The present invention relates to thermoelectric power generation, and particularly to methods and systems for domestic thermoelectric power and heat generation.

Thermoelectric materials are those which exhibit the thermoelectric effects known as the Peltier, Seebeck and Thomson effects. Such materials are capable of converting a temperature difference into an electrical potential via the Seebeck effect. A thermocouple comprises a pair of different thermoelectric materials joined at two locations. When a temperature gradient exists across the materials (e.g. when the two junctions are at different temperatures) a voltage develops.

A typical semiconductor thermocouple 10 is shown in Figure 1. The thermocouple comprises a pair of p-type and n-type semiconductor materials (12, 14). The thermocouple is shown sandwiched between two thin ceramic wafers 16 which are electrically- insulating and thermally-conducting. Energy from a heat source 18 can be converted into electricity by the thermocouple in the presence of a temperature gradient 20.

Thermocouples are robust, light weight, and reliable, as they are solid state and have no moving parts. These properties mean that they are widely used for temperature sensing and temperature-based control systems. However, they are less widely used for power generation, as the amount of electricity generated per thermocouple is small (in the order of micro Volts), and their efficiency is relatively low. A plurality of thermocouples may be connected electrically together into an array of thermocouples known as a thermopile.

It is becoming increasingly popular to generate electricity on a small scale (e.g. in a home, apartment, apartment block, office, etc). Such small scale electricity generation is described herein as 'domestic' . Current domestic power generation mostly relies on photovoltaic panels, which are able to convert solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. The photovoltaic effect is the generation of a voltage in a material exposed to light where electrons are ejected from the material's surface in the presence of sufficient light radiation. Photovoltaic panels are perceived as being environmentally friendly as they do not require fossil fuel to operate. However, significant carbon dioxide is emitted during the production of the silicon cells on which the operation of the photovoltaic panels depends. In addition, such panels tend to have a relatively inflexible design, and relatively high cost. Furthermore, their operation is dependent on the presence of sufficient solar radiation and so they can be unreliable services of power.

An object of the present invention is to provide alternative systems and methods for domestic power generation.

According to a first aspect of the invention there is provided a thermoelectric power generation system comprising a plurality of thermoelectric generating elements having a first region and a second region and being operable to generate electricity when a temperature gradient exists between the first and second regions, the system comprising a first thermoelectric subsystem provided with one or more of the thermoelectric generating elements, and a second thermoelectric subsystem provided with one or more thermoelectric generating elements, and wherein the second region of one or more thermoelectric generating elements of the first subsystem is in thermal communication with the first region of one or more thermoelectric generating elements of the second subsystem.

The system may be arranged so that, in use, the first region of each respective thermoelectric generating element is hotter than the second region of that thermoelectric generating element, and so that the first subsystem operates at a generally higher temperature than the second. Waste heat output from the first subsystem is thus used as input heat in the second subsystem, allowing power to be generated from the waste heat by the second subsystem, so increasing the efficiency of the thermoelectric power generation.

A heat transfer system may be provided between the second region(s) of the thermoelectric generating element(s) of the first subsystem and the first region(s) of the thermoelectric generating element(s) of the second subsystem. The heat transfer system may comprise one or more heat pipes. The system may comprise one or more further thermoelectric subsystems (e.g. a third subsystem, a fourth subsystem, etc), each further subsystem being arranged so that the first regions of the thermoelectric generating elements of that subsystem are in thermal communication with the second regions of the thermoelectric generating elements of one or more of the preceding subsystems.

Each subsystem may comprise a plurality of thermoelectric generating elements. The thermoelectric generating elements may comprise thermocouples. Each subsystem may comprise a heat collection mechanism, which may be operable to supply heat to the first region(s) of the thermoelectric generating element(s) of that subsystem. The heat collection mechanism may be operable to supply heat from more than one source (e.g. solar radiation and/or boiler exhaust gases). Each subsystem may comprise a heat removal mechanism, which may be operable to remove heat from the second region(s) of the thermoelectric generating element(s) of that subsystem.

