directly into electrical energy, that is, an intermediate conversion of heat to potential and subsequently mechanical energy as in heat engines does not take place. Thermoelectric generators make use of a phenomenon known as the Seebeck effect, which, although present in all materials, is only technologically relevant in certain metals and semiconductors: If a temperature difference is applied across two points of an object made of such material an electromotive force separates charge carriers along the direction of the imposed temperature gradient within the object. This results in a difference of the electrical potential between the two points. If the two points are then connected to an electrical load, an electrical current flows and electrical power is generated. Thermoelectric generators can operate with low temperature heat sources. Furthermore they do not have any moving parts, can be produced in batch processes, and can be easily integrated in many machines, such as heat exchangers, in which temperature gradients are present. The output voltage and power of single thermoelectric elements is low. For this reason, thermoelectric generators typically comprise a plurality of thermoelectric elements electrically connected in series.

The output power of thermoelectric generators is proportional to the square of the temperature difference across the thermoelectric elements but also depends on the internal electrical resistance of the thermoelectric generator, which comprises several of those elements. The material, the geometry, and the arrangement of the thermoelectric elements in the thermoelectric generator as well as the thermal conductivity between the thermoelectric generator and the heat source and sink, respectively, are thus crucial for the thermoelectric generator' s output power and thermal efficiency .

In order to exploit the full available temperature difference between the heat source and sink, the temperature should drop mainly across the thermoelectric elements, whereas the temperature drop across all other parts of the device should be minimized. Furthermore, the transport of heat to the hot side and away from the cold side of the device needs to be maximi zed .

In the present invention hydroelectric and thermoelectric power plants are thermally coupled. The thermoelectric power plant is integrated in the heat exchanger that is used for cooling lubricant oil from fluid bearings in the hydroelectric power plant. These heat exchangers provide a favorable heat source and sink for thermoelectric energy production for several reasons: 1. The temperature difference is approximately 50°, with the oil and water temperatures being approximately 60 °C and 10°C, respecti ely. Such small temperature differences can be used, unexpectally for energy production, allowed by inventive arrangement.

2. Oil is a fluid that can effectively transport heat by means of convection to the hot side of the thermoelectric generator.

3. Water is a fluid that can effectively transport heat by means of convection away from the cool side of the thermoelectric generator.

4. The temperature range allows using state-of-the-art thermoelectric materials, such as Bismuth-Tellurite- based semiconductors.

5. The hot oil and cold water are in the heat exchangers in close spatial proximity and only separated by a thin heat-conducting wall. A thermoelectric generator can be easily integrated in the heat-conducting wall in standard heat exchanger designs.

Moreover, the water used as heat sink flows through an open system and unused waste heat is disposed with the outflow of water. Furthermore, integrating thermoelectric generators in the heat-conducting wall of the oil-water heat exchanger does not compromise the operation safety of the hydroelectric power plant. Effective heat exchange between oil and water is guaranteed irrespective of whether the thermoelectric generator is operational or not.

The following is a brief description of an embodiment of the invention .

Fig.: 1: Schematic diagram of a combined hydroelectric- thermoelectric power plant; Fig. 2: Cross-sectional view of the lubricant oil-to-water heat exchanger;

Fig. 3: Enlarged cross-sectional view of the thermoelectric generator;

Fig. 4: Perspective view of a non-planar thermoelectric heat-conducting wall;

Fig. 5: Enlarged cross-sectional view of a non-planar thermoelectric heat-conducting wall.

Fig. 1 is a schematic diagram depicting one embodiment of the combined hydroelectric-thermoelectric power plant of the present invention. Other water sources or ways of connecting pipes are possible too. A relevant oil pump and pressure- reducing elements for a cooling water system not added to the drawing .

A hydroelectric power plant 1 partially converts potential and kinetic energy of water into electrical energy 6. Cold or cool water enters the hydroelectric power plant 1 through a pipe system 3 with high potential and/or kinetic energy flows through a turbine-generator assembly 8, and leaves the hydroelectric power plant 1 through a second pipe system 4 with less potential and/or kinetic energy. The turbine- generator assembly 8 converts strictly less than 100% of the energy difference between in- and out-flowing water into electrical energy 6. During electricity generation, the lubricant oil of fluid bearings 10 of the common shaft 9 of the turbine-generator assembly 8 in the hydroelectric power plant 1 is heated by dissipative losses above the temperature of the cold water. In the closed pipe system 5 the lubricant oil circulates between the fluid bearings in the hydroelectric power plant 1 and a heat exchanger 11 in the thermoelectric power plant 2. Water with high potential and/or kinetic energy enters the thermoelectric power plant 2 through a pipe system 3 and leaves it through the second pipe system 4 with less energy. The thermoelectric power plant converts a fraction of the heat exchanged between the hot lubricant oil and the cold water in the heat exchanger 11 into electrical energy 7, thereby increasing the overall efficiency of a stand-alone hydroelectric power plant.

