Patent Application
20140230869
Thermoelectric converters, such as solar thermoelectric converters are known in the art. These converters rely upon the Seebeck effect to convert temperature differences into electricity. A portion of the thermoelectric converter may be directly or indirectly heated by a heat source to create the necessary temperature difference. The efficiency of the energy conversion depends upon the temperature difference across the thermoelectric converter and the thermoelectric material's figure of merit, ZT. Greater temperature differences and greater ZT allow for greater conversion efficiency.
Embodiments may include a self-powered boiler comprising a burner that is adapted to burn a fuel to produce a hot combustion product, a hot combustion product conduit, and a thermoelectric generator (TEG) system comprising a first side in thermal communication with the hot combustion product conduit and a second side in thermal communication with a lower temperature region of the boiler, and a plurality of thermoelectric converters disposed therebetween, the thermoelectric converters comprising a nano structured thermoelectric material, wherein electric power generated by the TEG system is equal to or greater than a total electric power consumed by the boiler under steady state operating conditions.
Further embodiments include methods of operating a self-powered boiler comprising burning a fuel to produce a hot combustion product, flowing the hot combustion product in thermal contact with a first side of a thermoelectric generator (TEG) system comprising a nanostructured thermoelectric material, and generating electrical power by the TEG system that is equal to or greater than a total electric power consumed by the boiler under steady state operating conditions.
The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention.
The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims.
Multiple methods exist for generating electricity from heat energy. Various embodiments may include thermoelectric conversion elements. Thermoelectric conversion relies on the Seebeck effect to convert temperature differences into electricity. Thermoelectric converters operate more efficiently under greater temperature differences.
Boilers are widely used to heat buildings and generate hot water. A boiler typically operates by combusting a fuel to heat water or another fluid. Operation of a boiler typically requires consumption of a certain amount of electric power to drive various components of the boiler system, such as a control unit, fan(s) or blower(s) (e.g., air or fuel), pumps, electrically actuated valves, etc. In some cases, the boiler may provide local (i.e., space) heating, in other cases the boiler may provide domestic hot water heating and/or one or more heating circuits via a header and control valves. The operating temperature and flow rates of the system may vary depending on the size and type of the heating system, as well as the type of heat emitter(s) utilized (e.g., radiator, radiant floor, fan coil-type, etc.).
The largest amount of power consumption in a boiler-based heating system is typically attributable to the pump or pumps used to displace a fluid medium (e.g., water) of the heating system through a heat exchanger in thermal contact with the hot combustion products of the boiler flame and to provide the heated fluid medium to a location external to the boiler for subsequent use. The pump may be responsible for upwards of half of the total power consumption of the boiler system. A typical pump in a domestic heating system may consume 50-150 W (e.g., 65-100 W) when running. A boiler may include a unit having burner and heat exchanger integral with the pump or a separate pump external to the burner/heat exchanger unit.
Many heating systems also use boiler fans, as it is typical for modern boilers to have fan-assisted flues rather than natural draught flues and a permanent pilot. The increasing gas flow resistance from larger heat exchange surface areas in high-efficiency boilers has increased the power requirements of the boiler fan. In condensing boilers, the boiler fan typically requires increased fan power to overcome the fluid losses across the extra heat exchanger surfaces. Modulating burners also require speed control to ensure the gas/air modulation ratio can be maintained. The boiler fan may be responsible for approximately a third of the total power consumption of the burner. A typical fan in a domestic heating system may consume 20-135 W, such as 30-50 W, when running, depending on the boiler size.
The control unit, which can be an electrical or electromechanical control unit, typically consumes a small amount of instantaneous power (e.g., 0.4-5.0 W, such as 2-4 W), but it is always on. Thus, the control unit may be responsible for between 5-10% (e.g., about 8%) of the total power consumption of the boiler. Many boilers also include components such as electronic thermostats, flame detection and spark ignition systems that utilize an on-board power supply that results in a quiescent or “standby” power consumption. This “standby” power consumption can be between 5-15 W (e.g., about 8 W), and may account for between 10-20 (e.g., 15-20%) of the total power consumption of the boiler. Some boilers may also use electric heaters (e.g., immersion heaters, trace heaters) to maintain the fluid medium (e.g., water) at a desired temperature (e.g., 60° C.) and reduce boiler warmup time. Such electric heaters, when used, may consume 15-30 W (e.g., about 25 W).
