Thermal storage systems for solar power plants using oxide (earth)alkali aluminate as a filler, its use and the use of oxide (earth)alkali aluminates as a filler for thermal storage materials

Abstract

Thermal storage system for solar power plants comprising a thermal storage material for a liquid salt storage system and at least one filler, wherein the storage material is a mixture of NaNO3 and KNO3 and the filler is an oxide (earth)alkali aluminate, characterized in that the filler is present in particulate form with a particle size between 1 mm and several cm or as a porous shaped body with open channel structures with diameters between 1 mm and several cm, wherein the liquid thermal storage material is located inside the channels.

Description

[0001] The present invention relates to a heat storage device comprising a thermal storage material and a filler, wherein the filler is an oxide (earth)alkali aluminate.

[0002] Concentrated solar power (CSP) plants play a crucial role in future international energy supply. Molten salts represent the state of the art for thermal storage in concentrating solar plants. When selecting suitable salt systems, both material properties (melting temperature, enthalpy of fusion) and process boundary conditions must be considered. Nitrate and nitrite salt moltens have proven particularly suitable. Salt system (in brackets, values ​​in wt%) Melting point in °C enthalpy of fusion in J / g KNO3-LiNO3 (67-33) 133 170 KNO3-NaNO2-NaNO3 142 80 LiNO3-NaNO3 (49-51) 194 265 KNO3-NaNO3 (54-46) 222 100 LiNO3 254 360 NaNO3 306 175 KNO3 337 100

[0003] Furthermore, ternary salt systems of nitrate salts (for example in DE 102014212051 A) or halogen salts (for example in WO 2017 / 093030 A) are also described as suitable materials.

[0004] Mixtures of NaNO3 and KNO3, which can be used between 290 °C and 560 °C, have become particularly established in recent years. Although these nitrate salts are considered inexpensive as bulk chemicals at a price of approximately €800 per ton, there is a need for further cost reductions. At the same time, the efficiency of the storage system should not be reduced, or only minimally. Ideally, the cost reduction should be accompanied by an increase in efficiency to ensure the competitiveness of solar thermal power plants compared to other renewable technologies.

[0005] One concept currently under discussion is the use of fillers to replace significant amounts of solar salt. A prerequisite for this is that these fillers or additives are stable at temperatures up to 560 °C. Possible fillers under discussion include basalt, quartzite, or even SiO2, Al2O3, or TiO2 (for example: Breidenbach et al., Energy Procedia 2016, 99, 120-129 or Chieruzzi et al., Nanoscale Research Letters 2013, 8:448).

[0006] US 4421661 A describes a high-temperature direct-contact thermal energy storage element for storing thermal energy at about 400 °C to about 2000 °F, consisting of: a safety container containing about 10 to about 90 percent by volume of a solid-liquid phase-change chemical with a phase-change temperature of about 400 °C to about 2000 °F, selected from the group consisting of alkali metal and alkaline earth carbonates, chlorides, nitrates, nitrites, fluorides, hydroxides, sulfates, and mixtures thereof, which are stored in the pores of a sensitive thermal energy storage support material through which thermal storage and heat recovery fluid can circulate, selected from the group consisting of metal oxides, aluminates, titanates, and zirconates, with a submicron particle size, the particles of which do not undergo significant thermal cycling up to about 2000 °F. coarsen,and with a surface area of ​​more than approximately 10 square meters per gram of storage medium, where the storage medium comprises approximately 10 to approximately 90 percent of the volume of the heat storage element.

[0007] DE 1519589 A1 describes a closed heat transfer system, which may include a container heated by a conventional heat source, and contains a heat transfer medium consisting primarily of alkali metal hydroxide, a non-reducing agent, and preferably also a corrosion inhibitor. The mixture is heated to a temperature between 120 °C and 600 °C during use. Sodium hydroxide is the preferred alkali metal hydroxide, but potassium, sodium, lithium, rubidium, and cesium hydroxides, as well as mixtures thereof, can also be used.The non-reducing agent may consist of an alkali metal or alkaline earth metal sulfate, phosphate, halide, carbonate, stannate, silicate, fluorosilicate, fluoroborate, tetraborate, metaborate, borate, aluminate, bismuth, borate, borite, sulfate, sulfite, molybdate, tungstate, vanadate, nitrate, nitrite, manganate, permanganate, chlorate, perchlorate, persulfate, chromate, dichromate, hypochlorite or oxide, or of mixtures of two or more such compounds.

[0008] DE 102012100820 A1 describes a separating element (102) for a heat storage device (100) for storing heat in a storage medium, wherein, by means of the separating element (102), a storage space (114) of the heat storage device (100), in which storage medium can be received, can be separated from a flow space (116) of the heat storage device (100), which can be flowed through by a flow medium, characterized in that the separating element (102) comprises a base body (117) which is provided with at least one protective layer (134) to protect the base body (117) from the storage medium and / or from the flow medium.

