GB2584350A - Electric generator - Google Patents

Abstract

Thermoelectric generator comprises a chamber having a vertically orientated perimeter wall 16, 22 and an inlet and outlet at the lower and top ends of said wall 16, 22. An outer part extends through the perimeter wall 16, 22 and an inner part 9 is located within the perimeter wall 16, 22. A means to adjust the inner part 9 to control the amount of gas flow. A burner is adapted to combust fuel into hot gases. A cooling section 12 on an outer side of the chamber is arranged to mate with the outer part. A thermopile is arranged between the cooling section 12 and outer part. The chamber may comprise two heat exchangers, each comprising fins, arranged parallel to one another so that respective fins interlock, overlap or interweave. The means to adjust the inner part 9 may comprise shifting the fins laterally, frontally, forward or backwards.

Description

ELECTRIC GENERATOR

Field of the Invention

The present invention relates to a device for generating small amounts of electrical energy from the burning of hydrocarbon fuels. More specifically the invention relates to using thermoelectric generation of electrical energy and by using a modular design. The device is especially useful for remote locations.

Background to the invention

It is recognised that generating electricity reliably in remote locations is particularly difficult, even at low levels of 10W to 400W. This invention proposes a device that overcomes some of the limitations faced, using thermoelectric generation devices with a modular design.

Existing technologies such as internal combustion engines, low-temperature fuel cells, high-temperature fuel cells, solar photovoltaic units, small wind turbines and portable batteries all have their limitations related to: * Scalability * Longevity * Intermittency of energy resources * Ability to cycle on demand * Energy density * Using fuel sources that are not commonly available, potentially dangerous, toxic or environmentally undesirable The common alternatives available are: Small electric generators powered by internal combustion engines are prohibitively expensive to scale down for a number of different reasons. Firstly, they have a large number of accurately engineered moving parts and therefore the cost of manufacture is relatively similar even for a scaled-down version. The large number of moving parts requires frequent and expensive maintenance. Inherently internal combustion engines are noisy and to be made quiet requires additional technology and cost. Small internal combustion engine powered generators do not lend themselves to frequent stop-start cycles and are typically are not built with starter motors. Small combustion engines are typically inefficient and therefore cannot meet the present environmental legislation requirement for limiting pollution. Typically, they have a lifetime of 2,000 hours although in special circumstances this can be extended to 10,000 hours. Considering that to provide a constant load may require 8,000 hours operation per year, the life of an internal combustion engine is too short for most industrial applications.

Low-temperature fuel cells can generate small amounts of electricity.

However, they tend to be expensive. The limiting factor for fuel cells is the lifetime expectancy for any unit, which typically is below 10,000 hours and is grossly affected by the purity of the fuel gas. Although longer generation times are achievable, this is often achieved through higher platinum loading of the fuel cell membrane. These units typically run on hydrogen gas which is either supplied as a compressed gas or in some circumstances is reformed on site from hydrocarbon fuels. If reformed gas is used, achieving high purity hydrogen is expensive and technically difficult. This can create issues related to impurities in the hydrogen gas that may cause poisoning of the fuel cell membranes. If this occurs, the life of fuel cell and its performance is reduced. Compressed hydrogen is currently not widely available on a global basis due to the economics of supply.

High-temperature fuel cells can generate power directly from hydrocarbon fuels. Typically they operate at temperatures above 600° C and use ceramic materials within their construction. The use of ceramic materials is typically problematic when trying to cycle a high temperature fuel cell. Ceramics inevitably will require metal to ceramic interfaces in order to retain the hot gases and for mechanical reasons. The differential contraction rates between the metal and ceramic components results in unequal stresses being set up within the differing materials and cause the brittle ceramics to break. If this interface is cycled repeatedly through high and low temperatures, the fuel cell potentially fails and the lifetime is reduced. Therefore, it is common practice to keep high-temperature fuel cells hot for the whole of their life irrespective of whether they are generating useful electricity in order to protect them from early failure. This continued elevated temperature reduces the overall efficiency of the device.

Solar photovoltaic units can generate sufficient power if there is sufficient light. In order to supply power at night, the derived power during the day has to be stored within a battery and supplied when the photovoltaics are not in use.

In the northern hemisphere during midwinter, there is limited light, the solar units have to be very large in order to generate sufficient energy. This problem will be exacerbated if the solar panel is covered in did or snow. As an example, a 25 W constant load would require between 2.7 and 5.4 m2 of solar panels. The night store lead acid battery unit which would support the load at night would be in the region of 64 kg and would have to be replaced on a regular basis. Batteries generally tend to perform less well during cold periods, which in many cases is when they are most required. Therefore, when the solar panels are at their lowest performing state, the battery is also at its performance lowest state.