The heat transfer system may comprise the or part of the heat removal mechanism of the first subsystem and the or part of the heat collection mechanism of the second subsystem. The heat removal mechanism of the first subsystem may comprise the or part of the heat collection mechanism of the second subsystem. The heat removal mechanism of the first subsystem may comprise one or more heat pipes. The heat collection mechanism of the first subsystem may comprise a first solar collector. The first solar collector may comprise a lens. The first solar collector may comprise a Fresnel lens. The first solar collector may comprise a convex lens, and may be part spherical. The heat collection mechanism of the first subsystem may alternatively or additionally comprise a hot fluid interface, which may be a conduit connectable to an exhaust outlet of a domestic boiler.

The first subsystem may comprise an oil tank, and may comprise a heat exchanger disposed in the oil tank. The heat collection mechanism may be operable to heat the oil tank, possibly via the heat exchanger. The oil tank may heat the first region of the thermoelectric generating elements of the first subsystem.

The heat exchanger may be in thermal communication with the hot fluid conduit. The heat exchanger may be mounted on the hot fluid conduit.

The heat collection mechanism of the second subsystem may comprise a second solar collector. The second solar collector may comprise a lens. The second solar collector may comprise a plurality of lenses, which may be linear lenses. The lens or lenses of the second solar collector may be arranged in a generally annular array. The first solar collector may be disposed within the outline formed by the array, when the system is viewed from the expected incident direction of solar radiation.

The heat collection mechanism of the second subsystem may alternatively or additionally comprise a hot fluid interface, and may be connectable to an exhaust outlet of a domestic boiler. The hot fluid conduit of the second subsystem may be in fluid communication with, and may be a continuation of, the hot fluid conduit of the first subsystem. The heat collection mechanism of the second subsystem may comprise one or more heat pipes in thermal communication with the first subsystem.

The second subsystem may comprise an oil tank, and may comprise a heat exchanger disposed in the oil tank. The heat collection mechanism(s) may be operable to heat the oil tank, possibly via the heat exchanger. The oil tank may heat the first region of the thermoelectric generating elements of the second subsystem.

In use, the first subsystem may be disposed above the second subsystem. The first subsystem may be stacked above the second subsystem.

The heat removal mechanism of the second subsystem may comprise a cold fluid conduit, and may be connectable at a first end to a cold water supply, such as a domestic water pipe. The cold fluid conduit may be connectable at a second end to a cold water inlet of a domestic boiler. According to a second aspect of the invention there is provided a method of generating domestically usable electricity comprising using waste heat output from a first thermoelectric generating element to provide a heat input to a second thermoelectric generating element, and generating electricity using the second thermoelectric generating element.

Preferably, the method also comprises generating electricity using the first thermoelectric generating element. Most preferably the method also comprises using waste heat output from the second thermoelectric generating element as a heat source, possibly for heating water.

There may be a plurality of first thermoelectric generating elements, and/or there may be a plurality of second thermoelectric generating elements. The method may comprise using a system in accordance with the first aspect of the invention, or any of the dependent paragraphs.

According to a third aspect of the invention there comprises a method of generating domestically usable heat comprising using waste heat output from a first thermoelectric generating element to provide a heat input to a second thermoelectric generating element, and using waste heat output from the second thermoelectric generating element as a heat source.

The method may comprise using the waste heat output from the second thermoelectric generating element to heat water, for example to preheat water prior to that water being supplied to a domestic boiler.

Preferably, the method also comprises generating electricity using the second thermoelectric generating element. Most preferably the method also comprises generating electricity using the first thermoelectric generating element.

There may be a plurality of first thermoelectric generating elements, and/or there may be a plurality of second thermoelectric generating elements. The method may comprise using a system in accordance with the first aspect of the invention, or any of the dependent paragraphs.