[0009] The use of inexpensive natural fillers proves impractical, however, as at least silicate-containing mineral raw materials are quickly corrosively attacked by the molten salts. Reactions with, for example, NaNO3 and / or KNO3 also alter the enthalpy of fusion and the melting point of the salt. Oxides, such as Al2O3 or TiO2, are more suitable as fillers. However, accelerated time tests also show significant reactions with the solar salt for Al2O3. While ZrO2 and TiO2 are stable as fillers in a NaNO3-KNO3 salt melt, their high cost precludes any cost reduction.

[0010] To date, no filler for thermal storage materials has been found that is inert to the commonly used nitrate and / or nitrite salts or halogen salts, possesses a high heat capacity, and is sufficiently cost-effective. The object of the present invention is therefore to provide a filler that avoids the disadvantages of the prior art. Surprisingly, it has been found that oxide aluminates with alkali or alkaline earth metals as cations do not exhibit the disadvantages of the prior art.

[0011] In a first embodiment, the problem underlying the present invention is solved by a heat storage device for solar power plants comprising a thermal storage material for a liquid salt storage device and at least one filler material, wherein the storage material is a mixture of NaNO3 and KNO3 and the filler is an oxide (earth)alkali aluminate, characterized in that the filler is present in particulate form with a particle size between 1 mm and several cm or as a porous shaped body with open channel structures with diameters between 1 mm and several cm, wherein the liquid thermal storage material is located inside the channels

[0012] Preferably, the aluminates are selected such that the cation corresponds to a cation in the thermal storage material. For example, if KNO3 and NaNO3 are used as liquid solar salt as the thermal storage material, sodium aluminate and / or potassium aluminate are preferably used as fillers. An alkali aluminate is therefore preferably used. An alkaline earth aluminate is also preferably used. According to the invention, it is possible to use only one aluminate. However, mixtures of two, three, or more aluminates can also be used. Different aluminates within the meaning of the present invention also include, for example, different forms of sodium aluminate. Wherever sodium aluminate is mentioned herein, this refers to all types of sodium aluminate.

[0013] Aluminates are formally salts of aluminum acid HAlO2:H2O, in which aluminum is a complex anion [Al(OH)4] -Sodium aluminates form with hydroxide ions as ligands, as well as salts in which the anion exists as a condensate of the aluminate ion. Sodium aluminates are mixed oxides of sodium and aluminum with the general formula Na⁻¹⁰. x Al y O z , which are also called anhydrous aluminates. Such solid, anhydrous aluminates are particularly preferred according to the invention. The general composition of such compounds is M I [Al(OH)4] with M as the monovalent cation. Fully condensed (anhydrous) compounds have the general composition M I AlO2 with AlO2 - as an anion.

[0014] Examples of suitable aluminates are NaAl(OH)4, NaAlO2 or NaAl 11 O 17 Other suitable aluminates include, for example, calcium aluminate Ca3[Al(OH)4]2(OH)4 or magnesium aluminate (MgAl2O4).

[0015] In the thermal storage unit, the filler partially replaces the storage material. The thermal storage unit thus comprises the thermal storage material and the filler; preferably, it consists of the thermal storage material and the filler. The total volume of the thermal storage unit is 100%. Preferably, the filler is present in a proportion of 10% to 80% by volume, more preferably 20% to 70% by volume, particularly 30% to 60% by volume, and more preferably 40% to 50% by volume, based on the total volume of the thermal storage unit. For example, proportions of 30% to 80% by volume, and particularly 40% to 70% by volume, are suitable. The solid filler is intended to reduce the cost of the thermal storage unit. The proportion of molten storage material ensures virtually undiminished heat transfer compared to pure molten salts.

[0016] The proportion of fusible storage material is therefore preferably between 20 vol.% and 90 vol.%, particularly between 30 vol.% and 80 vol.%, and preferably between 40 vol.% and 70 vol.%. A proportion of thermal storage material of 30 vol.% to 60 vol.% is particularly suitable.

[0017] According to the invention, the storage material is a mixture of NaNO3 and KNO3. Other compounds containing oxyanions, such as other nitrates, nitrites, sulfates, and carbonates, are not considered storage materials according to the invention. Mg, Ca, Ba, Sr, K, Na, and Li can be selected as cations. Other storage materials not according to the invention include, with the exception of mixtures of NaNO3 and KNO3, (earth)alkali nitrate salts and / or (earth)alkali nitrite salts, whereby salts from the K / Na nitrate / nitrite material system can also be used. Typical compositions are 57 to 63 wt% NaNO3 and 37 to 43 wt% KNO3 with a nitrite content of 0.5 to 8 wt%.

[0018] Alternatively, the storage material not according to the invention can be selected from alkali halogen salts and / or alkaline earth halogen salts. These are preferably anhydrous. MgCl₂ is suitable, for example.

