1. Field of the Invention
The present invention relates to improved thermoelectrics for converting a temperature differential to electrical energy with greater efficiency.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the properties of certain materials to develop an electrical potential across their terminals in the presence of a temperature differential across the TE. Conventional thermoelectric devices utilize P-type and N-type semiconductors as the thermoelectric material within the device. Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in Angrist, Stanley W., Direct Energy Conversion, 3rd edition, Allyn and Bacon, Inc., Boston, Mass. 2210, USA, (1976). The most common configuration used in thermoelectric devices today is illustrated in FIG. 1. Generally, P-type and N-type thermoelectric elements 102 are arrayed in an assembly 100 between two substrates 104. The thermoelectric elements 102 are connected in series via copper shunts 118 soldered to the ends of the elements 102. A temperature differential is applied via the thermal source 106 at temperature TH and a thermal sink 108 at temperature TC across the device. The Peltier effect causes a voltage 110 (V) to be generated at the device terminals 116 that can be used to drive a current 112 (I) through a load 114 (R0).
FIG. 2 shows the flow of power within the system of FIG. 1. For simplicity, only two TE elements 202 are shown. The TE elements 202 are sandwiched between hot and cold substrates 204 and are electrically connected in series by shunts 218. The source 206 of input heat energy is maintained at temperature TH and the cold side source 208 is maintained at TC. Power is extracted at the terminals of the shunts 218 and provided to the load where work (W) 214 is done. Heat QH enters at the left with waste heat QC leaving at the right. Internal losses I2R are distributed evenly, half each to the hot and cold sides.
The basic equations for TE power generating devices in the most common form are as follows:
qC=xcex1ITC+xc2xdI2R+Kxcex94Txe2x80x83xe2x80x83(1)
qH=xcex1ITHxe2x88x92xc2xdI2R+Kxcex94Txe2x80x83xe2x80x83(2)
W=qHxe2x88x92qC=xcex1Ixcex94Txe2x88x92I2R=I2RLxe2x80x83xe2x80x83(3)
where qC is the heat exiting from the cold side, qH is the heat entering at the hot side, and W is the power dissipated in the load, wherein:
xcex1=Seebeck Coefficient
I=Current Flow
TC=Cold side absolute temperature
TH=Hot side absolute temperature
xcex94T=THxe2x88x92TC, the temperature difference
R=Electrical resistance of the thermoelectric device
K=Thermal conductance
RL=Electrical resistance of the external load
Herein xcex1, R and K are assumed constant, or suitably averaged values over the appropriate temperature ranges. It is also assumed that heat and current flow are one-dimensional, and that conditions do not vary with time.
Further, to quantify the performance of the generator, the efficiency is given by:                     η        =                  W                      q            H                                              (        4        )            
combining (2) and (3) yields:                     η        =                                            I              2                        ⁢                          R              L                                                                          α                ⁢                                  xe2x80x83                                ⁢                                  IT                  H                                            -                                                1                  2                                ⁢                                  I                  2                                ⁢                R                            +                              K                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                T                                      ⁢                          xe2x80x83                                                          (        5        )            
To achieve maximum performance, the generator internal resistance must be suitably matched to that of the load. Introducing:                     m        =                              R            L                    R                                    (        6        )            
as the ratio of load resistance to internal resistance Equation (5) can be rewritten as:                     η        =                              m            ⁢                                                            xe2x80x83                                ⁢                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  T                                                            T                H                                                                        (                              1                +                m                            )                        -                                          1                2                            ⁢                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  T                                                  T                  H                                                      +                                                            (                                      1                    +                    m                                    )                                2                                            ZT                H                                                                        (        7        )            
where;                     Z        =                              α            2                    RK                                    (        8        )            
is a material property known as the figure of merit
The optimum value of m is:
mmax={square root over (1+ZTA)}xe2x80x83xe2x80x83(9)
wherein:
TA=xc2xd(TH+TC)xe2x80x83xe2x80x83(10)
the average temperature
Substituting (9) in (7), the maximum efficiency achieved is therefore:                               η          max                =                                            (                                                m                  max                                -                1                            )                        ⁢                          xe2x80x83                        ⁢                                          Δ                ⁢                                  xe2x80x83                                ⁢                T                                            T                H                                                                        m              max                        +                                          T                C                                            T                H                                                                        (        10        )            
FIG. 3 depicts the efficiency of a thermoelectric generator for different hot side temperatures and different values of the figure of merit, Z. As can be seen from the graph, high values of Z and TH are needed to make thermoelectric generators efficient. Commercially available materials have ZTA≈1 and some new, experimental materials have ZTA≈1.5. Materials commonly used in thermoelectric generators include suitably doped lead telluride (PbTe) for TH≈500xc2x0 C. or silicon germanium (SiGe) for TH≈1000xc2x0 C. Generally, as better materials may become commercially available, they do not obviate the benefits of the present inventions.