According to the fourth aspect of the invention there comprises a domestic structure (for example, a house, apartment, apartment block or office) comprising a thermoelectric generating system in accordance with the first aspect of the invention, or any other dependent paragraphs.

The invention will now be more fully described, by way of example only, with reference to the following Figures:

Figure 1 is a schematic illustration of a semiconductor thermocouple;

Figure 2 is a schematic illustration of a thermoelectric power generation system; and

Figure 3 shows a first embodiment of a thermoelectric power generation system. Referring firstly to Figure 2, a thermoelectric power generation system 100 is shown. The system 100 includes a first thermoelectric subsystem 102 and a second thermoelectric subsystem 104. Each of the thermoelectric subsystems 102, 104 includes one or more thermoelectric generating elements 1 10 each having a first region (a 'hot side' 106) and a second region (a 'cold side' 108). Each thermoelectric generating element 1 10 of a respective subsystem is operable to generate electricity 1 12 via the Seebeck effect when a temperature gradient exists between the first and second regions of that element, and in particular when the first region 106 is hotter than the second region 108. The second region of each thermoelectric generating element of the first subsystem is in thermal communication with the first region of a thermoelectric generating element of the second subsystem.

Each of the first and second subsystems 102, 104 comprises a heat collection mechanism 1 14 and a heat removal mechanism 1 16. The heat collection means 1 14 of each subsystem is operable to convey heat to the first region of the thermoelectric generating elements of the respective subsystem, so as to raise the temperature of the first region of each thermoelectric generating element in relation to the temperature of the second region of that thermoelectric generating element. The heat removal means 1 16 of each subsystem is operable to remove heat from the second region of each thermoelectric generating element. When the heat collection mechanisms and heat removal mechanisms are operating a temperature gradient 1 18 is maintained across the thermoelectric generating elements in each subsystem, and electricity is produced.

The first and second subsystems are thermally linked by a heat transfer system 1 13, which comprises the heat removal mechanism 1 16 of the first subsystem and at least part of the heat collection mechanism 1 14 of the second subsystem. Thus 'waste' heat output from the first subsystem can be used to contribute to establishing a temperature gradient in the second subsystem.

The heat collection mechanism 1 14 of each subsystem might comprise multiple components, and might be operable to obtain heat from multiple sources. This is indicated in the example shown in Figure 2 by the presence of multiple input arrows 1 14 to the first subsystem. A first arrow 1 14a indicates a solar collector operable to draw heat from solar radiation. A second arrow 1 14b indicates a heat exchanger operable to draw heat from exhaust from a domestic boiler.

Similarly, the heat removal mechanism 1 16 of each subsystem might comprise multiple components.

The thermoelectric generating elements 1 10 in each subsystem are disposed such that the first side 106 of each thermoelectric generating element is heated by the heat collection mechanism 1 14 of that subsystem to a greater degree than the second side 108 of the thermoelectric generating element (which is cooled by the heat removal mechanism 1 16 of that subsystem). Thus, when a supply of heat is provided by the heat collection mechanisms electricity 1 12 is generated from the resulting temperature difference.

The system 100 is arranged so that, in use, the first subsystem operates at a generally higher temperature than the second. As shown in Figure 2, the second region 108 of the thermoelectric generating element of the first subsystem 102 is in thermal communication with the first region 106 of the thermoelectric generating element of the second subsystem 104. In particular, the heat removal mechanism 1 16 of the first subsystem functions as at least part of the heat collection mechanism of the second subsystem so as to provide at least a partial heat input to the second subsystem. In this way, waste heat from the first subsystem is used to raise the temperature of the second subsystem. The second subsystem generates electricity from the waste heat of the first subsystem. Waste heat output from the second subsystem can be used as a heat source, for example to heat or preheat domestic water.