This creates uncertainty of energy supply.

Wind units are very intermittent in terms of power generation. In order to supply a constant amount of power, the battery set would have to be extremely large and prohibitively expensive. Typically they will only generate power 25 to 30% of the time requiring a very large energy store. Often wind is often used in conjunction with solar panels because wind generation potentially continues during the night time and more wind is generated during wintertime.

Both wind and solar generation is limited by global and local geographic location. A solar panel must be located where it has access to as much light as possible. Therefore, in built-up urban areas, it can be difficult to place solar units in areas on the ground that are not subject to shading and sufficient sunlight is provided. Wind turbines also require open space where there is free movement of air they get maximum access to wind. Both solar and wind can be subject to local planning permission and be quite intrusive to local residents in terms of the visual impact and particularly the noise from wind turbines.

Battery technologies do not have high enough energy densities (Wh/kg) required so that a single person can practically and economically carry sufficient power in order to provide small amounts (>10W) of constant power. For example, traditional lead acid batteries typically have an energy density of 3050 Wh/kg. When compared to the energy density of a hydrocarbon fuels, batteries have an extremely low energy density. For example, liquid propane gas cylinders used for barbecues, have an energy density of approximately 10,000Wh/kg. Therefore, it is impractical to use batteries for anything other than very low wattages.

Higher energy density batteries are very expensive and presently still do not have the power densities required to be a practical alternative. In addition, higher energy batteries by definition store of high amounts of chemical energy, this often results in these types of batteries being energy dense in terms of their chemistry and therefore can be unstable and prone to fire risks. Typically these type of batteries are auto thermic if the contents are exposed to air.

Thermoelectric technology or Seebeck technology, such as the technology described in this patent, is an alternative method to generate power but for technical reasons hitherto has not been widely adopted. This is a power generating technology where electricity is generated as a result of heat flux passing through a thermoelectric device. By definition, a thermoelectric device will have a hot side and a cool side. This temperature difference has to be maintained to induce heat flux and therefore generate electricity. Thermoelectric units are made from thermopiles using the Seebeck effect.

Broadly speaking thermoelectric devices that use the Seebeck effect can be characterised by the operating temperature and each has its own limitations.

There are two classes of device and these are high and low temperature.

High-temperature thermoelectric devices are used on a limited basis for providing power on oil and gas lines where there is low cost methane gas available and efficiency is not a primary concern. These devices are not widely used because they require a constant supply of methane gas and are relatively expensive to install and maintain. This type of thermoelectric generator if cycled from low-temperature to high-temperature on a regular basis will incur damage and/or reduce lifetime of the units. Typically, this type of thermoelectric device is not efficient because the device ideally has to remain hot at all times.

Low-temperature (less than 400° C) thermoelectric devices that use Seebeck effect have been manufactured and distributed at scale. Typically, the heat source for these devices is scavenged from excess or waste heat. If given the perfect operating conditions, these devices will turn a maximum of 6% of the heat flux energy into electrical power. Their operating condition would typically be most efficient at 300° C on the hot side and 50° C on the cold side.

Examples of the use of thermoelectric devices would be a car exhausts, wood burning stove fans or a camping pot phone charger. The amount of power of these devices generates tends to be below 10W and is sufficient only to charge a phone or other similar device. The reason for the low power output is that the efficiency is often sub optimal due to lack of temperature control of both the hot side of the thermoelectric device and of the cold side. Therefore, a thermoelectric device attached to a camping stove would typically be operating at less than 2% efficiency. Assuming it is generating 5W, it would need to pass 250W on the cooling heat exchanger.

One of the previous limitations of using thermoelectric generators is that they are inherently difficult to electrically connect together. Each thermoelectric generator needs to experience extensively the same operating condition so that the internal resistance and electrical characteristics are correspondingly the same. If the units are in a various state of operating condition, the voltage and resistance also be various, resulting in an electrical unit inefficiency.

Therefore typically, thermoelectric devices are used on a singular basis. Multiple thermoelectric devices tend to operate in a sub optimal condition. By way of an example, if three thermoelectric devices were attached to a single heat exchanger that extracts heat from a single heat source in a co-current format, it is inevitable that this type of installation would operate in a sub optimal condition and create efficiencies substantially less than the maximum achievable 6%. The reason for this is that whilst the first thermoelectric device may experience optimal conditions, by way of the fact it is extracting heat, it will reduce the temperature for the second thermoelectric device and make it operate sub optimally. The third thermoelectric device is further encumbered by the extraction of heat from the previous two units and will operate at a further reduced efficiency. If each device is then connected directly in an electrical circuit, their differing conditions will cause further electrical inefficiencies. Therefore, the overall unit efficiency would be low.

