METHOD AND DEVICE FOR CONVERSION OF TERMAL ENERGY INTO ELECTRIC ENERGY AND ELECTRIC ENERGY INTO TERMAL ENERGY.

FIELD OF THE INVENTION

The present invention relates to the methods and the devices for direct conversion of thermal energy into electric energy and electric energy into thermal energy (cooling and heating). In particular, the present invention is related to the devices, generating electric energy without the need to create outside temperature difference (temperature gradient). The invention also is related to the devices having a reversible conversion and can be utilized to produce electric energy, as well as cooling and heating.

BACKGROUND OF THE INVENTION

The thermal energy can be converted directly into electric energy by applying several methods.

There exist methods and devices for direct conversion of thermal energy into electric energy and vice versa, which utilize the Peltie and Seebeck effects [ 1-4, 15,16]. The disadvantage of these converters is the low coefficient of efficiency and the inevitable presence not only of a source of heat, but also of a cooling element with lower temperature than that of the source of heat.

There is another well known of class converters of thermal energy into electric energy, called thermionic converters [ 5-9, 14,16 ]. They are utilized generally to conversion of thermal energy into electric energy in cases of higher temperatures. Recently converters, based on semiconductors have appeared in lower temperatures. In these converters there are also two regions with different temperatures.

A third class of thermal converters exists, which generate electric energy and they do not need two different temperatures in order to function. Similar converters are considered in [10-13]. Usually the converters of this type have very little output power.

The structure of thermionics converters is considered in [14]. Such a thermionic converter contains first electrode, called also emitter or cathode,

second electrode, called frequently collector or anode. Between the two electrodes a potential barrier is created, letting through predominantly the electrons, having higher kinetic energy, from the first electrode to the second electrode. The first electrode, the second electrode and the potential barrier are placed in a container. The first electrode and the second electrode are connected with suitable conductors to the output terminals of the thermal converter. The first electrode is thermally connected to a source of thermal energy, and the second electrode is thermally connected to a cooling element. An electrical load is connected to the output terminals of the device.

The essential process taking place in the thermionic converter is the following: the source of heat warms the first electrode to the necessary working temperature and together with the cooling element sustain a temperature difference between the first and the second electrode. As a result of the heating, the electrons, participating in the conductivity in the first electrode, acquire bigger kinetic energy. The electrons with the biggest kinetic energy from the first electrode, succeed to overcome the potential barrier and get into the second electrode. The cooling element sustains the temperature of the second electrode lower than this of the first electrode. The quantity of electrons from the second electrode, which manage to pass to the first is smaller, and because of that between the first electrode and the second electrode a potential difference is created, which increases till the number of passing electrons over the potential barrier in both directions is equal. When the electric chain between the first and the second electrodes is closed across an external electric load, as a result of the created potential difference, electric current flows across the thermionic converter.

The potential barrier usually is realized as a narrow vacuumed gap between the electrodes. This type of thermionic converter operate at temperature of the first electrode in the range of 1500-2000 ⁰ K, and temperature of the second electrode in the range of 500-1000 ⁰ K. To decrease the operating temperatures and increase the coefficient of efficiency of the converter, the first electrode is realized with low work function of electrons and the narrow gap is filled with suitable vapors ( for example cesium ).

Decrease of the operating temperatures and increase of the coefficient of efficiency with thermionic converters is achieved when they are made of semiconductor materials. Usually the first electrode is an n-type semiconductor, the second electrode is made from a p-type semiconductor,

and the narrow gap frequently is filled with suitable dielectric or semiconductor. These converters operate at temperature of the first electrode around 600 ⁰ K, and temperature of the second electrode around 300 ⁰ K. There are the semiconductor thermal converters which operate on the basis of the combined principle, thermionic, combined with effects of Peltie and Seebeck.

Thermionic converters which have found practical application so far are characterized by relatively high operating temperature of the first electrode and comparatively low coefficient of efficiency.

SUMMARY OF THE INVENTION

The problem that is resolved by the invention is to create a method and device for direct conversion of thermal energy into electric energy and electric energy into thermal energy. So that the process of energy conversion would not require two different temperatures, and conversion would be possible at lower operating temperatures of the device.

The problem of the invention is solved by a method and a device, using this method for conversion of thermal energy into electric energy and electric energy into thermal energy.

