DE102010051754A1 - Electrical energy store for storing electrical energy generated from e.g. wind turbine, has positive and negative electrodes that are in contact with high and low work function arresters respectively - Google Patents

Abstract

The electrical energy store has a positive electrode (2) and a negative electrode (4) that are separated by a dielectric material (3). The positive electrode consisting of an n-type semiconductor, is in contact with a high work function arrester (1). The negative electrode consisting of n-type semiconductor, is in contact with a low work function arrester (5).

Description

With the desired transition of the supply of electrical energy from fossil fuels and nuclear energy to renewable energy production by wind turbines and photovoltaic electricity generated, a previously unsolved problem arises:
Regenerative power generation depends on solar radiation and wind speeds and is therefore not continuous. As a result, these forms of energy generation as such are not eligible for baseload. To meet the demand for the supply of energy, very high storage capacities are required for the electric current. Until now, this has been done to an insufficient degree by pumped storage power plants, which have efficiencies of around 80%.

Studies at European level show that the construction of new pumped storage power plants in Europe is very limited; There are no geological and hydrological boundary conditions for the construction of large additional pumped storage power plants.

All other possibilities of energy storage are not yet suitable for economically saving energies in the range of megawatts or even gigawatts. Compressed air storage, despite heat recovery losses by 30 to 40%. They require extensive storage for thermal energy and large underground caverns for storing the compressed air. Such caverns do not exist in arbitrary volumes; They also want to use them for storing carbon dioxide, which you want to separate from the exhaust of fossil-fueled power plants and store there.

One would like to use such caverns but also for the storage of hydrogen or methane. Finally, there is too little storage volume.

As further ways of storing electrical energy, the electrolysis of water to hydrogen and oxygen is discussed. The efficiency of this electrolysis is a maximum of 70%, because the energy fraction bound in the oxygen can not be used. As soon as the hydrogen is recycled by combustion in turbines, a loss of efficiency of 50 to 60% is incurred, which means a total loss of around 65%. If you wanted to convert the hydrogen back to electricity by means of a fuel cell, then the total loss would be slightly lower, by 55%. However, it has been found that fuel cell technology is uneconomical for the size of the quantities of electricity to be stored, and it has not even proven to be economical to drive vehicles in the kilowatt hour range.

The chemical conversion of hydrogen with carbon dioxide to methane, which can be transported through existing pipeline networks and could be used as a cheap storage medium, is associated with considerable conversion losses. In the chain electricity-hydrogen-methane-electricity the total loss is about 65 to 75%.

The storage of energy in magnetic fields is limited to small amounts of energy. The storage capacity of superconducting magnetic fields is much too low, the superconductivity is also destroyed by high magnetic fields. Therefore, this type of energy storage has not come out over small demonstration plants in the last twenty years.

Even electrical capacitors including the double-layer capacitors have far too low energy densities. The energy content of capacitors can not be increased much further, because only the surface of the capacitor electrodes can be used and because by influence obviously only about one electric charge can be stably stored on an area of ten by ten nanometers square; Otherwise, the repulsion potential of the charges of the same name becomes too large.

The area of the electrodes, in today's double-layer capacitors already around 1,000 square meters per milliliter, can also hardly increase, otherwise the electrical conductivity of the carbon used as well as its mechanical stability are unduly reduced. For these reasons, it has not been possible to significantly increase the volume capacity of these double-layer capacitors in recent years despite all efforts.

Flywheels are the mechanical analogue of capacitors. They are capable of providing high power in the shortest possible time, thus compensating for short-term power failures. However, they are unable to store large amounts of energy.

As storage for large amounts of energy electrochemical storage are discussed, the electrolyte can be stored separately in tanks (redox flow principle). In principle, reversible chemical reactions are carried out in a reversible battery, an accumulator, on electrodes, which are subject to the thermodynamics of chemical reactions. While an oxidation takes place at one electrode, an electrochemical reduction takes place at the counterelectrode. Even a very expensive reversible battery would be economical if it allowed a virtually infinite number of charge and discharge cycles.

Unfortunately, the chemical reactions taking place in every reversible battery are not completely reversible. Due to the thermodynamic conditions, undesirable by-products always occur, which concentrate with increasing number of cycles of charge and discharge and thus reduce the capacity of the battery from cycle to cycle.