Commonly when designing heat exchangers capture heat for extracting heat from a gas flow there are certain design criteria; the heat exchangers have sufficient surface area to extract the heat from the heat source. At the same time, the heat exchangers have to have sufficient interspace, i.e. open space, to allow the hot gas to travel past the heat extracting surface area with minimal obstruction and impingement. If the gases are obstructed by insufficient free space, then the hot buoyant gas will not travel unheeded and could backup and possibly spill out through the bottom of the heat exchanger. This causes a design dilemma in that heat exchangers have to be open enough to pass the exhaust gas but at the same time provide a large enough surface area to capture the sufficient heat. However the more open the heat exchange the less efficient the heat extraction process becomes and the larger the surface area required.

To transfer the heat once captured, sufficient heat conducting metal is required to transport that heat to the point of use, in this case a thermoelectric device. The net result is typically heavy and large heat exchanger with a correspondingly large thermal mass.

Ideally, an electrical generator is required to generate power as quickly as possible and at the optimum efficiency. Therefore, to ensure the thermoelectric device reaches the optimum temperature as quickly as possible, the heat exchangers on the hot side have to have the lowest thermal mass as practically possible.

It follows that to rapidly reach operating temperature, a boost of excess heat energy is initially required over and above the steady state condition. This boost, when burning fuels, creates larger amount of exhaust gas compared with the steady-state operation. As an example, the combustion gas flow may vary as much as 10:1, exhaust gas flow, between the heating up condition and the steady-state condition. Therefore, the heat exchangers need to be designed to operate successfully in both these high flow and low flow operating conditions.

This invention provides a solution to the above problems and describes a thermoelectric generator for continuously and reliably generating amounts of electrical energy in the range of 10 to 400 W from the controlled burning of hydrocarbon fuels.

Statement of the Invention

According to an aspect of the invention there is provided a thermoelectric generator comprising: at least one chamber having, a vertically orientated perimeter wall, an inlet opening at a lower end of the perimeter wall, an outlet opening at a top end of the perimeter wall, at least one heat exchanger arranged to partially allow gas flow between the inlet and outlet opening having, an outer part extending through the perimeter wall and an inner part located within the perimeter wall; means to adjust the inner part so as to control the amount of gas flow; a burner adapted to combust combustible fuel into hot gases, such that the hot gases enter into the chamber via the inlet opening and exit via the lower outlet opening; at least one cooling section located on an outer side of the chamber arranged to mate with the outer part of the heat exchanger; at least one thermopile located between the cooling section and the outer part of the heat exchanger; means of cooling the cooling section; and an electrical output port with wiring to the or each thermopile.

The thermoelectric generator may have two or more chambers which are aligned vertically above one another so that hot gases flow from the outlet opening of the lower chamber into the inlet opening of a consecutive chamber.

Preferably the inner part of the or each heat exchanger may comprise a set of fins.

Preferably the or each chamber may have two heat exchangers arranged parallel to one another so that their respective set of fins are arranged to interlock, overlap or interweave.

Preferably the or each chamber has four heat exchangers may be arranged parallel to one another so that their respective set of fins are arranged to interlock, overlap or interweave.

The thermoelectric generator may further comprise means to adjust each or the inner part/s shift the sets of fins laterally.

is The means to adjust the or each inner part/s shift the fins may do so frontally, forwards or backwards in relation to the centre of the chamber.

In another aspect, each or the inner part/s of each chamber may be adjusted so that the amount of hot gasses flowing and thus of heat extracted from the or each heat exchanger/s is distributed vertically in substantially equal quantities.

Preferably the generator may comprise adjustable and resilient means to retain the, or each, thermopile between the cooling section and the outer part of the heat exchanger/s.

The thermoelectric generator may further comprise a boost combustion controller for rapid temperature increase and a steady state fuel controller to maintain temperature within a defined temperature range.

Preferably the boost and fuel controller may employ either or both an on-off temperature gas bulb expansion control valve or an electrical solenoid valve.

Alternatively the boost combustion controller may employ a variable control valve connected to an electronic temperature control unit.

The thermoelectric generator may employ passive cooling in the cooling section.

Alternatively, the cooling section may employ variable speed fans and means to maintain a desired temperature, which fans are powered from the electrical power generated.