Electrically actuated valves include motorized zone valves (e.g., two-port and three-port type valves). Many domestic boiler heating systems use motor open, spring return-type valves, which may consume about 5 W continuously while the valve is open. Other valves, such as motor open motor off (MOMO) type valves may consume about 6 W during a valve state change. Typically, the motorized zone valves may account for about 4-6% of the total power consumption of the boiler. A separate valve system for the burner gas may include a pair of solenoid valves (for safety reasons) which may consume around 6 W while the burner is firing. The gas valve may account for between about 2-5% (e.g., around 3%) of the total power consumption of the boiler.
Thus, a typical boiler for heating and/or hot water as described above may require at least about 100 W of electric power for operation, such as 100-250 W (e.g., 150-200 W, such as 175 W, or 200-250 W, such as 225 W). Some boilers may require at least about 250 W of electric power, such as 250-500 W, for operation, such as 250-400 W (e.g., 250-350 W, such as 300 W, or 350-450 W, such as 400 W or more).
In various embodiments, a boiler includes an integrated thermoelectric power generation (TEG) system for generation of thermal and electrical energy. The thermoelectric power generation (TEG) system may use a temperature gradient within the boiler, such as between a hot combustion product (e.g., a flame) and a lower-temperature fluid (e.g., water) to provide a temperature difference across one or more thermoelectric conversion elements and thereby generate electricity. In embodiments, the boiler may be a self-powered boiler, meaning that the net electrical power produced by the thermoelectric power generation system is equal to or greater than the total electric power consumed by the boiler under normal (e.g., steady state) operating conditions. Electricity from the TEG system may be provided to boiler electrical components.
In operation, fuel and air are fed to burner 101, which combusts the fuel/air mixture to produce the hot combustion product 103. A pump 133 circulates water 134 through the boiler 100 (e.g., through conduit 114 and heat exchanger 112) to transfer heat from the hot combustion product 103 to the water 134. The heated water 134, which may be vaporized, exits heat exchanger 112 for use in heating, residential, office or industrial hot water supply and/or other applications (e.g., power generation). After the water 134 cools, and optionally condenses, the water 134 may be recirculated to pump 133. A control unit 131 may include circuitry/logic for monitoring and controlling operation of the boiler 100 by, for instance, regulating the water temperature by controlling the operation of pump 133 and/or the fuel and air feeds to the burner 101 via electrically actuated valve 135 and fan/blower 137, respectively.
The TEG modules 109 may include a plurality of thermoelectric converters (e.g., one or more “couples” or interconnected pairs of p-type and n-type legs of thermoelectric material) each including a first (hot) side in thermal communication with the high-temperature conduit 105, a second (cold) side in thermal communication with the lower temperature region 107. Electrically conductive leads 117 may provide appropriate electrical coupling within and/or between thermoelectric converters, and may be used to extract electrical energy generated by the converters. The electrical energy generated by the TEG modules 109 may be provided over leads 117 to a load, which may be one or more components of the boiler 100, such as the control unit 131, electrically actuated valve(s) 135, fan or blower 137, water pump 133, a safety device, etc. The load powered by the TEG modules 109 may further comprise other components external to the boiler. At least a portion of the electrical power from the TEG modules 109 may be provided to an energy storage device (e.g., rechargeable battery, ultracapacitor, etc.) for later use or for start-up of the boiler. In some embodiments, at least a portion of the electrical power from the TEG modules 109 may be provided/sold to a power grid. The control unit 131 may be configured to operate the boiler 100 to provide a power output from the TEG modules 109 in excess of the power required to operate the boiler 100, and the excess power may be used to power one or more additional devices and/or to satisfy transient power demands on an as-needed basis. In embodiments, the TEG modules 109 may generate more than 400 W of electricity, which is more than twice the power needed to drive a typical boiler. In embodiments, the TEG modules 109 may use thermal energy from the boiler to generate electric power in an amount equal to at least the total amount of electric power required by the boiler for operation, such as more than 100 W of electric power, such as 100-250 W (e.g., 150-200 W, such as 175 W, or 200-250 W, such as 225 W), or more than 250 W (e.g., 250-500 W) of electric power, such as 250-400 W (e.g., 250-350 W, such as 300 W, or 350-450 W, such as 400 W). At start-up, the boiler 100 may plug into an outlet or use battery power or a pilot light. A control system may then switch to TEG power when it detects sufficient power at steady state. The boiler 100 of the present invention may be powered by the TEG modules 109 when other source(s) of power are not available, such as during a power outage, and may be used in areas where grid power is not available.