This includes chemical changes in the electrolytes as well as unwanted oxidation states, as well as unwanted changes to the electrode surfaces or, in the case of intercalation electrodes, undesirable changes in the volume of the electrodes. The thermodynamic boundary conditions led and lead to the fact that, despite intensive research and development, there are no economic electrochemical power storage units for the operation of vehicles as well as for the storage of electrical energy in the public networks and will not exist today.

The lack of economic power storage has also led to the grotesque situation that with the expansion of wind turbines and photovoltaic systems parallel power plants must be built, which must meet the current demand quickly with decrease in renewable electricity generation. These are essentially natural gas-fired power plants that can be started up quickly.

Since the running costs of these power plants such as capital costs, maintenance or personnel continue to run during downtimes, these costs must be allocated to the terms. This makes their electricity more expensive the shorter their working hours are. The protection of the base load thus leads to a disproportionate increase in the total electricity costs as the proportion of regeneratively generated electricity increases, partly due to the standstill costs of the "stand-by power plants" and partly due to the higher electricity generation costs of the regenerative generation.

In times of high power generation by wind energy, the European network sells the surplus of wind power at low prices to other countries and, at times of declining wind energy, otherwise produces electricity there, eg from nuclear energy, at high prices and thus the goal of a high share of economic regenerative energy Energy counteracted.

It was therefore an object of the invention to find a power storage unit that combines the advantages of a reversible battery with those of a capacitor. It should be a power storage with high energy density are found in which no chemical reactions take place and thus irreversible reactions are impossible.

So it was to find a power storage in which no mass transfer via ions, but only a charge exchange takes place. In addition, no toxic materials should be used for the construction of the power storage and only those that are accessible everywhere and therefore very inexpensive. Rare earth elements or other rare materials should not or only very sparingly be used in very thin layers.

This goal of finding a capacitor which does not have the low energy density of the prior art is not new. So will with the WO 2010/114600 a capacitor claimed with an extremely increased geometric surface of the electrodes. However, these surfaces are produced by nanopatterning microelectronics and are thus hardly economically transferable to larger energy contents. With the WO 2010/083055 Nanostructured particles, so-called "quantum confinement species" are incorporated into the dielectric. Charge carriers which tunnel through the dielectric should adhere to these particles and thus increase the energy density.

The inventive idea of the present invention is quite different and consists in carrying out the charge storage not only on the surface of capacitor electrodes but also within their volume. The enclosed sketches illustrate the inventive solution of the problem:
The current storage device according to the invention consists of five flat layers of different thickness, the flat levels of which touch, with no point contacts are present. ( 1 ) and ( 5 ) are live metallic conductive arrester, at the electrode ( 2 ) is an n-type semiconductor. The layer ( 3 ) is a conventional prior art dielectric. The counterelectrode ( 4 ) consists of an n-type semiconductor.

The combination of arresters ( 1 ) and electrode ( 2 ) is characterized in that the electronic work function of the semiconductor ( 2 ) is lower than that of the surface of the arrester which contacts the semiconductor.

The difference in the work function should be at least 1 electron volt (eV), larger differences are more favorable.

The non-conducting dielectric ( 3 ) may consist of organic as well as inorganic materials such as organic polymers or inorganic compounds such as titanates. Preferred are films of organic polymers such as polypropylene, polystyrene or polyethylene terephthalate because of their good accessibility, their high dielectric strength and their chemical inertness against the contacted electrode materials. Among the polymers, polypropylene films are preferred because of their high dielectric strength of about 650 volts per micron. Such films are commercially available from film thicknesses of 2 microns.

The combination of counterelectrode ( 4 ) and arresters ( 5 ) is characterized in that the arrester surface containing the counter electrode ( 4 ), a lower work function than the semiconductor ( 4 ) having. The difference in the work function should be at least 1 eV, larger differences are more favorable.

The functionality is explained in the sketches:

Sketch 1 shows the structure of the memory according to the invention without charge. From the n-type semiconductor ( 2 ) enter into the arrester surface due to its lower work function. This causes the semiconductor to charge positively and the arrester negatively.

In the semiconductor, a thin zone is formed, which is free of mobile charge carriers or contains only a few mobile charge carriers. This zone is characterized by the forming separation surface ( 2a ) separated from the high carrier concentration volume.