A method for conversion of thermal energy into electric energy and electric energy into thermal energy utilizing first electrode, second electrode and potential barrier. The method is characterized by the fact that the first electrode made from an electrically conductive material, containing electrons of conductivity with kinetic energy in interval with maximum energy (Emaxl+Efl) and minimum energy (Eminl+Efl), and the second electrode made from electrically conductive material, containing electrons of conductivity with kinetic energy in interval with maximum energy (Emax2+Ef2), are connected though the potential barrier with height (Ep +EfI) and width Lp so that the electrons whit bigger kinetic energy from the first electrode pass predominantly though the potential barrier, get into the second electrode and create potential difference between the first electrode and the second electrode. At flow of current as a result of the potential difference across an outside electrical load, the first electrode, the second electrode and the potential barrier cool and consume thermal energy from an external source of thermal energy, and in an electrical way this energy is deliver to an external electric load, and when there is flow of current from an external source of electric energy against the potential difference, in the

first electrode, second electrode and potential barrier thermal energy is released, which is received from an external source of electric energy and by a thermal way is transmitted to an external consumer of thermal energy.

The present method for conversion of energy permits two modes of operation, the first mode permits production of electric energy and cooling, and the second permits conversion of electric energy into thermal energy.

The present method permits reversibility, conversion of thermal energy into electric energy and electric energy into thermal energy.

The device for conversion of thermal energy into electric energy and electric energy into thermal energy in conformity with the present invention realizing the present method, includes first electrode, second electrode and potential barrier, placed in a corpus. Conductors connect the first electrode and the second electrode with output terminals. The device is characterized by the fact that the first electrode made from electrically conductive material, containing electrons of conductivity with maximum kinetic energy (Emaxl+Efl) and minimum kinetic energy (Eminl+EfL), the second electrode made from electrically conductive material, containing electrons of conductivity with maximum kinetic energy (Emax2+Ef2) and potential barrier is with height (Ep+Efl) and width Lp, letting through predominantly the electrons with bigger kinetic energy from the first electrode to the second electrode. The first electrode, the second electrode, the potential barrier and the corpus are thermally connected.

In order to obtain potential difference between the electrodes in thermionic converters a potential barrier and different temperatures of the electrodes are used, to sustain different energy of the electrons of electric conductivity in them, while the present invention instead of temperature difference uses electrodes, having different distribution of the energy of the electrons of electric conductivity in them, at equal temperature.

The advantages of the present method and device for conversion of thermal energy into electric energy and electric energy into thermal energy are, that two different temperatures for conversion of energy are not required. The conversion of energy is reversible, the same device can be utilized for the production of electric energy and at the same time for cooling, as well as for heating, while consuming electric energy. The invention is also applicable at comparatively low working temperatures. The fact that two different temperatures are not used, permits easy

realization of battery of serial, parallel or mixed connected devices in one common corpus.

BRIEF DESCRIPTION OF DRAWINGS

FIG.l schematically illustrates a device for conversion of thermal energy into electric energy and electric energy into thermal energy in accordance with the present invention.

FIG.2 schematically illustrates the present device in mode of conversion of thermal energy into electric energy.

FIG.3 schematically illustrates the present device in mode of conversion of electric energy into thermal energy.

FIG.4 represents the energy states of the electrons in metal.

FIG.5 represents the energy states of the electrons in heavily doped n- semiconductor.

FIG.6 represents the energy states of the electrons in heavily doped p- semiconductor.

FIG.7 represents the energy states in a device on the base of metal and heavily doped p-semiconductor in mode of short circuit connection.

FIG.8 represents the energy states in a device on the base of metal and heavily doped p-semiconductor in mode of idle running.

FIG.9 illustrates a device, realized on the base of semiconductor technology.

FIG.10 illustrates a device with adjustable output voltage.