From FIG. 3 it can be seen that theoretical efficiencies over 25% are possible. Practical considerations of unavoidable losses, present material limitations, and reliability have limited actual efficiencies to 4% to 8%. For today""s materials, thermoelectric devices have certain aerospace and some commercial uses. However, usages are limited, because system efficiencies are generally too low to compete with those of other types of electrical generators. Nevertheless, several configurations for thermoelectric devices are in current use in applications where benefits from other qualities of TEs outweigh their low efficiency. These include applications requiring multi-year reliability without maintenance, heat flux sensing, conversion of waste heat, and power supplies for certain interplanetary spacecraft. In sum, in conventional devices, conditions can be represented by those described above.
The commercial devices have in common that the heat transport within the device is constrained by the material properties of the TE elements. None of the existing devices modifies the heat transport within the TE assembly.
An improved efficiency thermoelectric power generator is achieved by generally steady state convective heat transport within the device itself. Overall efficiency may be improved by designing systems wherein the TE system (elements or arrays) are configured to permit to the flow of a heat transport fluid, transport thermal energy to a moving substance, or move the TE material itself to transport heat. As an alternative to, or in combination with improved efficiency, generally steady state convection can be employed to reduce qC, the heat flux to the waste (cold) side.
One aspect of the present invention involves a thermoelectric power generation system using at least one thermoelectric array. The array may be made up of a plurality of individual elements, or one or more arrays. The array has a hot side and a cold side exhibiting a temperature gradient between them during operation. In accordance with the present invention, at least a portion of the thermoelectric array is configured to facilitate convective heat transport through at least one array. To accomplish this, the array is configured to permit flow of at least one convective medium through at least a portion of the array to provide generally steady-state convective heat transport from one side to the other side of at least a portion of the array. In one embodiment, the flow is from the cold side to the hot side of at least a portion the thermoelectric array.
In one embodiment, the convective medium flows through at least some of the thermoelectric elements or along (between and/or around) the thermoelectric elements. In another embodiment, the convective medium flows both along and through the thermoelectric elements. In one preferred embodiment, to permit flow through the thermoelectric elements, the elements or the arrays may be permeable or hollow. A combination of both permeable and hollow elements may also be used in an array. In one embodiment, the elements are porous to provide the permeability. In another embodiment, the elements are tubular or have a honeycomb structure. All such flows may be generally within or along the length of the thermoelectric elements (including in a spiral) or a combination thereof.
In one particular embodiment, at least some of the thermoelectric elements form concentric tubes with convective medium flow between the concentric tubes. In one embodiment, a first set of concentric tubes forms a thermoelectric element, with each tubular portion made from thermoelectric material of the same conductivity type as the next tubular portion in the set of concentric tubes. In such an embodiment, a second set of concentric tubes is formed of a thermoelectric material of a different conductivity type from the first set. Alternatively, the tubes may concentrically alternate between p-type thermoelectric material and n-type thermoelectric material.