The boiler 100 may include heat exchange elements to conduct heat from the combustion product 103 of burner 101 to the “hot side” of the thermoelectric modules 109. For example, the high-temperature conduit 105 may include a plurality of thermally conductive plate elements 119 to provide a plate-type heat exchanger, as shown in
Other heat exchange configurations, such as a fin type heat exchanger, may also be employed. A fin type head exchanger may include a plurality of plate fins, pin fins, or both. When a fin type heat exchanger is utilized, a packing fraction of the fins may vary from a first packing fraction proximate the inlet to conduit 105 to a second denser packing fraction proximate the outlet of conduit 105, in order to provide a substantially uniform temperature to the hot sides of TEG modules 109, such as described in U.S. application Ser. No. 13/924,826 filed Jun. 24, 2013, the entire contents of which are incorporated herein by reference for all purposes.
In various embodiments, the hot combustion product 103 (i.e., flame) entering conduit 105 may have a temperature of greater than about 1000° C., such as about 1200° C. or more. The temperature on the “hot” side of the thermoelectric modules 109 may be 400° C. or more (e.g., 500° C. or more), such as 600-700° C. The temperature on the “cold” side of the thermoelectric modules 109 may be 200° C. or less, such as 150° C. or less (e.g., 100-130° C., such as ˜115° C.). The hot combustion product 103 leaving the conduit 105 may have a temperature greater than about 650° C., such as between 670-900° C.
In various embodiments, the hot combustion product 103 exiting the conduit 105 may optionally be provided to a heat exchanger 112 where thermal energy from the combustion product 103 may be transferred to a fluid 134, such as water or molten salt (e.g., water in a hot water heating boiler). As shown in
In this embodiment shown in
As is shown in
The heat exchanger 817 in this embodiment comprises a plurality of fins 825 directly attached to the module cover 821. The heat exchange fins 825 in this embodiment comprise plate type fins, although pin type fins and combinations of plate and pin type fins could also be used. In addition, this embodiment the plate fins 825 are evenly spaced and oriented generally parallel to the direction of fluid flow, although it will be understood that other configurations may be used. For example, a gradient fin heat exchanger may be used where the fin packing fraction is varied along the direction of fluid flow and/or in a direction transverse to fluid flow, as described above.
The fins 825 may be made of a thermally-conductive material, such as a metal or metal alloy, and may be made from the same or different material than the portion of the cover 821 to which they are attached. The fins 825 may be thermally matched to the cover 821 (e.g., made from a material having a coefficient of thermal expansion (CTE) within about 10%, such as 0-5%, including 0-1% of the cover material). In embodiments, direct attachment of fins 825 to the module cover 821 may eliminate thermal interface problems between the heat exchanger and the thermoelectric generator module 809, and may significantly enhance the performance of the module 809. The fins 825 may be attached to the cover 821 using any suitable technique, such as via brazing, soldering, welding, solid state diffusion, use of a high-temperature adhesive and/or via mechanical fasteners.
In embodiments, a plurality of modules 809 having heat exchangers 817 directly attached to the module cover 821 as shown in
The thermoelectric converters of the TEG modules 109, 809 according to any of the above-described embodiments may be made from a variety of bulk materials and/or nanostructures. The converters preferably comprise plural sets of two converter elements—one p-type and one n-type semiconductor converter post or leg which are electrically connected to form a p-n junction. The thermoelectric converter materials can comprise, but are not limited to, one of: half-Heuslers, Bi2Te3, Bi2Te3-xSex (n-type)/Bix Se2-xTe3 (p-type), SiGe (e.g., Si80Ge20), PbTe, skutterudites, Zn3Sb4, AgPbmSbTe2+m, Bi2Te3/Sb2Te3 quantum dot superlattices (QDSLs), PbTe/PbSeTe QDSLs, PbAgTe, and combinations thereof. The materials may comprise compacted nanoparticles or nanoparticles embedded in a bulk matrix material. For example, see U.S. patent application Ser. No. 11/949,353 filed Dec. 3, 2007, which is incorporated herein by reference for all purposes, for a description of exemplary materials.
As used herein, a “nanoparticle” or “nanosized” structure, generally refers to material portions, such as particles, whose dimensions are less than 1 micron, preferably less than about 100 nanometers. For example, nanoparticles may have an average cross-sectional diameter in a range of about 1 nanometer to about 0.1 micron, such as 10-100 nm. Nanostructured bulk materials which comprise compacted semiconductor and/or intermetallic nanoparticles are attractive since the materials are in a form that is compatible with use in a boiler yet have a relatively high figure-of-merit (ZT) and are economical. Such nanostructured bulk materials can be compacted from nanoparticles of the same material (e.g., SiGe, BiTe, half-Heusler material, etc.), or compacted particles of different materials, in which nanoparticles of one material form a host matrix and the nanoparticles of the second material form inclusions in the host matrix. The particles may be consolidated (compacted) using hot pressing or direct current induced hot pressing. The consolidated, dense bulk materials are nanostructured with grains having at least one of a median grain size and a mean grain size less than one micron, such as grains having a mean grain size less than 300 nm in which at least 90% of the grains are less than 500 nm in size. In one embodiment, the grains have a mean grain size in a range of 10-300 nm. In one embodiment, the grains have a mean size of around 200 nm. Typically, the grains have random orientations. Further, grains may include 5-50 nm size (e.g., diameter or width) nanodot inclusions within the grains.