Between ( 1 ) and ( 2 ) forms an electric field E2, which the charge transfer of ( 2 ) to ( 1 ) and this ends when an equilibrium is reached. Finally, in the semiconductor ( 2 ) formed a thin zone on the arrester side, which contains no or very few mobile charge carriers.

On the other side of the dielectric come out of the surface of the arrester ( 5 ) due to their lower work function electrons in the contacted semiconductor ( 4 ) one. The arrester ( 5 ) charges positively, the electrode ( 4 ) negative. At the contact to the arrester ( 5 ) arises in the semiconductor ( 4 ) a thin zone of increased charge density. Thus, the electric field E4 is built, which is the carrier current of ( 5 ) to ( 4 ) inhibits and arrives in equilibrium to a standstill. The zone of increased charge density ( 4a ) in the semiconductor ( 4 ) is stabilized by field E4.

Sketch 2 shows the processes during loading:
Via the current source S is connected to the arrester ( 1 ) the positive potential is applied to the arrester ( 5 ) the negative potential. In the charge carrier-poor zones, the electric fields E2 and E3 corresponding to the distances and the applied voltage are formed. Under the influence of the electric field E2, the moving negative charges in the semiconductor ( 2 ) to the arrester ( 1 ) and through the power source to the arrester ( 5 ).

The current source S applies the energy necessary for the movement of the charge carriers. This energy corresponds to the energy stored in the power storage. As the storage progresses, the interface shifts ( 2a ) in the semiconductor ( 2 ) more and more in the direction of the dielectric ( 3 ).

From the surface of the arrester ( 5 ), the charge carriers enter the semiconductor ( 4 ) one. Under the influence of the electric field E3, the charge carriers concentrate in the vicinity of the dielectric, and further charge carriers can be introduced into the semiconductor ( 4 ) "Pressed". With every electron coming out of the Electrode ( 2 ) Is "pulled", the field E3 increases, whereby the accumulation of charge carriers in ( 4 ) is stabilized.

Sketch 3 shows the state of the electricity store after the charge:
The semiconductor ( 2 ) is strongly depleted of negative charge carriers. Retained are the equivalent positive charges of the material, in intrinsic semiconductors atoms with lower oxidation state, in doped semiconductors, the positive charges of the doping elements. The negative charge carriers were subjected to the energy expenditure of the current source in the semiconductor ( 4 ), where they are stabilized by the electric field E3.

When unloading, sketch 4 , it comes to the charge balance between ( 4 ) and ( 2 ), the charge carriers move through the load L from the electrode ( 4 ) to ( 2 ), doing work on the load L and using the stored energy.

In a very simplified mechanical picture one can imagine the functioning of the energy storage as follows:
One volume ( 2 ) and a volume ( 4 ) each contain a working gas under the same pressure. A pump sucks the gas out of the volume ( 2 ) and press it into the volume ( 4 ). At the end of the process, the volume contains ( 2 ) a negative pressure and the volume ( 4 ) an overpressure. When pressure is equalized, the gas flows from the volume ( 4 ) into the volume ( 2 ) and can do work.

The function of the current memory according to the invention depends to a great extent on the materials used and their combinations. The surface of the arrester ( 1 ) should have the highest possible work function. For example, it can be used: metal Work for work (eV) Cu 5.0 Ni 5.2 Co 5.0 Fe 4.7

The cheapest is the use of nickel. It is quite sufficient to coat a support material, copper or aluminum, with a non-porous nickel layer, wherein the thickness of the nickel layer may be below 1 micrometer.

Are you worried about a chemical reaction with the semiconductor ( 2 ), it is advisable to coat the metallic arrester with a very inert layer which has a high work function, for example with tantalum carbide, which has a work function of 5.0 eV, with the niobium carbide Nb ₂ C having a work function of 5 , 2 eV, with the niobium carbide NbC (4.9 eV), with nickel silicides with work functions of 4.5 to 5 eV or titanium diboride with a work function of 4.7 to 5.0 eV.