DETAILED DESCRIPTION OF THE INVENTION

A method for conversion of thermal energy into electric energy and electric energy into thermal energy, utilizing first electrode, second electrode and a potential barrier. The method is characterized by the fact that the first electrode (1), made from electrically conductive material, containing electrons of conductivity with kinetic energy in interval with maximum

energy (Emaxl+Efl) and minimum energy (Eminl+Efl), and the second electrode (2), made from electrically conductive material, containing electrons of conductivity with kinetic energy in interval with maximum energy (Emax2+Ef2), which are connect through the potential barrier (3), with height (Ep +EfI) and width Lp so, that the electrons with bigger kinetic energy from the first electrode (1) pass predominantly through the potential barrier (3), get into the second electrode (2) and create potential difference between the first electrode (1) and the second electrode (2), and at flow of current as a result of the potential difference across an external electrical load (8), the first electrode (1), the second electrode (2) and the potential barrier (3) cool and consume thermal energy from an external source of thermal energy(7), and by electrical way this energy is delivered to an external electric load (8), and at flow of current from an external source of electric energy (9) against the potential difference, thermal energy is given off in the first electrode (1), second electrode (2) and the potential barrier (3), which is received by an external source of electric energy (9) and by thermal way is transmitted to an external consumer of thermal energy (10).

At flow of current in the closed electric circuit (FIG.2.), as a consequence of the created potential difference in the area of the first electrode (1), the second electrode (2) and the potential barrier (3) thermal energy is consumed by the external source of thermal energy (7), which by electric way is transmitted to the external electrical load (8). If that entering quantity of thermal energy from the source of thermal energy (7) is insufficient to sustain the temperature of the device, then its temperature begins to decrease. The generation of electric energy is connected with the absorption of thermal energy and the device operates in cooling mode. The present method for conversion of energy in this operating mode permits the production of electric energy and cooling (refrigeration ).

At flow of current (FIG.3.) from an external electric source (9) in the opposite direction of the one , created by the potential difference between the first electrode (1) and the second electrode (2), in the device except the Joule heat an additionally quantity of heat is given off, proportional to the product of the flowing current and the potential difference between the first electrode (1) and the second electrode (2), that energy is from the external electric source (9) and is transmitted to the external consumer of thermal energy (10). The present method for conversion of energy in this operating

mode permits the conversion of electric energy into thermal energy (heating).

The present method permits reversibility, conversion of thermal energy into electric energy and electric energy into thermal energy.

Distribution N(E) of electrons with kinetic energy around the Fermi level which participate in the electric conductivity is a function of the properties of the material and the temperature [16]. This function has two components. One component, which is the function of Fermi F(E), gives the probability for a given approved energy level around the Fermi level to be occupied by an electron and is a function of temperature. The other component S(E) gives the distribution of the approved energy levels around the Fermi level and is a function of the material. Non occupied approved energy levels are described by the function P(E).

For metals (FIG.4.) it can be accepted with approximation that the distribution of the accepted energy levels S(E) around the Fermi level Ef is uniform and the distribution N(E) of the electrons according to energy around the Fermi level Ef has the form of the Fermi function F(E). Electric conductivity is realized generally in the energy zone ((Emax+Ef)- (Emin+Eβ), where there are electrons, as well as non occupied energy levels. For greater clarity Emax and Emin are given in relation to the Fermi level.

When temperature in metals increases the number of electrons with bigger kinetic energy over the Fermi level also increases and approved energy levels under the Fermi level are released.

Electrically conductive materials with forbidden zone around the Fermi level don't have approved energy levels and respectively electrons with energy in the area of the forbidden zones.

It is well known that with semiconductors with n-type conductivity (FIG.5.), a forbidden zone (Ec-Ev) exists under the zone of the electric conductivity. With these electrically conductive materials, the electrons participating in the electric conductivity are in the area of the high kinetic energy of the Fermi distribution. The existence of the forbidden zone under the zone of conductivity determines a more narrow energy zone of electric conductivity ((Emax+Ef)-(Emin+Ef)) compared to that in metals, and participation in electric conductivity of electrons with higher kinetic energy

than the Fermi level Ef The electrons participating in the electric conductivity are with energies from the upper part of the spectrum of the Fermi distribution.

In the semiconductor with p-type conductivity (FIG.6.), the electric conductivity is realized in the upper energy spectrum of the valence zone and the forbidden zone (Ec-Ev) is situated over this zone. In these electrically conductive materials the electrons, participating in the electric conduction are in the area of the low kinetic energy of the Fermi distribution. The existence of a forbidden zone over of the zone of the electric conductivity determines a more narrow energy zone of the electric conductivity ((Emax+Ef)-(Emin+Ef)) compared to this in metals and participation in the electric conductivity of electrons with lower kinetic energy in relation to the Fermi level Ef. The electrons, participating in the electric conductivity are with energy from the bottom section of the spectrum of the Fermi distribution.