The heat collection mechanisms 1 14 are passive, in that the system does not require a powered heat source such as an electric heating element. The heat collection mechanisms are instead arranged to draw heat from the environment external to that subsystem, such as the sun, or waste heat from a domestic boiler, or another subsystem. The heat collection mechanism might be as simple as a surface (e.g. a black surface) arranged to absorb solar radiation, or might be a lens arranged to concentrate solar radiation. Various example heat collection mechanisms are described later with reference to the specific example shown in Figure 3.

The heat removal mechanisms 1 16 may also be passive, and may comprise heat pipes and/or cold water from a normal domestic supply. The heat removal mechanism may, in certain applications, comprise ambient air surrounding the second region of the thermoelectric generating elements of the system.

The greater the temperature gradient induced across the thermoelectric generating elements the more electricity that is produced by the thermoelectric generating elements. It is therefore desirable to arrange each subsystem so that as large a temperature difference as possible is established between the hot side 106 and the cold side 108 of the thermoelectric generating elements.

If the thermoelectric module (comprising a pair of thermocouples (having a thermoelectric element) works at 180°C for its hot side and 80°C for its cold side, then the power output for a thermoelectric module is about 1.30W. For example, the power outputs at temperature difference 90°C and 1 10°C are about 1.05W and 1.58W, respectively. An example thermoelectric power generating system will now be described with reference to Figure 3.

Figure 3 shows a thermoelectric generation system 200 which generates electrical power and hot water from two subsystems, a first 'high' subsystem 202 and a second 'low' subsystem 204. The two systems generate electrical power individually, and the high subsystem 202 supplies partial heat to the low subsystem 204 by taking heat away from the cooler side of high subsystem. The system is arranged so that, in use, the high subsystem is located above the low subsystem. The system might, for example, be located on the roof of a building, or within an attic or roof space (where suitable provision, such as a window, is made for the input of solar radiation). Waste heat from either subsystem, and in this case from the low subsystem, can be used as a domestic heat source. It will be appreciated that where the words 'high' and ' low' are used herein this is for convenience only, and refers to the layout shown in the example given in the drawings. The high subsystem need not, in use, necessarily be located at a greater height than the low subsystem - all that is necessary is that the 'high' subsystem is thermodynamically upstream of the 'low' subsystem, so that waste heat from the high subsystem is used to provide at least partial heat to the low subsystem. The 'high' subsystem might be located beneath the 'low' subsystem.

The high subsystem 202 includes a first heat collection mechanism 214A. In this example, the first heat collection mechanism 214A is arranged to extract heat from two sources, and comprises a first solar collector 220, in the form of a lens (in this case a hemispherical or ball-shaped lens), and a hot fluid interface 222, in the form of an exhaust gas conduit connectable to an exhaust outlet of a domestic boiler 224.

The high subsystem 202 also includes a first oil tank 226A and a first heat exchanger 228A immersed in the oil tank. The heat collection mechanism provides heat to the oil tank via the first heat exchanger 228A, which absorbs heat to raise the temperature of the oil tank. The first heat exchanger is shaped so as to present a large surface area towards the solar collector to maximise the heat absorbed. The part-spherical shape of the solar concentrator means that solar energy incident on the concentrator is directed onto the first heat exchanger irrespective of the time of day (and hence the angle of the sun), eliminating the need for a solar tracking system.

