Thermoelectric power generator

Details Thermoelectric power generator Thermoelectric power generator

2002-03-01

2004-04-06

Ryan, Patrick (Department: 1745)

Batteries: thermoelectric and photoelectric

Thermoelectric

Electric power generator

C136S236100, C257S930000

Reexamination Certificate

active

06717043

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermoelectric power generator and more particularly to a thermoelectric power generator capable of generating power not only from solar heat and geothermal heat but also from a heat source of medium or low temperature which has been impossible to be utilized by conventional arts, with high efficiency of thermoelectric conversion.
2. Description of the Related Art
At the present time, heat energy is converted into electric power mainly by a heat engine in which the process is: heat energy→high pressure steam→turbine→generator→electric power.
This method of converting heat energy into electric power have greatly contributed as power sources for supporting people's life in society, but there is a problem that it accompanies a large waste heat with its thermal efficiency of 45% at the best.
To combine a gas turbine to this method has already been attempted to good purpose, and it is applicable to thermal power generation using fossil fuels but not applicable to nuclear power generation.
Power generation by fuel cells seems like a promising one from a point of view of largely improving thermal efficiency, however at present the usable fuel is limited to hydrogen which is comparatively expensive and there still remain problems.
On the other hand, the conversion of heat energy into electric power using devices based on Seebeck effect is already established, but its thermal efficiency is 20% at the best and not in the state of general use on a large scale.
Electric power will continue without doubt to be necessary as an important energy to support people's life in society in the future. A requisite for the process of obtaining electric power from heat energy is to attain thermal efficiency as high as possible now that global environment crisis is strongly acknowledged.
However, there is a theoretical upper limit which can not be exceeded in thermal efficiency of each process of converting heat energy into electric power, and in any of the processes its thermal efficiency has reached near the upper limit by continued effort. Therefore, a leap in the concept of the method of thermoelectric conversion itself is necessary to get a quantum leap in thermal efficiency.
The inventors have investigated the operating mechanism of already-existing solar cells and devices utilizing Seebeck effect and worked toward development of a thermoelectric conversion device which operates at room temperature and moreover without large temperature difference in the device.
Two inventions made heretofore were applied for patent; the first one is disclosed Japanese Unexamined Patent Publication 6-151978 and the second is disclosed Japanese Unexamined Patent Publication 8-306964.
SUMMARY OF THE INVENTION
Here, the idea and invention obtained through a series of studies made previously will be described, in which mention will also be made of the significance of the present invention.
The basic configuration of the thermoelectric power generator according to the present invention is as follows:
Basic operation is as follows:
1) Electrons are thermally excited from valence band to conduction band in the semiconductor.
2) When an appropriate electric field exists in the semiconductor, the thermally excited electrons gather in region (C) of conduction band while holes gather in the region (A) of valence band. This is charge separation by the built-in electric field.
3) However, in the state cited above, the electrons and holes are in the state of thermal equilibrium, therefore the Fermi level is the same between region (A) and region (C).
4) If the electrons gathering in region (C) of conduction band can rise to higher Fermi level than that in region (A), then the electrons obtain the capability of flowing in an external circuit from cathode to anode and working electrically at the load.
Thus, the heat for thermal excitation of electron from valence band to conduction band in the semiconductor is converted into electric power. A temperature difference between both electrodes is not necessary, which is different from the case of Seebeck effect. Therefore, as the heat energy flowed into the body does not flow out to anywhere but converted into electric power, thermal efficiency would be 100%. This is the idea of thermoelectric conversion that occurred to the inventors. The inventors have made repeated studies to realize the idea, and made several key inventions cited below.
The band gap of semiconductor is desirable to be equal or under 1 eV in order to induce the thermal excitation of electrons in between the energy bands at room temperature or a little higher temperature, which is well known. The condition for building in an appropriate internal electric field to separate the carriers excited in on the energy bands is also publicly known. That is, in the configuration of an device shown below,
the condition for building in appropriate internal electric field to gather holes to region (A) of valence band and electrons to region (C) of conduction band is:
with n-type semiconductor;
&Dgr;&phgr;
A
=&phgr;
AN
−&phgr;
n
>(
Eg
−0.2)/
q
  [1]
&Dgr;&phgr;
C
=&phgr;
n
−&phgr;
CA
>0  [2]
with p-type semiconductor;
&Dgr;&phgr;
A
=&phgr;
AN
−&phgr;
p
>0  [3]
&Dgr;&phgr;
C
=&phgr;
p
−&phgr;
CA
>(
Eg
−0.2)/
q
  [4]
where symbols denote
&phgr; work function (v)
Eg band gap (eV)
q charge of an electron
A position (A)
C position (C)
n n-type semiconductor
p p-type semiconductor
AN anode
CA cathode.
The present invention establishes a Fermi level difference between region (A) and (C) by increasing the minority carrier density in either plane of a semiconductor beyond the thermal equilibrium state by external action.
The configuration of a device the inventors proposed as a means for realizing the idea mentioned above is that, tellurium (Te) is used as a semiconductor, copper (Cu) as an anode, aluminum (Al) as a cathode, The anode and cathode each is brought into close contact with the solid tellurium, and further glycerol is contacted to the cathode side. Properties of matter are as follows:
Te:
type of conduction ; p-type
Eg
; 0.32 eV
&psgr;:
Cu
; 4.86 V
Te
; 4.70 V
Al
; 4.25 V.
These values of properties suffice the required conditions [3] and [4]. Further, electrons liberated by the reaction of Al with glycerol are injected into tellurium (Te) at the cathode.
According to the idea of the inventors, the electron, which is minority carrier in tellurium (Te), externally injected with high electrochemical potential exceed the equilibrium state in both energy level and density, and would raise the Fermi level in region (C). The idea was verified by the experiments and the inventors disclosed it in Japanese Unexamined Patent Publication 6-151978. Though the invention enabled the device for thermoelectric conversion, further increase of power output was required.
Further, crystalline semiconductor such as tellurium is not suitable for producing a sheet-like semiconductor of large area. Producing a semiconductor in a sheet of large area is necessary for mass production of thermoelectric power generator, and a semiconductor suitable for this purpose should be selected.
The inventors hit upon an idea of using sulfide semiconductor. This is based on the characteristic that sulfide semiconductor is of ionic bonding and is functioning well by a comparatively easy production method. An idea of producing a sheet of large area is that the fine particles of sulfide semiconductor obtained by liquid phase reaction at normal temperature are shaped into a solid matter and hardened using an appropriate supporting material and binder.
It is necessary that the sulfide semiconductor is in a state of low hydration, and the fact that it contains water achieves an important role as mentioned later. The electron affinity x of sulfide semiconductor was assumed to be 3

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