Preferably the fans may be arranged horizontally so that the cooling is unaffected when two or more chambers are located vertically above one another.

Alternatively or in addition the cooling section may employ active liquid heat transfer means.

The thermoelectric generator may further comprise means to recirculate at least is part of the exhaust hot gases into the burner and/or the inlet opening.

Brief Description of the Drawings

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a graph of the power generation efficiency achieved by a thermoelectric generator, in accordance with of the invention, given various temperature conditions; Figure 2 shows a graph of the ideal temperature profile versus a temperature profile achieved by a burner module of the thermoelectric generator of the invention; Figure 3 shows an illustration of the burner module gas handling arrangement in accordance with the invention; Figure 4 shows a top view illustration of a first subunit of a thermoelectric module of the generator, in accordance with the invention, further showing a hot side heat exchanger on the right side of the illustration; Figure 5 shows a vertical side view illustration of the first subunit of the thermoelectric module of the generator, in accordance with the invention; Figure 6 shows an illustration of the relative proximity adjustment of the hot side heat exchangers; Figure 7 shows a top view illustration of two thermoelectric subunits joined together to complete a single thermoelectric module, in accordance with the invention; Figure 8 shows an illustration of a stacking arrangement of three thermoelectric modules in a proposed embodiment to form a thermoelectric generator, in accordance with the invention; Figure 9 shows a wiring arrangement of three generator modules when stacked to form a thermoelectric generator.

Detailed Description of the Invention

This invention solves the problem of generating small amounts of electricity, IOW to 400W, based on low-temperature thermoelectric principals. The invention proposes a modular and stackable design that can create generators of multiple sizes to suit different industrial applications.

Referring initially to Figures 5 and 8, the apparatus as described in the invention 20 comprises two main modules: Firstly, a burner module which has been specially designed and built to provide heat outputs to match the performance characteristics required. Second, a modular generator that can be manufactured from a series of similar interlocking generator subunits. These generator modules have heat 25 exchangers that can be adjusted to create open space for the gas flow and to purposely reduce efficiency in order to share heat and maintain optimum operating conditions for the thermoelectric devices stacked above.

In the interests of clarity, the following naming convention and reference numerals are used in the description and figures.

Thermopile A thermoelectric device that has the capability of generating electricity through the Seebeck effect Thermoelectric subunit A unit containing at least one thermopile located between a hot and cold heat exchanger Hot side heat exchanger A heat exchanger through which the hot gases pass and heats the thermoelectric device Cold side heat exchanger A heat exchanger which cools the thermoelectric device Thermoelectric module A unit containing two or more thermoelectric subunits wherein their respective hot side heat exchangers have been interlocked Thermoelectric generator One or more stacked thermoelectric modules Burner A single component that mixes the air and fuel prior to combustion Burner module A unit that safely controls the fuel and air flow including the burner Reference Numerals 1 Gas inlet solenoid valve 2 Spark ignition 3 Flame sensor 4 Boost circuit needle valve Gas thermostat 6 Gas safety controller 7 Steady-state needle valve 8 Burner 9 Hot side heat exchanger Thermal bridge 11 Thermoelectric device or TEG 12 Cold side heat exchanger 13 Cooling fan 14 Tie rods Retaining bolt 16 End insulation 17 Primary heat exchanger 18 Secondary heat exchanger 19 Tertiary heat exchanger Insulation 21 Gas bulb control thermostat 22 Insulation 23 Cooling air 24 Exhaust gas DC to DC buck boost converter 26 Temperature set point fan speed controller The general concept of thermoelectric devices is that they use a plurality of thermocouples to form a thermopile which has the capability to generate electrical current when there is a temperature gradient created across them. Typically these devices are used to scavenge heat and power electric devices.

Figure 1 shows that thermoelectric devices need to be operated at a narrow optimum temperature range. Below this optimum temperature range the thermoelectric device will generate electricity inefficiently and convert only a small amount of the heat flux passing through them into electricity. Above the optimum temperature range the thermoelectric device will generate power inefficiently and there is a danger that the thermoelectric generating module device will be damaged. Therefore it is an important feature of the invention that the apparatus heats up rapidly to the required optimum temperature and maintains that temperature for the duration of the generation cycle. Therefore the operational purpose of the burner module is to safely heat up the heat exchangers as rapidly as possible within the constraints of exhaust gas flow passing the heat exchangers to reach the optimum temperature and then hold that temperature whilst generating electricity.