There are electrically conductive materials, in which the electric conductivity is limited in the narrow energy spectrum between two forbidden zones. In these materials the zone of the electric conductivity is comparatively most narrow and can be located over the Fermi level, under it or covers it. Respectively the energies of the electrons of the electric conductivity are from the upper, the bottom or the middle part of the spectrum of the Fermi distribution.

At the electric contact between two electrically conductive materials with Fermi levels EfI and Ef2 it is known that the Fermi levels in the materials become equal. In the zone of the contact an electrical layer is created, compensating the contact potential difference (EfL - Ef2), which equalizes the number of the passing electrons through the zone of the contact in the two directions

When we connect a first electrode (1) made from electrically conductive material, containing electrons of conductivity with kinetic energy in the interval with maximum energy (Emaxl+Efl) and minimum energy (Eminl+Efl) and a second electrode (2) made from electrically conductive material, containing electrons of the conductivity with maximum kinetic energy (Emax2+Ef2) through a potential barrier (3) with height (Ep +Efl) and width Lp ( have in mind rectangular potential barrier with height (Ep +EfI) and width Lp) , letting through predominantly electrons with higher kinetic energy, in consequence of the changed condition of passing of the

electrons with different kinetic energy through the potential barrier (3), potential difference Up between the Fermi levels in the two electrodes arises.

On FIG.7 the energy diagram of a first electrode (1) made from metal, a second electrode (2) made from heavily doped p-semiconductor, separated with potential barrier (3) is shown. For simplification of the diagram, the Fermi levels in the two electrodes are represented as equal. When there is external short circuit between the two electrodes, the Fermi levels Ej ^' in them are sustained equal. As a consequence the electrons with energy bigger than the height (Ep +EfI) of the potential barrier (3) pass predominantly from the first electrode (1) to the second electrode (2), and a current flows through the closed electric chain.

In order the potential barrier to have (3) such properties, it is necessary its height (Ep +EfI) to be lower than the maximum energy of the electrons of conductivity (Emaxl+Efl) in the first electrode (1) and bigger than the minimum energy of the electrons of conductivity (Eminl+Efl) in the first electrode (1), and maximum energy of the electrons of conductivity (Emax2+Ef2) in the second electrode (2) also to be lower then the height of the potential barrier (Ep +EfI). In this case it is supposed, that the width Lp of the potential barrier is large enough and the tunnel effects are negligible. The output potential difference Up is function of this placement of the potential barrier (3) and depending on the operating temperature of the device is in the order of millivolts to hundred millivolts at high temperatures.

When there is an open electric circuit (FIG.8.), the potential difference Up increases while the flow of the electrons with lower kinetic energy from the second electrode (2) to the first electrode (1), caused by the potential difference Up is equal to the flow of the electrons from the first electrode (1) to the second electrode (2), and dynamical equilibrium is established. The increase of the potential barrier Up leads to deformation of the form and changing of the height of the potential barrier.

When current flows through an external load, Up is changed so that new dynamical equilibrium is achieved depending on the flowing current.

When structure with first electrode (1), made from heavily doped n- semiconductor, second electrode (2), made from heavily doped p- semiconductor and potential barrier (3), made from intrinsic semiconductor or a less doped n-semiconductor than the first electrode (1), made from the

same semiconductor material, as a result of the contact potential difference between the first electrode (1) and the second electrode (2), the potential barrier (3) and at short circuit it is different from the rectangular and has similar shape to that of FIG.8., and its maximum height exceeds the maximum energy of the electrons (Emaxl+Efl) in the first electrode (1). In this case the potential barrier (3) is made narrow enough so that the electrons with maximum energy (Emaxl+Efl) from the first electrode (1), to go tunneling through it.

When current IL flows through the device, thermal energy is generated in it having the power P = Up . IL. The generated thermal energy is with sign and depends on the direction of the current IL in relation to the created potential difference Up. If the current IL in the circuit is caused by the created potential difference Up, then thermal energy coming from the external source (7) is absorbs by the element with power P = Up . IL , and that energy by electric way is transmitted to the external electrical load (8). If the direction of the current is opposite to the created potential difference, then thermal energy with power P = Up . IL that is consumed by the electrical source (9) is generated in the device and is transmitted to the consumer of thermal energy (10).