[0019] The intended operating temperatures are particularly in the range of 130 °C to 700 °C, preferably in the range of 170 °C to 560 °C. It has been shown that the filler according to the invention is stable at these temperatures and does not react with the storage material.

[0020] The filler according to the invention is a solid. It is present, for example, in particulate form, particularly as a powder, spherical particles, or granules. Particle sizes between 1 mm and several cm are preferred. It is also preferred that the filler is present as a larger molded body with open channel structures, wherein the thermal storage material (liquid) is located within channels. Suitable diameters for these channels are also between 1 mm and several cm.

[0021] In a further embodiment, the problem underlying the present invention is solved by using oxide (earth)alkali aluminates as a filler for molten salt storage in a solar power plant. The fillers and storage material (molten salt) are preferably as described above.

[0022] In the following exemplary embodiment, the present invention is further explained in a non-limiting manner. Examples: Example 1: X-ray diffractometry

[0023] To investigate the chemical stability of aluminates towards thermal storage materials, sodium aluminate powder with a particle size <100 µm was stored in a NaNO3-KNO3 (60-40) melt for one month at 560 °C. Subsequently, the salt mixture was examined by X-ray diffractometry and analyzed using the Rietveld method. The measurements are presented in Fig. 1 shown. The X-ray diffractogram showed no evidence of reaction products of sodium aluminate with NaNO3 or KNO3.

[0024] The following table shows the percentage shares obtained using the Rietveld method before and after outsourcing: At the beginning of the experiment Analysis after outsourcing Mass [g] Mass [%] Mass [%] (Rietveld) NaNO3 2,4000 39,99 34,31 KNO3 1,6004 26,67 28,30 NaAlO2 2,0006 33,34 35,87 NaNO2 1,53

[0025] The nominal increase in NaAlO2 is due to the fact that the salt content in the non-hermetically sealed experimental setup decreases by 0.32 g, or 5.5%, through decomposition (new formation of NaNO2) and evaporation. Therefore, relating the unchanged mass of NaAlO2 to the reduced mass of the salt (3.68 g) results in a value of 35.3%, which corresponds well with the experimentally determined value within the margin of error. Thus, there is no evidence of a loss of NaAlO2. Example 2: EDX analysis

[0026] A pressed sodium aluminate (NaAlO2) pellet was positioned on a platinum mesh and immersed in molten solar salt at 560 °C (duration: 1 month). The pellet was removed, and the remaining solar salt was analyzed by EDX. The EDX analysis showed no evidence of aluminum and therefore no indication of sodium aluminate dissolution. The measurement is in Fig.Figure 2 shows the small peak at approximately 2.1 keV, which is due to platinum. The SEM samples were coated with platinum to prevent charging effects.

Claims

[1] Thermal storage system for solar power plants comprising a thermal storage material for a liquid salt storage system and at least one filler, wherein the storage material is a mixture of NaNO3 and KNO3 and the filler is an oxide (earth)alkali aluminate, characterized by , that the filler is present in a particulate form with a particle size between 1 mm and several cm or as a porous molded body with open channel structures with diameters between 1 mm and several cm, wherein the liquid thermal storage material is located inside the channels. [2] Heat storage device according to claim 1, characterized by that the filler is present in a proportion of 10 vol.% to 80 vol.%, preferably 20 vol.% to 70 vol.%, in particular 30 vol.% to 60 vol.%, preferably 40 vol.% to 50 vol.% based on the total volume of the storage material. [3] Heat storage device according to claim 1 or 2, characterized by that at least one alkali aluminate is used as a filler. [4] Heat storage device according to claim 1 or 2, characterized by that at least one alkali aluminate or alkaline earth aluminate is used as a filler. [5] Heat storage device according to any one of claims 1 to 4, characterized by , that NaNO3 and KNO3 are present in a weight ratio of 60 to 40. [6] Heat storage device according to any one of claims 1 to 5, characterized by that the filler includes and in particular consists of sodium aluminate. [7] Heat storage device according to any one of claims 1 to 6, characterized by that the filler is in the form of a powder, spherical particles or granules. [8] Use of a heat storage device according to any one of claims 1 to 7 in a solar power plant at temperatures of 700 °C or less. [9] Use of oxide (earth)alkali aluminates as a filler for thermal storage materials in a solar power plant, wherein the storage material is a mixture of NaNO3 and KNO3, characterized by, that the filler is present in a particulate form with a particle size between 1 mm and several cm or as a porous molded body with open channel structures with diameters between 1 mm and several cm, wherein the liquid thermal storage material is located inside the channels. [10] Use according to claim 9, characterized by that NaNO3 and KNO3 are present in a weight ratio of 60 to 40 and that the (earth)alkali aluminate is present in particulate form.

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