The ideal temperature profile, as illustrated in Figure 2, is essentially a square wave temperature profile. This illustrates how it is absolutely critical that the thermoelectric device is heated up to its optimum temperature as quickly as possible to attain the most efficient operating conditions. Once the generation cycle is over, it is best practice to extract the residue heat as quickly as possible and extract the energy remaining in the thermal mass into electricity. The transition temperatures, i.e. below the optimum temperatures, will still generate electricity but at a lower efficiency. Therefore lower than optimal operating temperatures, whilst unavoidable in transition, are best avoided to increase overall efficiency.

Burner module: Figure 3 shows a burner module gas handling arrangement in accordance with the invention. The burner module arrangement in this embodiment can burn any type of gaseous hydrocarbon fuels.

In other embodiments, the burner may be adapted to burn liquid or injected and atomised fuels.

The burner has two main operating gas flow circuits. The purpose of the first circuit is to provide maximum permissible gas flow, i.e. be a combustion boost initiator, to increase the temperature as rapidly as possible. The second circuit gas flow, or steady state fuel feeder and controller, is controlled through pre-set orifices to hold the temperature just below, e.g. at 10° C below, the ideal operating temperature requirement of the hot side heat exchanger.

Referring again to Figure 3, fuel enters the burner module via an electrical solenoid valve (1). Ignition may be achieved through high-voltage spark discharge (2). The solenoid valve (1) is kept open for a short amount of time, e.g. 3 seconds, by a fuel safety controller (6) whilst there is a flame is established, using a flame sensor (3). Initially most of the fuel will pass through a boost circuit via a pre-set needle valve (4) and via a thermostat valve (5). The thermostat valve has a pre-set on/off function set to stop the boost flow within 10°C of the optimum temperature. Preferentially the thermostat would be a gas bulb technology (21) that requires no electrical energy to reduce the parasitic electrical load of the control unit. The unit will continue to heat up through the boost circuit until the optimum temperature has been reached, at which time the thermostat will close the boost circuit. The flame is then solely reliant on fuel provided by the control circuit and is thus reduced to a steady-state condition provided via a pre-set needle valve (7) that supplies slightly less fuel than is required to maintain set-point temperature. Over time the temperature will ideally reduce, at which time the thermostat valve will reopen and boost the temperature. This boost follow by steady state gas flow cycle will continue whilst in operation.

A unit of this nature, that generates only small amounts of power (<400 W) therefore cannot carry a high parasitic load to power instrumentation and control units. It follows that any control circuit within the burner ideally wants to be using no, or extremely low, energy units. As a result, the control method of choice in this embodiment is a control thermostat that utilises gas bulb (21) technology. In this embodiment the gas bulb is inserted into one of the two hot side heat exchangers (9) nearest to the burner. In this embodiment, the target temperature is between 300 and 340° C but will depend on the choice of thermoelectric device. It is anticipated that the boost circuit may alternatively utilise other technologies such as gas actuated control valves or an electrically powered solenoid valve to achieve a desired set point temperature.

A combustible mixture is created in the burner (8) by combining e.g. gaseous fuels and air through a jet and venture effect prior to being ignited.

Once mixed with air the fuel/gas mixture is ignited in this case by a spark ignition unit (2).

Generator subunit: Referring now to Figures 4 and 5, a generator subunit and its construction principles in accordance with one embodiment of the invention are shown.

Heat from the burner carried by an exhaust hot gas, rises vertically due to buoyancy created by the heat and passes through the hot side, also referred to as the inner part (9), of a heat exchanger. Typically this hot side heat exchanger would be manufactured from aluminium although other high heat conducting metals such as copper could be used. The heat extracted by the hot side heat exchanger (9) is transported through the insulation layer (22) using a thermal bridge (10) which in this example is also manufactured using aluminium. the insulation layer (22) forms part of a vertical perimeter wall which encases the inner part of the heat exchanger into a chamber. The thermal bridge (10), also referred to as an outer part (10) of the heat exchanger extends through the insulation layer (22), i.e. through the perimeter wall.

At least one thermopile (11) is in close contact with the thermal bridge ensuring that it is intimately in contact with the hot surface (> 300° C). To provide heat flux, the thermopile is cooled using a cold side heat exchanger (12), also referred to as a cooling section (12).

Therefore, the one or more thermopiles are located between the cooling section 30 and the outer part of the heat exchanger.

The cooling section may be a cooling heat exchanger that employs a plurality of aluminium fins. It may use passive cooling or utilises a fan (13) to extract waste heat.

It is envisaged that the cooling section (12) could be a passive unit comprising of heat pipes, copper and aluminium fins for instance. The cooling section could be equally an active closed loop unit using a pumped heat transfer liquid and radiator devices. The cooling section fans may be replaced by combining the required air flow by being ducted to or from a central air movement device.