The absorption of thermal energy when current flows, different from that in transition between two electrodes without potential barrier is due to the fact, that from the first electrode to the second electrode predominantly electrons with higher kinetic energy pass and cool the first electrode, passing through the created potential difference they lose part of their energy and get into the corresponding energy levels in the second electrode. The coefficient is different from the Peltie coefficient for the electrodes with direct contact.

A device for the conversion of thermal energy into electric energy and electric energy into thermal energy in accordance with the present invention and realizing the present method comprises first electrode (1), second electrode (2) and a potential barrier (3), placed in a corpus (4). Conductors (5) connect the first electrode (1) and the second electrode (2) with output terminals (6). The device is characterized by the fact that the first electrode (1) is made from electrically conductive material, containing electrons of conductivity with kinetic energy in interval with maximum energy (Emaxl+Efl) and minimum energy (Eminl+Efl), the second electrode (2) is made from electrically conductive material containing electrons of conductivity with maximum energy (Emax2+Ef2), and potential barrier (3) with height (Ep +EfI) and width Lp, letting through predominantly the

electrons with bigger kinetic energy from the first electrode (1) into the second electrode (2). The first electrode (1), the second electrode (2), the potential barrier (3) and the corpus (4) are thermally connected.

Depending on the operating temperature of the device, the realization of the separate elements is possible in different ways. At high temperatures over the 600 ⁰ K it is necessary to be used for the first electrode (1) suitable metals or n-type semiconductors with enough width of the forbidden zone, ensuring the necessary characteristics of the semiconductor at this temperature. For the second electrode (2) p-type semiconductors can be used with enough width of the forbidden zone, ensuring the necessary characteristics of the semiconductor at this temperature. The potential barrier (3) can be a vacuum barrier or similar to the barriers, used in the classical thermionic converters. At these temperatures it is possible also to use a dielectric layer, forming a high potential barrier (3), through which the electrons with high kinetic energy tunnel. It is also possible to have a potential barrier (3) made of intrinsic semiconductor or low doped n- semiconductor

At temperatures lower than 600 ⁰ K it is preferable the device to be made as a single (solid state) element with the aid of the technologies, used in semiconductor engineering. The structure of such a device is shown in FIG.9, where on a metal plate (11), a layer of heavily doped n- semiconductor, forming the first electrode (1), thin layer of intrinsic semiconductor forming the potential barrier (3) and a layer of heavily doped p-semiconductor, forming the second electrode (2) are laid N times. On the last layer of heavily doped p-semiconductor a metal layer (12) is formed, connected by one of the connecting conductors (5) to the output terminals (6) of the device. The other connecting conductor (5) is connected to the metal plate (11). The device is put in a corpus (4). The above device has N times higher output tension than the output tension of one element.

In FIG.10 the structure of a device for the conversion of thermal energy into electric energy and electric energy into thermal energy is shown, including the device, shown in FIG.2, and characterized by the fact that the insulating (dielectric) layer (13) is placed over the first electrode (1), the second electrode (2) and the potential barrier (3), over the insulating layer (13) the steering electrode (14) is placed, while between the first electrode (1) and the steering electrode (14) an adjustable source of tension (15) is connected.

Depending on the potential of the steering electrode (14) in relation to the first electrode (1), the potential barrier (3) and the second electrode (2), under the steering electrode (14) a zone (16) is formed, in which the electrons are attracted or repulsed by the steering electrode (14), at which their passability through the barrier (3) is changed. In order to be possible to obtain full change of the output tension from Up = 0 to Up = Um, where Um is the maximum output tension of the device, it is necessary at zero tension of the steering electrode (14) the potential barrier (3) to be high enough not to let electrons pass in both directions. After feeding a positive potential to the steering electrode (14), a zone (16) is formed under it, and with the change of the potential of the steering electrode (14) the permeability of the potential barrier (3) is changed. In case that at zero tension the potential barrier (3) lets part of the electrons pass , and the zone (16) does not extend through the whole barrier (3), then with the steering electrode the influence can only be in the direction of decrease of the output tension Up of the device.

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