In another embodiment, at least part of the convective medium is thermoelectric material. The convective medium thermoelectric material then forms at least some of the thermoelectric elements. In addition, in one embodiment, at least part of the convective medium is thermoelectric material, with the convective medium thermoelectric material forming a first portion of at least some of the thermoelectric elements, and a solid thermoelectric material forming a second portion of the same thermoelectric elements. For example, the solid thermoelectric material is tubular or otherwise hollow, and the convective medium thermoelectric material flows through the solid thermoelectric material. The combination forms at least some thermoelectric elements. In one embodiment, the convective medium is a fluid, such as air, a solid or a combination of a fluid and solids such as a slurry.
In one configuration, a first plurality of the thermoelectric elements are configured for convective heat transport of a first type and a second plurality of the thermoelectric elements are configured for convective heat transport of a second type. For example, the first plurality of thermoelectric elements may be permeable, and the second plurality may be thermoelectric elements made from the convective material moving through the array. An example of a division of elements is the first plurality being thermoelectric elements of a first conductivity type and the second plurality being thermoelectric elements of a second conductivity type.
In another embodiment, at least some of the thermoelectric elements do not utilize convection, while others are configured for convection. For example, the thermoelectric elements that do not utilize convection are of a first conductivity type and the thermoelectric elements that utilize convection are of a second conductivity type.
Preferably, at least a portion of the array has at least one heat transfer feature that improves heat transfer between at least some of the convective medium and at least some of the thermoelectric elements or arrays. For example, where the thermoelectric elements or arrays are tubular or otherwise hollow, the heat transfer feature is inside at least some of the tubular thermoelectric elements. Where the convective medium flow along the outside of the thermoelectric elements, the heat transfer feature is between at least some of the thermoelectric elements. An example of such heat transfer feature is a convective medium flow disturbing feature.
In one further embodiment, at least one co-generator is configured to operate in conjunction with the thermoelectric power generation system. In one embodiment, at least a portion of the co-generator comprises at least one combustion process involving combustion of the convective medium. The combustion process may take place in at least one internal combustion engine, or at least one external combustion engine. Other combustion processes are also possible. For example, in one embodiment, the co-generator comprises at least one turbine generator. The turbine generator may operate using at least one expansion process involving expansion, phase change process of the convective medium, or combustion process. For example, the working fluid may be water changing phase to steam.
In one embodiment, the co-generator comprises at least one electrochemical process using the convective medium, such as a fuel cell. In another embodiment, a heating and/or cooling system, such as an absorption system is configured to operate at least in part with the convective medium from the thermoelectric power generator.
Another aspect of the present invention involves a method of improving efficiency in a thermoelectric power generation system having at least one thermoelectric array having at least one first side and at least one second side exhibiting at least one temperature gradient between them during operation of the thermoelectric power generation system through the introduction of heat to the system. The method involves the steps of actively convecting thermal power through at least a portion of the array in a generally steady-state manner and generating power from the at least one thermoelectric array.
In one embodiment, the step of convecting heat comprises flowing at least one convective medium through at least a portion of the at least one thermoelectric array. In a further embodiment, a further step is provided of co-generating power at least in part with the at least one convective medium. In one embodiment, the step of co-generating comprises combusting at least a portion of the at least one convective medium in at least one co-generator. In one embodiment, the co-generator comprises at least one turbine generator. The step of co-generating may alternatively comprise an expansion of at least a portion of the at least one convective medium. Similarly, the co-generating step may additionally or alternatively involve at least one electrochemical process, such as a fuel cell, with at least a portion of the at least one convective medium.
In yet another embodiment, the method further involves the step of heating or cooling at least in part with the at least one convective medium. An example of a cooling and/or heating system is an absorption system operation at least in part with the convective medium flowing through the thermoelectric power generator.