Managing the efficient transfer of heat to the thermoelectric devices once the heat is extracted from the combustion gases is critical. Therefore, all surfaces which have to transfer heat efficiently from surface to surface are polished and flattened within tolerances (flat to within 0.80 pm and polished to a #8 surface) and have a heat transfer medium between them to maximise thermal contact. In between the hot side heat exchanger (9) and the thermal bridge (10) would preferably be a high-temperature graphite foil. In between the thermal bridge (10) and the thermoelectric device would preferably be a high-temperature graphite foil. In between the thermoelectric device (11) and the cold side heat exchanger (12) could be either a low-temperature heat exchange silicon grease or a graphite foil In order to maintain thermal contact, the unit is ideally compressed to an optimum level. This compression force has to be maintained in all conditions and in this example, the thermoelectric device has to be compressed to approximate 125,000 kg/m2. When the unit is hot there is inevitably thermal expansion of the components corresponding to their operating temperature. In this embodiment a floating spring arrangement has been incorporated to avoid excessive compression force when the unit is hot and the metal elements are fully expanded. Figure 4 shows in this particular design there are 4 tie rods (14) that have a floating spring mounted arrangement that maintains the optimum force on the thermoelectric device (11) to achieve maximum heat transfer.

Therefore, adjustable and resilient means retain the, or each, thermopile between the cooling section and the outer part of the heat exchanger, to control and/or to maximise heat transfer.

A further improvement to the unit is to have the plurality of the springs acting on a plate that carries the force to above and centre of the thermoelectric device. This plate then ensures the force is evenly distributed across the whole area of the thermoelectric device. The spring or springs can be replaced by any compressive device such as hydraulics, Bellville or spring washers providing it maintains the force required. The number of tie rods can be varied providing the total compressive force on the thermoelectric device is maintained.

When the hot side heat exchanger (9) is at its full operating temperature (330° C), the thermal bridge (10) will be operating at a comparatively similar temperature (circa 320 ° C). The thermoelectric generator will then generate power providing that the temperature provided by cold side heat exchanger (12) is in the region of 50 to 80° C (varying with the incoming ambient air temperature).

Figure 5 shows a horizontal side view of the completed generator subunit and the preferred cold side heat exchanger (12) attached to the thermoelectric generating device (11). In this embodiment the cold side heat exchanger is manufactured from aluminium and the heat transfer fins run horizontally towards and away from the thermoelectric generator to maximise heat transfer. The outside fins of the heat exchanger form a box section to carry the cooling air (23).

Thermoelectric Module: One of the limitations of using thermoelectric generators is that that they are inherently difficult to electrically connect together without creating electrical inefficiencies. Each thermoelectric generator needs to experience extensively the same operating condition and if they are to be electrically connected together in the same circuit. If the thermoelectric devices are producing various amounts of resistance, voltage and current, then an electrical inefficiency will be created.

An important aspect of this invention is the control and manipulation of heat exchange efficiencies and hot gas flow from the burner. This is achieved by adjusting the flow of hot gases in the chamber, e.g. the inner part may yaw, pitch or roll. Alternatively it may shift laterally or frontally, forwards or backwards.

Figure 6 shows an idealised and illustrative view of triangular interwoven and overlapping heat exchange fins that can be adjusted in proximity to increase or decrease the interspace in between. By making this relatively small adjustment the amount of interspace for the hot gas to pass through is significantly increased or decreased thereby increasing or decreasing the efficiency of the heat exchange unit. This adjustment may be fixed on manufacture, manually adjusted or automated.

Figure 7 shows the generator module and how this invention solves of this problem by overlapping and interweaving the hot side heat exchangers (9). Two mirrored and similarly engineered thermoelectric subunits are brought together in order to form one thermoelectric module. The hot side heat exchangers (9) need to provide surface area to extract sufficient heat from the burner gas and at the same time provide sufficient interspace to allow the exhaust gas to pass vertically through. In this embodiment, the hot side heat exchangers are extensively triangular in nature primarily to provide sufficient heat conducting metal to transfer the heat to the base plate. Advantageously, by overlapping and interweaving the triangular fin of the hot side heat exchangers (9), their proximity can be adjusted to allow a greater or lesser amount of interspace and therefore adjust the heat extraction efficiency and control the gas flow. By way of example, where the hot side heat exchangers are slightly pulled apart by adjustment, a more open interspace is provided allowing a proportion of the hot gas to travel unhindered through the hot side heat exchanger without the heat being extracted. By making this adjustment heat can be passed vertically from one set of heat exchangers to distribute the heat to the next generator vertically above.