Such nanostructured thermoelectric materials have been shown to have a relatively high figure-of-merit, ZT, defined by ZT=(S2σ/κ)T, where σ is the electrical conductivity, S the Seebeck coefficient, κ the thermal conductivity, and T the absolute temperature. The nanostructured materials generally have a power factor (S2σ) comparable to the same materials produced using conventional techniques (i.e., non-nanostructured materal), but exhibit a much lower thermal conductivity κ. The significant reduction in thermal conductivity in the nanostructured materials may be attributed to the increased phonon scattering at the numerous interfaces of the random nanostructures. The combination of relatively high power factor and low thermal conductivity make the nanostructured thermoelectric materials an excellent candidate as a power generator for a self-powered boiler 100.
In preferred embodiments, the thermoelectric elements used in the boiler 100 employ half-Heusler materials. Suitable half-Heusler materials and methods of fabricating half-Heusler thermoelectric elements are described in U.S. patent application Ser. No. 13/330,216 filed Dec. 19, 2011 and Ser. No. 13/719,966 filed Dec. 19, 2012, the entire contents of both of which are incorporated herein by reference for all purposes. Half-Heuslers (HHs) are intermetallic compounds which have great potential as high temperature thermoelectric materials for power generation. HHs are complex compounds: MCoSb (p-type) and MNiSn (n-type), where M can be Ti or Zr or Hf or combination of two or three of the elements. Sn and Sb can be substituted by Sb/Sn. They form in cubic crystal structure with a F4/3m (No. 216) space group. These phases are semiconductors with 18 valence electron count (VEC) per unit cell and a narrow energy gap. The Fermi level is slightly above the top of the valence band. The HH phases have a fairly decent Seebeck coefficient with moderate electrical conductivity. The performance of thermoelectric materials depends on ZT, defined by ZT=(S2σ/κ)T, where σ is the electrical conductivity, S the Seebeck coefficient, κ the thermal conductivity, and T the absolute temperature. Half-Heusler compounds may be good thermoelectric materials due to their high power factor (S2σ).
The dimensionless thermoelectric figure-of-merit (ZT) of conventional HHs is lower than that of many other state-of-the-art thermoelectric materials. Recently, enhancements in the dimensionless thermoelectric figure-of-merit (ZT) of n-type half-Heusler materials using a nanocomposite approach has been achieved. A peak ZT of 1.0 was achieved at 600-700° C., which is about 25% higher than the previously reported highest value. The materials may be made by ball milling ingots of composition Hf0.75Zr0.25NiSn0.99Sb0.01 into nanopowders and hot pressing (e.g., DC hot pressing or without application of current) the powders into dense bulk samples. The ingots may be formed by arc melting the constituent elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.
By using a nanocomposite half-Heusler material, a greater than 35% ZT improvement from 0.5 to 0.8 in p-type half-Heusler compounds at temperatures above 400° C. has been achieved. Additionally, a 25% improvement in peak ZT, from 0.8 to 1.0 at temperatures above 400° C., in n-type half-Heusler compounds by the same nanocomposite approach has been achieved. The ZT enhancement is not only due to the reduction in the thermal conductivity but also an increase in the power factor. These nanostructured samples may be prepared, for example, by hot pressing a ball milled nanopowder from ingots which are initially made by an arc melting process. The hot pressed, dense bulk samples may be nanostructured with grains having a mean grain size less than 300 nm in which at least 90% of the grains are less than 500 nm in size. In some cases, the grains have a mean size in a range of 10-300 nm, such as a mean size of around 200 nm. Typically, the grains have random orientations. Further, many grains may include 10-50 nm size (e.g., diameter or width) nanodot inclusions within the grains.