The hot fluid conduit 222 also heats the first heat exchanger 228A. The hot fluid conduit is in thermal communication with the first heat exchanger, which comprises a plurality of fins in contact with (and in this example mounted on) the fluid conduit. The fins increase the contact surface area between the first heat exchanger and the oil in which it is disposed. The solar concentrator is able to heat the oil tank 226A, on a typical UK summer's day, to around 80°C. Exhaust gases typically leave a domestic boiler at around 150°C. In combination, the solar collector and the hot fluid passing through the heat exchanger are able to maintain the oil tank of the high subsystem at a temperature of around 150- 180°C, depending on whether or not the boiler is in operation and how much solar radiation is available. Oil has a high thermal inertia which means that once the oil tank has heated up then it takes a long time to cool down. Thus the tank remains hot for a time even whether there is no significant heat input, ensuring stable operation of the system. The high subsystem also includes a heat removal mechanism 216A in the form of one or more heat pipes 230. A heat pipe is an efficient heat transfer device that operates to transfer heat from a hot interface to a colder interface. A typical heat pipe comprises a hollow tube containing a low pressure fluid in both gas and liquid phases. Heat is transferred from the hot interface to the cold interface by fluid vapour moving toward the cold interface, condensing, and moving back toward the hot interface where it vaporises again. Latent heat is released from the fluid as it condenses, transferring heat from the hot interface to the cold interface. In the system of Figure 3, a plurality of heat pipes 230 are provided to remove heat from the high subsystem and convey it to the low subsystem.

Between the heat pipes 230 and the heat exchanger 228 a plurality of thermoelectric modules 232A are located. The modules comprise one or more pairs of thermocouples which have thermoelectric generating elements. Typically tens or hundreds of thermocouples may be provided in a module. Each thermoelectric generating element comprises a first, 'hot' region 206 on the side closest to the first heat collection mechanism 214A (i. e. the side which is heated by the heat exchanger and oil bath), and a second, 'cold' region 208 on the opposite side (i. e. the side which faces away from the first heat collection mechanism, and is not heated by it to a significant extent). Electricity is generated by the thermoelectric generating elements when a temperature gradient exists across them, i.e. when the 'hot' side is warmer than the 'cold' side. The electricity can be provided for domestic use and/or fed back into the local electrical grid, as required.

The thermoelectric elements 232A are in this case thermocouples. Each thermocouple is adapted to operate at relatively high temperatures (which in this case is less than 200°C, and generally between 150- 180°C). There are, in this example, 15 thermoelectric modules, each having 127 pairs of thermocouples. For each module the electricity output is about 2.28V (close circuit voltage) with the temperature difference at 70°C. Obviously, we could have more or fewer pairs of thermocouples (e.g. 100 + 10, or 20, or 30, or 40 or 40+), and the temperature difference could be greater or less (e.g. 70°C + 5, 10, 15, 20, 25, or 25 + °C.

The second, 'low' subsystem 204 has a similar construction to the high subsystem in that it includes a second heat collection mechanism 214B, second thermoelectric generating elements 232B and a second heat removal mechanism 216B.

The second heat collection mechanism 214B in this example includes a second solar collector 236 in the form of four linear lenses disposed in a roughly annular array. The array is arranged around the first solar collector in a nested configuration, so that when viewed in plan (arrow A) the first solar collector appears to be inside the outline formed by the second solar collector. Arrow A denotes the expected direction of the majority of the solar radiation to be collected by the device. This direction will vary dependent on the location in which the system is installed, but might be substantially vertical, or might be aligned with the expected position of the sun at a particular time of day (e.g. noon).

The second heat collection mechanism 214B also includes a hot fluid conduit 222, which in this case is a continuation of the exhaust gas conduit passing through the high subsystem (as this will still contain significant residual heat), as well as the cool interfaces of the heat pipes 230. Thus the heat pipes 230 serve as a heat transfer system between the first and second subsystems.

The second heat collection mechanism 214B is operable to heat a second heat exchanger 228B located in a second oil tank 226B. The heated oil from the high subsystem supplies partial heat to the oil of the low subsystem via the heat pipes 230. The low subsystem oil tank 226B gets further heated by the exhaust gas pipe 222 from the high subsystem as well as by the four linear solar concentrators 236 which concentrate solar radiation from four sides, ensuring the heat transfer oil in the second oil tank is heated for as long as possible without needing a solar tracking system. The second heat collection mechanism is operable to heat the first, hot side of the second thermoelectric generating elements 232B. The second, cold side of the second thermoelectric generating elements is cooled by a second heat removal mechanism 216B, in order to establish a temperature gradient across the second thermoelectric generating elements. The second thermoelectric generating elements are thus arranged to generate additional electricity to supplement that generated by the first thermoelectric generating elements.