Equally advantageously, by this method, each generator subunit by adjustment can experience the exactly the same temperature and therefore provides extensively the same operating conditions.

Once the adjustment is made, the two generator subunits are interlocked together using bolts (15) and in this embodiment has a dual purpose of holding the end insulation (16). The layers of insulation (22) and end insulation (16) forms a contiguous barrier in which the hot gases are contained.

It is also envisaged that instead of two separate hot side heat exchangers overlapping and weaving this could be manufactured in one box section with internal interconnecting aluminium fins providing sufficient interspace to allow the hot gases from the burner to pass through it. However, this would not allow for variable heat extraction and combustion gas flow adjustment.

This invention uses some counterintuitive thinking. Firstly to maximise the heat extraction from the hot gases from the burner, a typical engineering solution would be to create a counter flow exchange unit to maximise the amount of heat extracted. However, it can be seen from figure 1 that thermoelectric devices have a narrow and specific operating regime. Therefore, extracting heat at anything other than the optimum temperature would not maximise the opportunity for generating electricity because of the performance profiles of the is thermoelectric devices.

The solution described by this invention creates a unit where each thermoelectric module has the capability to allow heat to pass through it and be recaptured by subsequent generating modules above it. Heat flux is effectively shared by each successive hot side heat exchangers equally. This engineered inefficiency creates a unit which is overall more efficient in terms of extracting electrical energy from hot gases. Advantageously, this single design concept lends itself to modularisation and standardisation of design to reduce manufacturing costs.

It is envisaged that whilst extensively the electrical generation modules are similar, the surface areas of the hot side heat exchanger might be varied in order to affect different outcomes. As an example, the primary module might have shorter vertical length hot side heat exchangers in order to only capture a lessor amount of the heat generated from the burner. Equally, it is envisaged that the final heat exchanger might be the largest in the apparatus in order to scavenge the maximum amount of heat before exhausting to atmosphere.

Advantageously, a single generating unit will successfully generate electrical energy as a stand-alone unit albeit at reduced efficiency. Higher efficiencies can be achieved using a plurality of generator modules stacked on top of one another.

Thermoelectric generator: Figure 8 shows one embodiment where three generating modules are be stacked on top of a single burner (8). The modules whilst mechanically separate, when stacked, the insulation (22) combined with the end insulation (16) around the hot side heat exchangers, forms a gas tight barrier that carries the heat from the burner vertically through the successive hot side heat exchangers to the exhaust (24). This contiguous installation barrier provides a collimated and enclosed duct that ensures that the hot gases are safely contained, and ensures the heat rises sufficiently to be passed through the hot side heat exchangers.

This insulated duct also protects the thermoelectric devices and other equipment from experiencing unwanted high temperatures and contributes to the overall efficiency of the generator.

In Figure 8 it can be seen that the heat initially passes through the primary module hot side heat exchanger (17). This hot side heat exchanger is used as the control point for the burner and is maintained at the operating temperature (circa 330 ° C) to avoid overheating of the unit. Heat rises to pass through the primary module and is then partially captured in the secondary module hot side heat exchanger (18). In this three-module embodiment example, heat is finally captured in the tertiary module in hot side heat exchanger (19) prior to the gas exhaust (24). In each case the temperature difference between the hot side of the thermoelectric device and cold side is within the optimum range to be sufficient to generate electricity at an acceptable efficiency. The position of the fins in each chamber is arranged so that the heat from the hot gases is allowed to flow uniformly from the bottom up, and thus aid so that each hot side heat exchanger is as close to the same temperature as possible. I.e. each level of interlocking hot side heat exchanger allows each chamber to operate at the same isothermal conditions so that the respective thermopiles of those chambers operate under exactly the same temperature and thus generate similar amounts of electrical energy. In one embodiment this is achieved by employing inside the hot side exchangers with interlocking triangular sets of fins that control flow to create perfect conditions for the next layer of generators modules. It also makes making similar modules easy. In addition triangular fins are also very efficient in transferring heat.

Figure 9 shows the electrical connections between the modules. In this embodiment, each module has two thermoelectric devices although it is envisaged that there may be a plurality of thermoelectric devices. Depending on their voltage output, these will be connected in either parallel or series. This voltage will be typically between 3 and 12 V. Below 3V it is difficult to increase the voltage electronically. This variable and unconditioned voltage is connected to a commonly available buck-boost DC to DC converter (25). Buck boost (25) electronic devices typically have the characteristic of either lowering or raising voltage depending on the incoming voltage to a specified target output voltage.