The semiconductor ( 2 ) should be compared to the arrester surface ( 1 ) have the lowest possible work function. Suitable materials are the following: n-type semiconductor Work for work (eV) CaB ₆ 2.9 SrB ₆ 2.7 BaB ₆ 3.4 Mg ₃ Sb ₂ 3.5-3.8 Mg ₂ Si 3.3-3.6

The hexaborides are sintered by ceramic techniques to the layers of the required thickness. Magnesium antimonide, melting point 1,245 ° C, or magnesium silicide, melting point 1,102 ° C, however, can be cast from the melt under inert conditions analogous to the production of polycrystalline silicon to form films or layers. Other suitable semiconductors ( 2 ) the silicides are CaSi (1280 ° C), Ca ₂ Si (1.257 ° C), Sr ₂ Si (1120 ° C, SiSr (1195 ° C), Si ₃ Sr ₅ (1150 ° C) and Ba ₂ Si (1160 ° C)) Further suitable antimonides are Ca ₃ Sb ₂ (1.123 ° C), the strontium and the barium antimonide.

With the listed materials, differences in the work function of the materials ( 1 ) and ( 2 ) up to 2.5 eV.

The semiconductor ( 4 ) should be compared to the arrester surface ( 5 ) have as high a work function as possible. The following materials are suitable: n-type semiconductor Work for work (eV) Melting point (° C) Si 4.85 1410 SnS 4.2 882 Bi ₂ S ₃ 4.9 685 Sb ₂ S ₃ 4.7-4.8 630 Mg ₃ Sb ₂ 3.5-3.8 1245 Mg ₂ Si 3.3-3.6 1102

Both silicon and the other semiconductors can be processed from the melt. For reasons of availability and very precise knowledge of the properties and dopability, it is obvious that as semiconductor ( 4 ) preferably use an n-doped silicon. Also for the semiconductor ( 4 ) are under material ( 3 ) are important, the difference between the work functions between ( 4 ) and the arrester surface ( 5 ).

The to the semiconductor ( 4 ) contacted surface of the arrester ( 5 ) should have the lowest possible work function. For this purpose, the usual construction metals are not suitable, they have too high work functions.

The easiest way is to thinly coat a metal surface with yttrium or rare earth metals. Their work functions are at 3 eV, which gives a difference of almost 2 eV to the silicon.

However, there are other ways to make surfaces with lower work functions, again without the massive use of rare earth metals. For this purpose, the support metal is coated with magnesium or aluminum and then calcium, strontium, barium or rare earth metals in concentrations up to 20 atomic percent diffuse into the magnesium or aluminum. The alkaline earth metals or rare earth metals transfer negative charge to the metals aluminum or magnesium with the effect that their work function drops to values below 2 eV.

However, with the materials based on doped aluminum or magnesium, the combination with silicon as ( 4 ) critical because components of ( 5 ) into the surface of ( 4 ) and could undesirably alter the properties. In this case, the use of the listed silicides or antimonides as semiconductors ( 4 ) is displayed.

Chemically very inert surfaces of ( 5 ) whose reactivity with the contacted semiconductor ( 4 ) is very low, are titanium carbide, TiC (3.35 to 3.8 eV) or yttrium silicide, YSi _1.7 , (around 3.8 eV). Although these materials do not have very low work functions, they are so inert that no reaction with the semiconductor ( 4 ), which leads to undesirable effects such as increase of the work function or re-doping of ( 4 ) being able to lead.

As discussed later, it is more practical, for practical reasons, to use the high work function coatings of the arrester ( 1 ) and low work function of the arrester ( 5 ) not on the carrier material of the arrester, but on the respective electrodes ( 2 ) and ( 4 ).

The charge carrier concentrations in the semiconductors ( 2 ) and ( 4 ) should be as large as possible, according to the storable charge amount. However, the charge carrier concentration must not be so great that the semiconductors become metallically conductive. The carrier concentrations, either intrinsic or doped, or by combination of intrinsic conductivity and doping, should be in the range of 10 ¹⁹ to 10 ²¹ per cubic centimeter.

The thickness of the different layers depends on the task of the current memory. The thickness of the arrester is dimensioned such that when charging or discharging predetermined resistances are not exceeded in order to keep ohmic losses low. The thickness of the dielectric ( 3 ) depends on the maximum working voltage, which can range from a few volts to several thousand volts. The thickness of the semiconductor layers depends on the height of the charge to be stored and may be from a few micrometers to a few millimeters.