Fig. 2 shows a cross-sectional view of the heat exchanger 11 according to the invention mentioned above. The cross- sectional view of the lubricant oil-to-water heat exchanger discloses the placement of thermoelectric heat-conducting walls within the heat exchanger.

The hot lubricant, for example lubricant oil enters the heat exchanger 11 through an inflow opening 12 and leaves it through an outflow opening 15. The inflow opening 12 for the lubricant oil and the outflow opening 15 for the lubricant oil are connected to the closed pipe system for the lubricant oil. The cold water enters the heat exchanger 11 through an inflow opening 13 and leaves it through an outflow opening 14. The inflow opening 13 for the cold water is connected to a first pipe system 3 and the outflow opening 14 for the cold water is connected to a second pipe system 4. Within the heat exchanger 11, one or more thermoelectric heat-conducting walls 16 separate the hot oil and the cold water. Heat- conducting wall 16 may be thermoelectric generator or part of it . Fig. 3 shows an enlarged cross-sectional view of a thermoelectric heat-conducting wall 16. The thermoelectric generator is integrated in the heat-conducting walls of the lubricant oil-water heat exchanger. Other variants, e. g. fins on the surface of the thermoelectric generator to further increase heat exchange may be selected too.

The thermoelectric heat-conducting wall comprises a plurality of thermoelectric elements 21 and 22, which are electrically connected in series by a first set of interconnects 23 and a second set of interconnects 24. It furthermore comprises a first wall 25, which faces the hot lubricant oil flowing through the heat exchanger 11, a second wall 26, which faces the cold water flowing through the heat exchanger 11, and a matrix material 27. The series circuit preferably comprises two different types of thermoelectric elements made of materials that have Seebeck coefficients of opposite sign. A pair comprising a first type of thermoelectric element 21, preferably made of a p-type Bismuth-Tellurite semiconductor (may be manufactured from other sources, like antimony also) , and a second type of thermoelectric element 22, preferably made of a n-type Bismuth-Tellurite semiconductor is connected by an electrically and thermally conductive interconnect 23, which is in thermal contact with the first wall 25. Many of such pairs of thermoelectric elements are connected by electrically and thermally conductive interconnects 24, which are in thermal contact with the second wall 26. The first wall 25 is made of a single material or a combination of materials with very low combined thermal resistance, in which all materials in direct contact with interconnects 23 are electrical insulators, preferably a polymer, diamond, or ceramic. The second wall 26 is made of a single material or a combination of materials with very low combined thermal resistance, in which all materials in direct contact with interconnects 24 are electrical insulators, preferably a polymer, diamond, or ceramic. The matrix material 27 is an electrical insulator with very high thermal resistance, preferably a polymer or gas.

Fig. 4 shows a non-planar thermoelectric heat-conducting wall 16. Fig. 5 illustrates an enlarged cross-sectional view of the non-planar thermoelectric heat-conducting wall. In a preferred embodiment of the thermoelectric heat-conducting wall the matrix material 27 is an elastic polymer and the first wall 25 and the second wall 26 are dimensioned to be mechanically flexible such that the thermoelectric heat- conducting wall 16 can be bend into non-planar shapes. This allows adapting the thermoelectric heat-conducting wall's shape to accommodate common heat exchanger designs, such as pipe heat exchangers or plate heat exchangers.

The terms "heat conducting wall" and "thermoelectric generator" as used in the description refer to the same object because their functions are combined in a single object according to the invention.

Reference signs

1 hydroelectric power plant

2 thermoelectric power plant

3 pipe system

4 pipe system

5 pipe system

6 electrical energy

7 electrical energy

8 turbine-generator assembly 9 shaft

10 bearing

11 heat exchanger

12 inflow opening

13 inflow opening

14 outflow opening

15 outflow opening

16 heat-conducting wall

21 thermoelectric element

22 thermoelectric element 23 interconnects

24 interconnects

25 wall

26 wall

27 matrix material