In this example the electrical output in these devices will set to either 12 or 24 V depending on the requirement.

In this embodiment, the six cooling fans (13) that provide the air movement through the cold side heat exchangers. These fans would typically be wired straight into the output of the buck-boost (25) and form part of the modular arrangement. To reduce parasitic load created by the fans, these fans may utilise pulsed width modulation to (26) modulate their speed and maintain a preset temperature without wasting energy. In this way, the fans increase or decrease the airflow rate to maintain the cold side heat exchanger(s) temperature. It is anticipated that the cooling fans could be consolidated into a smaller number of fans and connected through a suitable ducting unit.

It should be noted that the term "comprising" does not exclude other elements, the term "a" or "an" does not exclude a plurality, a single feature may fulfil the functions of several features recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims. It should also be noted that the Figures are not necessarily to scale; emphasis instead generally being placed upon illustrating the principles of the present disclosure.

Claims (17)

1. CLAIMS: 1. A thermoelectric generator comprising: at least one chamber having, a vertically orientated perimeter wall, an inlet opening at a lower end of the perimeter wall, an outlet opening at a top end of the perimeter wall, at least one heat exchanger arranged to partially allow gas flow between the inlet and outlet openings having, an outer part extending through the perimeter wall and an inner part located within the perimeter wall; means to adjust the inner part so as to control the amount of gas flow; a burner adapted to combust combustible fuel into hot gases, such that is the hot gases enter into the chamber via the inlet opening and exit via the outlet opening; at least one cooling section located on an outer side of the chamber arranged to mate with the outer part of the heat exchanger; at least one thermopile located between the cooling section and the outer part of the heat exchanger; means of cooling the cooling section; and an electrical output port with wiring to the, or each, thermopile.
2. 2. The thermoelectric generator according to claim 1, having two or more 25 chambers which are aligned vertically above one another so that hot gases flow from the outlet opening of the lower chamber into the inlet opening of a consecutive chamber.
3. 3. The thermoelectric generator according to claim 1 or 2, wherein the inner part of the, or each, heat exchanger comprises a set of fins.
4. 4. The thermoelectric generator according to any of the above claims, wherein the, or each, chamber has two heat exchangers arranged parallel to one another so that their respective set of fins are arranged to interlock, overlap or interweave.
5. 5. The thermoelectric generator according to claim 3, wherein the, or each, chamber has four heat exchangers arranged parallel to one another so that their respective set of fins are arranged to interlock, overlap or interweave.
6. 6. The thermoelectric generator according to any of the above claims, wherein the means to adjust each, or the, inner part/s shift the sets of fins laterally.
7. 7. The thermoelectric generator according to claims 4 or 5, wherein the means to adjust the, or each, inner part/s shift the fins frontally, forwards or backwards, in relation to the centre of the chamber.
8. 8. The thermoelectric generator according to claims 2 to 7, wherein each, or the, inner part/s of each chamber are adjusted so that the amount of hot gasses flowing and thus of heat extracted from the, or each, heat exchanger/s is distributed vertically in substantially equal quantities.
9. 9. The thermoelectric generator according to any of the above claims, wherein adjustable and resilient means retain the, or each, thermopile between the cooling section and the outer part of the heat exchanger/s.
10. 10. The thermoelectric generator according to any of the above claims, further comprising a boost combustion controller for rapid temperature increase and a steady state fuel controller to maintain temperature within a defined temperature range.
11. 11. The thermoelectric generator according to any of the above claims, wherein the boost and fuel controller employ either or both an on-off temperature gas bulb expansion control valve and/or an electrical solenoid valve.
12. 12. The thermoelectric generator according to any of the above claims, wherein the boost combustion controller employs a variable control valve connected to an electronic temperature control unit.
13. 13. The thermoelectric generator according to any of the above claims, wherein the cooling section is passive.
14. 14. The thermoelectric generator according to claim 12, wherein the cooling section employs variable speed fans and means to maintain a desired so temperature, which fans are powered from the electrical power generated.
15. 15. The thermoelectric generator according to claim 14, wherein the fans are arranged horizontally so that the cooling is unaffected when two or more chambers are located vertically above one another.
16. 16. The thermoelectric generator according to claim 12 or claim 15, wherein the cooling section employs alternatively or in addition an active liquid heat transfer means.
17. 17. The thermoelectric generator according to any of the above claims, further comprising means to recirculate at least part of the exhaust hot gases into the burner and/or the inlet opening/s.