Embodiments of the half-Heusler materials may include varying amounts of Hf, Zr, Ti, Co, Ni, Sb, Sn depending on whether the material is n-type or p-type. Other alloying elements such as Pb may also be added. Example p-type materials include, but are not limited to, Co containing and Sb rich/Sn poor Hf0.5Zr0.5CoSb0.8Sn0.2, Hf0.3Zr0.7CoSb0.7Sn0.3, Hf0.5Zr0.5CoSb0.8Sn0.2+1% Pb, Hf0.5Ti0.5CoSb0.8Sn0.2, and Hf0.5Ti0.5CoSb0.6Sn0.4. Example n-type materials include, but are not limited to, Ni containing and Sn rich/Sb poor Hf0.75Zr0.25NiSn0.975Sb0.025, Hf0.25Zr0.25Ti0.5NiSn0.994Sb0.006, Hf0.25Zr0.25NiSn0.99Sb0.01 (Ti0.30Hf0.35Zr0.35)Ni(Sn0.994Sb0.006), Hf0.25Zr0.25Ti0.5NiSn0.99Sb0.01, Hf0.5Zr0.25Ti0.25NiSn0.99Sb0.01 and (Hf,Zr)0.5Ti0.5NiSn0.998Sb0.002.
The ingot may be made by arc melting individual elements of the thermoelectric material in the appropriate ratio to form the desired thermoelectric material. Preferably, the individual elements are 99.9% pure. More preferably, the individual elements are 99.99% pure. In some cases, two or more of the individual elements may first be combined into an alloy or compound and the alloy or compound used as one of the starting materials in the arc melting process. Ball milling may result in a nanopowder with nanometer size particles that have a mean size less than 100 nm in which at least 90% of the particles are less than 250 nm in size. In one example, the nanometer size particles have a mean particle size in a range of 5-100 nm.
It has been discovered that the figure of merit of thermoelectric materials improves as the grain size in the thermoelectric material decreases. In one example of a method for fabricating thermoelectric materials, thermoelectric materials with nanometer scale (less than 1 micron) grains are produced, i.e., 95%, such as 100% of the grains have a grain size less than 1 micron. Preferably, the nanometer scale mean grain size is in a range of 10-300 nm. This method may be used to fabricate any thermoelectric material and includes making half-Heusler materials with nanometer scale grains. The method may be used to make both p-type and n-type half-Heusler materials. In one example, the half-Heusler material is n-type and has the formula Hf1+δ-x-yZrxTiyNiSn1+δ-zSbz, where 0≦x≦1.0, 0≦y≦1.0, 0≦z≦1.0, and −0.1≦δ≦0.1 (to allow for slightly non-stoichiometric material), such as Hf1-x-yZrxTiyNiSn1-zSbz, where 0≦x≦1.0, 0≦y≦1.0, and 0≦z≦1.0 when 8=0 (i.e., for the stoichiometric material). In another example, the half-Heusler is a p-type material and has the formula Hf1+δ-x-yZrxTiyCoSb1+δ-zSnz, where 0≦x≦1.0, 0≦y≦1.0, 0≦z≦1.0, and −0.1≦δ≦0.1 (to allow for slightly non-stoichiometric material), such as Hf1-x-yZrxTiyCoSb1-zSnz, where 0≦x≦1.0, 0≦y≦1.0, and 0≦z≦1.0 when 8=0 (i.e., for the stoichiometric material).
The thermoelectric material (i.e., legs) of the TEG modules may be formed of a nanostructured material with grains having at least one of a median grain size and a mean grain size less than one micron, such as grains having a mean grain size less than 300 nm in which at least 90% of the grains are less than 500 nm in size. In one embodiment, the grains have a mean grain size in a range of 10-300 nm. In one embodiment, the grains have a mean size of around 200 nm. Typically, the grains have random orientations. Further, grains may include 5-50 nm size (e.g., diameter or width) nanodot inclusions within the grains. As discussed above, such nanostructured materials may provide a relatively high power factor at elevated temperature (e.g., 450-900° C., such as 600-800° C.) with low thermal conductivity. Because of this low thermal conductivity, even when the TEG modules are located upstream of and/or are co-located with the heat exchanger between the hot burner gas and the boiler water, sufficient heat remains in the hot burner gas to efficiently heat the boiler water. In embodiments, the TEG modules located upstream of and/or co-located with the boiler heat exchanger (i.e., heat exchange region 206 in
The foregoing method descriptions are provided merely as illustrative examples and are not intended to require or imply that the steps of the various embodiments must be performed in the order presented. As will be appreciated by one of skill in the art the order of steps in the foregoing embodiments may be performed in any order. Words such as “thereafter,” “then,” “next,” etc. are not necessarily intended to limit the order of the steps; these words may be used to guide the reader through the description of the methods. Further, any reference to claim elements in the singular, for example, using the articles “a,” “an” or “the” is not to be construed as limiting the element to the singular.
Further, any step or component of any embodiment described herein can be used in any other embodiment.
The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
This application claims the benefit of priority to U.S. Provisional Application No. 61/766,300 filed on Feb. 19, 2013, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61766300 | Feb 2013 | US |