In principle, there is a capacitor in whose electrode volumes electrical charges are stored. After the energy content of a capacitor through E = ½C × U ² is given (C = capacity, U = voltage), one will work with the highest possible voltages in the range of several hundred volts or in the kilovolt range. Finally, there is no limit to the operating voltage through electrochemical potentials.

The dimensioning of the components of the memory is carried out according to the requirements of operating conditions and types.

The electrode materials according to the invention are thermodynamically stable. They can be formed from the melt phase, either by knife-coating the melt onto a support, with the exception of the borides, or by thermal spraying, such as plasma spraying, including borides. In these cases, it is appropriate to cool the backside of the carrier sheet, for example by passing the sheet over a chill roll.

The electrode materials ( 2 ) and ( 4 ) are compact, an inner surface is not needed to perform their function. Due to the manufacturing process, for example by coating with finely ground material, the material layers may contain pores which are distributed in the material.

Since materials with low work function are chemically very reactive, their electrons easily give off oxygen, the individual stages of the production of the energy storage according to the invention under inert conditions, ie to perform under exclusion of oxygen and moisture exclusion. This applies in particular to the execution of the contacts of ( 1 ) to ( 2 ) and from ( 4 ) to ( 5 ). Here no oxide layers or other impurities may affect the transfer of the charge carriers.

For example, it is useful when used as an electrode ( 4 ) Silicon is used to clean the silicon wafers in a sputtering apparatus from their superficial oxide layer by bombardment with argon ions and then to sputter the cleaned surface with yttrium or the other listed low work function materials.

If, for example, disks of alkaline earth hexaborides are used as electrode material ( 2 ), then it is opportune to also clean them in a sputtering apparatus of surface oxides and then coat them, for example, with titanium diboride.

The finished arrays of arrester electrode assemblies are stacked and pressed, including the dielectric. The arresters of the same polarity are connected in parallel. From the base forth arbitrary designs are possible. After the manufacture of the finished energy storage these are hermetically sealed and protected their interior so from the ambient atmosphere.

An approximate calculation results in an energy density in the range of one to two kilowatt hours per liter at a charge density of 10 ²⁰ per cubic centimeter and 1,000 volts charging voltage.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2010/114600 [0019]
• WO 2010/083055 [0019]

Claims (4)

Electric energy storage high energy density, characterized in that electric charges are stored in the volume of semiconductor electrodes, said electrodes are not microporous, optionally contain production-related pores, loading as unloading run without mass transport and that all components of the memory are solid. Electrical energy store according to claim 1, characterized in that it consists of a metallically conductive arrester ( 1 ) having a high work function surface for electrons, an electrode ( 2 ) of an n-type low work function semiconductor material for electrons, a dielectric ( 3 ), a counter electrode ( 4 ) of an n-type semiconductor material of high work function and a metallic conductive arrester ( 5 ) with a surface of low work function. Electrical energy store according to claims 1 and 2, characterized in that the work function of the electrode ( 2 ) is at least one electron volt lower than the work function of the electrode contacted to this surface of the arrester ( 1 ) and that the work function of the surface of the arrester ( 5 ) is lower by at least one electron volt than the work function of the counterelectrode ( 4 ). Electrical energy store according to claims 1 to 3, characterized in that the surface of the metallically conductive arrester ( 1 ) consists of nickel, nickel silicides, niobium carbides, tantalum carbide or titanium diboride that the n-type semiconductor electrode ( 2 ) consists of the hexaborides of calcium, strontium or barium or of the silicides or of the antimonides of the alkaline earth metals such as Mg ₂ Si or Mg ₃ Sb ₂ , that the dielectric ( 3 ) consists of polypropylene, polystyrene or polyethylene terephthalate, that the n-type counter electrode ( 4 ) of silicon, tin (II) sulfide SnS, bismuth sulfide Bi ₂ S ₃ , antimony sulfide Sb ₂ S ₃ , or from the silicides or the antimonides of alkaline earth metals such as Mg ₂ Si or Mg ₃ Sb ₂ and which is the surface of the metallic conductive Arrester ( 5 ) of yttrium, titanium carbide, TiC, yttrium silicide, YSi _1.7 , rare earth metals or aluminum doped with calcium, strontium, barium or rare earth metals or magnesium doped with calcium, strontium, barium or rare earth metals.

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