The second heat removal mechanism 216B in this example comprises a coolant manifold connectable to a cold water supply. Cold water from the domestic supply runs through the coolant manifold prior to entering the domestic boiler. Thus, as well as generating electricity, the system is operable to heat the coolant water. The heated water is then supplied to the domestic boiler 224 as preheated water, meaning that less energy is needed to heat the water to required temperature level by the boiler.

The low subsystem 204 operates at a lower temperature than the high subsystem, and so the thermoelectric generating elements 232B of the low subsystem are optimised to operate at a lower temperature (in this case around 90- 120°C) than the thermoelectric generating elements 232A of the high subsystem.

We have found it useful to select thermoelectric generating elements based on the temperature range in which they are expected to operate because thermoelectric generating elements give a higher energy conversion efficiency when operating in their optimised temperature range. In addition, low temperature thermoelectric generating elements are cheaper than high temperature thermoelectric generating elements, resulting in a more economical system if the thermoelectric generating elements are chosen selectively based on the expected operating temperature.

The system shown in Figure 3 is totally static, and has no moving parts (with the exception of any convective circulation of oil in the tanks, and the flow of exhaust gases). The arrangement shown eliminates the need for a power-consuming circulation system by transporting the heat from the high subsystem to the low subsystem without the need for a pump. The system has a compact structure, as one subsystem is disposed above the other. The system is lightweight and can be fitted to a domestic roof by one or two installers. If required, the oil tanks might be provided with an inlet allowing them to be filled when the system is in location, to reduce installation weight. The system is operable to produce domestically usable electricity and heat in the form of preheated water using only free input heat. The two-stage nature of the system means that it is more efficient than conventional thermocouple power generating systems. The system shown in Figure 3 is able to achieve approximately about a 10% energy conversion. The ability to draw heat from multiple sources means that the system is highly versatile, and is suitable for use in locations where the amount of solar radiation is variable over the year.

Due to the characteristics of thermoelectric generation, if a large power output is to be produced, a large temperature difference is needed across the thermoelectric elements, which consequentially demands a high operating temperature. The use of heat transfer oil and heat pipes make the system more reliable by avoiding high operating pressure and extra energy consumption in the system.

The systems described herein are able to generate electricity from waste, surplus or unused heat. They are robust, hardwearing, and in some instances contain no moving parts, making them highly reliable and cheap to run. Because of their two stage construction, the systems are significantly more efficient than prior thermoelectric generating systems, as less heat is wasted. The systems are highly versatile because of their ability to draw heat from more than one external source. In the summer, the system can be heated primarily by solar radiation to provide electricity and preheated water for domestic use. In the winter, the system can be heated primarily by boiler exhaust gases. It will be appreciated that providing preheated water to a domestic boiler means that the boiler does not consume so much energy in bringing the water up to the required temperature, reducing the gas/electricity consumed by the boiler in heating the water. A system in accordance with the present invention might for example raise the temperature of the water provided to the boiler from ambient temperature (which might be around 20°C or less) to around 40°C.

The systems are compact, lightweight and can be fitted in any domestic building, such as a house, apartment or office. It will be appreciated that the exact construction of the heat collection means and the heat removal means does not matter, so long as a temperature difference can be created across the thermoelectric generating elements. Other heat sources/heat sinks to those described herein and shown in the drawings could be used if required. It will further be appreciated that the system is not limited to a two-stage thermoelectric generation system. Additional, for example third and fourth, subsystems might be provided if required. In such a multi-stage system, the second system might be arranged to supply heat or partial heat to the third subsystem, and the third subsystem might be arranged to supply heat or partial heat to the fourth subsystem. Waste heat from the fourth subsystem (as well as from any of the first to third subsystems) might be used as a heat source.